Unveiling Magma Reservoir Depths Beneath Venusian Coronae: Insights from the Parga Region
Authors: De Toffoli Barbara , Plesa Ana-Catalina , Mazzarini Francesco , Ghail Richard , Breuer Doris 
Published: July 2024

IntroductionVenus represents a terrestrial planet similar to Earth in size and mass, but its evolution is characterized by a lack of plate tectonics as observed on Earth. On Venus a rich variety of tectono-magmatic structures were detected. In this context, major zones of young extensional structures, called chasmata, are present on the Venus surface (e.g., Ivanov and Head, 2011). The origin of these rift systems is still debated. Suggested hypotheses include diapiric upwelling (Stofan et al., 1992) and lithospheric extension over the heated mantle (Phillips and Hansen, 1998; Hansen and Phillips, 1993).Parga Chasma is a discontinuous rift system extending for 10,000 km from Atla Regio to Themis Regio (Chapman and Kirk, 1996). On Earth, large-scale rifting is usually associated with plume centers, or igneous provinces, with faulting and fracturing spreading around volcano-magmatic centers forming triple-junction rifting. Comparable features in the Parga region are magmatic centers constituted by coronae or large volcanoes (Graff et al., 2018). Coronae are circular to elongated features surrounded by concentric fractures that reach diameters of hundreds of kilometers (Basilevsky &amp; Head, 2003) and are mostly located along rifts (Stofan et al., 1997). Coronae are likely the result of the mantle plumes&#8217; rise and collapse that generates typical radiating volcanic flows and concentric annular structures (Basilevsky &amp; Head, 2003). These features can be found in large numbers making them an outstanding tool for the lithosphere thickness investigation (Johnson and Solomon 1996).MethodsWe collected existing datasets of mapped coronae produced using Magellan SAR data sets (Gazetteer of Planetary Nomenclature USGS-NASA; Martin et al., 2007; Stofan et al., 2001). For the investigation of the regional-scale evolution of the study area, coronae included in the analyses needed to be on the rift or in a 1,500 km range distance (Martin et al., 2007; Hamilton and Stofan, 1996). As a result, the analyzed population was of 141 coronae identified in QGIS software as point-shaped features localized at the center of coronae&#8217;s anuli. Accordingly, all analyses were performed on datasets extracted through QGIS software tools for distance and Nearest Neighbor determination obtained by handling the point shapefile identifying coronae locations.In cases where a substantial number of features are surveyed on large areas, like Parga, point clustering can be conducted using MINITAB&#174; software and an agglomerative hierarchical clustering method (Mazzarini, 2007). Clustering provides a statistically significant subdivision of the original population allowing further analyses to explore the possibility of having multiple reservoirs feeding the vents of interest.&#160;The presence of numerous volcanic features suggests the existence of subcrustal/crustal magma reservoirs formed by magmatic upwelling. The Rayleigh Taylor (R-T) gravitational instability theory, applied to Venusian coronae, establishes a relationship between the spacing of volcanic structures at the surface and the depth of underlying magmatic reservoirs. This theory describes the upwelling of less dense material when overlain by denser layers, leading to gravitational instability and vertical spread of material. Depending on melt viscosity, instabilities may develop, resulting in upwelling and penetration of less dense material into the overlying fluid at evenly spaced points (Whitehead et al., 1984; Tackley et al., 1992). This approach, previously applied to volcanic clusters on Earth, including the Newer Volcanic Province in Southern Australia (Lesti et al., 2007) and the Main Ethiopian Rift (Mazzarini et al., 2013), is employed to identify the depth of thermal anomalies acting as magma source regions beneath Venusian coronae. The investigation utilizes a sinusoidal projection centered on the Parga region to minimize spatial distortion, with the average separation between coronae implemented in the R-T gravitational instability function to estimate the anomaly's wavelength and depth.In order to investigate the depth of magmatic reservoirs we also performed thermal evolution modeling. To this end we used the geodynamic code GAIA (H&#252;ttig et al., 2013) in a 2D spherical annulus geometry (Fleury et al., 2024). Our models include a variable thermal conductivity and thermal expansivity (Tosi et al., 2013) and we test different scenarios varying the magmatic style from fully intrusive to fully extrusive magmatism (Herrera et al., this meeting). In addition, we varied the depth at which melt remains trapped in the subsurface. The depth of the thermal anomaly obtained from the R-T analysis is compared to the depth of melting in our geodynamic models. Additionally, the spacing of volcanic structures observed at the surface is compared to the spacing obtained in our&#160;models between small-scale convection structures to select the scenarios that fit best the geological analysis.&#160;&#160;&#160;Preliminary ResultsOur findings provide insights into the spatial distribution and depths of thermal anomalies beneath Venusian coronae in the Parga region. Cluster analysis unveiled distinct spatial groupings of coronae, with notable polarization observed towards the northwest and southeast ends of the rift. Despite spatial clustering tendencies, the average distances between coronae remained remarkably consistent across the whole population and individual clusters. Utilizing the R-T gravitational instability theory, we estimated depths of thermal anomalies beneath coronae, providing constraints for the underlying magmatic processes. Specifically, we obtained a thermal anomaly depth of 117 &#177; 10 kilometers for the total population; spanning between 111 kilometers for the northwest cluster and 126 kilometers for the southeast cluster. Moreover, along Parga, an increase of the size of the coronae is observed from the north-west sector toward the southeastern one. Comparing the thermal anomaly depth obtained from the R-T analysis and the spacing between coronae in Parga Chasma with our thermal evolution models we find that best fit scenarios require less than 40% of the melt generated in the interior to reach the surface, indicating that Parga Chasma is dominated by magmatic intrusions.ImplicationsIn conclusion, our study establishes a direct link between coronae formation and the location of magmatic reservoirs beneath Venus&#8217; surface. By elucidating this relationship, our research sheds light on the dynamic processes shaping the Venusian lithosphere and offers valuable insights into the underlying mechanisms driving its tectonic evolution.AcknowledgementThis work is funded by the European Union &#8211; NextGenerationEU and by the 2023 STARS Grants@Unipd programme &#8211; &#8220;HECATE project&#8221;.