NEOGENE MAGNETOSTRATIGRAPHY OF THE GUIDE BASIN: MID-MIOCENE CLOCKWISE ROTATION AND IMPLICATIONS FOR UPLIFT OF THE NE TIBETAN PLATEAU

The mechanisms of growth of the Tibetan Plateau are still poorly understood because of a lack of accurate constraints on the timing of deformation in the Plateau, especially in its northeastern part. Here, oblique-slip deformation dominates, and a fold-thrust belt is even now undergoing shortening along its edge. Associated with the deformation, large sedimentary basins have developed. We report here a magnetostratigraphic study of the Guide basin that has allowed us to determine the ages of sedimentary sections in the middle of the NE Plateau. The Guide Basin is a typical intramontane molasse basin in NE Tibetan Plateau. It is controlled by two large fault systems, the sinistral Qilian and Kunlun Faults to the north and south, accompanied by two dextral NW-SE trending faults, the Wenquan Fault to the west and the Haiyan Fault to the east. Incision by the Huang He (Yellow River) and its tributaries has exposed a thick sedimentary sequence with numerous index fossils. With bio- and magneto-stratigraphic age constraints, we find that sedimentary infilling of the basin occurred during ca. 29 Ma to 1.8 Ma. At least three episodes of high accumulation rate have been observed at 6.3 – 5.9 Ma, 3.2 – 2.9 Ma and 1.95 –1.8 Ma. Two periods are particularly noteworthy for their massive conglomerates at ca. 20.9 - 17.3 Ma and 3.6 - 2.6 Ma; the sediments of the latter episode are of similar age as conglomerates in many other NE Tibetan basins. These massive conglomeratic influxes and the higher than usual sedimentation rates are likely reflecting episodes of increased tectonic activity and uplift in NE Tibet. Our study reveals a clockwise rotation of ca. 30 degrees (+/- 7 degrees) of the basin during the Middle Miocene, but shows no significant rotation afterwards, implying that the strike-slip faults were particularly active during 17.3-11.5 Ma. Average inclinations in sediments older than 17 Ma are shallower (by more than 15 degrees) than those of younger strata, but we do not believe that this indicates large-scale northward transport. Rather, we interpret this as having been caused by greater post-depositional compaction of the older strata.