UC San Diego

Thousands of kilometers of possible subduction sites have been identified on Venus near certain quasi-circular surface features called corona and near branches of rift zone trenches called chasmata. Subduction on Earth is driven by negative plate buoyancy with respect to the underlying mantle, however lithosphere on Venus may be significantly more buoyant than on Earth. On Earth, only 6-7 km of positively buoyant crust compete with the negative thermal buoyancy of the lithospheric mantle, but on Venus the enhanced positive buoyancy from 30 km of globally-averaged crust would significantly inhibit subduction initiation. Plume-lithosphere interactions have been proposed as a mechanism for subduction initiation, but the relatively high bending moments and elastic thicknesses near subduction sites indicate the lithosphere is thick and may not be easily penetrated by thermal upwellings. The following chapters present a series of 2D and 3D numerical models using the code, StagYY, to investigate the dynamics of regional-scale lithospheric recycling initiated at chasmata trenches on Venus with realistic crustal densities. Rather than subduction, a tectonic regime called ``peel-back delamination" (PBD) was observed in which the lithospheric mantle decouples and peels away from the overlying positively buoyant crust. Though net-negative lithospheric buoyancy is not a requirement for delamination, PBD is driven by the negative thermal buoyancy of the lithospheric mantle and resisted by the positive compositional buoyancy of the crust and the strength of the lithosphere. Aided by the pre-existing lithospheric weakness at chasmata, plume-rift interactions may accelerate timescales of delamination initiation. A thermal upwelling is shown to destabilize even thinner, more positively buoyant rift-adjacent lithosphere than in models without plume-rift interactions. Plume-induced PBD may require a minimum lithospheric thickness of approximately 150-200 km, and therefore may be most applicable to the inferred thick lithosphere near the Dali-Diana chasmata system. 3D delamination models show a radial retrograde migration of a trench and flexural bulge, compatible with the style of surface deformation observed at Artemis Corona in the Dali-Diana chasmata region of Venus.