Mesozoic Basin prototypes of the Hongshan and Huobuxun sags in the eastern segment of the northern Qaidam Block

Abstract

The prototypes and evolutionary history of the Mesozoic basins in the eastern segment of the northern Qaidam Block are controversial, and the stress field is unclear with temporal and spatial variation. First, we divide the tectonic units systematically according to orogenesis, orogen stratigraphy, radioisotope geochemistry, petrology, and tectonics in the study area. The tectonic framework of “two belts and two blocks” formed in the study area before the Jurassic. The intersection of the NW‐trending and NE‐trending basement‐involved faults influences the formation of the Mesozoic prototype basins in the northern Qaidam Block. Second, we use a balanced cross‐section restoration method to retrieve the Mesozoic evolutionary history and determine the prototypes of the Hongshan and Huobuxun sags. The two sags were pulled apart by transtension during the Early–Middle Jurassic, initiating the Mesozoic evolutionary stages. The Hongshan and Huobuxun sags were under different stress states at different tectonic evolutionary stages during the Mesozoic. The Hongshan Sag was controlled by transtension with the NE‐directed extension ratio larger than the NW‐directed one during the Early–Middle Jurassic. The relatively weak extension led to an initial depression during the Early Jurassic. With an intense extension, the subsequent faulted Hongshan Sag deepened during the Middle Jurassic. The tectonic reversal initiated at the beginning of the Late Jurassic, resulting in the formation of the compressive Hongshan Sag, with the NE‐directed shortening ratio larger than the NW‐directed one during the Late Jurassic. The Hongshan Sag was in a relatively weak compressive stress state during the Cretaceous. The initial depression of the Huobuxun Sag, also under transtension, began to develop into an extensional faulted basin at the beginning of the Middle Jurassic, with the NE‐directed extension ratio much larger than the NW‐directed one. The tectonic reversal led to the formation of the compressive Huobuxun Sag during the Late Jurassic, with the NE‐directed shortening ratio much larger than the NW‐directed one. The Hongshan Sag was under the stronger extension during the Early–Middle Jurassic and the stronger compression during the Late Jurassic than the Huobuxun Sag.