Indentation-induced tearing of a subducting continent: Evidence from the Tan–Lu Fault Zone, East China

Abstract

Vertical tearing of oceanic slabs has been well documented at subduction zones. It remains unclear whether a subducting continent can tear vertically. Origin of the continental-scale Tan–Lu Fault Zone (TLFZ) in East China provides an example of the vertical tearing of the subducting continent at the convergent stage. The Dabie and Sulu orogens between the North China Plate (NCP) and the Yangtze Plate (YZP) are left-laterally offset about 400km along the NE-striking TLFZ, but the fault zone terminates abruptly at the southeastern corner of the Dabie Orogen, suggesting unique origin of the fault zone. We present structural evidence to show that the TLFZ initiated as a sinistral fault zone that is exposed in ductile shear belts just to the east of the Dabie Orogen. Our muscovite 40Ar/39Ar dating results, coupled with existing age data, indicate that the TLFZ formed at 240–230Ma (Middle Triassic), when the YZP continental crust was subducted beneath the NCP along both the Dabie and Sulu sutures. We also show that SE-directed extrusion of the subducted crust in the Dabie Orogen occurred during 230–209Ma (Late Triassic) and overprinted contractional deformation in the TLFZ to the southeast of the Dabie Orogen. Syn-collisional folds and thrusts in the YZP, to the east of the TLFZ, exhibit evidence for large-scale dragging by the sinistral fault zone, whereas those in the NCP to the west are perpendicular to the TLFZ without obvious evidence for drag. Combining these lines of evidence with published data on the tectonic evolution of the two orogenic belts, we propose an indentation-induced continent-tearing model for the origin of the TLFZ. We suggest that the present southern boundary of the NCP represents its original shape, with a promontory in front of the Dabie Orogen. At the oceanic subduction stage, the overriding NCP promontory led to vertical tearing of the subducting oceanic plate along the TLFZ, which formed the eastern boundary of the promontory. Once the rigid NCP indenter collided with the passive YZP along the Dabie Orogen, the oceanic slab tear propagated into the YZP lithosphere, resulting in long-distance, low-angle subduction of the YZP underneath the NCP indenter and NNE-ward, horizontal motion of the torn YZP east of the TLFZ until the final collision at the Sulu Orogen. Thus, the indentation induced the vertical tearing of the subducting YZP along the TLFZ.