[1] Crustal plateaus, dominant physiographic features on Venus, likely originate through dynamic mantle processes, although a debate exists on whether they formed by mantle upwellings or downwellings. Regardless of the mode of formation, several observations led to the hypothesis that viscous relaxation may be the driving force behind the apparent evolutionary sequence from a high-standing plateau to a low-standing plateau with elevated margins. We apply analytic and finite element models to test this hypothesis for isostatically compensated topography, as modeling of gravity data suggests that crustal plateaus are presently supported by crustal roots. Geotherm values 5 K km À1 combined with a surface temperature of 740 K preclude relaxation within 10 9 years, while geotherm values !20 K km À1 can yield relaxation times of 10 8 years or less. Hence significant relaxation requires hot conditions in order to occur within the appropriate 1-Gyr timescale set by crustal plateau ages. We also show that a compensated plateau can either retain its shape as it relaxes or become more domical in appearance. Mantle temperatures <1400 K allow strong crust-mantle coupling that hinders flow of crustal material from the center of the plateau and produces domical relaxed profiles. Higher temperatures lead to a relatively inviscid mantle and relaxation that is largely insensitive to topographic wavelength, thus preserving the original topographic shape during relaxation. In either case, relaxation of compensated plateaus does not yield elevated rims. We propose that the state of compensation must be considered variable.

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Abstract. Mechanical decoupling between the crust and upper mantle has been proposed as an explanation for anomalously low effective elastic thicknesses (<20 km) locally associated with thermally mature (~ 1 Ga) continental lithosphere. The processes and consequences of crustmantle decoupling are investigated with a fully dynamic elastic-viscous-plastic (EVP) finite element model of orogenic loading and foreland basin subsidence. The basic dependencies of continental flexure on lithospheric thermal state, crustal thickness, and load magnitude are determined and are characterized by the best fit elastic thickness for the simulated deflections. Additional variables such as loading rate, viscoelastic stress relaxation, the boundary condition at the underthrust edge of the plate, and the lithospheric rheology also influence the flexural signature, and these sensitivities are likewise defined by EVP simulations. Decoupling can result in substantial reductions in the effective elastic thickness---up to a factor of 2 for lithosphere with a thermal age of 1 Ga. Elastic-viscous-plastic models show that lower crustal weakening occurs by locally enhanced creep driven by high shear stresses in the middle lithosphere at the load margin. Crustmantle decoupling in these models is fundamentally controlled by the temperature and the rheological contrast at the Moho. Previous studies of flexural decoupling have employed simplified multilayered elastic or elastic-plastic methods, the latter using a yield strength envelope with a prescribed strain rate and an assumption of complete slip at the Moho. However, lithospheric rheology is inherently stress and time dependent, with the deformation rate varying both spatially and temporally. Comparison of elastic-plastic solutions to the EVP simulations indicates that the former method is satisfactory for oceanic and coupled continental lithosphere but performs poorly with decoupled lithosphere. The EVP model is marginally successful at achieving effective elastic thicknesses <20 km for ~ 1-Ga lithosphere; additional reductions in the thickness may be achieved by a more thorough treatment of the thermal evolution of underthrust continental lithosphere.

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We have generated an elastoviscoplastic (EVP) rheological model of the lithosphere with an extended Maxwell model containing (in series) a linear elastic component, a creep component based on a flow law for dislocation creep in olivine, and a frictional component simulating Drucker-Prager plasticity based on Byerlee's rule. Finite element analyses for topographic loading of this oceanic lithosphere were carried out with two separate final loads (100 and 150 MPa) that were reached by four different load growth times (0, 0.1, 1, 10 Myr). Our results for the stress state and deformation of loaded lithosphere at 41.7 Myr into the model run are compared to results generated by the mechanical response of a time-independent elastic-perfectly plastic (EP) lithosphere, using a moment-curvature relationship based on the constant strain-rate yield strength envelope (YSE) and adopting a strain-rate representative of the EVP solution at 41.7 Myr. With identical flexural loading and material parameters, the deflection profiles of the EVP and EP solutions are quite similar, but it is unclear how the EP strain rate could be selected a priori without guidance from the EVP solution. For example, this uncertainty translates to about a 5 per cent error per decade of strain rate in the temperature gradient obtained by matching maximum moment and curvature in our EP models. The stress distributions of the time-dependent EVP model show deviations from the EP model (as defined by the YSE and an elastic core) in crystal plastic (macroscopically continuous dislocation creep) regions, where we observe vertical, lateral and temporal variations in the strain rate. At times much greater than the load growth time, the stress distribution in the lithosphere is independent of the loading rate and depends on the load magnitude only in that portion of the lithosphere that yields to frictional slip. After loading ceases, residual creep zones develop (in the vicinity of the brittle-plastic transition and the elastic-creep transition), driven by high stress in these regions.