CAN FOLDING TAKE UP LARGE CONTRACTION STRAINS ON EUROPA?

The surface of Europa is dominated by tectonic features that apparently formed in extension. Some of these features imply large strains: the formation of bands alone (by lithospheric opening) implies 5% compensating contraction of the rest of the lithosphere (and likely larger regional values). Whereas freezing and thickening of Europa’s ice shell results in net surface tension, the strains associated with the resulting surface uplift are small (~0.5% globally) and thus of minor importance to the global strain balance. Yet, few tectonic features of clear contractional origin have been identified. Thus, there appears to be an imbalance between the extensional and contractional strain observed. Here we investigate whether low-amplitude lithospheric folding can accommodate relatively large contractional strains, and provide a heretofore under-appreciated mechanism for accommodating Europa’s large extensional strains (Bland and McKinnon, submitted). Subtle folds with amplitude of ~200 m and wavelength of ~25 km are recognized at Astypalaea Linea; however, only small contractional strains (0.02% – 2%) have been inferred previously. We reexamine the amount of strain required for fold formation on Europa using a two-dimensional finite element model that allows the examination of fold formation at large strains. 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 modeling conditions, creating peak-to-trough fold amplitudes of 200 m 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 ~2% strain. Our results suggest that small but significant amounts of contraction can be taken up predominantly through uniform thickening of the lithosphere during fold “nucleation” without the formation of obviously visible 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.