Kinematic analysis and mechanical modeling of roof sequence response to blind thrusting

A major unresolved problem with blind duplex development is the mechanisms by which the roof sequence rocks accommodate duplex emplacement without being part of the fault system. This problem stems from a lack of detailed kinematic data from rocks adjacent to blind duplexes and the very limited number of mechanical models for evaluating the relative importance of possible kinematic responses as a function of controlling mechanical parameters. Forethrusting, backthrusting, and local compensation are alternative kinematic responses that have been proposed for the roof sequence behavior. This dissertation focuses on the behavior of the Paleozoic roof sequence in the Appalachian Plateau of West Virginia during emplacement of the Wills Mountain duplex. Geologic analyses and finite element models provide a kinematic description of roof sequence behavior and the mechanical controls on deformation processes. The Wills Mountain duplex accommodated 17.5 km of Alleghanian blind thrusting by flat-on-flat duplication of the thick Cambre-Ordovician carbonate section. Kinematic analysis of the Ordovician through Mississippian stratigraphy between Wills Mountain and the Elkins Valley anticline reveals that the roof sequence records at least two-thirds of this shortening. Mesoscale and smaller processes including grain-to-grain pressure solution, twinning, and cleavage formation account for over 75% of the shortening with the remainder accommodated by macroscale folding and faulting. Forethrusting was the dominant kinematic response to emplacement of the blind duplex based on the magnitude of roof sequence shortening. The shortening imbalance between the Cambre-Ordovician section and the younger roof sequence rocks indicates that additional forethrusting occurred further out in the foreland. Finite element modeling of roof sequence deformation during blind duplex emplacement reveals that the mechanical evolution is controlled by the frictional resistance on sliding interfaces, and to a lesser degree, the amount of thrust displacement. Roof sequence deformation is accomplished by a combination of rigid-body translation and internal distortion where the relative amount of each mechanism is controlled largely by the frictional resistance of the sliding interfaces. A combination of forethrusting and backthrusting of the roof sequence in the models occurs to accommodate the imposed displacement. Together, geologic analyses and finite element models suggest that the forethrusting-dominated response observed in the study area reflects the availability of weak decollements in the foreland of the Wills Mountain duplex. Results of a model with two sliding interfaces suggests that the Martinsburg decollement in the study area was much weaker than the Millboro decollement. Models also provide insights into the mechanics of detachment folding and the ductile-bead that precedes a propagating fault. The stress-strain relationship derived from Greenbrier twinning indicates that: (1) the yield stress assigned to the upper part of the roof sequence in the finite element models was too low; and (2) a small amount of plastic strain hardening would be appropriate. Future work can incorporate these changes and also address issues such as pressure-dependent yield, material anisotropies, and deformation with volume change.