ABSTRACT Small shield volcanoes with basal diameters <20 km represent the most abundant type of volcano on Venus. These shield volcanoes number >>10 6 in population and often occur in clusters known as shield fields, which have been interpreted to be analogous to basaltic volcanic fields on Earth. Despite previous work on shield fields, questions related to edifice morphology and magma viscosity, timing relations of events across an individual field, volume of erupted material, and the role of tectonic structures are still unresolved. Here, we address those questions through geologic mapping, volumetric calculations, and statistical analysis of possible edifice alignments in six venusian shield fields: Asherat Colles, Chernava Colles, Monoshi Tholus, Nordenflycht Patera, Ran Colles, and Urutonga Colles. Our results indicate that all of these shield fields and their associated deposits are younger than the surrounding units within the mapping areas, and each field displays overlapping temporal relations with local extensional and contractional structures. Each field also displays a lack of a consistent pattern in the temporal distribution of volcanism with regards to edifice type. Analyses of possible edifice alignments suggest edifice trends that are consistent with mapped tectonic structures within all studied fields except Asherat Colles. Comparison of these six venusian fields to terrestrial basaltic volcanic fields shows that venusian fields may be up to two to three orders of magnitude larger in their areal expanse and volume of erupted material. Our results are consistent with previous interpretations of venusian shield fields representing low rates (likely <5 × 10 −4 km 3 /yr) of magma supply feeding these magmatic centers and highlight the effects of the resolution limit of the Magellan data set on interpreting fundamental geologic processes on the venusian surface.

Popular models of slow unidirectional evolution of each planet are based on dogmatized 1970s–1980s speculations that Earth has a perpetually hot core that drives narrow vertical plumes of unfractionated mantle which produce volcanoes, propel lithosphere plates, and are compensated by subduction. Long-lasting hot cores, plumes, and minimal fractionation were dogmatized also for Venus and Mars, by analogy, but with a different stagnant-lid conjecture, rather by disrupted-lid plate tectonics, for each. Physics, empirical data, and planetary imagery disprove all three mutually incompatible models. Radiogenic heat, ~5× greater than now, forced synaccretionary magma-ocean fractionation of each planet before 4.5 Ga. This produced thick mafic protocrusts, concentrated radioactivity at shallow depths, and permanently depleted lower mantles. On Earth, the protocrust lay directly above refractory dunite, in turn above denser fractionates. The shallow concentrations of radioactivity allowed deep interiors to cool quickly. Venus and Mars have never since had hot cores or asthenospheres, and their “volcanoes” and other features popularly attributed to plumes are products of bolide impacts on internally inactive planets. Only Earth had enough radioactivity to remain warmer and to generate partial melts from protocrust to make Archean, and possibly Hadean, felsic crust. Dense garnet-rich residues of protocrust delaminated, sank through the low-density dunite, and began upper-mantle re-enrichment. Archean cratons stabilized where sinking of residua left derivative felsic crust directly upon sterile buoyant dunite. Where some protocrust remained, Proterozoic crustal activity ensued. This was mostly in the form of basin filling atop Archean felsic crust, commonly followed by radioactive heating, partial melting of basement plus fill, and structural inversion. Top-down enrichment of the upper mantle by evolving processes reached the critical level needed for plate tectonics only ca. 0.6 Ga. Plate motions are driven by subduction, which rights the density inversion due to top-down cooling of asthenosphere to lithosphere. Circulation is closed within the upper mantle. Primary fractionation was hot and dry. The inner planets may have received most of their water in a barrage of icy bolides, centered ca. 4.1 or 4.0 Ga, best dated on Mars and Venus but in accord with terrestrial and, possibly, lunar data. Earth's new water may have enabled formation of Archean tonalite-trondhjemite-granodiorite from protocrust. Increasing downward cycling of volatiles into Earth's upper mantle ever since has been essential for continuing tectonism and magmatism.