青藏高原东缘龙日坝断裂带的规模与初始活动时限

The Ms>7.0 earthquakes, occurred during last two decades in the Tibetan Plateau, has mainly focused on the boundary faults of the Bayankara Terrane, which is considered to be the response of the great stress formed by the Indian-Asian collision in these faults. However, how to adjust the enormous stress resulted by the Indian-Asian collision within the Bayankara Terrane is still unsolved. The NE striking Longriba fault zone, located ~200 km west of the Longmenshan fault zone in the eastern Tibetan Plateau, is 1) an important geomorphic boundary, 2) material boundary and 3) plate motion rate changing boundary in the eastern Tibetan Plateau. It controls the strain characteristics and stress conduction on the eastern Tibetan Plateau. And it is closely related to Ms>8.0 earthquake in the NE striking of the northeast Tibetan Plateau. Therefore, studying the formation and evolution of the Longriba fault zone is a breakthrough in studying the dynamics of the uplift and expansion of the eastern margin of the Tibetan Plateau. Moreover, this is also a key point for understanding the stress transmission and strain distribution in the eastern Tibetan Plateau. This application takes the Longriba fault zone as research object. We plan to do geological mapping works which aim to decipher geometry, kinematics and scale of the Longriba fault, in particular to explore how the fault extended into the southwest direction. The activity history and erosion process of the fault will be revealed by illite K-Ar dating and low temperature thermochronology. The tectonic significance of the Longriaba fault zone for the growth and rise of the eastern Tibetan Plateau will be discussed based on combined structural geology and geochronology results.

青藏高原近二十年来Ms>7.0的大地震主要集中在巴彦克拉地体的边缘断裂带，这被认为是印度-亚洲碰撞在这些断裂带的响应，然而巴彦克拉地体内部是如何调节印度-亚洲碰撞形成的巨大应力的依然是未解之谜。青藏高原东缘龙门山断裂带以西200km处NE走向的龙日坝断裂带是高原东缘重要的地貌边界、物质边界和速率变化边界，它控制着高原东缘的应变特征和应力传导，并且和高原东缘北东向Ms>8.0的大地震有着密切的联系。因此，研究龙日坝断裂带的形成演化是研究青藏高原东缘隆升扩展动力学机制的突破口，也是理解高原东缘应力传导和应变分配的关键点。本申请以龙日坝断裂带为研究对象。通过活动构造填图来刻画其几何学、运动学及规模，尤其详细探究其向西南的延伸及性质。利用原生伊利石K-Ar定年和低温热年代学方法揭示断层的活动历史和剥蚀过程。综合构造地质学和构造年代学的研究讨论龙日坝断裂带在青藏高原东缘生长崛起过程中的构造意义。

青藏高原东缘龙门山地区的特征地貌是何时如何形成的依然存在较大争议。越来越多的大地测量学和地貌学证据表明：青藏高原东缘以西约~150 公里处平行于龙门山冲断带的龙日坝断裂带在青藏高原东缘构造、地貌演化中发挥了重要作用。本项目通过沿龙日坝断裂带不同分支断裂的精细构造填图，发现龙日坝断裂带（北东段）北西分支龙日曲断裂倾向北西，主要为走滑断裂带；龙日坝断裂带南东分支毛儿盖断裂倾向南东，主要为逆冲断裂。龙日坝断裂带（西南段）两侧围岩的三组白云母/黑云母 40Ar/39Ar 年龄显示其经历了 156-136 Ma冷却事件，且由于断层两侧无年龄差异，证明断层在此时还未启动。龙日坝断裂带内新生伊利石 K-Ar 年龄 (~38.7-34.7 Ma) 反映了断层初始活动时的矿物生长，记录了断层的初始活动时限。来自龙日坝断裂带（北东段）北东分支的热年代学数据记录了 ~30-20 Ma 的快速冷却，冷却速率~0.6 mm/yr。龙日坝断裂带（北东段）南东分支上盘经历了自 ~15 Ma 以来的快速折返，折返率为 ~0.73 mm/yr。.综合龙日坝断裂带新获的年龄和区域年代学数据，总结得出青藏高原东缘经历了四期的构造演化。（1）青藏高原东缘主要由松潘-甘孜复理石组成，在240-180 Ma期间发生了地壳缩短增厚、花岗岩侵入和角闪岩相变质作用。（2）早白垩世冷却年龄反映了青藏高原东缘边界断层的再活动和地壳的再次增厚，这可能导致了青藏高原东缘早期高地形的形成。（3）在印度-亚洲初始碰撞阶段，青藏高原东缘断层在 ~55-40 Ma 期间从边界断裂带传播到高原内部。（4）自 ~25 Ma 以来，区域折返和冷却受控于在不同时期形成的断层的再活动。此外，基于断层时空关系，我们提出龙日坝断裂带和龙门山逆冲断层带组成逆冲-反冲系统，导致了晚新生代青藏高原东缘的整体抬升。板块会聚的远程应力控制青藏高原东缘的断层运动、断层再活动和断层扩展。因此，持续的断层活动可能是青藏高原东缘地壳增厚和隆升的主因。