青藏高原羌北地块志留纪至泥盆纪古地理位置的古地磁制约

The northern Qiangtang Block of the Tibetan plateau is a large and stable block with a Precambrian crystalline basement and intact Phanerozoic sedimentary cover. Quantitative descriptions of the Paleozoic paleogeography and kinematic evolution of this block will provide key constraints in the overarching endeavor to better understand the history and tectonic architecture of the Tibetan plateau (for example by establishing the early paleogeographic framework and by constraining the initiation, growth and destruction of the Tethys). This project therefore aims to obtain precise and quantitative Silurian and Devonian paleomagnetic data from the northern QiangtangBlock. Our work will combine a variety of research methods, including classic applied paleomagnetism paired with rock and mineral analysis, but also more exotic techniques, such as AMS (anisotropy of magnetic susceptibility) and E-I (Elongation-Inclination) analysis, to ensure that we deliver reliable data of the highest-quality. Sampling will be coordinated at two sections in Silurian and Devonian strata in the eastern and middle regions of the northern Qiangtang Block, which have been selected due to their abundant fossils (thereby providing good age control), clear formational contacts, and the absence of deformation and metamorphism (thereby increasing the chances that a primary magnetization has been preserved). Our results, combined with previously published paleomagnetic data, will allow the construction of an apparent polar wander path (APWP) for the northern Qiangtang Block, with which we can reconstruct the paleogeography and kinematic history of this critical region of the Tibetan plateau. More specifically, this work will provide key paleomagnetic constraints on the rift-to-drift timing for the blocks that split from northern Gondwana in the mid-to-late Paleozoic, creating the Neotethyan Ocean and ultimately setting in motion the events that gave rise to the formation of the Tibetan plateau. Our results will therefore also further contribute to ongoing discussions about the nature of the Tethys and the location of key boundaries between Gondwana and Eurasia.

羌北地块是青藏高原面积较大，具有古老基底且古生代以来沉积盖层发育较完整的稳定块体之一。定量描绘出其古生代以来的古地理位置和运动学过程，可为研究青藏高原古构造格局、显生宙特提斯演化和古地理重建等提供重要基础和关键制约。本项目拟主要利用构造磁学研究方法，结合岩矿分析、磁化率各向异性（AMS）测试、各种岩石磁学测试、E-I检验及各种可行的地质检验等技术方法，在羌北地块西部和中部化石丰富、时代依据充分、接触关系清楚、没有变形和变质的志留和泥盆系沉积地层中，分别选择两条连续完整的剖面开展系统古地磁研究，获得羌北地块志留和泥盆纪可靠古地磁数据。再结合前人研究结果，获得羌北地块志留纪以来的古地磁视极移曲线，重建青藏高原腹地关键地块在地质历史中的古地理位置和运动学特征，为再造冈瓦那大陆北缘块体裂离和欧亚大陆南缘块体拼贴的地质历史过程，为研究青藏高原特提斯演化和冈瓦纳与欧亚大陆界线位置等提供关键古地磁制约。

青藏高原位于环球纬向特提斯构造域东段，是一个由多条缝合带和被他们分隔开的地块组合而成的巨型造山拼贴体，它的形成与特提斯洋演化及多地块从冈瓦纳大陆到欧亚大陆的离散-聚合过程密切相关。羌北地块位于青藏高原中部，是组成青藏高原地块中面积较大、具有古老基底且古生代以来沉积盖层发育较完整的稳定地块。一般认为，青藏高原中部的羌塘（包括羌北和羌南）和拉萨地块可能是“基梅里大陆”的组成部分，晚古生代自冈瓦纳大陆北缘裂离并向北漂移，早中生代拼合至劳亚大陆南缘，造成北侧古特提斯洋关闭和南侧新特提斯洋扩张。所以，利用古地磁方法定量确定出羌塘和拉萨地块古生代以来的古地理位置和漂移历史，可为研究东特提斯域演化和古大陆重建等提供重要依据和关键制约。本项目①获得了羌北地块首个志留纪和泥盆纪的古地磁数据，填补了青藏高原中部地块上该时段古地磁数据的空缺；②获得了羌北地块中二叠和晚二叠世以及羌南地块中二叠世的古地磁新数据，为重建该地块古位置提供了重要依据；③结合发表的古地磁数据绘制了羌北地块志留至三叠纪的视极移曲线，重建了羌北地块在这一关键时段的北向漂移运动学过程。研究表明，①羌北地块古生代期间位于南半球中-低纬度，晚二叠世至晚三叠世快速北向漂移（平均运动速率约为11.2cm/yr），晚三叠世之前拼贴至塔里木/华北地块南缘；②羌北地块在志留纪可能已经不属于冈瓦纳大陆，晚古生代与华南和印支地块古地理位置关系密切。晚古生代期间羌北地块与塔里木地块之间的古特提斯洋纬向宽度达到5500km左右，而其与冈瓦纳大陆之间的古特提斯洋（龙木措-双湖洋）纬向宽度相对较小（约2000km）；③羌南地块在约265Ma时位于南纬22°，可能与从土耳其/伊朗延伸到滇缅马泰的更广阔的基梅里大陆一起，二叠纪早期开始从冈瓦纳分裂并向北漂移打开了新特提斯洋，最终在三叠纪晚期与劳亚大陆的南缘合并，古特提斯洋关闭。本项目研究结果可为研究东特提斯演化过程和古大陆重建等提供了重要依据和定量约束。