Late Neogene magnetostratigraphy in the western Qaidam Basin (NE Tibetan Plateau) and its constraints on active tectonic uplift and progressive evolution of growth strata

摘要

The Qaidam Basin as the largest intermontane basin of the NE Tibetan Plateau is the ideal place to provide constraints on depositional and tectonic patters. To determine its tectonic deformation history and progressive evolution of growth strata we conducted paleomagnetic study on the late-Neogene stratigraphic section in the western Qaidam Basin. A magnetostratigraphic study of the well exposed 805. m Qigequan section at the Qigequan anticline in the western Qaidam Basin reveals twelve pairs of normal and reversed polarity zones which can be readily correlated with chrons C1n-3Ar of the Geomagnetic Polarity Time Scale (GPTS). From this correlation we can conclude that the Shizigou and the Qigequan formations were formed at > 6.9 Ma-2.5. Ma and 2.5-~ 0.4. Ma, respectively. Accumulation rates determined from our chronology, together with the occurrence of unconformities suggest four phases of tectonic uplift which began at about 3.6 Ma, 2.5 Ma, 1.1 Ma and 0.8 Ma. The results also suggest that offlap growth strata according to the limb rotation model on the anticlinorium started to occur at ~ 8.2. Ma. They progressively become younger from the frontal region of the Altyn Tagh Mts. (~ 8.2. Ma) to the southwestern basin (~ 2.5 Ma) and to further east of the Qaidam Basin (< 2.5 Ma), caused by fault-propagation-folding in the Qaidam Basin, rapid uplift and fast exhumation of the NE Tibetan Plateau at those times.