The Study Revelation Of The Altyn Tagh Fault Basing On The Discovery Of The Unconformity In The Qaidam Basin

The Altyn Tagh fault constrains the north margins of the Tibetan Plateau, and also delimits the northwest of the Qaidam basin. The Cenozoic activity of the Altyn Tagh falut controlled the deposition and structures of the west Qaidam basin. This study, which focuses on the sedimentary sequences and structural characteriscs, will provide new constraints for the evolution of the Alytn Tagh fault.The idea of coupling between orogen and basin has been applied to this study. The Cenozoic structural deformation and deposition in the Qaidam basin have been used to reveal the evolution of the Altyn Tagh orogen.Based on the geological survey, synthesis of geophysical and drilling data, two unconformities, between upper and lower Ganchaigou Formation and between upper and lower Youshashan Formation, respectively, have been discovered in the west Qaidam basin, which have been considered to relate to the activity of the Altyn Tagh fault. The analysis of regional cross sections, sedimentary facies and paleo-structures in the south Altyn Tagh fault reveal the distribution of those two unconformities, which provide constraints for the spatial impact and active timing of the Altyn Tagh fault.The formation of large-scaled uplift marked the early Cenozoic activity of the Alyn Tagh fault. The erosion of the uplift area yielded the unconformity between upper and lower Ganchaigou Formation. The present Altyn Tagh fault was formed by later stage of uplift. During this stage, the activity of faults released the compressional stress, which slowed down the uplift rate to even cease uplifting. Sequentially, new deposit overlapped the older, and the unconformity between upper and lower Youshashan Formation formed as well during this stage.The discoveries above provide implications for the Cenozoic evolution model of the Altyn Tagh fault as followed. Since late Oligocene, large-scaled uplift occurred around the Altyn Tagh area, till mid-Miocene, the uplift rate slowed down, and the activity of the Altyn Tagh fault displayed as strike-slipping. Lately, stages of strike-slipping prevailed this area.These results provide new constraints for the active timing and characteriscs of the Altyn Tagh fault and Cenozoic evolution of the Qaidam basin. The research method, using the basin record to reveal the orogen evolution, provides new example for basin-orogen coupling between the Altyn Tagh fault and the Qaidam basin.