Geometric and kinematic evolution of basement-cored structures: intraplate orogenesis within the Yanshan orogen, northern China

Detailed surface mapping, lithology facies analysis, drillcore data and aeromagnetic interpretation reveal the geometries of the basement-involved structures of Laowopu, Nantan, Liudaohe, Xinglong and Malanyu, to have been generated by folding and thrusting. The resulting basement-cored anticlines are asymmetric, with a shallowly dipping top and backlimb, and a steep to overturned forelimb and/or frontal fault, linked with a gently dipping synclinal limb. Both the fold axes and frontal faults trend roughly east-west, suggesting north-south shortening. North- to northeast-trending faults behave as tear, lateral or oblique ramp structures and formed coevally with their frontal faults. Their trends may have been inherited from Proterozoic synsedimentary faults and Archaean basement ductile shear zones, foliations and mafic dikes. Thin-skinned thrusts and their three-dimensional geometry are locally controlled by detachment within mechanically incompetent layers. Four types of interrelationships between the thick- and thin-skinned structures occur including (1) lateral changes along the length of a fault, (2) displacement transfer between thick- and thin-skinned structures, (3) buttressing of thin-skinned thrusts against the basement-involved structures, and (4) generation of an out-of-the-syncline thrust under thick-skinned shortening. Most of the deformation within the study area involved brittle behaviour, but ductile shear zones developed where fluid access was possible. Both the basement-involved structures and the thin-skinned structures formed in the Early Mesozoic as constrained by cross-cutting relationships, isotopic ages of igneous rock and syntectonic basins. The model of geometric and kinematic evolution of the basement-involved structures, and their relationship with the thin-skinned structures defined in this paper, provides an alternative to the current metamorphic core complex model used for the Yanshan intraplate orogen. (C) 1998 Elsevier Science B.V. AU rights reserved.