Post-collisional orogen-parallel extension in the Trans-North China Orogen: Evidence from syn-kinematic pegmatite dikes

Abstract

Orogen-parallel extension is common in collisional settings, but its cause is debated. The Paleoproterozoic Trans-North China Orogen (TNCO) provides insights into the process of orogen-parallel extension. We present structural and geochronological data from pegmatite dikes in the Xiaoqinling region of central China to constrain the time and nature of Paleoproterozoic extension in the TNCO. The pegmatite dikes strike roughly E–W, have various dip angles, and are either deformed or undeformed. A progression from ductile to brittle deformation is commonly observed in the deformed dikes, suggesting synchronous orogenic uplift probably driven by isostatic adjustments. The deformation structures demonstrate that the intrusion of the pegmatite dikes was associated with a single phase of regional extension, thus making them syn-kinematic. Both ductile fabrics and fault-slip data from the dikes indicate that N–S (orogen-parallel) extension was responsible for dike deformation. Zircon U–Pb ages for the syn-kinematic pegmatite dikes constrain the time of extension to between 1840 and 1781 Ma, suggesting a duration of ∼ 59 Myr for the orogen-parallel extension. These data suggest that the extension postdated the syn-collisional shortening in the TNCO, which occurred between 1863 and 1840 Ma. The orogen-parallel extension, the synchronous isostatic uplift, and the intense magmatism in the TNCO are consistent with a gravitational collapse model in a post-collisional setting.