TECTONIC EVOLUTION OF BELL-REGIO, VENUS - REGIONAL STRESS, LITHOSPHERIC FLEXURE, AND EDIFICE STRESSES

In order to understand the relationship between volcanic and tectonic processes and the stress state in the lithosphere of Venus, we analyze d the stress environments and resulting tectonic features associated w ith the major volcanic edifices in Bell Regio, using Magellan syntheti c 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 ed ifices on Venus, The most prominent distinction is the lack of large r ift zones within the over all 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 surr ound volcanoes on Earth and Mars, Tepev and Nyx Montes exhibit both th e radial tectonic features associated with other Venusian edifices and numerous concentric graben. Nyx Mons implies a more distributed magma tic system by its broad shape, radial chains of pit craters, and expan sive flow fields, whereas Tepev Mons is a more centralized volcanic sy stem, with limited associated long flows. We investigate the regional stresses associated with Bell Regio and structural features believed t o be a consequence of lithospheric flexure due to volcanic loading, mo deling both Nyx Mons and Tepev Mons as axisymmetric loads with Gaussia n 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 anal ysis. Using topography data to model the shape of the volcano, we dete rmine 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 sh ift in volcanic style within Bell Regio from an early phase of broad, low shield formation to later steep-sided, more centralized edifice de velopment, Such changes are consistent with an increase in the thickne ss of the lithosphere over time.