The influence of lithospheric flexure on magma ascent at large volcanoes on Venus



[1] Large volcanoes on Venus exert large vertical loads on the lithosphere, which responds by deflecting downward. Stresses induced by this lithospheric flexure can have a strong influence on magma ascent pathways from the mantle source region to the surface. Here we propose that flexural stresses exert control over the shapes of volcanic edifices on Venus, applying criteria for magma ascent expressed in terms of stress orientations (can vertical dikes form?) and gradients (is magma squeezed upward or downward in a vertical dike?) to determine favored magma ascent paths and locations. For conical edifices emplaced on lithosphere with high elastic thickness Te, (e.g., > 40 km) both sets of magma ascent criteria are satisfied over the entire lithosphere, allowing essentially unimpeded ascent of magma to the surface and the formation of relatively steep edifices. However, for lower values of Te, high adverse stress gradients tend to cut off magma ascent beneath the summit, instead favoring lateral transport of magma at depth to distal regions with gentler stress gradients, resulting in domical edifice shapes. At the lowest values of Te (< 10 km), large short-wavelength deflections of the lithosphere tend to produce narrow and widely spaced zones of magma ascent: Such zones may produce annular ridges of volcanic material, thereby generating forms characteristic of a subset of features known as “coronae” on Venus. Another subset of coronae may form by intrusive-based generation of annular fractures at the edge of the summit region of domical edifices, as proposed for Alba Mons on Mars.