Fault Formation and Evolution on Icy Satellites as a Result of Bidirectional Cyclical Shear: Insights from Physical Analog Experiments
Thursday, 18 December 2014
Stresses resulting from time-varying tidal forces have been advocated as a primary driver of tectonism on icy satellites in orbits with sufficient eccentricities. Diurnally varying tidal stresses can cause both dextral and sinistral shear during one orbit, presumably contributing to fault initiation and propagation in the icy shell. Observed strike-slip structures indicate the occurrence of mode II shearing; however, the role of bidirectional cyclic shear in fault initiation and development is poorly constrained. To more fully understand the kinematic evolution of faults as a consequence of diurnal bidirectional shear, we perform a set of analog experiments. We use a tectonic apparatus to induce cyclic bidirectional shear in unconsolidated heterogeneous sand, which has similar material properties (i.e., a Coulomb material) to ice when scaled. Using a suite of initiation directions and offset ratios (with a subsequent suite of net offsets), we perform experiments with dry sand and repeat with wet sand (10 wt. %) overlain by dry sand. Our results indicate that, regardless of the offset ratio, after only 1-2 cycles bidirectional shear results in the development of linear (dry sand) to curvilinear (wet/dry sand) fault traces with topographically low damage zones and high fault-flanking ridges. We consider that such ridges may be analogous to those found on Europa and Enceladus. Initial fault-flanking ridges are discontinuous, as is observed for some younger faults on Europa. After additional cycles, ridges mature into more continuous fault-flanking features, suggesting a possible granular mechanical origin for ridges on icy satellites. The rate of ridge maturation is dependent on displacement per offset and net offset per cycle. Experiments with larger displacements per offset, as well as those with net offset, result in more rapid ridge maturation. By utilizing estimated fault displacements and net offsets per orbital cycle (based on existing mechanical models), and assuming ridge formation is a mechanical process as our model suggests, we may infer tentative lower limits for the orbital ages of ridged faults on icy satellites. We compare our model results to a variety of features on Europa, the icy satellite displaying the most diverse expressions of deformation, and find geomorphic and structural similarities.