HR: 0800h
AN: P31A-0959    [Abstracts]
TI: The Importance of Resolved Shear Stress and Dilation at the Instant of Cycloid Cusp Formation on Europa
AU: * Marshall, S T
EM: mars0776@uidaho.edu
AF: Dept. of Geological Sciences, University of Idaho, PO Box 443022, Moscow, ID 83844-3022 United States
AU: Kattenhorn, S A
EM: simkat@uidaho.edu
AF: Dept. of Geological Sciences, University of Idaho, PO Box 443022, Moscow, ID 83844-3022 United States
AB: Chains of arcuate fractures called cycloids or flexi are nearly ubiquitous on the ice surface of Europa and are typically among the youngest lineaments visible. Cycloids, by definition, consist of two or more arcuate segments and at least one cusp, although many cycloids may form long chains containing several segments and cusps. Cycloid segments meet at cusps, which are typically defined by a sharp kink. The current model for cycloid formation suggests that cycloids grow as tensile fractures in the diurnal tidal stress field on Europa, which constantly changes in magnitude and orientation, rotating clockwise in the southern hemisphere and counter-clockwise in the northern hemisphere. In this model, cycloids form perpendicular to the maximum tensile stress direction and grow in a curving path as the principal stresses rotate. Cycloids form cusps since a number of hours pass between the time when a segment is inactive and the time when the next segment initiates. This tensile crack model has seemingly isolated the dominant driving mechanism for cycloid growth (i.e. diurnal stress). Nonetheless, this model fails to account for the likely mechanics of crack development in the extant stress field at cycloid cusps. Our data indicate that cusp angles (the angular change from one segment to the next) are all less than $90\deg$, which geometrically necessitates resolved shear stress on an existing cycloid segment at the time of cusp formation. The only loading condition in which there would be zero resolved shear stress (for cusp angles $<$ $45\deg$) would be if the two horizontal principal stresses coincidentally resolved equal and opposite shear stresses on the existing segment, causing the resolved shear stresses to balance out. This scenario is impossible for the average cusp angle of $56\deg$ (based on 126 measured cusps), as it would geometrically require the minimum tensile stress to be more tensile than the maximum tensile stress. The existence of resolved shear stress must therefore be accounted for in cycloid growth models. When shear stress is resolved onto a crack, tailcracks may form in the extensional quadrants. For pure strike-slip sliding, tensile stress is theoretically maximized at $70.5\deg$ from the trend of the slipping feature in the extensional quadrants. We surveyed strike-slip faults on Europa having observable offsets and associated tailcracks and measured a wide variability in tailcrack angles with respect to fault strike, averaging $54\deg$ and ranging from 30 to $80\deg$. This variability in tailcrack angle can be mathematically attributed to concurrent opening and shearing along faults on Europa. High-angle tailcracks reflect pure strike-slip motions, whereas progressively lower angle tailcracks indicate increasing ratios of opening to strike-slip motion of the crack surfaces at the instant of tailcrack development. We advocate that rotating diurnal stresses can similarly form cycloids by initiating a cycloid segment in the form of a tailcrack, thus forming a cusp. The sense of shearing that is geometrically required during cusp formation is consistent with the formation of cusps by tailcrack initiation. The formation of cycloid cusps by tailcracking is also consistent with the observation that average cycloid cusp angles are almost identical to average tailcrack take-off angles along ridge-like strike-slip faults on Europa. These results suggest that a component of opening must accompany shearing during the formation of both cycloid cusps and tailcracks along strike-slip faults, but that the mechanical development of these two features is nonetheless identical.
DE: 8010 Fractures and faults
DE: 8020 Mechanics
DE: 6218 Jovian satellites
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting