Major Rifts on Venus: Lithospheric Properties and Formation of Parga and Hecate Chasmata

Parga and Hecate Chasmata are complex rift systems each approximately 10,000 km in length. Each chasmata has multiple branches at varying angles to the main trend, and over a hundred associated coronae. Their admittance signatures are distinct from the surrounding plains, and from each other. At Hecate Chasmata, admittance signatures and estimated elastic thicknesses are relatively uniform along several of the rift segments, while at Parga, most of the rift segments display variable admittance signatures. In addition, at Hecate, thinner elastic lithosphere correlates with coronae that are stratigraphically younger than the rift. The correlation between lithospheric properties and rift segments at Hecate suggests that rift formation at this location may be influenced by lithospheric properties, as occurs on Earth. Applying simple models of rift formation based on uniform regional extension (Buck, 1991) and comparing with estimated crustal and elastic thicknesses shows that both these rifts should be considered as wide rifts, which is favored for a strong crustal rheology. This is consistent with their average widths of approximately 150 km and even wider zones of fractures. The implication of the wide rift designation is that spreading velocities are on the order of 1 cm/yr, which is faster than might be expected for a stagnant lid convective regime. Agreement between models, lithospheric properties and stratigraphy at Hecate suggests that uniform extension may explain rifting at Hecate. However Parga does not display such correlations, possibly indicating modification since rifting occurred. The relationship between coronae and rifts remains enigmatic, and could have a significant effect on lithospheric properties.