Numerical Models of Folding on Europa, and the Mystery of Europa's Surface Area Balance

Details Numerical Models of Folding on Europa, and the Mystery of Europa's Surface Area Balance Numerical Models of Folding on Europa, and the Mystery of Europa's Surface Area Balance

Sep 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.6606b&link_type=abstract

American Astronomical Society, DPS meeting #41, #66.06

Astronomy and Astrophysics

Astronomy

1

Scientific paper

Europa's surface is dominated by extensional tectonic features. While contractional features have been identified (e.g., folds, convergence bands, and even take-up across sheared double ridges), it is unclear whether the sum of such convergence can match the amount of surface extension represented by dilational bands alone (of order 5%) (see Kattenhorn and Hurford, in Europa, in press). Dombard and McKinnon used a semi-analytical model to examine the growth of folds on Europa and found that Astypalaea-like folds can form with a thermal gradient of 20 K/km (high) and 2% contraction (low) for reasonable strain rates, but generally only for low surface temperatures (i.e., high latitudes). We revisit folding of an ice lithosphere using the finite element code Tekton; the "Astypalaea” problem is especially appropriate as surface strain appears low and localized brittle faulting minimal. Initial benchmark simulations using a constant lithospheric strength with depth (as required by the semi-analytic model) gives results consistent with previous work, with only modestly smaller instability growth rates. Employing the more realistic strength profile of Beeman et al., however, suppresses fold growth rates, a known result from terrestrial studies. In these cases forming Astypalaea-like folds requires 10% lithospheric contraction for thermal gradients of 20 K/km, or 5% contraction for thermal gradients of 30 K/km. In the absence of high thermal gradients contraction results in near-uniform thickening of the lithosphere, and large local strains may be accommodated without producing large-amplitude deformation. These models support a hypothesis that large contractional strains can be "hidden” in long-wavelength, low-amplitude folds, but the large compressive stresses necessary (of order 10 MPa) are problematic. We are presently exploring whether such "cryptic compression” is possible in the presence of strain weakening (Bland and McKinnon, submitted) and whether a "semibrittle” failure limit allows greater fold growth at lower strains and stresses.

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