Mountain Building on Io – Part 2: Effects of Preexisting Faults and Pore Sulfur

Introduction: A majority of the mountains on Io are tectonic, upthrusted blocks [1-8]. The mechanism behind their formation, however, is unknown. Mountain formation may be indirectly related to the prolific volcanism on Io [3]. Three major mountain formation hypotheses following from this idea include: • convection-modified subsidence: global compressional stresses (subsidence) are created by the continual burial of older volcanic layers by new volcanism [3] and are further modified by degree-2 mantle convection [4] • plume-focused subsidence: subsidence stresses are focused by upwelling mantle plumes impinging on the base of the crust [5,6] • thermally-modified subsidence: subsidence stresses are combined with thermal stresses due to local or regional reductions in eruption rates, which increases the amount of heat conducted through the crust [7,8]. Our previous work demonstrated the plausibility of the thermally-modified subsidence hypothesis. First, we showed that thermal stress with some subsidence stress is large enough to fault Io’s crust [7-9]. Furthermore, our spherical harmonic statistical analysis found that regions of mountain concentrations are anticorrelated to regions of volcano concentrations at any low harmonic degree [9,10]. This distribution is consistent with that predicted by the thermally-modified subsidence hypothesis. Finally, we have shown the observed pattern of mountain strike orientations within the regions of mountain concentrations is more-or-less circumferential to the centers of the regions. This indicates a stress pattern for Io’s crust consistent with subsidence stresses possibly acting along with thermal stresses [9,11]. Here we extend our previous thermal and subsidence stress modeling [7-9] to include the pore pressure effects of liquid sulfur in the crust and change the boundary condition from horizontally confined to unconfined. These changes relax some simplifications in our previous modeling. The first is that Io’s crust is purely composed of basalt. Only including sulfur is still a simplification, as other substances are likely found in Io’s crust (e.g., SO2), but it is a step forward. The second is that the horizontally confined boundary condition physically equates to preexisting faults not relieving stress or not being there at all. This is sensu stricto unlikely (except for maybe just after Io’s crust forms!), therefore we now model the crust with a “free” horizontal boundary condition to simulate stress being relieved, on average, on preexisting faults. Methods: Unconfined Horizontal Boundary. If the stress (σ) in Io’s crust is relieved on preexisting faults, then the depth-integrated (average) stress should be equal to zero. Therefore, at each time step the depthintegrated stress is calculated with [12]