Magnetostratigraphy of Paleogene sediments from northern Qaidam Basin, China: Implications for tectonic uplift and block rotation in northern Tibetan plateau

We have carried out a detailed magnetostratigraphic study of Tertiary sediments in the northern margin of the Qaidam Basin, northern Tibetan plateau. A total of 400 paleomagnetic samples (with an interval of about 2 m) were collected from the Shanggancaigou and Xiagancaigou Formations in the Hongsanhan section. Stepwise thermal demagnetization was used to isolate the magnetic components in these samples. A higher temperature component (HTC) was separated between 300 and 680 °C from most of samples. Positive fold and reversal tests suggest a primary origin for the HTC. A magnetic polarity sequence is established and then compared with the geomagnetic polarity time scale. Ostracoda assemblages and also a fission track age from an adjacent section suggest a Tertiary age of the Shanggancaigou and Xiagancaigou Formations in the Hongsanhan area. The magnetozones in the Xiagancaigou Formation may correspond to subchrons C16n-C18 ranging from > 40 to 35.5 Ma (Mid-Eocene), and the Shanggancaikou Formation to subchrons C8.2n-C15 (age from 35.5 to 26.5 Ma Late Eocene to Late-Mid Oligocene). The accumulation of sandy conglomerate and conglomerate layers on top of the Shanggancaikou Formation, and an abrupt increase in sedimentation rates are probably resulted from the main uplift of the north Qaidam at about 30 Ma. When compared with coeval directions expected from Eurasia, the mean declinations indicate a slight vertical-axis rotation of the northern Qaidam Basin since the Mid-Eocene. Sinistral shear strain may be concentrated on the Altyn Tagh Fault (ATF).