Brittle Ice Tectonics within Enceladus: Analogous Behavior within Shallow Terrestrial Rocks

Abstract:

Shallow ice within Enceladus experiences oscillating tidal stresses and ambient stresses from lateral buoyancy contrasts between warm and cold ice. Ice does not distinguish between these remote sources of stress. Frictional failure occurs when the total stress on a crack surface exceeds its frictional strength. Failure on pervasive distributed cracks has the net effect of creep from internal friction. Strong tidal stresses drive net strain within the ice mass with the orientation to relieve modest buoyancy stresses and hence plate-like convection. Analogous processes occur when strong surface waves pass through the shallow subsurface. (1) The upmost 10s of meters moves down a shallow slope from ambient topographic stresses during shaking. The net effect, a sacking, is a slow deep landslide over many earthquakes. (2) The shallow ambient tectonic stress is relieved by distributed frictional slip during shaking. Deeper tectonic movements then restore the stress. Repeated events facilate long-term shallow tectonic deformation as on Enceladus. (3) The impinging surface waves impose strain boundary conditions on the shallow subsurface. Stiff rock fails in friction and cracks. The resulting damaged rock is more compliant than intact rock and experiences lower stresses during the net shaking. A seismic regolith where the shear modulus increases linearly with depth eventually self-organizes so the regolith layer barely reaches frictional failure during strong events. A tidal regolith of this type may exist on Enceladus. It barely fails down to its base at times of peak tidal stress. (4) Ice, like rock, becomes weak once frictional slip is underway. Large stress drops and even overshoot occur. Local residual stresses from one event become prestresses for the next event. Favorably oriented stress patches fail from tidal stresses that are below the nominal frictional criterion.