70-10 Relative roles of slab age, convergence rate, upper plate thickness, and asymmetric erosion in Cordilleran-type orogens: insights from numerical modeling

Abstract:

Long-lived Cordilleran-type orogens display diverse structural styles and orogenic wedge geometries resulting from interactions among converging plates. Along-strike variations in climate, plate properties, and convergence rates may influence upper plate deformation. We use a visco-elasto-plastic thermo-mechanical numerical model to simulate the effects of end-member subducting slab age (20 and 50 Ma), convergence velocity (2 and 7 cm/yr), and initial upper plate crustal thickness (30 and 50 km). We also couple the geodynamic model with a surface-process model to simulate end-member asymmetric erosion (10 times higher) focused on the foreland, focused on the forearc, or minimal erosion, to assess the effects of climate gradients. The parameter space is motivated by the Andes, where along-strike variations in orographic gradients, oceanic plate age, convergence velocity, and upper plate thickness result in an important transition toward a smaller and thinner orogen in the south (Patagonian Andes) compared with the Central Andes. Among our results, we show that oceanic plate age plays a major role in the amount of upper plate lithosphere removed by subduction erosion, with the older oceanic plate (50 Ma) transporting more upper plate material into the asthenosphere. Initial upper plate crustal thickness strongly controls upper plate deformation, where the thinner crust (initially 30 km thick) results in greater lithospheric bulk strain, with folding and tilting of the lithosphere. The initially thick crust, instead, results in a much more stable continent with only minor deformation. The effects of convergence velocity are more important with an initially thin crust, where faster convergence rates (7 cm/yr) result in greater deformation and subduction erosion. Effects of asymmetric erosion are only visible when enough topography is generated by deformation, as erosion is scaled to be a function of the topographic slope and drainage area, and thus, in several models it is negligible. A few models show that forearc-focused erosion helps prevent subduction erosion to some extent, whereas foreland-focused erosion can generate tilting and strain concentration in the foreland region. We thus highlight the importance of subducting plate age on subduction erosion, which has been largely overlooked in previous studies, and reinforce that lithosphere with thinner crusts are more prone to deformation, whereas asymmetric erosion exerts only a secondary influence under the explored conditions.

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