A New View On The Mechanism Of Chengde Thrust Structure In The Central Part Of The Yanshan Orogenic Belt

In the central part of the Yanshan orogenic belt an attractive tectonic terrain is located in the northeastern part of Chengde County, Hebei province. It is exposed as a narrow, arcuate, fault-bounded body dominated by Meso-Proterozoic strata in the southern, northern and eastern boundaries in the published geologic maps. A deformed"Chengde Thrust Fault"model was proposed by Davis et al. in 1998 which treated the unique tectonic terrain as a synformal structure derived from folding of a northward thrust and nappe tectonics with displacement over 40km and the Proterozoic strata and Jurassic units in its hanging wall. However, more and more research results in sedimentology and tectonic deformations in Chengde basin and adjacent areas have been queried in last several years against the tectonic model by Davis et al., meanwhile no a more reasonable tectonic model has been proposed based on new findings and more detailed geologic mapping and tectonic deformation analysis is needed. Chengde Thrust Fault model by Davis et al. (1998) is open to challenge.This research focuses on the geometric and kinematic significances of"Chengde Thrust Fault". The observations indicate"Chengde Thrust Fault"by Davis et al. (1998) is actually not a single northward thrust, but a deformation system consists of a syncline and several faults formed in its limbs, which will be called as Chengde Thrust Structure in this paper. Shuangmiao thrust in the northern limb of syncline is a northward thrust, compared with Jiyuqing fault in the southern limb which has the steep northward dipping and is kinematically southward thrust. The tow thrusts formed in the northern and southern limb of syncline respectively do not be joined in the hinge of syncline. The ages of syncline and thrusts are almost in the same period. That means"Chengde Thrust Fault"model by Davis et al. is not sound in terms of tectonic deformation analysis.A new view on the mechanism of Chengde Thrust Structure, an out-of-syncline thrust model is proposed in this paper based on the observations of field work and Fold-accommodation faults model developed by Mitra in 2002, in which the thrusts in the limbs of Chengde syncline are due to the category of out-of-syncline thrust developed as a way to solve the space problem in the core of syncline during folding. This model provides a relatively reasonable interpretation for the arcuate shape of Proterozoic strata in Chengde Thrust Structure, the relations between Chengde syncline and thrusts and the kinematic process of these thrusts. This research indicates that although contractional deformation exists widely in the Yanshan orogenic belt there is not a convictive example in support of horizontally large-scaled shortening.As a tectonic-scale fold-accommodation fault, Chengde Thrust Structure is introduced in detailed in this paper. On the basis of observations together with the published data, the main significances and identification criteria in a geological map for the tectonic-scale fold-accommodation fault, specially, for out-of-syncline thrust and into-anticline thrust are concluded in this paper, several possible problems in the practical research and the important implications born by tectonic-scale fold-accommodation faults for the research on regional tectonics evolution are discussed as well. This research is much helpful not only for a better understanding of fold-accommodation faults model but also for the future research on the similar tectonics deformation analysis.