Mathematics – Logic
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.p31c..07k&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #P31C-07
Mathematics
Logic
6020 Ice, 6218 Jovian Satellites, 8010 Fractures And Faults, 8020 Mechanics
Scientific paper
Secondary cracks are commonly produced at stress concentration points at the tips of faults. These so-called tailcracks form at an angle to the fault trace, with locations about the fault tips that indicate whether slip was left-lateral or right-lateral. Tailcracks are widespread on the surface of Jupiter's moon, Europa, and attest to the common occurrence of strike-slip faults. The crust of Europa is an ice shell at least several kilometers thick, potentially underlain by liquid water. The ice shell is constantly flexed by tidal forces that have been sufficiently high in the geologic past to pervasively fracture the ice. At any point on Europa, the tidal stresses are constantly rotating, causing shear stresses to be resolved onto pre-existing lineaments. There is clear evidence of lateral offsets across many large Europan lineaments, such as the 810 km long, right-lateral, Astypalaea Linea. We have identified tailcracks along many Europan strike-slip faults, with geometries compatible with the sense of slip. Nonetheless, the "take-off" angles between many of the faults and their associated tailcracks are less than the theoretical, and commonly observed, 70° angle that characterizes terrestrial tailcracks, which form along fault surfaces that remain in contact during slip (mode II in fracture mechanics terminology). Several tailcracks at the eastern tip of the 1500 km long Agenor Linea are oriented 30-35° to fault strike. Agenor has experienced at least 20 km of right-lateral motion, but has also apparently dilated and been infilled with material from below the ice shell to form a fault zone about 20-30 km wide. Low-angle tailcracks have also been identified along other dilated faults on Europa. We used linear elastic fracture mechanics models to test the effect on tailcrack angles by dilation during fault slip events (mixed-mode I-II). Our result for the pure mode II case predicts a take-off angle of 70° . However, as the amount of fault dilation increases during slip (increasing mode I/mode II ratio), tailcracks develop broader curvatures and lower take-off angles, similar to the tailcracks at the tip of Agenor Linea, which resemble the result for a mode I/mode II ratio of 2. This implies that Agenor may have dilated during slip by a factor of twice the amount of strike-slip motion. Such behavior is consistent with the observed evidence of dilation along Agenor and other faults with low-angle tailcrack geometries. Thus, dilation may be an important component of the process by which slip accumulates along many (but not all) Europan strike-slip faults. Such dilation during fault motion is compatible with the conceptual "tidal walking" model for Europan strike-slip faults, which hypothesizes repetitive cycles of opening and closing during shear motion, allowing faults to accumulate shear offsets like a ratchet. Finally, tailcracks on Europa commonly occur along lineaments that show no evidence of lateral offsets. The resolution of Europa images may thus be insufficient to resolve small strike-slip offsets along many lineaments that nonetheless slipped a sufficient amount to generate tailcracks at their tips. Therefore, there may be many more strike-slip faults on Europa than can be determined from lateral offset evidence. Accordingly, features that have conventionally been assumed to be pure mode I (tensile) fractures, such as cycloidal ridges (which commonly exhibit takeoff angles in the 60-70° range), may actually be associated with small shear motions and associated tailcracking.
Kattenhorn Simon A.
Marshall Scott T.
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