Cordilleran orogens, such as the central Andes, form above subduction zones, and their evolution depends on both continental shortening and oceanic plate subduction processes, including arc magmatism and granitoid batholith formation. Arc and batholith magma compositions are consistent with partial melting of continental lithosphere and magmatic differentiation, whereby felsic melts rise upward through the crust, leaving a high-density pyroxenite root in the deep lithosphere. We study gravitational removal of this root using two-dimensional thermal-mechanical numerical models of subduction below a continent. The volcanic arc position is determined dynamically based on thermal structure, and formation of a batholith-root complex is simulated by changing the density of the arc lithosphere over time. For the model lithosphere structure, magmatic roots with even a small density increase are readily removed for a wide range of root strengths and subduction rates. The dynamics of removal depend on the relative rates of downward gravitational growth and lateral shearing by subduction-induced mantle flow. Gravitational growth dominates for high root densification rates, high root viscosities, and low subduction rates, resulting in drip-like removal as a single downwelling over 1–2.5 m.y. At lower growth rates, the root is removed over >3 m.y. through shear entrainment as it is carried sideways by mantle flow and then subducted. In all models, >80% of the root is removed, making this an effective way to thin orogenic mantle lithosphere. This can help resolve the mass problem in the central Andes, where observations indicate a thin mantle lithosphere, despite significant crustal shortening and thickening.

Abstract We investigate the differential loading and pore-fluid pressure required for failure and subsequent prolonged gravitational spreading of passive margin shale basins using two-dimensional analytical limit analysis and plane-strain finite element modeling. The limit analysis, supported by the models, indicates that narrow margins (slope regions that are approximately 50 to 100 km [31 to 62 mi] wide) require pore-fluid pressures that are 80-94% of the overburden weight for failure to occur, whereas for wider margins (~400 km [~250 mi] wide) like the Niger Delta, the corresponding values are 95-99%; these ranges depend on the intrinsic strength of sediments. In the large deformation models, gravitational spreading in response to sedimentation-induced overpressure caused by delta progradation is investigated. Shale is modeled as a visco-plastic Bingham fluid that is frictional-plastic below yield and has a yield criterion that depends on the effective pressure (mean stress minus pore-fluid pressure). The velocity of the postyield flow of the shale is limited by the viscosity of the Bingham fluid, chosen for this study to be 10 18 Pas. Pore-fluid pressure is predicted parametrically to be proportional to the local sedimentation rate during progradation, where the proportionality constant, k c , depends inversely on the hydraulic conductivity. Varying the sediment progradation rate, the depth of onset of excess pore pressure, and k c produces model deformation patterns consistent with seaward-directed squeeze-type flow of overpressured shale (Poiseuille flow) or wholesale sea-TOC ward motion of the shale and overburden (Couette flow), depending on the overall mobility of the model.

In order to investigate the development of structures at scales smaller than that of an entire belt, we examined aspects of the mechanics of thin-skinned fold-and-thrust belts in cross section using an arbitrary Lagrangian-Eulerian frictional-plastic finite-element model. A series of models, beginning with the deformation of a thick uniform layer above a thin weak layer on a fixed base, sequentially illustrates the effects of including flexural isostatic subsidence, strain-softening, multiple layers of strong and very weak materials, and finally erosion and sedimentation. These continuum models develop thin shear zones containing highly sheared material that approximate fault zones. The corresponding structures are similar to those in fold-and-thrust belts and include: far-traveled thrust sheets, irregular-roof and smooth-roof duplexes, back thrusts, pop-ups, detachment folds, fault-bend folds, break thrusts, and piggyback basins. These structures can develop in-sequence or out-of-sequence, remain active for extended periods, or be reactivated. At the largest scale, the scale of the wedge, the finite-element model results agree with critical wedge solutions, but geometries differ at the sub-wedge scale because the models contain internal structures not predicted by the critical wedge stress analysis. These structures are a consequence of: (1) the complete solution of the governing equations (as opposed to a solution assuming a stress state that is everywhere at yield), (2) the initial finite-thickness layers, (3) the spatial and temporal variations of internal and basal strength, and (4) the coupling between surface processes and deformation of the wedge. The structural styles produced in models involving feedback with surface processes (erosion and sedimentation) are very similar to those mapped in the foothills of the southern Canadian Rockies and elsewhere. Although syndeformational sediments have been removed by postorogenic erosion across the foothills belt, evidence of the interaction between surface processes and deformation is preserved in the structural style.

Abstract Plane-strain, thermo-mechanical, finite element model experiments of lithospheric extension are used to investigate the effects of strain softening in the frictional-plastic regime and the strength of the lower crust and mantle lithosphere, respectively, on the style of extension. Crust and mantle lithosphere strength are varied independently. A simple scaling of wet quartz and dry-olivine rheologies is used to examine crust and mantle lithosphere strength variations. Cases are compared where the crust is strong (η wet quartz x 100), weak (η wet quartz ), or very weak (η wet quartz /10), and the mantle lithosphere is either strong (η dry olivine ) or weak (η dry olivine /10). Strain softening takes the form of a reduction in the internal angle of friction with increasing strain. Predicted rift modes belong to three fundamental types: (1) narrow, asymmetric rifting in which the geometry of both the upper and lower lithosphere is approximately asymmetric; (2) narrow, asymmetric, upper lithosphere rifting concomitant with narrow, symmetric, lower lithosphere extension; and (3) wide, symmetric, crustal rifting concomitant with narrow, mantle lithosphere extension. The different styles depend on the relative control of the system by the frictional-plastic and ductile layers, which promote narrow, localized rifting in the plastic layers and wide modes of extension in the viscous layers, respectively. A weak, ductile crust-mantle coupling tends to suppress narrow rifting in the crustal layer. This is because it reduces the coupling between the frictional-plastic upper crust and localized rifting in the frictional-plastic upper mantle lithosphere. The simple strength variation may be taken to represent end-member thermal and/or compositional conditions in natural systems and the relevance for rifting of old, strong, and cold cratonic lithosphere as compared to young, “standard”, and moderately weak Phanerozoic lithosphere is discussed.

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 ( V sp = 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/m 3 . 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.