Cenozoic Tilting History Of The Altvn Slope:Implications For The Kinematics Of The Altyn Tagh Fault

The Altyn Tagh Fault(ATF)plays a significant role in the northward growth of the Tibetan Plateau,but its inception and kinematics in the Cenozoic remain debated.In this paper,we identified a transition zone between the ATF and the Qaidam Basin interior,based on a dense network of 2D/3D seismic reflection profiles and isopach maps.Termed as the Altyn Slope,its formation and evolution is intimately related to the Cenozoic strike-slip motion of the ATF.Thus we depicted its structural geometry and studied its Cenozoic evolution,giving us implications of the Cenozoic kinematics of the ATF.Below are several new insights:1)Located at the northwestern margin of the Qaidam Basin,the Altyn Slope distributes along the ATF with the width of 30-80 km.The slope increases the erosion and the uplift amplitude of its southeastward tilted pre-Cenozoic basement toward the ATF.Tilted by a series of EW-trending transpressional faults that constitute the positive flower structure of the ATF,the current geometry of the Altyn Slope is characterized by a southeast-dipping slope with its southeastern boundary exhibiting as a wavy line with peaks coincidentally located at the major anticlinal belts in the basin.2)We proposed a quantitative method to restore the Cenozoic tilting history of the Altyn Slope during different time periods in the Cenozoic by identifying growth-strata geometry from latest isopach maps.The results show that the Altyn Slope was initiated in late Eocene(ca.40 Ma)and continued to expand until mid-Miocene(ca.15 Ma)with a different shape from that of the current one.The Altyn Slope suffered significant NE-SW-directed folding since the mid-Miocene(ca.15 Ma),resulting in its southeastern boundary becoming wavier and shrinking towards the ATF.3)Two prominent fault systems,i.e.the EW-striking and NW fault systems,developed in the Altyn Slope with clear otherness and intimate relationship.The EW-trending faults locate to the western rim of Qaidam Basin,oblique to the strike of ATF with small angle;while the NW fault system causes the formation of folding in the basin and is composed of two groups of faults that develop in the deep and shallow layers with right-lateral transpressional attributes,respectively.The differences of geometric distribution,initiation age and nature between these two fault systems are transparent,but the associated relationship sterming from the slip motion of the ATF is also striking.A mid-Miocene structural transformation took place in the Qaidam Basin can be inferred from the change of the dominant role of these two fault systems,which reflects that they developed in different regional stress field or a conversion of the stress state occurred at ca.15 Ma.4)Correspondingly,the left-slip motion on the ATF is inferred into two distinct stages.During the first stage of ca.40-15 Ma,the ATF was activated with slow slip rate,and most of the transpressional stress was converted to vertical strain raising the Altyn Slope instead of strike-slip motion.During the second stage since ca.15 Ma,faster sinistral strike-slip motion on the ATF took place,releasing the stress beneath the Altyn Slope and inducing intense NE-SW-directed shortening within the Northern Tibetan Plateau.