Tectonic evolution of Bell Regio, Venus: Regional stress, lithospheric flexure, and edifice stresses

Abstract

In order to understand the relationship between volcanic and tectonic processes and the stress state in the lithosphere of Venus, we analyzed the stress environments and resulting tectonic features associated with the major volcanic edifices in Bell Regio, using Magellan synthetic aperture radar (SAR) images and altimeter measurements of topography. The major volcanoes of Bell Regio, Tepev Mons and Nyx Mons, exhibit tectonic characteristics that are unique relative to other volcanic edifices on Venus. The most prominent distinction is the lack of large rift zones within the overall highland uplift, which characterize many other highland rises on Venus. Also, previous studies have determined that many large Venus volcanoes exhibit radial tectonic structures on their flanks but generally lack the circumferential graben which surround volcanoes on Earth and Mars. Tepev and Nyx Montes exhibit both the radial tectonic features associated with other Venusian edifices and numerous concentric graben. Nyx Mons implies a more distributed magmatic system by its broad shape, radial chains of pit craters, and expansive flow fields, whereas Tepev Mons is a more centralized volcanic system, with limited associated long flows. We investigate the regional stresses associated with Bell Regio and structural features believed to be a consequence of lithospheric flexure due to volcanic loading, modeling both Nyx Mons and Tepev Mons as axisymmetric loads with Gaussian mass distributions on an elastic plate. The relationship between the tectonic features surrounding Tepev Mons and stresses associated with magma chamber inflation are also examined through finite element analysis. Using topography data to model the shape of the volcano, we determine that a horizontally ellipsoidal or tabular reservoir at a range of depths from approximately 20 to 40 km can satisfy the locations of graben formation observed in Magellan images. These results imply a shift in volcanic style within Bell Regio from an early phase of broad, low shield formation to later steep-sided, more centralized edifice development. Such changes are consistent with an increase in the thickness of the lithosphere over time.