Neogene Magnetostratigraphy of the Guide Basin and its Implications for Pliocene Evolution of the NE Tibetan Plateau

In order to provide age constraints to the Cenozoic clastic sedimentary rocks in the intramontane Gonghe and Guide Basins, we carried out a lithostratigraphic and magnetostratigraphic investigation of two sections north of the town of Guide. The resulting correlation of the magnetic stratigraphy with the geomagnetic polarity time scale and the lithological changes along the sections, allow us to propose new unit divisions of the sedimentary infilling of the Guide Basin. The strata are divided into two groups: the Oligocene Xining Group and the Miocene to Pliocene Guide Group. The Guide Group can now be further divided into five informal formations (with ages shown in parentheses): the Amigang (1.8 to 2.6 Ma), Ganjia (2.6 to 3.6 Ma), Herjia (3.6 to 7.8 Ma), Ashigong (7.8 to >12 Ma), Garang (<16 to 18.4 Ma) and the Guidemen Formation (18.4 to 20.5 Ma). Three episodes of rapid (>20 cm/ka) sediment accumulation are observed in our section during the late Miocene to latest Pliocene, with ages of about 6 Ma, 3 Ma and 1.9 Ma. Massive conglomerates of the Ganjia Formation characterize the 3.6 to 2.6 Ma interval. Massive gravels of similar ages appear to be widespread in the nearby basins of Linxia, Jiuquan (northern Qilian Shan), Hexi and Qaidam, documenting enhanced Pliocene tectonic activity and erosion in the NE Tibetan Plateau. Five S-vergent thrust faults, affecting the Miocene and Pliocene molasse sediments, caused shortening of the basin section estimated at about 25 km since 8 Ma.