CONTRACTIONAL TO EXTENSIONAL EVOLUTION OF A ≥40 MA MID-CRUSTAL SHEAR ZONE, YANSHAN FOLD-THRUST BELT, NORTHERN CHINA

Research in the Yanshan fold-thrust belt of northern China reveals its long and evolving history of Late Jurassic-Early Cretaceous mid-crustal ductile shear with a duration of ≥40 Ma. The capture of this shear zone in the footwalls of two Early Cretaceous mcc’s (Yunmengshan, Yiwulushan), 400 kms apart, has provided rare tectonic windows into the Yanshan middle crust. The Hefangkou detachment fault of the Yunmengshan mcc near Beijing has exhumed the deepest exposed levels of the Yanshan belt — including the overturned limb of a crustal scale, south-vergent recumbent nappe. This structure, the Archean basement-cored Sihetang anticline, lies above the Miyun Reservoir thrust and a lower plate plutonic assemblage of Late Jurassic-Early Cretaceous age. Both plates were overprinted, at a probable depth of 20 to 25 km, by the 6 km-thick Sihetang ductile shear zone. This contractional amphibolite-grade shear zone was active by 155 Ma and persisted until ~127 Ma, the latter age based on the late stage deformation within it of an Early Cretaceous pluton. At about this time the NE Asian region experienced a remarkably brief and still cryptic transition between contractional and extensional crustal deformation. Extension led to the formation of mcc’s in 3 former collisional orogenic belts (Mongol-Okhotsk, Yanshan-Yinshan, Dabie-Sulu) and widespread normal faulting between them. Yunmengshan mcc studies demonstrate that its SE-dipping Hefangkou detachment fault rooted into the Sihetang shear zone at ~125 Ma and flattened within the zone's former boundary between upper and lower plates (Davis and Xia, 2013). Displacement on the detachment persisted to ~115 Ma. A similar temporal and geometric history between mid-crustal contractional shear and Early Cretaceous Waziyu detachment faulting in the more eastern Yiwulushan mcc has recently been described (Lin et al., 2013, Gondwana Research). In summary, the mid-crustal Sihetang shear zone evolved from deep, ductile and contractional (≥155 to ~127 Ma) to a shallower, semiductile zone (~125-115 Ma) that spawned the Hefangkou and Waziyu mcc detachment faults. Given this Yanshan belt example, the possible role in other orogens of pre-existing mid-crustal anisotropy for controlling younger detachment-related extension deserves scrutiny.