Prediction of fracture-induced permeability and fluid flow in the crust using experimental stress data

The results of a series of two-dimensional photoelastic experiments on dilational jogs cut in PERSPEX (super TM) sheets have been used to determine the second-order fracture patterns and fluid-flow network associated with tectonically loaded dilational jogs. The stress data derived from these experiments, orientation of the principal stresses, and the distributions and magnitudes of the differential stress and the mean stress are used in combination with the theory of brittle failure to predict the orientation, approximate distribution, and likelihood of formation of second-order brittle structures associated with the modeled jog geometries. The orientations of brittle structures predicted compare favorably with those found in natural large-scale dilational jogs, such as the Vienna Basin, the Dead Sea Basin, and the Brawley jog (California), as well as in mesoscopic examples and those formed in analog models. From the experimental results it is possible to determine areas of high and low mean stress, and by assuming that fluid migration occurs in response to mean stress gradients it is possible to predict the fluid migration associated with the development of a dilational jog. In this way, it is possible to show that the intrajog region of "underlapping" and "neutral" dilational jogs will not be favorable sites for fluid ingress. When the jog has an "overlapping" geometry, the intrajog region is a mean stress low and, consequently, fluid ingress into the jog is more likely. In addition, as the fault overlap increases the distribution of mean stress, and thus the associated fluid flow, in the intrajog region increases in complexity. By combining the predicted second-order fracture patterns and the mean stress distribution derived from the photoelastic analysis, it is possible to construct a map showing sites of likely fluid accumulation and mineralization. These sites correspond to regions of low and high fluid throughflow, respectively. Predictions based on this model compare well with natural examples of fluid accumulation and mineralization in dilational jogs.