LINEAMENT ORIENTATIONS THROUGH TIME NEAR EUROPA'S LEADING POINT: IMPLICATIONS FOR STRESS MECHANISMS AND ROTATION OF THE ICY SHELL

Nonsynchronous rotation stress patterns have swept across the surface of Europa, with principal stresses represented as orthogonal directions of least and greatest compression. Many researchers have assumed that lineaments have formed as tension cracks. This assumption breaks down in two antipodal football-shaped zones in the equatorial zone of Europa, in which both principal stresses are compressional. Lineaments in the equatorial compressional zones (ECZs) may form as conjugate pairs of strike-slip faults, with predicted orientations of ~30° from the greatest compressional stress direction. Predicted differential stress magnitudes in the ECZs are sufficient to cause failure in shear if lithospheric strength is frictionally controlled. Based on the hypothesis of conjugate shear failure, we have evaluated the stratigraphy of lineaments near Europa's leading point (lat 0-30°, lon ~80°), a region predicted to be just emerging from an ECZ. We have mapped >50 lineaments in the Galileo E15REGMAP02 observation at ~235 m/pixel. Lineaments have been grouped into inferred stratigraphic intervals based on the number of overcuttings versus undercuttings of other mapped lineaments. Lineaments formed as conjugate shears are expected to be oriented at a range of angles, with two orientations dominant in each stratigraphic level, exceptions being N-S and E-W orientations. Lineament orientations do fit the predicted pattern of conjugate shears. The predicted dominant E-W orientation of lineaments if due to either diurnal stress patterns or equatorial tensile zone stresses is completely lacking in the mapped area. This places an upper limit of ~90° of nonsynchronous rotation for the icy shell, and supports the hypothesis of conjugate shear failure. We continue to test the hypothesis of conjugate shear failure with modeling of surface stresses and areas in which failure of the icy shell can occur, and eventually through generation of a precise stratigraphy for regions of Europa in order to compare with modeling predictions.