MID-CRETACEOUS EXTENSION SUPERIMPOSED UPON THE JURASSIC-CRETACEOUS YANSHAN FOLD-THRUST BELT, NORTHEAST CHINA

An extensive rift system, comparable in size to the Basin and Range province, developed in northeast China and southern Mongolia during Late Jurassic-Early Cretaceous time. This rift system is superimposed on and interacts with a number of pre-extensional zones of intracontinental shortening, including the Jurassic-Cretaceous Yanshan fold-thrust belt of northeast China. Throughout most of its length, the Yanshan belt (and its Yinshan counterpart further west) is characterized by north and south-vergent structures that occupy a strongly linear, east-west trending belt of shortening. In its eastern segment, however, Jurassic fold and thrust structures are strongly southeast vergent and the belt trends to the northeast. The Cretaceous extension direction, as derived from stretching lineations in mylonites, fault striae, and normal fault orientation, was oriented northwest-southeast. Where extension was imposed upon pre-rift fabrics trending orthogonal to the extension direction, such as in the eastern, northeast-trending segment of the Yanshan belt, a system of lacustrine half-graben basins formed. Basin-bounding normal faults are localized along older southeast vergent thrust structures, and reactivate them in a normal sense. Many of these normal faults had anomalously low (ca. 20-30°) dips that we attribute to their localization along older thrust planes. Mid-Cretaceous extension in the Yanshan region immediately followed or overlapped with contractile tectonism in the area. Although the timing of the onset of extension is at present only poorly constrained, extension may have began as early as 130 Ma. Major north-south contractile deformation in the eastern Yanshan belt continued until at least 127 Ma. The drivers of extension in this region remain unclear.