CONTRASTING EXHUMATION HISTORIES ALONG A CRUSTAL-SCALE STRIKE-SLIP FAULT ZONE: THE EOCENE TO MIOCENE AILAO SHAN-RED RIVER SHEAR ZONE IN SOUTHEASTERN TIBET

Four metamorphic massifs in southeastern Tibet – the Xuelong Shan (XLS), Diancang Shan (DCS), Ailao Shan (ALS) and Day Nui Con Voi (DNCV) – have been suggested to constitute coherent parts of the Ailao Shan-Red River (ASRR) shear zone. Results of ⁴⁰Ar/³⁹Ar dating, zircon and apatite fission track reveal the cooling/exhumation histories of the crustal-scale shear zone by determining the cooling paths of the ALS massif. The ⁴⁰Ar/³⁹Ar results of amphibole and biotite from the southern ALS massif yield amphibole ages between 27.5 and 29.2 Ma and biotite ages between 23.10 and 26.30 Ma. One zircon sample yields a fission track age of 23 Ma, and three apatite samples yield fission track ages of 9.7 ± 1 Ma, 13 ± 2 Ma, and 8.9 ± 2 Ma, respectively. The amphibole and biotite ⁴⁰Ar/³⁹Ar ages do not exhibit obvious age-elevation correlations along the ENE-WSW sections across the ALS massif, indicating that the massif was likely homogeneously exhumed with respect to the E-W sections. Combining the present results with previously published ages, the results show that the entire ALS massif has experienced two stages of diachronous cooling histories, i.e., an early rapid cooling of ca. 45 to 715°C/Myr and a late slow cooling of 10 to 17°C/Myr, and the transition from the early rapid cooling to the late slow cooling occurred at a temperature of ca. 300°C across almost the entire ALS massif. Comparing these paths to those of the DCS massif, which featured an early slow cooling of 11 to 17°C/Myr and a late rapid cooling of 30 to 140°C/Myr, the cooling history of ALS differ from that of the DCS massif along the same shear zone.

This study confirms that differing cooling/exhumation histories exist along the same crustal-scale shear zone. From the contrasting cooling/exhumation histories, we would argue that the ALS and DCS massifs are isolated units with different thermal histories during the Oligo-Miocene tectonic evolution of southeastern Tibet. Detailed structural analysis of the massifs along the Ailao Shan – Red River (ASRR) shear zone is necessary to elucidate the structural evolution and structural relationship between the massifs.