The fold-thrust belt stress cycle: Superposition of normal, strike-slip, and thrust faulting deformation regimes

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.