Lithospheric rheology and flexure at Artemis Chasma, Venus

Artemis Chasma is an arcuate, 2000-km-diameter zone of convergence and lithospheric underthrusting on Venus. Inelastic flexure modeling of the topography at Artemis, combined with observations of trench tectonics, allow us to document local temperature gradients below 4 K km(-1) and an immense compressive in-plane force at the trench. Lithospheric rheology on Venus may be stronger than would be predicted from conventional extrapolations of rock mechanics experiments; in particular, the brittle surface strength must reach a few tens of mega-pascals to satisfy the observed lack of flexurally induced surface faulting. Elastic plate bending models provide an adequate estimate of the bending moment at Artemis, but they fail to constrain-or even recognize-the in-plane force. The inelastic analysis implies an in-plane force of the order of -1 x 10(14) N m(-1); a potential driving force is thermal thinning of regionally thick lithosphere in the highlands to the north. The low heat flux at Artemis, which is a comparatively young structure, is compatible with the notion that Venus has experienced a prolonged period of cooling in the last several hundred million years. The inference of such exceptionally low thermal gradients embraces three end-member possibilities: (1) the surface age is >600 Ma, (2) the lithospheric thermal age is greater than the surface age, and/or (3) the upper mantle temperature is anomalously low (similar to 1550 K).