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Oxygen, carbon, and strontium isotope variations in vein-filling calcite and quartz cements and their host rocks are used to elucidate the origin, spatial and temporal evolution, and migration pathways of fluids in the detachment Nuncios fold complex, northeastern Mexico. The folded Mesozoic sedimentary sequence contains two regional paleohydrostratigraphic units separated by a unit of low permeability. Two main generations of cements are present in both paleohydrostratigraphic units. Distinct differences exist between [[delta].sup.18]O, [[delta].sup.13]C, [sup.87]Sr/[sup.86]Sr, and fluid-inclusion temperatures of early vein-filling cements in the lower and the upper units. These differences, together with a strong correspondence between early cement and host-rock [[delta].sup.18]O and [[delta].sup.13]C values, suggest that early diagenetic fluids were compartmentalized between the two units. Late vein-filling cements have isotopic compositions and fluid-inclusion temperatures that converge to similar values, indicating a change to open fluid flow between the lower and upper units. We hypothesize that the fluid history of the Nuncios fold complex evolved in two main stages: (1) burial diagenesis and early folding, during which fluids were confined within individual units, and (2) late-stage folding, during which increased deformation associated with fold tightening caused the expulsion of fluid from the lower unit into the upper unit. Keywords: fluid migration, evaporite, isotopes, detachment fold, Sierra Madre Oriental.

In many sedimentary basins of the world the minimum horizontal stress, [S.sub.h], is greater in overpressured zones than in normally pressured zones at equivalent depths. A common explanation is that the frictional slip on listric normal faults keeps the difference between vertical stress, [S.sub.v], and [S.sub.h] within certain bounds, and the difference is smaller under lower effective stress (i.e., higher pore pressure, [P.sub.p]). However, in the overpressured parts of the central North Sea graben, United Kingdom, and the Sable subbasin of the Scotian Shelf, Canada, conventional friction envelopes underestimate the magnitude of [S.sub.h]. These data instead indicate that [S.sub.h] increases at a rate proportional to but less than the rate of increase of [P.sub.p], a condition consistent with a [P.sub.p]-induced deformation of the rock called poroelastic behavior. This paper argues that, whereas friction may govern [S.sub.h] in normally pressured basins, poroelastic behavior is responsible for the unusually high Sh in the overpressured parts of these same basins. Data on the [P.sub.p] and [S.sub.h] gradients from these basins suggest that [Delta][S.sub.h]/[Delta][P.sub.p] [approximately] 0.7.

We treat layers of sedimentary rock as elastic plates and predict the orientations of joints by assuming that they open parallel to the maximum instantaneous stretch of a layer. Because the direction of maximum instantaneous stretch is parallel to the maximum curvature of a surface, we hypothesize that joints will trend parallel to the minimum curvature of an elastically deformed layer. After constructing pseudo--three-dimensional trishear models of Laramide-style uplifts that grow self-similarly, we calculated the direction of minimum-curvature axes during the evolution of the fold. Our analysis of minimum-curvature axes in evolving folds suggests several important characteristics for fold-related joint sets: (1) joints that are neither parallel nor perpendicular to the fold axis may be induced by local fold-related strains; (2) at any time during folding, joint orientations may vary according to the structural position on a fold; (3) at any location on a fold, joint orientation may depend on when a joint forms during the evolution of the fold; and (4) joint patterns in trishear folds may vary with stratigraphic position. Natural folds that evolve along simple geometric pathways may develop fold-related joint sets, the orientation, dominance, abutting relationships, spacing, and continuity of which will vary systematically throughout the structure. This variation in joint-system architecture may reflect the history of fold growth. Keywords: curvature, fracture, prediction, trishear, fold, modeling.

K.M. Brown et al. have committed an error in using effective stress in analyzing hydraulic fracturing. The analysis of hydrofracture dilatancy of existing cracks using an effective stress criterion can be successful only if the minimum principal total stress in the tectonic stress state is vertical. It would be more accurate to use total stress criterion in the analysis. Moreover, the workers have also used a wrong equation in relating horizontal effective stress to change in pore pressure.