Shield Fields and Shield Plains on Venus: Contrasting Volcanic Units Exemplified in Shimti Tessera (v-11) and Vellamo Planitia (v-12) Quadrangles

Introduction: The geologic features “shield fields” and “shield plains” (or “shield terrain”) have been used by a number of Venus researchers and authors of Venus geologic quadrangle maps. However, there has been occasional confusion concerning the two terms and the geologic units they define. Stratigraphic interpretation may vary due to the identification of clusters of small shields as either shield fields or isolated patches of shield plains [1]. Vellamo Planitia Quadrangle (V-12) includes the originally defined type example of shield plains [2]. Shimti Tessera Quadrangle (V-11) [3] includes a shield field that is younger than the regional wrinkle-ridged plains as well a regional shield plains unit (with partially buried outliers) that is older than the regional wrinkle-ridged plains. Detailed analyses of these quadrangles provide a useful contrast and comparison between the two geologic units and their interpretations. Shield Fields: Enhanced concentrations of small Venus volcanoes distributed over a quasi-circular region from 100 to 150 km in diameter are called “shield fields” [4], following terrestrial volcanological usage of the term “volcanic field” that may contain clusters of 10s to 100s of vents. Shield fields have vent densitiers of >10,000 vents/10 6 km 2 . Shield fields have been considered to be a distinct type of volcanic center on Venus, possibly arising from conditions of low magma supply rates in which the magma flux from a melting anomaly is less than that necessary to sustain shallow magma reservoirs [5], analogous to the current interpretation of terrestrial volcanic fields with clustered monogenetic vents [6].