IMPLICATIONS OF A CRUSTAL RECYCLING STRESS STATE ON IO FOR MAGMA ASCENT PATHWAYS, LARGE MOUNTAINS, AND THICKNESS OF THE LITHOSPHERE

While Jupiter’s moon Io is the most volcanically active body in the solar system, the largest mountains seen on Io are created by tectonic forces rather than volcanic construction. Pervasive compression, brought about by subsidence induced by sustained volcanic resurfacing and aided by thermal stress, creates the mountains, but at the same time inhibits magma ascent in vertical conduits (dikes). However, the superposition of stress states from crustal resurfacing/recycling and mountain loading can result in viable pathways of magma ascent through nearly the entire lithosphere. The viability of these pathways appears to be strongly related to the thickness of the mechanical lithosphere on Io, which is thought to correspond closely to the thickness of Io’s crust (pervasive melt generation in Io’s upper mantle limits the strength of any mantle contribution to the lithosphere). We superpose stress solutions for subsidence and thermal stress (from resurfacing) in Io’s lithosphere with stresses from Io mountain-sized loads (in a shallow spherical shell solution) in order to evaluate magma ascent pathways. We use stress orientation (least compressive stress horizontal) and stress gradient (compression decreasing upwards) criteria to identify ascent pathways through the lithosphere. For resurfacing stress states alone, the ascent criteria are satisfied only in a narrow (5 km or so), roughly mid-lithosphere band. When these stresses are superposed with those from loading of a 150-km-radius, 15-km-tall (pre-flexure) mountain on a lithosphere with elastic lithosphere thickness Te = 50 km, a U-shaped ascent-favorable (AF) zone is produced connecting the deep lithosphere beneath the center of the mountain with the surface at the mountain’s periphery. Such zones are seen over a range of mountain radii and heights at Te = 50 km. However, AF zones are disrupted or absent entirely for thinner (Te = 25 km) or thicker (Te = 75 km) lithospheres: for the former, high-magnitude compressive mountain-loading stresses in the upper lithosphere inhibit magma ascent, while for the latter mountain-loading stresses are too low to change the crustal recycling stress state appreciably. Thus, we conclude that there is an optimum lithospheric (and thus, crustal) thickness that facilitates magma ascent pathways associated with Io’s mountains.