The tectonics and volcanism of Venus: New modes facilitated by realistic crustal rheology and intrusive magmatism

Abstract

To explain Venus’ young surface age and lack of plate tectonics, Venus’ tectonic regime has often been proposed to be either an episodic-lid regime with global lithospheric overturns, or an equilibrium resurfacing regime with numerous volcanic and tectonic activities. Stratigraphic analysis suggests that Venus’ surface tectonics could be a combination of these two end-member models with a global resurfacing event that created most of the crust followed by tectonic and volcanic activities until now. Recent analyses of Venus’ satellite images also suggest widespread lithospheric mobility in the lowland basins that does not fit the episodic-lid tectonic regime. Here, we use global 2-D thermochemical convection models with realistic parameters, including rheology (dislocation creep, diffusion creep, and plastic yielding), an experiment-based plagioclase (An75) crustal rheology, and intrusive magmatism, to investigate the tectonics and mantle evolution of Venus. We find that surface tectonics is strongly affected by crustal rheology. With a “weak” plagioclase-rheology crust, models exhibit episodic overturns but with continuously high surface mobility and high distributed surface strain rates between overturns, leading to a new tectonic regime that we name “deformable episodic lid”. On the other hand, olivine-crustal-rheology models exhibit either standard episodic-lid tectonics, i.e. with mobility that is high during overturns and near zero between overturns, or stagnant-lid tectonics, i.e. with near-zero mobility over the entire model time. In models with plagioclase-rheology crust, magmatic resurfacings are short-lived and randomly located, leading to a rather uniform resurfacing rate. Also, a combination of plagioclase crustal rheology and dislocation creep can weaken the lithosphere sufficiently to facilitate lithospheric overturns without applying plastic yielding. Internally, the composition-dependent density profile results in a “basalt barrier” at the mantle transition zone, which strongly affects Venus’ mantle evolution. Only strong plumes can penetrate this basalt barrier and cause global lithospheric overturns. This basalt barrier also causes global internal episodic overturns that generate global volcanic resurfacing in stagnant-lid models, which suggests a new resurfacing mechanism that does not involve lithospheric overturns. We name this regime “stagnant episodic-volcanic-resurfacing”. Results in models with plagioclase crustal rheology and low eruption efficiencies best fit current estimates of crustal thickness, surface age, and tectonic structures based on observations of Venus. The crustal thickness in models with an olivine-rheology crust is limited by the basalt–eclogite phase transition and is higher than observation-based estimates. Higher intrusion rates better fit these estimates, as models with high eruption efficiency predict very low surface age (< 100 Myr).