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B.R. Tufts, R. Greenberg, G. Hoppa, P. Geissler (Lunar and Planetary Lab, University of Arizona)
Astypalaea Linea is an 810 km-long gray band located near the south pole of Europa, found from Voyager 2 images to be a strike-slip fault with 42 km of right-lateral offset. Recent Galileo high resolution images captured a 290 km-long portion of the fault, revealing its inner structure, and enabling inferences to be drawn regarding the nature of the subsurface. The fault began as a curved or cycloidal crack, which then underwent shear. The irregular path of this original crack meant that strike-slip occurred along intermittent segments of the structure, while dilational regions opened where the crack formed releasing bends between those zones of shear. The resulting band consists of en echelon, overlapping pull-aparts, bounded on their peripheries by the sides of the original crack and separated from one another by the strike-slip segments of the fault. The structure on which motion occurred (and the successor to the original crack) is recorded as a curving break which traverses the length of Astypalaea Linea viewed in the Galileo images. The right shear characterizing Astypalaea Linea was probably driven by "walking," an effect of changing diurnal tidal stress. Walking occurred in the strike-slip segments of the fault, explaining the presence of "Class 1"-like doublet ridges (Greenberg taxonomy) there. The configuration of these ridges is probably inconsistent with formation by linear diapirism. The en echelon pull-aparts dilated in response to walking on the strike-slip segments of the active crack. Shear was not generated externally, as evidenced by the lack of simple shear structures adjacent to the fault. Displacement of Astypalaea Linea requires a subsurface decoupling layer to which the fault penetrates. For Class 1 ridges to form along strike-slip segments of the fault suggests that the decoupling layer is liquid.
The author(s) of this abstract have provided an email address for comments about the abstract: rtufts@pirl.lpl.arizona.edu