藏南床得剖面古地磁结果对印度-亚洲碰撞方式的约束  

Paleomagnetic results from Jurassic-Cretaceous strata in the Chuangde area of southern Tibet constrain the nature and timing of the India-Asia collision system

作　　者：张波兴 李永祥 胡修棉[1] ZHANG BoXing LI YongXiang HU XiuMian(State Key Laboratory for Mineral Deposit Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China)

机构地区：[1]南京大学地球科学与工程学院,内生金属矿床成矿机制研究国家重点实验室,南京210023

出　　处：《科学通报》2017年第4期298-311,共14页Chinese Science Bulletin

基　　金：国家自然科学基金(41274071,41525007);国家重点基础研究发展计划(2012CB822001)资助

摘　　要：特提斯喜马拉雅位于大印度最北端,其构造演化是理解印度-亚洲大陆碰撞过程的关键之一.为了更好地限定特提斯喜马拉雅中生代以来的古地理位置及其漂移历史,本文对藏南江孜床得剖面侏罗系下热组、维美组和白垩系床得组开展了古地磁研究.古地磁样品取自39个采样点,其中下热组5个,岩性为青灰色安山岩;维美组25个,岩性为粉砂岩和砂岩;床得组9个,岩性为灰岩.对118个古地磁柱状样品进行的逐步热退磁和交变退磁结果显示,下热组和维美组样品未能分离出稳定的特征剩磁;床得组样品分离出了稳定的特征剩磁(Ch RM),且具有正反极性,并通过了倒转检验.结合沉积学和岩石磁学结果,床得组样品的特征剩磁很可能为原生剩磁,其年龄为86.3~74.0 Ma.床得组样品特征剩磁的平均方向为Ds=152.0°,Is=?52.9°,?=18.0,?95=5.9°.该数据表明,特提斯喜马拉雅江孜地区在晚白垩世(80.1±6.2 Ma)所处的古纬度为南纬33.1°±5.6°,与印度板块北缘同时代所处的古纬度一致(参考点28.0°N,88.5°E),说明二者至少在晚白垩世之前从运动学意义上仍然是一个块体,缺乏大洋分隔的证据.这为理解印度-亚洲大陆碰撞方式提供了新的古地磁约束.The Tethyan Himalaya sits between the India craton and the Lhasa block, the contact being marked by the Indus-Tsangpo suture zone（ITSZ）. The amalgamation of the Tethyan Himalaya with the Lhasa block has been traditionally regarded as representing the India-Asia collision. Recently, however, a dual-collision model has been proposed, in which Tethyan Himalayais was considered as a separate block that rifted from Greater India＇s northern margin in Late Cretaceous. The Tethyan Himalaya subsequently underwent two episodes of collision, i.e., the first with Lhasa block to the north -50 Ma and the second collision with India to the south -23 Ma. The dual-collision model hinges on the speculated rifting of Tethyan Himalaya from Greater India in Late Cretaceous, which, however, has not been paleomagnetically constrained because the Late Cretaceous paleomagnetic data from Tethyan Himalaya are lacking. Clearly, elucidating the Mesozoic, particularly the Late Cretaceous, tectonic evolution of Tethyan Himalaya is crucial to understanding the India-Asia collisional processes. To better constrain the paleogeographic position and drift history of Tethyan Himalaya during the Mesozoic, particularly in Late Cretaceous, a paleomagnetic investigation was conducted of the Xiare and Weimei（Jurassic） and Chuangde（Cretaceous） formations in the Chuangde areas of southern Tibet. Paleomagnetic samples were collected from 39 sites, among which 5 sites were from grey andesite of the Xiare Formation, 25 sites were from siltstone and sandstones of the Weimei Formation, and 9 sites were from limestones of the Chuangde Formation. A total of 118 paleomagnetic specimens were subjected to stepwise thermal or alternating field demagnetization. The demagnetization results show that samples from the Xiare Formation and the Weimei Formation did not yield characteristic remanent magnetization（Ch RM）. Specimens from the Chuangde Formation yield stable Ch RMs upon demagnetization that display both normal and reversed polarities, and passe

关 键 词：藏南 床得剖面 特提斯喜马拉雅 晚白垩世 古地理 古地磁 

分 类 号：P318.44[天文地球—固体地球物理学] P542[天文地球—地球物理学]