Philippe Herve Leloup

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

[1]xa0Exposures 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 40Ar/39Ar 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.

High-temperature metamorphism in a major strike-slip shear zone: the Ailao Shan—Red River, People's Republic of China

Abstract Petrographic and thermobarometric analysis provides constraints on thePT path of the mylonitic gneisses of the left-lateral Ailao Shan-Red River shear zone which has accommodated the lateral extrusion of Indochina during the Tertiary. Two different paragenesis, P1 and P2, are coeval with this deformation and correspond respectively to the amphibolite and greenschist facies. Microprobe analysis reveals that P1 garnets bear a chemical zonation from core to rim. This zonation indicates a temperature increase during garnet growth. Conditions of formation of garnet rims (P1b), which are estimated using biotite-garnet and plagioclase-garnet thermobarometers, are close to the granitic solidus (710 ± 70°C and4.5 ± 1.5 kbar). P2 conditions are estimated to be approximately 500°C and

Bounds on strain in large Tertiary shear zones of SE Asia from boudinage restoration

We have used surface-balanced restoration of stretched, boudinaged layers to estimate minimum amounts of finite strain in the mylonitic gneisses of the Oligo-Miocene Red River-Ailao Shan shear zone (Yunnan, China) and of the Wang Chao shear zone (Thailand). The layer-parallel extension values thus obtained range between 250 and 870%. We discuss how to use such extension values to place bounds on amounts of finite shear strain in these large crustal shear zones. Assuming simple shear, these values imply minimum total and late shear strains of, respectively, 33 ± 6 and 7 ± 3 at several sites along the Red River-Ailao Shan shear zone. For the Wang Chao shear zone a minimum shear strain of 7 ± 4 is deduced. Assuming homogeneous shear would imply that minimum strike-slip displacements along these two left-lateral shear zones, which have been interpreted to result from the India-Asia collision, have been of the order of 330 ± 60 km (Red River-Ailao Shan) and 35 ± 20 km (Wang Chao).