Research on the Closure Polarity of the Paleo-Asian Ocean: Evidence From the Three-Dimensional Lithospheric Resistivity Structure of Central Asian Orogenic Belt

Abstract

The Central Asian Orogenic Belt (CAOB) is a Phanerozoic accretionary orogen with a complex formation process. This study imaged the three-dimensional electrical structure of the lithosphere using the magnetotelluric data collected covering the southern margin of the CAOB. The resistivity model shows alternating high and low resistivities along the southern margin of the CAOB in the north, with the low resistivities in the middle and lower crust interpreted as remnants of the subducted crust of the Paleo-Asian Ocean (PAO). The belt-like low-resistivity body encased by high-resistivity bodies on both sides is interpreted as residual back-arc oceanic crust. The Alxa Block (AB) in the south exhibits overall high-resistivity characteristics, but a large low-resistivity area exists near the southeastern connection to the Ordos block. Based on the characteristics of the resistivity model, we propose that the final closure position of the PAO in the southern margin of the CAOB is the Enger Us fault zone, with a north-south bidirectional subduction mode. The widespread low-resistivity anomaly near the Badain Jaran fault zone is interpreted as residual subduction of the back-arc oceanic crust. The subduction of the PAO is a typical “trench-arc-basin” model. The variation in the subduction angle of the PAO along the strike may indicate that the later stage of the southern margin of the CAOB was subjected to the convergence effect of northeast-directed stress from the northeast margin of the Tibetan Plateau and the associated tectonic activities.