THE AILAO SHAN-RED RIVER SHEAR ZONE (YUNNAN, CHINA), TERTIARY TRANSFORM BOUNDARY OF INDO-CHINA

The Red River Fault zone (RRF) is the major geological discontinuity t hat separates South China from Indochina. Today it corresponds to a gr eat right-lateral fault, following for over 900 km the edges of four n arrow (< 20 km wide) high-grade gneiss ranges that together form the A ilao Shan-Red River (ASRR) metamorphic belt: the Day Nui Con Voi in Vi etnam, and the Ailao, Diancang and Xuelong Shan in Yunnan. The Ailao S han, the longest of those ranges, is fringed to the south by a strip o f low-grade schists that contain ultramafic bodies. The ASRR belt has thus commonly been viewed as a suture. A detailed study of the Ailao a nd Diancang Shan shows that the gneiss cores of the ranges are compose d of strongly foliated and lineated mylonitic gneisses. The foliation is usually steep and the lineation nearly horizontal, both being almos t parallel to the local trend of the gneissic cores. Numerous shear cr iteria, including asymmetric tails on porphyroclasts, C-S or C'-S stru ctures, rolling structures, asymmetric foliation boudinage and asymmet ric quartz (c) axis fabrics, indicate that the gneisses have undergone intense, progressive left-lateral shear. P-T studies show that left-l ateral strain occurred under amphibolite-facies conditions (3-7 kb and 550-780 degrees C). In both ranges high-temperature shear was coeval with emplacement of leucocratic melts. Such deformed melts yield U/Pb ages between 22.4 and 26.3 Ma in the Ailao Shan and between 22.4 and 2 4.2 Ma in the Diancang Shan, implying shear in the Lower Miocene. The mylonites in either range rapidly cooled to approximate to 300 degrees C between 22 and 17 Ma, before the end of left-lateral motion. The si milarity of deformation kinematics, P-T conditions, and crystallizatio n ages in the aligned Ailao and Diancang Shan metamorphic cores, indic ate that they represent two segments of the same Tertiary shear zone, the Ailao Shan-Red River (ASRR) shear zone. Our results thus confirm t he idea that the ASRR belt was the site of major left-lateral motion, as Indochina was extruded toward the SE as a result of the India-Asia collision. The absence of metamorphic rocks within the 80 km long ''Mi du gap'' between the gneissic cores of the two ranges results from sin istral dismemberment of the shear zone by large-scale boudinage follow ed by uplift and dextral offset of parts of that zone along the Quater nary Red River Fault. Additional field evidence suggests that the Xuel ong Shan in northern Yunnan and the Day Nui Con Voi in Vietnam are the northward and southward extensions, respectively, of the ASRR shear z one, which therefore reaches a length of nearly 1000 km. Surface balan ce restoration of amphibolite boudins trails indicates layer parallel extension of more than 800% at places where strain can be measured, su ggesting shear strains on the order of 30, compatible with a minimum o ffset of 300 km along the ASRR zone. Various geological markers have b een sinistrally offset 500-1150 km by the shear zone. The seafloor-spr eading kinematics in the South China Sea are consistent with that sea having formed as a pull apart basin at the southeast end of the ASRR z one, which yields a minimum left-lateral offset of 540 km on that zone . Comparison of Cretaceous magnetic poles for Indochina and South Chin a suggests up to 1200 +/- 500 km of left-lateral motion between them. Such concurrent evidence implies a Tertiary finite offset on the order of 700 +/- 200 km on the ASRR zone, to which several tens of kilomete rs of post-Miocene right-lateral offset should probably be added. Thes e results significantly improve our quantitative understanding of the finite deformation of Asia under the thrust of the Indian collision. W hile being consistent with a two-stage extrusion model, they demonstra te that the great geological discontinuity that separates Indochina fr om China results from Cenozoic strike-slip strain rather than more anc ient suturing. Furthermore, they suggest that this narrow zone acted l ike a continental transform plate boundary in the Oligo-Miocene, gover ning much of the motion and tectonics of adjacent regions. 700 and 200 km of left-lateral offset on the ASRR shear zone and Wang Chao fault zone, respectively, would imply that the extrusion of Indochina alone accounted for 10-25% of the total shortening of the Asian continent. T he geological youth and degree of exhumation of the ASRR zone make it a worldwide reference model for large-scale, high-temperature, strike- slip shear in the middle and lower crust. It is fair to say that this zone is to continental strike-slip faults what the Himalayas are to mo untain ranges.