At the Valley of Fire State Park, southeastern Nevada, there is a unique relationship between deformation bands and the distribution of diagenetic mineralization in the Aztec Sandstone, a reservoir and aquifer analog exposed at the surface. Distinct diagenetic alteration fronts are refracted where they cross deformation bands. Modeling this refraction as resulting from the advective transport of a nonreactive solute, we are able to back out that deformation band permeability is reduced by an average of 1.3 orders of magnitude relative to the surrounding rock with a range of 0.7 to 2.1 orders of magnitude. The geometric relationship utilized in our approach formed in the subsurface during diagenesis and provides an in situ measurement of permeability not affected by uplift and erosion or by damage associated with collecting a sample from a well bore. Our estimation is about 1 order of magnitude lower than values reported in the literature for similar rock types. ¿ 2000 American Geophysical Union