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Abstract

Salt tectonics in passive continental margin settings is investigated using a 2D vertical cross-sectional finite element numerical model of frictional-plastic sedimentary overburden overlying a linear viscous salt layer. We present preliminary results concerning the effects of sediment progradation over salt, buoyancy driven flow owing to density contrast between the sediment and salt, regional tilt of the salt layer, and local isostatic adjustment of the system. Sediment progradation causes a differential load on the underlying salt, which can cause the system to become unstable, leading to landward extension accommodated by seaward distal contraction. Slow progradation (Vsp = 0.5 cm/yr) of slightly aggrading sediments gives a diachronous evolution comprising four main phases: (1) initiation of salt channel flow and the formation of mini-basins and associated diapirs; (2) onset of listric normal growth faulting and extension of the sedimentary overburden; (3) large-scale evacuation of the salt, formation of prerafts and rafts, and inversion of the mini-basins; and (4) formation of a contractional allochthonous salt nappe that thrusts over the depositional limit of the salt.

Buoyancy effects are investigated using models with density contrasts between overburden and sediment of 0, 100, and 400 kg/m3. Although the lateral flow driven by differential loading dominates in all cases, the form of the mini-basins, the overall salt evacuation, and style of diapirism are sensitive to buoyancy forces, as the large density contrast produces the most developed diapiric and mini-basin structures.

A regional seaward tilt of 0.2° (of the type that may be produced by thermal contraction of the rifted margin) enhances and accelerates the overall seaward flow of the unstable slope leading to much earlier overthrusting of the distal depositional limit of the salt. The added down-slope gravitational component also modifies the style of the mini-basins by enhancing the horizontal channel flow by comparison with the vertical buoyancy driven flow. This reduces the apparent efficiency of diapirism.

Local isostatic adjustment, owing to overburden and water loading, introduces a landward tilt of the system, thereby requiring salt to flow updip against gravity during evacuation. Isostasy also changes the overburden geometry and, therefore, modifies the stability and flow velocity of the extending overburden through the increased strength of the isostatically thickened proximal overburden, and through the modified differential pressure acting on the salt under these circumstances. The seaward flow of the unstable slope region is slower for the same overburden progradation velocity, more salt remains beneath the shelf as rollers and pillows during evacuation, counter-regional faults are more pronounced, and the allochthonous salt nappe progressively climbs above the isostatically adjusting sediments as it overthrusts.

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