ABSTRACT

The growth mechanisms of the southeastern Tibetan Plateau are debated between models of Oligocene shortening and lateral extrusion of coherent crustal blocks versus Late Miocene flow of low-viscosity lower crust. To test these models, we examine brittle deformation of the north-striking Anninghe fault, which has been acting as a left-lateral strike-slip fault since Late Miocene time, using structural and bedrock low-temperature thermochronologic methods. Different from previous interpretations of Late Miocene strike-slip motion, our structural observations indicate westward contractional dip-slip along the southern segment of the fault. New zircon and apatite (U-Th)/He thermochronologic data along a vertical profile of Precambrian volcanoclastic cover both sides of the Anninghe fault. The age-elevation relationships and inverse thermal modelling suggest a phase of Oligocene-Early Miocene (~24-18 Ma) differential rock exhumation and cooling across the fault, with a mean erosion rate of ~0.30 ± 0.12 km/m.y. and ~0.007 ± 0.007 km/m.y. in the hanging wall and footwall, respectively. Consistent with the structural observations, the higher rate of rock exhumation in the hanging wall indicates an Oligocene-Early Miocene phase of west-verging reverse faulting. Combined with previously published data from adjacent sites, we suggest that the Anninghe fault has experienced a tectonic transition from Oligocene-Early Miocene east-west contraction to Late Miocene strike-slip motion. The early to Mid-Cenozoic crustal shortening contributed to the uplift of the southeastern Tibetan Plateau, supporting the crustal shortening and extrusion model.

摘要

青藏高原东南部的生长机制在渐新世缩短和连贯地壳侧向挤压模型与晚中新世低粘度下地壳流动模型之间存在争议。为了检验这些模型，我们使用构造和基岩低温热年代学方法研究了北向安宁河断裂的脆性变形，该断裂自晚中新世以来一直是左侧走滑断裂。与之前对晚中新世走滑运动的解释不同，我们的构造观测表明沿断层南段向西收缩倾滑。沿前寒武纪火山碎屑垂直剖面的新锆石和磷灰石 (U-Th)/He 热年代学数据覆盖了安宁河断层两侧。年龄-高程关系和逆热模型表明，断层存在渐新世-早中新世（~24-18 Ma）差异岩石折返和冷却阶段，平均侵蚀速率为~0.30 ± 0.12 km/my 和~0.007 ±上盘和下盘分别为 0.007 km/my。与构造观测结果一致，上盘岩石折返率较高表明为渐新世-早中新世西向逆断层相。结合之前公布的相邻站点数据，我们认为安宁河断裂经历了从渐新世-早中新世东西向收缩到晚中新世走滑运动的构造转变。早至中新生代地壳缩短促进了青藏高原东南部的隆升，