Novae on Venus: Geology, classification, and evolution

[1] Novae are "radially fractured centers'' 100-300 km in diameter, and 64 have been identified on Venus. Dense radial fracturing and upraised topography are common, and massive amounts of volcanism are seen in some. Novae are interpreted to be the result of updoming and fracturing of the surface due to interaction of mantle diapirs with the lithosphere and radial fracturing caused by dike emplacement. We analyzed the topography and character of deformational and morphological structures of all 64 novae, leading to a better understanding of their evolution and relation to diapir dynamics, and their age. We subdivided novae into four topographic classes: ( 1) upraised, ( 2) annular, ( 3) flat and negative, and ( 4) plateau-like. Novae are located mostly in areas of regional rises and rift zones, with a small number in the lowlands. About one half the novae studied show association with rifts; in some of these the rift troughs surround nova rises and form plateau-like novae. Plateau- like novae may predate, postdate, or form simultaneously with rifts. Our analysis generally confirms and updates previous models of novae evolution, with the following geological factors being important: ( 1) depth of the neutral buoyancy level of the rising mantle diapir; ( 2) rheological characteristics of the overlying lithosphere; ( 3) whether or not the visco-plastic material above the diapir undergoes spreading; and ( 4) the nature and influence of regional stress and rift structures. These combined factors exert control on the direction of nova evolution and appear to determine the primary characteristics of nova, such as topography, tectonics, and volcanism. We found clear evidence for ( 1) the important role of dike swarm emplacement in the formation of radial patterns of novae and ( 2) the formation of concentric tectonic features during the relaxation stage of novae. Novae have multiple evolutionary stages and are long-lived structures. Stratigraphic analysis of all 64 local areas showed a similarity in the sequence of regional geologic units. We found that 40.3% of the novae population started to form before emplacement of regional plains with wrinkle ridges and that 11.3% completed their activity before this time; 88.7% of the population of novae was active after regional plains formation. In contrast to novae, coronae activity was greatest before formation of regional plains, which may be due to thickening of the lithosphere with time. Detailed structural analysis shows that novae evolution does not always lead to the formation of corona-like features.