I. EXECUTIVE SUMMARY
III. ADMAP OVERVIEW
IV. STATUS OF REGIONAL MAGNETIC ANOMALY MAPPING
VI. MAGNETIC ANOMALY COMPILATION: PROCEDURES, SOFTWARE
VII. SATELLITE MAGNETIC ANOMALIES
VIII. COMPLEMENTARY DATA SETS
X. FUTURE PLANS
XII. REFERENCES CITED
B) Members of the Working Group
C) Catalogue of Magnetic Data Sets for the Antarctic
D) Workshop Participants
E) Workshop Agenda
F) Workshop Abstracts
I. EXECUTIVE SUMMARY
A second workshop (ADMAP II) was held at the Instituto Nazionale di Geofisica in Rome, Italy, from 29 Sep. to 2 Oct. 97 to consider progress on the development of a digital magnetic database for the production of an Antarctic magnetic anomaly map. ADMAP II attracted 40 scientists from 8 countries with magnetic programs in the Antarctic. In addition to 33 scientific presentations on this international effort, the following actions were taken:
1) The protocols from ADMAP I were reaffirmed, whereby existing
Antarctic magnetic data holdings will be made available to the World Data
Centers (WDCs) by 1999 for inclusion in the digital magnetic database and
anomaly map. Magnetic data collected by any future program will be
made available to the WDCs within 6 years of the completion of the field
2) The Working Group (WG) will work to complete by Spring '99 the integration of near-surface and satellite survey data into regional compilations for the Weddell Sea sector [(15°-135 °) E], the East Antarctic sector [(135°-255°) E], and the Ross Sea sector [(155°-15°) E].
3) In Spring '99, the production and analysis of these 3 regional compilations will be featured at the ADMAP III workshop at the Byrd Polar Research Center in Columbus, Ohio, and in a special session of the American Geophysical Union meeting in Boston, MA.
4) To facilitate the production and geologic applications of the Antarctic digital magnetic database, the WG established subcommittees for a) gravity data compilation, b) rock physical properties compilation, c) advising on mapping procedures, d) magnetic reference field development, and e) developing funding opportunities for the WG's proposals.
5) Ash Johnson stepped down as ADMAP co-chair and was replaced by Peter Morris at the Aug '97 meeting of the IAGA WG V-9 Subcommittee on Polar Magnetic Anomalies in Uppsala, Sweden. ADMAP's current co-chairs are Ralph von Frese (BPRC) and Peter Morris (BAS).
6) Papers related to ADMAP's activities will be prepared and submitted to peer-review for publication in a special issue of the journal ANNALI DI GEOFISICA.
The Antarctic is the most poorly understood region of the planet. However, the geology of the Antarctic maintains an important record of Gondwana and Rodinia evolution, and hence it is the focus of extensive international scientific inquiry. Geologic studies of the Antarctic are greatly aided by magnetic anomaly data because of the region's nearly ubiquitous cover of snow, ice, and sea water. Consequently, numerous near-surface magnetic surveys have been carried out for site-specific geologic objectives by the international community.
As a result of the first ADMAP workshop (ADMAP I) in Cambridge, UK, it became clear that these individual magnetic surveys may be combined into a regional magnetic synthesis that would further enhance their utility for geologic studies (Johnson et al., 1996; 1997). Accordingly, ADMAP was launched in 1995 to compile and integrate into a digital database all existing near-surface and satellite magnetic anomaly data collected in Antarctica and surrounding oceans south of 60° S.
This multinational project is encouraged by resolutions (see Appendix XII.A) of the Scientific Committee on Antarctic Research (SCAR) and the International Association of Geomagnetism and Aeronomy (IAGA). An international Working Group (WG) was established to implement ADMAP's objectives that reports to SCAR and the IAGA WG V-9 (Magnetic Anomalies: Land & Sea) Subcommittee on Polar Magnetic Anomalies. An initial co-chair of the WG was Ash Johnson who stepped down at the Aug. '97 assembly of IAGA in Uppsala, Sweden, and was replaced by Peter Morris. Current co-chairs of the ADMAP WG are Ralph von Frese and Peter Morris. Appendix XII.B lists the current WG members of ADMAP.
As a result of ADMAP I (Johnson et al., 1996; 1997), protocols were adopted by the WG for making existing and future magnetic data sets available to this international effort. In particular, by 1999, existing Antarctic magnetic data holdings are to be deposited in the World Data Centers and any future data set is to be made available to the WDCs within 6 years of the completion of the field survey.
A plan for implementing ADMAP's objectives was also developed involving local-, regional-, and continental-scale compilation efforts (Johnson et al., 1996; 1997). At the local scale, investigators are working with their agencies to format their current data holdings for access and use by the international community. At the regional scale, investigators are working in international collaborations to integrate their holdings into regional maps for the Weddell Sea sector [(15°-135°)E], the East Antarctic sector [(135°-255°)E], and the Ross Sea sector [(155°-15°)E]. At the continental scale, these regional compilations will be combined into a compilation for the entire hemisphere south of 60° S. Satellite magnetic data are being processed for their crustal components to facilitate merging the airborne, marine and terrestrial magnetic data sets, and to help supplement gaps in their coverage. An updated catalogue of magnetic data sets that are available for this effort is given in Appendix XII.C. Figure 1 shows the current track-line distribution of near-surface magnetic surveys from our catalogue.
To enhance geologic applications of the digital magnetic database, ADMAP I also recommended extending the WG's compilation efforts to incorporate the gravity anomaly data into a digital database and gravity anomaly map for the Antarctic south of 60° S (Johnson et al., 1996; 1997).
To consider the progress and problems of this international effort, a second ADMAP workshop (ADMAP II) was held at the Istituto Nazionale di Geofisica (ING) in Rome, Italy from 29 Sep. to 2 Oct. 97. ADMAP II involved the participation of 40 geoscientists from 8 countries with magnetic programs in the Antarctic (see Appendix XII.D for a list of the participants). The workshop featured 33 technical papers with the first scientific results produced under the ADMAP banner. These presentations considered the geological analysis and interpretation of Antarctic satellite, airborne and shipborne magnetic anomalies and also reviewed the availability of these data sets for ADMAP. The agenda and related abstracts for ADMAP II are given in Appendices XII.E and XII.F, respectively.
In the sections that follow, we review the activities of each workshop session and summarize the actions taken by the SCAR/IAGA Working Group as a result of the ADMAP II meeting.
III. ADMAP OVERVIEW
The co-chairs summarized the evolution and future prospects of ADMAP. R. von Frese traced the history of ADMAP to long standing efforts to improve the magnetic databases for both polar regions. He pointed out that funding requests for major geophysical field equipment and programs to agencies of governments with interests in both the Arctic and Antarctic would benefit greatly if the support of both polar communities of geoscientists could be mustered. Major equipment items such as aircraft or ships, for example, can be implemented to service the requirements of both Arctic and Antarctic geoscientists because the related field seasons are mutually exclusive. In particular, it would be very desirable to field long-range aircraft to collect regional tie-lines for the Arctic and Antarctic that facilitate the merger of disparate magnetic surveys into effective regional compilations. However, because the field seasons are different, the two polar communities rarely come together at the same national and international scientific meetings, and hence efforts to coordinate their activities in support of common interests have been very limited to date.
A. Johnson summarized the activities of ADMAP since the Cambridge workshop. He noted that the clock had started on the implementation of the data exchange protocols, and that significant progress was being made by the WG on compiling the regional databases for all three Antarctic sectors. He also reported that he was leaving the British Antarctic Survey for private industry where his capacity to help lead ADMAP's activities would be significantly limited. Accordingly, he would be stepping down as co-chair upon the conclusion of the ADMAP II workshop. This development was considered earlier at the Aug'97 meeting of the IAGA WG V-9 Subcommittee on Polar Magnetic Anomalies in Uppsala, Sweden, where P. Morris was asked to replace him and serve as ADMAP co-chair with R. von Frese.
IV. STATUS OF REGIONAL MAGNETIC ANOMALY MAPPING
This session considered geological applications of aeromagnetic and shipborne magnetic surveys.
A) Tectonic Interpretation of Aeromagnetic Anomalies
S. Golynsky presented aeromagnetic data and their interpretation over portions of the East Antarctic Shield, Weddell Sea Embayment, the Mesozoic magmatic arc of the Antarctic Peninsula and the Precambrian Haag Nunataks. These data indicated that the East Antarctic magnetic zone may be divided into four distinct units that include the Archean-to-Middle Proterozoic Grunehogna Province, the Middle-to-Late Proterozoic metamorphic belt of the Maudheim Province, Jurassic intrusions, and a marginal mobile belt of an ancient cratonic fragment located in Coats Land. Magnetic highs in the Weddell Sea region may reflect magmatic rocks related to the initial rifting of Gondwana. Cretaceous plutons also appear to correspond to aeromagnetic highs. Based on the aeromagnetic data, the Precambrian rocks of the Haag nunataks are interpreted to underlie parts of the Ellsworth-Whitmore Mountains, the Ronne Ice Shelf and southern Palmer Land.
J. Ferris showed the interpretation of aeromagnetic data from a 1997 survey that were merged with earlier data sets over the Weddell Sea at the margin of the Antarctic Peninsula. The 1997 aeromagnetic survey was flown over the Black Coast area of the southeast Antarctic Peninsula that straddles the junction between stretched continental crust and true oceanic crust. Linear belts of positive aeromagnetic anomalies are interpreted to reflect volcanic piles formed as a result of rifting. Prominent linear offsets in the anomaly patterns may be related to faults. The large Orion anomaly may be caused by a thick section of basalt that reflects the beginning of true oceanic crust.
M. Ghidella presented possible tectonic and structural interpretations of the magnetic anomalies on the Antarctic Peninsula area. A large amplitude positive anomaly along the Pacific margin of the Antarctic Peninsula may have been produced by calc-alkaline intrusions that formed the roots of a Cretaceous magmatic arc. In the Bransfield Basin area, the anomaly is dissected by a magnetic low that may reflect a basin which formed possibly as a result of rifting during the opening of the Scotia Sea and extended the older magmatic arc.
J. Behrendt showed how the widely-spaced aeromagnetic data from the late 1970's was used to site the surveys that were flown in the late 1980's and during the 1990's. He also showed data from the 1995/96 field season that are being incorporated into an extensive 5 km aeromagnetic anomaly database for a large part of the West Antarctic Rift System. These data facilitate geological interpretations that range from continental scale tectonic features to more local structures such as the subglacial presence of active volcanoes. The latter proposal, made possible owing to coincident ice-sounding radar data, has implications for both the formation of the ice sheet and it's recent history.
F. Ferraccioli presented data from the 1996/97 German-Italian GITARA expedition in the Ross Sea area, which is a prime example of the collaboration between institutes from different nations that can lead to major advances in Antarctic research. The data were interpreted as imaging geological structures that vary from Palaeozoic suture zones to Cenozoic alkaline volcanics. The accuracy of these modern data enabled confident correlations to be made between mapped geological features and the magnetic anomalies, such as for the Jurassic basalts of the Rennick Graben. These new data provide an important contribution to the ongoing magnetic anomaly compilation for the Ross Sea sector, which among other things will address issues like the evolution of the Palaeo-Pacific margin of Gondwana.
In the final presentation of this session, J. Ferris showed reprocessed data over the Dufek intrusion and surrounding areas of the Filchner and Ronne ice shelves. Regional data from American, British, Danish, and Russian reconnaissance surveys were merged with more closely spaced lines in the area of the Dufek Massif. A new aeromagnetic map of the area resulted with improved data density so that a re-assessment of the previously proposed extent of the Dufek Massif was suggested. The recompiled data also enabled a timing sequence of emplacement to be proposed, involving several phases of extension in the southern Weddell Sea Embayment during the initial stages of Gondwana break-up. The presentation provoked lively discussion that further emphasized the need for acquiring new aeromagnetic and gravity data over this and surrounding regions - which of course is a prime objective that ADMAP seeks to encourage.
B) Analysis and Interpretation of Marine Magnetic Surveys
Five talks were presented on the reduction and interpretation of Antarctic marine magnetic data. J. Labrecque presented a new technique for compilating existing geomagnetic data sets of the marine and continental shelf areas of the Antarctic. P. Morris demonstrated a new compilation of marine magnetic data for the region north of the Antarctic Peninsula. E. Lodolo described marine magnetic data collected by the R/V OGS-Explora since 1987 in the Ross Sea, the southwestern Pacific Ocean, the Scotia Sea, and along the Pacific margin of the Antarctic Peninsula and Southern Chile.
Y. Nogi presented an interpretation of vector magnetic anomalies that were measured in 1988 by the icebreaker Shirase in the Southern Indian Ocean. T. Ishihara described the compilation of marine magnetic and gravity data for the Prydz Bay area, East Antarctica. The magnetic anomaly map of the Prydz Bay and adjacent slope area was also presented.
V. EXTERNAL AND INTERNAL MAGNETIC FIELD PROBLEMS
The accurate estimation of magnetic anomalies requires the effective removal of the core and external field components from the magnetic observations. J. Torta discussed the important role of the Spanish Antarctic geomagnetic observatory in reducing aeromagnetic survey data. He pointed out the need to to take into account the average crustal magnetic anomaly level of the area in which the observatory is located.
A. De Santis discussed the need for improving the accuracy of the Antarctic core field. He also considered the possibility of developing a Laplacian core field model from Antarctic geomagnetic observatory data and near-surface and satellite surveys using spherical cap harmonic analysis.
G. Gregori reviewed the sources of the polar external magnetic fields and efforts to model them for their complex geometric and time varying properties. In a second presentation, he considered the separation of core and lithospheric magnetic field components according to the so called power spectrum of the geomagnetic field.
The main purpose of this session was only partly accomplished by the four presentations. To cover these fundamental and complex problems adequately, more discussions were necessary that might also have included Arctic perspectives. Future workshops should continue assessing these problems and the progress in resolving them.
VI. MAGNETIC ANOMALY COMPILATION: PROCEDURES, SOFTWARE AND APPLICATIONS
Merging disparate polar magnetic surveys into regional compilations involves relatively unique digital data processing problems and considerations. C. Flinn shared the experiences of the USGS in integrating magnetic surveys of portions of the northwestern US into coherent digital compilations. She pointed out the advantages of an accurate geomagnetic reference surface for minimizing level shifts between data sets.
S. Maschenkov described the experiences and facilities of VNIIOkeangeologia in St. Petersburg, Russia for compiling polar magnetic survey data into regional databases and maps. These facilities were largely put together in collaboration with the efforts of the Geological Survey of Canada to compile the Arctic magnetic anomaly map. A. Golynsky reported on the use of these facilities to produce magnetic anomaly grids from combined Russian, British, Australian and German survey data. Maps from these data have been produced for areas that include the southern Weddell Sea and Queen Maud Land, Enderby Land, and the region of the Prince Charles Mountains.
M. Chiappini reported on the progress that had been made to date in acquiring, re-processing and compiling aeromagnetic anomaly data of the GITARA and GANOVEX surveys for the integrated Transantarctic Mountains and Ross Sea area magnetic anomaly project (INTRAMAP). D. Damaske reported on the completion of the magnetic data grids from the aeromagnetic surveys of GANOVEX.
A. Johnson focused on industry procedures for merging magnetic anomaly data grids. This approach may be used to great advantage where pre- existing, internally consistent survey grids are available such as have been or are being produced for the Antarctic. C. Jewel described the on-line database system developed by Leeds University that could prove useful for ADMAP's data compilation and dissemination efforts. Adopting the Leeds' system would also facilitate incorporating ADMAP's data sets into an international program that is working to produce the global magnetic anomaly map.
Discussions related to this session resulted in the recommendation that a subcommittee be formed to advise ADMAP particiapants on the use of these procedures for producing maps and related digital databases from polar magnetic survey data.
VII. SATELLITE MAGNETIC ANOMALIES
The crustal components in satellite magnetic observations can yield insight on the evolution of the Antarctic lithosphere. They can also facilitate efforts to adjust near-surface magnetic survey data for errors in line and survey leveling, as well as in core and secular field corrections.
R. von Frese showed how regional gravity fields and spectral correlation theory may be applied to separate crustal, core, and external field effects in satellite geomagnetic observations. By this approach, he estimated the crustal components in the Antarctic Magsat data due to crustal thickness differences and intracrustal variations of magnetization. Geological applications of the Antarctic Magsat data are problematic, however, because the mission was flown in the austral Summer and Fall periods when highly agitated external fields operated to strongly contaminate the observations. Hence an important test of the veracity of the Antarctic crustal Magsat map will be provided by the upcoming Ørsted satellite magnetic mission that will obtain minimally contaminated data from the austral Winter and Spring periods.
M. Purucker described the newly available magnetic data set from the POGO satellites that operated from 1967 until 1971. These satellites flew at significantly higher altitudes than Magsat, but they also operated over several austral winters when the south polar region was in darkness. Hence they provide data to test the longer wavelength components of the Antarctic crustal Magsat map. Preliminary analyses show that there are sufficient data to make separate total field anomaly maps for the 400-500 km altitude range and the 500-600 km altitude range. In general, POGO anomaly features compare well with a number of Antarctic Magsat anomalies.
VIII. COMPLEMENTARY DATA SETS
Geological analyses of the digital magnetic database will be greatly facilitated by the availability of related geophysical and rock property data for the region south of 60° S.
A) Compilation of Gravity Anomaly Data
R. Bell reported on a new U.S. initiative to compile a digital gravity database and map of the Antarctic. The goal is to develop an on-line Antarctic gravity database that will provide access to improved high resolution satellite gravity models, in conjunction with shipborne, airborne and land based gravity measurements for the continental regions.
R. von Frese reported on the status of satellite measurements of gravity over the Antarctic. After reviewing the gravity coverage available from the EGM96 model, he outlined the improvements that are expected from the upcoming CHAMP and GRACE missions. Estimates of the shorter wavelength anomalies of the marine gravity field from Geosat altimetry were found to compare well with gravity anomalies measured from shipborne surveys.
A. Golynsky reviewed the extensive land and airborne gravity measurements that are available over the Weddell Sea and East Antarctic regions and presented a series of new maps and interpretations for these areas.
B) Other Data Sets
E. Tabacco discussed the possibility of integrating the magnetics with radio echo sounding (RES) measurements that are being compiled within the framework of the Antarctic BEDMAP project (ref.???). He presented new Italian airborne RES data in the Dome C area that indicated the possible presence of subglacial bedrock faults. Studies of tectonic features such as these would be more effective where the RES are also constrained by magnetic data.
L. Sagnotti discussed the fascinating possibility of using the magnetics for paleoenvironmental studies. He reported on a magnetic study of the CIROS-1 core that was recovered in the Victoria Land Rift Basin. It showed climatic changes at the Late Eocene-Early Oligocene boundary when major tectonic and magmatic events also occurred in the Ross Sea region.
R. Bell presented examples of using of CASERTZ magnetic anomaly mapping for studying the tectonic controls on the advance of the West Antarctic ice-sheet. She also presented results from a new collaborative US-Italian-German aerogeophysical project for the Transantarctic Mountains and Wilkes Basin, and the Pensacola Pole Basin.
Discussion at the end of this session focused on the necessity to include all available rock magnetic measurements in the ADMAP database to enhance the utility of the magnetic anomaly data for crustal studies. P. Taylor and others recalled how the aeromagnetic anomaly compilation efforts by the Geological Survey of Finland had benefitted from the availability of rock magnetic property information.
IX. OPEN DISCUSSIONS
This was a mixed session with longer periods of discussion and with presented papers.
The utility of aeromagnetic, satellite, ground magnetic, marine magnetic
measurements in the study of several differing but fundamental topics concerning
the Antarctic continent and surrounding oceans was discussed.
The Antarctic continent is traditionally subdivided in two major geographical and geological provinces: East Antarctica and West Antarctica. East Antarctica is a large Precambrian shield forming the core of Gondwana and possibly of Rodinia. Magnetics and gravity over
East Antarctica are a tool to be used to better constrain the extent and character of individual Archean cratons and Proterozoic to Paleozoic mobile belts
fringing or reactivating the cratons.
West Antarctica is believed to represent a collage of independant the regional magnetic anomaly maps for the three sectors be joined at the boundaries?
As a result of these discussions, the following conclusions were made regarding the present status of the three regional compilations:
1) For the Weddell Sea sector [(255°-15°) E], BAS hopes to be able to coordinate the compilation of all available airborne, shipborne and land-based data sets within the next two years. The role of satellite magnetic data in adjusting the compilation for long wavelength errors is currently being evaluated at BPRC.
2) For the East Antarctic sector [(15°-135°) E], the compilation involves mainly Russian airborne and land-based data as there is very little else available. The compilation of these data has been pretty much completed by VNIIOkeangeologia. Marine data need to be included, but the coverage is sparse. Efforts to incorporate satellite data into the compilation have not yet been initiated.
3) For the Ross Sea sector [(135°-255°) E], ING through the INTRAMAP Project (Chiappini et al., 1997) plans to coordinate the compilation of available airborne, shipborne and land-based data sets within the next two years. Efforts to incorporate satellite data into the compilation have not yet been initiated.
X. FUTURE PLANS
In view of the considerable progress that was demonstrated at ADMAP II in achieving the goals of the project, the WG expects to have the major elements of the digital magnetic database completed by Spring '99. The WG plans to present these results at the ADMAP III workshop and the Spring '99 AGU meeting.
ADMAP III will be held at the Byrd Polar Research Center of The Ohio State University just before the AGU meeting at which the WG also plans to have a session devoted to ADMAP's activities.
A report on the activities of the ADMAP II workshop will be completed for dissemination by the next meeting of the WG at the Spring '98 AGU meeting in Boston (USA).
The proceedings of the ADMAP II workshop will be published in a special issue of the journal ANNALI DI GEOFISICA (AdG). This publication is also open to ADMAP-related papers from those who could not attend ADMAP II. Manuscripts should be submitted to the guest editors, M. Chiappini and R. von Frese, by 15 Apr. 98. Instructions for authors as well as other information are available from the ADMAP website at the URL:
A schedule of the planned activities of the SCAR/IAGA WG through Spring '99 is given in Table 1 below.
||ADMAP II Workshop at Rome, I|
||Submit EOS article on ADMAP II Workshop|
||Circulate preliminary draft of ADMAP II report
Submit AdG-papers to Guest Editors
||Deadline for feedback on first draft of ADMAP II report|
||Approve final draft of ADMAP II report at Spring '98 AGU meeting in Boston, MA|
||Print and distribute ADMAP II report|
||Complete reviews of AdG-papers|
||Complete revisions of AdG-papers|
||ADMAP III Workshop at BPRC, Columbus, OH
ADMAP Session at Spring'99 AGU in Boston, MA
||ADMAP Session at 8th ISAES in Wellington NZ|
To facilitate the production and geological applications of the digital magnetic database, the WG established the following subcommittees:
1) GRAVITY DATA COMPILATION, chaired by Robin Bell (LDO) with members Phil Jones (BAS), John Labrecque (JPL), Ralph von Frese (BPRC), Patrick Taylor (NASA), Sergei Maschenkov (VNIIOkeangeologia), Takemi Ishihara (GSJ), Marta Ghidella (IAA), Emanuele Bozzo (UNIGE), and Uwe Meyer (BGR), will work to compile gravity anomaly data into a digital database and anomaly map for the Antarctic south of 60° S.
2) ROCK PHYSICAL PROPERTIES, chaired by Anne Grunow (BPRC) with members Mike Purucker (NASA), Peter Wasilewski (NASA), Juha Korhonen (GSF), and Minoru Funaki (GSJ), will work to compile rock magnetic and other physical properties into a database for the Antarctic.
3) MAPPING ADVISORY, chaired by Carol Finn (USGS) with members Peter Morris (BAS), Massimo Chiappini (ING), Fausto Ferraccioli (UNIGE), and Sergei Maschenkov (VNIIOkeangeologia), will help to coordinate ADMAP compilation procedures, data formats, merging techniques, etc.
4) MAGNETIC REFERENCE FIELD, chaired by Angelo De Santis (ING) with members Ralph von Frese (BPRC), Mike Purucker (NASA), Miquel Torta (OE), Massimo Chiappini (ING), Patrick Taylor (NASA), and Giovanni Gregori (IFA-CNR), will coordinate efforts to develop a more accurate Antarctic core field model for a variety of applications including better determination of crustal magnetic anomalies.
5) OPPORTUNITIES ADVISORY, chaired by Ralph von Frese (BPRC) with members Antonio Meloni (ING), Sergei Maschenkov (VNIIOkeangeologia), and Peter Morris (BAS), will work to identify funding and other opportunities for proposals from the WG, and any of its members.
Each subcommittee is charged to advise the WG in the area of its focus and to contribute progress reports to ADMAP workshops and reports. Subcommittee members, however, have considerable flexibility in organizing their subcommittee's activities and scope.
The Working Group expresses its heartfelt thanks to Ash Johnson for his tireless efforts to get this project off the ground and running strongly. ADMAP has also benefited from the efforts of Colin Reeves (Chair, IAGA Div. V on Observatories, Instruments, Surveys, and Analyses), Patrick Taylor (Chair, IAGA WG V-9 Subcommittee on Polar Magnetic Anomalies), and Jacob Verhoef (GSC Project Leader of the Arctic/North Atlantic Magnetic Anomaly Compilation). Support for the Working Group's activities has been provided by the Scientific Committee on Antarctic Research (SCAR), Istituto Nazionale di Geofisica (ING), the National Science Foundation (NSF) of the USA, the British Antarctic Survey (BAS) of the Natural Environment Research Council (NERC), and the Byrd Polar Research Center (BPRC) and the Center for Mapping (CFM) of The Ohio State University, and the Ohio Supercomputer Center.
XII. REFERENCES CITED
Chiappini, M., Ferraccioli, F., Bozzo, E., Damaske, D., and J. up V.9 subcommittee on magnetic anomalies of the polar regions for the production of a database for a magnetic anomaly map of Antarctica and its surrounding oceans.
2) SCAR Recommendation XXIII SEG-5 (Rome, Italy; August, 1994)
Recognizing the value of regional magnetic compilations for enhancing the understanding of continental-scale geological features and encouraging further work, and
noting IAGA WGV.9 Resolution #4 (1993),
the Solid Earth Geophysics Working Group recommends the creation
and publication of a magnetic anomaly map and digital database for the
Antarctic continent and surrounding oceans, and encourages all countries
holding magnetic data to contribute those data to the project.
3) IAGA Resolution #1 (Uppsala, Sweden; August, 1997)
noting the ability of satellites to provide unparalleled spatial and temporal coverage of observations of the Earth's magnetic and gravity fields, and
recognizing the revolutionary contribution that an extended time-series of such observations would make to a wide spectrum of geoscientific and space science studies, and
welcoming the present plans by several nations to launch potential-field satellites within the next 5 years,
considers that now is a favorable time for an international effort to promote and coordinate satellite surveys to achieve, for the first time, continuous monitoring of geopotential field variability in the near-Earth environment, and
recommends that an "International Decade for Geopotential-Field Research" be declared to provide an international focus for such efforts.
4) IAGA Resolution #2 (Uppsala, Sweden; August, 1997)
recognizing the importance of obtaining world-wide coverage of gravity and magnetic data, and
recognizing the technical difficulties of routing satellites over regions close to the geographic poles,
urges the community to consider using conventional land,
marine, and airborne methods for completing gravity and magnetic anomaly
coverage in the polar regions.
D) Workshop Participants
|Dr. John BEHRENDT
INSTAAR - University of Colorado
Boulder, CO 80309-0450
Tel: (1) 303 4923733
Fax: (1) 303 4926388
|Dr. Robin E. BELL
Lamont-Doherty Earth Observatory of
P.O. Box 1000
Torrey Cliff Road - 61 Route 9W
Palisades, NY 10964-8000
Tel: (1) 914 3658827
Fax: (1) 914 3658179
|Prof. Emanuele BOZZO
Dipartimento di Scienze della Terra - DISTER
Università degli Studi di Genova
V.le Benedetto XV, 5
Tel: (39) 10 3538095
Fax: (39) 10 3538055
|Dr. Giuliano BRANCOLINI
Osservatorio Geofisico Sperimentale
P.O. Box 2011 - Borgo Grotta Gigante
Tel: (39) 40 2140251
Fax: (39) 40 327307
|Mr. Giorgio CANEVA
Dipartimento di Scienze della Terra - DISTER
Università degli Studi di Genova
V.le Benedetto XV, 5
Tel: (39) 10 3538093
Fax: (39) 10 3538095
|Dr. Massimo CHIAPPINI
ING - Istituto Nazionale di Geofisica
Via di Vigna Murata, 605
Tel: (39) 6 51860313
Fax: (39) 6 5041181
|Dr. Detlef DAMASKE
Bundesanstalt für Geowissenschaften und Rohstoffe
Tel: (49) 511 6432692
Fax: (49) 511 6432304
|Dr. Angelo DE SANTIS
ING - Istituto Nazionale di Geofisica
Via di Vigna Murata, 605
Tel: (39) 6 51860327
Fax: (39) 6 5041181
|Dr. Fausto FERRACCIOLI
Dipartimento di Scienze della Terra - DISTER
Università degli Studi di Genova
V.le Benedetto XV, 5
Tel: (39) 10 3538092
Fax: (39) 10 3538055
|Dr. Julie FERRIS
British Antarctic Survey
High Cross, Madingley Road
CAMBRIDGE CB3 OET
Tel: (44) 1223 251582
Fax: (44) 1223 362616
|Dr. Carol A. FINN
U.S. Geological Survey
MS 964, Box 25046
Denver Federal Center
Denver, CO 80225
Tel: (1) 303 2361345
Fax: (1) 303 2361708
|Dr. Fabio FLORINDO
ING - Istituto Nazionale di Geofisica
Via di Vigna Murata, 605
Tel: (39) 6 51860385
Fax: (39) 6 5041181
|Dr. Marta E. GHIDELLA
Dto. de Ciencias de la Tierra
Instituto Antartico Argentino
1010 BUENOS AIRES
Tel: (54) 1 8126313
Fax: (54) 1 8122039
|Dr. Alexander GOLYNSKY
Dept. of Antarctic Geology
1, Angliysky pr.
ST. PETERSBURG 190121
Fax: (7) 812 1141470
|Prof. Giovanni P. GREGORI
IFA - CNR
Solar-Terrestrial Relations & Global Change
P.le Sturzo, 31
Tel: (39) 6 59293017
Fax: (39) 6 5915790
|Dr. Takemi ISHIHARA
Geological Survey of Japan
Marine Geology Department
TSUKUBA Ibaraki 305
Tel: (81) 298 543591
Fax: (81) 298 543589
|Mr. Chris JEWELL
Dept. of Geophysics
86 Brookside Road
NW11 9NG LONDON
Tel: (44) 181 4585557
Fax: (44) 181 4551037
|Dr. Ash JOHNSON
Geosoft Europe Ltd
Airborne and Marine Applications
20/21 Market Place - First Floor
Oxfordshire OX10 ODY
Tel: (44) 1491 835231
Fax: (44) 1491 835281
|Dr. John LABRECQUE
Jet Propulsion Laboratory
California Institute of Technology
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Tel: (1) 818 3547827
Fax: (1) 818 3934369
|Dr. Emanuele LODOLO
Osservatorio Geofisico Sperimentale
Dept. of Geophysics of the Lithosphere
P.O. Box 2011 - Borgo Grotta Gigante
Tel: (39) 40 2140359
Fax: (39) 40 327307
|Prof. Carlo LUSETTI
Istituto Idrografico della Marina
Passo Osservatorio, 4
Tel: (39) 10 24431
Fax: (39) 10 261400
|Prof. Sergei MASCHENKOV
1, Angliysky pr.
ST. PETERSBURG 190121
Tel: (7) 812 1145892
Fax: (7) 812 1142088
|Dr. Antonio MELONI
ING - Istituto Nazionale di Geofisica
Via di Vigna Murata, 605
Tel: (39) 6 51860317
Fax: (39) 6 5041181
|Dr. Andrea MORELLI
Istituto Nazionale di Geofisica
Via di Vigna Murata, 605
Tel: (39) 6 51860443
Fax: (39) 6 5041181
|Dr. Peter MORRIS
British Antarctic Survey
Dept. of Geosciences
CAMBRIDGE CB3 OET
Tel: (44) 1233 251574
|Ms. Silvia NARDI
ING - Istituto Nazionale di Geofisica
Via di Vigna Murata, 605
Tel: (39) 6 51860405
Fax: (39) 6 5041181
|Dr. Yoshifumi NOGI
National Institute of Polar Research
Division of Research - Earth Science
1-9-10 Kaga, Itabashi
Tel: (81) 3 39624789
Fax: (81) 3 39625741
|Ms. Loredana PROTO
ING - Istituto Nazionale di Geofisica
Via di Vigna Murata, 605
Tel: (39) 6 51860318-319
Fax: (39) 6 5041181
|Dr. Michael PURUCKER
NASA - Goddard Space Flight Center
Geodynamics Branch and Hughes STX
Code 921 - NASA/GSFC
Greenbelt, MD 20771
Tel: (1) 301 2864736
Fax: (1) 301 2861616
|Prof. Carlo Alberto RICCI
Dipartimento di Scienze della Terra
Università degli Studi di Siena
Via delle Cerchia, 3
Tel: (39) 577 298818
Fax: (39) 577 298815
|Dr. Leonardo SAGNOTTI
ING - Istituto Nazionale di Geofisica
Via di Vigna Murata, 605
Tel: (39) 6 51860321
Fax: (39) 6 5041181
|Prof. Ezio TABACCO
Università di Milano
Via Cicognara, 7
Tel: (39) 2 23698406
Fax: (39) 2 7490588
|Dr. Patrick TAYLOR
NASA - Goddard Space Flight Center
Code 921 - NASA/GSFC
Greenbelt, MD 20771
Tel: (1) 301 2865412
Fax: (1) 301 2861616
| Dr. Miquel TORTA
Observatori de l'Ebre
Roquetes - Tarragona
Tel: (34) 77 500511
Fax: (34) 77 504660
|Prof. Ralph VON FRESE
The Ohio State University
Byrd Polar Research Center & Dept. of Geological Sciences
125 S. Oval Hall
Columbus, Ohio 43210-1308
Tel: (1) 614 2925635
Fax: (1) 614 2927688
| Ing. Mario ZUCCHELLI
ENEA - Progetto Antartide
Via Anguillarese, 301
00060 S. Maria di Galeria (RM)
P.O. Box 2400
Tel: (39) 6 30484939
Fax: (39) 6 30484893
E) Workshop Agenda
Monday 29 September 1997
||M. Chiappini Istituto Nazionale di Geofisica
(ADMAP Local Organizing Committee)
A. Meloni Istituto Nazionale di Geofisica
C.A. Ricci Universitá di Siena
M. Zucchelli ENEA-PNRA
|10:30-10:55||R.R.B. von Frese
Polar Magnetic Anomaly Maps: Problems and Progress
ADMAP: An Introduction to the Antarctic Digital Magnetic Anomaly Project
1.1: Tectonic Interpretation of Aeromagnetic Anomalies
Chair: C. Finn
Magnetic Anomaly Imprints of the Major Tectonic Provinces in the Weddell Sea Region
Interpretation and Merging of 1997 Aeromagnetic Data with Existing Data Sets over The Weddell Sea, Antarctic Peninsula Margin
Structural Magnetic Anomalies on the Antarctic Peninsula area: Possible Tectonic Implications
Chair: A. Johnson
dAeromagnetic Evidence for a Volcanic Caldera(?) Complex beneath the Divide of the West Antarctic Ice Sheett
A Tectonic Perspective of Newly Acquired GITARA 5 (96/97) Aeromagnetic Data and its Utility for Ongoing Magnetic Anomaly Compilation of Victoria Land (Antarctica)
Form and Extent of the Dufek Intrusion, Antarctica, from Newly Compiled Aeromagnetic Data
Chair: S.P. Mashenkov
|15:15 - 15:40|| J.L. LaBreque
Integration and Error Correction on Circum-Antarctic Marine and Continental Shelf Magnetic Anomaly Data Set
|16:00 - 16:10||P. Morris
Compilation of Marine Magnetic Data North of the Antarctic Peninsula
|16:10 - 16:35||E. Lodolo
The Osservatorio Geofisico Sperimentale marine magnetic surveys in the Antarctic Seas
|16:35 - 17:00||Y. Nogi
Vector Magnetic Anomalies in The Southern Indian Ocean
|17:00 - 17:25||T. Ishihara
Compilation of shipborne magnetic and gravity data of the Prydz Bay area, East Antarctica
|17:25 - 17:50||Open Discussion on Session 1|
Tuesday 30 September 1997
Session 2: EXTERNAL AND INTERNAL MAGNETIC FIELD PROBLEMS
Chair: A. Meloni
|9:00-9:25||G. P. Gregori
Modeling the external magnetic sourcesi
A New Geomagnetic Observatory at Livingston Island (South Shetland Islands). Implications for Future Regional Magnetic Surveys
|9:45-10:00||G. P. Gregori
The separation of fields originated in the core, in the asthenosphere, and within the crust
|10:00-10:20||A. De Santis
Proposal for a geomagnetic reference field of Antarctica
Session 3: MAGNETIC ANOMALY COMPILATION: PROCEDURES, SOFTWARE AND
Chair: R.R.B. von Frese
Procedures for Merging Magnetic Surveys into a Digital Regional Antarctic Compilation
Computer Processing for Magnetic Anomaly and Other Geodata in the Polar Regions
Merging aeromagnetic data collected at different levels: the GEOMAUD Survey
Magnetic Compilation: a grid-based approach
First stages of the INTRAMAP: INtegrated Transantarctic Mountains and Ross Sea Area Magnetic Anomaly Project
Magnetic Anomaly Map of Antarctica: first compilation based on the Russian and British aeromagnetic data
Managing ADMAP data
|15:00-15:30||Discussion on Session 3|
Session 4: SATELLITE MAGNETIC ANOMALIES
Chair: A. De Santis
|16:00 - 16:25||R. R.B. von Frese
Satellite Magnetic Anomalies of the Antarctic Crust
|16:25 - 16:50||M. E. Purucker
A New Satellite Anomaly Map of the Antarctic from POGO Data
|16:50 - 17:30||Discussion|
Wednesday 1 October 1997
Session 5: COMPLEMENTARY DATA SETS
Chair: M. Purucker
5.1 Compilation of Gravity Anomaly Data
|9:00-9:35||R. E. Bell
Proposal for Development of a New Generation Gravity Map of Antarctica
|9:35-10:00||R. R. B. von Frese
Satellite Mapping of the Antarctic Gravity Field
Gravity Studies of the Weddell Sea Region and the Central Sector of East Antarctica: Free-air and Bouguer Anomaly Map Compilation
5.2 Other Data Sets
Chair: F. Ferraccioli
|10:45 - 11:10||E. Tabacco
Latest improvements and future perspective for the echo sounding system of the glaciologic Italian group in Antarctica
|11:10 - 11:25||L. Sagnotti
Environmental Magnetic Study of the CIROS-1 Sequence: Evidences for Antarctic Climatic Changes During the Late Eocene/Early Oligocene
|11:25 - 11:50||R. Bell
Collaborative Aerogeophysical Studies of the Transantarctic Mountains and the Wilkes Basin
|11:50: - 12:40||Discussion|
Afternoon - evening: Social Events
Thursday 2 October 1997
Chair: P. Taylor
|9:00-10:30||Producing ADMAP with maximum utility for geological applications|
|10:50 - 12:40||Producing ADMAP with maximum utility for geological applications|
Chair: P. Morris
|14:00 - 15:40||Open questions|
Chair: R.R.B. von Frese
|16:00 - 17:45||Future planning|
F) Workshop Abstracts
As a part of extensive earth science explorations conducted by "Sevmorgeologia", PMGRE in Antarctica, regional gravity surveys were accomplished over vast areas in the Weddell Sea sector westward 6W and in the central sector of East Antarctica (20 -90 E) between 1972 and 1991. These surveys comprised land gravity measurements supplemented by seismic soundings and, beginning from 1978, airborne gravity observations. In the course of multi-years studies 2400 land-gravity stations have been obtained over the total area of 780,000 km2 with spacing every 10-30 km, and approximately 100,000 km of aerogravity profiles with 20 km line spacing have been recovered over coastal and adjacent sea regions. The land base data sets are characterized by standard errors estimated less than 1 mGal for absolute gravity values, about 2.5 mGal and over 4 mGal for free-air and Bouguer anomalies respectively. Internal crossover errors for aerogravity data sets widely range from 4.0 mGal to 10 mGal owing to a number of technical difficulties aroused during flights. Regardless of great magnitude of errors decreasing reliability of acquired aerogravity data, they have an obvious significance providing insight into general structural features and crustal morphologies especially in poorly studied regions, e.g. the southern Weddell Sea.
With a view to compile the coherent gravity maps of surveyed regions the available data sets collected over a number of years and preserved at present in tabular catalogues form have been digitized and reprocessed. All gravity data are referred to IGSN71. The 1930 International Gravity Formula was used for the latitude corrections. Reprocessing was mainly concerned the aerogravity data. After internal adjustment and correction for latitude separate data sets were merged and levelled. To adjust the discrepancies among the incorporated data intersection values were minimized applying constant gravity offsets to lines. The uniformly processed data were averaged to an equally spaced grid values by means of minimum curvature algorithm. The grid interval was selected as a one half of line spacing - 10 km. The land gravity data sets were treated separately and converted to a grid form using 5km and 10 km interval proceeding from spacing between stations of observations in studied areas. Before contouring all grid data sets were transformed to grid with 5 km interval and filtered by running mean technique with a 15 km radius. Data processing and mapping was made by using software developed in Geological Survey of Canada-Atlantic.
Newly compiled gravity maps displaying the general distribution of obtained Free-Air and Bouguer anomalies over vast areas enable to define their relationships with major crustal features in studied regions, as well as to judge and evaluate isostatic balance of recognized crustal units. In the coastal regions Free-Air anomalies in the main closely correspond with sub-ice topography. The mountained areas of the western and eastern Dronning Maud Land, of the Enderby and the MacRobertson Lands with elevations over 1000 m above sea level are marked by high amplitude up to 160 mGal positive anomalies. Broad and elongated gravity lows reaching in places -80 mGal outline sub-ice bedrock depressions and extented valleys of major outlet glaciers ( e.g. Shirase and Lambert Glaciers) with depths varied from -500 m to -1500 m. While gravity high of up to 60 mGal mapped over coastal plain in the Ingrid Christensen Coast apparently associated with high density middle crust as evidenced by deep seismic soundings. The most conspicuous feature in Free-Air anomaly map is the elongated dipolar anomaly stretching along the continental margin. Along strike it varies in width and total high-low amplitude primarily reflecting morphologic features of the shelf edge/slope area. The prominent gravity highs of this marginal anomaly with amplitudes from 100 to 140 mGal, observed in Riiser-Larsen Sea margin and in eastern Weddell Sea margin, are supposedly attributed to high density material intruded into lower/middle crust and related to rifting events during separation Gondwana assemblages. The pronounced Bouguer anomalies of up to 90 mGal mapped in Ronne-Filchner Ice Shelf area are coincided with a chain of very deep troughs over -1200 m and are supposedly caused by rising of upper mantle as an isostatic compensation of graben-rift structures.
In June 1997 a proposal was submitted to the U.S. National Science Foundation to support the development of a new generation gravity map of Antartica. The requested funding will cover the development of a web based access tool and the entry of data as it becomes available. We hope to have a response to this proposal by the early fall. To be sucessful this project must be closely coordinated with ADMAP and BEDMAP as well as SCAR.
The goal of this project is to develop an on-line Antarctic gravity database which will facilitate access to improved high resolution satellite gravity models, in conjuction with shipboard, airborne and land based gravity measurements for the continental regions. This compilation will provide an important new tool to the Antarctic Earth science community from the geologist needing to place field observations in a regional context to the seismologist studying continental scale mantle structure. The proposed gravity database will complement the parallel projects underway to develop new continental bedrock (BEDMAP) and magnetic (ADMAP) maps of Antarctica.
An international effort will parallel these ongoing projects in contacting the Antarctic geophysical community, identifying existing data sets, agreeing upon protocols for the use of data contributed to the database and finally assembling a new continental scale gravity map. The proposed project has three principal stages. The first stage will be to investigate the accuracy and resolution of currently available high resolution satellite derived gravity data and quantify spatial variations in both accuracy and resolution. High quality, GPS navigated marine gravity data will be used for this assessment as well as for a detailed investigation of the recoverability of short wavelength gravity anomalies in the satellite gravity model. In addition, we will use the satellite gravity data to assess the quality of archival, pre-GPS shipboard gravity data by identifying offset anomalies arising from navigation errors. The second stage of this project will be to develop an interactive method of accessing existing satellite, shipboard, land based and airborne gravity data via a Web based interface. The Lamont-Doherty Earth Observatory RIDGE Multi-beam bathymetry database will be used as a template for this project. The existing on-line RIDGE database allows users to access the raw data, the gridded data and raster images of the seafloor topography. A similar structure will be produced for the existing Antarctic gravity data. The third stage of this project will be to develop an international program to compile existing gravity data south of 60° S. This process will begin by presenting the concept to both the ADMAP and BEDMAP communities and to the appropriate working groups of SCAR. The goal will be to present a preliminary map of existing data at the Antarctic Earth Science meeting in Wellington in 1999. A gravity working group meeting will be held in conjunction with the Wellington meeting to reach a consensus on the protocols for placing data into the database. By the end of this project we aim to complete the preliminary basemap based on presently accessible data, the identify the existing gravity data, and the resolve international protocols for placing data in this on-line database.
The divide of the West Antarctic Ice Sheet, at an elevation of about 1800 m, is underlain by the 400-km-long Sinuous Ridge, which rises several hundred meters above sea level and bisects the Byrd Subglacial Basin. This structure was first defined by widely-spaced aeromagnetic and radar ice-sounding flights made in 1978-79.
Aeromagnetic data acquired in 1995-96 by the US Support Office for Aerogeophysical Research (SOAR), show a prominent circular pattern of anomalies over the Sinuous Ridge, suggesting a volcanic caldera(?) complex about 70 km diameter. An interesting alternate speculation that the circular feature is evidence of a meteorite impact structure cannot be ruled out. However, the volcanic interpretation is more likely considering exposed late Cenozoic volcanoes about 300 km distant. We calculated magnetic- and pseudogravity-terrace maps from these data, which significantly enhance the prominent circular structure.
Magnetic models, constrained by radar ice sounding data, show that the sources of these anomalies are at the base of the ice sheet overlying the Sinuous Ridge. The prominent positive and negative anomalies range from 1200 to -500 nT, very high considering that the shallowest possible sources lie at a minimum of 1.2 km below the survey aircraft.
Although the magnetic models all fit the data using present field directions, both normal and reversed magnetizations are required. This indicates that the sources were emplaced during periods when the earth's field was both normal and reversed.
Because the Antarctic plate has been essentially stationary for the past 100 Ma we cannot infer age from the direction of magnetization. However, based on previous interpretations of magnetic anomalies over the West Antarctic Ice Sheet it is likely that the sources beneath the ice divide are late Cenozoic volcanic rocks associated with the West Antarctic rift system. Active volcanism was previously interpreted from the CASERTZ survey in the area of the rift shoulder and beneath the Ross sea continental shelf, but we cannot make such an inference from available data over the Sinuous Ridge.
Previously published interpretations of closely spaced aeromagnetic surveys over the CASERTZ and Ross Sea-Ross Ice Shelf areas of the West Antarctic rift system showed a linear rift fabric. In contrast, the magnetic survey over the Sinuous Ridge area of the West Antarctic Ice Sheet shows a pronounced circular character and no apparent linearity to the distribution of magnetic anomalies. This observation, combined with the 2-km structural relief of the Sinuous Ridge suggests a different (younger?) age for the feature. Is it possible that uplift of the Sinuous Ridge forced the advance of the West Antarctic Ice Sheet when the ridge was subaerial, before the onset of glaciation over the present Byrd Subglacial Basin?
The INTRAMAP (INtegrated Transantarctic Mountains and Ross sea Area Magnetic Anomaly Project) aims, by means of a compilation of existing aeromagnetic, ground magnetic and marine magnetic data, to contribute to the study of a tectonically crucial area located between the active West Antarctic Rift System, the old Ross Orogen tectonic province and the East Antarctic Craton. Geological and geophysical studies along the Transantarctic Mountains (TAM) and over the ice-covered Polar Plateau margin (where only few seismic lines exist, gravimetric coverage is incomplete and sparse and outcrop is mainly at the coastal regions) rely extensively on airborne and ground magnetic surveys. In the Ross Sea area aeromagnetic and marine magnetic data support the extensive seismic findings. The integrated datasets will offer the unique opportunity to study the magnetic signature of the tectono-dynamical relationship between the uplifted TAM rift shoulder and the subsided Ross Sea. The project will be distinguished into several phases: i) acquisition, re-processing and compilation of aeromagnetic anomaly data of the GITARA and GANOVEX surveys as well as of ground magnetic data; ii) extension of the compilation to marine magnetic data; iii) regional interpretation of the main tectonic and geologic features off and onshore; iv) contribution to continental scale efforts such as the ADMAP (Antarctic Digital Magnetic Anomaly Project).
INTRAMAP not only will create a uniform magnetic dataset for the Ross Sea - Transantarctic Mountains but ultimately will try and lead to fundamental improvements in the tectonic and geologic knowledge of the area.
The data used in the present paper belong to the GANOVEX (IV) surveys, flown at different altitudes, in three separate sections, owing to topographic elevation, and to the GITARA 1, 2 and 3, flown at same altitude.
Ground magnetics collected at Terra Nova Bay area, during 1985-1989 seasons were used as well, covering about 15,000 km2.
Different processing techniques were undertaken prior to our reprocessing.
Base station correction with a low pass filter; IGRF correction; levelling (least square method (ties and profiles) and statistical tie line levelling); gridding using minimum curvature (ties and profiles and only profiles) with different grid cell size ( 440 and 880 m); microlevelling (frequency domain directional filtering) and Empirical corrections (DC level shifts applied to single sectors in some cases).
From a general point of view, several factors can be responsible for significant differences in magnetic anomaly values relative to adjacent surveys: errors in navigation and positioning; discrepancies in elevation; IGRF and empirical corrections; levelling procedures and edge effects. Currently there is a widespread use of digital enhancement techniques for improved interpretation of regional magnetic images; thus, it is necessary to minimize such differences along survey boundaries.
Whatever technique one wishes to apply to achieve this, the general criteria in integration of the datasets are: i) continuity in values, gradients and curvature of regional and, if possible, local anomalies across survey boundaries; ii) minimal distorsion of original information contained in the datasets; iii) visual and spectral continuity.
We followed three different independent approaches: linking lines, grids, and "an integrated approach".
The properties of the earth's core magnetic field and its secular variations are poorly known for the antarctic. The increasing availability of magnetic observatory data and repeat stations, as well as magnetic observations from airborne and satellite surveys offers the promise of greatly improving our understanding of the antarctic reference core field. We investigate the possible development of a laplacian model field from these observations using spherical cap harmonic analysis.
Possible uses and advantages of this approach relative to the standard global reference field will also be considered.
A new aeromagnetic survey (1996/97) has been performed in the XII Antarctic Italian expedition by the GITARA (German Italian Aeromagnetic Research in Antarctica) Group in the Salamander Range- Evans Névé sector of Northern Victoria Land, in the framework of international geophysical investigations of the Ross Sea area; 36,000 Km2 were successfully covered. Flight altitude was 9000 ft for most of the survey, with profile line spacing of 4.4 km for the regional grid and 2.2 km for the detailed one; tie line interval was 22 km. Two main objectives were given a high priority in the GITARA 5 planning and execution: i) to contribute to the study of the “local” tectonic structures; ii) to link as best as possible existing regional GANOVEX surveys, thus contributing to ongoing magnetic anomaly compilation efforts, which represent an important approach for describing the “regional” geologic setting of the Transantarctic Mountains and adjacent Ross Sea.
From a tectonic point of view the surveyed area is characterized by two main structures: a) the Paleozoic suture zone between the Wilson, Bowers and Robertson Bay Terranes; b) the central part of the Mesozoic-Cenozoic Rennick Graben. The southeastern sector is also marked by rift shoulder Cenozoic alkaline volcanics (The Pleiades). The hybrid magnetic anomaly- geologic images we produced provide new effective tools to map and study these features. The most prominent high frequency magnetic anomalies occur over the Pleiades (with peaks above 1000 nT) and over the axial Jurassic basalts of the Rennick Graben (> 500 nT), while more moderate anomalies (< 100 nT) correspond to Jurassic Ferrar sills. A broad positive anomaly may be correlated to the Devonian- Carboniferous Admiralty Intrusive Complex stiching the terranes. Subtle high frequency anomalies are aligned with the NW trending Bowers-Robertson Bay suture zone (Millen Schist?). Several sets of magnetic trends (NW, NNW, NE-SW) are further compared to geologically mapped or inferred faults. In particular, by studying magnetic lineaments and anomaly patterns due to the Jurassic tholeitic rocks, we are starting to propose a redifinition of the master faults and of the internal structure of the Rennick Graben compared to previous geologic interpretations. The “graben” has recently been described from geologic evidence as a pull-apart basin of Cenozoic or Cretaceous age, likely tectonically related to the offshore Victoria Land Basin, one of the major rift basins of the Ross Sea.
This recent re-interpretation, which requires a transtensional kinematic regime is “locally” compatible with the new magnetic results. It therefore now appears even more necessary that the study of the entire Rennick Graben-Victoria Land Basin system from the Pacific Coast to the Ross Sea should be a major target of ongoing magnetic anomaly compilation. Previous magnetic (aeromagnetic and ground magnetic) studies of the Northern Victoria Land terranes did not reveal remarkable features over the Cambro-Ordovician suture zones. Our study does show linear trends especially between the Bowers and Robertson Bay Terranes. Thus we propose that the compilation should also try to address the signature or lack of signature of the terrane boundaries in a broader context of the accretionary paleo-Pacific margin of Gondwana and, if possible, seek for magnetic evidence for transtensional reactivation of these inherited basement structures as recently proposed for example from seismic evidence offshore.
The Black Coast area of the southeast Antarctic Peninsula is a key to our understanding of the geology of the region as it straddles the junction between the Antarctic Peninsula crustal block, the Weddell Sea, and the Weddell Sea embayment. The Weddell Sea embayment has previously been interpreted from reconnaissance aeromagnetic data, as stretched continental crust, comprising a number of N-S trending horst blocks. The boundary between this stretched crust and the true oceanic crust of the Weddell Sea, correlates with the E-W trending Orion anomaly, interpreted as resulting from extensional volcanism, in a similar setting perhaps to the Voring Plateau, offshore Norway. Information about this margin, its shape, and affect on the mainly N-S trends of the Antarctic Peninsula block, is critical to our understanding of Gondwanaland breakup in this region.
A data gap existed along the whole of the Black Coast between the British Antarctic Survey onshore surveys and the Russian reconnaissance data offshore. A survey was flown during the 1996/97 season to cover this data gap, using a long-range aircraft flying from Rothera research station.
The flight line direction intersected both the N-S and the E-W trends. The survey was drape-flown over the mountains and flown at variable barometric altitudes, according to cloud heights over the sea ice. The magnetic field within the survey area itself varies from wavelengths of less than 1 km over the mountains of the Antarctic Peninsula to more than 10 km over the Weddell Sea embayment, where the thickness of the sediment is estimated to be up to 15 km. The new data set has been merged with the offshore reconnaissance data, whilst preserving maximum detail over the Black Coast.
Interpretation of the merged data has thrown up a number of new features. The Orion anomaly appears to be just one of a number of similar sub-parallel features; these have been interpreted to be caused by volcanic piles, marking earlier or simultaneous rifting attempts. The line of discrete anomalies following the Black coast are now known to continue for some distance offshore. As several of the topographic features have been mapped as Late Cretaceous? Intrusions, the string of islands following the coastline were all interpreted to be intrusions. However, the magnetic field is flat over Dolleman Island, and so it is unlikely to represent an intrusion of similar magnetic properties to those of the Eilson Peninsula.
A lineation is seen to cross the Weddell Sea, offsetting the Explora anomaly, several of the anomalies sub-parallel to the Orion anomaly, and truncating the Black Coast intrusion anomalies. At the point where it intercepts the coast, a fault is already mapped, with a similar trend. This lineament joins the Peninsula at the geomorphological change between Palmer Land and Graham Land.
Comparisons with the satellite gravity data show a good correlation between the gravity continental margin anomaly and a long wavelength magnetic anomaly parallel with the Black Coast. The gravity anomaly bends to follow the Orion anomaly offshore, but some 100 km to the South of it. This might be expected as the gravity anomaly should represent the edge of the Continental crust, whilst the Orion magnetic anomaly probably represents, a pile of basalts forming the beginnings of true oceanic crust.
The Dufek intrusion, in the northern part of the Pensacola Mountains, Antarctica, is part of an extensive, Middle Jurassic igneous province that was related to, and emplaced just prior to Gondwana break-up. It has been described as one of the largest layered gabbro intrusions in the world, and is exposed as two non-overlapping gabbro sequences in the Dufek Massif and the Forrestal Range. A new compilation of aeromagnetic data over the Dufek intrusion and surrounding region is based on mapping at two scales, a 2 km grid spacing over the Dufek intrusion, and a 6 km grid spacing over the surrounding region. Advanced processing of the data is used to produce an analytical signal and a terrace map as a basis for a new interpretation.
Modelling and interpretation of the data show that the Dufek intrusion is not as extensive as was thought, and that it is more similar in size to the Stillwater Intrusion in North America, than to the Bushveld Complex in South Africa to which it has previously been compared. The reduced size of the Dufek intrusion is due to reinterpretation of an anomaly over Berkner Island, to the north of the Pensacola Mountains, as that of an uplifted basement block rather than a continuation of the Dufek intrusion. The Dufek intrusion is modelled as two separate intrusive phases of a composite intrusion. The main part is a dipping intrusive sheet displaced by a normal fault which accounts for two parallel magnetic anomalies over the Forrestal Range. The minor part forms the Dufek Massif itself, which the magnetic data show was emplaced on a separate trend and is of a shallower origin than the Forrestal phase. Magnetic lineaments on the compilation map are used to establish a possible chronology of events. We suggest that the emplacement of both phases of the Dufek intrusion was preceded by a period of extensional block faulting which uplifted the Berkner Island basement block, and was succeeded by a further period of extensional faulting involving a component of strike-slip deformation during the initial stages of Gondwana break-up.
There are several important steps required to complete a digitally `merged aeromagnetic map. All programs used in the merging process are available at the U. S. Geological Survey (USGS), Denver, CO.
2. Grid all data sets at the appropriate spacing (.33 to .2 of the original line spacing) with the same projection.
3. Remove the International Geomagnetic Reference Field (IGRF) from the gridded data using the same flight altitude as the observed data. The USGS uses the definitive IGRF (DGRF). DGRF model years include 1945, 1950, 1960, 1965, 1970, 1975, 1980, 1985, 1990 and 1995. Fields in intervening years are calculated by linear interpolation of the coefficients from the surrounding models. All surveys should be reduced using the same model.
For data corrected with a reference field different from the accepted model, the old reference field should be added and the appropriate DGRF removed. One of the biggest sources of shifts between adjacent surveys is the difference in the reference model removed.
4. It may be necessary to level individual flight lines within a survey area if there were navigation problems or the diurnal was not adequately removed. This is either done by using crossovers at tie lines to adjust the level or by comparing the observed data to a reference surface (like a smoothed version of the original data if tie lines are unavailable).
5. Continue all data sets to the same level. Aeromagnetic surveys can be flown two ways: level (constant elevation) or draped (flown a constant clearance above terrain). To prepare these grids for merging, one must mathematically generate the magnetic field as it would be at a determined flight altitude, either level or draped. Some grids need to be continued up or down by a constant amount; other grids will need to be converted from one surface to another (drape-to-level or level-to-drape). The USGS typically continues surveys to an elevation of 300 m above the terrain surface. Regional topographic data are available for much of Antarctica and can be used to create a flight elevation grid. For the Antarctic compilation it might be easiest to pick a clearance above the surface (whether it be rock, ice or water), because of the lack of bedrock elevation and bathymetric data. Upward continuing data sets to the level of the highest survey is not a good option because high resolution data are degraded. If any of the continuation operations involve downward continuation, the data should be filtered during or after continuation. Aliasing may occur during downward continuation a distance greater than one grid interval, resulting in rings of anomalies. Regridding to a coarser grid interval may help, but causes a loss of resolution.
6. Regrid all continued data sets to the same grid interval and origin increments. The choice of grid interval will depend on the desired resolution of the final map and capabilities of computer programs to handle large grids.
7. Create a reference grid (optional). It is useful to have a grid to which the individual surveys can be referenced to minimize level shifts between data sets and to have a common magnetic datum. The satellite data, continued to the selected flight elevation, might provide a suitable datum.
8. Determine a constant difference across survey boundaries and add or subtract this difference from one of the grids. In a perfect world, the leveled grids should differ along boundaries by a constant value. This is usually not the case. It is possible to remove surfaces from data sets to force them to match along boundaries but is recommended only if the user understands the source of the surface (rarely the case).
9. Merge grids. Blend adjacent grids by splining along the boundary. Add new grids to each blended grid until the final product is achieved. Where possible, merge the lowest resolution data first so that the highest resolution data is given priority.
10. Make the map of the merged data sets. It may also be useful to merge the original data together with a constant removed and the survey boundaries marked.
Along the Pacific margin of the Antarctic Peninsula there is a strong lineated positive magnetic anomaly (PCMA) that has been detected by several aerosurveys, and interpreted as produced by a calco-alkaline suite caused by the long history of subduction along that margin. From about latitude 65 to the north, there is a second branch to this anomaly at the seaward side. This branch has its more intense expression on the South Shetlands Is. area, and continues until past King George Is., where it abruptly ends. The first branch, however, continues farther until the western border of the Powell Basin. North of the Hero Fracture Zone, the two branches enclose the Bransfield Basin, where the magnetic field has a negative signature with low amplitude but distinctive relative maxima that are the response to a volcanic axis that runs from Deception to Bridgeman Islands. South of Clarence Is., another structural magnetic anomaly appears to be a displaced fragment of the second branch of the PCMA; it stretches farther beyond the end of the first one, bordering northern margin of the Powell Basin.
One possible interpretation for this pattern of structural magnetic anomalies is that subduction related extension was already active at the Pacific margin of the Antarctic Peninsula when the Scotia Sea opening started.
The discontinuity of the second branch seems to be another feature that contributes to mark the particular evolution of the North Bransfield Basin, related to the observed instability of the Antarctic - Scotia plate boundary in this area.
The Weddell Sea region is of crucial importance to understanding the structure and tectonic evolution of the Antarctic continent and its relation to the Gondwana assemblages. The most controversial questions are concerned with original position of the Ellsworth-Whitmore Mountains crustal block before Gondwana separation and recent proposed geological continuity between Laurentia and Gondwana during the Middle Proterozoic (Dalziel, 1991). Based on the Russian aeromagnetic data acquired in this region undergoing a complex tectonic history we intend to present the possible evidences for existence of the Precambrian basement under largest part of the Weddell Sea Embayment and to discuss another geological results of magnetic anomaly field structural interpretation.
Presented magnetic anomaly map of the Weddell Sea region incorporates all available data sets collected in 1977-1989 over area comprising the East Antarctic Shield, Weddell Sea Embayment, Mesozoic magmatic arc of the Antarctic Peninsula and Precambrian Haag Nunataks block. Airborne magnetic surveys were carried out with different flight-line spacing and elevations, measuring equipment and navigation systems, therefore the map was produced by removal of the most appropriate geomagnetic reference field - IGRF-1985, no attempt was made to continue analytically the anomaly data to a common altitude. Data processing was made by applying the GSC (Atlantic, Geological Survey of Canada) geophysical image processing and visualization tool package.
The adjusted magnetic data of each individual regional survey (20 km flight-line spacing) were interpolated onto 5 km grid using a minimum curvature technique. The magnetic data of the detail surveys (5 km flight-line spacing) were gridded with 1.5 km interval and subsequently were transformed to 5 (5 km grid in order to integrate them with regional datasets. Detail aeromagnetic surveys covering the southern Palmer Land, Pensacola Mountains and western Dronning Maud Land considerably overlap adjacent areas mapped by regional surveys. Therefore for merging all grids into one dataset the detail data were used in the invariable form as basis level for adjoin areas, along with regional aeromagnetic data obtained during 1989 field season when digital registration and Global Positioning System satellite navigation were used. Before contouring, the resulting grid was filtered with a 7.5 km radius running mean technique. The final grid for whole surveyed area was used for calculation pseudogravity and horizontal gradient of pseudogravity, first vertical derivative and horizontal gradient. Since the traditional methods of presentation filter out much of the original information of the data relating to geological structures and trends we used a variety of image processing techniques to enhance the structural grain of magnetic anomalies. To create colour shaded-relief maps of the Weddell Sea region the processed data were transferred into the ER Mapper software system, where grid values were scaled to a range of 256 using histogram-equalization and were converted into a 3-band pseudo-coloured image by using the RGB (Red, Green, Blue) combinations. All colour shaded-relief images were produced by using a sun-angle routine with artificial illumination from north and an inclination of 45 together with applying a two-dimensional transform to the gray-scale images.
The main goal of our research was application of the aeromagnetic data sets for defining and outlining the major crustal structures in geologically known tectonic provinces of both West and East Antarctica and in the Weddell Sea Embayment (WSE) and for subsequent identification their relationships. The amplitudes and wavelengths of anomalies exhibited on the compiled map vary considerably within the study area. The highest values with amplitude up to 5000 nT are associated with the Cretaceous mafic intrusions of the Antarctic Peninsula, whereas lowest values (25-50 nT) can be observed over the Bormassivet intrusions. The Weddell Sea Embayment is clearly distinct from surrounding areas showing the low-amplitude and broadened magnetic anomalies with wavelength up to 100-150 km. On the basis characteristics anomaly wavelength and amplitude can be distinguish a number of magnetic patterns attributed to different crustal units and combined into three major zones associated with the West and East Antarctic terranes and the Weddell Sea Embayment area.
The East Antarctic magnetic zone (EA) can be subdivided into four distinct magnetic units. Magnetic low occupied the Princess Martha Coast and linear short-wavelength magnetic anomalies over the Ahlmanruuryggen- Borgmassivet area are corresponded to an Archaen to Mid-Proterozoic Grunehogna Province (Groenewald et al., 1991). A broad linear belt of magnetic highs and lows observed over the H.U. Sverdrupfjella and Kirwanveggen is associated with Middle- to Late Proterozoic metamorphic belt (Maudheim Province) (Groenewald et al., 1991). It continuously traced through Vestfjella to continental slope where the well-known Explora Anomaly (Kristoffersen & Hinz 1991) cuts it abruptly. The discrete high-amplitude anomalies in Vestfjella (1900 nT) and H.U. Sverdrupfjella (800 nT) correspond to Jurassic intrusions of olivine gabbro and Stramsvola-Tvora alkaline complexes respectively. The Coats Land magnetic unit exhibit broken anomaly pattern with isolated short- wavelength highs. In the south it is bounded by the prominent Shackleton Range magnetic anomaly and on the west by less intensive magnetic anomaly named as Druzhnaya Anomaly. These two anomalies supposedly represent single tectonic zone evolved as marginal mobile belt of the ancient cratonic fragment located in the Coats Land. We suggest that magnetic anomalies of the northern Berkner Island are also associated with this zone.
Magnetic field of the WSE zone is relatively smooth and of low amplitude, while there are some outstanding magnetic anomalies which provide clues to the nature of underlying basement. The most conspicuous feature of the WSE zone is a wide curvilinear belt of positive magnetic anomalies running parallel with the coast of western Dronning Maud Land and Coats Land southwards to the Berkner Island. It includes the Explora Anomaly, the Druzhnaya Anomaly and the Berkner Island Anomaly and marks the transition between the WSE and EA magnetic zones. We believe that these anomalies are caused by the sources originated from different tectonic events during the evolution of the Antarctic continental margin in this region. On the basis of the magnetic anomaly grain we suppose that the Explora Anomaly apparently is rather produced by magmatic rocks intruded basement then volcanic sedimentary sequences forming the Explora Wedge and related to the initial rifting events of Gondwana separation. Another striking feature of the WSE zone is linear magnetic low with minimum amplitude less then -300 nT and average width about 100 km runs southwards from continental rise to the coast-line. It is associated with Weddell Sea failed rift. Low-amplitude magnetic anomalies running roughly N-S over Ronne Ice Shelf toward the northern edge of the ice shelf are sub-parallel to the eastern structural boundary of the Haag Nunataks block. This suggests that they may be interpreted due to a continuation of the Precambrian basement exposed at Haag Nunataks under the Ronne Ice Shelf.
West Antarctic magnetic zone comprises the southern Palmer Land Unit and the Haag Nunataks Unit. The first one is dominated by the Pacific Margin Anomaly (PMA) (Maslanyj et al., 1991), Central Plateau Magnetic Anomaly (CPMA) and East Coast Magnetic Anomaly (ECMA) which are caused by Cretaceous plutonic rocks. The aeromagnetic data provide with additional information on the extent of the Haag Nunataks (HN) block. In accordance with our interpretation, crystalline basement of HN may also lie at least beneath the Sentinel Range of Ellsworth-Whitmore Mountains, the Ronne Ice Shelf up to 59W in the east and northward to coast line and the southern Palmer Land. This is severe constraint for any paleoreconstructions which does not consider a possible extent of the HN crustal block. Our interpretation contradict to suggestion that the Coast Land area is a continuation of the Mazatal-Yavapai- Grenville provinces of North America. We also suggest that Ross-age suture resulting from Gondwana coalescence not presented in the western Dronning Maud and Coats Lands, it must lie southwards of the current exposures of the Shackleton Range.
Dalziel, I. W. D. 1991. Pacific margins of Laurentia and East Antarctica-Australia as a conjugate rift pair: evidence and implications for an Eocambrian supercontinent. Geology, 19, 598-601.
Garrett, S. W., Herrod, L. D. and Mantripp, D. R. 1987. Crustal structure of the area around Haag Nunataks, West Antarctica: new aeromagnetic and bedrock elevation data. In Gondwana Six: Structure, Tectonics, and Geophysics, ed. G. D. McKenzie, Geophysical Monograph 40, Washington, D.C., American Geophysical Union, 109-115.
Groenewald, P. B., Grantham, G. H. and Watkeys, M. K. 1991. Geological evidence for a Proterozoic to Mesozoic link between southeastern Africa and Dronning Maud Land, Antarctica. In: Findlay, R. H., Banks, H. R., Veevers, J. J. and Unrug, R. (eds) Gondwana 8: assembly, evolution and dispersal. A.A. Balkema, Rotterdam.
Kristoffersen, Y. & Hinz, K. 1991. Evolution of the Gondwana plate boundary in the Weddell Sea area. In: Thomson, M. R. A., Crame, J. A. and Thomson, J. W. (eds) Geological Evolution of Antarctica. Cambridge University Press, 225-230.
Maslanyj, M. P., Garrett, S. W., Johnson A. C., Renner, R. G. B. and Smith, A. M. 1991. Aeromagnetic Anomaly Map of West Antarctica (Weddell Sea Sector). In: BAS GEOMAP Series, Sheet 2, 1: 2,500,000 (with Supplementary text, 37p.), Cambridge, British Antarctic Survey.
The aeromagnetic surveying by Russian expeditions commenced in 1955 has resulted in over 475 000 line kilometers of data flown in West and East Antarctica. Aeromagnetic surveys were carried out in the coastal and adjacent shelf regions with flight-line spacing 20 km. The mountainous areas of Western Dronning Maud Land, the southern Palmer Land of Antarctic Peninsula and the Prince Charles Mountains and Princess Elizabeth Land have been flown with flight lines 5 km apart. The East Antarctica interior was investigated with line separation averaging 50 km. The datasets originally obtained in analog form have been digitized. Available aeromagnetic datasets were used to produce a number of magnetic anomaly maps of 1:1 000 000 and 1:2 500 000 for the Weddell Sea Region and Central Sector of East Antarctica. Aeromagnetic data from surveys flown in the Central Sector of East Antarctica were processed in a joint Russian/Australian project. Part of the compiled dataset for this sector was published by Golynsky et al. (1996). The Russian aeromagnetic data with flight-line spacing 20 km collected in West Antarctica were used in a joint British/Russian project to create an internally coherent magnetic data base of the Weddell Sea sector. Regional aeromagnetic data obtained by the British and Russian expeditions were reduced to IGRF 1985, adjusted, levelled, gridded and published by Johnson et al. (1992) and subsequently by Hunter et al. (1996).
Recently within a joint Russian/Germany project of processing and interpretation of geophysical data in the southern Weddell Sea, the Russian regional datasets which were previously processed by BAS were combined with detailed datasets (5 km) collected in the Western Dronning Maud Land and the Southern Palmer Land to produce the magnetic anomaly map of the Weddell Sea region. Subsequently this map was supplemented by the aeromagnetic data collected by the British Antarctic Survey over the Antarctic Peninsula and adjacent areas (Johnson & Smith, 1992, Maslanyj et al., 1991).
To produce the first complete version of the 1:10 000000 Magnetic Anomaly Map of Antarctica two final grids from West and East Antarctica were combined with the data acquired in 1955-1962 during earlier Russian expeditions around Mirnyy and Novolazarevskaja stations along with shipborne data from the Prydz Bay area. The merged data sets were gridded at intervals of 5x5 km and contoured at 100 nT intervals. Over the areas where the network of lines was not sufficient for gridding profiles are presented. Data processing and mapping was made by using geophysical image processing and visualization tool package developed in Geological Survey of Canada, Atlantic. The produced map, despite the inconsistent characteristics of the surveys from which it was compiled, is useful in studying the distribution and character of regional geologic features such as geologic provinces, for defining major rifts both along the continental margin and in the interior of Antarctica. It is also valuable tool to understand the structure and tectonic evolution of the Antarctic continent.
Golynsky, A. V., Masolov, V. N., Nogi, Y., Shibuya, K., Tarlowsky, C. and Wellman, P. 1996. Magnetic anomalies of Precambrian terranes of the East Antarctic Shield coastal region (20E-50E). In: Proc. NIPR Symp. Antarct. Geosci., 9, 24-39.
Hunter, R. J., Johnson A. C. and Aleshkova, N. D. 1996. Aeromagnetic data from the southern Weddell Sea embayment and adjacent areas: synthesis and Interpretation. In: Storey, B. C., King, E. C. and Livermore, R. A. (eds) Weddell Sea Tectonics and Gondwana Break-up, Geological Society Special Publication No. 108, 143-154.
Johnson, A. C., Aleshkova, N. D., Barker, P. F., Golynsky, A. V., Masolo v, V. N. and Smith, A. M. 1992. A preliminary aeromagnetic anomaly compilation map for the Weddell Province of Antarctica. In: Yoshida, Y., Kaminuma, K. and Shiraishi, K. (eds) Recent Progress in Antarctic Earth Science. TERRAPUB, Tokyo, 545-553.
Johnson, A. C. and Smith, A. M. 1992. New aeromagnetic map of west Antarctica (Weddell Sea Sector): Introduction to important features. In: Yoshida, Y., Kaminuma, K. and Shiraishi, K. (eds) Recent Progress in Antarctic Earth Science. TERRAPUB, Tokyo, 552-562.
Maslanyj, M. P., Garrett, S. W., Johnson A. C., Renner, R. G. B. and Smith, A. M. 1991. Aeromagnetic Anomaly Map of West Antarctica (Weddell Sea Sector). In: BAS GEOMAP Series, Sheet 2, 1: 2,500,000 (with supplementary text, 37p.), Cambridge, British Antarctic Survey.
The separation of the different sources of the internal field can be accomplished by means of the so-called spatial spectrum of the internal origin field. It is shown how such a rationale, when suitably interpreted, allows for recognizing the field that is apparently originated by currents flowing either on the inner-core boundary, or on the core-mantle boundary, or in the asthenosphere. It appears important, however, to rely on satellite measurements, as ground-based and airborne records are normally severely perturbed by the crustal field. Therefore, on the basis of a critical reconsideration of some most relevant papers appeared on such an item, it is shown that the best approach appears to be in avoiding mixing altogether satellite measurements and airborne and observatory measurements. Rather, satellite data appear best suited for recognizing the dynamo field (originated at different depths within the Earth). Instead, the other afore-mentioned records, which are measured much closer to the crustal sources, appear best suited, after subtraction of the satellite-derived dynamo field, for inferring an accurate world map of the geomagnetic anomalies that are likely to be associated with the crustal sources alone.
The physical system composed of the electric currents that flow within the solar wind, the magnetosphere, and the ionosphere is per se extremely complex, and subject to very rapid variations in time. The unique way of getting rid of such a difficult drawback for ground based and air-borne magnetic surveys is in envisaging a clear conceptual scheme for modeling the different possible sources, in terms of a few suitably simple elementary phenomena that superimpose over each other. The detail of the model of every such phenomenon occurring at any given time instant is a function of its degrees of freedom, that correspond to the number of observational inputs that are available for computing the model itself.
Much like a zoologist who must distinguish different phila prior to making statistics on the characteristics of living beings (mammals ought to be distinguished from reptiles, etc.), we have to recognize a few typical different configurations that provide with a broad separation between physical situations that cannot be physically compared with each other.
The magnetosphere/ionosphere system can be described in terms of different observables (e.g. light, magnetic field, electric field, electric currents, particle fluxes, energy contents, e.m. waves, Alfvén waves, etc.). Either one of them is eventually suited for recognizing different physical basic configurations.
For example, quiet time and magnetic storm time situations are a well known classical distinction. Light (polar auroras of the oval) allowed for recognizing substorms. Electric fields appear very erratic, although there is a specific rationale, and their diagnosis can be most effectively afforded by means of satellite-borne observations of the polar auroras displaying inside the oval, etc.
The recognition of different basic configurations relies mostly on a topological (rather than geometric) description of the magnetosphere. The Hamilton variational principle plays a fundamental role. The description of the magnetosphere in terms of electric currents results very profitable and effective. The energetic constraint leading to the formation of the neutral sheet is a clear inference of such arguments. Also the long-debated reconnection of magnetic field lines, a paradox per se in conflict with Maxwell’s laws, appears as a most simple consequence of such a rationale.
Summarizing, it is shown how a suitable combination of different observational inputs can afford in providing with a few basic indications by which the very complex set of phenomena that eventually occur above the polar caps (and also at lower latitudes) can be computed by means of the exploitation of a few models dealing with some comparably “simple” typical phenomena. No general model, however, can be provided that can be suited for every time instant. The entire system is very complex and it allows for no easy-to-handle solution. The detail that one affords in managing such a difficult task depends on the degrees of freedom that he wants to allow for within the model.
Moreover, such degrees of freedom must correspond to the prime available set of observations that are simultaneously available at every given time instant of concern. Therefore, it is up to the experimenter who carries out the magnetic survey to decide what is the most convenient detail that he can attain in interpreting his observations by means of his available budget and facilities.
Shipborne magnetic data collected by Russian, Australian and Japanese survey cruises were compiled to make a magnetic anomaly map of the Prydz Bay and adjacent slope area (63 S - 69.5 S, 68 E - 80 E).
The original Russian data had been already corrected for time variation. Hourly values of Mawson station, which is located about 500 km west of the centre of the compiled area, were used to correct the magnetic time variations in all the Australian data and in the Japanese data collected in 1984-85, while one-minute values of Mawson were used for the Japanese data collected in 1989-90. High amplitude (up to 700 nT) daily variations were observed at Mawson especially in February 1982, when considerable amount of Australian data were collected. Most of variations with period of 6 hours or longer were probably removed from the survey data by this correction, but shorter period variations were difficult to correct, because the station is not close enough from the area.
More than 58000 profile data along 150 track lines remained after removing several bad data segments. The standard deviation of 340 cross over differences was 117.3 nT, but it reduced to 54.4 nT after giving a constant bias to each track line to minimize the standard deviation. A gridded data set was created by applying minimum curvature method to the corrected profile data, and a preliminary magnetic anomaly map was made using this gridded data.
Shipborne gravity data collected by Russian and Japanese survey cruises were used to compile gravity anomaly map of the same area. More than 18000 profile data along 103 track lines were used.
The free air anomaly values of the original Japanese data had been calculated using the 1967 gravity formula, while those of the Russian data had been calculated using the 1930 gravity formula. The Russian data were recalculated using the 1967 gravity formula. However, large cross over differences still remained after this conversion: the standard deviation of 246 cross over differences was 8.1 mGal. It reduced to 1.2 mGal after giving a constant bias to each track line. A gridded data set was created using these profile data, and a free air anomaly map was made using this gridded data.
The most prominent feature in the free air anomaly map is a broad negative anomaly belt extending in the NNE direction from the Amery Ice Shelf. This negative anomaly belt corresponds to a graben structure of the basement which is interpreted as a part of a rift zone continued from the Lambert Graben south of the area. A positive anomaly belt extending in the NE direction is observed northwest of the negative anomaly belt, and another positive anomaly belt extending in the E-W direction exists at the shelf edge further northwest.
The most prominent feature in the preliminary magnetic anomaly map is positive anomalies distributed roughly along the free air positive anomaly belt northwest of the graben structure. These anomalies have rather short wavelengths and suggest existence of igneous rocks below the anomalies.
The Antarctic Digital Magnetic Anomaly Project (ADMAP) is a multinational project to compile near-surface and satellite magnetic anomaly data into a digital database and map for the Antarctic continent and surrounding oceans south of 60S. The map and database can then be used by a variety of researchers investigating the structure, geologic processes and tectonic evolution of Antarctica.
The Antarctic continent is 99% covered with ice, which can be up to four kilometres thick in places. Remotely-sensed data, such as magnetic anomaly data provide one of the few ways to obtain geological information over much of the continent. A large quantity of magnetic data have been collected to the south of 60S since the International Geophysical Year of 1958 (Johnson et al., 1997). The magnetic data will help to delineate major structural components of the continent, such as terranes, mobile belts and rifts. Relative timing and kinematics of the evolution of the continent can also be addressed, along with super-continental reconstructions, palaeogeography and boundary conditions at the base of the ice sheets. The Antarctic magnetic map will provide the final piece needed to complete the magnetic map of Gondwana.
ADMAP is a joint project of the Scientific Committee on Antarctic Research (SCAR) and the International Association of Geomagnetism and Aeronomy (IAGA). Following a resolution at the 1993 IAGA meeting and a supporting recommendation from the SCAR 1994 meeting, the first ADMAP Workshop was held in Cambridge, UK, during September 1995 (Johnson & von Frese, 1996). The workshop was hosted by the British Antarctic Survey, and made possible with funding from SCAR and the National Science Foundation Office of Polar Programs. At the workshop, a set of protocols was adopted, based on the successful SCAR-ANTOSTRAT guidelines (SCAR, 1992). These protocols can be summarized as follows: New magnetic data may remain in local archives for up to three years from the date of collection. After three years, data will become available for collaborative research within the ADMAP project, with the agreement of the data collection agency.
Six years after collection, the data will become available publicly e.g. through a World Data Centre. Existing Antarctic magnetic data holdings will be made available with minimal restrictions e.g. through a World Data Centre within 3 years of the date of the First Workshop report. The actual compilation of the data has been divided into three sectors, with those participants active in each sector being responsible for the work. These sectors are the East Antarctic Sector (15E to 135E), Ross Sea Sector (135E to 255E) and the Weddell Sea Sector (255E to 15E). Work is already in progress in each of these sectors. For East Antarctica, a large amount of Russian and Australian data have been recovered, digitised, reprocessed and compiled (Golynsky et al., 1996); for the Ross Sea Sector, work has begun on the INTRAMAP project, merging Italian, German and US digital data (Chiappini et al., 1997) and for the Weddell sea Sector the compilation process is well established with primarily Russian and British data (Johnson et al., 1992).
The Second ADMAP Workshop in Rome, Italy, 29 September 1997 to 2 October 1997 will provide the focus for completing the compilation of these sectors and planning the merging of each sector with satellite magnetic data into a single map. A number of interpretation projects arising from collaboration with the ADMAP Working Group will also be presented during the meeting, and discussions will begin on a complementary gravity compilation.
Chiappini, M., Ferraccioli, F, Bozzo, E., Damaske, D. and Behrendt, J. 1997. INTRAMAP: Integrated Transantarctic Mountains and Ross Sea Area Magnetic Anomaly Project. In. Abstracts, 8th Scientific Assembly of IAGA, 511.
Golynsky, A. V., Masolov, V. N., Nogi, Y., Shibuya, K., Tarlowsky, C. and Wellman, P. 1996. Magnetic anomalies of Precambrian terranes of the East Antarctic Shield coastal region (20E-50E). Proceedings of the NIPR Symposium on Antarctic Geoscience, 9, 24-39.
Johnson, A. C., Aleshkova, N. D., Barker, P. F., Golynsky, A. V., Masolov, V. N. and Smith, A. M. 1992. A preliminary aeromagnetic anomaly compilation map for the Weddell Province of Antarctica. In: Yoshida, Y., Kaminuma, K. and Shiraishi, K. (eds) Recent Progress in Antarctic Earth Science. TERRAPUB, Tokyo, 545-553.
Johnson, A.C. and von Frese, R.R.B. 1996. Report of the SCAR/IAGA Working Group on the Antarctic Digital Magnetic Anomaly Map. 26pp.
Johnson, A.C. , von Frese, R.R.B. & the ADMAP Working Group. 1997. Magnetic Map Will Define Antarctica's Structure. Eos, Transactions of the AGU. 78, 185. SCAR, 1992. A SCAR seismic data library system for cooperative research: Summary report of the International Workshop on Antarctic Seismic Data - Convener Alan K. Cooper and the ANTOSTRAT Steering Committee. SCAR Reports, 9, 15 pp.
The merging of magnetic data into continental compilations has long been recognised as a useful tool for large-scale geologic interpretation and continental reconstructions. The recent session at the IAGA Scientific Assembly in Uppsala 'Towards a Digital Magnetic Anomaly Map of the World' demonstrated the utility of both continental-scale compilations and a 'compilation of compilations'. This type of project can be achieved in two ways, either from a profile-based or a grid-based approach. Although the profile-based approach is perhaps the more rigorous, it is certainly more time-consuming and difficult than using pre-existing, internally consistent grids. Problems merging grids have a variety of causes, affecting a range of data wavelengths. Long wavelength problems can arise from (but are not limited to):
Different survey parameters e.g. flight heights
Different line-processing parameters e.g. reduction to different IGRF coefficients and different levelling strategies
Different grid processing strategies e.g. trend removal. Shorter wavelength problems can include diurnal problems, edge effects and data spikes.
The following is one of a number of approaches to solving these grid merging problems, which can be easily achieved within the OASIS montaj system (Geosoft, 1997). The method will work best with two grids that have a reasonable (say 5%) overlap. The first stage is to remove zero- or possibly first-order trends from each grid to attempt to bring each grid to a consistent long-wavelength level. The area of overlap between the two grids is then examined. Two profiles are digitized along each edge of the overlap area, along the whole of the area. All the points along one profile are set to zero, and along the other to unity. These two lines are then gridded, and the final grid is trimmed to the size of the overlap. This creates a correction grid. The next stage is to multiply the first grid by the correction grid, and the second grid by (1-the correction grid). The two corrected overlap grids thus created are then added together into a final overlap grid. This overlap grid is then Boolean added to the two original grids. The effectiveness of the merging process can easily be assessed using a shaded grid of the merged area. If necessary, a polygonal area from around the join can be extracted (with the edge in long-wavelength areas), filtered and then replaced. As with all grid manipulation techniques, care must be taken when interpreting the final results, particularly when looking at wavelengths of the order of the component grid size.
Geosoft Inc. 1997. OASIS montaj Data Processing and Analysis (DPA) System for earth Science Applications Version 4.1 User Guide. Geosoft Inc., Toronto, Canada, 290 pp.
An effort is underway to produce a compilation of all available geomagnetic data sets over the marine and continental shelf areas of the Circum-Antarctic. The digital compilation of marine magnetic and aeromagnetic data will include a grading system for data quality in three areas: navigation, external geomagnetic activity, and digital sampling quality. Errors in the data sets will be corrected whenever possible by using the original data sets. The compilations will include along track identifications of the seafloor spreading magnetic anomaly wherever applicable as well as a filtered version of the anomaly in the 300 to 3000 km bandpass for comparison to the satellite anomaly fields.
Examples of the data products from this compilation will be presented.
Since 1987, the R/V OGS-Explora conducted several geophysical cruises in the Ross Sea region, South-western Pacific Ocean, Scotia Sea, Pacific margin of the Antarctic Peninsula, and Southern Chile margin. The amounts of marine magnetic data (acquired with a proton magnetometer and a proton gradiometer in the three last cruises) for each geophysical survey are summarized in the following Table:
|Ross Sea||2317 km||4080 km||2516 km||554 km||1575 km|
|S-W Pacific Ocean
|3042 km||1273 km||2510 km||1194 km
Pac. marg. Ant. Pen.
South Chile margin
All the surveys were conducted in the frame of the Italian "Programma Nazionale di Ricerche in Antartide", and some of them were performed in collaboration with international scientific institutions. The majority of the ship-track magnetic data have been processed, and for the areas of the South-western Pacific/northern Ross Sea and for the western Scotia Sea, tectonic reconstructions mainly based on magnetic anomaly identifications have been proposed (Lodolo et al., 1996; Lodolo et al., 1997). The main scientific results from our study in the two areas will be summarized.
South-western Pacific/northern Ross Sea Magnetic and bathymetric measurements carried out in the southwest Pacific between New Zealand and Antarctica south of 60° S, combined with the satellite-derived gravity map, add new information on the structural fabric and tectonic development of this remote region of the Southern Ocean. The Pacific-Antarctic plate boundary includes in its northern part a series of short spreading centers offset by NNW-SSE trending fracture zones. Towards the south, it becomes structurally more complicated and appears to be formed by extension along an ancient strike-slip lineation. The data set allowed the compilation of a new map of Chrons (C1-C20), spanning the region comprised between the easternmost Australian-Antarctic ridge segment and the western Ross Sea. Rates of oceanic crustal accretion have been computed, and polynomial analysis conducted on the velocities distribution have been performed. The resulting instantaneous velocities testify that spreading processes in this sector of the Southern Ocean are not uniform. Substantial asymmetry of spreading between the northern and southern flanks of the ridge axis has been revealed in the 2.15-4.29 Ma time interval. A 20 Ma period in the spreading velocity trend has been found, with maximum values positioned at 10 Ma (4.8 cm/yr) and 31 Ma (6.1 cm/yr); the minimum coincides with 21.5 Ma (1.5 cm/yr). These results are in agreement with those obtained by computation of instantaneous velocities for the conjugate northern flank of the Australian-Antarctic ridge segment. The identification of marine magnetic lineations crossing the Pacific-Antarctic plate boundary testifies that a general tectonic reorganization occurred as a consequence of a change in the Pacific-Antarctic relative motions that resulted in a geometric readjustment of the plate boundary. The newly created crust is not older than Late Miocene and is presently surrounded by oceanic regions where the magnetic anomalies range from 19 to 24 (~41-53 Ma) northeastward, and from 7 to 9 (~24.5-28 Ma) southwestward. This segment of the plate boundary formed along an older fracture zone that was pulled-apart by the new plate motions, and the segmented ridge-transform system then evolved.
The western Scotia Sea developed since about 30 Ma, as a consequence of the pulling away of the two major plates of South America and Antarctica, that definitively separated Antarctica from the other continental masses. Marine magnetic identifications revealed that spreading processes stopped along the WSW-ENE-trending spreading system at about 7 Ma (anomaly C5). The western part of the Scotia Sea has been extensively surveyed, combining marine magnetic profiles, multichannel seismic reflection profiles, and satellite-derived gravity anomaly data. We identified and mapped the oldest part of the oceanic crust at the corner of the Shackleton Fracture Zone with the South Scotia Ridge; in this sector, the age of the crust is older than chron C10 (28.7 Ma). This rhomboid-shaped area is surrounded by structural features clearly imaged by seismic profiles, corresponding to gravity lows. An oceanic area with analogous age has been found in the north-western sector of the Scotia Sea, just to the south of Tierra del Fuego margin. We interpret these areas (Lodolo et al., submitted), now dispersed at the corners of the western Scotia Sea margin, as relicts of oceanic crust formed during an earlier, possibly chaotic episode of spreading during the first opening of the Drake Passage.
Presently, we are focusing on the southern part of the Central Scotia Sea north the South Orkney block, where the Protector Basin and Pirie Bank (Tectonic map of the Scotia Arc, 1985) are located. The origin of these basins is enigmatic. Looking at the eastern part of the Scotia Sea spreading system, it is clearly evident that these two areas cannot be related to the spreading processes that accompanied the western Scotia Sea accretion, because they represent an additional part of oceanic crust not presenting any counterpart in the conjugate northern flank of the ridge system. Some authors propose that this seafloor was created by N-S- trending spreading centers, others suggest alternative hypothesis such as older ocean lithosphere capture or presence of pre-Drake Passage opening crust. The analysis and interpretation of the acquired magnetic anomaly data will probably allow to identify the main events responsible for the Central Scotia Sea tectonic development.
Lodolo E. and Coren F. 1997. A Late Miocene plate boundary reorganisation along the westernmost Pacific Antarctic ridge, Tectonophysics, 274, 295-305.
Lodolo E., Schreider A.A. and Coren F. 1996. Sea-floor spreading in the eastermost Indian Ocean reveals cyclicity in ocean crust accretion, Marine Geology, 134, 249-261.
Lodolo E., Schreider A.A. and Coren F. (submitted). Oldest South-western Scotia Sea: tectonic implications, Marine Geophysical Researches. Tectonic map of the Scotia Arc (1985). 1:3,000,000 Misc. 3, British Antarctic Survey, Cambridge.
It is well known that the geological structures of the Polar regions (Arctic and Antarctic) contain potential answers to major problems in the Earth Sciences and that many of them relate to questions that are of global scientific significance. However because of ice cover, harsh climate, enormous fieldwork costs and the absence of research platform suitable for many type of investigations these regions are the most poorly studied on Earth. In these conditions potential field data provide the geoscientists with essential information to the understanding of the deep geological structure and the geodynamic evolution of the lithosphere.
The paper presents computer system for processing, mapping and interpretation of magnetic anomaly, gravity and bathymetry data developed recently in the VNIIOkeangeologia (St. Petersburg, Russia) on the basis of several UNIX workstations (both HP and Sun), network of PC’s and powerful peripheral equipment. This system is widely used for reprocessing and mapping of potential field and bathymetry data, collected by Russian agencies in the Arctic and Antarctic during the last tree decades. Software packages supporting the system include the programs for processing and presenting of geophysical data, developed in Geological Survey of Canada - Atlantic (Verhoef, Usov, 1995), several commercial mapping tools (ErMapper, Surfer, MapView) and original programs written at VNIIOkeangeologia (Korneva, 1995 - 1997, Andrianov, 1997). Major technical problems revealed in digitized data previously collected in Arctic and Antarctic such as corrections due to poorly accurate navigational systems, relevelling, unclear reference field etc., were solved with the presenting system. Some mapping and interpretative results are discussed.
As part of its preparations for the ADMAP compilation the British Antarctic Survey has recently made a new compilation of marine magnetic data from the Scotia and Bellingshausen seas. Within the ADMAP region the data is generally sparse except for the area just to the north of the Antarctic Peninsula. The excellent definition of the oceanic anomaly pattern here based on marine data alone suggests that a final map including the large number of aeromagnetic tracks across the area should be spectacular.
Marine magnetic anomalies in the Southern Ocean are vital to understanding the geological processes involved in the breakup of Gondwana. However, marine magnetic data are still sparse and magnetic anomaly lineations remain undetermined, especially in the Southern Indian Ocean.
Measurements of the vector geomagnetic field have been carried out by a shipboard three-component magnetometer (STCM; Isezaki, 1986) on board the Japanese icebreaker Shirase since 1988 for identifying magnetic anomaly lineations in the Southern Indian Ocean.
Vector measurements of the geomagnetic field with STCM provide more detailed information than measurements of total intensity with a proton magnetometer to reveal the magnetic structures of the oceanic crust. STCM was developed recently and used in many oceanic regions to measure successfully geomagnetic filed vectors. Although sparse proton magnetometer observations do not allow us to identify the magnetic anomalies, the STCM vector geomagnetic anomalies allow us to infer the strike of two-dimensional magnetic structures, such as magnetic anomaly lineations and fracture zones, even with one observation line. Moreover, STCM can easily be installed on any ships and operated even under severe weather conditions, because the sensors of this system can be fixed on the deck.
Vector anomalies of the geomagnetic field were obtained in the Enderby Basin, Southern Indian Ocean from 1988 on board the icebreaker Shirase. The strikes of the two-dimensional magnetic structures, such as magnetic anomaly lineation and fracture zone trends, could be determined according to the method of Seama et al., (1993), together with the sea surface and satellite gravity anomaly data.
Relative change in total intensity geomagnetic field calculated from vector geomagnetic field by STCM was in good agreement with that measured by the proton magnetometer (Nogi et al., 1990). However, absolute values of total intensity geomagnetic field calculated from vector geomagnetic field is not reliable, possibly due to unpredictable change of the viscous remanent magnetization of the ship. Therefore, absolute values of total intensity geomagnetic field calculated from vector geomagnetic filed should be carefully used when merged with those measured by the proton magnetometer, and be constrained from total intensity geomagnetic field measured by the proton magnetometer and/or satellite derived magnetic anomalies.
We present here the results of vector geomagnetic anomaly field obtained in the Enderby Basin, Southern Indian Oceans. We also discuss application of total intensity geomagnetic field calculated from vector geomagnetic filed for the Antarctic digital magnetic anomaly map (ADMAP).
Nogi, Y., N. Seama & N. Isezaki. 1990. Preliminary report of three components of geomagnetic field measured on board the icebreaker Shirase during JARE-30, 1988-1989, Proc. NIPR Symp. Antarct. Geosci., 4, 191-200.
Seama, N., Y. Nogi & N. Isezaki .1993. A new method for precise determination of the position and strike of magnetic boundaries using vector data of the geomagnetic anomaly field. Geophysical Journal International. 113, 155-164.
The magnetic field satellite Magsat collected magnetic field data over the Antarctic from Oct, 1979 until May, 1980. This time period is centered on the austral summer when the south polar region was in daylight. This is not an optimal time for collecting magnetic field information as external fields are most dynamic during daylight and least disturbed at midnight. Many of the reproducibility problems encountered in producing crustal ONT>
The onset of continent-wide glaciation in Antarctica is still poorly understood, despite being one of the primary palaeoclimatic events in the Cenozoic. The Eocene/Oligocene boundary interval has been recently recognized as a critical time for Antarctic climatic evolution, and it may mark the preglacial-glacial transition. We report results of an environmental magnetic study of the lower sequence (Late Eocene/Early Oligocene) of the CIROS-1 core that was recovered from the Victoria Land Basin (VLB) beneath McMurdo Sound, Antarctica.
Measured magnetic properties include magnetic susceptibility, intensity of natural and artificial remanences, hysteresis parameters and magnetic anisotropy. The sediments have a clear magnetic signature, with alternating intervals of high and low magnetic mineral concentrations. The boundaries of these intervals do not correspond to lithostratigraphic units in the core. Pseudo-single domain magnetite is the main magnetic mineral throughout the sequence.
Sharp decreases in magnetite concentration correspond to changes in the clay mineralogy beneath and at the Eocene/Oligocene boundary. We conclude that large amounts of detrital magnetite were shed from the continent into the VLB, which resulted from more intense weathering of the Ferrar Group, during periods when the Antarctic climate was warmer than today. During intervals when the climate was relatively cool, less detrital magnetite was shed from the continent as chemical weathering of the Ferrar Group was arrested and deposition in the VLB slowed.
By this interpretation, the rock magnetic properties may be used to trace the alternation of gross and small scale fluctuations in the Antarctic paleoclimatic regime. The lithology of the core indicates that major ice sheets reached the CIROS-1 site only after the Early/Late Oligocene boundary. Environmental magnetic results also indicate that a cold and dry climate was not established in Antarctica until the Eocene/Oligocene boundary. However, this was preceded by some cold intervals which indicate that climate had begun to deteriorate by the Middle/Late Eocene boundary.
The past Greenland experiences confirmed the topical significance of surface topography, bed morphology, and internal layer geometry, in order to select the best site for deep drilling ice project.
The ideal drill site would be marked, at the same time, by a flat surface at the regional summit (topographic dome), by a flat bed morphology, by continuos and horizontal internal layering, and by absence of basal melting areas. In the frame of EPICA- DOME C project, the surface and bottom topography of Dome C area was determined from GPS and airborne and ground-based radio echo sounding over a 120 km by 80 km grid centered on the 1994 "summit" site (75° 06' S, 123° 23' E). The total airborne survey length was about 2800 km; the ground-based one about 300 km. The surface topography with decimetric accuracy, and the true summit location, were determined. High quality radar data were obtained over all the grids. The ice thickness and bottom topography is accurate within +/- 25 m. Measured ice thickness ranged between 2500-4000 m. Internal layering was detected normally between 800-2800 m depth. Four subglacial lakes in the marginal grid area were located.
Evidence of correlations between slope surface and bed features has been found, moreover a drilling site is proposed. The measurements were carried out by a 60 MHz radar instrument designed by M. Gorman (Scott Polar Research Institute). Hardware and software improvements on the system were performed. A new digitising board and the related software have been developed. The sampling rate of the analog signal has been doubled increasing the radar resolution. Data files acquired with the new system have been real time corrected with actual position and time according with GPS files. A new PLL frequency synthesiser has been designed and manual corrections are now possible during the measurements. In this way it is possible to avoid the reflected peak power from the folded dipole antenna due to mismatching. In order to adapt the radar system to the several condition of scattering (surface roughness and volume inhomogeneities) a multi-frequency system will be studied.
A new geomagnetic observatory started its operations during the 1996-1997 Austral summer survey at the Spanish Antarctic Station Juan Carlos I. The Station is located at Livingston Island, in the South Shetland Islands. The magnetic observatory is based on an automatic vector magnetometer, in which a proton precession magnetometer, acting as the magnetic sensor, is adequately exposed to bias fields by means of two mutually perpendicular pairs of Helmholtz coils. When deployed, the coils are aligned to measure changes in declination (D) and Inclination (I) (deltaD/deltaI configuration), and total field (F). Absolute measurements are independently taken with a D/I fluxgate theodolite. As the site is only manned during the Austral summer all scientific staff departed at the end of February but the magnetometers were left recording and we hope to retrieve the data recorded throughout the winter at the beginning of the next survey season in December 1997. Thereafter we intend to install a satellite transmitter to automatically retrieve the data in quasi real-time.
The continuous records that this observatory is providing will be very useful for any kind of geomagnetic study and, specially, as a reference level to correct magnetic surveys in the South Shetland Islands - Antarctic Peninsula region. This is particularly important now that the geomagnetic observatory of the Polish Arctowski Station at King George Island is closed, and the operation of Argentine Islands observatory in the former UK Faraday Station is uncertain, after it was taken over by the Ukrainian Antarctic Research Centre and renamed Vernadsky.
However, since the region is immerse in a very complex magnetic anomaly structure and tectonically it is perhaps the most active region in Antarctica, those records must be taken with some care because they content an important anomaly bias and their variations might be not sufficiently representative to satisfy the assumption of regional uniformity of temporal variations of the geomagnetic field.
There exist several marine and airborne magnetic surveys covering the South Shetland Islands - Bransfield Strait - Northern Antarctic Peninsula region. The most striking feature shown by these surveys is a high intensity magnetic anomaly broad belt (50-100 km width), termed as Pacific Margin Anomaly (PMA) or West Coast Magnetic Anomaly (WCMA), depending on the authors. This belt, extending along the Pacific margin of the Antarctic Peninsula, divides into two branches northeastwards of Adelaide Island and one of them extends along the South Shetland Islands.
Although it apparently corresponds to an anomaly high in the total field, both three above mentioned observatories show important lows in this element with respect to the global models that range from about 500 to 1000 nT, indicating that the anomaly pattern is much more complex and/or that there are interferences form shorter wavelength constituents. The best survey data for the South Shetland region seems to correspond to a Chilean aeromagnetic survey carried out during 1983 and 1984 (Parra et al., 1984, 1988). A first inspection to the anomaly contour map from these data certainly shows how the anomaly highs are distributed along the Pacific margin of the islands, while in the Bransfield Strait margin -where Arctowski and Livingston Island observatories are located- the highs alternate with lows.
The above mentioned complex pattern, which is specially evident at Livingston Island, is interpreted by previous authors as originated by a series of longitudinal fractures parallel to the axis of the oceanic subduction trench. This might explain the 1000 nT in total field or the 2 degrees in declination biases with respect to IGRF observed from the new observatory. To confirm that, it is our intention to downward continue the aeromagnetic anomaly map as well as to derive an estimate of the other elements. This will also serve to check whether the assumption of taking magnetizations parallel to the present field is correct or not. By translating the field components to present epochs (we verified that the secular variation of IGRF works properly on the area) a reference field model for the region will be evaluated. The inclusion of the new geomagnetic observatory data in the derivation of global models which incorporate a solution for the anomaly field at each observatory (by using satellite and surface data together) will provide a better estimation of the actual observatory anomaly bias. The uniformity of the variations should be studied by analyzing simultaneous data from an array of magnetometers, which seems to not be attainable at present.
Parra, J.C., G. Yanez & Grupo de Trabajo USAC (1988). Reconocimiento aeromagnetico en la Peninsula Antartica y mares circundantes, integracion de la informacion obtenida a diferentes alturas, Ser. Cient. INACH, 38, 117-131.
The Antarctic Digital Magnetic Anomaly Project (ADMAP) has its origins in a 1987 meeting of the International Lithosphere Program Coordinating Committee-5 (Data Centers and Data Exchanges) at the IUGG in Vancouver, British Columbia. The ILP CC-5 recommended the production of magnetic databases for the polar regions. Since then, this initiative has spawned 5 resolutions of the international geoscience community and obtained the support of IAGA, SCAR, and other scientific organizations for its activities. These developments are reviewed to provide perspective on the efforts and motivations that influence ADMAP's present and future activities. ADMAP organizational issues and activities to the year 2000 are also considered.
Spatially and temporally static crustal magnetic anomalies are contaminated by static core field effects above degree 12 and dynamic, large-amplitude external fields.
To extract crustal magnetic anomalies from the measurements of NASA’s Magsat mission, we implemented procedures to separate crustal signals from both core and external field effects. In particular, we define Magsat anomalies relative to the degree 12 core field and use spectral correlation analysis to reduce them for external field effects. We obtain a model of Antarctic crustal thickness by comparing the region’s terrain gravity effects to free-air gravity anomalies derived from the Earth Gravity Model 1996 (EGM96). To separate core and crustal magnetic effects, we obtain the pseudo-magnetic effect of the crustal thickness variations from their gravity effect via Poisson’s theorem. We compare the pseudo-magnetic effect of the crustal thickness variations to core field differences between degrees 12 and 14 by spectral correlation analysis. Residual core field effects in the Magsat anomalies that had been defined relative to the degree 12 core field are thus identified and removed. The resultant anomalies reflect the contrasts due both to crustal thickness and intracrustal variations of magnetization. The crustal anomalies also compare well with the near-surface magnetic survey data of the Antarctic Peninsula, Weddell Sea, and Dronning Maud Land. However, the validity of this approach is limited by the poor quality of the Antarctic Magsat data which were obtained during austral Summer when the activity of the south polar external fields is maximum.
Hence an important test of the crustal Magsat anomaly map for the Antarctic will be provided by the pending Ørsted mission which will yield coverage over austral Winter when external field activity is minimum.
The production and interpretation of the Antarctic digital magnetic anomaly map will be greatly facilitated by the availability of complementary gravity anomaly data. Combining terrain gravity effects with free-air gravity anomalies is useful for obtaining an effective model of crustal thickness variations. The pseudo-magnetic effects of this model may be obtained via Poisson’s theorem for effective separation of crustal and core field effects. Thus gravity data may be used to develop a crustal magnetic reference map that can be useful for correcting long-wavelength errors in an Antarctic compilation of disparate near-surface magnetic surveys. Additionally, gravity anomalies provide important constraints for limiting ambiguities in the geological interpretation of magnetic anomaly data.
Gravity coverage of the Antarctic is available as 1° predictions of free-air gravity anomalies from the spherical harmonic expansion to order 360 known as the Earth Gravity Model 1996. EGM96 gravity predictions satisfy 30-arc-minute averages of marine gravity anomalies derived from satellite radar altimetry (Geos 3, Seasat, Geosat GM), airborne gravity measurements over the Weddell Sea, terrestrial gravity observations where available, and gravity anomalies computed from orbital analyses of 21 satellites using laser tracking data, the global positioning system, Doppler data, and range-rate as well as satellite-to-satellite tracking and optical data. Within the region south of 60° S, roughly 75% of the 30’ blocks incorporate surface or near-surface gravity observations. This coverage includes 91% of the marine blocks, 61% of the terrestrial blocks of Antarctica, 86% of the West Antarctic blocks, and nearly 50% of the East Antarctic blocks. Gravity estimates for unsurveyed 30’ blocks are constrained only by gravity predictions to degree 50 from satellite orbit analyses in combination with predictions of the topographic-isostatic potential based on the Airy-Heiskanen isostatic model. The lithospheric utility of the EGM96 coefficients will clearly benefit greatly from additional gravity observations over the unsurveyed regions of the Antarctic, such as will be obtained from the pending low-orbit (i.e., 150 - 400 km) CHAMP and GRACE satellite missions of ESA and NASA, respectively.
Shorter wavelength anomalies of the marine gravity field are available from Geosat-Geodetic Mission (GM) and ERSI&2 radar altimetry data. Over open waters away from the shoreline, satellite altimetry-derived gravity anomalies tend to compare very well with gravity anomalies obtained by ship surveys. An example over the Gunnerus Ridge offshore of Japan’s Syowa Station is given where we obtain geoid undulations from Geosat-GM data. We transform these undulations directly into gravity anomalies using a frequency domain gradient filter based on Bruns’ formula. The resultant free-air gravity anomaly predictions at a 3-5 km grid interval have an accuracy of about 3 mGal or less when compared with modern, good-quality shipborne data.