Numerical Modeling of Salt Tectonics on Passive Continental Margins: Preliminary Assessment of the Effects of Sediment Loading, Buoyancy, Margin Tilt, and Isostasy

Steven Ings¹, Christopher Beaumont², and Lykke Gemmer²

¹Department of Earth Sciences, Dalhousie University, Halifax, NS, B3H 3J5, Canada

sings@dal.ca

²Department of Oceanography, Dalhousie University, Halifax, NS, B3H 4J1, Canada

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 the 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 may lead to instability of the system with landward extension accommodated by seaward distal contraction. Slow progradation (V_sp = 0.5 cm/yr) of aggrading sediments gives a diachronous evolution comprising four main phases: 1) initiation of salt channel flow and the formation of minibasins 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 pre-rafts and rafts, and inversion of the minibasins; 4) formation of a contractional allochthonous salt nappe that overthrusts 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 minibasins, the overall salt evacuation, and style of diapirism are sensitive to buoyancy forces, with the large density contrast producing the most developed diapiric and minibasin structures.

A regional basinward 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 region of the model leading to much earlier overthrusting of the distal depositional limit of the salt. The added downslope gravitational component also modifies the style of the minibasins 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.