Shuang Dai

Tibetan plateau aridification linked to global cooling at the Eocene-Oligocene transition

Continental aridification and the intensification of the monsoons in Asia are generally attributed to uplift of the Tibetan plateau and to the land–sea redistributions associated with the continental collision of India and Asia, whereas some studies suggest that past changes in Asian environments are mainly governed by global climate. The most dramatic climate event since the onset of the collision of India and Asia is the Eocene–Oligocene transition, an abrupt cooling step associated with the onset of glaciation in Antarctica 34 million years ago. However, the influence of this global event on Asian environments is poorly understood. Here we use magnetostratigraphy and cyclostratigraphy to show that aridification, which is indicated by the disappearance of playa lake deposits in the northeastern Tibetan plateau, occurred precisely at the time of the Eocene–Oligocene transition. Our findings suggest that this global transition is linked to significant aridification and cooling in continental Asia recorded by palaeontological and palaeoenvironmental changes, and thus support the idea that global cooling is associated with the Eocene–Oligocene transition. We show that, with sufficient age control on the sedimentary records, global climate can be distinguished from tectonism and recognized as a major contributor to continental Asian environments.

Late Cenozoic deformation and uplift of the NE Tibetan Plateau: Evidence from high-resolution magnetostratigraphy of the Guide Basin, Qinghai Province, China

The Cenozoic intramontane Gonghe–Guide Basin in Qinghai Province, China, is tectonically controlled by the sinistral strike-slip framework of the Kunlun and Altyn Tagh–South Qilian faults in the northeastern Tibetan Plateau. The basin is filled with thick Cenozoic clastic sedimentary formations, which provide important evidence of the deformation of this part of the plateau, although they have long lacked good age constraints. Detailed magnetostratigraphic and paleontologic investigations of five sections in the Guide Basin and their lithologic and sedimentary characteristics allow us to divide a formerly undifferentiated unit (the Guide Group) into six formations (where ages are now magnetostratigraphically well established, they are given in parentheses): the Amigang (1.8–2.6 Ma), Ganjia (2.6–3.6 Ma), and Herjia formations (3.6 to ca. 7.0–7.8 Ma), and the older Miocene Ashigong, Garang, and Guidemen formations. These rocks document a generally upward coarsening sequence, characterized by increasing accumulation rates. Increasing gravel content and sizes of its components, changes of bedding dips and source rock types, and marginal growth faults collectively reflect accelerated deformation and uplift of the NE Tibetan Plateau after 8 Ma, punctuated by a sharp increase in sedimentation rate at ca. 3.2 Ma that reflects the boulder conglomerates of the Ganjia formation. Interestingly, much of the vergence of the compressional deformation in the basin is to the south, accommodated by a sequence of six thrusts (F1–F6), which become active one by one progressively later toward the south, undoubtedly contributing to the uplift of this part of the plateau. F1 likely initiated the Guide Basin due to crustal flexure in the Oligocene, F2 was active in the early Miocene, F4 and F5 at ca. 3.6 Ma, and F6 was active in the early Pleistocene. The detailed late Miocene and younger magnetostratigraphy allows us to place much improved time constraints on the deformation and, hence, uplift of northeastern Tibet, which, when compared with ages for events on other parts of the plateau, provides important boundary conditions for the geodynamical evolution of Tibet.

Oligocene slow and Miocene–Quaternary rapid deformation and uplift of the Yumu Shan and North Qilian Shan: evidence from high-resolution magnetostratigraphy and tectonosedimentology

Abstract Most existing tectonic models suggest Pliocene–Quaternary deformation and uplift of the NE Tibetan Plateau in response to the collision of India with Asia. Within the NE Tibetan Plateau, growth of the terranes was suggested to progress northeastward with the Yumu Shan (mountain) at the northeasternmost corner of the Qilian Shan (mountains) being uplifted only since about 1 Ma ago. Here we present a detailed palaeomagnetic dating and tectonosedimentological measurement of Cenozoic sediments in the eastern Jiuquan Basin related to the deformation and uplift of the North Qilian Shan and Yumu Shan. The results show that the eastern Jiuquan Basin is a Cenozoic foreland basin and received sediments at about 27.8 Ma at the latest. Eight subsequent tectonic events at about 27.8, 24.6, 13.7–13, 9.8–9.6, 5.1–3.6, 2.8–2.6, 0.8 and 0.1 Ma demonstrate the development of the foreland basin in response to Oligocene–Quaternary uplift of the North Qilian Shan and subsequent propagation of thrust–fold system owing to collision of India with Asia. The Yumu Shan is the late phase of deformation front in the thrust–fold system and commenced rapid uplift at about 9.8–9.6 Ma at the latest. A rigid block-floating model is proposed to interpret the mechanism of this deformation and uplift history.