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

Abstract

[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 T e , (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 T e , 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 T e (< 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.

Signatures of Lithospheric Flexure and Elevated Heat Flow in Stereo Topography at Coronae on Venus

“…If many craters on Venus are volcanically modified, then the average surface age—at least of the uppermost few hundred meters—is quite young even with conventional assumptions about the impactor population (Herrick & Rumpf, ; O'Rourke et al, ). Given their abundant volcanic and tectonic signatures, above average heat flow locally at coronae is perhaps unsurprising in any case (e.g., McGovern & Solomon, ; McGovern et al, ).…”

“…Lateral crustal flow and associated deformation then produces topographic features observed at coronae today. Similarly, McGovern et al () argued that loading of the lithosphere by large volcanoes could produce a subset of the features classified as coronae. In regions of low elastic thickness, control of magma ascent zones by lithospheric flexural stresses may produce annular ridges of volcanic material matching topographic signatures of some coronae.…”

The history of tectonism on Venus: A stratigraphic analysis

“…Thus, these broad outlines of the geologic history of tectonism, together with the geological history of volcanism (Ivanov and Head, 2013) and the global geological map of Venus (Ivanov and Head, 2011), provide the basis to help distinguish among the range of proposed models for the geodynamic evolution of Venus (Shalygin et al, 2012;McGovern et al, 2013;Driscoll and Bercovici, 2014;Gillmann and Tackley, 2014;Taylor, 2014;Campbell et al, 2015).…”

The eruptibility of magmas at Tharsis and Syrtis Major on Mars

“…The wall rocks are expected to have a Maxwellian viscoelastic rheology, and consequently, the maximum overpressure in the chamber will depend on buoyancy, recharge rates, chamber volume, and the temperature‐dependent wall rock viscosity [ Dragoni and Magnanensi , ; Jellinek and DePaolo , ; Karlstrom et al ., ; Karlstrom and Richards , ; Degruyter and Huber , ; Caricchi et al ., ]. Surface loading also modulates overpressure [ Pinel and Jaupart , ; Grosfils , ; McGovern et al ., ; de Silva and Gregg , ], and regional stresses and strain rates can influence magma ascent [e.g., Delaney et al ., ; Rubin , ; Buck et al ., ; Menand et al ., ; Daniels and Menand , ]. In the limit where viscous relaxation quickly alleviates lithospheric stresses from changes in magma volume or from loading and regional tectonics, buoyancy from volatile exsolution during decompression or second boiling controls eruptibility [ Tait et al ., ; Caricchi et al ., ; Malfait et al ., ; Degruyter and Huber , ].…”