Geology and mapping of dark terrain on Ganymede and implications for grooved terrain formation

Geological mapping of regional and high-resolution Galileo images reveals a variety of units and structures within Ganymede's dark terrain. We have ma de a detailed study of areas within Galileo, Marius, and Nicholson Regiones in order to investigate the style of tectonic deformation experienced by d ark terrain adjacent to swaths of grooved terrain. Dark terrain appears to become fractured as a precursor to grooved terrain formation; in places, da rk fractured swaths are recognized which have similar characteristics to br ighter grooved terrain swaths. Tectonic deformation may be, but is not alwa ys, focused through preexisting weaknesses caused by impact craters and fur rows, A prominent groove lane, Anshar Sulcus, is inferred to have formed by the process of hanging wall rollover, accompanied by a small amount of rig ht-lateral horizontal shear offset resulting from NE-SW extension.