Formation mechanisms of Europan ridges with apparent lateral offsets

Introduction: Ridges are the most common linear and tectonic surface features on Europa [1,2,3,4] and apparent lateral offsets are often observed along them [4,5]. Ridges have formed throughout Europa’s geologic history, creating identifiable crosscutting relationships [3,4]. These crosscutting relationships offer us an opportunity to determine the mechanisms responsible for ridge formation [6,7]. Recent ridge formation models characterize them as shearing-related [8]. We show evidence for both lateral shearing and ridge-orthogonal motion along ridges, indicating that apparent lateral offsets are not purely the result of strike-slip motions. First, apparent offsets along a ridge (or band) of interest must be large enough to overcome Galileo’s image resolution constraints. Therefore, our analysis required us to perform a search of high resolution Galileo images to focus on ridges with these characteristics. Second, in order for our analysis to accurately determine the displacement ratio (DR) of strike-slip offset motion to ridge-orthogonal motion, a ridge must have offset features along its length with both low and high-angle orientations, !, relative to the ridge. At both small and large orientations (0° to 30° and 160° to 180°), apparent offsets due to convergence across the ridge may become relatively large compared to where these angles are closer to 90° (for which pure-ridge orthogonal motion gives an apparent offset of zero). Finally, using ISIS software, each image chosen for our analysis was photometrically corrected and reprojected to preserve local angular relationships and line lengths using a transverse mercator or an orthographic projection, respectively [9]. A technique developed by [6] uses simple geometric measurements to provide a clear mode of fracture development and allows us to create a verifiable model for ridge (and band) evolution. We present an analysis of an unnamed band as a test for our method for determining displacement ratios, and to quantify the deformation mechanisms of both a band and a ridge. Analytical Technique: The technique uses three parameters that we can measure from each crosscut feature across a ridge or band of interest. Separation (sep) is the perpendicular distance between a displaced feature and its corresponding projection from the opposing side of the ridge or band. Strike-slip offset (w) is the measurable distance along a ridge between two parts of a displaced feature and alpha (!) is defined as the clockwise angle between the ridge or band of interest and the displaced feature. Fault-orthogonal motion is defined by dilation (+d) or contraction (-d). The measured separation and alpha angles are normalized and corrected to account for the limited range of alpha angles found along the ridge of interest (as detailed in [6]). These data sets are then compared to unique analytical curves of various displacement ratios (DR). Figure 1 illustrates several examples of right-lateral motion plus contraction displacement ratios, and shows that each DR’s curve is unique. Analogous curves have been produced for leftlateral motions and for oblique dilations.