STRUCTURAL, PETROLOGICAL AND THERMAL EVOLUTION OF A TERTIARY DUCTILE STRIKE-SLIP SHEAR ZONE, DIANCANG SHAN, YUNNAN

The Diancang Shan, a horst massif within the Red River fault zone in Yunnan, People's Republic of China, preserves a structural, petrological, and thermal record of two distinct phases of tectonic activity: a left-lateral ductile shear that terminated between 20 and 17 Ma and a ductile-to-brittle phase of normal faulting which began at 4.7 Ma and remains active. Mylonitic rocks in the core of the range display an early, steep, high-temperature (HT), schistosity and a horizontal stretching lineation that are both parallel to the trend of the belt. Kinematic indicators indicate that shear was left-lateral. The complex shape of the HT schistosity at the southern termination of the massif likely results from a large-scale, oblique, left-lateral C' shear plane that dismembered the shear zone and separated the Ailao Shan and the Diancang Shan as left-lateral deformation terminated. Thermochronological and thermobarometric results suggest that the gneisses were partially unroofed during this event. Along the eastern edge of the Diancang Shan, the HT fabrics were overprinted by low-temperature structures during activation of east clipping normal faults. Cooling associated with this normal/right-lateral faulting along the Diancang Shan (and perhaps activation of the right-lateral/normal movement on the Range Front fault farther south along the Ailao Shan) began at 4.7+/-0.1 Ma. These results tend to support the view that extrusion of Indochina occurred along the left-lateral Red River shear zone between 35 and 19-17 Ma. Initiation of right-lateral/normal slip during the late Miocene may relate to eastward extensional collapse of the thickened Tibetan crust or more probably, to initiation of the second phase of extrusion.