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 This field guide describes three accessible sites along the Dandridge-Vonore fault zone in the Eastern Tennessee seismic zone. These sites reveal bedrock faulted against Quaternary river sediments, including (1) a thrust fault on the Little River near Alcoa, Tennessee; (2) a series of thrust faults exposed in a drainage ditch that thrust Conasauga Shale against Quaternary colluvium in the footwall; and (3) a normal fault at Tellico Lake near Vonore, Tennessee, with Quaternary sediments faulted against Conasauga Shale.

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.

The deformed wedge of Paleozoic sedimentary rocks in the southern Appalachian foreland fold-thrust belt is defined by the configurations of the undeformed basement surface below and the base of the Blue Ridge–Piedmont megathrust sheet above, together with the topographic free surface above the thrust belt. The base of the Blue Ridge–Piedmont sheet and undeformed basement surface have been contoured using industry, academic, and U.S. and state geological survey seismic-reflection and surface geologic data. These data reveal that the basement surface dips gently SE in the Tennessee embayment from Virginia to Georgia, and it contains several previously unrecognized normal faults and an increase in dip on the basement surface, which produces a topographic gradient. The basement surface is broken by many normal faults beneath the exposed southern Appalachian foreland fold-thrust belt in western Georgia and Alabama closer to the margin and beneath the Blue Ridge–Piedmont sheet in Georgia and the Carolinas. Our reconstructions indicate that small-displacement normal faults form beheaded basins over which thrust sheets were not deflected, whereas large-displacement normal faults (e.g., Tusquittee fault) localized regional facies changes in the early Paleozoic section and major Alleghanian (Permian) structures. These basement structures correlate with major changes in southern Appalachian foreland fold-thrust belt structural style from Virginia to Alabama. Several previously unrecognized structures along the base of the Blue Ridge–Piedmont sheet have been interpreted from our reconstructions. Large frontal duplexes composed of rifted-margin clastic and platform rocks obliquely overridden along the leading edge of the Blue Ridge–Piedmont sheet are traceable for many kilometers beneath the sheet. Several domes within the Blue Ridge–Piedmont sheet also likely formed by footwall duplexing of platform sedimentary rocks beneath, which then arched the overlying thrust sheet. The thickness and westward limit of the Blue Ridge–Piedmont sheet were estimated from the distribution of low-grade foot-wall metamorphic rocks, which were observed in reentrants in Georgia and southwestern Virginia, but are not present in simple windows in Tennessee. These indicate that the original extent of the sheet is near its present-day trace, whereas in Georgia, it may have extended some 30 km farther west. The southern Appalachian foreland fold-thrust belt consists mostly of a stack of westward-vergent, mostly thin-skinned thrusts that propagated westward into progressively younger units as the Blue Ridge–Piedmont sheet advanced westward as a rigid indenter, while a few in northeastern Tennessee and southwestern Virginia involved basement. Additional boundary conditions include temperatures <300 °C and pressures <300 MPa over most of the belt. The southern Appalachian foreland fold-thrust belt thrusts, including the Blue Ridge–Piedmont megathrust sheet, reach >350 km displacement in Tennessee and decrease both in displacement and numbers to the SW and NE. Much of the Neoproterozoic to Early Cambrian rifted-margin succession was deformed and metamorphosed during the Taconic orogeny, and it is considered part of the rigid indenter. Only the westernmost rocks of the rifted-margin succession exhibit ideal thin-skinned behavior and thus are part of the southern Appa-lachian foreland fold-thrust belt. Palinspastic reconstructions, unequal thrust displacements, and curved particle trajectories suggest that deformation of the belt did not occur by plane strain in an orogen that curves through a 30° arc from northern Georgia to SW Virginia. Despite the balance of many two-dimensional cross sections, the absence of plane strain diminishes their usefulness in quantifying particle trajectories. Coulomb behavior characterizes most individual faults, but Chapple's perfectly plastic rheology for the entire thrust belt better addresses the particle trajectory problem. Neither, however, addresses problems such as the mechanics of fault localization, out-of-sequence thrusts, duplex formation, three-dimensional transport, and other southern Appalachian foreland fold-thrust belt attributes.

Multiple levels of frontal ramps and detachment flats accommodate tectonic shortening in contrasting deformation styles at different levels in a mechanically hetero geneous stratigraphic succession in a foreland thrust belt. The late Paleozoic Appalachian thrust belt in Alabama exhibits a balance of shortening in contrasting deformation styles at different stratigraphic levels. The regional décollement is in a weak unit (Cambrian shale) near the base of the Paleozoic succession above Precambrian crystalline basement rocks. Basement faults, now beneath the décollement, controlled the sedimentary thickness of the Cambrian shale and the location of high-amplitude frontal ramps of the regional stiff layer (Cambrian-Ordovician massive carbonate); shortening in a mushwad (ductile duplex) from thick Cambrian shale is balanced by translation of the regional stiff layer at a high-amplitude frontal ramp above a basement fault. A trailing, high-amplitude, brittle duplex of the regional stiff layer has a floor on the regional décollement and a roof that is also the floor of an upper-level, lower-amplitude, brittle duplex. The roof of the upper-level brittle duplex is a diffuse ductile detachment below an upper-level mushwad, with which parts of the brittle duplex are imbricated. The basal detachment of the upper-level mushwad changes along strike into a frontal ramp at a location coincident with a sedimentary facies change in the weak shale unit that hosts the mushwad. The roof of the upper-level mushwad is a brittle massive sandstone. Shortening on the regional décollement is balanced successively upward through contrasting tectonic styles in successive mechanically contrasting stratigraphic units.

Controversy is common concerning the sequence of thrust fault imbrication on the scale of one or several quadrangles. Regional thrusting sequences in young orogenic belts are generally from the hinterland to the foreland. This is contrary to the previously proposed regional progression of thrusting for the southern Appalachian Valley and Ridge province. This paper uses cutoff-line maps to systematically examine some of the map patterns and cross-sectional interpretations used as evidence for the foreland-to-hinterland sequence of thrusting. Idealized examples of cutoff-line maps and cross-sectional patterns for both truncated structures and stair-stepped structures can be compared with observed map patterns and previously proposed cross-sectional interpretations. This provides critical evidence for interpreting the map data. Additional critical observations can be made as to the extent that faults may be folded by underlying structures, rather than truncating them. Overall, the cutoff-line approach and the folded fault approach document that the truncated folds expected in map-pattern for a foreland-to-hinterland thrust sequence do not occur in the east Tennessee area. Folded faults and westward-younging cutoff-line patterns indicate that later faults were in front of, and beneath, earlier ones in a hinterland-to-foreland sequence.