Seismic velocities from the Barbados Ridge Complex: Indicators of high pore fluid pressures in an accretionary complex
Seismic velocities from the Barbados Ridge Complex: Indicators of high pore fluid pressures in an accretionary complex
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巴巴多斯海岭杂岩体的地震速度：增生杂岩体中高孔隙流体压力的指标
DOI：
10.1029/jb095ib06p08767
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发表时间：
1990
影响因子：
--
通讯作者：
P. Buhl
中科院分区：
文献类型：
--
作者：
N. Bangs;G. Westbrook;J. Ladd;P. Buhl
在垂直于Barbados Ridge增生杂岩走向的16°12′n（465线）、14°23′n（480线）和13°20′n（484线）处拍摄了3条宽孔径（0-5 km偏移）地震剖面，对增生杂岩的深部结构进行了成像，并获得了地震速度数据。利用全叠共深度点（CDP）数据集的速度分析，沿每条线提供距离为1 km的速度/深度函数，绘制速度随深度和距离的变化。地震速度的增加和孔隙度的降低表明，杂岩内沉积物的横向固结程度随着杂岩年龄的增加而增加。沿480线楔形底部的速度在趾部为~ 2.4 km/s。在距离脚趾约60公里的楔形底部，楔形厚度从1.8公里增加到约5.0公里，速度增加到大于3.0公里/秒。然而，在三个剖面中，前缘和下逆冲层序内的速度都很低，比同等厚度的未变形沉积层序的速度低1.0 km/s。低速区被解释为具有高孔隙度的沉积物。在推断孔隙度高的地方，孔隙流体压力也被推断为超过流体静力学，孔隙流体实际上支持最近逆冲和复杂岩层增厚所产生的增加的静岩载荷。最快速增厚区域与速度异常之间的相关性表明，相对于孔隙流体排出速率的快速加载速率产生了这些超压沉积物。在逆冲构造层序中，楔形沉积物的加载历史与逆冲构造层序之间的差异最大，但速度足够低，足以产生速度反演。低速也发生在与复合体相邻的区域，在那里孔隙流体压力可能被相邻楔块产生的压力升高。在楔形西部，沿480号线的弧前盆地，靠近地震成像的泥底辟，在海底以下3.5公里的深度，速度比在更正常的沉积剖面中预期的速度低0.5公里/秒。在复合体的直接向海方向，最深的未变形层序的速度异常为- 0.5 km/s。在向海方向至少12公里的地方，流速仍然很低，但离楔体较远的地方，流速变得更接近正常的沉积物区。
Three wide-aperture (0–5 km offset) seismic profiles were shot perpendicular to the strike of the Barbados Ridge accretionary complex at 16°12′N (line 465), 14°23′N (line 480), and 13°20′N (line 484) to image the deep structure of the accretionary complex and obtain seismic velocity data. Velocity analyses on full-fold common depth point (CDP) gathers were used to provide velocity/depth functions at 1 km separation along each line to map the variation in velocity with depth and distance across the accretionary complex. The sediments within the complex show consolidation increasing laterally with greater age of the complex as indicated by the increase in seismic velocity and the inferred decrease in porosity. The velocity at the base of the wedge along line 480 is ∼2.4 km/s at the toe. The velocity at the wedge base at ∼60 km from the toe, where the wedge thickness has increased from 1.8 km to ∼5.0 km, has increased to greater than 3.0 km/s. However, the velocities within the leading margins and within undeithrust sequences in each of the three sections are low, as much as 1.0 km/s less than velocities found in undeformed sedimentary sequences of equivalent thickness. The low-velocity regions are interpreted as sediments with high porosity. Where porosity is inferred to be high, pore fluid pressures are also inferred to exceed hydrostatic and pore fluids actually support the increased lithostatic load produced by recent thrusting and thickening of the complex. The correlation between the most rapidly thickened regions and the velocity anomalies suggests that a rapid rate of loading relative to the rate of pore fluid expulsion produced these overpressured sediments. Velocities in the underthrust sequences, where the differences between loading histories of wedge sediments and the underthrust sequences are largest, are sufficiently low to produce velocity inversions. Low velocities also occur in regions adjacent to the complex where the pore fluid pressures may be elevated by the pressure produced in the adjacent wedge. West of the wedge, in the forearc basin along line 480, and near a seismically imaged mud diapir, velocities at a depth of 3.5 km below the seafloor are 0.5 km/s less than what is expected in a more normal sedimentary section. Immediately seaward of the complex, the deepest undeformed sequences have a velocity anomaly of −0.5 km/s. Velocities remain low for at least 12 km seaward of the complex, but they become closer to a more normal sediment section farther from the wedge.