Reconstruction of geologic history of Indus suture zone of Ladakh, Indian Himalayas, by high resolution fossil and radiometric ages

本项目的主要成果总结如下：1.晚三叠世至早白垩世放射虫产于印度河缝合带上同碰撞磨拉石砾岩中的硅质岩碎屑中。由于该硅质岩被认为是大洋板块盖层沉积物的一部分，这些化石证据表明，拉达克喜马拉雅地区的塞诺特提斯(从冈瓦那北缘开始的喀喇昆仑地块的前晚三叠世裂谷)在晚三叠世之前就已经打开了。Shyok缝合带陆架相海相地层可细分为中侏罗世(卡洛维亚)含菊石的Soltak组和中白垩世含Albian Orbitolina的Shyok组。这些化石强烈地表明，Shyok缝合带沿线的海洋在中白垩世后关闭了。拉达克弧深成岩是由塞诺特提斯洋板俯冲形成的，其矿物学和地球化学特征为钙碱性，K-Ar年龄为50~51 Ma。弧体北缘还分布有具有钙碱性特征的花岗岩和闪长岩，并具有-66K-Ar年龄。这些证据清楚地表明，在欧亚大陆和印度碰撞之前，在50-66 Ma期间，弧形岩浆活动至少持续了1600万年。沿喀喇昆仑断裂的糜棱岩具有垂直线理和伸展变形结构，以及右旋走滑剪切指示器，指示了与现代活动相同的意义。到目前为止，这些构造显示了长达1500多万年的复杂变形历史。动态重结晶石英颗粒的长径比表明，随着断裂带温度的降低，剪切带宽度变窄。

Main results of this project are summarized as follows.1. Late Triassic to Early Cretaceous radiolarians are found from chert clasts in the Hemis Conglomerate (syn-collisional Molasse sequence) along the Indus suture zone. Since the chert is considered to have been part of the ocean-plate cover sediments, these fossil evidence shows the pre-Late Triassic opening of the Ceno-Tethys (pre-Late Triassic rifting of Karakoram block from the northern margin of Gondwana) in Ladakh Himalayas.2. Shelf-facies marine formations along the Shyok suture zone are subdivided into the Middle Jurassic (Callovian) ammonoids-bearing Soltak Formation and Middle Cretaceous Shyok Formation with Albian Orbitolina. These fossils strongly indicate that the ocean along the Shyok suture was closed in the post-Middle Cretaceous time.3. Plutonic rocks of the Ladakh arc which were formed by the subduction of oceanic plate of Ceno-Tethys are calc-alkaline on the basis of their mineralogy and geochemistry, and have K-Ar ages of 50-51 Ma. Granite and diorite with calc-alkaline signature and 64-66 K-Ar ages are also distributed on the northern margin of the arc. These lines of evidence clearly show that the arc magmatism had continued at least for sixteen million years during 50-66 Ma before the collision between Eurasia and India.4. Mylonites along the Karakoram fault have vertical lineation and extensional deformation structures together with the right-lateral strike-slip shear indicators designating same sense as the present-day activity. These structures show a long and complicated deformation history more than fifteen million years until now. Aspect ratios of dynamically recrystallized quartz grains suggest that the width of shear zone has narrowed with the decreasing temperature of the fault zone.