Thrust Faults, Folds, Both, or Neither? Accommodating Lithospheric Shortening on Icy Worlds.

Abstract:

Enceladus’ surface exhibits numerous tectonic features interpreted to form by lithospheric shortening, including long-wavelength folds, smaller-scale folds and ridges, branching ridges with 1 km of relief (dorsa), and curvilinear ridge belts [1]. The morphology of many of these features suggests that thrust faulting plays a dominant role in their formation [2,3]. In contrast, Europa, where lithospheric shortening must have occurred [4], exhibits just a few putative contractional features: subtle long-wavelength folds [5], and regions of missing surface area that may have been subsumed into the interior [6]. Here we utilize numerical models of lithospheric shortening on icy satellites to elucidate the processes and conditions that lead to faulting and folding. These simulations indicate that cold surface temperatures (~70 K), a thin lithosphere (rapid viscosity decrease with depth due to a high heat flux and/or low thermal conductivity), and low surface gravity promote localization of strain into fault-like deformation bands. Warmer surface temperatures (~100-120 K), thicker lithospheres, and weaker rheological transitions promote folding or uniform thickening. With its generally colder surface and potentially high heat flow of >100 mW m^-2 [7,8,9,10], our models predict that faulting should occur more readily on Enceladus than Europa, which is consistent with observations. In particular, localizing contractional strain into fault-like zones on Europa is challenging due to its warmer surface temperature, and may require very high heat flows. The difficulty in forming large-scale thrust faults presents a challenge to the hypothesis that regions of the Europa’s surface have been subsumed into the interior. We continue to evaluate the role of ice shell thickness on lithospheric shortening. [1] Crow-Willard and Pappalardo, 2015. JGR 120, doi:10.1002/2015JE004818. [2] Pappalardo et al. 2014. LPSC #2143. [3] Beddingfield et al. 2013. LPSC #1254. [4] Bland and McKinnon 2012. Icarus 221, 694-709. [5] Prockter and Pappalardo, 2000. Science 289, 941-943. [6] Kattenhorn and Prockter, 2014. Nat. Geo. 7, 762-767. [7] Bland et al. 2007. Icarus 192, 92-105. [8] Giese et al. 2008. GRL 35, L24204. [9] Bland et al. 2012. GRL 39, L17204. [10] Bland et al. 2015, Icarus 260, 232-245.