Mesozoic contraction deformation in the Yanshan and northern Taihang mountains and its implications to the destruction of the North China Craton

Mesozoic contraction deformation in the Yanshan and Taihang mountains is characterized by basement-involved thrust tectonics,basement-cored buckling anticlines and ductile thrust and nappe tectonics.Most of these deformations are orientated west-east,west-northwest and northeast to north-northeast.The contraction deformations began in the Permian,continued through the Triassic and Jurassic and terminated in the Early Cretaceous,and constitute an important part of the destruction of the North China Craton.It is estimated,from balanced cross-section reconstructions,that the north-south shortening of the central part of the Yanshan belt before 135 Ma was around 38%.The initial crust thickness,pre-dating the major contraction deformation in late Paleozoic and early Mesozoic,was estimated to be around 35 km based on paleogeographic characteristics.Assuming that the inferred depth of ductile thrusting deformation,20-25 km,was the crust thickness involved in the contraction deformation,and also assuming that the N-S contraction deformation was accommodated by vertical crust thickening,the thickness of the crust after the contraction deformation was expected to be around 47-50 km.This was the approximate crust thickness required for the eclogitization of the lower crust for delamination.The gravity potential accumulated by the isostatic uplift of the thickened crust,together with the decrease in crustal strength caused by the coeval magmatisms associated with the contraction deformation,led to the subsequent extensional collapse of the middle and upper crust although the regional stress regime associated with the plate interactions remained constant.It is inferred that the Mesozoic contraction deformations in the Yanshan and Taihang mountains were not only a significant tectonic process contributing to the destruction of the craton in middle and upper crust but also stimulated delamination at a deep level and the extension of the shallow crust.In other words,both the suspected delamination of the lower crust and upper mantle and the well constrained extension deformations of the shallow crust in the eastern North China Craton during the late Mesozoic are a consequence of crust thickening due to previous contractions.Extensional deformations could be expected to occur independently in the shallow crust,and are not necessarily associated with or responding to delamination at a deep level.