哥斯达黎加汇聚边缘俯冲剥蚀的古地磁学制约-IODP344航次后研究

Subduction erosion at convergent margins removes material from the front and base of the overriding plate, causing the subsidence in the forearc area and the landward migration of trench. It is critical to quantify the mass loss by subduction erosion as it is closely related to crustal recycling and mass balance. Because the part of the convergent margin that is affected most by subduction erosion is typically under the sea, it is difficult to study the effect of subduction erosion by direct observations. Previous studies of subduction erosion often estimate the subduction-caused mass loss on the basis of biostratigraphy as an age control that is low in resolution and can lead to large errors in the estimates. Integrated Ocean Drilling Program Expedition 344 cored at the subducting Co-cos ridge and the forearc area that is directly affected by the subduction at the Costa Rican convergent margin. And the cores recorded the vertical motions at the convergent margin that were interpreted to result from the subduction erosion. Here we propose to conduct a detailed study of magnetostratigphy and rock magnetism that is based on down hole variation of rock magnetic parameters of the cores from three representative sites, namely, Site U1381 from the subducting plate, Site U1380 from the middle slope, and Site U1413 from the upper slope of the forearc area. Together with the constraint from biostratigraphy, the paleomagnetic work will aim to establish high-resolution timescales for these three sites. The high resolution timescales will provide an important basis for examining the impact of the subduction erosion by the Co-cos ridge on the Central American convergent margin, and contribute to better quantify the mass loss by subduction erosion at the convergent margin.

汇聚边缘的俯冲剥蚀作用将上覆板块的前缘和底部物质带回到地球内部，从而导致弧前区沉降和海沟陆向迁移。因此，定量估算俯冲剥蚀产生的物质亏损对于理解地壳物质再循环至关重要。然而，由于受俯冲剥蚀影响的汇聚边缘通常处于海面以下，不易直接取样观察，而研究程度较低；而且,现有的研究中，地层时代通常由生物地层控制而较粗略，并导致对俯冲剥蚀的估算误差较大。IODP 344航次在哥斯达黎加汇聚边缘实施了钻探，直接从位于俯冲板块之上的科科斯脊及受其俯冲剥蚀影响的弧前区钻取了岩芯；而且,所取岩芯清晰记录到了俯冲剥蚀引起的垂向运动。本项目拟对该航次的U1381、U1380和U1413等三个有代表性站位的岩芯开展详尽的磁性地层和岩石磁学综合研究；同时，结合生物地层，为这些站位的岩芯建立高分辨率的年代标尺, 从而为探讨科科斯脊俯冲剥蚀对中美洲汇聚边缘的影响, 以及为准确地估算俯冲剥蚀引起的物质亏损提供可靠的年代学基础。

汇聚边缘俯冲剥蚀作用将上覆板块的前缘和底部物质带回到地球内部，从而导致弧前区沉降和海沟陆向迁移。然而，定量估算俯冲剥蚀产生的物质亏损却非常困难，从而制约了准确理解地壳物质平衡与再循环过程。这是因为，受俯冲剥蚀影响的汇聚边缘通常处于海面以下，不易直接取样观察，而研究程度较低；而且由于俯冲剥蚀是对岩石记录的破坏与消减，使得对俯冲剥蚀过程的认知不足。本项目选取IODP 344航次在哥斯达黎加汇聚边缘直接从位于俯冲板块之上的科科斯脊及受其俯冲剥蚀影响的上覆板块弧前区钻取的三个有代表性站位（即U1381、U1380，U1413）的岩芯开展详细的磁性地层、岩石磁学以及与生物地层等相结合的综合研究。项目研究旨在为这些站位的岩芯建立高分辨率的年代标尺, 为探讨科科斯脊俯冲剥蚀对中美洲汇聚边缘的影响, 以及为准确地估算俯冲剥蚀引起的物质亏损提供可靠的年代学基础。研究发现：科科斯脊上U1381站位沉积间断的时限为1.52-9.61Ma，而且综合分析发现沉积间断呈区域性出现，很可能是由科科斯的初始俯冲引起的。因此，科科斯初始俯冲不晚于1.52Ma，并提出科科斯脊初始俯冲的两种沉积响应机制-科科斯脊本身发生了挠曲(buckling)和底流(bottom current) 侵蚀。在上覆板块斜坡中段U1380的~490-550米处识别出~1.94-1.83Ma的强变形层段，而且发现强变形发生得很快（~1.83-1.78Ma）。与斜坡上段和下段的沉积、构造等结果对比分析发现，弧前强变形层段具有区域性和准同时性（~2.0-1.83Ma）特征，提出发生于~1.80Ma的弧前强变形是科科斯俯冲过程中俯冲剥蚀与强烈的近水平剪切变形耦合作用的结果。由于俯冲剥蚀导致剥蚀型汇聚板块边缘沉降以及海沟的陆向迁移，这种耦合机制不只局限于中美洲汇聚边缘，很可能也存在于其它剥蚀型汇聚板块边缘。