Central Eistla Regio: Origin and relative age of topographic rise

Large topographic rises on Venus pose a number of fundamental questions concerning the mantle processes responsible for them. Eistla Regio, a 7500 km‐long elevated region extending WNW from the west end of Aphrodite Terra, is a particularly interesting example. The Sappho Patera quadrangle, 0–25°N, 0–30°E, includes all of central Eistla Regio, a region dominated by two interesting, apparently hybrid structures (Irnini and Anala Montes) with characteristics of both shield volcanoes and coronae. Mapping and tectonic analysis within this region indicate that this part of the rise experienced progressive deformation extending from prior to emplacement of the regional plains to after emplacement of young lavas; that is, for most of the stratigraphically defined history of Venus. Estimates of the relative roles of uplift and volcanic construction in the evolution of Irnini and Anala Montes are critically dependent on whether they are primarily anomalous coronae that have been uplifted, or whether they are true shield volcanoes that have evolved into coronae. Studies elsewhere, as well as most models of corona evolution, favor the first alternative; the geology of central Eistla Regio favors the second, suggesting that local thinning of the lithosphere allowed shield volcanoes to evolve into coronae. Because these hybrid edifices are not typical of either most coronae or most shield volcanoes on Venus, they deserve detailed modeling. For now, the relative roles of uplift and volcanic construction in central Eistla Regio cannot be even approximately estimated.