Strike-slip fault evolution on Europa: evidence from tailcrack geometries

Secondary cracks are commonly produced at stress concentration points at the tips of slipping interfaces such as faults. These so-called tailcracks form an antisymmetric pattern at opposite tips of the fault with a fracture geometry that is a mechanical indicator of the sense of slip, whether left-lateral or right-lateral. I present descriptions of tailcracks along numerous strike-slip faults on Europa. Two distinct styles of strike-slip faults are identified: ridge-like and band-like. The angles between faults and tailcracks are variable and are commonly less than the theoretical 70.5° angle that approximately characterizes many terrestrial examples involving fault surfaces that remain in contact during slip. Median tailcrack angles are lower for band-like faults (30°) than ridge-like faults (52°). In addition, the sense of curvature of band-like fault tailcracks is characteristically opposite to that of ridge-like faults. Analytical models of stress orientations around strike-slip faults on Europa indicate that these effects result from dilation during strike-slip motion. Band-like faults characteristically underwent concurrent dilation and shearing but this dynamic coupling is not characteristic of ridge-like faults. The implication is that strike-slip faulting was not a globally homogeneous process on Europa and as a result the morphologies of strike-slip faults are variable. Furthermore, band-like faults appear to corroborate the predictions of the tidal walking theory, exhibiting predominantly right-lateral motions in the southern hemisphere and left-lateral motions in the northern hemisphere. However, ridge-like faults do not obey this slip-sense convention, suggesting that the tidal walking theory may be appropriate for describing the evolution of dilational band-like faults but that ridge-like faults may have resulted from a different driving mechanism.