ANATOMY OF A VENUSIAN HOT-SPOT - GEOLOGY, GRAVITY, AND MANTLE DYNAMICS OF EISTLA REGIO

Eistla Regio is a series of broad swells, each up to a few thousand kilometers in diameter, in the equatorial highlands of Venus. It is characterized by strong positive free-air gravity anomalies, shield volcanoes, and rift systems. We present a two-part study of the western and central portions of this Venusian hot spot. First, Magellan radar images were mapped in order to understand the general geologic history of the region. Radial fracture systems both on the rises and volcanoes indicate uplift and associated faulting accompanied volcanic construction. Prominent fracture zones strike WNW to NW, parallel to the long axis of the highlands. The largest of these, Guor Linea, displays a progressive deformation history that may include minor clockwise rotation in addition to bulk NNE-SSW extension. Other regional structural patterns also suggest this sense of rotation. The parts of Eistla studied here appear to be structurally distinct from surrounding regions, although volcanic radar-bright plains and ridge-and-groove deformation to the west indicate an earlier, more diffuse thermal uplift. In the second part of our analysis, Pioneer Venus line-of-sight accelerations were inverted for vertical gravity which, when combined with topography, were used to solve for mass anomalies on the crust-mantle boundary and in the upper levels of the mantle convective system. Both western and central Eistla appear to be sites of vigorous mantle upwelling. A linear upwelling appears beneath Guor Linea, indicating that rifting is actively driven by mantle flow. Minor crustal thinning may be present beneath western Eistla and the northern plains, whereas some thickening occurs beneath central Eistla and the southern plains. Heng-0 corona, in particular, may be the site of a small shallow mass anomaly of either compositional (thickened crust) or thermal origin. Mantle return flow is asymmetrically distributed about Eistla, being much stronger to the south than the north, and little flow is inferred at all to the NE in Bereghinya Planitia. These differences introduce a clockwise rotation to finite strain, in agreement with observations.