Abstract

The 1000 km long NW–SE-striking, left-lateral Ailao Shan–Red River shear zone runs from the southeastern corner of Tibet to the Gulf of Tonkin and the South China Sea. It has been used as the prime example of a lithospheric-scale strike-slip fault that has accommodated between 500 and 1000 km of southeastwards extrusion of Indo-China away from the Indian plate indentor. Central to the model of continental extrusion is that such faults cut through the entire lithosphere, that shear heating resulted in high-grade metamorphism and local anatexis, and that the ages of sheared granites along the fault also date the timing of strike-slip shearing. However, structural data from the Red River shear zone clearly show that vertical strike-slip faulting post-dated metamorphism and granite emplacement. Most granites along the shear zone are mantle-related granodiorites or within-plate alkali granites formed prior to shearing along the Red River shear zone. Left-lateral kinematic indicators are ubiquitous within the Red River mylonites, but they are always lower-temperature fabrics, formed after peak sillimanite metamorphism and after granite crystallization. It is suggested that left-lateral strike-slip shearing along the Red River shear zone started after 21 Ma, not at 35 Ma as previously thought, and the fault was purely a crustal structure. None of the geological features used to propose the 500–1000 km offsets are robust, and the total finite offset remains unknown.

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