STRUCTURAL AND GEOCHRONOLOGICAL CONSTRAINTS ON THE EXHUMATION OF METAMORPHIC ROCKS ALONG THE CHONG SHAN SHEAR ZONE, WESTERN YUNNAN, CHINA

A significant part of the post-collisional northward indentation of India into Asia has been accommodated by the Tertiary Gaoligong Shan and Chong Shan shear zones (CSSZ), as well as by the left-lateral displacement along the better known Ailao Shan-Dianchang Shan shear zones that lie farther E in Yunnan, China. A striking feature of the geology in this region is that all of the ductile shear zones composed of mylonitized mid-crustal rocks, metamorphosed at middle to upper amphibolite facies, are structurally high relative to adjacent low-grade to unmetamorphosed rocks. We present a structural and geochronological study of the CSSZ that constrains the deformation-time path of progressive transpression and exhumation. The CSSZ is composed of mylonitic augen gneiss, migmatite, and metasedimentary rocks. Foliation dips moderately to steeply (>60º) west. Stretching lineations are sub-horizontal (0 - 15º), consistent with dominantly strike-slip shear. Kinematic indicators such as rotated feldspar porphyroclasts and S-C fabrics are common at microscopic and mesocopic scales and indicate dextral and sinistral shear. Boudinage structures, indicating both horizontal and vertical stretching, affected compositional bands in mylonitic rocks formed during earlier stages of deformation and leucogranite sills emplaced during later stages of deformation. Monazite with weak sector zoning from a syn-deformational leucogranite yielded ²⁰⁷Pb/²³⁵U dates of ca. 38 - 34 Ma. ⁴⁰Ar/³⁹Ar dates of biotite from this sample suggest relatively slow cooling after emplacement, cooling below ~370ºC by ca. 20 Ma. We hypothesize that mid-crustal material along the CSSZ moved vertically by both thrusting and upward extrusion during transpressive shortening following India-Eurasia collision. The CSSZ is presently bounded by sub-vertical and discrete faults on either side, the eastern one being continuous for >1000 km. The contribution of these faults to the exhumation of the CSSZ remains unclear. However, it is clear that the CSSZ is the third major middle Cenozoic shear zone east of the Eastern Himalayan syntaxis.