CENOZOIC DEFORMATION OF CENTRAL AND SOUTH VIETNAM

The pre-Tertiary basement of central and south Vietnam is affected by pervasive strike-slip and normal faulting, which appears to control th e shape of the present continental margin of eastern Indochina. Using remote sensing and field studies, we show the existence of two superpo sed strike-slip fault systems which were probably active during the Pa leogene and early Neogene, respectively, The older system consists of large NW-SE left-lateral strike-slip faults, parallel to the Red River Fault, compatible with an E-W maximum shortening axis, In south Vietn am, conjugate N50 degrees E right-lateral faults are also present, rea ctivating pre-existing Paleozoic and Mesozoic faults, The younger faul t system consists of dominant N160 degrees E to N-S right-lateral faul ts, compatible with a N10-30 degrees E maximum shortening axis, These N-S-trending dextral strike-slip faults are parallel to the escarpment limiting the continental margin of Vietnam, south of Da Nang. Some of the N-S and N50 degrees E faults have been reactivated locally as nor mal faults, especially during the uplift of central and south Vietnam, which was associated with voluminous late Neogene-Quaternary basaltic volcanism. These new field data show that eastern Indochina was affec ted by the collision of India with Eurasia, first through pervasive NW -SE left-lateral strike-slip faulting compatible with the extrusion of Indochina, and then through N160 degrees E to N-S right-lateral fault ing, Thus, a 90 degrees rotation of the strain pattern occurred over I ndochina as the collision proceeded and as the eastern syntaxis of the Himalayas migrated northward. We propose that the younger strain patt ern is compatible with the existence of a large right-lateral sub-meri dian shear zone over the eastern margin of Indochina as the South Chin a Sea basin was opening.