Strain Accommodation by Fracturing During Folding of Sedimentary Rock

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The PI's propose to investigate the mechanics of fracturing during folding because field data indicate this is a significant strain-accommodating mechanism of the brittle crust. Fractures nucleate and propagate as a result of stress concentrations, and the stress fields within folds can be constrained by understanding the associated fracture evolution in space and time. Previous efforts have argued for simple, symmetric and homogeneous stress-fold relationships, and various models of fracture formation in such idealized stress fields have been proposed. These methods have in common that they ignore pre-folding structures such as joints, which can perturb the stress field within the fold, if they are reactivated. New fractures then would form in a stress field different from that predicted by the simple models. At the Emigrant Gap anticline, a Laramide fold located near Casper, Wyoming, the simplified relationships of previous models are not capable of explaining the majority of the observed fractures. The PI's preliminary field observations show that two tectonic joint sets existed prior to folding, and that most fractures observed on the fold are sub-parallel to these two sets. Some of the older joints show shear offsets, arguing for their reactivation during folding. Associated with the reactivation is the formation of new fractures with strikes that parallel those of sheared joints rather than the fold axis. Measured fracture intensities for both fracture sets on the fold are approximately four times the intensities of the respective joint sets in unfolded strata of the same formation.The PI's will determine the influence of pre-folding joints, bedding interfaces, and contrasting constitutive properties of the strata on fracture development and strain accommodation during folding. They plan to integrate detailed field observations with mechanical modeling of these structures. Their specific objectives are: l) Map and document fractures and fold geometry of various sandstone units at the Emigrant Gap anticline, including fracture geometries in 3-D, the 3-D shape of the folded strata, and evidence for bedding-parallel shearing within the shale units. 2) Design and execute large-strain numerical experiments using the FEM code ABAQUS to understand the underlying mechanical process of strain accommodation by fracturing during folding as motivated by the field observations. 3) Integrate the insights gained from the field observations and numerical analyses to construct a conceptual model to predict fractures in similar geologic and tectonic settings. This model will modify and, in cases such as the Emigrant Gap anticline, replace existing models based on symmetric fold-fracture relationships.