Kinematics and Geochronology of Late Paleozoic–Early Mesozoic Ductile Deformation in the Alxa Block, NW China: New Constraints on the Evolution of the Central Asian Orogenic Belt

Abstract

AbstractStrike-slip faults are widely developed throughout the Central Asian Orogenic Belt (CAOB), one of the largest Phanerozoic accretionary orogenic collages in the world, and may have played a key role in its evolution. Recent studies have shown that a large number of Late Paleozoic–Early Mesozoic ductile shear zones developed along the southern CAOB. This study reports the discovery of a NW–SE striking, approximately 500 km long and up to 2 km wide regional ductile shear zone in the southern Alxa Block, the Southern Alxa Ductile Shear Zone (SADSZ), which is located in the central part of the southern CAOB. The nearly vertical mylonitic foliation and subhorizontal stretching lineation indicate that the SADSZ is a ductile strike-slip shear zone, and various kinematic indicators indicate dextral shearing. The zircon U-Pb ages and the 40Ar/39Ar plateau ages of the muscovite and biotite indicate that the dextral ductile shearing was active during Middle Permian to Middle Triassic (ca. 269–240 Ma). The least horizontal displacement of the SADSZ is constrained between ca. 40 and 50 km. The aeromagnetic data shows that the SADSZ is in structural continuity with the coeval shear zones in the central and northern Alxa Block, and these connected shear zones form a ductile strike-slip duplex in the central part of the southern CAOB. The ductile strike-slip duplex in the Alxa Block, including the SADSZ, connected the dextral ductile shear zones in the western and eastern parts of the southern CAOB to form a 3000 km long E-W trending dextral shear zone, which developed along the southern CAOB during Late Paleozoic to Early Mesozoic. This large-scale dextral shear zone was caused by the eastward migration of the orogenic collages and blocks of the CAOB and indicates a transition from convergence to transcurrent setting of the southern CAOB during Late Paleozoic to Early Mesozoic.