TOPOGRAPHIC ANALYSIS OF SAMARIA FOSSA: A NORMAL FAULT ON ENCELADUS

The ice shell of Enceladus has experienced widespread and diverse tectonic deformation. In the northern trailing hemisphere, we have identified a large normal fault, recently named Samaria Fossa by the IAU. Samaria Fossa (SF) is 195 km long, roughly oriented N–S, is centered at ~27.2º N, 134.9º E, and has a relief of ~550 m. Based on crater count-derived ages, the unit it cuts may be between 4.6 and 0.9 Ga, but crosscutting relationships indicate that SF is geologically young.

On terrestrial and icy bodies, normal fault topography has been used to infer properties of the ice shell at the time of fault formation. Here we use two methods to evaluate Samaria Fossa’s preserved fault topography, derived from stereo images, to estimate the fault dip angle, slip magnitude, and depth of faulting via mechanical modeling, and to calculate the local elastic thickness (Te) of the ice shell through flexure analysis, assuming a broken elastic plate.

The fault topography was inverted using the mechanical modeling program Coulomb, which calculates fault-related deformation in an elastic half-space. The model results suggest a fault dip of 75º, a maximum depth of faulting of ~7.5 km, and a maximum slip at depth of ~730 m. The local effective Te of the ice shell was estimated by comparing the idealized shape of a flexed plate to the observed topography. Values of Te are calculated to be 450–525 m for a Young’s modulus (E) of 1 GPa and 275–400 m for E = 9 GPa. Flexural profiles are sensitive indicators of the vertical thermal profile and associated heat fluxes. Corresponding heat flux values are high and range from 9.9–124 mW/m² and 310–928 mW/m², depending on the method used.

Mechanical modeling implies that the brittle portion of Enceladus’s ice shell was at least 7.5 km thick at the time of faulting. However, the calculated Te values are less than the observed fault throw, indicating that flexure analysis does not give the thickness of the ice shell. This suggests that deformation in this location is not well approximated by a broken elastic plate, implying that SF does not fully penetrate the elastic ice shell and that deformation in an elastic half-space is instead more representative. Flexure-derived very thin ice shells require potentially unrealistically high heat fluxes, implying that flexure analyses should be interpreted and applied with caution.