Mathematics – Logic
Scientific paper
May 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agusm.p33a..01m&link_type=abstract
American Geophysical Union, Spring Meeting 2009, abstract #P33A-01
Mathematics
Logic
5418 Heat Flow, 5462 Polar Regions, 6008 Composition (1060), 6020 Ices, 6055 Surfaces
Scientific paper
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 its South Pole that supply material to Saturn's 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 that include mountain ranges and massive grabens. On Earth, new lithosphere is created at spreading centers and consumed at subduction zones, a process enabled by differences in composition, density, thickness and mineral properties between continental and oceanic crust. However, the Enceladan lithosphere is made entirely of water ice, so any newly formed crust would have the same composition but lower density due to higher temperature (being more recently solidified), making subduction and consequently spreading, as we understand it on Earth, unlikely. Geometrically, the absence of fold-thrust belts and transform faults, and the widespread presence of normal faulting and extensional structures, implies extension without corresponding shortening elsewhere. This is not possible in a conventional (terrestrial) plate tectonic regime, as surface area is not conserved; 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. We hypothesize that the orogenic zone surrounding the SPT is an extensional phenomenon, broadly analogous to terrestrial basin and range topography, formed by the "calving" of blocks at the periphery of the SPT. Superficially resembling the seracs in a glacial icefall, these tilted ice blocks remain essentially stationary, while the basal detachment (possibly a listric normal fault) progresses outward from the SPT through time, effectively marking the expanding transition from the thin, warm ice of the SPT basin to the cold, thick ice of the northern plains. As the heat affected zone spreads outward over time, more proximal blocks undergo gradual viscoelastic relaxation until they are topographically indistinguishable from the complexly-oriented ridged units associated with the "tiger-stripe" sulci. We interpret these ridges as cryovolcaniclastic levees, based on recent high-resolution Cassini-ISS imagery. The presence of a solid-liquid phase transition below the south polar terrain provides a mechanism for producing a basin over a positive thermal anomaly, as a consequence of the unusual behaviour of water ice compared to silicate rock on Earth. We 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.
Lageson D. R.
McLeod S. S.
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