NEW CONSTRAINTS ON EXHUMATION OF THE AILAO SHAN SHEAR ZONE FROM TERTIARY SEDIMENTS, YUNNAN PROVINCE, CHINA

The mid-Cenozoic Ailao Shan shear zone figures prominently in interpretations of Southeast Asian tectonics. The shear zone nominally extends ~1000 km from the South China Sea into southeast Tibet, and its northeastern margin is coincident with the Red River and the active Red River fault. Tertiary sediments preserved to the northeast record the nature and timing of deformation within the shear zone. Field based mapping indicates that similar units along the entire length of the Red River are dominantly conglomeratic, inter-bedded with rare sands and fine-grained fossil leaf-bearing intervals. Sediment derived from shear zone gneisses is found only in the upper part of the Tertiary section, which contains Miocene fossils. These sediments are fluvial in origin, intermingled with alluvial material shed from the adjacent shear zone. Sediments are a few km thick at most, confined to within 5 km of the Ailao Shan. Our observations are consistent with geochronologic results indicating that deformation began as early as 34 Ma (Gilley et al., 2003) with the onset of rapid cooling and exhumation ~25 Ma (Harrison et al., 1996). Exhumation must have ended before the formation of a regional low-relief erosion surface and onset of brittle deformation along the Red River fault, probably in the early Pliocene.

Rapid exhumation of the Ailao Shan shear zone has been interpreted to have occurred in transtension, accommodated by a normal fault on the northeast margin of the shear zone. This is supported by the rapid and extensive nature of exhumation, the sharp contrast in metamorphic grade along the northeast boundary of the shear zone, and thermochronologic results (Harrison et al., 1996). However, we observe consistent and pervasive shortening structures within the Tertiary section. Gouge zones within weak Tertiary lithologies dip 45° to the southwest, and are distinct from vertical shear deformation associated with the active Red River fault. Tertiary strata are folded northeast of these gouge zones, and synclinal troughs form where rocks from the northeast are thrust over the basin short distances to the southwest on steep faults. This syn-depositional deformation indicates significant shortening during exhumation of the shear zone. Our field data thus suggest mid-Tertiary transpressional exhumation of the Ailao Shan shear zone.