Constraining the contribution of isostasy and dynamic uplift at Venusian volcanic rises and tessera terrain: implications for rifting and volcanism

Venus and Earth share a similar size, mass, density and volume, however surface conditions on each planet are very different. Venus has a surface temperature of ~472ºC, surface pressure over 90 bar, lacks surface water, and plate tectonics. These conditions make obtaining information about the interior difficult. Furthermore, a lack of integrated cross disciplinary studies of Venus's internal dynamics and topographic support mechanisms have resulted in highly unconstrained models of lithospheric structure. This study aims to provide the first complete analysis of the processes compensating the observed topography at Atla Regio, Beta Regio, and Fortuna Tessera by modelling their lithospheric structure and internal dynamics. Atla and Beta Regio are rift-dominated volcanic rises and Fortuna Tessera is a crustal plateau. Atla Regio exhibits evidence for current or recent geological activity, whereas Beta Regio has the highest Geoid-to-Topography Ratio (GTRs), size, and volume of all the volcanic rises. In contrast, Fortuna Tessera comprises one of the oldest terrain units on Venus, but may provide long-wavelength support to the adjacent mountain belt, Maxwell Montes. Both short and long wavelength processes can contribute to the support of the topography observed at each area. Airy isostasy and regional isostasy (flexure) provide short wavelength support, while thermal isostasy and dynamic uplift provide long wavelength support. By modelling the lithospheric structure below each area, we constrained the contribution of Airy isostasy, thermal isostasy, regional isostasy and dynamic uplift in shaping the topography observed within each area. Lithospheric modelling found the thickest crust (~65-80 km) is located at Fortuna Tessera, followed by the volcanoes (~30-75 km) at Atla and Beta Regio. The crust below the central rift trough, Ganis Chasma, in Atla Regio is < 1 km in thickness. This compares to the crust below Devana Chasma in Beta Regio, which has a thickness of ~15-25 km. The thermal lithosphere is thin below the rises (~50-100 km), but is thicker (~150-340 km) below Fortuna Tessera. The topography observed at each modelled area is primarily supported by a combination of Airy and thermal isostasy. Additional compensation mechanisms, however, are required within these areas such as regional isostasy and dynamic uplift. The dynamic support (at spherical harmonic degrees < ~40) required at each volcanic rise is further supported by a best-fit global convection model with an effective Rayleigh number of 2.5x10⁷, a non-dimensional internal heating of 40, a non-dimensional activation energy of 10, and a lower mantle three times more viscous than the upper mantle. These findings have important implications for the current state of localised rifting and volcanism on Venus. At Atla Regio, the thin thermal lithosphere (~50-100 km), thin crust (< 1km) along the central rift trough of Ganis Chasma, dynamic compensation (up to 1.5 km), and high surface heat flows (~26 mW m⁻²) may facilitate ultra-slow rifting at a rate of ~5-12 mm yr⁻¹ along Ganis Chasma. This rifting and the associated lithospheric thinning at Ganis Chasma, as well as the upwelling mantle plume below Atla Regio, results in pressure-release melting, which may be efficiently extracted to form the crust along this rift at a rate of 0.78 km³ yr⁻¹. In comparison, the thick crust (~15-25 km) below the central rift trough of Devana Chasma in Beta Regio suggests rifting may not be active. Furthermore, the localised dynamic compensation (~0.3 km) found for this area implies Beta Regio's high GTR, size, and swell volume are most likely caused from long wavelength thermal thinning of the lithosphere rather than a broad upwelling mantle plume presently located below the rise. This study demonstrates how rifting and tectonic deformation may exist on a planet without plate tectonics and has implications for studies of exoplanets and Earth's early evolution.