The complicated and enigmatic Jurassic folds and thrusts around Chengde Basin in central intraplate Yanshan Orogenic Belt were previously interpreted as a synclinal deformed thrust sheet with large-displacement(~>40-45km) formed after Late Jurassic Tuchengzi Formation.This model has been challenged by many new findings in paleogeographic and provenance analysis of Tuchengzi Formation and Proterozoic Changcheng System,together with the research result in tectonic deformations.Meanwhile no new model has been proposed so far to account reasonably for the structural style and tectonic framework as well as the evolution process of this structurally complicated region.Detailed mapping have been conducted in last several years to reveal systematically the geometric and kinematic features of the Chengde syncline and thrust faults developed in its both limbs and around its exposed hinge zone to the east.U-Pb age dating with LA-MC-ICPMS method on zircons from key terranes of Chengde Basin has been employed to constrain the timing of major deformations.We found that the thrust faults in northern and southern limbs of the Chengde syncline are independent thrust faults moving away from the core of the syncline.They died out around the hinge zone separately other than connected to each other to form a unified thrust fault as expected by synclinal deformed thrust sheet model.Age dating results suggested that both the Chengde syncline and the major thrust faults formed around 139-136 Ma.Based on recent research results aforementioned,an out-of-syncline thrust model has been proposed in this paper.The complicated geometric and kinematic features of folds and thrust faults in Chengde Basin as well as the provenance features of Tuchengzi Formation can be interpreted reasonably with this new model.This research implies that there is no large-scale thin-skinned thrust tectonics in intraplate Yanshan Orogenic Belt as those in foreland fold and thrust belt of collision orogenic belt.Fold-accommodation fault is supposed to be treated as an important alternative model in dealing with the relationship between large-scale folds and thrust faults as well as tectonic framework and evolution in contraction regime.

燕山-太行山中生代收缩构造变形主要表现为基底卷入的逆冲构造、基底为核的大型纵弯褶皱构造,以及韧性逆冲推覆构造.构造形迹展布方向主要有近WE,NWW和NE-NNE向.在总体构造线呈NNE向展布的太行山构造带和辽西燕山东段,均发现有近WE向和NWW向收缩构造变形.收缩变形发生在二叠纪晚期、三叠纪、侏罗纪及早白垩世.它们的发生已经使克拉通遭受破坏.燕山中部近NS向构造剖面复原表明,在135 Ma之前的构造变形缩短率约为38%,华北东部晚古生代和早中生代岩相古地理研究显示,收缩变形前的地壳厚度约为35 km.如果将上述地质历史时期韧性剪切收缩变形反映的变形深度(20～25 km)作为卷入收缩变形的地壳厚度,并假定水平缩短变形量由垂向地壳加厚所调节,则在南北向缩短变形之后地壳厚度可达47～50 km,已经接近拆沉构造模型下地壳榴辉岩化所需的地壳厚度.同时,加厚地壳均衡抬升产生的重力势能差,与地壳加厚期间持续不断的岩浆活动导致的岩石圈强度弱化,为在区域构造应力状态不发生改变的情形下产生中浅部地壳的伸展垮塌创造了充分条件.因此,燕山-太行山中生代收缩构造变形,一方面直接导致了克拉通岩石圈浅层稳定状况的破坏,另一方面为在深部可能发生的拆沉作用和在浅部产生强烈伸展变形创造了有利条件.拆沉作用和伸展变形可能同是早期收缩变形导致地壳强烈加厚的结果.伸展变形既可以与拆沉作用相伴发生,也可以单独出现,不宜将浅层伸展变形作为深部拆沉作用曾经发生的直接证据.

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.