CENOZOIC TECTONIC EVOLUTION OF THE NORTHERN TIBETAN PLATEAU INFERRED FROM MAGNETOSTRATIGRAPHY AND MAGNETIC ANISOTROPY SUSCEPTIBILITY, DAHONGGOU, NE QAIDAM BASIN

As the largest active intermountain basin in the northern Tibetan Plateau, the Qaidam Basin was filled with over 10 km of fluvio-lacustrine sediments during the Cenozoic. It has great potential for deciphering the environmental, geomorphologic and tectonic evolution of the NE Tibetan Plateau. Although substantial research has been done in the Qaidam Basin and surrounding ranges, the absence of an age frame still hinders our understanding on these questions. Here we present a magnetostratigraphy and anisotropy of magnetic susceptibility (AMS) from the >5 km Dahonggou section in the NE Qaidam Basin. We match our magnetostratigraphy to the Ogg12 geomagnetic polarity time scale and establish a time scale of the studied section ranging from ~52 Ma to ~7 Ma. Sedimentation rates increased at 25–16 Ma, implying the faster erosion of the South Qilian Range at that time. After 16 Ma, sedimentation rates also increased at ~13 Ma and ~9.5 Ma. Rock magnetic analyses indicate that paramagnetic and antiferromagnetic minerals are the dominant contributor to AMS and thus can be used to inquire tectonic strain. Equal-area stereographic projections of AMS show that the maximum axes of susceptibility (K_max) are clustered and parallel to the bedding plane in an east-west direction, but perpendicular to the paleocurrent, while the minimum axes of susceptibility (K_min) are nearly perpendicular to the bedding plan. This indicates that the AMS of the studied section is a combination of sedimentary compaction and mild S-N tectonic strain. Potential changes in κ, P_j, and Toccurred at ~52–46 Ma and ~21–16 Ma, suggesting two marked increases in tectonic strain in the NE margin of the Qaidam Basin. Based on comprehensive analysis of changes in sedimentation rates and the AMS sequence, we conclude that the northern margin of the Qaidam basin has experienced the following stages: (1) initial and intense deformation at ~>52–46 Ma, synchronous with the start of India-Eurasia collision; (2) the second phases of deformation and erosion at ~25 Ma, synchronous with other tectonic studies from the N and NE Tibetan Plateau. Climate also began to show large changes in central and eastern Asia after that time; (3) tectonic strain increased at ~21 Ma; (4) increases in sedimentation rates at ~13 Ma and ~9.5 Ma may reflect minor erosion, possibly local events.