Episodic uplift of the Tianshan Mountains since the late Oligocene constrained by magnetostratigraphy of the Jingou River section, in the southern margin of the Junggar Basin, China

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Thick Cenozoic sediments in the southern margin of the Junggar Basin accumulated in response to the uplift and deformation of the Tianshan Mountains. To better constrain this mountains' tectonic history, we report a detailed magnetostratigraphic study of the Jingou River section, located in the northern piedmont of the Tianshan Mts. A total of 1133 samples were used to establish the magnetostratigraphy of a more than 3000 m thick section from the exposed base of the Anjihaihe Formation to the middle of the Xiyu Formation. Two vertebrate fossil sites and a good correlation with the CK95 geomagnetic polarity timescale suggest that the section was deposited from ∼30.5 to ∼4.6 Ma and the age of the top of the Xiyu formation is ∼2.6 Ma based on an extrapolation of the sedimentation rates. Our results show that significant increases in sedimentation rates as well as notable changes in depositional environments occurred at ∼26–22.5 Ma, ∼13–11 Ma and ∼7 Ma, which represent the initial uplift of the Tianshan Mts. and two subsequent rapid uplift events. In addition, changes in sedimentation rates imply that the uplift of the Tianshan Mts. was episodic, and relatively short, with intervals of intensified deformation with long periods of tectonic weakness or quiescence.