Gravity analysis of Parga and Hecate chasmata: Implications for rift and corona formation

Abstract

[1] The two largest rift systems on Venus, Parga, and Hecate chasmata contain one third of all coronae. We map variations in elastic thickness and apparent depth of compensation (ADC) in these two regions using the admittance function for gravity and topography. We examine the relationship between rifting and coronae by comparing lithospheric structure with corona characteristics including volcanism, topographic shape, fracture pattern, diameter, and stratigraphic age. At Hecate chasmata, both ADC and elastic thickness correlate with two main rift branches, the fracture style and corona characteristics. Hecate chasma thus appears to be dominated by extensional processes. Parga chasma shows little correlation between lithospheric properties and the location of coronae or rift segments. The ADC at Hecate and Parga chasmata is mostly less than 75-100 km, implying that there is not large-scale upwelling underlying the rift systems. The variations in the corona population sizes between Parga chasma, which has 131 coronae, and Hecate chasma, with 50 coronae, suggest differences in the evolution of the two rift systems. The much larger population of coronae at Parga contains more small coronae, many of which have relatively low volcanism and post date the rift. Further, many of the small coronae occur in chains and have similar diameters, as predicted by Rayleigh-Taylor type instabilities. These characteristics suggest that the smaller coronae may form via a later, secondary decompression melting phase. Thus, Hecate chasma may be younger than Parga chasma and could experience a secondary stage of corona formation in the future.

Themis Regio, Venus: Evidence for recent (?) volcanism from VIRTIS data

“…In terms of trying to better understand the evolution of a venusian hotspot, this study is consistent with previous work ( Martin et al, 2007;Smrekar et al, 2010b ) that there is no apparent sequence of volcano or corona formation in Themis Regio. The results indicate that recent volcanism has occurred both on and off the rise, and both at volcanoes and coronae.…”

“…The regional map of crustal and elastic thickness correlates well with the region of high emissivity anomalies. Smrekar et al (2010b) produced maps of elastic and crustal thickness focusing on Hecate and Parga Chasmata, but covering most of Themis Regio and the southern end of Parga Chasma. In their analysis, the Themis regions with high emissivity correspond to areas of low elastic thickness ( ∼10 to 30 km) and compensation depths of ∼75 to 105 km, in general agreement with more localized studies.…”

Rheological and petrological implications for a stagnant lid regime on Venus

“…Differences in lid extension rates are immediately apparent; major melting extends for more than 1500 km from Parga Chasma but only 750 km from Devana Chasma. The extensive area of melt available under Parga and Hecate Chasmata may explain their greater concentration of coronae (Smrekar et al, 2010a), and within the Beta-Atla-Themis ("BAT") region as a whole. Beyond the area of asthenosphere the crust and mantle are relatively strong and coupled.…”

Coronae formation on Venus via extension and lithospheric instability

“…At Hecate, the lithospheric structure clearly reflects the extensional environment. In contrast, at Parga, there appears to be little correlation between the apparent depth of compensation and elastic thickness with rift branches, fracture style, and corona characteristics [ Smrekar et al , ].…”

“…They have low levels of volcanism, are smaller in diameter, and have topographies consistent with delamination, such as depressions and outer rings [ Martin et al , ]. These coronae have an average diameter of 175 km (compared to an overall average diameter of 200 km) and may be characterized as a particular step in the process of the interaction of a mantle plume with the lithosphere: a late secondary decompression melting phase [ Smrekar and Stofan , ; Smrekar et al , ]. And so we pose the question: is there a way to connect the mantle convection associated with the rift to the coronae and associated volcanism we observe more than 100 km away from the rift?…”