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.

Abstract Clastic foreland basin stratigraphy is primarily determined by the relative rates of first-order basin controlling processes; the rate of mass accretion to an orogen by thrust tectonics, the rate of mass redistribution by surface processes, the rate of fiexural isostatic compensation, and the rate of absolute sea level change. We have developed a composite planform foreland basin model to look for model stratigraphic signatures which reflect either the dominant influence of one of these basin controlling processes or interaction among several processes. The foreland basin model links component models of orogen tectonics, surface processes, lithospheric flexure and eustasy in an internally consistent manner. The tectonic model uses critical wedge principles to construct a doubly-vergent wedge-shaped orogen. The flexural isostasy model uses either an elastic or a thermally-activated linear visco-elastic lithospheric rheology. The surface processes model couples hillslope (mass diffusion) and climate-mediated fluvial (mass advection) transports to erode, redistribute and deposit mass across the orogen, foreland basins and peripheral bulges. Preliminary results are presented from two models which illustrate the terminal stages of ocean closure, the ensuing continent-continent collision, and the kinematic growth of an orogen with two flanking foreland basins. In the first model, there is no significant strike variation in model processes, therefore, cross-sections of any model are sufficient to analyze the basins and compare (hem with previously published results. The contrasting stratigraphic architecture of the basins is controlled by the inherent tectonic asymmetry and by the erosion and sediment flux which become progressively asymmetric as a consequence of the relative positions of the basins on the windward and leeward sides of the growing orogen. The second model demonstrates the complexities that result when there is a significant strike variation in tectonic processes. This model takes the form of a diachronous continent-continent collision between two continental margins inclined at an angle of ~25°. The model collision zone evolves in time and along strike from accretionary prism to orogen. Sediment flux into the windward foreland basin is greatest adjacent to the largest part of the orogen. This region of the basin becomes subaerial first and the drainage network develops a longitudinal trunk river system, similar to those common to many foreland basins. The combination of lateral and longitudinal fluvial transport results in diachronous filling of the marine basin by an assemblage of fluvial and marine facies which prograde down the basin axis.

Abstract Foreland-basin stratigraphy and orogen state are determined by the rate of mass accretion to an orogen by thrust tectonics, the efficiency of mass redistribution by surface processes, and lithospheric flexure. Orogen state can be characterized as constructive, steady, or destructive depending on the mass net balance in the orogen (Jamieson and Beaumont, 1988, Tectonics, v. 7, pp. 417-445). We have constructed a kinematic planform foreland-basin model to look for stratigraphic relationships between synthetic foreland basin stratigraphy and orogen state. The foreland-basin model links thin-skinned tectonic development of an orogen, lithospheric flexure (Beaumont and others, 1988, Tectonics, v. 7, p. 389-416) and mass redistribution by surface processes (Beaumont and others, 1992, Thrust Tectonics, p. 1-18). The tectonic model uses critical wedge principles to construct a two-sided wedge-shaped orogen. Sediments are accreted to the toe of each wedge at a rate proportional to the convergence rate of each leading slip line with the adjacent autochthon. The wedges, which need not be symmetric, grow in proportion to the net rate of mass influx. Their geometry is consistent with flexural adjustment of the lithosphere, conservation of mass, the criticality of each Coulomb wedge and match of wedge heights at their interface. The lithospheric-flexure model includes elastic or thermally activated linear viscoelastic rheologies. The surface process model couples climatic, hillslope (mass diffusion) and fluvial (mass transport) processes to erode, redistribute, and deposit mass across the orogen, its foreland basin and peripheral bulge. Synthetic stratigraphic assemblages are constructed for a range of tectonic, lithospheric, and surface process model parameters, to determine under what circumstances an assemblage can be considered diagnostic of an orogens's state, or change in state.

Abstract Two competing hypotheses can explain distal unconformities in foreland basin stratigraphy: peripheral-bulge migration with stress relaxation in the lithosphere (Quinlan and Beaumont, 1984, Canadian Journal of Earth Sciences, v. 21, p. 973) or orogen tectonics and basin-filling mechanisms (Flemmings and Jordan, 1990, Geology, v. 18, p. 335; Sinclair and others, in review). We investigate synthetically the origin of unconformities using the planform model (Johnson and Beaumont, abstract above). The figure shows an unconformity bounded sequence for one tectonic cycle, on a stress-relaxing lithosphere. I-type erosion occurs when constructive orogenic loading outstrips basin-filling and lithospheric relaxation. F-type erosion occurs when peripheral bulge migration with lithospheric-stress relaxation dominates. I- and F-type unconformities remain distinct when intervening sediments are preserved, otherwise the composite unconformity reflects the superposition of tectonic and relaxation dominated phases that may span several tectonic cycles. Does synthetic basin stratigraphy provide the evidence to distinguish I- from F-type erosion and to determine which dominates?

Abstract Contrasting styles of lithospheric extension are investigated with two-dimensional plane-strain finite-element models in which the brittle and duc-tile behavior of lithospheric rocks are represented, respectively, by elastoplasticity and thermally acti-vated power-law viscoelasticity. Parameter values were chosen to agree with laboratory measure-ments. Emphasis is placed on a comparison between the postcompressional extension of tectonically thick-ened lithosphere and the extension of normal thick-ness lithosphere containing a weak zone. The former may represent extension of the Basin and Range province, whereas the latter may correspond to extension at a simple rifted continental margin. Models of the collapse of a region of tectonically thickened lithosphere, like Tibet, under forces from its own excess potential energy (f) and from in-plane stress transmitted horizontally through the lithosphere (f) predict that the strain rate and total extension are sensitive functions of its thermal state. Three time scales are critical: (1) the time available for thermal equilibration of the tectoni-cally thickened lithosphere before the compressive stress is removed; (2) the time that f3 acts alone; and (3) the time that f and f3 act together. Extension is maximized when thermally equilibrated lithosphere collapses and rupture can then be achieved by the addition ofanf tensile force which is as small as 20 MPa for the parameter values chosen. By contrast, normal lithosphere which has been weakened by a factor of 100 requires f = 90 MPa to cause rupture. Inherited tectonic characteristics, like the initial distribution of thickened lithosphere, and the three time scales may govern the extensional style. Both pure shear and “pinch-and-swell” styles are pre-dicted, and large-scale simple shear occurs when laterally offset weak zones are present. Many of the differences between the style of Basin and Range extension and that of typical rifted margins may be attributed to tectonic inheritance, as can the strike variability of rifted margins. If the Basin and Range province was similar to Tibet before extension, caution is required in applying models that are appropriate for this tectonic setting to typical continental margins which may have very different initial lithospheric properties.