Exploring the disparate tectonic manifestations of lithospheric shortening on ocean worlds: Enceladus and Europa

Abstract

The moons Europa and Enceladus both have water-ice dominated shells tens of kilometers thick overlying global oceans. Both moons are tidally heated and have high rock mass-fractions relative to other moons in their system, and both have youthful, tectonically deformed surfaces. Despite their many commonalities, the surfaces of the two moons bear little resemblance to one another. Europa's tectonics are dominated by extensional features, including wide bands that imply extremely large surface strains. Features formed by the requisite lithospheric shortening accompanying such extension are largely absent, but long-wavelength folding may have occurred in some locations. Enceladus' tectonics include features formed in both extension and contraction. Unlike Europa, folds, fold and thrust belts, and isolated thrust faults are plentiful there. In this investigation we use numerical simulations of the shortening of ice lithospheres to bring new insight into why two moons with so much in common appear so different from each other. We find that the temperature structure of the lithosphere controls the style of deformation that results from shortening. Warm conditions resulting from higher surface temperatures and heat fluxes produce folding, which is essentially a viscous response to shortening. Cold temperatures resulting from lower surface temperatures and heat fluxes results in faulting, which is a brittle response. Intermediate conditions can lead to complex combinations of folding and faulting. We hypothesize that Europa's surface is generally warm enough for shortening to be accommodated by subtle folds that are difficult to detect in the sparse topography data currently available, whereas Enceladus' colder temperatures favor a combination of folding and faulting. We argue that the dependence of tectonic style on lithospheric temperature self-consistently explains the pattern of tectonics observed across Enceladus, and we emphasize that non-rigid deformation (folding) is not inconsistent with the emerging paradigm of plate tectonics on Europa.