Paleozoic evolution of the Valley and Ridge thrust belt in Alabama

Subsidence curves, compiled for each structural/stratigraphic belt of the foreland and palinspastically restored to their prethrust positions, depict the changing basement configuration through time. Changes in basement configuration indicate four historical phases: (1) a miogeoclinal phase (Early Cambrian-Early Ordovician) characterized by linear subsidence and development of a southeast-facing carbonate shelf, complicated by abortive local rifting (Middle Cambrian); (2) subsidence of the southeastern part of the shelf associated with development of a Middle Ordovician foredeep; (3) an epeiric phase (Silurian-Early Mississippian) characterized by minimal subsidence and differential uplift of the eastern Valley and Ridge; and (4) extreme subsidence of the entire basin to form an Early Mississippian-Early Pennsylvanian foredeep. Simultaneous initiation of subsidence throughout the latter foredeep, and differential uplift during the epeiric phase, indicate that the lithosphere behaved as an elastic material over short periods of stress application (< 14 my), and viscoelastically over longer periods (> 30 my).

Gross structure of the thrust belt is atypical, in that the dominant features are overthrusts discordant to structure in both the upper and lower plates. Mesoscopic fabric data and map relationships indicate that these faults represent the terminal phase of a thrust-fold-thrust kinematic cycle which migrated toward the foreland. Recognized sequences are of two types: (1) development of northwest vergent faults, followed by decapitation by northwest vergent faults soling rearward of the first faults; and (2) development of southeast vergent listric thrusts, later decapitated by northwest vergent thrusts. In both cases major transport is accomplished by the later, discordant, thrusts. Local thinning of stratigraphic units in the Early Pennsylvanian foredeep fill indicates that thrusting was penecontemporaneous with sedimentation. Sequential cross-sections, based on regional and local kinematic relationships, illustrate the sequence of Late Paleozoic thrusting.