the Polar Marine Geological Research Expedition (PMGRE) in the Prince Charles Mountains (PCM) and surrounding areas was to contribute data on major aspects of Antarctic geology: the tectonics of the mobile belts and cratonic fragments; the crustal structure of the Lambert-Amery rift system and its development; the determination of the boundaries between different tectonic terranes and the tracing known structures underneath the ice-sheet so that they can be arranged in the regional framework.
The aeromagnetic data of the Lambert Glacier-PCM area provide a rather complex but surprisingly coherent image for studying the geology and tectonic history of this region. Several distinct structural units may be differentiated in the magnetic anomaly data. In the northeastern block intense short-wavelength, high-amplitude positive anomalies extend around the Vestfold Hills and are associated with high-grade metamorphic Early Archean rocks known as the Croocked Lake and Rauer Group orthogneisses. Northern MacRobertson Land is characterized by predominantly low-amplitude anomalies within an area of low gradients. From its pattern and position, this unit may be caused by the Rayner Complex, which displays a similar magnetic response in Enderby Land. The southern boundary of the Larsemann magnetic sub-unit is outlined by the striking Amery Lineament (AL) that runs continuously from MacRobertson Land across the Amery Ice Shelf and further eastward possibly delineating the southern boundary of the Vestfold Hills crustal block. The nature of the AL within the coastal outcrops of southern Prydz Bay is related with the early Neoproterozoic orthogneissic basement rocks (S~strene Orthogneiss). Metasedimentary cover sequences (Brattstrand Paragneiss) of this area known to largely reflect metamorphism and deformation at -500 Ma do not show any outstanding magnetic responses and mainly associated with the negative anomalies of a local appearance. The NPCM displays a predominantly northeasterly trending magnetic fabric (Beaver magnetic unit, BMU) that continues to the western shoulder of the Lambert Rift. The negative and positive anomalies reflect Athos and Astronomov supracrustals and Porthos orthogneisses, respective-ly .. Elongate and moderate magnetic banding appears to characterize the Mesoproterozoic rocks of the Fisher Massif. The Beaver magnetic unit might be evident on the eastern side of the Lambert Rift.
Here, the magnetic anomalies are less variable with relatively diminished amplitudes. The prominent alternating system of linear NE-SW positive and negative anomalies over the eastern shoulder of the Lambert Rift may reflect the eastern boundary of the BMU. The Grove Mountains (GM) magnetic unit is clearly differentiated from the adjacent the BMU by an uncomplicated anomaly pattern of low-amplitude anomalies. The origin of the GM magnetic unit is not constrained due to insufficient geologic data; it is assumed that this unit may mark a distinct terrane similar to those observed in the Vestfold Hills or in the SPCM. Two different magnetic anomaly patterns are well distinguished over the southern PCM and the southern Mawson Escarpment. The Ruker terrane of the southern PCM and southern Mawson Escarpment are mainly associated with low or moderate amplitude anomalies in a low-gradient area. The central Mawson Escarpment and to a lesser degree its northern part of the Lambert Terrane is largely characterized by negative anomalies produced by Late Proterozoic granite-gneiss-schist sequences. Majorities of positive anomalies over the SPCM are associated with co-called Mawson orthogneiss. The most prominent magnetic lineament of the SPSM is the Ruker Anomaly that is related with a banded iron formation. The Ruker Anomaly extends 130 km westward of Mount Ruker with amplitudes up to 2600 nT.
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-Analysis of the aeromagnetic data and the produced scheme of interpretation clearly evidenced that the Pan-African mobile belt in Prydz Bay was obviously not linked with Ltitzow-Holm Bay or may have extended inland towards the Mawson escarpment or Grove Mountains, thereby indicating that the East Gondwana might not be divided into lndo-Antarctic and Australo-Anrarctic sectors, as recently was suggested by a number of authors. The allocation of basement outcrops to specific struc-tures based solely on similarities in the age is dangerous in the absence of evidence for tectonic continuity. In this respect the aeromagnetic data are the best tool to make or break tectonic correla-tions.
AD MAP - a Digital Magnetic Anomaly Map of the Antarctic (poster p.)
A.V. Golynsky1, M. Chiappini2, D. Damaske3, F. Ferraccioli
4,
C. Fino5, T. Jshihara6, V.N. Masolov7, P.Morris4, Y. Nogi8 & R.R.B. von Frese9
1VNII0keangeologia. I Angliysky Ave., 190121 St. Petersburg, Russia; <sasha@vniio.nw.ru>;
2ING. via di Vigna Murata 605. 00143 Roma. Italy; <chiappini@ingrm.it>;
3Federal Institute for Geosciences and Resources, Stilleweg 2, D-30655, Honnover, Germany;
<d.damaske@bgr.de>;
4British Antarctic Survey, High Cross, Madingley Road Cambridge CB3 OET, UK; <ffe@bas.ac.uk;
r,mor@bas.ac.uk>;
US Geological Survey, Box 25046 Denver, CO 80255, USA; <cfinn@usgs.goV>;
6GSJ, 1-1-3, Higashi, SUKUBA lbaraki 305. Japan; <t-ishihara@aist.go.jp>;
7PMGRE, 24 Pobeda St., 189510 Lomonosov.Russia;<antarctida@peterlink.ru>;
8NIPR, 1-9-10, Kaga. Itabashi, Tokyo 173, Japan; <nogi@nipr.ac.jp>;
90hio State University, 381 Mendenhall Lab 125 S. Oval Mall. Columbus, OH 43210-1398, USA;
<vonfrese@osu.edU>.
The AD MAP project was initiated in J 995 after encouragement from IAGA and SCAR to produce the first version of a unified magnetic anomaly map of the whole Antarctic region south of 60°S and to incorporate the available magnetic data into a digital database. This effort integrated near surface anomaly surveys acquired by the international community for site-specific geologic objectives since the International Geophysical Year 1957-58 through the year 2000 with lithospheric anomaly estimates from the Magsat mission has been successful; the composite magnetic anomaly map of the Antarctic is now completed and a 5 km grid covering the entire region was produced. A printed version of the map has recently been produced which is being distributed as the British Antarctic Survey publication (GOLYNSKY et al. 2001 ). The aeromagnetic and marine data in the compilation were processed from digital profiles and grids, as well as from manually digitized graph maps. The near surface magnetic data were separately compiled for the Weddell Sea, East Antarctic and Ross Sea sectors. 1n each sector, the magnetic data were edited for high-frequency errors, levelled and adjusted and assessed for data quality by statistical analysis of the crossover errors. The three regional sector grids were merged to create the master grid over the Antarctic with minimal mismatch between adjacent data sets. The final master grid of the Antarctic was filtered to remove residual low frequencies before being added to a long wavelength regional grid developed from the joint inversion of the Magsat anomalies and the low-passed filtered ~400 km) near surface anomalies.
The map shows a wide variation of magnetic anomaly patterns, trends and types reflecting the diversity of the geologic terranes of varying ages, degree of reworking, lithological and metamorphic variations. Not all changes in the apparent magnetic fabric signify differences in magnetic properties, some may be caused by variations in survey specifications. The new map readily portrays the first-order magnetic differences between oceanic and continental regions. The magnetic anomaly pattern over the continent reflects many phases of geological history whilst that over the abyssal plains of the surrounding oceans is dominated by simpler patterns of linear seafloor spreading anomalies and
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-fracture zones. The Antarctic compilation reveals terranes of varying ages, including Proterozoic-Archaean cratons, various Proterozoic-Palaeozoic mobile belts, Palaeozoic-Cenozoic magmatic arc systems, the boundary between East and West Antarctica, continent-ocean transitions, and other important crustal features. The map delineates basement structural trends, suture zones, the basement terranes, intra-continental rifts and major rifts along the Antarctic continental margin, and the regional extent of plutons and volcanics, such as the Ferrar dolerites and Kirkpatick basalts. The magnetic anomaly map of the Antarctic together with other geological and geophysical information provides new perspectives on the break-up of Gondwana and Rodinia evolution.
The Antarctic anomaly map is limited by the highly variable specifications of the surveys and regional gaps in coverage of the near surface surveys. ADMAP is working to improve the compilation for the 501h anniversary of the IGY with additional high-resolution magnetic data sets and by using CHAMP magnetic observations as they become available.
Golynsky, A., Chiappini, M., Damaske. D .. Ferraccioli, F., Ferris, J .. Finn, C., Ghidella, M., Ishihara, T., Johnson, A., Kim, H.R .. Kovacs, L., LaBrecque, J., Masolov, V., Nogi, Y .. Purucker, M., Taylor, P. & Tona. M. (2001): In: Morris, P. & R. von Frese, eds., BAS (Misc.) JO, Cambridge, British Antarctic Survey.
Magnetic anomaly pattern of the Grove Mountains region:
Implications for the tectonic correlations
(poster p.)
A.V. Golynsky1, V.N. Masolov2, V.S. Volnukhin2 & D.A. Golynsky3
1VNIIOkeangeologia, 1 Angliysky Ave., 190121 St. Petersburg, Russia; <sasha@vniio.nw.ru>.
2PMGRE, 24 Pobeda St., 189510 Lomonosov, Russia; <antarctida@peterlink.rU>;
3SPbSU, 7/9 Universitetskaya nab., 199034 St. Petersburg, Russia; <dmitry_gol@yahoo.com>.
The Grove Mountains are located eastward from the Prince Charles Mountains (PCM) and Mawson Escarpment and about 350 km south from the Prydz Bay coast. Up to now these isolated mountains remained one of the last unstudied regions of exposed rocks in East Antarctica despite were visited by several field parties. The geological and tectonic settings of the GM rocks have remained poorly understood. In accordance with several authors they might be correlated with: the southern (PCM) or with northern PCM; the intensity of the Pan-African event shows some affinities with the Prydz Bay coast area and they might be considered as a distinct terrane (MIKHALSKY et al. 2001).
It is believed that aeromagnetic surveys are a powerful tool in mapping basement structures and particularly useful in areas of poor outcrop for unraveling the structural history of a region. They provide a means to extrapolate known geology exposed in widely separated outcrops into broad-covered areas and help to delineate structural features that are not clearly recognizable from outcrop mapping. The aeromagnetic data in the GM area (71.9-73.6°S, 72-76°E) were collected by the PMGRE in 2000 along flight lines 5 km apart provide the only geophysical information of the upper crustal structure of this region. The magnetic anomalies of the GM and surrounding areas are characterized by a simultaneously simple and complex fabric. Four major magnetic anomaly patterns are well distinguished (Northern, Central, Gale and Law Plateau sub-units) which were further subdi-vided into a number of areas with outstanding magnetic grain. One of the essential features of the GM and surrounding areas is the regulation of magnetic anomalies along its periphery and the absence of intensive magnetic anomalies in the northern part of region. The most intensive anomalies (up to 500-600 nT) are distinguished within the Robertson Lineament in the north-western comer and along the western boundary of the survey area. The oblique character of the magnetic anomalies observed along the south-eastern, western and north-western boundary of the study area in respect to its central part (taking into account additional data from the eastern shoulder of the Lambert rift and located northward from 72°S; GOLYNSKY et al. 2002) clearly evidenced that the basement of the northern part
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-of the GM is seen to comprise much older crust then in the neighboring terranes with Mesa- to early Neoproterozoic high-grade metamorphic rocks. The existence of two ancient cratonic blocks in the SPCM and Vestfold Hills allow assuming that this region may contain the Archaean- to Paleopro-terozoic juvenile crust. This is supported by preliminary U-Pb dating of GM rocks which allowed to conclude that the protolith of metasediments are either Paleoproterozoic age or derived from source rocks of this age (MIKHALSKY et al. 2001). To clarify our assumption it would be reasonable more extensive geological studies to carry out visiting northern group of outcrops (Cook Peak and Vukovich Peaks) which magnetically appear to be unaffected by younger events. As to the GM crustal block itself, it is clearly discernible in the aeromagnetic data and can be considered as a region underwent either Grenvillian or Pan-African or both tectonism and reworking. Magnetic anomaly patterns of the GM and NCPM are not compatible and therefore the GM rocks not related to the Mesoproterozoic mobile belt of the NPCM, although Proterozoic histories of these regions have at least some features in common. The absence of any visible magnetic trends running towards the Prydz Bay coast area precludes any clear correlation with this region. Similarities of the magnetic patterns of the study area together with northern territory and the SPCM are rather distinctive suggesting that both regions apparently could form one indivisible terrane of Archean to Paleoproterozoic age. A continued southwards trajectory of the Gale magnetic sub-unit leads apparently to the Gamburtsev Subglacial Mountains although insufficient data are available to constrain a lateral extent of this distinct terrane in accordance with our interpretation. Whatever courses of the observed magnetic anomaly patterns they can not be discounted in any models for the reconstruction of both Rodinia and Gondwana.
Golynsky, A.V .. Alyavdin, S.V .. Masolov, V.N .. Tscherinov, A.S. & Volnukhin, V.S. (2002): Tectonophysics 347: 109-120.
Mikhalsky, E.V., Sheraton, J.W. & Bcliatsky, B.V. (2001): Terra Antarctica 8: 3-10.