Evidence for a Locally Thinned Lithosphere Associated with Volcanism at Aramaiti Corona

Topographic lithospheric flexural signatures at chasmata and coronae on Venus have allowed the effective elastic thickness of the lithosphere, h_e, and the corresponding surface heat flow to be estimated. Most flexural modeling has been restricted to large features due to the limited spatial resolution of Magellan altimetry data. High resolution topography derived from stereo Magellan Synthetic Aperture Radar (SAR) images has recently allowed the systematic study of smaller coronae (diameters 100 -800 km), many of which show substantially thinner lithosphere and correspondingly larger heat flows than estimated at other locations.

Here, we compare axisymmetric elastic flexure model predictions to the stereo topography surrounding Narina Tholi, a steep-sided dome ~40 km across that is spatially associated with the tectonic annulus of Aramaiti Corona. Results using the canonical value for Young's modulus, E, (65 GPa) indicate a best-fit elastic thickness of about 3.5 km. This suggests a thinned lithosphere and elevated heat flow in the vicinity of the tholi compared to an earlier study of Aramaiti Corona that inferred h_e = 13 ± 3 km.

Horizontal radial stresses derived from the best-fit flexural model are ~180 MPa and predict concentric fracturing over an extended radial distance around Narina Tholi. However, only one partial, single concentric fracture associated with the flexure and collocated with the peak stresses is present in the SAR images. This places quantitative constraints on the cohesive strength and E of the surface material. A smaller value for E, motivated by terrestrial in-situ experiments on rock masses weakened by discontinuities, results in reduced peak stresses that are more consistent with observations. For E = 30 GPa, stresses are reduced by 40% and elastic thickness values are correspondingly larger (~4.5 km), but still indicate a thinned lithosphere and elevated heat flow compared to the surrounding region.

Our results shed light on rock properties and lithospheric thickness, by combining topography with SAR image observations. Future satellite missions that yield topographic and image coverage at higher resolution will permit investigation of additional small features.