Structural constraints on the timing of left-lateral shear along the Red River shear zone in the Ailao Shan and Diancang Shan Ranges, Yunnan, SW China

Abstract

The >1000-km-long Oligocene—Miocene left-lateral Red River shear zone (RRSZ) and metamorphic belt and the Pliocene—active right-lateral Red River fault (RRF), stretching from SE Tibet to the South China Sea, has been cited as one of the primary examples of a lithospheric scale strike-slip fault that has resulted in syn-kinematic metamorphism and partial melting and accommodated several hundred to a thousand kilometers of horizontal motion as a result of the indentation of India into Asia. Alternatively we interpret the metamorphic complexes along the RRSZ as exhumed metamorphic core complexes of older rocks, subsequently affected by Oligocene–Early Miocene left-lateral shear and localized partial melting (leucogranite dykes), Miocene low-angle normal faulting along margins (Range Front faults), and Pliocene active dextral strike-slip faulting (RRF). Along the Ailao Shan (ALS) and Diancang Shan (DCS) ranges in Yunnan, SW China, early amphibolite facies metamorphic rocks were intruded by K-feldspar orthogneisses of Triassic age (Indosinian). LA-ICP-MS U-Pb zircon dating reveals a complex history with zircon cores showing evidence of Indosinian (∼239–243 Ma) to Neoproterozoic magmatism. Zircon rims show an Oligocene (∼26 Ma) magmatic or metamorphic overprint. Biotite granodiorites and syenites of mantle origin intruded the gneisses during the Oligocene (∼35 Ma). Later biotite leucogranites intruded the orthogneisses and migmatite host rocks before a significant phase of tight to isoclinal folding. Ductile, left-lateral strike-slip shear fabrics were superimposed on all lithologies at high temperature (∼500–550 °C) for the ALS and lower temperatures (∼250–150 °C) after peak metamorphism and after granite intrusion. A few very small biotite (±Grt ± Tur) leucogranite veins and dykes crosscut the ductile strike-slip shear fabrics at Yuanjiang, in the Ailao Shan. Low-angle normal faulting along the margins of the metamorphic massif accommodated final exhumation of the Red River gneisses. Using published U-Th-Pb and 40 Ar/ 39 Ar ages of granites along the shear zone, the age of left-lateral ductile shearing along the RRSZ can be constrained as between the earlier deformed leucogranites (31.9–24.2 Ma) and the later crosscutting dykes (21.7 Ma) with exhumation-related cooling continuing until ∼17 Ma.