THE ROLE OF EXTENSIONAL INSTABILITY IN CREATING GANYMEDE GROOVED TERRAIN - INSIGHTS FROM GALILEO HIGH-RESOLUTION STEREO IMAGING

Galileo stereo images covering about 1500 km(2) of Uruk Sulcus on Gany mede have revealed two scales of ridges; (1) large-scale ridges and tr oughs spaced similar to 6 km apart, corresponding to the ''grooves'' s een in Voyager images, and (2) small-scale ridges spaced hundreds of m eters apart superimposed on the large-scale ridges. We interpret the s mall-scale ridges to be the result of tilt-block normal faulting of th e surface brittle layer, while the large-scale ridges may be due to ne cking of the brittle layer over a ductile substrate. The geometry of t he tilt blocks revealed in Galileo images leads to a minimum estimatio n of 51% to 58% extensional strain in the area. The strain estimate, w hen incorporated into a model for the formation of grooved terrain by necking of a brittle layer undergoing extension, leads us to estimate a thermal gradient of similar to 20 K/km and a strain rate of similar to 10(-14) s(-1) during groove formation.