HR: 0800h
AN: T41F-1280    [Abstracts]
TI: Growth and Displacement-Length Scaling of Grabens: Examples of Fault Restriction and {\it D-L} Saturation on Mars
AU: * Polit, A T
EM: polita@mines.unr.edu
AF: Geomechanics-Rock Fracture Group, Dept. of Geol. Sciences/172, Mackay School of Earth Sciences and Engineering, Univ. of Nevada, Reno, NV 89557 United States
AU: Schultz, R A
EM: schultz@mines.unr.edu
AF: Geomechanics-Rock Fracture Group, Dept. of Geol. Sciences/172, Mackay School of Earth Sciences and Engineering, Univ. of Nevada, Reno, NV 89557 United States
AB: A fault in a layered sequence grows with a constant {\it D-L} ratio until restricted from further down-dip propagation by a change in the mechanical properties of the sequence. The transition from a "small" to "large" fault is marked by a change from a peaked to a flat-topped displacement profile. We measure the throw distribution on faults in an exceptionally well exposed population of grabens in northeast Tharsis on Mars. We find a transition from peaked to flat-topped profiles as a function of fault length. The graben-bounding faults with flat-topped profiles exhibit a relatively constant displacement ({\it D-L} saturation), indicating these faults are restricted by a mechanical boundary within the Martian crust. Faults less than 25 km in length exhibit peaked (triangular) displacement profiles and {\it D-L} ratios consistent with those of other measured normal faults on Mars ($\sim$1$\times$10$^{-3}$). Graben-bounding faults greater than 60 km clearly exhibit flat-topped displacement distributions, with a relatively constant throw of $\sim$45m (there is a lack of data on faults between 25 and 60 km). Restriction likely controls the nearly constant graben spacing. The effects of interaction and fault linkage can be seen within the displacement profiles. Many of the faults have asymmetric displacement distributions, with the point of maximum displacement skewed towards the central segments of the grabens of Tantalus Fossae (to the south). Some of the grabens with skewed profiles are in close proximity and appear to be in the process of linking with other grabens along-strike. Reductions in the amount of displacement can be correlated with faults that have linked but have not yet accumulated enough displacement to erase all evidence of this linkage. Changes in graben character, such as an increase in width, correlate with an increase in displacement. Additionally, our preliminary data suggest that as one follows the segmented grabens to the south (away from their distal edges), the stress perturbations caused by the faults overcome the strength of the mechanical barrier and displacements increase until the graben-bounding faults are again restricted by a deeper mechanical boundary.
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
DE: 8149 Planetary tectonics (5475)
DE: 6225 Mars
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting