HR: 10:35h
AN: T51E-08
TI: Crustal Structure of an Active Backarc Basin at the Rift-Drift Transition: Bransfield Strait, Antarctica.
AU: Barker, D H
EM: danb@ig.utexas.edu
AF: University of Texas Institute for Geophysics, 4412 Spicewood Springs Rd., Bldg. 600, Austin, TX 78759-8500 United States
AU: * Christeson, G L
EM: gail@ig.utexas.edu
AF: University of Texas Institute for Geophysics, 4412 Spicewood Springs Rd., Bldg. 600, Austin, TX 78759-8500 United States
AU: Austin, J A
EM: jamie@ig.utexas.edu
AF: University of Texas Institute for Geophysics, 4412 Spicewood Springs Rd., Bldg. 600, Austin, TX 78759-8500 United States
AB: Bransfield Strait is a backarc basin situated between the northern Antarctic Peninsula (AP) and the South Shetland Islands (SSI). Ocean bottom seismograph (OBS) data acquired in 2000 and older complementary multichannel seismic (MCS) reflection data document the crustal structure of this complex extensional basin. Prior analysis of MCS data documents extensional structure in the upper crust, including features that are interpreted to show: (i) extension propagates from NE-to-SW; (ii) volcanics of the axial neovolcanic zone are more voluminous in the NE than in the SW; (iii) detachments may signify basinwide simple-shear extension that is in part responsible for strong basinal asymmetry; (iv) Bransfield Strait is extensively segmented along-strike. New OBS data provide information about deeper crustal structure supporting several of these interpretations, particularly regarding propagation of rifting, distribution of magmatism, and along-strike segmentation. Data were acquired on five strike profiles, each approximately 240 km long using 11 OBSs at 20 km spacing, and three dip profiles, each approximately 150 km long using 11 OBSs at 15 km spacing. These data show (i) alternating zones of fast and slow velocity regions along strike; and (ii) spatial distribution of crustal thinning in the basin. Moho depth is constrained by observed {\it PmP} arrivals. Mirroring the basin's bathymetric asymmetry, Moho deepens gradually from 12-16 km beneath the axial neovolcanic zone to 18-20 km beneath the AP margin, but deepens more abruptly to 21-26 km beneath the SSI margin. The strike profiles show overall NE-to-SW deepening of Moho by 2-5 km occurring over a zone approximately 50 km wide in the central basin. This Moho geometry is consistent with NE-to-SW evolution of rifting, although crust is actually thinnest in the central basin and thickens slightly to the NE. Alternating fast-slow regions occur along strike. Fast regions appear to correlate with basement highs observed in MCS data on the AP margin, and with proximity to presumed intrusive/extrusive basalts towards the axial neovolcanic zone, while slower regions appear to correlate with half-grabens on the AP margin and ponded sediments in the axial deep. Boundaries between fast and slow regions beneath the conjugate SSI margin appear to correlate with structural boundaries between faster, older arc-related plutonic and volcanic rocks and slower, recent backarc-related volcanics and/or rifted Palaeozoic accreted forearc sediments. Using OBS, MCS, multibeam bathymetry and onshore geological observations, some structural boundaries may be traced margin-to-margin across the basin.
DE: 3025 Marine seismics (0935)
DE: 8105 Continental margins and sedimentary basins
DE: 8109 Continental tectonics--extensional (0905)
DE: 8150 Plate boundary--general (3040)
SC: T
MN: 2001 AGU Fall Meeting