EVIDENCE FOR ACTIVE HOTSPOTS ON VENUS FROM ANALYSIS OF MAGELLAN GRAVITY-DATA

The 500-Myr average crater retention age for Venus has raised question s about the present-day level of tectonic activity. In this study we e xamine the relationship between the gravity and topography of four lar ge volcanic swells, Beta, Atla, Bell, and Western Eistla Regiones, for clues about their stage of evolution. The Magellan line-of-sight grav ity data are inverted using a point mass model of the anomalous mass t o solve for the local vertical gravity field. Spectral admittance calc ulated 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 in cludes top loading by volcanoes and subsurface loading due to a deep, low density mass anomaly. The coherence is also calculated in each reg ion, but yields an elastic thickness estimate only at Bell Regio. In a ll models, the long wavelengths are compensated locally. The long-wave length estimates of the deep compensation depth for Bell, Atla, Wester n Eistla, and Beta Regiones are 125, 175, 200, and 225 km, respectivel y, with an error of approximately +/- 35 km. The flexural model is rej ected primarily because it gives values of effective elastic thickness of approximately 100 km, which is at least a factor of two larger tha n estimates of effective elastic thickness obtained from the coherence spectra at Bell Regio and from other studies that model the topograph ic expression of flexure at Bell Regio and in other tectonic settings globally. Assuming a crustal thickness of 30 km, the fit to the hotspo t model at Atla Regio gives an effective elastic thickness of 30 +/- 5 km. Atla Regio is interpreted as an active hotspot because of the dee p compensation depth and a strong subsurface loading signature. At Bel l Regio, effective elastic thickness is 30 +/- 5 km at short wavelengt hs and 50 +/- 5 km at long wavelengths, using a crustal thickness of 1 0 km. The 30-km value is interpreted as the effective elastic thicknes s at the time when the volcanoes are emplaced; the 50-km value is beli eved to reflect the present-day effective elastic thickness. Bell Regi o has a relatively shallow compensation depth, 125 km, and a weak bott om-to-top-loading ratio determined from the coherence spectra. These r esults may indicate a relatively old, possibly inactive plume. The dat a at Beta and Western Eistla Regiones are of relatively poor quality a nd do not permit detailed interpretations. These areas are interpreted as active hotspots because of their large compensation depths, greate r than typical of the rest of the planet. (C) 1994 Academic Press, Inc .