Abstract:

Abstract: Three geometric models concerning the development mechanisms of foldaccommodation faults, namely concentric folding, kinkband folding, buckle flexuralslip folding, are summarized. Five wellexposed metrescale folds with minor faults are analyzed in detail. They developed within Meso and Neoproterozoic sequence in central intraplate Yanshan Orogenic Belt, northern North China Craton. These observations provide us opportunities to a better understanding of factors controlling the development of foldaccommodation faults and time sequence between scale compatible folds and faults formed within contraction regime. The distancedisplacement diagrams are employed to define the sequential development of faults relative to their hostfolds together with the geometric and kinematic relations between them. It is found that faults spatially closerelated folds in contraction deformation could form previous to, synchronous with or after the folding deformation. Only those developed synchronous with folding in response to the strain contrast related to structural position and stratigraphic mechanics could be treated as foldaccommodation faults. Some preexisting faults may serve as nucleation sites during followed folding deformation and finally lead to the formation of hinge wedges. Development of foldaccommodation faults may be effectively influenced by buckling flexuralslip folding, tightness change of folds and escape of incompetent units from cores of folds. (1)Accumulated layerparallel shearing in limbs associated with buckling flexuralslip folding may give way to faults of cutupsection when layerparallel movement was blocked. Traceable faults formed in limbs or/and hinge zones, such as wedge thrusts, outofsyncline and intoanticline thrusts, could form as a result of this mechanism. (2)Curvature change of folds derived from various factors served as another important factor in the development of foldaccommodation faults. Stratigraphic levels where convergence of axial planes occurred are expected to be potential position for the initiation of foldaccommodation faults. (3)During progressive contraction deformation of interbedded competent and incompetent units with higher competency contrast and larger distance between competent units, escape of incompetent units from the core of fold following the previously intensive concentration would result in the formation of foldaccommodation faults.

Key words: foldaccommodation faults, buckling flexuralslip folding, curvature change, competency contrast, fault distancedisplacement diagram