HR: 16:00h
AN: T32F-01    [PDF]
TI: Bransfield Basin and Cordilleran Orogenesis
AU: * Dalziel, I W
EM: ian@ig.utexas.edu
AF: University of Texas, Jackson School of Geosciences Institute for Geophysics 4412 Spicewood Springs Road, Building 600, Austin, TX 78759-8500 United States
AU: Austin, J A
EM: jamie@ig.utexas.edu
AF: University of Texas, Jackson School of Geosciences Institute for Geophysics 4412 Spicewood Springs Road, Building 600, Austin, TX 78759-8500 United States
AU: Barker, D H
EM: d.barker@gns.cri.nz
AF: Institute of Geological & Nuclear Sciences, P.O. Box 30368, Lower Hutt, New Zealand
AU: Christensen, G L
EM: gail@ig.utexas.edu
AF: University of Texas, Jackson School of Geosciences Institute for Geophysics 4412 Spicewood Springs Road, Building 600, Austin, TX 78759-8500 United States
AB: 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 degrees 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 degrees S, separating the South Shetland Islands from the Antarctic Peninsula, constitutes a modern 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 United States 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 flanking the South Shetlands island arc and gently dipping listric normal faults along the Antarctic Peninsula margin. Normal fault polarity reversals appear to be related to distributed loci of magmatic activity within the basin. This architecture 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 constitute ideally oriented surfaces for reactivation as thrust faults, leading to obduction of the basin floor rocks during any ensuing compressional event. Seismic refraction studies of Bransfield basin using Institute for Geophysics ocean bottom seismographs 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 of the 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.
DE: 8100 TECTONOPHYSICS
SC: Tectonophysics [T]
MN: 2003 Fall Meeting