CRUSTAL EXTENSION AND VOLCANISM INTERLEAVED WITH JURASSIC SHORTENING IN THE YANSHAN FOLD-THRUST BELT, NORTH CHINA

The North China block is a world-class example of a reactivated Archean craton. The Yanshan fold-thrust belt records a complex Mesozoic history of repeated extensional and contractile events associated with this reactivation. At least three tectonic events affected the central Yanshan during Jurassic time: 1) Early-Middle Jurassic south-directed thrusting, 2) Middle-Late Jurassic extension and volcanism, and 3) Late Jurassic-Cretaceous bivergent thrusting. These events preceded a phase of mid-Cretaceous extension and magmatism that affected all of eastern Asia.

Several generations of Early-Middle Jurassic south-directed folds and thrusts deform Archean-Proterozoic strata in the central Yanshan. These structures lie to the immediate north of a deformed basin containing north-derived Lower Jurassic conglomerate. Nearly flat-lying volcanic and sedimentary strata (158.9 ±2.2 Ma) unconformably overlie structures associated with this deformation; tightly folded Middle Jurassic strata (164.3 ±1.1 Ma) underlie this unconformity. Strata and structure of Early-Middle Jurassic age are cut by two major normal faults that bound a narrow graben cut into folded Proterozoic sedimentary strata and filled with Middle-Late Jurassic (160-152 Ma) andesitic volcanics. Extension of the region ended in Late Jurassic time (ca. 152 Ma), when thrust faulting resumed. Widespread conglomerate of the Late Jurassic to Early Cretaceous (>135 Ma) Tuchengzi Formation overlies this graben and is thought to represent syntectonic sedimentation associated with this younger phase of thrusting. Tuchengzi strata above the graben are folded into a 20-km wide, southwest-trending syncline (the Chengde syncline) with a steeply-dipping to overturned southern limb that is very likely associated with north-vergent thrusting. Tuchengzi strata within this syncline show little evidence for intrabasinal unconformities associated with folding. However, in Shouwangfen Basin, southwest of the Chengde syncline, Tuchengzi strata are folded into a growth syncline in the footwall of the south-directed Gubeikou thrust, and are clearly related to south-vergent thrusting. Middle-Late Jurassic extension and magmatism in the Yanshan may be related to localized lithosphere loss in an overall compressive regime.