Evidence for Active Hotspots on Venus from Analysis of Magellan Gravity Data

Abstract

The 500-Myr average crater retention age for Venus has raised questions about the present-day level of tectonic activity. In this study we examine the relationship between the gravity and topography of four large volcanic swells, Beta, Atla, Bell, and Western Eistla Regiones, for clues about their stage of evolution. The Magellan line-of-sight gravity data are inverted using a point mass model of the anomalous mass to solve for the local vertical gravity field. Spectral admittance calculated from both the local gravity inversions and a spherical harmonic model is compared to three models of compensation: local compensation, a "flexural" model with local and regional compensation of surface and subsurface loads, and a "hotspot" model of compensation that includes top loading by volcanoes and subsurface loading due to a deep, low density mass anomaly. The coherence is also calculated in each region, but yields an elastic thickness estimate only at Bell Regio. In all models, the long wavelengths are compensated locally. The long-wavelength estimates of the deep compensation depth for Bell, Atla, Western Eistla, and Beta Regiones are 125, 175, 200, and 225 km, respectively, with an error of approximately ± 35 km. The flexural model is rejected primarily because it gives values of effective elastic thickness of approximately 100 km, which is at least a factor of two larger than estimates of effective elastic thickness obtained from the coherence spectra at Bell Regio and from other studies that model the topographic expression of flexure at Bell Regio and in other tectonic settings globally. Assuming a crustal thickness of 30 km, the fit to the hotspot model at Atla Regio gives an effective elastic thickness of 30 ± 5 km. Atla Regio is interpreted as an active hotspot because of the deep compensation depth and a strong subsurface loading signature. At Bell Regio, effective elastic thickness is 30 ± 5 km at short wavelengths and 50 ± 5 km at long wavelengths, using a crustal thickness of 10 km. The 30-km value is interpreted as the effective elastic thickness at the time when the volcanoes are emplaced; the 50-km value is believed to reflect the present-day effective elastic thickness. Bell Regio has a relatively shallow compensation depth, 125 km, and a weak bottom-to-top-loading ratio determined from the coherence spectra. These results may indicate a relatively old, possibly inactive plume. The data at Beta and Western Eistla Regiones are of relatively poor quality and do not permit detailed interpretations. These areas are interpreted as active hotspots because of their large compensation depths, greater than typical of the rest of the planet.