The tectonic and volcanic evolution of Ganymede and its implications for the satellite's internal structure and evolution

Volcanism and tectonic deformation on Ganymede were separated on the basis of relative age into three broad episodes, each of which was characterized in detail. These observations were used to test multiple models of the relationship of surface features to the satellite's internal structure and evolution.; This analysis results in the following model for Ganymede's volcanic and tectonic history: Dark terrain was emplaced at least 3.8 Gyr ago, by accumulation of dark volcanic materials to a global average thickness of about 5 km. Two large systems of radial and concentric furrows, one each in the sub-Jovian and anti-Jovian hemisphere, formed throughout this period by endogenic extensional tectonic reactivation of impact-generated fracture zones. Younger and thicker dark volcanic materials accumulated within a geologically "anomalous" region around 25{dollar}spcirc{dollar}S,122{dollar}spcirc{dollar}W, within which tectonic deformation was also more concentrated. At the end of dark-material volcanism a third furrow system formed, and is characterized by a radius of over 5000 km and an arrangement of troughs radial to the coordinates given above.; Ganymede underwent a fundamental change in the style of its tectonics and volcanism at least 3.8 Gyr ago. Initially, large lithospheric blocks underwent significant shear offsets, accompanied by pervasive deformation of reticulate and lineated terrains. Light material then began to be emplaced, in close association with formation of U-shaped tectonic grooves. The oldest or nearly oldest light materials accumulated in the geologically anomalous region as a distinct terrain type. As grooved and light terrain formation became widespread, they occurred in a three-stage sequence that is recognizable across large areas. Groove formation was initiated by reactivation of preexisting structures, including both furrows and an additional global lithospheric "fabric" that may be related to tidal despinning.; It is proposed that this sequence of tectonism and volcanism resulted from long-term warming of an initially cold, undifferentiated satellite interior consisting of silicates and water-dominated ices. Tectonism is interpreted to have resulted primarily from global expansion, and to a lesser extent from mantle convection. Volcanism is interpreted to have resulted from removal from the warming interior of a limited amount of ammonia-water solution.