Forming Europa’s folds: Strain requirements for the production of large-amplitude deformation

Abstract

The surface of Jupiter’s iconic icy satellite Europa is dominated by tectonic features that have formed in extension. In contrast, features that have formed in contraction appear to be rare. Thus there exists an apparent imbalance between the observed surface expression and relative magnitudes of extensional and contractional strain. In this paper we reexamine the amount of strain that can be accommodated during fold formation on Europa using a two-dimensional finite element model that allows the examination of fold formation at large strains and under a broad range of initial conditions. We find that, in contrast to previous assessments, relatively large strains can be accommodated by the development of relatively low-amplitude folds. Under our nominal conditions, creating peak-to-trough fold amplitudes of ∼200m—consistent with Europa’s Astypalaea folds—requires ∼7% contraction of the lithosphere. Decreasing either the strain rate or the ice grain size allows the formation of larger-amplitude folds at smaller strains, but we find no cases where 200-m-amplitude folds form at less than ∼3% strain. Only if both of these conditions are met simultaneously do large amplitude folds form at contractional strains ⩽3%. The requirement of larger strains relative to previous analytical models results from the combined effects of an initial period of fold nucleation (generally 1–2% contractional strain) in which limited amplification occurs, and smaller amplitude growth rates overall. The requirement of several percent strain to produce even moderate-amplitude folds is consistent with previous numerical models of single-layer and lithospheric-scale viscoelastic folding under terrestrial conditions. Because the formation of large-amplitude folds requires relatively large strain, small but significant amounts of contraction can be accommodated predominantly through uniform thickening of the lithosphere during the nucleation phase of fold growth without the formation of obvious surface deformation. The accommodation of contractional strains via such pre-folding lithospheric thickening provides at least a partial solution to Europa’s apparent strain imbalance.