FORMATION OF GANYMEDE GROOVED TERRAIN BY SEQUENTIAL EXTENSIONAL EPISODES - IMPLICATIONS OF GALILEO OBSERVATIONS FOR REGIONAL STRATIGRAPHY

High-resolution Galileo images of grooved terrain on Ganymede have rev ealed details of the morphology of grooves and their stratigraphic rel ationships. Based on lower resolution Voyager images, complex areas of grooved terrain in Uruk Sulcus were interpreted in the context of str ucturally isolated grooved polygons which were resurfaced by cryovolca nism and individually deformed by local processes. In Galileo images o f grooved polygon boundaries, the faults comprising the groove lanes b etween the polygons are observed to truncate features within the polyg ons, and incipient fractures from these groove lanes modify the edges of the polygons, indicating that these groove lanes are younger than t he polygons. In the Uruk Sulcus region, tectonic resurfacing by the fo rmation of younger sets of grooves appears to be obscuring older patte rns of deformation. The grooved polygons are thus remnants of terrain affected by older episodes of deformation isolated from each other by areas of more recent deformation. The stratigraphy of grooves can be d etermined for small areas covered by high-resolution Galileo images (s imilar to 100 m/pixel) and then generalized to larger areas imaged by Voyager at lower resolution (similar to 1 km/pixel). This method has b een applied to the regions around Uruk Sulcus and Nippur Sulcus, resul ting in two preliminary conclusions about the nature of grooved terrai n deformation: (1) the style of deformation has changed through time a nd (2) the orientation of least compressive stress has changed through time. Throughout the mapped region, the stratigraphically oldest groo ves are closely spaced, subdued structures which indicate least compre ssive stress oriented NW-SE, while the youngest grooves exhibit morpho logy consistent with tilt block normal faulting, with least compressiv e stress oriented NE-SW. These observations are consistent over an are a of 1.6 million square kilometers and therefore do not favor highly l ocalized or low-strain mechanisms for grooved terrain formation, such as the cooling of cryovolcanic flows or the surface expression of diap irs. Instead, the formation of grooved terrain on Ganymede may be domi nated by global expansion due to tidal heating and/or differentiation, possibly organized into coherent regional patterns by transient low-o rder convection or changes in the satellite's figure. (C) 1998 Academi c Press.