Rotation of Crustal Blocks as an Explanation of Oligo-Miocene Extension in Southeastern Tibet—Evidenced by the Diancangshan and Nearby Metamorphic Core Complexes

Abstract

The Diancangshan (DCS) metamorphic core complex (mcc) along the Red River Fault Zone (RRFZ) is an incomplete mcc composed of two units with distinct characteristics, i. e. a lower plate of amphibolite grade tectonites intruded by synkinematic monzogranite, and a greenschist grade detachment fault zone. Lower plate rocks possess high temperature (high- T) structures and L or L>>S mylonites with left-lateral strike-slip shear sense. The detachment fault zone is a low temperature (low- T) shear zone with top to the E to SE extensional shearing and has typical S-L mylonites with both shearing and flattening strains. Low- T tectonites also include several mylonitized porphyritic monzogranite intrusions in the lower plate. The mcc is juxtaposed by non-metamorphosed Mesozoic sedimentary rocks to the west, and cut by Quaternary faults in the east, presenting a Pleistocene to Holocene sedimentary basin along Er Hai Lake. Through structural analysis of the DCS mcc, together with the geochronological and paleomagnetic analysis of mcc's in contiguous areas, we would argue that the occurrence of mcc's in a fan-shaped area between the Red River fault zone and Sagaing fault zone in SE Asia is related to regional extension during Oligo-Miocene period. The extension is attributed to the differential rotation in the Indochina block, caused by the clockwise rotation and rollback of the obliquely subducting Indian plate since about 33 Ma.