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).