Formation of Ganymede Grooved Terrain by Sequential Extensional Episodes: Implications of Galileo Observations for Regional Stratigraphy

High-resolution Galileo images of grooved terrain on GanGrooved terrain covers over half of Ganymede’s surface ymede have revealed details of the morphology of grooves and their stratigraphic relationships. Based on lower resolution and records a period of intense surface deformation at Voyager images, complex areas of grooved terrain in Uruk some point in the satellite’s history. Ganymede’s neighSulcus were interpreted in the context of structurally isolated boring satellite Callisto does not display any non-impactgrooved polygons which were resurfaced by cryovolcanism and related tectonic features, despite the gross similarities in individually deformed by local processes. In Galileo images of size, density, and location in the Solar System of Ganymede grooved polygon boundaries, the faults comprising the groove and Callisto. Some process has occurred on Ganymede lanes between the polygons are observed to truncate features and not on Callisto to form the grooved terrain, and the within the polygons, and incipient fractures from these groove determination of this process has major implications for lanes modify the edges of the polygons, indicating that these groove lanes are younger than the polygons. In the Uruk Sulcus the interior evolution of these two bodies (cf. McKinnon region, tectonic resurfacing by the formation of younger sets and Parmentier, 1986). Understanding the events that led of grooves appears to be obscuring older patterns of deformato the formation of the grooved terrain is essential for tion. The grooved polygons are thus remnants of terrain affected understanding much of the unique geological history of by older episodes of deformation isolated from each other by Ganymede, and how its evolution diverged from that of areas of more recent deformation. The stratigraphy of grooves its twin Callisto. can be determined for small areas covered by high-resolution It is generally agreed based on Voyager images that most Galileo images (p100 m/pixel) and then generalized to larger grooves are extensional tectonic structures (Shoemaker et areas imaged by Voyager at lower resolution (p1 km/pixel). This method has been applied to the regions around Uruk al. 1982) and Galileo images have confirmed this general Sulcus and Nippur Sulcus, resulting in two preliminary concluinterpretation (Belton et al. 1996, Pappalardo et al. 1998). sions about the nature of grooved terrain deformation: (1) the As extensional features, the orientations of the grooves style of deformation has changed through time and (2) the are dominantly controlled by the orientations of the orientation of least compressive stress has changed through stresses that formed them. Different models to describe the time. Throughout the mapped region, the stratigraphically olddriving mechanisms of groove formation predict different est grooves are closely spaced, subdued structures which indipatterns of stress within Ganymede’s lithosphere, and the cate least compressive stress oriented NW–SE, while the younggrooves are a surface manifestation of the strain associated est grooves exhibit morphology consistent with tilt block normal faulting, with least compressive stress oriented NE–SW. These with these stresses. Thus, measurements of groove orientaobservations are consistent over an area of 1.6 million square tion are essential for examining the strain history of Gankilometers and therefore do not favor highly localized or lowymede and constraining the driving mechanism of strain mechanisms for grooved terrain formation, such as the groove formation. cooling of cryovolcanic flows or the surface expression of diaThe collection of groove orientation data is relatively pirs. Instead, the formation of grooved terrain on Ganymede straightforward in swaths of grooved terrain which only may be dominated by global expansion due to tidal heating display one orientation of grooves. However, many regions and/or differentiation, possibly organized into coherent regional of grooved terrain, such as Uruk Sulcus, display complex patterns by transient low-order convection or changes in the arrays of grooves oriented in many directions (Fig. 1). In satellite’s figure.  1998 Academic Press Uruk Sulcus and other regions of complex grooved terrain,