New field, structural and geochronological data from the Shyok and Nubra valleys, northern Ladakh: linking Kohistan to Tibet
New field, structural and geochronological data from the Shyok and Nubra valleys, northern Ladakh: linking Kohistan to Tibet
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来自拉达克北部什约克和努布拉山谷的新野外、结构和地质年代学数据：连接科希斯坦和西藏
DOI：
10.1144/gsl.sp.2000.170.01.14
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发表时间：
2000
期刊：
影响因子：
--
通讯作者：
M. Whitehouse
中科院分区：
文献类型：
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作者：
Roberto F. Weinberg;W. Dunlap;M. Whitehouse
拉达克北方的努布拉-希约克汇合区是研究西北喜马拉雅构造演化的关键地区，也是巴基斯坦地质与西藏地质联系的基础。汇流区的地质一直是许多猜测的主题，主要集中在蛇绿岩的存在及其区域意义上。这些蛇绿岩被认为代表了Shyok缝合带（SSZ）的向东延伸，该缝合带将德拉斯岛弧与欧亚大陆的南部边缘分开，并被沿着Khalsar逆冲断层（通常被认为代表了喀喇昆仑主逆冲断层的东部延伸）的运动所覆盖。该地区的地质相对复杂，现有资料很少，妨碍了区域地质对比。喀尔萨逆冲断层（KT）和喀喇昆仑右旋断层（KF）是喜马拉雅西北部的两个区域构造特征，在交汇处合并，形成一个三相点和三个区块：南部的拉达克区块，西北部的萨尔托罗区块和东北部的喀喇昆仑区块。在三相点附近，KF改变了走向和运动方向。三相点的运动矢量分析表明，KT和KF的两个部分可以同时移动，并允许跨断层的运动矢量的预测。KT和KF剪切优先火山沉积岩的Shyok和Nubra组，分别。与以前的解释相反，这些剪切岩石并不代表破碎的蛇绿岩。然而，区域构造重建需要拉达克地块和欧亚大陆之间的过渡带，巴尔蒂斯坦SSZ的走向表明，缝合带可能会出现在KT北部，或者沿着萨尔托罗山脉的南坡，或者沿着沿着萨尔托罗山谷更北。在我们能够访问的萨尔托罗地块的少数露头中，我们没有发现蛇绿岩的证据。相反，我们发现了钙碱性的提利特岩基的露头。虽然我们的观察没有证实在萨尔托罗地块南部存在与缝合线有关的岩石，但不能排除这种可能性。锆石U-Pb离子探针年龄为68 ± 1 Ma。相似范围的模式组成和年龄的提克里特和拉达克岩基表明，他们是同一岩浆事件的一部分。这一结果和其他一些观测结果表明，75 Ma后的拉达克和萨尔托罗地块的地质是相似的。因此，如果在Saltoro地块南部存在缝合带，那么，正如其他人沿巴基斯坦西部相同构造边界沿着得出的结论，在75 Ma之前就发生了断裂。KF代表了一个更年轻的喀喇昆仑边界，将拉达克和萨尔托罗地块的岩石与喀喇昆仑山脉的岩石并列。沿沿着KF未发现与大陆板块有关的岩石。产于喀喇昆仑山南部的喀喇昆仑浅色花岗岩的锆石U-Pb年龄为15.0 ± 0.4Ma（2σ）。由于这些浅色花岗岩没有发现南KF，这个故障必须启动后浅色花岗岩侵入，因此必须小于15 Ma的年龄。在汇合处，KF穿过区域岩石序列，从科希斯坦进入巴尔蒂斯坦，进入汇合区。断层上的运动使层序位移约150 km，到达西藏东南部，在那里可以恢复区域岩石层序。
Abstract The Nubra-Shyok confluence in northern Ladakh is a key area for understanding the tectonic evolution of NW Himalaya and provides the basis for linking the geology of Pakistan to that of Tibet. The geology of the confluence area has been the subject of much speculation centred mainly on the existence of ophiolites and their regional significance. These ophiolites are thought to represent the eastward extension of the Shyok Suture Zone (SSZ), which separates the Dras island arc from the southern margin of Eurasia, and which was overprinted by movement along the Khalsar Thrust (often thought to represent the eastern continuation of the Main Karakoram Thrust). The geology of the area is relatively complex and the little information available has hampered regional geological correlations. The Khalsar Thrust (KT) and the dextral Karakoram Fault (KF), two regional tectonic features of NW Himalaya, merge at the confluence defining a triple point and three blocks: the Ladakh block to the south, the Saltoro block to the northwest, and the Karakoram block to the northeast. Close to the triple point, the KF changes strike and movement direction. Movement vector analysis of the triple point indicates that the KT and the two parts of the KF could have moved contemporaneously, and allows prediction of the movement vectors across the faults. The KT and KF shear preferentially volcano-sedimentary rocks of the Shyok and Nubra formations, respectively. Contrary to previous interpretations, these sheared rocks do not represent disrupted ophiolites. Regional tectonic reconstructions, however, require suturing between the Ladakh block and Eurasia and the strike of the SSZ in Baltistan suggests that the suture zone might crop out north of the KT, either along the southern slopes of the Saltoro Range or further north along the Saltoro valley. In the few outcrops of the Saltoro block we were able to visit, we found no evidence of ophiolitic rocks. Instead we found outcrops of the calc-alkaline Tirit batholith. Although our observations do not confirm the presence of the suture-related rocks in the southern Saltoro block, this possibility cannot be ruled out. Zircons from a sample of Tirit granite (U-Pb ion-microprobe age) yielded an age centred at 68 ± 1 Ma. The similar range of modal composition and age of the Tirit and Ladakh batholiths suggest that they are part of the same magmatic event. This result and a number of other observations indicate that the post-75 Ma geology of the Ladakh and Saltoro blocks is similar. Thus, if there is a suture zone in the southern Saltoro block, suturing must have occurred before 75 Ma, as concluded by others along the same tectonic boundary to the west in Pakistan. The KF represents a much younger terrane boundary, juxtaposing rocks of the Ladakh and Saltoro blocks to those of the Karakoram terrane. Rocks related to suturing of continents were not found along the KF. Karakoram leucogranites cropping out in the southern part of Karakoram terrane yielded a U-Pb zircon age centred at 15.0 ± 0.4 Ma (2σ). Because these leucogranites were not found south of the KF, this fault must have initiated after leucogranite intrusion and must therefore be younger than 15 Ma old. At the confluence the KF cuts across the regional rock sequence than can be followed from Kohistan into Baltistan and into the confluence area. Movement on the fault displaces the sequence by approximately 150 km to southeastern Tibet where the regional rock sequence can be regained.