Effects of faulting on crustal stresses during mountain formation on Io

Abstract

The vast majority of mountains observed on Io are tectonic, upthrusted blocks – a product of compressional forces. The mechanism behind their formation is generally understood to be related to Io's heat-pipe volcanism and crustal subsidence. Compressional thermoelastic stresses from sustained local or regional shut down of Io's heat-pipe volcanism could be potentially important orogenic drivers as well (Kirchoff and McKinnon, 2009, Formation of mountains on Io: Variable volcanism and thermal stresses, Icarus, 201, 598–614). At the same time, extensional tectonic features (rifts, pull-apart basins, simple graben) are found on Io. Furthermore, results from a two-dimensional finite element model of thrust fault formation in the ionian environment show that natural kinematics of fault motion creates localized regions of near-surface extension, while compressional stresses at depth are greatly reduced (Bland and McKinnon, 2016, Mountain building on Io driven by deep faulting. Nat. Geosci. 9, 429–432). In light of these observations and model results, we refine our previous one-dimensional, advective heat-pipe model of Io's crustal stresses, exploring another horizontal boundary strain end-member, which is the laterally unconfined (or integral stress) condition first suggested by Turcotte (1974, Are Transform Faults Thermal Contraction Cracks? J. Geophys. Res. 79, 2573-2577). We also explore the effects of directly incorporating brittle failure limits and liquid sulfur – through pore pressure – on these failure limits. Horizontally unconfined, vertical boundaries serve as an end-member representation for stress release on preexisting faults, and in the context of Io, predict extensional failure in the upper crust in response to deeper compression of either subsidence and/or thermal origin. Including these modifications greatly decreases the size of the region in compressional failure in the lower crust, unless the effects of pore sulfur are included. As in our previous work, the most favorable condition for mountain formation on Io is suggested to be a combination of subsidence stress, thermal stress from a decrease in volcanic heat piping, and crustal thinning due to basal melting.