An early Jurassic dextral strike-slip system in southern South China and its tectonic significance

摘要

Ductile shear structure of the Guanging-Bobai fault belt in the SW part of the Qin-Hang belt, South China, has been investigated and interpreted in terms of radiometric dating data. A series of ductile shear zones occurs within the fault belt with deformation and metamorphic features indicative of formation under medium temperature and pressure conditions. The foliation such formed is steeply dipping and bears a gently plunging lineation, which along with the dextral kinematic indicators determines a broad NE-SW-trending dextral strike-slip regime responsible for the development of these shear zones. Field overprinting relationships indicate that this dextral shear is pre-dated by the Indosinian(P_2-T) NNE-verging thrusting and post-dated by the Yanshanian (J_2-K_2) SE-verging thrusting, and thus occurred during a transitional period between the two orogenies. ~(40)Ar-~(39)Ar radiometric dating on muscovite from mylonites further constrains timing of the dextral shear to the Early Jurassic (187-193 Ma). Based on the coeval tectonic framework of eastern Asia, we propose here that the dextral strike-slip system was initiated by the far-field oblique stress field from the incipient subduction of the Izanagi oceanic plate. An offset continental margin of the South China plate may have been created in response to this transcurrent movement along the fault belt, dissecting and displacing not only a relict Caledonian foreland basin, but also an Indosinian magmatic belt.