Rifting and Faulting on icy Satellites

Two kinds of rifting have been identified on the icy Galilean satellites [1,2]. Europa possesses ∼10~km wide extensional bands, characterized by very high degrees of local extension, internal deformation on a lengthscale of ∼~100~m, and a general resemblance to mid-ocean ridges on Earth [3]. Ganymede has ∼100~km wide areas of grooved terrain, characterized by km-scale tilted fault blocks [4,5], lower degrees of local extension (stretching factor <1.6 [5]) and a general resemblance to continental rifts on Earth [1]. The characteristic spacing of faults on Europa and Ganymede has been used to infer the depth to the brittle-ductile transition (BDT), which depends on the strain rate and the shell thickness [4,6]. Here I present another constraint on these quantities, obtained by considering the circumstances under which narrow (Europa-style) or wide (Ganymede-style) rifts may form. The model is based on an analysis of terrestrial continent rifting [7]. When an ice shell is extended, the thermal gradient increases and it becomes weaker, favouring further extension. The extension also gives rise to lateral shell thickness variations, which oppose further extension. However, these lateral thickness variations may be removed if the base of the ice shell can flow rapidly. If lateral flow is rapid, narrow zones of extension and high stretching factors are generated. If lateral flow is slow, wider rifts and lower stretching factors are favoured. Thick ice shells or high strain rates favour narrow rifts; thin ice shells or low strain rates favour wide rifts. The existence of wide rifts on Ganymede is consistent with a conductive shell thickness of 4-8~km at the time of rifting, and agrees with previous estimates of strain rates [8]. To produce narrow rifting and the inferred BDT depth on Europa requires a larger shell thickness (8-20~km) and a strain rate >= 10^-15~s^-1. Based on the likely shell thicknesses, the inferred strain rates for Europa and Ganymede can be explained by differing mean stresses: 0.1-0.2~MPa for Ganymede and 0.3~MPa for Europa. These values are comparable to estimates of stress levels derived from flexural features [9,10]. The maximum strain a fault can withstand before breaking depends on the stress drop and the shear modulus [11]. Assuming that the stress drop is comparable to the remote stresses derived above, then the critical strain is ∼ 10^-4, similar to terrestrial values. For a strain rate of 10^-15~s^-1 the recurrence interval is thus ∼3000~yrs for each fault. The moment release for a 10~kmx3~km fault plane is 10¹⁷ N~m, equivalent to a M_w = 5.3 terrestrial earthquake. [1] Pappalardo et al.,Icarus 135, 276-302, 1998. [2] Sullivan et al., Nature 391, 371-372, 1992. [3] Prockter et al., JGR 107, 5028, 2002. [4] Patel et al., JGR 104, 24057-24074, 1999. [5] Collins et al., GRL 25, 233-236, 1998. [6] Pappalardo et al., JGR 104, 24015-24055, 1999. [7] Buck, JGR 96, 20161-20178, 1991 [8] Dombard and McKinnon, Icarus 154, 321-336, 2001. [9] Nimmo et al., GRL 29, 1158, 2002. [10] Nimmo et al., GRL 30, 1233, 2003. [11] Scholz, Mechanics of earthquakes and faulting, CUP, 1991.