Yanshanian deformation along the northern margin of the North China Craton: Constraints from growth strata in the Shiguai Basin, Inner Mongolia, China

The evolution of Yanshanian structures, especially the timing of onset, is a key problem in reconstructing the Late Mesozoic structural framework of the North China Craton. Growth strata and progressive unconformities, coupled with related structures, have been recognized for the first time in the Jurassic Shiguai Basin, thus allowing us to constrain the timing of Yanshanian structures with greater precision. Growth geometries and detailed structural analysis indicate that the late Middle Jurassic Changhangou Formation can be considered as the earliest sedimentary response to the onset of Yanshanian deformation during 170-160 Ma. At this time interval, shortening deformation was dominantly controlled by N-S compression driven by the closure of the Mongol-Okhotsk Ocean, and growth structures constituted a roughly forward thrusting sequence with a slow uplift rate in the early stages and a suddenly accelerated uplift rate in the final stage. Subsequently, due to the superposition of NW-SE compression during the Late Jurassic (160-145 Ma), the initial E-W-trending structures were converted into a stepped geometry marked by several structural reentrants developing along strike. In addition, out of sequence thrusts represented by the Beilinshan thrust and its splays began to be active in the middle portion of the previous forward thrusting sequence. Northwestward subduction of the Izanagi plate along the eastern margin of Asia is generally considered to be responsible for this phase of deformation.