EVOLVING GEOMETRY OF THE HUHHOT METAMORPHIC CORE COMPLEX, INNER MONGOLIA, CHINA

The Huhhot metamorphic core complex lies ~400 km west of Beijing along the northeast margin of the Ordos plateau. With a minimum along strike length of 100 km, the Early Cretaceous Huhhot core complex is a major component of the entire eastern portion of the Daqing Shan. The complex can be divided into southern and northern portions, separated by a major, E-W trending antiform. A major south-dipping, low-angle normal fault, the Huhhot detachment, lies along the southern flank of the Daqing Shan antiform. Along most of its length it separates a mylonitic footwall of Early Cretaceous to Archean crystalline rocks from a brittlely deformed upper plate of pre-Jurassic basement rocks and Jura(?)-Cretaceous cover strata. Abundant and consistent kinematic indicators demonstrate top to the southeast displacement. The northern flank of the Daqing Shan antiform contains two major low-angle detachment faults that are synformally folded. The lowermost detachment juxtaposes lower plate mylonitic and higher, non-mylonitic crystalline rocks, whereas the upper detachment separates the crystalline rocks from Late Jurassic(?) and Early Cretaceous volcanic and sedimentary strata. The stacked detachments exhibit upper plate movements to the southeast. We interpret the multiple detachment faults to represent the structural evolution of one detachment system. The single Huhhot detachment along the southern flank of the range is the master fault while the synformally folded faults to the north are splays (lowermost the oldest) that were only active for part of the core complex evolution. We estimate a total horizontal component of NW-SE extension of >40 km.

The Huhhot metamorphic core complex is superimposed on the E-W trending Yinshan fold and thrust belt of Jura-Cretaceous age. Development of the Yinshan belt thickened the North China crust, possibly leading to post orogenic collapse and core complex formation. Early Cretaceous extension in the Daqing Shan appears to be broadly synchronous with the Yagan metamorphic core complex to the west and the Yunmeng Shan, north of Beijing, to the east.