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Influence of a Non-rigid Backstop on Subduction Processes at the Northern Lesser Antilles Convergent Margin John S Stachowiak 1 (5122323254; jss@ig.utexas.edu) Nathan L Bangs 1 Gail L Christeson 1 Thomas H Shipley 1 Yosio Nakamura 1 1The University of Texas Institute for Geophysics, 4412 Spicewood Springs Rd. Building 600, Austin, TX 78759, United States The backstop plays a fundamental role in the development and deformation of an accretionary wedge. Mechanical and numerical modeling of the Lesser Antilles subduction zone incorporates a rigid backstop for simplicity and shows relatively little deformation occurring within the overlying forearc basin sediments. To investigate the role of the backstop in forearc deformation, seismic profiles of the Lesser Antilles subduction zone between $16\deg$ 7' N and $16\deg$ 22' N were acquired during a 1998 seismic survey. They reveal a complex interplay between the seaward-dipping backstop, the forearc basin and the accretionary wedge. The 2-D seismic reflection profiles include five dip lines (roughly perpendicular to the deformation front) spaced 5-10 km apart and eight strike lines (parallel), spaced 10 km apart. A refraction study complements the multichannel seismic data and provides an enhanced delineation of primary structural boundaries, allowing a detailed look at interaction of the backstop and the colliding plate. The data acquired on this cruise portray a backstop that is non-rigid. It is characterized by folds and isolated domes, several kilometers wide, underlying highly contorted forearc basin sediments. The axis of backstop folding appears to be more or less parallel to the strike of the Barbados Ridge and the domes seem to be randomly distributed. The rough and varied topography of the non-rigid backstop causes an uneven transmission of stresses within the forearc basin and the accretionary wedge, resulting in differential deformation. We speculate the primary causes of backstop deformation to be a combination of compression induced by the collision of ocean floor topography and material underplating. The subduction of a raised feature such as a basement ridge could produce the broad, linear folding visible in the backstop. Localized underplating of lithified sediments could account for the dome-like structures superimposed on the folds. |
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Last Modified: October 8, 1999
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