Structural evolution of the Nam Con Son Basin: quantitative fault analysis applied to a 3-simensional seismic dataset

Abstract

The Nam Con Son Basin is one of several Tertiary rift basins located on the continental shelf, offshore Indochina. It has received relatively little attention until recently and remains a poorly understood sedimentary basin. The discovery and production of significant volumes of hydrocarbons from within fault-bounded structural highs has spurred a drive to better understand the basin structure. Previous interpretations of regional 2-dimensional seismic data suggested a complex structure, with E-W trending Eocene-Oligocene faults, overprinted by a rhomboidal pattern of mainly N-S and NE- SW trending Miocene faults and the possible involvement of compressional and/or strike-slip tectonics. The driving mechanism for extension on the continental shelf is not fully understood, but the Cenozoic structural evolution was undoubtedly influenced by a heterogeneous crust cut by a number of pre-existing Palaeozoic and Mesozoic structures. Crucially, the western extent of South China Sea rifting in the region may have been controlled by the -N-S trending edge of the Indochina craton, a major rheological boundary that likely underlies the Nam Con Son Basin. In addition to its economic importance, the proximity of the basin to the tip of the South China Sea, the possible involvement of wrench tectonics and the potential influence of pre-existing fabrics, makes it an ideal target for academic study. A 3-dimensional seismic dataset from the centre of the Basin has been used to build a fault/horizon model of the Early-Middle Miocene syn-rift sequence. The faults have a wide range of orientations and, in all cases, the hanging wall is down-thrown consistent with apparent normal offsets; there is no evidence for strike-slip or reverse faulting. By accurately modelling the fault surfaces and the fault/horizon intersections, the dip, strike, dip-azimuth and offset of 225 faults have been calculated and used to quantitatively analyse the fault sample. The timing of fault activity has been constrained for each fault and shows that all the faults are broadly contemporary but that the number of active faults increased through time. This is partly attributed to continuing subsidence on graben bounding faults" which focussed faulting and extension within the graben. There is no correlation between fault strike and fault age and the faults show no significant rotation about vertical axes with progressive deformation. The range of fault orientations likely reflects the conflicting influences of the -NW-SE regional extension direction and the dominant -N-S trending basement fabric. A systematic variation in maximum dip-slip fault offset (defined as the distance between the footwall and hangingwall cutoffs measured parallel to the fault surface) with fault strike has revealed an 'ideal' fault orientation, which can be used to infer the direction of maximum horizontal extension. The relationship between this extension direction, the dominant pre-existing basement fabric and the regional displacement direction is consistent with a model of moderately extension-dominated dextral transtension for the Nam Con Son Basin synchronous with the opening of the South China Sea. At a local scale, the influence of the underlying Eocene-Oligocene structures has produced offset depocentres, along strike fault polarity flips, and complex accommodation zones.