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Previous studies of jointing in sedimentary rocks have shown that joint-network architecture is controlled by mechanical stratigraphy, which is described by (1) the thickness and rigidity of stratigraphic units and (2) the nature of interfaces between beds. Using joint pattern to infer rigidity contrast between beds is complicated by the possibility that material properties of the beds may change between fracturing events. Observation of both an early bed-contained joint set and a differently oriented late throughgoing joint set in the same beds of growth strata in the Oliana anticline in the Spanish Pyrenees may reflect changing mechanical properties (i.e., via diagenesis) during the folding process. Using a Schmidt hammer, we assess the rigidity contrast between the individual units and show that the present-day contrast is not great enough to terminate joints at interfaces. This result is consistent with an interpretation that the late-stage throughgoing joints formed in strata with conditions similar to those of the present day and that the early bed-contained joints formed when the rigidity contrast between beds was significantly greater than that of the present day. For example, differential diagenesis rates between layers of differing grain size would produce temporally changing mechanical stratigraphy. We propose that changes in mechanical stratigraphy may have previously been unrecognized in other stratigraphic sections and that these changes affect joint-network architecture and subsequent fluid-flow pathways.

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Many algorithms assume plane strain to construct, model and restore fault-cored folds. Using mechanical models that allow heterogeneous transport in three dimensions, we explore the distribution and magnitude of out-of-plane transport in plunging fault-cored anticlines and provide guidelines of where plane strain should and should not be applied. We developed a new technique of incrementing infinitesimal elastic strains to produce folds with aspect ratios similar to natural folds. Map views of displacement vectors show that in general, out-of-plane displacement is localized near the lateral fold tips. Cross-sections show that out-of-plane transport is depth dependent with out-of-plane displacement increasing toward the surface. Flexural slip surfaces compartmentalize out-of-plane transport within distinct mechanical units, with the maximum outof-plane displacement near the tops of mechanical units. Two-dimensional models with additional frictionally slipping bed contacts suggest that freely slipping contacts can approximate the deformation of many frictionally slipping contacts. We show that out-of-plane transport is significant in the simplest non-cylindrical folds, and suggest that complex non-cylindrical structures should not be modeled using plane strain exclusively. We also show that flexural slip surfaces exert a significant control on the magnitude and structural position of out-of-plane transport in our models.

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