Flexural Stresses and Magma Ascent at Large Volcanoes on Venus

Introduction: The Magellan mission to Venus [1] revealed the presence of at least 144 large volcanic edifices (i.e., those with flows extending > 100 km in diameter; Figure 1) [2-4]. Numerical models of coupled edifice growth and lithospheric flexure on Venus [5] revealed that horizontal compression in the upper lithosphere gets transmitted into the edifice, tending to halt magma ascent (and thereby, edifice growth) in the absence of mitigating circumstances such as thick elastic lithosphere or buoyant subsurface loading (mantle dynamic or crustal underplating). Here I examine in detail the ways in which lithospheric flexural stresses influence magma ascent at Venusian volcanoes. In particular, I present two forms of magmastalling “stress traps” and discuss how they may affect the growth of several types of volcano on Venus. Data: Large volcanoes on Venus are typically shallowly sloped (generally < 5 degrees) domes or cones covered by numerous radially oriented lava flows that extend outward to a nearly flat flow apron, as seen at Sapas Mons and Irnini Mons (Figure 1). Sapas Mons exhibits very narrow annular zones of extensional faulting on the eastern and western mid-flanks [6]. Radial fractures extend from the lower flanks and flow apron, preferentially to the north-northeast, southsoutheast and southwest directions. Sapas Mons thus qualifies as a radiating fracture system [7]. Irnini Mons (Fig. 1) exhibits the radial flows typical of large volcanoes, but also has a prominent tectonic annulus surrounding an annular ridge [3, 8]. The tectonic annulus and ridge qualify this feature for inclusion in a database of coronae [e.g. 9], a class of volcano-tectonic feature defined by topographic and/or tectonic annuli. However, the topography of the lower flanks of Irnini Mons and the radial flows in the distal flow apron [e.g., 4] suggest that this construct is representative of a hybrid class of structures transitional between volcanoes and coronae. [10,8,5]. Models: Finite element models of the interaction between edifice growth and lithospheric flexure at large volcanoes on Venus showed that flexurally induced horizontal compression in the upper lithosphere was transmitted into the growing edifice [5]. Given that dikes tend to form perpendicular to the least compressive principal stress [11], the modeled stress states in the upper lithosphere and edifice predicted lateral magma emplacement in sills rather than vertical ascent in dikes (as predicted for the lower lithosphere). The horizontal compressive stresses thus form a “stress trap” for magma, preventing ascent and thereby inhibiting edifice growth. However, several factors mitigate this stress trap [5]: the magnitude of horizontal compression decreases with increasing elastic lithosphere thickness Te, and horizontal compression is relieved by uplift from underplating, dynamic support, or intrusion. Recent advances in modeling techniques [12] allow the introduction of intrusions as displacements enforced between adjacent rows of finite elements. Sill-induced uplift further alters the lithospheric stress state by relieving horizontal compression in material above the sill [12] possibly re-opening magma ascent paths to the surface. Further, sill intrusion can generate mid-flank slope breaks and annular or radial fault systems at the surface of the volcano, as observed at Alba Patera on Mars [12] and Irnini Mons on Venus (Fig. 1). These results suggest the importance of subsurface buoyant and intrusive loading at volcanoes like Irnini Mons. Despite recent progress, there are still further conditions to consider for models of magma ascent through the lithosphere. The three sources of pressure available to drive magma flow in vertical dikes (Eqn. 7 of [13], modified to account for our “extension positive, z positive upward” sign conventions): (dP/dz + ρm g) = -Δρ g + dΔσy/dz + dΔP/dz (Eqn. 1).