The Role of the Crust and Elastic Lithosphere in the Formation and Evolution of Venusian Type 2 Coronae

The population of over 500 coronae provide a useful means of probing the structure of the lithosphere on Venus. We examine the admittance signature for all the 37 Type 2 coronae (defined as having less than 50% complete fracture annuli) that are well resolved in the gravity data using both Cartesian and wavelet admittance approaches. We obtain a wide range of elastic thicknesses, from 10 to 85 km. Larger values are derived from bottom loading models, which have not been used in most prior studies of Venus. Estimates of crustal and elastic thicknesses obtained for Type 2 coronae span the range obtained for Venus globally. Neither the thickness of the elastic lithosphere nor the crust appear to control whether Type 1 coronae, Type 2 coronae, or volcanoes form over small scale mantle upwellings. Using the estimated lithospheric properties, loading signature, and geologic characteristics, we examine the factors controlling corona morphology, size, and fracture pattern (Type 1 versus Type 2). Elastic thickness has no correlation with diameter, and thus does not limit the location of coronae. The ratio of the crustal thickness to plume diameter does not control morphology as a function of size, as predicted by a spreading drop model. However, rim only coronae, which are predicted to form via isostatic rebound of crust thickened by delamination are clearly supported by a density interface. This interface could be the basalt-eclogite transition, which would favor delamination and might be more common in the plains. Lithospheric properties for Type 2 coronae vary with topographic morphology. The complex processes that form different morphologies preclude a simple increase in elastic thickness over time. Overall the stage of evolution and formation mechanism account for the majority of variability found in Type 2 corona morphology, and probably for Type 1 coronae as well. The role of a density interface and delamination of the lower lithosphere are likely more important in understanding the evolution of the Venusian lithosphere than variations in elastic thickness.