Geologic evolution of Galileo Regio, Ganymede

The geology of Galileo Regio is characterized by: (1) two principal morphologic terrain units (rough and smooth terrains); (2) craters in various states of preservation; and (3) a tectonic framework consisting of three furrow systems of different ages. The three furrow systems transect the rough terrain but predate all other terrains as well as the oldest craters larger than about 10-km diameter. A broadly arcuate NW-SE furrow system dominates the tectonic pattern and is intermediate in age between widely spaced NE-SW and N-S furrow systems. The furrow systems were probably produced by extensional stresses that occurred during crustal solidification but before large craters could be retained. Smooth terrain is concentrated in the southern part of Galileo Regio and probably resulted from fluid extrusions along fractures associated with the dominant furrow system. The distribution of smooth terrain on Galileo Regio suggests that the ancient crust of Ganymede was relatively thin in the equatorial region and thickened poleward in this area. The origin of the furrow systems is very uncertain. The age relationships, morphology, and geometry of the furrow systems do not favor an origin by impact or tidal stressing. A possible, but speculative, origin is crustal uplift caused by a plume-like convection cell in a fluid mantle underlying a thin crust. Stratigraphic and morphologic relationships among furrows and crater palimpsests suggest that palimpsest morphology is largely the result of impact into a rheologically weak crust rather than viscous relaxation.