GEOMETRY AND SENSE OF MOTION ALONG THE VIETNAM CONTINENTAL-MARGIN - ONSHORE OFFSHORE DA NANG AREA/

The continental margin of the Indochina peninsula has recorded the tec tonic processes related to the opening of the south China Sea Basin. T he geometry and chronology of the Cenozoic structures linked to the op ening of this basin were studied in detail in Da Nang area (central Vi etnam). An integrated onshore and offshore study was conducted, combin ing onland Landsat-Spot imagery interpretation and fracture analysis w ith marine geophysical data collected during the 1993 Ponaga cruise (R /V L'Atalante) and completed by previously unpublished industrial data . On land, two superposed fault sets were observed on remote sensing d ata: large N140 degrees E trending strike-slip faults are observed to link with E-W trending normal faults, and major N-S trending strike-sl ip faults are connected to N050 degrees E faults. At this scale of obs ervation, no significant displacement can be demonstrated along these two strike-slip fault systems which show a cross-cutting map geometry without visible offset. The data collected offshore demonstrate that t he N-S fault pattern is developed strongly, but there is no evidence o f major N140 degrees E fault orientations. Important crustal extension on shallow-dipping fault planes striking N050 degrees E and connected to the N-S strike-slip faults, is compatible with dextral motion alon g these N-S faults. The discrepancy between the amount of motion obser ved onshore and offshore may be explained by the presence of a major N -S trending shear zone along the Vietnam margin. This shear zone is lo cated in the region of the N-S Qui Nhon ridge, which is blanketed by M iddle Miocene carbonates. Dextral motion along this shear zone became insignificant regionally during the late early Miocene (around 20 Ma), when carbonates deposition started. We suggest that central Vietnam r ecords two distinct, and partly coeval, events of the strike-slip faul ting that have contributed to the opening of the south China Sea: sini stral NW shear, then dextral N-S shear south of Hainan.