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ROBERTS,ALAN, Badley Earth Sciences, UK; LENG SIANG GOH, MAT DE JONG, HAGE VELTMEYER, Conoco Vietnam,The Netherlands; ROBERT HOOPER, Conoco UK, Aberdeen

Abstract: Quantitative Analysis of Structural Development and Subsidence in the Nam Con Son Basin, Offshore Vietnam: Differential Stretching ahead of a Propagating Ocean?

As part of Conoco's exploration of the Nam Con Son Basin, offshore southern Vietnam, a number of regional geological profiles have been analysed using quantitative techniques for structural and stratigraphic basin-modelling. The modelling aims were two-fold. First, to estimate fault-block extension, footwall-uplift and erosion, in order that their likely impact on prospectivity would be known. Second, to assess the magnitude of basement driving-subsidence and related heat-flow anomaly in the basin for input to thermal modelling studies.

The most obvious structures in the basin are of Middle Miocene age, defining classic tilted fault-block topography. More poorly defined evidence of older fault movement, thought to have been Eocene/Oligocene in age, is also apparent.

With respect to the Middle Miocene rift the stratigraphy of the basin can be assigned to three tectonostratigraphic sequences. A pre-rift sequence of Oligocene-Early Miocene age (thought to rest on basement), a syn-rift sequence of Middle Miocene age and a post-rift sequence of Late Miocene-Recent age.The latter sequence includes the Mekong Delta.

Forward modelling of the fault geometries associated with the Middle Miocene rift shows it to be a typical intra-continental rift, associated with maximum stretching factors (b) of ~1.15 (15% extension). Maximum predicted footwall uplift of the major fault block crests is of the order of 1km. This moderate amount of stretching cannot, however, explain the thickness of the Late Miocene-Recent post-rift sequence, backstripping of which requires larger stretching factors, nor can it explain the presence of the thick Oligo-Miocene pre-rift sequence.

Previously the Oligocene-Early Miocene sequence has been interpreted as the post-rift sequence to the poorly-defined Eocene/Oligocene rift. Analysis of the thickness of this sequence on our profiles, however, shows that towards the SW of the Nam Con Son Basin driving-subsidence of at least b=2 is required to generate the observed stratigraphic thickness, while further to the NE a b of at least 5 is required to explain the thickness of both the pre-rift and the younger Mekong Delta (post-rift). It is not possible to reconcile a b=2 with the sketchy observations of the early rift, let alone an extreme value of b=5.

A recent paper by Huchon et al (1998) has confirmed large values of both stretching and crustal thinning in the Nam Con Son Basin. Our structural and stratigraphic work is consistent with the regional gravity interpretation of these authors.

We suggest that something other than "simple" intra-continental rifting has controlled subsidence of the Nam Con Son Basin. Our principal observation is that stretching factors derived by subsidence analysis (backstripping) are larger than stretching factors derived by analysis of fault-block extension. This observation has recently been made at a number of other basins adjacent to passive margins (, Roberts et al 1997).

The suggestion put forward to explain this observation at passive margins is that the effects of differential stretching within the crust and upper mantle are being documented, with the ductile lower crust and mantle extending more than the brittle upper crust. The differential stretching is related to the dynamics of continental break-up. The Nam Con Son Basin lies.ahead of the SW-propagating tip of the Eastern Sea or Bien Dong (or South China Sea) oceanic crust (33-17Ma in age). In the Nam Con Son Basin we may (for the first time) be observing the subsidence effects of differential stretching ahead of a propagating ocean rather than at its flanks. On our profiles the anomalously-large subsidence is greater towards the NE, nearer the ocean, in keeping with a dynamic link related to ocean formation.

AAPG Search and Discovery Article #[email protected] International Conference and Exhibition, Birmingham, England