The icy crust of Ganymede comprises bright and dark areas. Investigation of Voyager 1 and 2 images has shown that bright terrain is grooved and separates dark polygons of cratered terrain. The grooved terrain contains alternating ridges and grooves in straight and curvilinear sets, which are locally interrupted by smooth patches and swaths. Cratered terrain, where 'it occurs in small wedges and slivers, has a pervasive grain of narrowly spaced furrows, and thus is transitional to grooved terrain. An analysis of the morphology of terrain types, and of superposition and cross-cutting relations, suggests that grooved terrain grew at the expense of cratered terrain, that tracts of cratered terrain were converted into grooved terrain in situ, and that vertical tectonism and shear movements dominated in the restructuring of Ganymede's surface. It is postulated that during a period in the planet's history when the lithosphere was thin, upwelling convection currents caused incipient rifting accompanied by intensive normal faulting; where rifting went to completion, crustal segments separated, locally spread apart, and sheared past one another. In places subduction and compression may have occurred, but the evidence is inconclusive. Thus, the grooved terrain on Ganymede may record an early phase of ice-plate tectonics that caused rifting and drifting of the icy lithosphere, but, unlike silicate plate tectonics on Earth, may have resulted in only minor vertical turnover.
