Abstract:

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 (~＞4045km) 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 139136 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.

Key words:  thrust tectonics, foldaccommodation fault, outofsyncline thrust, Chengde, Yanshan