Late Cretaceous to Late Eocene Exhumation in the Nima Area, Central Tibet: Implications for Development of Low Relief Topography of the Tibetan Plateau

Abstract

Much of the interior of the Tibetan Plateau is characterized by internal drainage, low relief topography, and high altitude. How and when this landscape formed is controversial. In this study, we use new zircon U‐Pb data and low‐temperature thermochronological data (apatite and zircon [U‐Th/He], apatite fission track [AFT]) from the Late Cretaceous to Cenozoic Nima Basin sedimentary rocks and Xiabie granite in the adjacent Muggar Thrust hanging wall (part of the regional Shiquanhe‐Gaize‐Amdo thrust system), to determine the paleodrainage and timing of exhumation in the region. Individual AHe and ZHe cooling ages range from 9 to 60 Ma and 58 to 118 Ma, and the AFT ages range from 30 to 90 Ma. The thermal history derived from the Northern Nima Basin sediments and Xiabie granite require a period of exhumation between 70 and 40 Ma in the thrust fault hanging wall, and 40 and 30 Ma in the Nima Basin. Across the region, this event was followed by low rates of exhumation and the deposition of locally sourced sediment, lacustrine, and evaporitic deposits that are indicative of an internal drainage system. We suggest that the exhumation event is associated with development of thrust‐elevated relief that may have disrupted the drainage network favoring the development of an endorheic system. This system, sediment accumulation, and/or post‐30 Ma tectonic quiescence led to the generation of low relief topography.

Plain Language Summary

The Tibetan Plateau is the highest altitude low topographic relief region on Earth. There is no consensus on how the flat topography formed. Low‐temperature thermochronology is widely used to establish the time of exhumation of rocks to the Earth's surface. In this study, we found that the rocks in the Nima area of the central Tibetan Plateau experienced moderate exhumation from Late Cretaceous to early Oligocene (70‐30 Ma), after which time the exhumation rate slowed. Such low rates imply slowing of local tectonic activity. The sedimentology and stratigraphy of the basin indicate that an internal drainage system developed in the Nima area since 30 Ma. Given the arid climate conditions, we therefore propose that the fault activity (70‐30 Ma) represented by rapid exhumation resulted in a change to topography that triggered the change of water systems. The low relief topography subsequently developed in the arid and tectonically quiescent environments in this area.

Key Points

• Low‐temperature thermochronology records moderate exhumation of the Nima area from 70 to 30 Ma, after which time exhumation slowed

• Generation of local thrust‐related relief over the period of 70‐30 Ma may have played a role in the development of internal drainage by 30 Ma

• The transition to tectonic quiescence and/or the establishment of internal drainage may have led to the development of low relief topography