LATE MIOCENE ONSET OF FAULT SLIP ALONG MARGINS OF THE GONGHE BASIN, NE TIBET: IMPLICATIONS FOR THE EVOLUTION OF TOPOGRAPHY IN NE TIBET

Characterizing the spatio-temporal pattern of deformation throughout the Indo-Asian collision can place constraints on the processes responsible for the development of high topography. In northeastern Tibet, the history of deformation appears to have been quite complex. Although a number of ranges around the northeastern margin of the plateau experienced a pulse of deformation in the Late Miocene (ca. 12-8 Ma), emerging evidence suggests that deformation may have begun as early as the Paleogene, in some cases coincident in time with the onset of the Indo-Asian collision. Here, we present new constraints on the timing and magnitude of deformation along the margins of the Gonghe basin complex. This basin occupies an interior portion of NE Tibet between the southern Qilian Shan and the central Tibetan Plateau, and is located along strike from the Qaidam basin and the West Qinling Shan, both regions of Paleogene deformation. The basin is overthrust by two regionally extensive fault systems, the Qinghai Nan Shan (QNS) fault system on the north side and the Gonghe Nan Shan (GNS) fault system on the south side. Deformation of Tertiary strata, sedimentary facies, and clast provenance are used to constrain the onset of growth of faults along the southern margin of the Gonghe basin. A combination of magnetostratigraphy, biostratigraphy and cosmogenic burial ages provides age control for the GNS fault system. A 250 m thick package of growth strata along the southern range front is between 3.4 – 0.5 Ma; preliminary results suggest that a ~500 m thick package of growth strata on the north side of the range is also Plio-Quaternary in age and that an underlying, ~1000 m thick package of pre-growth sediments is Neogene in age. Structural restorations of the western QNS fault system suggest that structural relief there is on the order of 1-3 km. (U-Th)/He low temperature thermochronology indicates that this structural relief developed during the Neogene, corroborating a companion study interpreting the onset of deformation along the QNS at ≥ 5-7 Ma by dating growth strata adjacent to the range. Our findings indicate that the structures surrounding the Gonghe basin complex evolved during the Neogene and suggest that the dominant episode of Himalayan orogeny-related topographic growth in interior NE Tibet occurred at this time.