青藏高原东缘中新生代龙门山前陆盆地动力学及其与大陆碰撞作用的耦合关系

Mesozoic-Cenozoic Dynamics of Longmenshan Foreland Basin along the Eastern Margin of the Tibetan Plateau and Its Coupled Relationship with Continnent Collision

位于青藏高原东缘的龙门山前陆盆地是中国典型的前陆盆地之一。自晚三叠世以来,该盆地充填了厚度大于1万余米的海相至陆相沉积物,以不整合面为界可将其划分为6个构造层序,根据几何形态将构造层序区分为两种类型,即楔状构造层序和板状构造层序,其中晚三叠世、晚侏罗世、晚白垩世—古近纪构造层序为楔状构造层序,其余为板状构造层序。研究结果表明楔状构造层序为逆冲构造负载的产物,板状构造层序为走滑剥蚀卸载的产物。本次以晚三叠世前陆盆地为典型的楔状前陆盆地开展了逆冲构造负载系统的弹性挠曲动力学模拟,以晚新生代龙门山前陆盆地为典型的板状前陆盆地开展了与走滑剥蚀卸载系统的弹性挠曲动力学模拟,并计算了龙门山构造负载系统向扬子克拉通的推进速率,结果表明龙门山造山楔的推进速率在早期较快(如晚三叠世最大推进速率达15mm/a),晚期较慢(如晚侏罗世、晚白垩世—古近纪最大推进速率仅为6.7mm/a)。进而推测龙门山幕式逆冲作用的构造驱动力来自于青藏高原中生代以来的基麦里大陆加积碰撞和印度与亚洲板块碰撞作用,其中晚三叠世楔状构造层序是羌塘板块与亚洲大陆碰撞的产物,晚侏罗世楔状构造层序是拉萨板块与亚洲大陆碰撞的产物,晚白垩世—古近纪楔状构造层序是科希斯坦板块、印度板块与亚洲大陆碰撞的产物。

The Longmenshan foreland basin is one of typical foreland basins along the eastern margin of the Tibetan Plateau. The Late Triassic-Quaternary strata there, more than 10000 m thick, can be divided into six tectonic sequences by unconformities. Two depositional patterns of tectonic sequences have been distinguished, one is wedge-shaped and the other is tabular-shaped. The wedge-shaped tectonic sequences include Late Triassic, Late Jurassic and Late Cretaceous-Eogene tectonic sequences. The tabular-shaped tectonic sequences include Early to Middle Jurassic, Middle Cretaceous and Neogene to Quaternary tectonic sequences. The results indicate that the wedge-shaped tectonic sequences are sedimentary response of active thrusting and the tabularshaped tectonic sequences are sedimentary response of strike slipping. Taking the Late Triassic the wedgeshaped foreland basin as an example of the typical wedge-shaped foreland basin, we used a one-dimensional analytical model to simulate the subsidence in the wedge-shaped foreland basin due to the emplacement of tectonic loads in the Longmenshan thrust belt on an elastic plate. Taking the Neogene to Quaternary tabularshaped foreland basin as an example of the typical tabular-shaped foreland basin, we used a one-dimensional analytical model to simulate the subsidence and uplift in tabular-shaped foreland basin due to the erosional unloading in Longmenshan on an elastic plate. The results indicate that the migration rate of the orogenic wedge in the early wedge-shaped foreland basin history （c. Late Triassic） was rapid （maximum of 15 mm/a） compared to the slow migration rate （maximum of 6. 7 mm/a） during the later stage of the wedge-shaped foreland basin evolution （c. Late Jurassic and Late Cretaceous-Eogene）. We infer that the episodic thrusting activity of Longmenshan was driven by the accretion of the Cimmerian continents and the India-Eurasia collision. The Late Triassic wedge-shaped tectonic sequence was a sedimentary response to the Qiangtang- Eurasian collision, the Late Jurassic wedge-shaped tectonic sequence was a sedimentary response to the Lhasa- Eurasian collision, and the Late Cretaceous-Eogene wedge-shaped tectonic sequence was a sedimentary response to the Kohistan-Eurasian collision and the India--Eurasian collision.