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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B11, 2518, doi:10.1029/2001JB000195, 2003

Thin layer tuned reflections from an anisotropic décollement zone

Sheila Peacock

School of Computer Science and School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK


Abstract

The décollement beneath the Barbados accretionary complex is a gently dipping shear zone 10–40 m thick within which slippage occurs between the overlying sediments of the accretionary wedge and the underthrust sediments on the subducting tectonic plate. The density and seismic velocity of the décollement, derived from modeling of “tuned” seismic P wave reflections from its top and base, are very low and indicate high porosity sustained by above-hydrostatic pore pressure. Previous modeling assumed that the décollement is isotropic. Here we investigate whether neglecting anisotropy might bias the interpretation. Likely causes of anisotropy are aligned cracks and aligned pores and grains within the clays. Unrecognized anisotropy might cause misestimation of porosity, and confusion of P – S – P mode converted waves with the tuned P wave reflections. Horizontal cracks within the décollement would cause lower vertical velocity than in an isotropic material with the same porosity; this reduces the interpreted porosity, but by only a few percent, because crack density is limited by the observed finite strength of the décollement. P – S – P mode-converted waves would be caused by anisotropy due to nonhorizontal alignment within the décollement or in the accretionary wedge above. Appreciable P – S – P conversion requires anisotropy too great to be caused by aligned cracks alone. A décollement of aligned anisotropic clay particles gives appreciable mode conversion when 50% of the clay particles are within 30° of perfect alignment. Reflections for incidence angles up to ∼26° from a strongly anisotropic décollement zone with horizontal alignment are indistinguishable from those from an isotropic zone.

Received 19 November 1999; accepted 27 July 2003; published 7 November 2003.

Index Terms: 0935 Exploration Geophysics: Seismic methods (3025); 3022 Marine Geology and Geophysics: Marine sediments—processes and transport; 3025 Marine Geology and Geophysics: Marine seismics (0935); 7203 Seismology: Body wave propagation.

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Citation: Peacock, S. (2003), Thin layer tuned reflections from an anisotropic décollement zone, J. Geophys. Res., 108(B11), 2518, doi:10.1029/2001JB000195.