Geometric and temporal evolution of an extensional detachment fault, Hohhot metamorphic core complex, Inner Mongolia, China

The Early Cretaceous Hohhot metamorphic core complex and its master Hohhot detachment fault are approximately 400 km west of Beijing in the Daqing Shan (Mountains) of Inner Mongolia. The complex developed across the east-trending Yinshan fold-and-thrust belt within <4 m.y. following cessation of thrusting ca. 125 Ma (see note added in proof in main text). Postcontractional extension was initiated within a mid-crustal zone of mylonitic and ductile shear that was in part controlled by Carboniferous(?) strata sandwiched between its Proterozoic and Archean crystalline basement and an overlying thrust sheet of similar crystalline rocks. The Hohhot detachment fault appears to have rooted into deep, kinematically active levels of the mid-crustal shear zone. Higher, inactive levels of the mylonitic section were transected by the fault and carried upward in its footwall. Geometries of the footwall mylonitic rocks indicate localized ramp-flat geometries of the fault within and across them. The crosscut top of the mylonitic sequence defines a mylonitic front that departs from the gently south dipping detachment fault and dips northward into its footwall. Early Cretaceous extension was widespread elsewhere in northern China, and was particularly pronounced in the Yunmeng Shan core complex north of Beijing. The gravitational collapse of orogenically thickened crust acting in concert with localized centers of deep-seated plutonism appear to have led to the development of isolated metamorphic core complexes within a broad region of more distributed extensional deformation.