Strike-slip faults on Europa: Global shear patterns driven by tidal stress

Diurnal tides due to orbital eccentricity may drive strike-slip motion on Europa through a process of "walking" in which faults open and close out of phase with alternate right- and left-lateral shear. Mapping of five different regions on Europa has revealed 121 strike-slip faults, including Astypalaea Linea, a 800-km-long fault with 42 km of right-lateral offset. At high southern latitudes near Astypalaea Linea all of the strike slip faults identified were right-lateral. Europa appears to preferentially form right-lateral faults in the southern hemisphere and left-lateral faults in the northern hemisphere, consistent with tidal walking. At the five locations, nonsynchronous rotation explains the azimuthal orientations and distribution of sense of shear, which fit formation ∼60° to 90° west of their current positions. Alternatively, stress due to differential rotation might also explain the observed shear patterns. Nearly all identified strike-slip faults were associated with double ridges or bands, but few were detected along ridgeless cracks (even older ones). Thus, cracks without ridges may not have penetrated to a decoupling layer, consistent with the models for ridge formation that require cracks to penetrate to a liquid water ocean.