Late Mesozoic tectonic evolution of the central Bangong–Nujiang Suture Zone, central Tibetan Plateau

The Bangong–Nujiang Suture Zone (BNSZ), located in the central Tibetan Plateau, experienced a Late Mesozoic transition from oceanic closure to continent–continent collision. Based on sandstone-composition analyses, U-Pb detrital-zircon dating and structural observations, we determined that this transition occurred coeval with the accretion of the Lhasa Terrane to Eurasia. According to our newly observed stratigraphic and geochronological data, we conclude the following: (1) the Upper Triassic-Lower Jurassic Muggar Gangri Group was mainly derived from the Qiangtang Terrane; (2) the Shamuluo Formation composed of Upper Jurassic-Lower Cretaceous shallow marine strata was sourced from both the Qiangtang Terrane and recycled Muggar Gangri Group; (3) the Lower Cretaceous (~125–110 Ma) Duoni Formation shows both Lhasa and Qiangtang detrital zircon fingerprints, indicating closure of the Bangong–Nujiang Ocean at ~120 Ma; and (4) the Upper Cretaceous Abushan Formation is a suite of intramontane molasse deposited after the Qiangtang–Lhasa collision. Therefore, these sedimentary strata record the transition from oceanic closure to continent–continent collision from 125 to 90 Ma within the central BNSZ. Moreover, based on relationships among strata distributions and thrust locations, we propose that thrust movements propagated southward during this transition and controlled sedimentary basin deposition. The Late Mesozoic tectonic transition within the BNSZ occurred well before the India-Asia collision and hence profoundly influenced the Cenozoic formation of the Tibetan Plateau.

位于青藏高原中部的班公湖-怒江缝合带（Bangong–Nujiang Suture Zone, BNSZ）经历了晚中生代从洋壳闭合到陆陆碰撞的构造转换过程。依托砂岩成分分析、U-Pb碎屑锆石定年（U-Pb detrital-zircon dating）与构造观测结果，我们判定该构造转换事件与拉萨地体（Lhasa Terrane）向欧亚大陆的增生作用同期发生。结合新观测到的地层学与年代学数据，我们得到如下结论：（1）上三叠统-下侏罗统木嘎岗日群（Muggar Gangri Group）的主要物源为羌塘地体（Qiangtang Terrane）；（2）由上侏罗统-下白垩统浅海相地层组成的沙木罗组（Shamuluo Formation），其物源兼具羌塘地体与再旋回木嘎岗日群；（3）下白垩统（约125~110 Ma）多尼组（Duoni Formation）同时具备拉萨地体与羌塘地体的碎屑锆石指纹特征，指示班公湖-怒江洋于约120 Ma完成闭合；（4）上白垩统阿布山组（Abushan Formation）为羌塘-拉萨碰撞后形成的一套山间磨拉石沉积。综上，上述沉积地层记录了班公湖-怒江缝合带中部在125~90 Ma期间从洋壳闭合到陆陆碰撞的构造转换历程。此外，基于地层分布与逆冲构造位置的关联关系，我们提出该构造转换阶段内逆冲活动向南迁移，并控制了沉积盆地的沉积作用。班公湖-怒江缝合带的晚中生代构造转换发生于印-亚碰撞之前，因此对青藏高原的新生代构造形成具有深远影响。