HETEROGENEOUS EXTRUSION ALONG THE TRANSPRESSIVE RED RIVER SHEAR ZONE, SOUTHEAST ASIA

The >1000 km long Red River shear zone (RRSZ) stretching from SE Tibet to the South China Sea is thought to have allowed the Palaeogene extrusion of the Indochina block to the southeast during the indentation of India into Asia. The deformation in the RRSZ is commonly thought to be pervasively distributed across a 10 km wide band of high grade mylonitic gneiss. This interpretation is made tentative by extensive Mesozoic and Cenozoic sedimentary cover over very limited outcrop of crystalline rocks that could preserve the history of deformation. However, an Eocene unconformity above the crystalline rocks in the Daincang Shang block 10 Km west of Dali, Yunnan province, China puts these rocks at the surface about 50 Ma. In this study we examine the microstructure and geochronology of this block exposed along the west wall of the RRSZ. Here we find cataclastic rocks derived from these gneisses to be overprinted by steeply eastward dipping lower greenschist facies phyllitic fabrics. To establish the thermal history of these rocks under the unconformity we analyze K-feldspar by ⁴⁰Ar/³⁹Ar methods. Steps in the age spectrum of one unaltered K-feldspar sample climb from 10-41 Ma establishing both Middle Eocene resetting and Eocene to Late Miocene cooling from about 250°Cto 150°C. White mica extracted from cross-cutting phyllitic folia produce an age spectrum that climbs from 19 to 22 Ma. This age range between the two K-feldspar cooling ages and demonstrates that this mica crystallized at about 20 Ma at about 200°C. We interpret these results to reflect slow exhumation from Middle Eocene to Late Oligocene. Here, steep phyllitic fabrics associated with the RRSZ did not develop until Early Miocene. These results are in contrast with geochronology from this and other crystalline rocks on and across strike where monazite grains and zircon rims yield Oligocene U-Pb ages suggesting lower crustal amphibolite facies conditions at this time. One way to reconcile these contrasting results is to infer that deformation along the RRSZ is causing the extrusion of several very thin (km- scale) fault slivers some exhuming much faster than the apparently thin sliver we have identified in this study.