Orogen-parallel mid-lower crustal ductile flow during the late Triassic Qinling orogeny: structural geology and geochronology

Abstract

ABSTRACT The North and South China Block collision in Triassic resulted in the Qinling orogeny with massif batholith intrusion. In this study, we investigated the deformation structures of the Foping domal zone, the Ningshan bedding-parallel shear zone, and the Yangtian-Tongchewan strike-slip shear zone in East Qinling, aiming at unravelling orogen-parallel mid-lower crustal ductile flow during this late-stage orogeny. Field measurements of foliated felsic migmatites in the Foping domal zone show a subhorizontal stretching lineation associated with top-to-east shearing, and the quartz c-axis fabrics of rock samples from leucosome display characteristic prism <c> slip system (>650°C), consistent with granulite-facies metamorphic condition. The ductile fabrics observed along the W-E extending Yangtian-Tongchewan mylonitic zone along the southern Foping domal zone indicate dextral strike-slip shearing. The Ningshan bedding-parallel shear zone was mapped in the Palaeozoic sedimentary strata along the southern rim of the Foping domal zone and it exhibits subhorizontal stretching lineation. Zircon U-Pb dating results of migmatite from the Foping domal zone, mylonite from the Yangtian-Tongchewan shear zone and syntectonic granitic dyke from the Ningshan bedding-parallel shear zone, together with mica 40Ar/39Ar dating results, allow to constrain the timing of coeval development of these structures in Late Triassic at 220–208 Ma, broadly consistent with the pluton intrusion. The overall pattern is interpreted to result from a local indentation of the Hannan Massif with a salient boundary geometry, which possibly controlled the lateral extrusion of the orogenic wedge, orogen-parallel crustal ductile flow as well as the Late Triassic plutonic emplacements in East Qinling.