MULTIDIRECTIONAL SHORTENING IN THE YANSHAN FOLD-THRUST BELT, CHINA

The Yanshan fold-thrust belt is located approximately 150 km northeast of Beijing in Hebei Province, China. The Yanshan, which developed on the former North China Craton, experienced polyphase, multi-directional deformation from several opposing forces during Mesozoic time. Development of the Solonker collisional suture to the north, the Paleo-Pacific subduction zone to the east, and the Qinling-Dabie suture to the south all potentially contributed to contractile Mesozoic deformation in the Yanshan. Several episodes of Mesozoic deformation are evident in the region east and south of the city of Xiabancheng (Chengde County). Early NW-SE contraction, likely related to Permo-Triassic formation of the Solonker Suture, created a regional northeast-trending, southeast-vergent anticline. Subsequent NE-SW compression, possibly related to early Paleo-Pacific subduction, refolded this anticline and formed several other northwest-trending folds. Lower Jurassic Xingshikou Formation conglomerate unconformably overlies one such fold. NW-trending folding also appears to predate development of a major south-vergent thrust. NE- and NW-trending strike-slip faults cut this thrust, the older folds, and the Xingshikou Formation. Following a Middle-Late Jurassic phase of extension and volcanism, all of these structures were cut by a system of E-W trending, Late Jurassic-Early Cretaceous thrust faults that include the north-vergent Chengde County Thrust, the south-vergent Pingquan Thrust, and the south-vergent Gubeikou Thrust. Following intrusion of several mid-Cretaceous plutons, some of these thrusts were reactivated on a smaller scale. Although it is unclear how extensive these events are throughout North China, the Yanshan Belt has apparently undergone a more complex history than is currently recognized.