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

Diurnal tides due to orbital eccentricity may drive strike-slip motion on E uropa through a process of "walking" in which faults open and close out of phase with alternate right- and left-lateral shear. Mapping of five differe nt regions on Europa has revealed 121 strike-slip faults, including Astypal aea 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 iden tified were right-lateral. Europa appears to preferentially form right-late ral faults in the southern hemisphere and left-lateral faults in the northe rn hemisphere, consistent with tidal walking. At the five locations, nonsyn chronous rotation explains the azimuthal orientations and distribution of s ense of shear, which fit formation similar to 60 degrees to 90 degrees west of their current positions. Alternatively, stress due to differential rota tion might also explain the observed shear patterns. Nearly all identified strike-slip faults were associated with double ridges or bands, but few wer e detected along ridgeless cracks (even older ones). Thus, cracks without r idges may not have penetrated to a decoupling layer, consistent with the mo dels for ridge formation that require cracks to penetrate to a liquid water ocean. (C) 1999 Academic Press.