TECTONIC SEDIMENT THICKENING, FLUID EXPULSION, AND THE THERMAL REGIME OF SUBDUCTION ZONE ACCRETIONARY PRISMS - THE CASCADIA MARGIN OFF VANCOUVER-ISLAND
TECTONIC SEDIMENT THICKENING, FLUID EXPULSION, AND THE THERMAL REGIME OF SUBDUCTION ZONE ACCRETIONARY PRISMS - THE CASCADIA MARGIN OFF VANCOUVER-ISLAND
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DOI：
10.1029/93jb02391
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发表时间：
1993-12-10
影响因子：
3.9
通讯作者：
SPENCE, GD
中科院分区：
文献类型：
--
作者：
HYNDMAN, RD;WANG, K;SPENCE, GD
用数值模型描述了卡斯卡迪亚北部俯冲带吸积棱柱的构造沉积增厚、流体排出和由此产生的热体制。沉积物的增厚和流体的排出都具有重要的热效应。模型的约束条件是：(1)从热控深度变化到模拟天然气水合物底部反射层(BSR)的大陆斜坡吸积带的详细热流剖面，以及(2)从P波速度-深度剖面的陆地变化推断的跨越吸积带的孔隙度变化。以形变锋面向陆地15公里为中心的棱柱快速增厚区域的热流比忽略沉积物吸积过程热效应的大尺度热力模式预测的热流低20%。与邻近深海卡斯卡迪亚盆地的相同深度相比，该地区的孔隙度也要高得多。如果来沙剖面最初在水平方向上被简单地构造缩短，在垂直方向上被加厚，从而形成一个高孔隙度的欠固结剖面，则模型结果与温度和速度数据都是一致的。再固结和流体排出对构造增厚的响应是缓慢的，主要发生在变形前缘向陆地20-30公里处。预测的最大排液速率约为4×10~(-11)m S~(-1)(1.3 mm a~(-1))。根据速度数据估算了变形前沿区的孔压分布，简化假设为速度只是有效压力的函数；孔压很高，在变形前缘陆地方向15~20公里处可达到静岩压力的80%。通过将数值模型中的孔压分布与速度数据进行匹配，得到了棱镜向海数十公里范围内的体积渗透率的估计值，范围从顶部附近的10(-16)m~2到底部附近的约10(-18)m~2。据推测，至少在向海的30公里范围内，棱镜沉积物的固结程度很低，孔隙压力很高。
Tectonic sediment thickening, fluid expulsion, and the resulting thermal regime of the northern Cascadia subduction zone accretionary prism have been described by numerical models. Both the sediment thickening and the fluid expulsion are found to have important thermal effects. Constraints on the models are provided by (1) detailed heat flow profiles across the continental slope accretion zone from variations in the thermally controlled depth to a gas hydrate bottom-simulating reflector (BSR), and (2) porosity changes across the accretion zone inferred from the landward variation in the P wave velocity-depth profiles. The heat flow in the region of rapid prism thickening centered 15 km landward of the deformation front is 20% below that predicted by a larger scale thermal model that ignores the thermal effects of the sediment accretion processes. The porosity also is substantially higher in this region compared to at the same depths in the adjacent deep sea Cascadia basin. The model results are in agreement with both the thermal and velocity data if the incoming sediment section is initially simply tectonically shortened horizontally and thickened vertically, generating a high porosity underconsolidated section. Reconsolidation and fluid expulsion in response to the tectonic thickening is slow, occurring mainly 20-30 km landward of the deformation front. The maximum predicted fluid expulsion rate is about 4 x 10(-11) m s-1 (1.3 nun a-1). The pore pressure distribution across the deformation front region has been estiniated from the velocity data using the simplifying assumption that velocity is only a function of effective pressure; pore pressures are high, reaching 80% of lithostatic 15 to 20 km landward of the deformation front. An estimate of the bulk permeability in the seaward tens of kilometers of the prism ranging from 10(-16) m2 near the top to about 10(-18) m2 near the base has been obtained through matching the pore pressure distribution in the numerical model to that obtained from the velocity data. The prism sediments are inferred to be substantially underconsolidated and pore pressures to be high for at least the seaward 30 km of the accretionary prism.