A system of arcuate furrows in Galileo Regio and Marius Regio, two large blocks of ancient dark terrain, has previously been proposed to be the remnant of an originally more concentric furrow set that was disrupted by shear. Estimates of the two regions' poles of furrow concentricity indicate a considerable westward offset of the Galileo Regio pole. These measurements suggest the possibility of about 500 km of left-lateral offset of the two areas; four additional, independent structural indicators support this hypothesis. There is evidence that shear deformation was concentrated along a major structural lineament which closely follows a small circle about 45¿ of arc of radius which encloses Galileo Regio, and that a smaller amount of distributed shear occurred across an adjacent 500- to 1500-km-wide band. There is also morphologic evidence for zones of minor right-lateral shear between central and southern Marius Regio and between Barnard and Nicholson Regio, and for a zone of minor left-lateral shear in Nun Sulci. Stratigraphic relations indicate that any major shear offsets occurred before and during the earliest stages of grooved terrain formation, before most grooves had formed and before virtually all light material had been emplaced. However, regionally dominant groove orientations are generally consistent with orientations expected for transtensional features: Either shear strain or the shear's driving mechanism may have had a long-term effect on patterns of deformation of Ganymede's lithosphere. ¿ American Geophysical Union 1988