The potential for crustal delamination at the base of crustal plateaus on Venus: The case of Western Ovda Regio

The crustal plateaus on Venus are highlands characterized by strongly deformed terrains, called tessera, and correspond to the stratigraphically oldest features on the planet [1]. Investigations of the gravity and topography signatures of the plateaus show that they are compensated by thickening of the crust and are consistent with being in an Airy isostasy state [2,3].  Recent crustal thickness estimates indicate that the base of the crust under plateaus can reach depths larger than 50 km [4], which correspond to pressures large enough to generate eclogite. A key property of eclogite is that it has a density larger than the surrounding mantle. This negatively buoyant material causes delamination and recycling of the crust. Hence, the depth of eclogite formation is commonly used to define the maximum possible crustal thickness of planets, and of Venus in particular [e.g. 5].

Nevertheless, the process of delamination of high-density eclogite under thickened crustal regions has not been numerically studied in detail using thermodynamic models that calculate the densities consistent with Venusian petrology and basalt-to-eclogite phase change. In this study, we combine depth-dependent crustal density profiles obtained with Perple_X considering the surface composition constrained by Vega 2 (see [6] for details) and the finite-element software COMSOL multiphysics  that couples heat transfer, creeping flow, and the level set modules to investigate the delamination process. We focus in particular  on the Western Ovda Regio, as this process could be relevant for  the crustal structure suggested at this location by gravity and topography studies [5].

Western Ovda Regio, shown in Figure 1, has a unique topographic signature among all crustal plateaus. It presents a high-elevation rim, reaching up to 2 km altitude in the eastern part, and a collapsed center heavily embayed by the volcanic plains. The processes that caused this central collapse is not well-understood and cannot be explained by viscous relaxation alone [e.g., 7, 8]. In this study we investigate if Western Ovda could have experienced  delamination of a high-density eclogitic material, which is thought to have formed at the base of the plateau as a consequence of the basalt-to-eclogite phase transition.

Our first results show that generating negatively buoyant eclogitic crust is not enough to trigger recycling. If the temperature is cold, this eclogitic crust would be trapped in the so-called stagnant lid, an immobile layer that forms as a consequence of the temperature-dependence of the viscosity.  When considering a linear thermal gradient of 5 K/km throughout the crust, negatively buoyant material can be found below 40 km depth, but this material is only recycled if the crust reaches about 100 km thickness. On the other hand, if the temperature is high enough, (i.e., about 1200K), the material is able to flow, and the deeper crustal layer could delaminate. For a thermal gradient of 10 K/km, the stagnant lid is thin enough to allow for high density materials that form at depths of around 60 km to delaminate. In conclusion, our results show a delicate interplay between the thickness of the crust, the thermal conditions of the lithosphere and the density contrast between the crust and mantle. In scenarios where delamination occurs, the models indicate that the high surface topography indeed collapses, as we expected. Next steps in this study will include testing a wider range of model setups, in particular testing different lithospheric thermal gradients. In addition, we will extend our analysis by estimating the gravity signature from these models and comparing them with the observations. 

References

[1] Ivanov and Head, 2011. PSS; [2] Grimm, 1994. Icarus; [3] Kucinskas and Turcotte, 1994. Icarus; [4] Maia and Wieczorek, 2022. JGR: Planets; [5] James et al., 2013. JGR: Planets; [6] Collinet et al., 2024. EPSC 2024 (this meeting); [7] Nunes et al., 2004. JGR; [8] Nunes and Phillips, 2007. JGR