NEOGENE MAGNETOSTRATIGRAPHY IN THE GUIDE-GONGHE BASIN (NE TIBET) AND THE TECTONIC UPLIFT OF THE TIBETAN PLATEAU

Most of the evidence for the uplift of the Tibetan Plateau comes from its southern and central parts. While the northern rim has been less studied, it may greatly contribute to the understanding of the mechanism and timing of the uplift, as it is the farthest deformed area from the Himalayas. Recent studies on Qaidam and the northeastern Plateau suggest that the uplift can largely be explained by Cenozoic thrusting and folding linked to the movement of the Altyn Tagh and Kunlun faults. In this scenario, the NW-SE trending mountain ranges have been explained as large-scale ramp anticlines, whereas the intermontane depressions are piggy-back basins caught between such rising ranges. Thus, diffuse shortening might be a better way to explain the thickening of the continental interior. This view contrasts with other models that look at the Tibetan Plateau as a piston that has risen as a single unit. The aggregate of basins around the northeastern rim of the Plateau can be envisioned as an amalgam of foreland depressions closely related to the propagation of bonding faults rather than a continuum. Data indicate two major pulses in the evolution of the Tibet: Eolian sequences reveal that Tibet's height was sufficient to induce aridification at around 7-8 Ma. In addition, different lines of evidence, including thick conglomerates, river terraces, and paleosol complexes, indicate an important uplift pulse of Tibet around 3-4 Ma. The terrigenous formations in the Guide-Gonghe Basin show several phases of deformation, and include thick deposits of conglomerates that are thought to be the result from an increase in sediment flux caused by tectonic uplift and erosion of source areas. Dating of these gravel deposits with magnetostratigraphic and faunal studies permits the precise timing of the tectonic uplift to be established. Our preliminary results indicate that the onset of the conglomerates as well as the low-angle thrusting occur in the Pliocene, an observation which is in agreement with those of the Linxia Basin, just east of the Plateau.