Characterizing the structure of large-displacement, plate-boundary fault zones iscentral to the MARGINS initiative to understand the mechanisms that allow con-tinental lithosphere to deform by weak tectonic forces, strain partitioning, andmovement of fluids during margin formation. At many boundaries, plate motionsprimarily are accommodated along large fault zones that achieve significant dis-placement over time. In addition, fluid migration through the crust often is intimatelylinked to the fluid-flow properties of these zones. At the crustal scale, faults maybe idealized as simple discontinuities, or surfaces, between relatively rigid blocksalong which shear displacement has occurred. Considerable work has elucidated thegeometry of faults and the shear-displacement distribution along faults, as well asscaling relations and the manner in which faults grow and link with strain (Cowieet al. 1996; Davison 1994). Nonetheless, continued effort to better define the struc-tural, physical, and chemical properties of fault zones is necessary to understandthe mechanical, fluid-flow, and geophysical properties of the lithosphere