Mesozoic-cenozoic Uplift And Slip History Along The Middle Segment Of The Altyn Tagh Fault System

The left-lateral Altyn Tagh fault system is one of the largest strike-slip faults on the Earth.It constitutes the northern boundary of the Tibetan Plateau,and is crucial to understanding the northward growth and underlying mechanism of the Tibetan Plateau.Based on detailed field sedimentological observations and apatite fission track analysis,this paper systematically studies the Mesozoic-Cenozoic sedimentary sequence and bedrock exhumation history on the southern side of the middle segment of the Altyn Tagh fault system,and reconstructs the Meso-Cenozoic strike-slip history of this fault system.The conclusions are summarized as follows.(1)The Meso-Cenozoic strata could be divided into three sets of structural sequences that are separated by three unconformities.The first set is the Lower Cretaceous Quanyagou Formation,which is separated from the underlying Jurassic lacustrine sandstone and mudstone by a disconformity.It consists mainly of fluvial to alluvial sandstone and conglomerate,with a coarsening-upward pattern.The second set is the Eocene Xiaganchaigou Formation to the Early Miocene Xiayoushashan Formation,which is separated from the underlying pre-Cenozoic by a disconformity or small-angle unconformity.From bottom to up,the lithology of this sequence gradually changes from lacustrine sandstone and mudstone,to conglomerate and sandstone deposited in braided rivers and alluvial fans,also representing a coarsening-upward pattern.This pattern was accelerated in the upper part of the Xiayoushashan Formation.The third set is the Middle Miocene to Pleistocene Upper Youshashan Formation and Shizigou Formation,which are separated from the underlying Lower Youshashan Formation by a local angular unconformity extending parallel to the Altyn Tagh fault.The lithology of this sequence is relatively composed of fine-grained sandstone and mudstone,with conglomerate locally occurring at the bottom of each stratum,representing multiple secondary sedimentary cycles from coarse-to fine-grained sediments.(2)Apatite fission track dating of bedrock samples on the southern side of the middle section of the Altyn fault system,together with related thermal modeling,revealed three rapid exhumation events.The first phase occurred in the Cretaceous roughly between 120-70 Ma,and is recorded by all samples.The second phase initiated in the Eocene and continued to the Early to Middle Miocene,and is revealed by some samples with good modelling results.The third phase occurred in the Early Middle Miocene,roughly starting at 20-15 Ma,and has continued to the present.These three rapid exhumation events coincide well with the above three sets of structural sequences.(3)Based on a systematical review of the depositional-tectonic events related to the activity of the Altyn Tagh fault system,we regard that our data herein support the Middle Miocene tectonic reorganization model of the Altyn Tagh fault system proposed by the predecessors.On this basis,we reconstructed the Meso-Cenozoic slip history of the Altyn Tagh fault system.The results show that the strike-slip rate of the middle segment of the Altyn fault system was about 2.2±0.9 mm/a in the Cretaceous,and3.1±2.9 mm/a from the Eocene to the Middle Miocene,but increased to 10±2mm/a since the Middle Miocene.We speculated that only the last stage of strike-slip movement mainly occurred on the currently active Altyn Tagh fault,but the first two likely occurred on the North Altyn fault.Our findings reflect the protracted and multistage complicated strike-slip history of the Altyn Tagh fault system.