New constraints on the structure, thermochronology, and timing of the Ailao Shan‐Red River shear zone, SE Asia

Abstract

New structural, petrographic, and 40 Ar/ 39 Ar data constrain the kinematics of the ASRR (Ailao Shan‐Red River shear zone). In the XueLong Shan (XLS), geochronological data reveal Triassic, Early Tertiary, and Oligo‐Miocene thermal events. The latter event (33–26 Ma) corresponds to cooling during left‐lateral shear. In the FanSiPan (FSP) range, thrusting of the SaPa nappe, linked to left‐lateral deformation, and cooling of the FSP granite occurred at ≈35 Ma. Rapid cooling resumed at 25–29 Ma as a result of uplift within the transtensive ASRR. In the DayNuiConVoi (DNCV), foliation trends NW‐SE, but is deflected near large‐scale shear planes. Stretching lineation is nearly horizontal. On steep foliations, shear criteria indicate left‐lateral shear sense. Zones with flatter foliations show compatible shear senses. Petrographic data indicate decompression from ≈6.5 kbar during left‐lateral shear (temperatures >700°C). 40 Ar/ 39 Ar data imply rapid cooling from above 350°C to below 150°C between 25 and 22 Ma without diachronism along strike. Along the whole ASRR cooling histories show two main episodes: (1) rapid cooling from peak metamorphism during left‐lateral shear; (2) rapid cooling from greenschist conditions during right‐lateral reactivation of the ASRR. In the NW part of the ASRR (XLS, Diancang Shan), we link rapid cooling 1 to local denudations in a transpressive environment. In the SW part (Ailao Shan and DNCV), cooling 1 resulted from regional denudation by zipper‐like tectonics in a transtensive regime. The induced cooling diachronism observed in the Ailao Shan suggests left‐lateral rates of 4 to 5 cm/yr from 27 Ma until ≈17 Ma. DNCV rocks always stayed in a transtensive regime and do not show cooling diachronism. The similarities of deformation kinematics along the ASRR and in the South China Sea confirms the causal link between continental strike‐slip faulting and marginal basin opening.