Shield fields and shield plains on Venus: Geologic characteristics and spatial relationships
Miller, Diana Morgana
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The Venusian surface exhibits a plethora of clusters of small shield (<20 km basal diameter) volcanoes and their abundance suggests that they played an important role in Venusian resurfacing. Shield clusters are of 2 types: shield fields and shield plains. Shield fields are compact clusters covering ≤ 10 4 km 2 , and shield plains contain small shield volcanoes dispersed over areas between 10 3 and 10 6 km 2 . Relative to shield fields, shields in shield plains have a more uniform distribution. I analyzed the stratigraphic characteristics of 3 shield plains and 4 shield fields and examined shield cluster contribution to resurfacing processes. This was done through geologic mapping of shields in the shield clusters, analyses of geologic features around the shield clusters, and spatial analyses. The results suggest that shield fields and shield plains are, in fact, separate geologic units with distinct characteristics. Shield fields have various stratigraphic positions relative to surrounding materials, but are stratigraphically above densely lineated plains and tesserae. Shield field edifice densities for the mapped shield fields average 14,713 (± 2735) edifices/10 6 km 2 . Shield field locations appear to be independent of other geological features (such as shield plains, volcanoes, or tesserae), but flows from individual edifices are observed to be topographically constrained. Some shield fields occur in close proximity to or within the boundaries of other geologic features, such as coronae. The Nearest-Neighbor (NN) spatial analyses reveal that shield field edifices exhibit a Poisson or Normalized Poisson distribution, suggesting that edifice distribution is indistinguishable from a random distribution. In turn, this suggests the underlying processes of shield formation have been erased by continuing activity or that shield field formation is indeed a random process. Shield plains also display various stratigraphic positions relative to surrounding units, and are also stratigraphically above densely lineated plains and tesserae. Shield plain edifice densities for the mapped shield plains average 3,902 (± 1896) edifices/10 6 km 2 . Shield plains mapped in this study are consistently located near tesserae and coronae. Although the edifices in a shield plain are not restricted by topography, the flows from an individual edifice may be topographically confined. The results of NN spatial analyses of shield plains are inconclusive because the analysis assumes the smallest possible perimeter around the shield plain. The actual perimeter of the shield plain is larger than the assumed perimeter, forcing the results to be more clustered relative to the PNN than it would be with the appropriate perimeter. The stratigraphy of shield fields and of shield plains suggests that neither geologic unit was emplaced all at one time. Both shield fields and shield plains exhibit varying stratigraphic positions relative to surrounding geologic units. Therefore, the stratigraphic positions of shield fields and shield plains support the equilibrium resurfacing hypothesis, which proposes resurfacing of Venus occurred in different places during the planet's history rather than globally all at one time.
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