Ganymede Tectonics: Insights from Galileo Imaging

R. T. Pappalardo, J. W. Head, G. C. Collins, L. M. Prockter (Brown Univ.), R. Greeley (ASU), C. R. Chapman (Southwest Res. Inst.), P. Helfenstein (Cornell Univ.), G. Neukum, R. Wagner (DLR), B. R. Tufts (Univ. of AZ), Galileo Imaging Team

During its four close passes by Ganymede, Galileo imaged samples of bright grooved terrain and dark terrain, revealing that each can be highly tectonically deformed. Tectonic deformation is expressed as sets of subparallel or cross-cutting ridges and troughs of a variety of specific morphologies. Ridges and troughs within fractured dark terrain can be widely spaced and somewhat anastomosing in planform. In intensely deformed regions, tectonism appears to increase the integrated brightness of dark terrain by increasing the relative fraction of steep, brighter slopes. Some craters have focused dark terrain tectonism, perhaps as crater topography relaxed. Locally high amounts of extensional strain are recognized in both bright and dark terrain, most clearly demonstrated where fracture zones have extended craters by tens of percent or more. Bright grooved terrain is generally pervasively tectonized, with morphologies and structural relationships indicative of extensional tectonism, with a minor component of shear. En echelon and sigmoidal structures are indicative of shear that is concentrated along narrow zones and distributed within fault duplexes. Two topographic wavelengths of grooved terrain deformation are recognized in stereo and other imaging. Texturally distinct smooth materials provide indirect evidence for icy volcanism within bright terrain. Lanes of smooth terrain generally display fine striations which may indicate fine-scale tectonism and/or shallow cryovolcanic burial of preexisting tectonic structures. Prominent bounding structures are common along the margins of grooved terrain but are absent in the case of fractured dark terrain, and these structures likely mark prominent bounding faults. In places, isostatic uplift of the adjacent footwall block is suggested.