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 revealed details of the morphology of grooves and their stratigraphic relationships. Based on lower resolution Voyager images, complex areas of grooved terrain in Uruk Sulcus were interpreted in the context of structurally isolated grooved polygons which were resurfaced by cryovolcanism and individually deformed by local processes. In Galileo images of grooved polygon boundaries, the faults comprising the groove lanes between the polygons are observed to truncate features within the polygons, and incipient fractures from these groove lanes modify the edges of the polygons, indicating that these groove lanes are younger than the polygons. In the Uruk Sulcus region, tectonic resurfacing by the formation of younger sets of grooves appears to be obscuring older patterns 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 determined for small areas covered by high-resolution Galileo images (similar to 100 m/pixel) and then generalized to larger areas imaged by Voyager at lower resolution (similar to 1 km/pixel). This method has been applied to the regions around Uruk Sulcus and Nippur Sulcus, resulting in two preliminary conclusions about the nature of grooved terrain deformation: (1) the style of deformation has changed through time and (2) the orientation of least compressive stress has changed through time. Throughout the mapped region, the stratigraphically oldest grooves are closely spaced, subdued structures which indicate least compressive stress oriented NW-SE, while the youngest grooves exhibit morphology consistent with tilt block normal faulting, with least compressive stress oriented NE-SW. These observations are consistent over an area of 1.6 million square kilometers and therefore do not favor highly localized or low-strain mechanisms for grooved terrain formation, such as the cooling of cryovolcanic flows or the surface expression of diapirs. Instead, the formation of grooved terrain on Ganymede may be dominated by global expansion due to tidal heating and/or differentiation, possibly organized into coherent regional patterns by transient low-order convection or changes in the satellite's figure. (C) 1998 Academic Press.