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Abstract: “Fault drag” (deflection of beds or other markers into folds that are convex in the direction of relative slip) is often interpreted as the product of frictional sliding along a fault and progressive tilting of beds with increased amount of displacement along a fault. We analyze two sets of normal faults, with throws ranging from 0.5m to 5m, and associated fault-related folds in mechanically layered upper Cretaceous carbonate, anhydrite, and shale in central Texas. For each fault set, we interpret the fault displacement and fault-related folds exposed in outcrop to represent different stages in the developmental sequence. In both fault sets, faults in dolostone or limestone lose displacement and tip into less competent anhydrite or shale where deformation is accommodated by folding and smaller scale deformation. Fold wavelength is established early and at small displacement (<1m throw). With increasing displacement the monoclinal fold limb steepens and is extended parallel to bedding, locally producing boudinage in the most competent bed between incompetent beds. Clay smear is well developed where a 0.35m thick clay shale is locally thinned to 0.1m associated with fault throws of 0.5 to 5m. Bed tilting and the development of apparent drag is not the product of frictional sliding but instead folding at the tip of an arrested, in this case upwardly, propagating normal fault. We conclude that synthetic dip associated with steep normal faults (i.e., fault drag) should not be assumed to be the product of frictional drag, but must be considered in the context of the mechanical stratigraphy. Instead, fault-tip folding in mechanically layered rocks produces synthetic dip (drag) early in the fault development history prior to propagation of the fault tip through the folded layer. [Copyright &y& Elsevier]

Abstract: Understanding stress states and rock mass deformation deep underground is critical to a range of endeavors including oil and gas exploration and production, geothermal reservoir characterization and management, and subsurface disposal of CO2. Geomechanical modeling can predict the onset of failure and the type and abundance of deformation features along with the orientations and magnitudes of stresses. This approach enables development of forward models that incorporate realistic mechanical stratigraphy (e.g., including competence contrasts, bed thicknesses, and bedding planes), include faults and bedding-slip surfaces as frictional sliding interfaces, reproduce the overall geometry of the fold structures of interest, and allow tracking of stress and strain through the deformation history. Use of inelastic constitutive relationships (e.g., elastic–plastic behavior) allows permanent strains to develop in response to the applied loads. This ability to capture permanent deformation is superior to linear elastic models, which are often used for numerical convenience, but are incapable of modeling permanent deformation or predicting permanent deformation processes such as faulting, fracturing, and pore collapse. Finite element modeling results compared with field examples of a natural contractional fault-related fold show that well-designed geomechanical modeling can match overall fold geometries and be applied to stress, fracture, and subseismic fault prediction in geologic structures. Geomechanical modeling of this type allows stress and strain histories to be obtained throughout the model domain. [Copyright &y& Elsevier]

Mesoscale deformation in fold-thrust belts and foreland areas occurs during the interaction of evolving stress regimes and mechanical stratigraphy, leading to a progressive deformation sequence. Tensile, hybrid, shear, compactive shear, and compactive failure may all occur in a thrust faulting stress and deformation regime, often contemporaneously in adjacent layers. Mesostructural studies, however, typically identify structural suites that appear inconsistent with a thrust faulting stress regime, such as vertical opening-mode fractures, normal faults, and strike-slip faults. We present a conceptual model that considers the progressive sequence associated with development and eventual waning of fold-thrust belt activity. The general sequence consists of five stages: (1) burial deformation in a normal faulting stress regime, (2) early horizontal contractional deformation in a strike-slip stress regime, (3) horizontal contractional deformation in a thrust faulting stress regime, (4) late horizontal contractional deformation and initial relaxation in a strike-slip stress regime, and (5) horizontal extensional deformation in a normal faulting stress regime. This sequence can be recognized in horizontal or subhorizontal strata in fold-thrust belts and foreland areas, and in more structurally complex structural positions. This model explains common suites of mesostructures, and can be used to (i) predict mesostructural suites in different structural positions, (ii) explain apparently anomalous, but inherited, structures, and (iii) infer paleostress history. F0B7 New conceptual model for progressive deformation sequence in fold-thrust belts and forelands. F0B7 Sequence explains structural suites that are inconsistent with a thrust faulting stress regime. F0B7 Tensile, hybrid, shear, compactive shear, and compactive failure expected. F0B7 Sequence includes normal-faulting, strike-slip faulting, and thrust-faulting stress regimes. F0B7 Model enables prediction of mesostructural suites in different structural positions. [ABSTRACT FROM AUTHOR]