Isostatically compensated extensional tectonics on Enceladus

Abstract

Saturn's moon Enceladus is the smallest body in the solar system known to be geologically active. Extensive, energetic resurfacing processes are ongoing and it possesses a system of geysers at the South Pole that supply material to the E-ring. The South Polar Terrain (SPT) is the youngest region on Enceladus and its contacts with the older cratered and grooved plains to the north are delineated by a variety of complex geologic features including mountain ranges and massive grabens. Many of the geologic features seen on Enceladus bear superficial resemblance to terrestrial structures associated with plate tectonics. A detailed structural geologic analysis, supported by crater counting studies, was used to determine whether the features seen on Enceladus are compatible with terrestrial-style plate tectonics. On Earth, new lithosphere is created at spreading centers and consumed at subduction zones, enabled by the compositional dichotomy between oceanic and continental crust. Enceladus's lithosphere appears to be made entirely of pure water ice, so any newly formed crust will have the same composition, but lower density due to higher temperature, making subduction and consequently spreading, as we understand it on Earth, impossible. Geometrically, the absence of fold-thrust belts and transform faults in the presence of normal faults and basin and range-style features implies extension without corresponding shortening elsewhere. This is not possible in a conventional (terrestrial) plate tectonic regime as surface area is not conserved; therefore, an alternate explanation is required. Topographic features associated with density contrasts between old and new terrain that are diagnostic of terrestrial spreading centers are also not observed on Enceladus. I conclude that features observed on Enceladus are inconsistent with terrestrial-style plate tectonic spreading, and represent a style of tectonism peculiar to bodies with icy lithospheres. I present an interpretation in which the cordillera surrounding the SPT is a broadly developed extensional regime, and describe a model for its formation that is consistent with the known physical properties of Enceladus, dependent on the presence of a water-ice phase transition below the south polar terrain.