Guy Simpson

Coupled model of surface water flow, sediment transport and morphological evolution

This paper presents a mathematical model coupling water flow and sediment transport dynamics that enables calculating the changing surface morphology through time and space. The model is based on the shallow water equations for flow, conservation of sediment concentration, and empirical functions for bed friction, substrate erosion and deposition. The sediment transport model is a non-capacity formulation whereby erosion and deposition are treated independently and influence the sediment flux by exchanging mass across the bottom boundary of the flow. The resulting hyperbolic system is solved using a finite volume, Godunov-type method with a first-order approximate Riemann solver. The model can be applied both to short time scales, where the flow, sediment transport and morphological evolution are strongly coupled and the rate of bed evolution is comparable to the rate of flow evolution, or to relatively long time scales, where the time scale of bed evolution associated with erosion and/or deposition is slow relative to the response of the flow to the changing surface and, therefore, the classical quasi-steady approximation can be invoked. The model is verified by comparing computed results with documented solutions. The developed model can be used to investigate a variety of problems involving coupled flow and sediment transport including channel initiation and drainage basin evolution associated with overland flow and morphological changes induced by extreme events such as tsunami.

Role of river incision in enhancing deformation

Many active compressional belts contain examples of transverse rivers intersecting anticlines at their highest structural and topographic position. This unusual association between rivers and doubly plunging anticlines at relatively small scales is usually explained by some form of antecedence or superposition. In this article I suggest an alternative explanation, i.e., that river erosion unloads the crust and that this process causes local deformation to be enhanced in the vicinity of rivers. I show the plausibility of this hypothesis with the aid of a coupled three-dimensional mechanical-surface process mathematical model. The conclusion is that river incision can have a major influence on deformation of the surrounding crust if incision occurs at the same time that the crust is deforming plastically in response to regional compression. In this case, incision amplifies background deformation at a relatively small scale, leading to the formation of noncylindrical folds with culminations coinciding with river incision. In the absence of regional deformation, the response of the crust to river incision is small and of long wavelength because it is governed by flexural isostasy of relatively rigid elastic crust. Thus, whether rivers exert an influence on local deformation depends critically on the timing between river incision and regional deformation. This point may explain why in some cases, rivers appear to have had no significant influence on local deformation, whereas in other cases, they have.

Model shows that rivers transmit high-frequency climate cycles to the sedimentary record

Rivers are a major component of sediment routing systems that control the transfer of terrigenous sediments from source to sink. Although it is widely accepted that rivers are perturbed by millennial-scale climatic variability, the extent to which these signals are buffered or transferred down river systems to be recorded in sediments at or beyond the river mouth remains debated. Here, we employ a physically based numerical model to address this outstanding issue. Our model shows that river transport strongly amplifies high-frequency sediment flux variations arising from changing water discharge, due to positive feedback between discharge and the channel gradient. This behavior is distinctly different from short-period sediment flux signals (with constant water discharge) where the output sediment flux is strongly dampened within the river, due to negative feedback between the channel gradient and sediment concentration. We conclude that marine sedimentary basins may record sediment flux cycles resulting from discharge (and ultimately climate) variability, whereas they may be relatively insensitive to pure sediment flux perturbations (such as for example those induced by tectonics).

Formation of accretionary prisms influenced by sediment subduction and supplied by sediments from adjacent continents

Mechanical models are used to investigate the formation of accretionary prisms and related basins fed by sediments supplied from adjacent continents and subjected to sediment subduction. Results show that prisms forming under the infl uence of a hinterland sediment fl ux exhibit markedly different characteristics compared to classic critical wedges with identical mechanical properties forming by frontal accretion of a preexisting layer. The main differences are reduced surface slopes, increased spacing between thrusts, widespread wedgetop basins, and the presence of buried structures. These differences are explained by the ability of continuous sedimentation to reduce differential stresses in the wedge, leading to stable (supercritical) geometries. Thus, in regions of active sedimentation, wedge geometries cannot be explained solely in terms of relative strengths of the wedge and its decollement, as is the case for critical wedges. Results also show that variations in the relative rates of sediment supply (e.g., linked to climate changes) and sediment subduction may lead to pulsed growth and decline of wedges though time, as has been evidenced for some natural wedges. Thus, rather than viewing compressive plate margins as accretionary versus erosive, the dominant mode may repeatedly switch back and forth through time in response to variations in relative rates of sediment supply and sediment subduction.

Permeability enhancement due to microcrack dilatancy in the damage regime

A three dimensional microscopically based permeability model incorporating inelastic deformation has been developed to account for the modification of transport properties due to fracturing. The basic hypothesis investigated is that permeability enhancement during brittle deformation is caused by the formation of dilatant microcracks which are associated with friction sliding on a preexisting random population of shear cracks. Additional dilatancy, produced as a result of frictional sliding over asperities, is also accounted for by the introduction of a crack roughness parameter. Linear elastic fracture mechanics is used to calculate the evolution of crack length and crack area as a function of applied stress and fluid pressure. After making a geometrical simplification the microcrack parameters derived from the deformation model can be used to calculate the permeability tensor assuming that fluid transport results from Poiseuille flow through a connected distribution of cracks. The model enables investigation of the macroscopic permeability variation as a function of two loading parameters, three constant material parameters, and the crack roughness parameter. Results demonstrate that permeability is a smooth but strongly increasing (near power law) function of the Terzaghi effective stress ratio and is strongly dependent on the initial crack density, Youngs modulus, the friction coefficient, and the effective confining pressure. Numerical results are in quantitative agreement with published experimental measurements and display behavior similar to results obtained with a relatively simple analytical model. The model enables calculation of the degree of stress-induced anisotropy, which is shown is be relatively small (< × 10) for resonable effective stress ratios. The modeling presented provides a quantitative tool with which the effects of microcrack-induced permeability enhancement can be investigated within the broader context of coupled fluid flow and brittle deformation in the crust.

Zircons reveal magma fluxes in the Earth’s crust

Magma fluxes regulate the planetary thermal budget, the growth of continents and the frequency and magnitude of volcanic eruptions, and play a part in the genesis and size of magmatic ore deposits. However, because a large fraction of the magma produced on the Earth does not erupt at the surface, determinations of magma fluxes are rare and this compromises our ability to establish a link between global heat transfer and large-scale geological processes. Here we show that age distributions of zircons, a mineral often present in crustal magmatic rocks, in combination with thermal modelling, provide an accurate means of retrieving magma fluxes. The characteristics of zircon age populations vary significantly and systematically as a function of the flux and total volume of magma accumulated in the Earth’s crust. Our approach produces results that are consistent with independent determinations of magma fluxes and volumes of magmatic systems. Analysis of existing age population data sets using our method suggests that porphyry-type deposits, plutons and large eruptions each require magma input over different timescales at different characteristic average fluxes. We anticipate that more extensive and complete magma flux data sets will serve to clarify the control that the global heat flux exerts on the frequency of geological events such as volcanic eruptions, and to determine the main factors controlling the distribution of resources on our planet.

Possible erosional control on lateral growth of the European Central Alps

Between the middle and late Miocene, the Central Alps of Switzerland and northern Italy underwent a major phase of lateral crustal growth as recorded by the formation of the southern Alps and the Jura fold-and-thrust belt. This period of dominantly outward-directed deformation differs from the late Oligocene and early Miocene deformation style, which was characterized mainly by rapid vertical exhumation. Sediment budgets from circum-Alpine sedimentary basins indicate a decrease in the erosional efficiency in the Alpine hinterland several million years before lateral orogen growth was initiated, and decreasing magnitudes of sediment discharge thereafter. This decrease in erosional efficiency of the hinterland coincides approximately with widespread exposure of the crystalline core in the Alpine hinterland as indicated by clast types in conglomerates of synorogenic deposits, and with a contemporaneous increase in the continental influence in the paleoclimate, as suggested by the fossiliferous plant record. We suggest that the observed decrease in erosional efficiency caused gravitational forces to increase relative to tectonic forces driving the orogenesis, which led to a transition from dominantly vertical to horizontally directed extrusion. The data thus can be interpreted to indicate an active link between surface erosion and orogenic evolution.

Neogene sediments and modern depositional environments of the Zagros foreland basin system

A sedimentological investigation of the Neogene deposits of the Zagros foreland basin in SW Iran reveals a continuous and largely gradational passage from supratidal and sabkha sediments at the base (represented by the Gachsaran Formation) to carbonates and marine marls (Mishan Formation with basal Guri carbonate member) followed by coastal plain and meandering river deposits (Agha Jari Formation) and finally to braided river gravel sheets (Bakhtyari Formation). This vertical succession is interpreted to represent the southward migration of foreland basin depozones (from distal foredeep and foredeep to distal wedge-top and proximal wedge-top, respectively) as the Zagros fold–thrust belt migrated progressively southward towards the Arabian foreland. This vertical succession bears a striking similarity to modern depositional environments and sedimentary deposits observed in the Zagros region today, where one passes from mainly braided rivers in the Zagros Mountains to meandering rivers close to the coast, to shallow marine clastic sediments along the northern part of the Persian Gulf and finally to carbonate ramp and sabkha deposits along the southeastern coast of the Persian Gulf. This link between the Neogene succession and the modern-day depositional environments strongly suggests that the major Neogene formations of the Zagros foreland basin are strongly diachronous (as shown recently by others) and have active modern-day equivalents.

Tempo of magma degassing and the genesis of porphyry copper deposits

Porphyry deposits are copper-rich orebodies formed by precipitation of metal sulphides from hydrothermal fluids released from magmatic intrusions that cooled at depth within the Earth’s crust. Finding new porphyry deposits is essential because they are our largest source of copper and they also contain other strategic metals including gold and molybdenum. However, the discovery of giant porphyry deposits is hindered by a lack of understanding of the factors governing their size. Here, we use thermal modelling and statistical simulations to quantify the tempo and the chemistry of fluids released from cooling magmatic systems. We confirm that typical arc magmas produce fluids similar in composition to those that form porphyry deposits and conclude that the volume and duration of magmatic activity exert a first order control on the endowment (total mass of deposited copper) of economic porphyry copper deposits. Therefore, initial magma enrichment in copper and sulphur, although adding to the metallogenic potential, is not necessary to form a giant deposit. Our results link the respective durations of magmatic and hydrothermal activity from well-known large to supergiant deposits to their metal endowment. This novel approach can readily be implemented as an additional exploration tool that can help assess the economic potential of magmatic-hydrothermal systems.

Rock uplift and exhumation of continental margins by the collision, accretion, and subduction of buoyant and topographically prominent oceanic crust

Understanding the causes of rock and surface uplift is important because they control the location of mountain building, depocenters, and drainage characteristics and can influence climate. Here we combine previous thermochronological data with field observations to determine the amount of exhumation, rock, and surface uplift that occurs in the upper plate of Central and South American subduction zones during the collision, accretion, and subduction of oceanic plateaus and aseismic ridges. The collision of buoyant and topographically prominent oceanic plateaus and ridges can drive at least 5 km of rock uplift within 2 Ma. Uplift appears to be an immediate response to collision and is generally independent of the slab dip. The amount of rock uplift is controlled mainly by excess topography associated with the ridge (ultimately linked to buoyancy) and erosion, while it is also influenced by the strength of the subduction interface related to the presence of volcanic asperities and overpressured sediments in the subduction channel. The quantity of exhumation is strongly dependant on climate-induced erosion and the lifespan over which the topography is uplifted and supported. Sediment draining into the trench may leave the elevated ridge axis sediment starved, increasing the shear stresses at the ridge subduction interface, leading to positive feedback between ridge subduction, rock uplift, and exhumation. Trench-parallel variations in exhumation have a direct impact on exploration paradigms for porphyry-related metalliferous deposits, and it is likely that porphyry systems are completely eroded by the impingement of plateaus and aseismic ridges within temperate and tropical climates.