Crustal displacement features on Europa

R. Tufts, R. Greenberg, P. Geissler, G. Hoppa (LPL, Univ. of Arizona), R. Pappalardo (Brown Univ.), R. Sullivan (Arizona State Univ.), Galileo Imaging Team

Significant crustal displacements of various types, including separations, shear and vertical motions have been identified on Voyager and/or Galileo images of Europa. A San Andreas-sized strike-slip fault near the south pole, Astypalaea Linea, exhibits a consistent 35km dextral offset over 810km of length (Tufts etal, 1996). "Ratcheting" or "walking" caused by rotating diurnal tidal stress is a plausible explanation for the large offset. Previously undetected dextral shear has recently been found adjacent to Astypalaea Linea, and a jagged crack seen near the north pole resembles early stages in the formation of a left-lateral fault, symmetrical with the southern hemisphere offsets. Block rotation and translation involving opening of gaps and infill from below has occurred in the antiJove region based on reconstruction of wedge-shaped bands viewed by Galileo, giving high-resolution confirmation of general deformation measured by Schenk and McKinnon (1989) in this region. Block motions were in two separate directions and, as with Astypalaea Linea, occurred approximately midway in a relative succession of ridge and terrain formation. High resolution observations of ridges have revealed extension (widening), and downslope lineations in the medial groove - suggesting mass wasting. Many ridges are flanked by parallel fractures, exploitation of which by later stages of ridge development probably creates the observed complex, braided morphologies. An example is Asterius/Minos Linea, which encircles the Europan globe almost completely. Some ridges visibly downwarp the underlying crust, which in one case is calculated to have a local elastic thickness less than 200m. Crustal fragments in Conamara Chaos include remnant ridges near their original location, arguing that a melting process caused their disruption.