LITHOSPHERIC FLEXURE: THE KEY TO UNDERSTANDING THE DIVERSE SHAPES OF LARGE VOLCANOES ON VENUS

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. 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. These volcanoes are large enough to induce flexure in the underlying lithosphere, inducing potentially large stresses there. Such stresses may, in turn, affect subsequent magma ascent paths through the lithosphere, thereby influencing the evolution and structure of the volcano. To examine this interaction, we calculate stresses via analytic models of axisymmetric lithospheric flexure, in conjunction two criteria for magma ascent, including a formulation that relates lithospheric stress gradients to magma ascent velocity in dikes. Our models demonstrate a strong influence of flexural stresses on the shape of a volcanic edifice. In particular, the thickness of the elastic lithosphere (T_e) modulates stresses magnitudes and flexural wavelengths of the response, such that each particular edifice shape (conical, domical, and annular) has a corresponding range of T_e (high, moderate, and low, respectively) that favors volcanic construction of that shape. This 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. Intra-lithospheric sills in particular appear to be crucial paths of magma transport for annular and domical edifices. Second, since T_e can be considered a proxy for the thermal gradient in the lithosphere, 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).