Implications of Volcanic Edifice Shapes and Structures for the Volcanological and Thermal Evolution of Venus

Introduction: Large volcanic edifices comprise a significant portion of the geologic record of Venus, as revealed by radar-based imaging and ranging of the surface of that planet [1]. These edifices display a range of shapes from conical to dome-shaped. Furthermore, the class of large volcanoes on Venus overlaps with a class of features, called coronae, that are typified by annular-shaped tectonics, topography, or both [2]. In a companion abstract [3], we demonstrate that lithospheric flexure can exert a strong influence on the shape of a volcanic edifice via the effects of flexural stresses on intrusive magma ascent pathways (dikes). In particular, the thickness of the elastic lithosphere (Te) modulates stresses magnitudes and flexural wavelengths of the response, such that each particular edifice shape (conical, domical, and annular) has a corresponding range of Te (high, moderate, and low, respectively) that favors volcanic construction of that shape [3]. The link between lithosphere thickness, magma ascent, and edifice shape has several implications. First, it provides a new scenario for construction of a subset of coronae, namely, those with substantial annular topography and lava flows associated with such relief. The structure and surficial geology of such coronae may then result from volcanic construction (by effusion and intrusion) rather than from surface deformation induced by subsurface flow (usually diapiric) in the crust and mantle, as posited by a class of popular models for corona formation [4-7]. Second, since Te can be considered a proxy for the thermal gradient in the lithosphere [e.g., 8], the discovery of the lithosphere-shape link raises the exciting prospect of using spatial variations in volcanic edifice morphology to map the thermal state of the lithosphere (corresponding to the time of volcanic construction). Data and Models: Synthetic aperture radar (SAR) images collected from the Magellan mission [9] reveal flows, faults, and vents used to identify volcanoes and coronae. The shapes and sizes of the features can be determined using Magellan altimetry data. More than 150 large volcanoes [1,10] and 500 coronae [2,11] have been identified. Image data are draped over topography for two large volcanic structures in Figure 1. Kokyanwuti Mons (Figure 1a) exhibits a domical profile, with a nearly flat summit region and gently sloping outer flanks. The summit has a fairly uniform radar-dark surface, whereas the outer flanks are draped with numerous digitate units of varying brightness, consistent with the presence of relatively young lava flows. Models of lithospheric flexure with Te in the range 20-30 km result in a shutoff of magma ascent beneath the summit plateau but allow ascent beneath the outer flanks [3], consistent with the pattern of flow units seen at Kokyanwuti Mons. Aruru Corona (Figure 1b) displays an annular topographic profile, with associated circumferential fracturing. Numerous lava flows appear to have resurfaced the outer slope of the annulus. Some flows appear to have emanated from the inner slope as well, covering the interior low region. Models with low Te (<= 15 km) produce two narrow zones of predicted magma ascent on the inner and outer slopes of the annulus [3], in accordance with the observed distribution of flows. For both Kokyanwuti Mons and Aruru Corona, models with Te outside the ranges given above predict magma ascent patterns that are at odds with the observed topography and flow distributions [3].