Cenozoic tectonic evolution of the Elashan range and its surroundings, northern Tibetan Plateau as constrained by paleomagnetism and apatite fission track analyses

Abstract

Magnetostratigraphic studies of sediments in the western Gonghe and eastern Qaidam Basins, together with apatite fission track (AFT) analyses in the Elashan Range, suggest that the Elashan range and its surroundings experienced a two-stage evolution: a protracted uplift from the Cretaceous to the early Tertiary (which explains the absence of the Paleogene strata in the region) and an accelerated growth of the Elashan range since the latest Oligocene caused by extensively distributed thrust faults in the region. This later phase of uplift resulted in the subsequent thick sediments within the both easternmost Qaidam and Gonghe Basins (basal ages of 21–22 Ma in the Wulan section, of ~ 20 Ma in the southern Gonghe section, and of > l2 Ma in the Chaka section. The two phases of thrust motion and range growth mean that the Qaidam and Gonghe Basins were never united during Cenozoic times. The strike–slip is likely to have occurred at ~ l0 Ma, as shown by the paleomagnetic declination data, which show a rapid transition in rotation sense (from counterclockwise to clockwise) at that time. This time is also similar to the 9 Ma age of initiation of the Wenquan fault determined by backward calculation based on its late Quaternary slip rate. Eventually, the Qinghai Nan Shan thrust belt began to develop at ~ 6 Ma.

Introduction

The NW-trending Elashan range and its surroundings, as an integral part of the northern Tibetan Plateau, figure significantly in the debates on the history and mechanism of formation of the northern Tibetan Plateau during the Cenozoic India-Asia collision. Although the eastward shift of the Cenozoic deposition and depocenter along the central axis of the Qaidam basin has been recognized by numerous studies (Metivier et al., 1998, Wang et al., 2006, Yin et al., 2008, Zhu et al., 2006), it still remains poorly understood as to when the sediment deposition reached the easternmost part of the Qaidam basin and whether the Qaidam and adjacent Gonghe basins were once connected as a larger basin during the Cenozoic (Fig. 1). Metivier et al. (1998) once envisaged that a river system sourced from the Kunlun range flowed from the Qaidam basin, through the Gonghe basin, and finally to the Yellow River prior to the late Miocene. Later (Pliocene) slip on the Wenquan fault and the accompanying mountain building processes might create the drainage divide (the Elashan range) between the Qaidam and Gonghe basins and thus induced rapid infilling of the peripheral basins (Fig. 1). Wang and Burchfiel (2004) observed that redbed sediments of the Wulan section were distributed along the thrust fault associated with the Wenquan fault, and thus argued that those sediments were syntectonic. Indeed, the timing of surface uplift of the intervening Elashan range and of the initiation of sedimentation within the Gonghe basin and the eastern margin of the Qaidam basin are central to elucidating those issues. However, controversies remain with respect to the age of the initiation of basin sedimentation. For example, Zhang et al. (2012) suggested ~ 11 Ma age of inception of sediment accumulation in Gonghe basin, whereas Craddock et al. (2011) indicated ~ 20 Ma age for it. Based on the late Quaternary slip rate of the Wenquan fault, Yuan et al. (2011) concluded that its timing of initiation of strike–slip motion likely occurred at ~ 9 Ma. However, it remains to be determined whether the subsidences within the easternmost Qaidam and Gonghe basins are predominantly controlled by the strike–slip motion of the Wenquan fault or by the widespread thrust faults across the Elashan range.

To clarify those issues, we conducted detailed magnetostratigraphic studies on two sedimentary sequences within the easternmost Qaidam and westernmost Gonghe basins, and low-temperature thermochronology (Apatite Fission Track—AFT) on a steep topographic transect in the northernmost Elashan range. The results, combined with published data, suggest that the Elashan range underwent protracted uplift from the Cretaceous to the early Tertiary and subsequent accelerated growth since the late Oligocene. The uplift history of the Elashan range well explains the lack of Paleogene strata and presence of Neogene strata within adjacent basins. The basin–mountain coupling process suggests that the Qaidam and Gonghe basins never formed an integrated basin during either the Cretaceous or Cenozoic.