THERMAL BUOYANCY ON VENUS - UNDERTHRUSTING VS SUBDUCTION

In an effort to define the conditions distinguishing underthrusting and subduction in the Venus environment, we have modeled the thermal and buoyancy consequences of the subduction endmember. Predictions of the thermal evolution of a slab subducting at a fixed angle into the Venusian mantle are used to find slab densities based on slab composition and changes of temperature, pressure, and phase. The sustainability of subduction is assessed by considering the effects of slab buoyancy and mantle flow on the subduction angle. Mantle flow induced by slab motion applies torques on the slab, which are compared to buoyancy torques. Flow torques tend to decrease the angle of subduction. Buoyancy torques also act to decrease the subduction angle when the slab is positively buoyant. The basalt-eclogite phase transition dominates the transformation of positively buoyant slabs to negative buoyancy. Slabs that have descended to depths less than about 275 km remain positively buoyant. Beyond 275 km slabs become negatively buoyant. We predict that the combination of flow and buoyancy torques will tend to force initially subducted slabs to assume an underthrusting position, leading to crustal thickening, deformation, melting, and volcanism. This may provide a model explaining the association of compressional mountain belts and tessera blocks with apparent flexural rises, foredeeps, and voluminous volcanic deposits. Only special circumstances appear capable of promoting the conditions for negative slab buoyancy. These might include one where an under-thrust slab becomes attached to or included in a densified crustal root created in a zone of crustal thickening. The subsequent delamination and sinking of the root may then lead to the subduction of the slab.