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

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.

Graphical Abstract

KEYWORDS:

• East Qinling orogeny
• Foping domal zone
• Bedding-parallel shear zone
• Orogen-parallel crustal ductile flow
• U-Pb and ⁴⁰Ar/³⁹Ar geochronology

Acknowledgments

Data supporting this article are available by contacting the corresponding author Yueqiao Zhang ([email protected]). This work was supported financially by National Key R＆D Program of China (Grant No. 2017YFC0601402), National Natural Science Foundation of China (Grant No. 41472178). Thanks are given to Jianhua Li for helpful suggestions and discussions, and to Drs. Erkang Qiu, Jun Ren for their assistance in geochronology analysis.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Highlights

1. Orogen-parallel crustal flow was documented in Foping migmatic domal zone of East Qinling and along the bedding-parallel Ningshan shearing zone.

2. Zircon U-Pb geochronology reveals coeval development of crustal anatexis, dextral strike-slip shearing and bedding-parallel shearing in Late Triassic at 220-208 Ma.

3. Overall deformation pattern and associated magmastim seem to be controlled by a salient geometry of the Hannan indentor.

Supplementary material

Supplemental data for this article can be accessed here.

Additional information

Funding

This work was supported by the National Key R & D Program of China [2017YFC0601402]; National Natural Science Foundation of China [41472178].