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The décollement under the Nankai Trough accretionary prism in the Leg 131 area is imaged seismically as a high-amplitude, reversed-polarity reflection. The décollement is a 19-m-tnick zone of intense brecciation where it was penetrated on Leg 131. Physical properties measurements at Site 808 indicate that the strata beneath the décollement have lower velocities and densities than overlying strata. The seismic signature of the décollement varies considerably along both strike and dip. The lateral amplitude variations could be due to: (1) tuning effects as the layer thickens and thins, (2) focusing and defocusing of seismic energy due to the complex overlying structure, or (3) velocity variations caused by changes in porosity as fluid pressures dilate zones within or beneath the décollement. Synthetic seismic models show that the décollement signature can best be modeled by a sharp decrease in velocity and density beneath the décollement. Numerous models with a range of reasonable overlying structures were explored to examine the lateral amplitude variations. Based on these models, the variations in amplitude cannot be completely explained by focusing/defocusing of rays by the complex overlying structure or by changes in velocity/density of the underlying strata. The thickness of the décollement zone is very close to its tuning thickness, so tuning effects can explain much of the amplitude variations. The changes in amplitude are therefore caused by lateral changes in velocity, density, and thickness of the décollement, which are in turn probably driven by fluid pressure variations within the décollement zone. INTRODUCTION One of the major targets of drilling on ODP Leg 131 was the décollement at the base of the accretionary prism. Seismic profiles show a prominent reflection along the basal décollement that extends seaward under the trench wedge and landward beneath the accretionary prism (Moore et al., 1990, 1991). Leg 131 drilling at Site 808 penetrated the décollement and sampled the strata above and below it (Figs. 1, 2). The décollement zone itself was a region of poor recovery, however, so the nature of the zone remains poorly understood. Physical properties measurements do not constrain the velocity of the décollement, so it could be a zone of either high or low velocity. The two have fundamentally different implications for fluid flow within and below the décollement. Drilling into the Barbados accretionary prism showed the décollement to be a zone of fluid transport (Moore et al., 1988). Bangs and Westbrook (1991) modeled this décollement reflection as a thin zone of low velocity and density. They attributed lateral changes in amplitude of the décollement reflection to lateral changes in velocity of the décollement zone. This paper investigates the character of the décollement in the Leg 131 area of the Nankai Trough. We combine the results of regional seismic interpretations of the décollement with synthetic seismic modeling, based on physical property measurements made at Site 808. DATA ACQUISITION AND PROCESSING The seismic data used in this study were collected on the Fred Moore in July 1987 (Fig. 1) with a 1065 in3 (17.5 L) array made up of six air guns of various chamber sizes. The seismic signals were received with a 68-channel hydrophone array with 33.3-m group spacing, sampled at 2 ms and recorded in demultiplexed format (SEG-D) on magnetic tape. Processing consisted of common midpoint (CMP) sort, resample to 4 ms, velocity analysis, normal Hill, LA., Taira, A., Firth, J.V., et al., 1993. Proc. ODP, Sci. Results, 131: College Station, TX (Ocean Drilling Program). 2 Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822. 3 University of Texas Institute for Geophysics, Austin, TX 78759-8345. moveout (NMO) correction, stack, deconvolution, filter, finite-difference time migration, and depth migration. Details of the acquisition and original processing are described in Moore et al. (1990 and 1991). Prior to the seismic modeling described below, reprocessing was undertaken to preserve as much of the original source waveform and reflection amplitude information as possible. This was accomplished by starting with the CMP-sorted data at 2-ms sample interval, careful editing of bad shots and traces, NMO correction, and stacking of only the near 12 traces of each CMP (170-703 m) to eliminate effects of CMP stretch and amplitude variations with offset. Deconvolution was eliminated to prevent wavelet modifications. Arelatively broad-band, time-varying filter (10-15-40-50 Hz at the level of the décollement) and spherical divergence (t2) gain were applied after stack (Fig. 3). Although the depth-migrated section was used to construct the seismic depth models, only the stacked section was used for comparison to the detailed models because of the possibility of amplitude and waveform distortion in the migration process. CHARACTER OF THE DECOLLEMENT Structural and Physical Properties Expression The décollement was penetrated at Site 808 and is defined structurally as a 19.2-m-thick zone of intense faulting and brecciation beginning at 945 m in Hole 808C (Shipboard Scientific Party, 1991). There is a marked structural contrast between the relatively undeformed underthrusting strata beneath the décollement and the more strongly deformed prism material above the décollement (Shipboard Scientific Party, 1991). The décollement is also easily identified in physical properties measurements (Shipboard Scientific Party, 1991). Velocities in samples above the décollement increase approximately linearly from about 1750 m/s at 400 mbsf to more than 2250 m/s just above the décollement. Velocities drop sharply to less than 2100 m/s just below the décollement. Bulk densities show a parallel increase to the top of the décollement and decrease below the décollement. Samples within the décollement zone have very high velocities (2300-2400 m/s) and high densities (2.26-2.35 g/cm). It is not clear, however, whether these velocities and densities reflect the bulk physical properties of the décollement zone or whether they are valid only for isolated coherent blocks.

Character of the Décollement in the Leg 131 Area, Nankai Trough