Lithospheric Flexure, Stress, and Volcanic Edifice Morphology: A Connection for a Subset of Coronae on Venus?

English Abstract:

The lithosphere responds to emplacement of volcanic materials by flexing. Flexure induces stress in the lithosphere that can influence subsequent ascent of magmas and thereby affect the shapes of the resulting volcanic edifices. Principal stress orientations and considerations of force balance on vertical dikes provide two stress-based criteria for determining where magmas can ascend. A key finding is that lithospheric thickness T_e and edifice shape are linked: thick lithospheres produce low stresses consistent with broad conical edifices, while thinner lithospheres produce higher stress magnitudes and shorter-wavelength flexural responses that can lead to construction of domical edifices and, at the lowest T_e values, annular-shaped topographic ridges. Annular structures are the primary characteristics of the morphometric (not genetic!) class of features known as coronae. Thus, I propose that construction of volcanic edifices upon lithosphere with low T_e can account for the formation of a significant fraction of coronae on Venus.

To constrain this fraction, I consider constraints on T_e from analysis of gravity and topography. Hoogenboom et al. [2004] used spatio-spectral localization of gravity/topography relationships to estimate T_e at 103 coronae on Venus, using top- and bottom-loading models of lithospheric flexure. Here I focus on the 65 coronae determined to be best-fit with a top-loading model, in accordance with the volcanic construction hypothesis. The overall distribution of best-fit T_e is skewed toward low values (mean 12 km, median 6 km), but with a long tail of values stretching up to 48 km. The high T_e values in this tail have been used to argue against the idea that thin lithospheres favor corona formation, but the strong skewing of the distribution toward zero supports this idea. Further, when coronae in the “Fracture Belt” geologic setting are excluded, the best-fit T_e distribution of the remaining 20 coronae has a mean of 6.4 km and median of 4.5 km, with all values less than 20 km (i.e., no tail). The distribution of the 45 coronae in the Fracture Belt setting, therefore, has all the members of the long tail. Further analysis of coronae in the Fracture Belt setting is warranted, to determine if the coronae composing the tail are anomalous in some structural or geographical manner.