(oral p.)
I.W.D. Dalziet1, J.A. Austin, Jr.1, D.H.N. Barker2 & G.L. Christesen'
1University of Texas, Jackson School of Geosciences, Institute for Geophysics, 4412 Spicewood Springs Road, Building 600, Austin, Texas 78759-8500.USA;<ian@ig.utexas.edu>, <jamie@utexas.edu1>.
<gail@ig.utexas.edu>;
21nstitute ofGeological and Nuclear Sciences. P.O. Box 30368, Lower Hutt, New Zealand;
<d.barker@gns.cri.nz>.
Tectonic uplift of the Andean Cordillera was initiated in the mid-Cretaceous with inversion of a composite marginal basin along 7500 km of the continental margin of South America, from Peru to Tierra del Fuego and the North Scotia Ridge. In the southernmost Andes, from 50-56°S, the quasi-oceanic floor of this basin is preserved in the obducted ophiolitic rocks of the Rocas Verdes (Green Rocks) Basin. We suggest that the basin beneath Bransfield Strait, 61-64°S, separating the South Shetland Islands from the Antarctic Peninsula. constitutes a modem analog for the Rocas Verdes Basin.
Marine geophysical studies of Bransfield Basin have been undertaken over the past 12 years by the Institute for Geophysics, University of Texas at Austin, under the auspices of the Ocean Sciences Division and US Antarctic Program, National Science Foundation. These studies have elucidated the structure and evolution of Bransfield Basin for comparison with the Rocas Verdes Basin, with a view to eventual forward modeling of the evolution of a hypothetical cordilleran orogen by compression and inversion of the basin. These are the processes that can be observed in the tectonic transformation of the Rocas Verdes Basin into the southernmost Andean cordillera, as South America moved rapidly westward in an Atlantic-Indian ocean hot-spot reference frame during the mid-Cretaceous.
Multi-channel reflection seismic data from the Bransfield Basin reveal an asymmetric structural architecture characterized by steeply-dipping normal faults f I anking the South Shetlands island arc
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-and gently dipping lislric normal faults along the Antarctic Peninsula margin. Normal fault polarity reversals appear to be related to distributed I 0 ci of magmatic activity within the basin. This archi-tecture is remarkably similar to that deduced from field structural studies of the Rocas Verdes Basin. Notably. the oceanward-dipping, low angle normal faults along the Antarctic Peninsula margin consti-tute ideally oriented surfaces for reactivation as thrust faults, leading to abduction of lhe basin floor rocks during any ensuing compressional event.
Seismic refraction studies of Bransfield Basin using Institute for Geophysics ocean bottom seismo-graphs indicate along-strike northeastward thinning of the crust associated with southwestward rift propagation, comparable to that indicated for the Rocas Verdes Basin by the results of U-Pb zircon dating ofthe ophiolitic rocks. Cross-strike segmentation of Bransfield Basin also mirrors the isolated ophiolitic complexes of the inverted Rocas Basin in South America and on South Georgia Island.
The results of our geophysical studies of Bransfield Basin reveal close similarities in structure and evolution with the Cretaceous Rocas Verdes Basin, and therefore encourage the next step of forward modeling in an effort to help elucidate fundamental cordilleran orogenic processes.
Aeromagnetic anomaly investigation over East Antarctica from northern Victoria Land to George V Land
{poster p.) D. Damaske1, E. Bozzo2 & F. Ferraccioli3
'Federal lnstitute for Geosciences and Natural Resources (BGR), Stilleweg 2, 30655 Hannover, Germany;
<D.Damaske@bgr.de>;
2DIPTERIS, Sez. Geofisica, Universita di Genova, V.le Benedetto XV 5, 16 I 32 Genova, Italy:
<bozzo@dipteris.unige.it>;
3British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 OET, UK; <ffe@bas.ac.uk>.
As part of the joint 1999-2000 German-Italian GANOVEX VITI-ITALIANTARTIDE XV Antarctic campaign, an integrated geophysical dataset including aeromagnetics, magnetic susceptibility, ground-based gravity, airborne radio-echo sounding, geomagnetic depth sounding and passive seis-mology was collected (Bozzo & DAMASKE 200 I) over northern Victoria Land, Oates Land and George V Land. The aim was to provide new constraints upon crustal and lithospheric structure and tectonic evolution for this part of East Antarctica. This key region includes the northermost segment of the Transantarctic Mountains and, within its "backside", the northernmost edge of the enigmatic Wilkes Subglacial Basin.
Here we focus upon some results of the aeromagnetic survey. The complete coastal region from Mertz Glacier to Yule Bay spans a length of approximately IOOO km. It was surveyed for the first time using the "Polar Duke" vessel as a mobile base. The helicopter-borne survey was flown with a profile spacing of 4.4 km and tie lines 22 km apart. Survey altitude was at a constant barometric level wilhin the individual sections. For the Lillie Glacier area the altitude was 2730 m; the Oates Coast section was flown at 1715 m. West of the Matusevich Glacier the survey level is 1425 m. Altogether 26,575 km of usable data on lines were collected over an area of 83,800 km2. The aeromagnetic data were processed by applying base station corrections. differential GPS, IGRF removal, and statistical levelling procedures.
The magnetic anomalies over the Lillie Glacier, Oates Coast and George V Coast survey sections pro-vide insight into geology of the region. Over the Robertson Bay Terrane or Zone (FINN et al. 1999), the Admiralty Intrusives of the Everett Range and of Yule Bay show contrasting magnetic signatures.
Just offshore Yule Bay, a near-circular magnetic anomaly overlies Unger Island and Surgeon Island.
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-It reveals a mafic(?) intrusion beneath this enigmatic Ross-age crustal fragment (FIORETII et al.
2002). Over the Wilson Terrane or Zone the most outstanding feature of the Oates Coast map, is a linear magnetic anomaly over the Matusevich Glacier. This anomaly might reflect Ross-age arc rocks emplaced along the Exiles Thrust Fault System (FLOTIMANN & KLEINSCHMIDT 1991, FERRA CCI OU et al. 2002). Just west of the Matusevich Glacier, a high-amplitude circular anomaly shows the extent of the Archangel Nunataks gabbro. Two broad positive anomaly complexes are imaged in the George V Coast area, which is almost entirely devoid of outcrop. One is located along the inferred north-easternmost edge of the Wilkes Subglacial Basin, over the Mawson Peninsula, and SCAR Bluffs area.
It may be caused by a buried Jurassic mafic intrusion, similar to the one interpreted from TAMARA aeromagnetics 1000 km further south (BEHRENDT et al. 2002). The second anomaly complex is located between Mertz and Ninnis Glacier. The trend of this anomaly is parallel to a major fault imaged from seismic data offshore (DE SANTIS et al. 2002). Along the northern edge of the Wilkes Subglacial Basin, high-frequency linear anomalies suggest that Ferrar sills and dykes may not be restricted to the Transantarctic Mountains. This new interpretation is consistent with magnetic model-1 ing results along the IT ASE traverse from Terra Nova Bay to Dome C (FERRACCIOLI et al. 200model-1).
Behrendt J.C., Damaske D., Finn C.A.. Kyle P. & Wilson T.J. (2002): J. Geophys. Res. B8: 10.l029/2001JB000376.
Bozzo E. & Damaske, D. (2001): Terra Antartica Rep. 5: 109 pp.
De Santis L., B. Brown & G. Brancolini (2002): Workshop Scienze della Terra in Antartide, Siena: 47.
Ferraccioli F .. Coren. F, Bozzo E., Zanolla C.. Gandolfi S., Tabacco I. & Frezzoui M. (2001): EPSL 197: 407-421.
Ferraccioli, F .. E. Bozzo & Capponi G. (2002): Geophys. Res. Leu. 29(10): I0.1029/2001GLOl4138.
Finn C., Moore D., Damaske D. & Mackey T. (1999): Geology 27: 1087-1090.
Fiorelli A.M., Capponi G., Black L.P., Varne R. & Visona' D. (2002): Gondwana 11.
Floumann T. & Kleinschmidt. G. (1991): Geology 19: 45-47.
Quartz grain surface textures as indicators of transport and depositional mechanisms in glacial marine sediments under the McMurdo/Ross Ice Shelf, Windless Bight
(oral p.)
Damiano Damiani1 &K-0422
1Dipartimento di Scienze della Terra, Universita di Siena, via Laterina 8, 53100 Siena, Italy;
<damiani@unisi.it>;
2The K-042 party comprised: Peter Barrett, Victoria University of Wellington, Scientific Leader; Gavin Dunbar (VUW); Giovanna Giorgetti (Siena University); Frank Niessen (A WI Bremerhaven); Uwe Nixdorf (A WI Bremerhaven); Natalie Robinson (VUW); Christina Riesselman (Stanford University); Alex Pyne (VUW, Field Leader): Erich Dunker (A WI): Jonothan Leitch (Antarctica NZ) and Dougal Mason (VUW).
Many studies on glacial marine sediments have established that provenance, processes of erosion and transport, and depositional environment all influence quartz grain morphoscopy and surface texture.
With this in mind quartz grains sampled from recent marine sediments from beneath the McMurdo, Ross Ice Shelf in Windless Bight were analysed to extract information on provenance and sedi-mentary processes.
This study is part of a New Zealand, German, USA and Italian research program. During the 2003 Antarctic campaign two 60-cm-wide holes were drilled through the ice shelf (70.5 m thick at Site 1 and 143.7 m-thick at Site 2) using a hot water drilling system. Water depth was around 920 mat both sites and cores over 60 cm long were collected from each site.
Quartz grains have been sampled from Site 1, which probably represents the longest time interval based on lithology and preliminary smear slide observations. Different stratigraphic units have been recognized from visual description and each of them has been sampled. Abrasion and corrosion features on quartz grains have been observed at magnification obtainable with scanning electron microscope (SEM) in the secondary electron mode.
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64-Quartz grains present a great variety of surface textures which reflect several transport and deposi-tional mechanisms. Four typologies are recognised, representing aeolian, glacial, glacio-fluvial and marine environments. The top layer of the core is characterised by a low biogenic content and a high quartz content; quartz grains from aeolian environment are more abundant than quartz grains with characteristic of glacio-fluvial and marine transport. In the central part of the core, characterise by low biogenic and quartz content, quartz grains from a glacial environment are dominant. The lower part of the core is characterised by the absence of biogenic material and a low quartz content. Here quartz grains from glacial environment and, subordinately, from an aeolian environment are present. The bottom part of the core is characterised by the absence of biogenic material and a high quartz content.
Here quartz grains with textures typical of glacio-fluvial and marine transports are dominant.
These preliminary results underline different sedimentary processes of transport and deposition under the McMurdo-Ross Ice Shelf. Further studies are needed to better constrain the past changes in the sedimentary environment linked to the ice shelf dynamic.