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We develop a two-dimensional plane strain large-deformation coupled poroelastoplastic finite element model to simulate initiation and rise of a salt wall from a flat salt body during sedimentation. We run transient analyses with high-permeability and low-permeability sediments in the model to simulate drained conditions and overpressure (or pore fluid overpressure). We investigate deformation, stress, and overpressure in the evolving suprasalt basin. Model results show that horizontal stress increases even higher than vertical stress at the flank of the salt wall and in the minibasin due to horizontal pushing out of the rising salt wall and that orientations of principal stresses rotate in the minibasin relative to far-field stress state. Model predicts that compared with far-field overpressure, the overpressure near the salt wall and within the minibasin is largely perturbed by the salt body. We find that perturbations of pore pressure (or pore fluid pressure) near the salt wall and within the minibasin cannot be resolved by traditional pore pressure prediction methods such as normal compaction trend approach and mean stress model approach. In order to predict pore pressure more accurately, especially in the regions near salt bodies or where stresses are perturbed, we need to apply a method that includes both effects of mean effective stress and deviatoric stress on pore pressure and compaction, for example, the Modified Cam Clay model approach, to pore pressure prediction. Our results provide geoscientists insights into evolution of salt basins, near-salt deformation, stress, overpressure, and overpressure prediction methods and have implications for near-salt wellbore drilling programs.

The 2-D concept of diapiric downbuilding is expanded to the third dimension. Our advanced approach shows which range of salt fluxes and sedimentation rates (time dependency included) can explain the formation of upward narrowing diapirs (previously thought of as R˙/A˙ 1), and vertical cylindrical diapirs (~ R˙/A˙ = 1). In addition to being a nonrigorous measure, R˙/A˙ does not account for the effect of a regional slope on diapir shape during downbuilding. When a regional topographic slope is absent and lateral changes in sedimentation rate are negligible, our models confirm downbuilding diapirs always develop axisymmetric shapes. However, when the salt source lies on a regional slope, asymmetric diapirs will form. The rate of salt supply from the source and the superposed gravity flow downslope control the flow lines and shape of the advancing salt sheet. In the 2-D gravity spreading plane, the ratio of source flow to gravity flow strengths can be characterized by a Rankine number. Obstruction of lateral spreading of salt issued from the feeder sources due to sedimentation lifts the salt from its 2-D spreading plane, which our model simulates by superposing a velocity component normal to the sedimentation surface. We next apply the 3-D analytical model to an offshore salt sheet in the Gulf of Mexico and analyze which conditions controlled its flow kinematics down the continental slope. Stratal cutoffs at the base of salt combined with the analytical gravity spreading model provide estimates for the dimensional flow rates and the effective viscosity of the Louann Salt.

Critical-state wedges, ice glaciers, and salt sheets have many geometric and mechanical similarities. Each has a tapering geometry and moves along a basal detachment. Their motions result from the combined effects of internal deformation and basal sliding. Wedge deformation and geometry, basal conditions, and overpressure (pore fluid pressure less hydrostatic pore fluid pressure) development within the substrate interact with each other in this mechanically coupled system. However, the nature of this interaction is poorly understood. In order to investigate this coupled system, we have developed two-dimensional poromechanical finite-element models with porous fluid flow in sediments. We have simulated the advance of a salt sheet wedge across poroelastic sediments in this study. We emphasize that our results have applications beyond salt wedges to both critical-state wedges and ice glaciers. Overpressure develops within the substrate over time during the advance of the wedge. The magnitude of the overpressure influences the wedge geometry and the wedge advance rate. Lower overpressure results in a thicker and steeper wedge geometry, and a slower advance rate, while higher overpressure favors a thinner, wider, and more flattened wedge geometry and a faster advance rate. This study provides key insights into the links between wedge geometry, basal shear stress, and overpressure in substrates.

Geological mass flows extruding from a point source include mud, lava, and salt issued from subsurface reservoirs and ice from surface feeders. The delivery of the material may occur via a salt stock, a volcanic pipe (for magma and mud flows), or a valley glacier (for ice). All these source flows are commonly skewed by a superposed far-field velocity vector imposed by the topographic slope and thus develop plumes having a wide range of shapes. The morphological evolution of the perimeter of the plumes (in plan view) can be simulated by varying the key parameters in a simple analytical flow description on the basis of Rankine equations. Our model systematically varies the strength of the point source relative to the downslope far-field velocity of its expelled mass. The flow lines are critically controlled by the relative speed of the two rates, which can be concisely expressed by the dimensionless Rankine number (Rk, introduced in this study). For steady flows, plume widths can be expressed as a function of Rk. The viscosity of the rock, mud, or lava mass involved in the gravity flow affects Rk and thus the appearance of the plumes. For unsteady source strength, Rk becomes time dependent and the plume width varies over time. The model flow shapes suggest that the plume shapes of natural gravity flows of terrestrial surface materials (mud, lava, salt, and ice) commonly express fast initial flux of the source, followed by an exponential decline of the source strength. Flows having initially higher Rk but otherwise equal life cycles create broader plumes. Peaks in the source flux due to magmatic pulsing during the eruption cycle can explain the formation of pillow lavas. Rather than instantaneously reaching full strength before declining, some natural source flows start by swelling slowly, leading to the creation of unique plume shapes like a flying saucer

[1] We explore exhumation in the coastal Kwanza Basin by combining analyses of Tertiary hiatuses and apatite fission tracks. Planktonic biozones show five major hiatuses in the Oligo-Miocene and Plio-Pleistocene. Between gaps, Oligo-Miocene strata accumulated under marine conditions. A marine setting refutes the idea of a massively raised coastal plateau in the mid-Tertiary. Marine conditions continued until ∼5 Ma. Fission track data suggest three thermal events: ∼150 Ma, during rifting and volcanism; ∼100–70 Ma, during shortening and volcanism; and ∼20–10 Ma, during exhumation. Tertiary uplift was spatially highly variable. For the Kwanza Basin, we infer that Tertiary uplift on the West African margin is indeed a fact but that estimates of uplift timing and size are unreliable when extrapolated to adjoining areas. Massive uplift (2000–4000 m) of the Precambrian craton had little structural effect in the outer basin. Instead, minor uplifts on the shelf drove late Tertiary deformation on the slope.