Effect of state of compensation on the relaxation of crustal plateaus on Venus

Abstract

Building on Nunes et al. (2004), which examined the end‐member case of viscous relaxation of fully compensated crustal plateaus on Venus, we now investigate viscous relaxation for the other end‐member case of uncompensated topography. The approach consists of modeling viscous flow in a crust‐mantle system by means of analytic and finite element methods. Differential stresses produced by uncompensated plateau topography concentrate beneath the entire domain of the plateau and extend deep into the mantle. Loss of topography occurs via fast subsidence due to isostatic adjustment, and a slower lateral flow due to spreading of the thickened crust. Rims arise from the isostatic adjustment, and rim dimensions depend on the thickness of the competent layer, controlled by the thermal state of the system. Rim dimensions after 1 Myr are ∼1 km high and ∼300 km wide for a surface temperature of 740 K and thermal gradient of <10 K/km, and ∼600 m high and ∼100 km wide for hotter crustal conditions. Although models predict the marginal circumferential (hoop) folding observed at crustal plateaus, they do not account for the large radial grabens also observed. Adding a subcrustal zone of time‐dependent buoyancy causes a delay in the isostatic adjustment. During this period of delay, viscous flow in a relatively hot crust leads to a reduction in rim amplitude once isostatic adjustment takes place. Only cooler conditions in this model can reproduce the wide and high rims observed at crustal plateaus on Venus. Consequently, phase transitions in the lower crust may produce sufficiently large variations in buoyancy to affect the surface topography and are more compatible with a cooler thermal state, but they cannot explain the thickened crust or the interpretation of gravity data showing current isostatic support. Lateral variation in heat flow (and thickness of the competent layer) may help address the latter and needs to be explored through both lithospheric and mantle convection models.