Deformational and sedimentary responses to Late Miocene (13-8.5 Ma) left-lateral oblique movement along the northern foreland of the Red River-Ailao Shan shear zone, Yunnan, China

The arcuate Ailao Shan-Red River shear zone, one of the largest Cenozoic strike-slip faults within the Tibetan plateau, experienced the left-lateral and right-lateral strike-slip motions, occurred in late Oligocene to Miocene time (24-17 Ma) and late Cenozoic time (2-4 Ma), respectively (Allen et al., 1984; Tapponnier et al., 1986; Harrison et al., 1996, Schoenbolhm, et al., 2004). The question underlying this study is what happened in the interval between these two events. Our reconstruction of the range-front deformation and sedimentation of the Ailao Shan shear zone (ASSZ) demonstrates that the eastern range-front of the southeastern segment with a strike of 290°-110° used to be dominated by low-angle normal faulting, which resulted in contemporaneously coarse-grained sedimentation associated with NE-SW shortening by gravitational gliding that occurred within a narrow half-graben along the foreland of the southeastern segment of the shear zone. These coarse-grained sediments, namely Ailao Shan conglomerate, are late Oligocene to Miocene in age, dated by fossils. They are in unconformable depositional contact on pre-Cenozoic rocks of the Yangtze block to the north, with the unconformity cropping out along the north side of the Red River valley, and consistently dip to the southwest against the mylonitic gneiss along the ASSZ with dipping angle decreasing upward from 60° to sub-horizontal, characteristic of growth strata. The Ailao Shan conglomerate in the Mosha area is dated as 11.8-8.5 Ma by paleomagstratigraphy, which is approximately coeval with a rapid cooling event of the ASSZ, occurred from 13 to 10 Ma (FT dating, Bergman et al., 1997). All these evidence suggest that the ASSZ was highly active during middle to late Miocene (13-8.5 Ma). We interpret that the low angle normal faulting to have been controlled by the change in geometric pattern of the ASSZ. The ASSZ lies within a wide zone of the north-south right-lateral shear, generated by the relative movement between the Indian subcontinent and the South China block, within which the crustal fragments are undergoing clockwise rotation (Wang and Burchfiel, 1997). Thus, the northwestern segment of the ASSZ with a strike of 320°-140° must have experienced clockwise bending. As the bending proceeded with time, the oblique extension was generated in a direction oblique to the southeastern segment of the shear zone, by which the low-angle normal faulting occurred along the range front of the southeastern segment. In summary, the ASSZ may have had two stages of development. The first stage occurred between 24-17 Ma, dominated by left-lateral displacement with a minor component of the normal displacement, which juxtaposed the mylonite with various rocks of the Yangtze block along a high-angle normal fault bounding the shear zone on the east. The second stage occurred between 13-8.5 Ma, dominated by the low-angel normal faulting along the range-front of the southeastern segment of the shear zone, associated with the contemporary coarse-grained sedimentation.