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 for mation 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 d uring SW-directed compression (D-1) to localized sinistral transcurren t shear (D-2) and finally to increasingly localized high temperature ( D-3p) and retrograde (D-3r) reverse shear in response to SE-directed c ompression. Both D-2 and D-3 deformations were co-planar and reactivat ion of the S-2 foliation led to the formation of a composite S-2-3 fab ric. However, inclusion trails in garnet porphyroblasts preserve evide nce of a transition between these strike-slip and reverse shear end me mbers, indicating that D-2 and D-3 were part of an episode of progress ive transpressional shear. Metamorphic grade increases from chlorite t o kyanite zone within the Wenchuan-Maowen;Shear Zone, reaching syn-D-3 p peak conditions of similar to 600 degrees C and 10 kbar. The D-3p an d D-3r reverse shears were responsible for tectonic exhumation of the shear zone and resulted in the formation of retrograde D-3r high strai n zones along the margins of the Xuelongbao Granite and adjacent to th e Wenchuan-Maowen Fault.