Regional topographic rises on Venus: Geology of Western Eistla Regio and comparison to Beta Regio and Atla Regio

Abstract

Regional topographic rises in the equatorial region of Venus (Western Eistla Regio, Beta Regio, and Atla Regio) form a unique class of structures which on the basis of geologic mapping and geophysical data are interpreted to be sites of mantle upwelling or plumes. The geologically least complex of these regions is Western Eistla Regio, an elongated domical rise that contains the volcanoes Sif and Gula montes. Sif Mons is a large volcano (diameter > 100 km) whose local stratigraphy indicates that it has undergone a history marked by initial radial fracturing, widespread outpouring of lava, additional fracturing along its northern flank, and extrusion of lava that is either extremely fluid or was erupted at a high rate of effusion. Sif contains a 40‐km‐diameter summit caldera within which are nested smaller calderas and pit structures. Theoretical considerations suggest multiple levels of magma emplacement within the volcano, with the primary reservoir residing in the crust below the edifice. Gula Mons is made up of multiple volcanic source regions and is associated with Guor Linea, a zone of lithospheric stretching, faulting, and rifting. The geologically complex highland of Beta Regio is a tectonic junction with three arms of Devana Chasma intersecting at Theia Mons. Magellan data reveal (1) the details of rifting and volcanism concentrated at Theia Mons, (2) that Rhea Mons is highly deformed and possesses characteristics of tesserae and whose origin as a volcano is placed in question, (3) that tesserae are a major unit, arrayed in a pattern around Theia from east to west starting on the eastern flank of Beta and continuing westward to the northern part of Asteria Regio, (4) that the northern periphery of the highland contains coronae connected by fractures forming chains, and (5) the presence of a deformed crater within Devana Chasma, split and extended in an east‐west direction, indicating that part of the rift has undergone as much as 20–30% extension. Like Beta, Atla Regio is a complex area of converging rifts (five separate arms) that are centered on the volcano Ozza Mons. The Magellan images show that volcanism and rifting have occurred contemporaneously at Ozza, with lava flows both covering and being crosscut by faults. A second volcano, Maat Mons, contributes to the infilling of the rift Dali Chasma and does not appear to be deformed by faulting. Two of the rifts at Atla contain chains of coronae, suggesting that upwelling along linear zones of extension has occurred. Beta, Atla, and Western Eistla are similar in that they are broad regional rises, are associated with major rift zones, have large apparent depths of isostatic compensation, and contain large volcanoes. Their associations with features mapped as coronae and tesserae are quite variable, suggesting that the detailed characteristics of the individual highlands are most likely linked to each areas local geologic environment.