A Geometric Analysis of Surface Deformation: Implications for the Tectonic Evolution of Ganymede
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ICARUS 60, 200--210 (1984) A Geometric Analysis of Surface Deformation: Implications for the Tectonic Evolution of Ganymede M. T. ZUBER AND E. M. PARMENTIER Department of Geological Sciences, Brown University, Providence, Rhode Island 02912 Received August 5, 1982: revised May 30, 1984 The visual nonalignment of the furrows and the circularity of impact craters are used to study surface deformation on Ganymede. The furrow system is examined to test the hypothesis that lateral motion has taken place between areas of dark terrain. Results show that while lateral motion cannot be ruled out, it may not be required to explain the geometry of the system. Initial noncon- centricity of the furrows or an early period of penetrative deformation shortly after furrow forma- tion could also account for the present configuration. Centers of curvature of the furrows in Galileo and Marius Regiones are numerically determined and it is shown that if lateral movement did occur, it is not possible to determine the amount of displacement. The axial ratios of impact craters in the Uruk Sulcus region which separates Galileo and Marius Regiones are determined and show that large scale shear deformation has not occurred in that area since bright terrain was emplaced. Deformation of impact craters within Galileo Regio suggests that Ganymede's lithosphere has behaved rigidly throughout most of the satellite's evolution. The shapes and orientations of impact craters in dark terrain around wedges of bright terrain are used to place an upper limit on the amount of extension associated with bright terrain formation. ,, 1984 Academic Prcs~. Inc. INTRODUCTION mation, although of importance in under- standing the mechanism of global rifting on The surface of Ganymede preserves the Ganymede, has yet to be determined. record of an active and diverse tectonic his- Evidence for shear deformation on Gany- tory. The major surface units consist of ap- mede is localized and small scale (Luc- proximately equal portions of dark, heavily chitta, 1980; Golombek and Allison, 1982; cratered polygons and bright, lightly cra- Parmentier et al., 1982). The most convinc- tered regions which penetrate and separate ing examples are several impact craters dark areas (Smith et al., 1979a,b). Funda- whose rims are offset by not more than a mental observations of the nature of defor- few kilometers. Lucchitta (1980) has cited mation within these units lead to basic con- the nonconcentricity of the Ganymede fur- clusions about the tectonic history of the row system as evidence for large lateral satellite. Deformation on Ganymede, at motion of dark terrain blocks. The exis- least since bright terrain formation, has tence of lateral displacements at this scale been primarily extensional. Compressional would place important constraints on the deformation has not been identified within global tectonic history of Ganymede. the dark terrain and is uncertain in bright Also of considerable importance would terrain. Parmentier et al. (1982) cite evi- be the recognition of smaller scale deforma- dence which suggests that bright terrain tion within individual terrain areas. Con- formed due to finite extension of the litho- straints on the style, degree, and timing of sphere, and Shoemaker et al. (1982) suggest deformation within isolated units has rele- that bright bands resemble normal fault pat- vance not only to the tectonics, but to the terns in terrestrial rift zones. The amount of physical properties of the lithosphere at the extension associated with bright terrain for- time of deformation. The manner in which 200 0019-1035/84 $3.[~) ('opyright ~ 1984 by Academic Press. Inc All righls of reproduction in any form reserved SURFACE DEFORMATION ON GANYMEDE 201 the near surface deforms throughout time the satellite's surface, as it predates essen- has implications for the rheological struc- tially all impact craters in the dark terrain ture of the lithosphere, the geothermal gra- (Smith et al., 1979b). dient, and possibly the source of stress. The pattern of furrows is most distinct in This study focuses on the identification Galileo Regio, shown in Fig. 1. Furrows in of various types of surface deformation on this region are approximately 10 km wide, Ganymede by a geometric study of the fur- spaced 50 km apart, and extend for hun- row system and an analysis of circularity of dreds of kilometers across the surface impact craters. First, we test the hypothe- (Smith et al., 1979b). A number of linear sis that the furrow system was initially con- bright bands along the southern border of centric and that the present configuration of Galileo resemble furrows in width, spacing, the furrows provides a measure of lateral and orientation. The system continues into motion of dark terrain. This is accom- adjacent Marius Regio, however furrows in plished by determining the present centers this area exhibit distinct morphological dif- of the furrow system in several dark terrain ferences compared to those in Galileo (Zu- blocks and observing the amount of defor- ber, 1982). The Marius furrows occur as mation in bright terrain regions which sepa- shorter segments spaced an average of 22 rate these blocks. Second, we place an up- km apart and 6 km in width. A faint second- per limit on the amount of extension ary trend in the strike of furrows can be associated with bright terrain formation by observed, where approximately nine per- using the deviation from circularity of im- cent of the Marius furrows trend at an angle pact craters to calculate the strain field greater than thirty degrees from the local around wedge-shaped regions of bright ter- mean orientation. These secondary furrows rain. Third, we attempt to identify penetra- are either contemporaneous or postdate tive deformation in Ganymede's litho- those of the main set; in no case do they sphere by examining the circularity of predate furrows of the main trend. There- impact craters in dark terrain. fore they must have formed concurrently or in a subsequent event to that which pro- FURROW CONCENTRICITY duced the main system. The reason for the differences in widths The Furrow System and spacings of furrows in adjacent areas is The furrow system is a regional array of unclear. From the theory of multiringed ba- subparallel, arcuate structures that traverse sin formation (McKinnon and Melosh, several areas of dark terrain on the satel- 1980; Melosh, 1982), ring spacing should in- lite's leading hemisphere. A furrow is char- crease with lithospheric thickness. If the acterized by a central depression flanked by furrow system was impact-produced, then raised rims with relief not exceeding a few the variation in furrow spacings may reflect hundred meters (Smith et al., 1979b). Fur- a difference in lithosphere thickness be- rows are locally discontinuous, but broad tween Marius and Galileo Regiones at the arcs can be traced up to hundreds of kilo- time of formation of the system. Differ- meters across individual dark areas. The ences in furrow geometry may instead be furrows have been interpreted as ring gra- related to inhomogeneous mechanical prop- ben formed as a consequence of a basin- erties in the Galileo and Marius areas. scale impact into Ganymede's lithosphere However, the regions are separated by a (Smith et al., 1979b; McKinnon and Me- relatively narrow band of bright terrain and losh, 1980), though evidence to substantiate a gradual transition in furrow widths and this hypothesis is inconclusive. Whatever spacings is not observed. It is enigmatic the origin, the formation of the furrow sys- that two adjacent areas of the lithosphere tem marks the earliest event preserved on should display such an abrupt change in ei- 202 ZUBER AND PARMENTIER FIG. 1. Part of the Ganymede rimmed furrow system as seen in Galileo Regio. Furrows continue into Marius Regio, which lies to the west of this region. The bright terrain unit Uruk Sulcus, shown in the lower left of this photograph, separates the Galileo and Marius Regiones (Voyager 2 frame 0104J2- O01L ther mechanical properties or lithosphere therefore asymmetrical structures cannot thickness. be predicted. Asymmetries have also been noted in the The furrows in Galileo and Marius are morphology of the Valhalla ring system on not concentric, and it has been suggested Callisto in areas of high resolution Voyager that some degree of right lateral motion coverage (Hale et al., 1980; Remsberg, and/or clockwise rotation of Marius terrain 1981). These studies report an azimuthal blocks would restore the system to a circu- transition from graben structures to out- lar geometry (Passey and Shoemaker, 1982; ward-facing scarps as opposed to a simple Shoemaker et al., 1982). If the two systems change in graben dimensions. Because of were initially concentric, then the present mathematical intractibility, axial asymme- orientation of the furrows in the displaced try has not been incorporated into present dark terrain blocks provides evidence for theoretical models of basin formation, lateral motion between adjacent blocks. If SURFACE DEFORMATION ON GANYMEDE 203 lateral motion has taken place, then the off- since a longer furrow defines a greater arc, set of the respective centers of curvature it provides a better constraint on the center provides a measure of the displacement of curvature. which has occurred between the areas since The linear least-squares formulation rep- their formation. Inherent in this approach resented in Eq. (1) assumes a Gaussian dis- are two basic assumptions: (1) the furrow tribution of errors, which is that observed systems were initially concentric, and (2) on a plane surface. However, as noted in dark terrain areas acted as rigid plates since various studies of terrestrial plate tectonics the formation of the system, such that pen- (e.g., Minster et al., 1974), the distribution etrative deformation could not explain any of errors on a sphere is characterized by the present deviation from circularity.