Constraints on the expansion of Ganymede and the thickness of the lithosphere

Grooves and furrows have been interpreted as grabens resulting from extension of Ganymede's lithosphere. Geologically reasonable estimates of the dip (90°–60°) and displacement (less than 1 km to a few hundred meters) of faults bounding grooves on Ganymede allow calculation of the extension and surface area increase (0.04–2%) required for the formation of all grooved terrain. The implied radius increase is thus limited between 0.02 and 1%; 1% planetary expansion is the maximum possible since formation of the grooved terrain began. Similar estimates for faults bounding furrows, the oldest preserved tectonic features on Ganymede, indicate a maximum surface area increase of ∼0.5% for the best developed furrow system, which is located in Galileo Regio. If equivalent furrow systems existed on all of the cratered terrain, then a maximum of∼0.5% increase in surface area of Ganymede is possible after the formation of the lithosphere and prior to the formation of grooved terrain. Faults bounding simple graben originate at a subsurface mechanical discontinuity and propagate upwards, resulting in grabens with similar widths and similar spacings between members of a set. The depth to this mechanical discontinuity for furrows and grooves on Ganymede must give at least a minimum lithosphere thickness, because it is impossible for faults to originate and propagate upwards from a point below the lithosphere. Therefore the minimum lithosphere thickness on Ganymede is 5 km and 9 km (in Marius and Galileo Regiones, respectively) at the time of furrow formation and 4 km at the time of groove formation.