Quantitative Evaluation of Fractures on Monkshood Anticline, a Detachment Fold in the Foothills of  Western Canada

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Monkshood anticline is a well-exposed surface anticline located in the Foothills in northeastern British Columbia. Extension fractures are well developed in the Prophet Formation (a carbonate-shale-chert sequence) throughout this fold. Virtually all of these fractures are filled with some combination of calcite and quartz cements. The fractures formed and the mineral cements were deposited during the fold development. Most fractures on Monkshood anticline have formed at high angles to bedding, and they commonly fall into two to three distinct orientation sets. The dominant fracture trend is aligned with the fold axis through the backlimb of the fold, but there is considerable variance in the dominant orientation in the forelimb. An order of magnitude variation is found in fracture densities across this fold, and over two orders of magnitude variation occur in mean fracture aperture. These variations in density and aperture do not correlate with particular structural positions. Fracture trace lengths exhibit power-law distribution patterns, suggesting fractal character. Fracture aperture displays a roughly linear correlation to volumetric fracture strain, but shows no consistent association with either density or trace length. Stochastic modeling of the fracture networks on Monkshood anticline suggests density is the primary factor affecting fracture network connectivity, but fracture size also can play an important supporting role. The occurrence of a dominant fracture orientation set can impart a significant directional bias to connectivity. Measurement stations on Monkshood anticline that have both high fracture densities (for connectivity) and substantial fracture apertures (for conductance) occur primarily in midlimb positions.