Low viscosity and highly intrusive magmatism beneath Venusian coronae in Parga Chasma

INTRODUCTION

Venus is a terrestrial planet similar to Earth in size and mass, but its geological evolution followed a very different path leading to a dense CO2 atmosphere and the absence of plate tectonics at present day. Radar data collected by NASA’s Magellan mission revealed a surface characterized by a wide variety of tectonic and volcanic features, including large extensional systems known as chasmata (Ivanov and Head, 2011), whose origin is still debated (Stofan et al., 1992, Phillips and Hansen, 1998; Hansen and Phillips, 1993).

One of the largest rift systems on Venus is Parga Chasma, which extends for about 10,000 km from Atla Regio to Themis Regio (Chapman and Kirk, 1996; Stofan et al., 1992) and contains several discontinuous rift segments associated with coronae and large volcanic centers (Graff et al., 2018). Coronae are circular to elongated structures surrounded by concentric fractures and are thought to form through the rise and collapse of mantle plumes (Basilevsky & Head, 2003). Since they are widespread along the rift, analyzing these features provides hints on the tectonic evolution and lithospheric structure of Venus.

Distinct spatial clustering of coronae along the rift and estimated thermal anomaly depths between ~110 and 130 km (De Toffoli et al., 2024) suggest that Parga Chasma is dominated by magmatic intrusions rather than volcanism. Building on these results, we extended this study to further constrain the rheological properties of the Venusian mantle beneath the Parga region varying the reference mantle viscosity and magmatic parameters related to the magmatic style and intrusions depth.

METHODS

We perform geodynamical simulations using the mantle convection code Gaia-v2 (Huttig et al., 2013). Our models are built with a 2D spherical annulus geometry (Fleury et al., 2024). While our model size spans a larger area than Parga Chasma, this choice was made to include the geodynamic processes of Parga Chasma in a more global geodynamic context, and avoid any effects that might be associated with model boundaries.

Our geodynamic models have a non-Newtonian rheology Assuming a depth- and temperature-dependent viscosity (Hirth & Kohlstedt, 2003), and dry dislocation creep (Karato et al., 1986; Karato & Jung, 2003). We also assume depth- and temperature-dependent thermal conductivity and expansivity (Tosi et al., 2013). We account for radiogenic heat decay (Moroz et al. 1980), core cooling (Steinbach & Yuen, 1994), and melting curves based on thermodynamic models for an Earth-like mantle composition (Stixrude et al., 2009).

We consider that the melt generated when the temperature exceeds the solidus melting curve can partially reach the surface (extrusive magmatism), but also part of this melt can remain trapped in the interior (intrusive magmatism).  Extrusive melts instantaneously cool to surface temperature, while magmatic intrusions cool adiabatically according to the intrusive melt depth assumed. Magmatic intrusions also evolve according to the rheological critical melt fraction (Arzi, 1978), distinguishing between solid- and melt-dominated regimes that respectively reduce the viscosity as a function of the melt fraction or increases the thermal conductivity to capture the effect of small-scale convection in the melt.

The intrusive to extrusive ratio in the Parga chasma region is unknown, and thus we vary the amount of extrusive melt between 0% (fully intrusive) and 100% (fully extrusive) in steps of 10%. Since the depth at which the melt remains trapped below the surface is poorly constrained, we also vary this parameter among cases between 10 km and 90 km in steps of 10 km, and the value is kept fixed within each case. We analyze this parameter space for two sets varying the reference mantle viscosity: one assuming 10²⁰ Pa s and 10²¹ Pa s, that could be attributed to differences in the mantle composition of Venus.

RESULTS

Models assuming a higher mantle viscosity predict melting regions located at depths significantly greater than the observed range in De Toffoli et al., (2024) and are therefore inconsistent with the inferred thermal structure. In contrast, several models with a mantle reference viscosity of 10²⁰ Pa s are compatible with the estimated depths. This lower viscosity is reasonable for a hot mantle domain beneath an active rift system, especially if partial melting and elevated temperatures reduce mantle strength. This reference viscosity value is consistent, although it lies at the lower end of the reference viscosity range inferred from tidal deformation studies of the interior of Venus (Walterová et al., 2024; Musseau et al., 2024).

Our best fitting scenarios also require a dominant contribution from magmatic intrusions rather than surface volcanism, supporting the interpretation that intrusive magmatism plays a major role in the evolution of the Parga rift system. Although our findings do not distinguish between suggested forming mechanisms of rift systems that point to diapiric upwelling (Stofan et al., 1992) or lithospheric extension over heated mantle (Phillips and Hansen, 1998; Hansen and Phillips, 1993), they help to narrow the parameter space of both hypotheses, suggesting the presence of a relatively weak and thermally active mantle beneath the rift, consistent with plume-related, tectono-magmatic activity and localized lithospheric weakening on Venus.

SUMMARY

Our analysis indicates that geodynamic models compatible with previous depth estimations are scenarios dominated by magmatic intrusions and lower mantle viscosities, further supporting the interpretation that intrusive magmatism plays a major role in the evolution of the Parga rift system.