Timing of deformation and location of the eastern Liaoyuan Terrane, NE China: Constraints on the final closure time of the Paleo-Asian Ocean

Abstract

The Liaoyuan Terrane (LT) in NE China formed as a result of accretion during the final closure of the Paleo-Asian Ocean between the North China and Siberian cratons. However, the timing of the deformation and the location of the eastern LT remain controversial. Here we present structural data, zircon U–Pb age dating, and Hf isotopic data to constrain the kinematics and timing of deformation, and the southern boundary of the eastern LT in relation to the NE–SW-striking Yilan–Yitong Fault (YYF), one of two branches of the continental-scale Tan–Lu Fault Zone in NE China. Pre-Jurassic rocks in the eastern LT were widely involved in ductile deformation within WNW–ESE-striking thrust belts. The microstructures in these shear zones indicate a top-to-the-SSW sense of shear and deformation temperatures of 450–500 °C. The northern margin of the North China Craton (NCC) was also affected by the NNE–SSW contraction, as shown by similar ductile deformation in the Archean gneisses and WNW–ESE trending folds in the Mesoproterozoic sedimentary cover. The ages of crystallization in deformed and undeformed igneous rocks in the LT constrain the deformation to an interval between 241 and 193 Ma. U–Pb dating of zircons in syn-kinematic migmatitic rocks indicates an age of 234–229 Ma for the deformation. We consider this Middle Triassic age to represent the final closure time of the Paleo-Asian Ocean along the eastern LT, and this timing is consistent with the eastwards scissor-style closure model. Lithological variations, zircon age spectra, εHf(t) values, and model ages confirm the eastward extension of the LT across the YYF. The data constrain the boundary between the LT and NCC to be roughly along the Liao River to the west of the YYF, and along the northern edge of the exposed Archean gneisses to the east. The boundary follows the Bayan Obo–Chifeng–Kaiyuan Fault, which was active as a thrust (thrusting of material to the SSW) during the Middle Triassic orogeny. The newly determined boundary location indicates a 35 km sinistral displacement along the YYF.