Data from thousands of coalbed methane wells, conventional oil and gas wells, and five regional seismic-reflection profiles show evidence of relationships among multiple extensional events and Appalachian thrusting in the area of the Alabama Promontory and Black Warrior Basin. The oldest extensional event is of late Precambrian to early Middle Cambrian age, associated with Iapetan rifting. Along the southeastern margin of the promontory, in the northeastern part of the Birmingham graben system, a fill sequence older than the Rome Formation (Early Cambrian) is inferred. Normal faults along both sides of the promontory were active from Early Cambrian to early Middle Cambrian, as indicated by expanded hangingwall sections of the Rome and Conasauga Formations. In the Black Warrior Basin, some basement-involved normal faults were active during deposition of the Ketona and Knox carbonates (Late Cambrian–Early Ordovician). Middle Cambrian to Early Ordovician extension coincided with the inception and opening of the Rheic Ocean. Small amounts of growth occurred on some normal faults and on folds at the leading edge of the Appalachians during deposition of the Pottsville Formation (early Pennsylvanian, Morrowan). Major thin-skinned and basement-involved normal faulting occurred in the Black Warrior Basin after deposition of the preserved Pottsville section, probably during Atokan time. The extensional thin-skinned detachments are in or at the base of the Pottsville Formation and the top of the Conasauga Formation. Major Appalachian thrusting occurred after the main episode of normal faulting, perhaps during the late Pennsylvanian.

Abstract Eastern North America holds clear records of two Wilson cycles and hints of two earlier cycles, through which tectonic inheritance is evident at multiple scales. Large-scale transform offsets of rifted margins indicate inheritance through multiple cycles; transform-parallel intracratonic fault systems suggest a transform-parallel fabric in the lithosphere. Rift segments of the continental margins did not inherit the locations of earlier rifts; synrift intracratonic fault systems follow earlier contractional fabrics of supercontinent assembly. Large-scale curves of the Appalachian–Ouachita orogenic belt (closing of the Iapetus Ocean) mimic the shape of the Iapetan rifted margin of Laurentia. Basins along the Iapetan rifted margin reflect inheritance from transform faults in the greater magnitudes of early post-rift thermal subsidence and later synorogenic tectonic loading and flexural subsidence. Older synrift basement faults buttressed the frontal ramps of Appalachian–Ouachita thin-skinned thrust faults. Basement fault blocks and associated synrift stratigraphic variations in the weak layers that host the regional décollement localized transverse alignments of lateral ramps, as well as tectonic thickening of a mud-dominated graben-fill succession in a ductile duplex (mushwad). The many examples of tectonic inheritance attest to the linkages between processes of successive opening and closing of oceans, as well as the break-up and assembly of supercontinents, through successive Wilson cycles.

ABSTRACT In a well-defined subrecess in the Appalachian thrust belt in northwestern Georgia, two distinct fold trains intersect at ~50° in the down-plunge depression of the Floyd synclinorium. A mushwad (ductile duplex) of tectonically thickened weak-layer rocks (primarily the shale-dominated Cambrian Conasauga Formation) filled the space beneath folds and faults of the overlying Cambrian–Ordovician regional stiff layer (mushwad roof). Measurements of the mushwad thickness from balanced cross sections provide the basis for three-dimensional (3-D) models. Tectonically thickened weak-layer shales in a model using a simple line-length balance of the stiff layer have a volume of ~64% of the volume in the deformed-state model, indicating that this balanced reconstruction is not appropriate. Previous work demonstrated deposition of a thick mud-dominated succession in a basement graben to balance the volume. A 3-D model incorporating a thick Conasauga Formation shale succession deposited in a basement graben yields good correspondence to the deformed-state mushwad volume. That model requires vertical separation on the graben boundary faults greater than the present small-magnitude separation; unconformable truncation of the upper part of the Cambrian–Ordovician carbonate succession documents Ordovician inversion of the graben boundary faults. In the 3-D models, the distribution of thickness in the deformed state suggests movement of weak-layer shale out of the planes of cross sections and up plunge away from the structural depression of the Floyd synclinorium. Out-of-plane tectonic translation is consistent with a relatively uniform depositional thickness of ~800 m, which allows calculation of the magnitude of vertical separation on basement faults during Conasauga Formation deposition.

Abstract In a well-defined small-scale recess in the Appalachian thrust belt in northwestern Georgia (USA), two distinct regional strike directions intersect at c. 50°. Fault intersections and interference folds enable tracing of both structural strikes. Around the recess, tectonically thickened weak stratigraphic layers – shales of the Cambrian Conasauga Formation – accommodated ductile deformation associated with the folding and faulting of the overlying Cambrian–Ordovician regional competent layer. The structures in the competent layer are analogous to those over ductile duplexes documented along strike to the SW in Alabama, where gas production has been established from the deformed shale. The analogy with structures in Alabama suggests a ductile duplex and natural gas potential within the recess in Georgia. The tectonic thickening of the weak-layer shales is evident in palinspastically restored cross sections, which demonstrate a nearly 100% increase in volume over the restored state cross sections. The dominant cause of the surplus shale volume is likely pre-thrusting deposition of thick shale in a basement graben that was later inverted. The volume balance of the ductile duplex is critical for palinspastic reconstruction of the recess, and for the kinematic history and mechanics of the ductile duplex.