Host rock fracture analysis: applying the deformation mechanics associated with shallow igneous intrusion to the fracture bridging theory, mckinney hills laccolith, big bend national park
Zimmerman, Nathan Marcus
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A detailed structural analysis of fractures, faults, and cleavage within the Cretaceous and Tertiary siliclastic host rocks along the semi-circular northwestern aureole of the McKinney Hills laccolith was performed to understand the relationship between regional tectonic strain and local emplacement-related strain, since the late Mesozoic. Local and regional effects of deformation are evident in the field that wrap around the laccolith (e.g. deflected host rocks, fractures) and cut through the laccolith (e.g. normal faults). Approximately 2000 fracture orientations, collected primarily from sandstone in host rock exposures within 5 structural domains and 6 stratigraphic stations in the Pen and Aguja Formations that span the western flank of the intrusion, reveal three different types of fractures in the host rocks based on bedding layer morphology and cross-cutting relationships: 1) cross-cutting, 2) confined, and 3) densely spaced. The most common type of fracture, 2) confined to a lithologic layer, are the opening mode I, extensional type fracture where no displaced markers indicate shear (e.g. slickenlines, calcite veins, displaced fossils, etc.). The confined sandstone fractures commonly either arrest at shale layers or bridge shale layers where the fracture surface continues into the overlying sandstone layer with no apparent fracture in the shale. Fracture orientations collectively analyzed in rose diagrams and stereonets reveal regional modes of deformation as well as local strain from laccolith emplacement. In all sub-domains and six measurement stations in the Pen and Aguja Formations, a primary NNW-trending and subordinate NE-trending fracture sets that coincides with predicted early Basin and Range and late Basin and Range extensional fracture directions for the Trans-Pecos region of West Texas, respectively. Theoretical laccolith emplacement-related fracture sets trend both sub-parallel to bedding strike and bedding dip. Secondary- and tertiary-trending sets of fractures that coincide with theoretical emplacement-related, extensional fracture trends occur in sub-domains IA/B, IIA/B, IIIA, IVA/B, and VC, and the six measurement stations. The fracture bridging model mimics commonly observed conditions of fracturing in the Pen and Aguja Formations, in which a fracture confined to a sandstone layer prior to laccolith emplacement is modeled. A layered model is utilized where closing tractions applied to the crack surface in the shale layer approximate mechanical differences in rock properties. Using a stress intensity analysis at the fracture tip the role to which a fracture will propagate as a function of depth within a laccolithic overburden is explored. A layer-parallel, tensile stress, associated with flexure (i.e. stretching and bending) of the McKinney Hills laccolithic overburden, acting as a bent elastic plate, is derived and applied to the fracture bridging theory. A subsequent plot of assumed values fall well within the critical sandstone failure envelope, using previously determined, mean fracture toughness values. Therefore, both orientation and numerical analyses confirm that smaller tertiary sets of fractures mapped in the Pen and Aguja Formations must be strain locally imposed by laccolith emplacement.