Formation of grooved terrain on Ganymede: Extensional instability mediated by cold, superplastic creep

The morphology of Ganymede's bright terrain is dominated at multikilometer wavelengths by sets of subparallel, linear-to-sinuous ridges and troughs, collectively known as grooves. It has been often suggested that the grooves are the result of a necking-type extensional instability, but the detailed analysis of Herrick and Stevenson (1990, Icarus 85, 191-204) concluded that such instabilities were not sufficiently strong to explain the observed topographic relief. We have reexamined this model, wherein an instability arises when a strong plastic or brittle layer and underlying viscous or ductile substrate undergo extension, by incorporating the recently determined flow laws for superplastic creep of cold ice at low stresses. The behavior of these flow laws is closer to Newtonian than to earlier determined, high-stress flow laws. The power-law creep of very cold ice at high stresses has also been remeasured to be very stress dependent, which decreases its influence with respect to other flow mechanisms at low stresses. In addition, because of a dimmer, younger Sun and potentially higher albedos of recently emplaced bright terrain material, surface temperatures in bright terrain were probably lower during the time of groove genesis than they are today. Both incorporation of closer-to-Newtonian ductile flow laws and lower temperatures serve to better decouple the strong, deforming layer from the substrate, resulting in the development of stronger instabilities. Consequently, we find that it is possible to generate sufficiently strong instabilities at topographic wavelengths consistent with the grooved terrain topography of Ganymede and at geologically plausible strain rates (similar to10(-16) to 10(-14) s(-1) for specific examples). With sufficient strain, it is even possible to form grooves in dark terrain. The required thermal gradients for groove formation in either bright or dark terrain are one-to-a-few x 10 K km(-1), which require exceptional heat flows with respect to the steady-state radiogenic background. (C) 2001 Elsevier Science (USA).