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

摘要

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.