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

Abstract

Progressive localization of strain within the Songpan—Garzê 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—Garzê 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 3 p ) and retrograde ( D 3 r ) 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 3 p peak conditions of ∼600°C and 10 kbar. The D 3 p , and D 3 r reverse shears were responsible for tectonic exhumation of the shear zone and resulted in the formation of retrograde D 3 r high strain zones along the margins of the Xuelongbao Granite and adjacent to the Wenchuan—Maowen Fault.