Sedimentary Formation and Tectonic Evolution on Both Sides of the Altun Fault in the Early Paleozoic Era

Abstract

Correlation of the geological characteristics on both sides of the Altun Fault is an important geoscientific question in the geology of western China. Based on sedimentary formation and geological structures combined with paleontological and paleomagnetic data， Early Paleozoic geological characteristics were studied in the Altun area. It was concluded that there are clear differences in the lithofacies and geological structures on either side of the Altun Fault， mainly manifested by the following aspects.（1） The Tarim area was subject to a background of tectonic spreading and lacked volcanic activity， whereas the Qilian area had mainly a contractive background with much volcanic activity during the Early Cambrian. Sediments bearing phosphorus， vanadium and uranium were deposited in the deep water basin located at the northern margin of the Tarim block， in comparison to the mainly volcanic rocks and ophiolites in the Qilian area during the Middle Cambrian. Reddish mudstone appears at the northern margin of the Qaidam block， but is not found in middle and late Cambrian rocks in the Tarim block. In the Qilian area the Cambrian tectonic characteristics were inherited in the Ordovician， but tectonic stress in the Tarim Basin changed from extension in the Cambrian to contraction in the Middle-to-Upper Ordovician. Back-arc spreading basins occurred in the north and south of the Tarim block and oceanic subduction seems to have faded in the Qilian area； back-arc marine foreland basins were formed in the Silurian， but only in the southern Qilian area.（2） Asian-Australian （southern type） biota appear in the Tarim block， but mixed northern China biota and South China biota are evident in the Qilian area during the Cambrian and Ordovician periods. The biota were typical of a temperate climate zone in the Tarim block， but typical of a subtropical⁃temperate climate zone in the Qilian area during the middle and late Silurian.（3） The Tarim， Qaidam⁃middle Qilian and Alashan blocks were all located in the low latitudes of the southern hemisphere in the Cambrian period. The Tarim block clearly moved southward in the Ordovician， while the Alashan and Qaidam blocks remained near the equator. From the Silurian to the Devonian， the Tarim block moved rapidly northwards by as much as 33 degrees of latitude， and the Qaidam and Alashan blocks moved northwards by about 5 degrees and 10 degrees， respectively. These blocks were all independent and moved along completely different tracks.（4） In the Cambrian， the Altun block was surrounded by the Altun Ocean， and subduction began to take place on both sides of the block in the later Cambrian and Early Ordovician periods.In summary， throughout the Early Paleozoic era the Tarim and Qaidamu⁃middle Qilian blocks followed completely different tectonic activity paths， and they possess different sedimentological and structural characteristics. Therefore， different exploration strategies are required for oil-gas and/or non-oil-gas exploration on either side of the Altun Fault.