Direct dating of left‐lateral deformation along the Red River shear zone, China and Vietnam

Exposures of high‐grade, midcrustal rocks within the Red River shear zone (RRSZ), which separates the Indochina and South China blocks, exhibit clear evidence of left‐lateral, ductile deformation. Assuming that the South China Sea represents a pull‐apart basin formed at the southeastern termination of the RRSZ, it has been argued that seafloor magnetic anomalies constrain the timing of sinistral slip accommodated by the RRSZ between ∼32 and 17 Ma at a rate of ∼4 cm/yr. While 40 Ar/ 39 Ar thermochronometry indicates that left‐lateral slip occurred along the RRSZ between 25 and 17 Ma, the timing of earlier high‐temperature deformation has not been directly constrained. In situ Th‐Pb ion microprobe dating of monazite inclusions in garnets allows direct assessment of the timing of amphibolite‐grade metamorphism and synchronous left‐lateral shearing. Results from northern segments of the RRSZ in Yunnan, China, indicate that synkinematic garnet growth occurred between 34 and 21 Ma and are the first to document late Oligocene metamorphism and left‐lateral shearing. Data from the southern RRSZ within Vietnam are complicated by Tertiary overprinting of rocks that experienced amphibolite facies metamorphism during the Indosinian orogeny (∼220 Ma). The period during which sinistral deformation is now constrained to have occurred along the RRSZ (i.e., 34–17 Ma) is essentially coincident with spreading of the South China seafloor (32–17 Ma). This temporal and kinematic link between left‐lateral shearing along the RRSZ and opening of the South China Sea supports the view that Indochina was extruded from Asia as a block along lithospheric‐scale strike‐slip faults.