CHARACTER OF THE DECOLLEMENT IN THE LEG 131 AREA , NANKAI TROUGH 1
CHARACTER OF THE DECOLLEMENT IN THE LEG 131 AREA , NANKAI TROUGH 1
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南海1号海槽131段区域的塌陷特征
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发表时间：
2006
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通讯作者：
T. Shipley
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作者：
G. Moore;T. Shipley
在leg131地区，南开海槽增生三棱镜下的胞体在地震上成像为一个高振幅、反极性反射。在131号腿被击穿的地方，有一个直径19米的剧烈角化区。808站点的物理性质测量表明，与上覆地层相比，dsamement下面的地层速度和密度较低。沿走向和倾斜方向，该矿区的地震特征变化很大。横向振幅变化可能是由于：(1)随着地层增厚和变薄而产生的调谐效应，(2)由于复杂的上覆结构而引起的地震能量的聚焦和散焦，或(3)由于流体压力在井筒内部或下方扩张而引起的孔隙度变化引起的速度变化。综合地震模型表明，用dsamicement下面的速度和密度急剧下降来模拟dsamicement的特征是最好的。探索了许多具有合理上覆结构范围的模型来检验横向振幅的变化。基于这些模型，振幅的变化不能完全用复杂的上覆结构或下伏地层的速度/密度变化引起的射线聚焦/散焦来解释。dsamicement区的厚度非常接近于它的调谐厚度，因此调谐效应可以解释振幅的变化。因此，振幅的变化是由dsamement的速度、密度和厚度的横向变化引起的，而这些变化又可能是由dsamement区域内的流体压力变化引起的。ODP Leg 131钻井的主要目标之一是吸积棱镜底部的dsamement。地震剖面显示，沿基底面有明显的反射，基底面向海延伸至海沟楔下，向陆延伸至增生棱镜下（Moore et al., 1990,1991）。在808地点进行的131号钻井穿透了该矿区，并对矿区上下的地层进行了取样（图1、2）。然而，dsamement区本身是一个恢复不佳的区域，因此对该区域的性质仍然知之甚少。物理性质测量不限制dsamement的速度，因此它可能是一个高或低速度的区域。这两种方法对胞体内部和胞体下方的流体流动有着根本不同的含义。对巴巴多斯增生棱镜的钻探表明，该区域是一个流体输送区（Moore等人，1988年）。Bangs和Westbrook（1991）将这种dsamicement反射建模为一个低速和低密度的薄区。他们将dsamicement反射振幅的横向变化归因于dsamicement区域速度的横向变化。本文研究了南开海槽131段海域的海洋环境特征。我们将区域地震解释的结果与基于808站点物理性质测量的合成地震建模相结合。本研究中使用的地震数据于1987年7月在Fred Moore上收集（图1），使用由6个不同腔室尺寸的气枪组成的1065 in3 （17.5 L）阵列。地震信号由68通道水听器阵列接收，组间距为33.3 m，采样时间为2 ms，并以解复用格式（SEG-D）记录在磁带上。处理包括共同中点（CMP）排序，采样至4 ms，速度分析，正常希尔，洛杉矶。， Taira， A., Firth, j.v.等人，1993。项目：ODP， Sci。结果，131：大学站，得克萨斯州（海洋钻探计划）。2夏威夷大学地质与地球物理系，夏威夷檀香山96822；3德克萨斯大学地球物理研究所，德克萨斯奥斯汀78759-8345；移出（NMO）校正，堆栈，反褶积，滤波，有限差分时间迁移和深度迁移。摩尔等人（1990年和1991年）描述了采集和原始处理的细节。在进行下面描述的地震建模之前，进行了再处理，以尽可能多地保留原始震源波形和反射振幅信息。这是通过以2毫秒的采样间隔从CMP排序的数据开始，仔细编辑不良拍摄和轨迹，NMO校正，并仅堆叠每个CMP （170-703 m）的近12条轨迹来消除CMP拉伸和振幅变化的影响。消除了反卷积以防止小波修改。相对宽频带，时变滤波器（10-15-40-50 Hz在d郁闷的水平）和球面发散（t2）增益叠加后应用（图3）。虽然采用深度偏移剖面构建地震深度模型，但由于偏移过程中可能出现振幅和波形畸变，因此仅采用叠加剖面与详细模型进行对比。滑脱层结构特征和物理性质表达：滑脱层在808井处被钻穿，在结构上被定义为从808C井945米处开始的一个19.2米厚的强烈断裂和角化带（Shipboard Scientific Party, 1991）。在dsamacement下方相对未变形的逆冲地层与dsamacement上方变形更强烈的棱柱材料之间存在明显的结构对比（Shipboard Scientific Party, 1991）。在物理性质测量中也很容易识别出dsamicement （Shipboard Scientific Party, 1991）。在dsamacement之上的样品中，速度从400 mbsf时的1750 m/s左右近似线性地增加到刚好高于dsamacement的2250 m/s以上。速度急剧下降到低于2100米/秒，刚好低于直径。堆积密度在dsamacrement上方呈平行增加，在dsamacrement下方呈下降趋势。在dsamicement区域内的样品具有非常高的速度（2300-2400米/秒）和高密度（2.26-2.35克/厘米）。然而，尚不清楚这些速度和密度是否反映了dsamicement区的整体物理特性，或者它们是否仅对孤立的相干块有效。
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.