The Evil Twin of Agenor: More Evidence for Tectonic Convergence on Europa

Background: Voyager images of Europa showed evidence of both strike-slip displacement and dilation along cracks [1]. Galileo images confirmed that dila-tion has been common and widely distributed. Large crustal plates have separated, with the space between them filled by new surface material creating dilational bands. Similarly, new surface has also been created along strike-slip faults, where curves in the faults have resulted in pull-apart zones [2,3], similar in appearance to the dilational bands. Terrestrial style convergence features have not been identified, raising the question: How and where has crust been removed to compensate for the new surface area that has been created? In principle, surface area could be reduced by cor-rugation-like folding, an idea based on examples of such folding that have been observed [4]. However, it is not clear whether that process could be quantitatively significant in reducing surface area. Alternatively , or in addition, surface contraction may be taken up within Europan chaotic terrain, especially if that terrain represents thermal perforation of the crust [5,6]. Patches of chaos cover about half the surface [7], so they could accommodate considerable surface contraction , but there is no compelling indication that they have played a major role. Increasing evidence is emerging that crustal surface is removed along several convergence bands, all with morphological similarities. From surface reconstructions of strike-slip displacement, Sarid et al. [3] identified two examples of linear surface features that appeared to have accommodated surface convergence. At these locations, band-like structures have an appearance distinctly different from dilational bands. The latter have parallel boundaries of identical shape, with intervening material covered by fine grooves parallel to the edges, and the pre-dilation configuration can be reconstructed by removing the intervening material and fitting together the adjacent crustal plates. In contrast, the convergence bands have curved boundaries that do not fit together. Bands with the general appearance of the convergence bands found in [3] are fairly common on Europa. Further support for the hypothesis that such bands generally represent convergence comes from consideration of 1400 km long Agenor Linea. As a long, bright, wide band, Agenor (including the nearby, associated , similar Katreus Linea) is unique on the relatively heavily imaged anti-jovian hemisphere. The proximity of the Wedges region, where considerable surface dilation has been a source of new surface, is