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 similar to 60 degrees to 90 degrees 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. (C) 1999 Academic Press.