Abstract

Symmetric relations between synfolding fractures and fold geometry have been used to explain the formation of fractures in folded sedimentary rocks. Based on field observations at the Emigrant Gap, a Laramide anticline located near Casper, Wyoming, we propose that the orientation of synfolding fractures was influenced dominantly by two tectonic joint sets, J1 and J2, which developed prior to folding. Rotations appropriate for unfolding the strata reorient most fractures such that they strike sub-parallel to one or the other of the two joint sets and oblique to the fold hinge line. On the fold limbs, these prefolding joints apparently propagated along strike and infilled during folding to form a denser network of joints bedding perpendicularly with spacing ∼1/5 to 1/10 that of the same joint set in nearby unfolded outcrops of the same formation. With proximity to the hinge line of the anticline, where bending of strata and surface curvature are elevated, the style of deformation is different. Whereas most J2-parallel fractures still are joints, fractures in the J1 orientation include sheared prefolding joints and bedding-inclined shear fractures. Apparent normal offsets of bedding by these shear fractures are an order of magnitude larger than offsets observed across sheared joints on the fold limbs. The proposed model of fracture-fold relationships differs from existing conceptual models because it postulates that folding-related fracture orientations are controlled by the orientations of prefolding fractures. For the Emigrant Gap anticline, these fractures are not oriented symmetrically with respect to the fold hinge line.

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