Modelling sedimentation in ocean trenches: the Nankai Trough from 1 Ma to the present
Modelling sedimentation in ocean trenches: the Nankai Trough from 1 Ma to the present
复制标题
海沟沉积模拟：1 Ma 至今的南海海槽
DOI：
--
复制
发表时间：
1996
期刊：
影响因子：
--
通讯作者：
G. Westbrook
中科院分区：
文献类型：
--
作者：
N. Mountney;G. Westbrook
位于积极增生辐合边缘的海沟内的沉积物堆积是由沉积物供应、覆盖增生杂岩趾部沉积物俯冲/增生和俯冲速率之间的相互作用决定的。对海沟沉积进行建模，可以深入了解主要的控制因素，并从沉积模式中推导出沉积物供应和俯冲速率等因素在海沟沉积物积聚期间的变化情况。南开海槽内2个DSDP-ODP钻孔点显示出粗化向上的大气层序，表明其沉积相由深海泥质-海沟外粉砂质-海沟内砂质递进过渡。通过模拟两个垂直海沟剖面的净泥沙通量变化，确定了海沟楔在过去1千万年中几何形状的变化。通过对模型的不断调整，模拟的海沟现今的几何形状与地震反射剖面上显示的几何形状相匹配。两个地点的结果都证实了一个<20 km mir -1的“缓慢”俯冲速率。在西南遗址（582），在过去的1万年里，沟楔的宽度从13公里到21公里不等。在东北，808站点，在过去的0.5 Myr中，宽度从7到13公里不等。这些沟楔宽度的变化主要是泥沙供给速率大变化的结果。与808遗址相邻的化石扩散中心——国脊的俯冲作用对海沟内的沉积方式产生了重大影响。
Abstract Sediment accumulation within ocean trenches located at actively accreting convergent margins is determined by an interplay between sediment supply, sediment subduction/accretion at the toe of the overriding accretionary complex and the rate of subduction. Modelling trench sedimentation provides insight into the principal controlling factors, and a means of deriving, from the pattern of sedimentation, how factors, such as the rates of sediment supply and subduction, have varied over the period of accumulation of the trench sediments. Two DSDP-ODP drill sites within the Nankai Trough reveal a coarsening-upward megasequence, indicating a progressive facies transition from abyssal muds to outer-trench silts to inner-trench sands. The changing geometry of the trench-wedge over the past 1 Myr has been determined by modelling variations in net sediment flux for two trench-perpendicular profiles. The models were constrained to fit the stratigraphy at the drill sites, and the simulated present-day geometries of the trench were matched with those shown on seismic reflection profiles by successive adjustment of the model. Results from both sites confirm a ‘slow’ subduction rate of <20 km Myr-1. At the south-western site (582), the width of the trench-wedge has ranged from 13 to 21 km over the past 1 Myr. To the north-east, at Site 808, the width has ranged from 7 to 13 km over the past 0.5 Myr. These changes in trench-wedge width are primarily the result of large changes in sediment supply rate. The subduction of the Shikoku Ridge, a fossil spreading centre adjacent to Site 808, has had a major influence on the style of sedimentation within the trench.
The style of accretion from the trench to the toe of the accretionary complex has important implications for geometrical adjustment of the trench-wedge. Thrust displacement lifts the protothrust region out of the trench, resulting in a decreased width. This is followed by a phase of increasing width as the trench-wedge adjusts towards a new equilibrium. The cyclical, episodic accretion process results in a periodic second-order variation in trench-fill size that is superimposed on primary trends determined by variations in sediment supply rates and subduction rates over time.