Cenozoic stratigraphic chronology and sedimentary-tectonic evolution of the Qaidam Basin

Abstract

<p indent="0mm">The Qaidam Basin hosts thick Cenozoic strata with an uninterrupted depositional time frame from early Cenozoic to the Quaternary, where remarkable number of mammalian fossils were discovered. These properties of the Cenozoic strata provide a unique chance to establish the basin-wide chronological framework by using palaeomagnetistratigraphy and biostratigraphy. Armed with the chronological framework, the sedimentary evolution and structural deformation of the Qaidam Basin and its adjacent region can be reconstructed to reveal the deformational processes and dynamic mechanism of the Tibetan Plateau expansion during the Cenozoic. According to the previous studies, the Cenozoic strata preserved in the Qaidam Basin, have been subdivided into seven primary progressively younger stratigraphic units: The Lulehe, Xia Ganchaigou, Shang Ganchaigou, Xia Youshashan, Shang Youshashan, Shizigou, and Qigequan formations. Despite the long study history of the Qaidam Basin, they are still controversial questions of basin-wide Cenozoic stratigraphic unit correlations and the time of formation of the Qaidam Basin. On the basis of the field observations, this paper summarizes relatively long magnetostratigraphic sections (thickness ≥ <sc>4500 m),</sc> such as the Huatugou, Lulehe, Honggou, Dahonggou, and Huaitoutala magnetostratigraphies in the Qaidam Basin from the west to east. The magnetostratigraphies/refined magnetostratigraphies dated the Lulehe Fm. to span from 31 to 23.7 Ma, Xia Ganchaigou Fm. from 23.7 to 17 Ma, Shang Ganchaigou Fm. from ca. 17 to ca. 12 Ma, Xia Youshashan Fm. from ca. 12 to ca. 10 Ma, Shang Youshashan Fm. from ca. 10 to ca. 7 Ma, and Shizigou Fm. is younger than ca. 7 Ma. Our interpretations suggest that the westernmost portion of the Qaidam Basin is probably formed in the Eocene, and the middle part of the basin began to accumulate sediments since the Oligocene (ca. 31–26 Ma). In the eastern part of the Qaidam Basin, Cenozoic subsidence was likely to initiate during the early to middle Miocene (ca. 16 Ma). Although onset of the Qaidam Basin expanded between Eocene and early Miocene from the west to east, significant portion of the basin seems to subsided after the Oligocene. This phenomenon indicates that the most profound episode of the tectonic deformation around the Qaidam Basin has occurred since the Oligocene. In the Qaidam Basin, the sediment accumulation rates over the basin had increased significantly during the middle Miocene (ca. 15 Ma). We rule out climate changes to drive the rapid middle-Miocene erosion and accumulation rates because of stepwise aridification of the northeastern Tibetan Plateau during the Neogene time interval. Thus, the increased accumulation rates imply direct relation to upward growth of the mountain ranges around the Qaidam Basin. There is abundant evidence demonstrates occurrences of the Oligocene (ca. 30 Ma) and middle-Miocene (10–15 Ma) stages of crustal shortening in many localities in and around the Tibetan Plateau, suggesting that both stages the deformation in ca. 30 and 10–15 Ma are synchronous and large-scale tectonic activation events. The regional synchronicity of plateau-wide deformational phases indicates significant changes in the dynamics of the plateau growths during the Oligocene and middle-Miocene. We suggest that ca. 30 and 10–15 Ma upward and outward growths of the Tibetan Plateau are likely to be driven by removal of mantel lithosphere beneath different portions of the Tibetan Plateau. Following the continuous convergence between the Indian and Eurasian continents, the lithosphere of the Tibetan Plateau was thickened. Removal of thickened, dense lithosphere could cause the uplift of the Tibetan Plateau, and then applied exert compressive stresses to the relatively lower relief region to trigger pulsed outward growth of the plateau to its recent margins.