(oral p.) S.D. Boger & C.J.L. Wilson
School of Earth Sciences, The University of Melbourne, Melbourne VIC 3010, Australia;
<sdboger@unimelb.edu.au>, <cjlw@unimelb.edu.aU>.
The Lambert Glacier - Amery Ice Shelf occupies a narrow NNE-SSW orientated fault bound depres-sion referred to as the Lambert Graben. Seismic reflection surveys have recognised deep faults associated with this structure and these data suggest that faulting associated with the Lambert Graben may extend at least 700 km inland from the Antarctic coast. These faults are interpreted to have
31
-initially developed during the Carboniferous and Permian, and are thought to have formed the depres-sion into which the Permo-Triassic Amery Group were deposited. Resent geological investigations from the Lambert Glacier -Amery Ice Shelf region has identified quartz- and calcite-bearing faults, inferred to represent the surface expression of the larger structures recognised geophysically. Kine-matic and palaeostress data from the exposed faults suggest that the majority of the preserved offset occurred in response to NW-SE directed extension, oblique to the axis of the graben. This resulted in predominantly dextral strike slip fault movement, accommodating components of both normal and reverse offset. Although the age of these structures is unconstrained, faults with this offset in the northern Prince Charles Mountains disrupt the Permo-Triassic Amery Group and juxtapose it against Proterozoic basement. Equivalent strike-slip faults in the southem Prince Charles Mountains produce dextrally offset tectonic boundaries and metamorphic isograds across the Lambert Glacier. These observations imply both a post-Triassic timing and regional significance for this period of faulting.
Given the implied post-Triassic timing of deformation, we suggest that these faults are related to the Cretaceous rifting of Gondwana. The remarkable similarity in orientation between the palaeostress field calculated for these faults and the Cretaceous divergence vector between India and Antarctica supports this inference. Although these results do not preclude an earlier phase (Carboniferous?) of failed rifting, they suggest the Cretaceous overprint was significant. The results of which now dominate the observed brittle structures and preserved fault offsets.
S.D. Boger
Barrovian-type metamorphism from the Archaean Ruker Terrane, southern Prince Charles Mountains, East Antarctica
(oral p.)
School of Earth Sciences, The University of Melbourne, VIC 3010; <sdboger@unimelb.edu.au>.
Mineral textures coupled with thermodynamic modeling in the K20-Fe0-Mg0-A'203-Si0i-H20 (KFMASH) model system of amphibolite facies metapelites from the Ruker Terrane, southern Prince Charles Mountains, point to the preservation of an up-temperature prograde metamorphic path, followed by rapid decompression (i.e. a clockwise P-T path). Textural evidence for the up-temper-ature path is given by the sequential growth of garnet, staurolite and kyanite in a number of rocks of different composition. The peak mineral assemblage consisted of garnet + kyanite ± biotite ± muscovite that formed at P-T conditions of approximately 700
·c
and 9 kbar. The subsequent growth of sillimanite, then cordierite (± K-feldspar), is interpreted to reflect close to 5 kbar of near isothermal decompression. In less aluminous assemblages, decompression resulted in the formation of partial melts. The observed mineral assemblages, P-T path, and inferred peak pressures and temperatures are typical of Barrovian metamorphic terranes. During the Phanerozoic, these types of belts are commonly inferred to reflect convergent collisional orogens, the result plate tectonics. However, orogenesis in the Ruker terrane occurred during the late-Archaean (2870 Ma), a period in the Earths history when heat production and crustal heat flow were considerably higher, and the role of plate tectonics remains debated.-32
-Development of Jane Basin by crustal fragmentation: southern margin of the South Orkney Microcontinent, Antarctica
(oral p.)
F. Bohoyo1, J. Galindo-Zaldivar2, A. Jabaloy2, A. Maldonado1, J.M. Martinez-Martinez1'2, J.
Rodriguez-Fernandez1, A.A. Schreider3 & E. Surifiach4
1 lnstituto Andaluz Ciencias de la Tierra, CSIC/Universidad de Granada, 18002 Granada, Spain:
<fbohoyo@ugr.es>, <amaldona@ugr.es>, <jmmm@ugr.es>, <jrodrig@ugr.es>;
2Departamento de Geodinamica, Universidad de Granada, 18071 Granada, Spain; <jgalindo@ugr.es>,
<jabaloy@ugr.es>;
3P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, 23 Krasikova, 117218 Moscow, Russia;
4Departament de Geologia Dinamica i Geoffsica, Universitat de Barcelona, 08028 Barcelona, Spain;
<emma@geo.ub.es>.
Jane Bank and Basin system is located along the southern margin of the South Orkney Microcontinent (SOM). This region constitutes the eastern seaward prolongation of the Antarctic Peninsula.
Geophysical data (magnetic, gravity, swath bathymetry and multichannel seismic profiles-MCS) along three profiles orthogonal to the main tectonic and bathymetric trends of the system were recorded during the SCAN97 cruise by the Spanish BIO "Hesperides". The new MCS profiles and gravity modelling reveal that these structures extend westwards beyond it was previously proposed.
We show, for the first time, linear sea floor magnetic anomalies in Jane Basin, which allow to date the oceanic crust. Spreading of Jane Basin began around 17.6 Ma, which is the age of the oldest magnetic anomaly. (chron C5Dn), and ended at about 14.4 Ma (chron C5ADn). Chron C6n (19.5 Ma) was identified as the youngest oceanic crust of the northern Weddell Sea, whose northern spreading branch was totally subducted. Magnetic anomalies of high intensities (over 400 nT) were detected, moreover, along the S-SE margin of the SOM, which may be attributed to basic plutonic rocks intruded during Mesozoic in the Pacific margin of the Antarctic Peninsula, and that form the Pacific margin anomaly belt.
The MCS profiles and gravity models show heterogeneous thinning of the continental crust at the SE margin of the SOM. Taking into account the probable continental nature of Jane Bank, the drifting of this bank to the southeast from the SOM was probably due to the same type of processes of continental fragmentation. MCS profile SM04 shows a good image of a spreading centre near the axis of Jane Basin, although this ridge is not well represented in the other sections. Important reversal faults in the oceanic crust of the Weddell Sea and Jane Basin were detected in the MCS profiles.
These structures are younger than the age of development of Jane Basin, and they probably mark the end of Jane Basin spreading due to a change of the tectonic regime in the area. A thrusting of the SOM over the oceanic crust of the Jane Basin and duplication of this oceanic crust are detected in the gravity modeling, which emphasizes the importance of these reverse faults.
The analysis of the different set of data and mainly the distribution of the magnetic anomalies, suggest that subduction of the spreading centre of Weddell Sea below the SE margin of the SOM plays an important control for the development of Jane Ba.sin. An important change in tectonic regime occurs at the Scotia-Antarctic plate boundary near 14.4 Ma being the responsible of the end of the Jane Basin spreading.
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