Deformation partitioning and foliation reactivation during transpressional orogenesis, an example from the central Longmen Shan, China

Progressive localization of strain within the Songpan-Garze Fold Belt, at the eastern margin of the Tibetan Plateau, has resulted in the formation of the Wenchuan-Maowen Shear Zone and preservation of excellent overprinting relationships between three major phases of ductile and semi-ductile deformation. These relationships show that deformation in the eastern Songpan-Garze Fold Belt evolved from crustal thickening during SW-directed compression (D-1) to localized sinistral transcurrent shear (D-2) and finally to increasingly localized high temperature (D-3p) and retrograde (D-3r) reverse shear in response to SE-directed compression. Both D-2 and D-3 deformations were co-planar and reactivation of the S-2 foliation led to the formation of a composite S-2-3 fabric. However, inclusion trails in garnet porphyroblasts preserve evidence of a transition between these strike-slip and reverse shear end members, indicating that D-2 and D-3 were part of an episode of progressive transpressional shear. Metamorphic grade increases from chlorite to kyanite zone within the Wenchuan-Maowen;Shear Zone, reaching syn-D-3p peak conditions of similar to 600 degrees C and 10 kbar. The D-3p and D-3r reverse shears were responsible for tectonic exhumation of the shear zone and resulted in the formation of retrograde D-3r high strain zones along the margins of the Xuelongbao Granite and adjacent to the Wenchuan-Maowen Fault.