Did the Kohistan-Ladakh island arc collide first with India?
Did the Kohistan-Ladakh island arc collide first with India?
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DOI：
10.1130/b26348.1
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发表时间：
2009-03-01
影响因子：
4.9
通讯作者：
Stockli, Lisa
中科院分区：
文献类型：
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作者：
Khan, Shuhab D.;Walker, Douglas J.;Stockli, Lisa
科希斯坦-拉达克地块位于喜马拉雅山脉的西北角，长期以来一直被认为是侏罗纪和白垩纪建造在海底的岛弧。由于科希斯坦-拉达克区块现在位于亚洲大陆，了解它是如何以及何时被夹在印度和亚洲其他地区之间的，这一点很重要。通过对古地磁资料的分析，我们发现，在晚白垩世-早古新世，科希斯坦-拉达克岛弧不可能远离赤道。印度靠近赤道，但亚洲南缘在北面3000多公里处。我们对Kohistein-Ladakh地块岩石进行的新的锆石U-Pb法测年结果表明，钙碱性火山弧火成活动在Kohistein-Ladakh弧处结束了61 Ma。我们将这一停止解释为科希斯坦与印度的碰撞。有证据表明，南半球富含杜帕尔地幔的源区参与了科希斯坦-拉达克弧白垩纪末期Teru火山岩的形成，证实了这一新的时代。古新世早期科希斯坦弧与印度碰撞的进一步证实来自蛇绿岩逆冲推覆的时间证据，以及碰撞后沉积岩与巴基斯坦西北部和西部印度大陆边缘的不整合。印度现在西北角承载着科希斯坦-拉达克区块，最终并入亚洲是在Shyok缝合线上。缝合时间约50 Ma的最好证据来自科希斯坦北部的两块碰撞后花岗岩(年龄分别为47 Ma和41 Ma)，它们的锆石同位素组成表明，在缝合形成之前，科希斯坦之下并不存在古亚洲陆壳的参与。Shyok对当时活跃的喀喇昆仑安第斯弧的缝合年龄为50 Ma，这与缝合线延伸到科希斯坦-拉达克地块东端以连接印度和西藏南部(拉萨地块)边缘之间的约51 Ma雅鲁藏布缝合线的时间吻合得很好，当时安第斯弧冈底斯弧也占据了这条缝合线。
The Kohistan-Ladakh block occupies the northwestern corner of the Himalayan mountains and has long been recognized to represent an island arc constructed on ocean floor during Jurassic and Cretaceous times. Because the Kohistan-Ladakh block now lies within the Asian continent, it is important to know how and when it became sandwiched between India and the rest of Asia. We have found from analysis of paleomagnetic data that in Late Cretaceous-early Paleocene times, the Kohistan-Ladakh island arc could not have been far from the equator. India was close to the equator, but the southern margin of Asia was more than 3000 km to the north. Our new U-Pb zircon age results from rocks of the Kohistan-Ladakh block show that calc-alkaline volcanic arc igneous activity ended in the Kohistan-Ladakh arc by 61 Ma. We interpret that cessation to date the collision of Kohistan with India. This new timing is confirmed by evidence that a Southern Hemisphere enriched DUPAL mantle source was involved in the generation of the latest Cretaceous Teru Volcanic Formation rocks of the Kohistan-Ladakh arc. Further confirmation of the collision of the Kohistan arc with India in early Paleocene times comes from evidence of the timing of obduction of ophiolites; and from the unconformity of postcollisional sedimentary rocks onto the Indian continental margin in northwestern and western Pakistan. Final incorporation of India, now carrying the Kohistan-Ladakh block in its NW corner, into Asia took place at the Shyok suture. The best evidence for the timing of that suturing ca. 50 Ma comes from two postcollisional granites (ages 47 Ma and 41 Ma) in northern Kohistan, which show in their zircon isotopic compositions evidence of the involvement of ancient Asian continental crust that did not exist under Kohistan before the suture formed. The 50 Ma age for Shyok suturing against the then-active Karakoram Andean arc fits well with the extension of the suture beyond the eastern end of the Kohistan-Ladakh block to join the precisely dated ca. 51 Ma Yarlung-Tsang Po suture between India and the southern (Lhasa block) margin of Tibet, which at that time was also occupied by an Andean arc, the Gangdese arc.