Reconciling Different Deformation Mechanisms in Adjacent Sedimentary Lithologies at Raplee and Comb Folds, Monument Upwarp, UT

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This project compares and evaluates the brittle deformation (jointing in siltstone and sandstone) and the ductile deformation (shear zone development with veins and solution seams in limestone) that occurred at the outcrop scale in adjacent strata during kilometer-scale folding on the Monument Upwarp during the Laramide orogeny. The goals of the research are: 1) to describe and map the joints and conjugate shear zones at Raplee anticline and Comb monocline in southeastern Utah; and 2) to construct and evaluate continuum mechanical models using finite element methods to investigate the brittle elastic and ductile plastic deformation associated with these structures. We seek to reconcile how and why these different mechanisms were activated in close proximity and under similar loading conditions during folding, and to deduce the stress-strain behavior of the host strata. The hypothesis is that different constitutive properties and strengths led to the activation of the different mechanisms. To test this hypothesis we compare the systematic geometric features of the structures (e.g. opening distributions and shapes of veins), to the results of the numerical models. We anticipate that certain ranges of constitutive properties and states of stress will result in similarity. This will provide important insights about the mechanical behavior of sandstone and limestone, not obtainable from laboratory testing at the decimeter scale in the absence of such structures. Folds are one of the most common geologic structures, occurring in all layered rock types at scales from hand samples to mountain ranges. Folds in sedimentary rock take on the added importance of being traps for hydrocarbons, and therefore a primary target for oil and gas exploration. Being familiar with the stiffness and strength of sedimentary rock slabs used as paving stones or counter tops, the general public must wonder how such strata is contorted into spectacular folds such as those Comb and Raplee anticlines. The proposed project addresses this question, which also is a fundamental issue for structural geology: what deformation mechanisms operate within sedimentary strata during kilometer-scale folding? One mechanism is obvious: sandstone layers are broken by brittle fractures (joints). A second mechanism is quite subtle: limestone layers lack joints, but contain shear zones, alignments of surfaces (solution seams) along which calcite has been dissolved, and orthogonal opening cracks (veins) filled with calcite. The juxtaposition of brittle (jointing) and ductile (shear zone) mechanisms in adjacent outcrops offers the opportunity to deduce how the mechanical properties of the respective formations varied at the time of folding. The interplay of detailed mapping and mechanical modeling promises to offer new insights and original concepts about rock deformation during folding and to contribute new insights for scientists working in the closely related disciplines of petroleum geology, rock mechanics, and geodynamics. In addition to the research objectives of this project, the award is supporting the training of a graduate student; includes a partnership with faculty at Dine College to organize field trips for Native American students who will participate in collecting data; and is contributing to the broadening of participation of underrepresented groups in the Earth sciences.