AN EARLY MIOCENE TRANSITION IN DEFORMATION REGIME WITHIN THE RED RIVER FAULT ZONE, YUNNAN, AND ITS SIGNIFICANCE FOR INDO-ASIAN TECTONICS

Recent work has tended to support the view that lateral extrusion of Indochina along the Red River fault zone was a significant factor in accommodating the convergence of India with Asia. To better understand the nature and timing of this feature, 40Ar/39Ar age spectrum analyses were undertaken on minerals from the Ailao Shan/Red River metamorphic belt in southern Yunnan, China. Interpreted using diffusion domain theory, these results yield thermal histories in the temperature range 550-150-degrees-C. Discontinuities in cooling rates observed from these results suggest a transition in deformation style in the shear zone during the early Miocene. At least in the vicinity of our samples, left-lateral, strike-slip ductile deformation appears to have ceased by about 20 Ma. The mylonitic gneisses in the shear zone were subsequently obliquely unroofed, probably due to a component of normal faulting along the eastern edge of the shear zone causing a brief (locally < 1 m.y.) but rapid (> 100-degrees-C/m.y.) cooling episode. This transition may have resulted from a component of northeast-southwest extension attributable to greater clockwise rotation of Indochina relative to South China after approximately 20 Ma than that consistent with pure strike-slip along the fault. K-Ar ages on clay minerals from gouge within the normal faults bounding the Ailao Shan and Diancang Shan yield a range of ages between 20 and 185 Ma but show a significant clustering at 20-25 Ma. These results suggest that early Miocene movement on a precursor to the Range Front normal fault in the Ailao Shan caused oblique unroofing of the shear zone. This early Miocene event corresponds in time with a transition from low to very high denudation rates throughout the Himalaya and southern Tibet. This coincidence may reflect that once left-lateral strike-slip motion on the Red River fault zone slowed or was terminated, accommodation of much of the continued northward convergence of India occurred on the Main Central Thrust resulting in rapid uplift in the Tethyan Himalaya and Gangdese belt.