Other
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p24a..08c&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P24A-08
Other
[5475] Planetary Sciences: Solid Surface Planets / Tectonics, [6280] Planetary Sciences: Solar System Objects / Saturnian Satellites
Scientific paper
The surface of Dione exhibits several zones of normal faults that have been imaged by the Cassini ISS. These faults occur in two main clusters. The best imaged cluster of fault zones is near the trailing point, and radiates from this area in the four ordinal directions. Another group near the leading point displays some unusual morphologies, but has not been extensively imaged. In between, near the sub- and anti-saturnian points, the cratered plains are crossed by only a few faint E-W lineaments. To test models for the origin of tectonic features on Dione, fault scarps and lineaments were digitized from the global image mosaic of Dione into GIS. These digitized features were then compared to various stress fields arising from figure change. Two methods of comparison were used. In the first method, the orientations of extensional structures were correlated to the local orientation of the most compressive stress. In the second method, the ratio of shear to normal stress on the fault planes (the slip tendency) was calculated, assuming normal faults. If the shear stress cannot overcome the friction on the fault planes, the faults will not slip. A number of different stress fields were examined: polar wander, nonsynchronous rotation, tidal recession, and despinning. These stress fields were examined with and without adding an isotropic stress from interior volume change, such as would be expected for freezing an internal ocean. Each stress field was also examined under a range of possible former spin axis and tidal axis positions. The best fits between stress trajectories and fault orientations are from nonsynchronous rotation models (NSR), with correlations r > 0.8. In the high correlation cases, the observed faults project from the areas of the equator that are under tension during NSR, and the cratered plains are largely under compression. This would be the case under normal (westward) NSR if the tidal axis was oriented over 90 degrees from the current axis, or alternatively by positing reverse (eastward) NSR with the tidal axis close to its current position. The slip tendency for NSR is too low below 30 degrees of reorientation to activate the faults. This means that either NSR stress built up over a large amount of reorientation, or isotropic stress from a freezing subsurface ocean aided faulting. The existence of faint lineaments in the expected extensional orientation within the nominally compressional areas of the cratered plains may argue for some isotropic tension added to the NSR stress. Strain measurements across fault zones on Dione (Tarlow & Collins, this meeting) imply less than 1% surface area expansion, which could be accomplished by freezing less than 50 km of ocean to the base of the ice shell (out of a total water/ice layer approximately 200 km thick).
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