The Nazca-South America convergence represents a unique natural laboratory to probe our understanding of subduction, mantle flow and stress coupling at Andean-type margins. Here, it is shown that the most fundamental balance of forces associated with the downgoing slab, the upper plates and the mantle can account for the Nazca plate motions, although it does not adequately explain the variations of the Cordilleran tectonics found along the 6000 km wide margin. Using three-dimensional numerical models it is shown that trench-parallel gradients in both the driving and resisting forces are an essential component of the force balance, and necessary to reproduce the macroscopic features observed. When along-trench buoyancy variations similar to the Nazca plate’s are included, the slab dips and upper plate deformations observed in the Nazca slab, in the Cordilleras and South American continent interiors can be reproduced. The models show that gradients in the resisting shear force along the trench can be as relevant, as they modulate the trench retreat to form the concave Bolivian Orocline. Pressure gradients in the mantle follow the Nazca buoyancy gradients, and effectively rearrange the flow introducing a trench-parallel component, similar to what suggested by seismic anisotropy in this area. Although they introduce only secondary variations to the primary subduction and mantle flow dynamics, the regional features of the Nazca and South American plates exert a primary control at the margin-local scale. This suggests that far-field forces, e.g. from spreading Atlantic or large-scale convection, should play a minor role in the formation of the Cordilleras.
SUMMARY Our understanding of the seismicity of continental interiors, far from plate margins, relies on the ability to account for behaviours across a broad range of time and spatial scales. Deformation rates around seismic faults range from the slip-on-fault during earthquakes to the long-term viscous deformation of surrounding lithosphere, thereby presenting a challenge to modelling techniques. The aim of this study was to test a new method to simulate seismic faults using a continuum approach, reconciling the deformation of viscoelastoplastic lithospheres over geological timescales. A von Mises yield criterion is adopted as a proxy for the frictional shear strength of a fault. In the elastoplastic fault models a rapid change in strength occurs after plastic yielding, to achieve stress–strain equilibrium, when the coseismic slip and slip velocity from the strain-rate response and size of the fault are calculated. The cumulative step-function shape of the slip and temporally partitioned slip velocity of the fault demonstrated self-consistent discrete fault motion. The implementation of elastoplastic faults successfully reproduced the conceptual models of seismic recurrence, that is strictly periodic and time- and slip-predictable. Elastoplastic faults that include a slip velocity strengthening and weakening with reduction of the time-step size during the slip stage generated yield patterns of coseismic stress changes in surrounding areas, which were similar to those calculated from actual earthquakes. A test of fault interaction captured the migration of stress between two faults under different spatial arrangements, reproducing realistic behaviours across time and spatial scales of faults in continental interiors.
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