Appalachian Fold-thrust Belt Research Articles

Automated identification and mapping of geological folds in cross sections

Abstract Cross sections carry information on the spatial distribution of rock strata and the development of geological structures, and it is an important data source for three-dimensional (3D) geological modeling. However, the interpretation and mapping of geological structures in sections by means of manual interpretation are inefficient and costly, and the performance varies greatly with the experts’ ability and experience. The objective of this article is to develop an automatic recognition and mapping method for folds in cross sections. This method mainly includes identifying folds based on stratigraphic sequence characteristics (symmetrical and repetitive), classifying fold types based on geometric attributes of folds (interval scheduling, strike, and section morphology), optimizing strata based on the superposition principle and area conservation principle, and constructing the polygon features of folds. Based on experiments in the Parallel Fold Belt of Eastern Sichuan and the central Appalachian fold-thrust belt in the Appalachian Mountains, the method presented in this article can effectively be used for automatic recognition and high-quality mapping of folds in the cross sections. The method provides a good source of geological cross-sectional data for the 3D modeling of geologic bodies.

Quantifying shortening across the central Appalachian fold-thrust belt, Virginia and West Virginia, USA: Reconciling grain-, outcrop-, and map-scale shortening

AbstractWe present a kinematic model for the evolution of the central Appalachian fold-thrust belt (eastern United States) along a transect through the western flank of the Pennsylvania salient. New map and strain data are used to construct a balanced geologic cross section spanning 274 km from the western Great Valley of Virginia northwest across the Burning Spring anticline to the undeformed foreland of the Appalachian Plateau of West Virginia. Forty (40) oriented samples and measurements of >300 joint orientations were collected from the Appalachian Plateau and Valley and Ridge province for grain-scale bulk finite strain analysis and paleo-stress reconstruction, respectively. The central Appalachian fold-thrust belt is characterized by a passive-roof duplex, and as such, the total shortening accommodated by the sequence above the roof thrust must equal the shortening accommodated within duplexes. Earlier attempts at balancing geologic cross sections through the central Appalachians have relied upon unquantified layer-parallel shortening (LPS) to reconcile the discrepancy in restored line lengths of the imbricated carbonate sequence and mainly folded cover strata. Independent measurement of grain-scale bulk finite strain on 40 oriented samples obtained along the transect yield a transect-wide average of 10% LPS with province-wide mean values of 12% and 9% LPS for the Appalachian Plateau and Valley and Ridge, respectively. These values are used to evaluate a balanced cross section, which shows a total shortening of 56 km (18%). Measured magnitudes of LPS are highly variable, as high as 17% in the Valley and Ridge and 23% on the Appalachian Plateau. In the Valley and Ridge province, the structures that accommodate shortening vary through the stratigraphic package. In the lower Paleozoic carbonate sequences, shortening is accommodated by fault repetition (duplexing) of stratigraphic layers. In the interval between the duplex (which repeats Cambrian through Upper Ordovician strata) and Middle Devonian and younger (Permian) strata that shortened through folding and LPS, there is a zone that is both folded and faulted. Across the Appalachian Plateau, slip is transferred from the Valley and Ridge passive-roof duplex to the Appalachian Plateau along the Wills Mountain thrust. This shortening is accommodated through faulting of Upper Ordovician to Lower Devonian strata and LPS and folding within the overlying Middle Devonian through Permian rocks. The significant difference between LPS strain (10%–12%) and cross section shortening estimates (18% shortening) highlights that shortening from major subsurface faults within the central Appalachians of West Virginia is not easily linked to shortening in surface folds. Depending on length scale over which the variability in LPS can be applied, LPS can accommodate 50% to 90% of the observed shortening; other mechanisms, such as outcrop-scale shortening, are required to balance the proposed model.

Unraveling the central Appalachian fold-thrust belt, Pennsylvania: The power of sequentially restored balanced cross sections for a blind fold-thrust belt

We present a kinematic model for the sequential development of the Appalachian fold-thrust belt (eastern U.S.) across a classic transect through the Pennsylvania salient. New map and strain data are used to create a balanced geologic cross section from the southern edge of the Valley and Ridge Province to the northern Appalachian Plateau. This region of the central Appalachian fold-thrust belt is an ideal location to illustrate the incorporation of strain data in balanced cross sections, because it cannot be balanced without quantifying grain-scale strain. We use a sequentially restored, balanced cross section to show how layer-parallel shortening (LPS) is distributed above and ahead of thrust and fold shortening and constrain the geometric and kinematic evolution of a passive roof duplex. By combining line length and area balancing of a kinematically viable cross section with LPS estimates in both the Valley and Ridge Province (20%) and Appalachian Plateau (13%), we document the total magnitude of shortening in both the folded cover sequence and the duplexed lower layer of the fold-thrust belt. Restoration of the cross section indicates a total of 77 km (22%) of shortening between the southern margin of the Valley and Ridge Province in central Pennsylvania and a pin line immediately north of the northern limit of documented LPS in the foreland. The 24 km (13%) of LPS on the Appalachian Plateau is interpreted as being above the Salina (salt) decollement. This magnitude of shortening is 14 km greater than the amount of displacement on the Nittany Anticlinorium, the northernmost structure of the fold-thrust belt that cuts upsection from the Cambrian Waynesboro Formation to the Silurian Salina decollement. Because the fault that cores the Nittany Anticlinorium can only facilitate 10 km of shortening on the plateau, an early history of Appalachian Plateau LPS in Silurian and younger rocks is required to balance the section. We propose that the additional 14 km of LPS on the plateau occurred early in the deformation history and was kinematically linked to two fault-bend folds that have a lower decollement in the Cambrian Waynesboro Formation and an upper, subhorizontal detachment in the Silurian Wills Creek Formation (in the Valley and Ridge) and the Salina Group on the Appalachian Plateau. This upper detachment feeds displacement from these early horses in the duplex system onto the Appalachian Plateau and is expressed there as LPS shortening. This early shortening is followed by the development of in-sequence horses that repeat the mainly thrust-faulted Cambrian–Ordovician sequence using both the main decollement in the Cambrian Waynesboro and the Ordovician Reedsville Formations as an upper detachment horizon. In the south, shortening in the Late Ordovician through Devonian layers is accommodated by both LPS and forced folding of the overlying folded cover sequence. We propose that the Reedsville Formation becomes weaker to the north, facilitating shorter wavelength detachment folds. The development of gentle open folds on the Appalachian Plateau, as well as the last 10 km of LPS on the plateau, is linked to the most forelandward horse in the duplex. This horse forms the broad Nittany Anticlinorium, the northern boundary of the Valley and Ridge.

Remagnetization in the Tennessee salient, Southern Appalachians, USA: Constraints on the timing of deformation

The Appalachian fold–thrust belt is characterized by a sinuous trace in map-view, creating a series of salients and recesses. The kinematic evolution of these arcuate features remains a controversial topic in orogenesis. Primary magnetizations from clastic red beds in the Pennsylvania salient show Pennsylvanian rotations that account for about half of the curvature, while Kiaman-aged (Permian) remagnetizations display no relative rotation between the limbs. The more southern Tennessee salient shows a maximum change in regional strike from ~ 65° in Virginia to ~ 10° in northern Georgia. Paleomagnetic results from thirty-two sites in the Middle to Upper Ordovician Chickamauga Group limestones and twenty sites from the Middle Cambrian Rome Formation red beds were analyzed to constrain the relative age of magnetization as well as the nature of curvature in the Tennessee salient. Results from three sites of the Silurian Red Mountain Formation were added to an existing dataset in order to determine whether the southern limb had rotated. After thermal demagnetization, all three sample suites display a down and southeasterly direction, albeit carried by different magnetic minerals. The syn-tilting direction of the Chickamauga limestones lies on the Pennsylvanian segment of the North American apparent polar wander path (APWP), indicating that deformation was about half completed by the Late Pennsylvanian. The Rome and Red Mountain Formations were also remagnetized during the Pennsylvanian. Both the Chickamauga limestones and Rome red beds fail to show a correlation between strike and declination along the salient, suggesting either that the salient was a primary, non-rotational feature or that secondary curvature occurred prior to remagnetization, as it did in Pennsylvania. Moreover, remagnetized directions from the Red Mountain sites show no statistical difference between the southern limb of the salient and the more northeasterly trending portion of the fold–thrust belt in Alabama. Thus, all of the studied units in the Tennessee salient are remagnetized and show no evidence for rotation. This confirms that remagnetization was widespread in the southern Appalachians and that any potential orogenic rotation must have occurred prior to the Late Pennsylvanian.

Comparison of Fry strain ellipse and AMS ellipsoid trends to tectonic fabric trends in very low-strain sandstone of the Appalachian fold–thrust belt

In moderately to highly strained sandstones, both the long axis of the bedding-parallel finite-strain ellipse, as calculated by the normalized Fry method, and the projection of the long axis of the AMS ellipsoid on the plane of bedding, align well with local “structural grain” (trends of cleavage, folds, and faults). This relationship implies that results of both 2D Fry and AMS analyses represent the local layer-parallel tectonic strain component. Do both methods provide comparable results for very low-strain sandstone (e.g., <5%)? To address this question, Fry and AMS analyses were conducted in very low-strain sandstone from two localities in the Appalachian foreland fold–thrust belt: near Rosendale in New York and the Lackawanna synclinorium of Pennsylvania. We compared the map projections of both bedding-parallel Fry ellipses and AMS ellipsoids to the local structural grain. In both study areas, projections of the long axis of Fry strain ellipses do not cluster in a direction parallel to structural grain, whereas the projection of the long axes of AMS ellipsoids do cluster closely to structural grain. This observation implies that in very low-strain sandstone, AMS analysis provides a more sensitive “quick” indicator of tectonic fabric than does normalized Fry analysis.

Structural controls on fractured coal reservoirs in the southern Appalachian Black Warrior foreland basin

Coal is a nearly impermeable rock type for which the production of fluids requires the presence of open fractures. Basin-wide controls on the fractured coal reservoirs of the Black Warrior foreland basin are demonstrated by the variability of maximum production rates from coalbed methane wells. Reservoir behavior depends on distance from the thrust front. Far from the thrust front, normal faults are barriers to fluid migration and compartmentalize the reservoirs. Close to the thrust front, rates are enhanced along some normal faults, and a new trend is developed. The two trends have the geometry of conjugate strike-slip faults with the same σ 1 direction as the Appalachian fold-thrust belt and are inferred to be the result of late pure-shear deformation of the foreland. Face cleat causes significant permeability anisotropy in some shallow coal seams but does not produce a map-scale production trend.

Nd isotopic constraints on sediment sources of the Ouachita-Marathon fold belt

Nd isotopes for the overthrust deep-marine Ouachita-Marathon sedimentary assemblage of Arkansas-Oklahoma and west Texas, and associated Paleozoic shelf and foreland deposits, resolve into three distinct populations: (1) Lower to Middle Ordovician, ϵ Nd = −13 to −16 (average T DM = 2.0 Ga); (2) Upper Ordovician to Pennsylvanian, ϵ Nd = −6 to −10 (average T DM = 1.6 Ga); and (3) Mississippian tuffs, ϵ Nd = −1 to −3 (average T DM = 1.1 Ga). A rapid shift in ϵ Nd from −15 (passive margin shales) to −7 (orogenic turbidites) in the Ouachita assemblage at ca. 450 Ma implies termination of craton-dominated sources and the emergence of the Appalachian orogen as the primary source of sediment for sea floor lying south of North America. This connection is reinforced by Nd isotopes in Ordovician-Silurian turbidites from both the Ouachita assemblage and the southern Appalachian Sevier-Martinsburg (Taconic) foredeep, which are identical (ϵ Nd = −7 to −9). The post–450 Ma Ouachita assemblage falls along a single Nd isotopic trend that, significantly, is not deflected by onset of Carboniferous flysch (ϵ Nd = −7 to −10) sedimentation nor by associated regional volcanism. The less negative ϵ Nd (−2) of Mississippian ash-flow tuffs that erupted from arc(s) to the south probably resulted from isotopic mixing of old (Precambrian) crust with young, mantle-derived components within a continental margin arc. There is little isotopic, trace element, or petrographic evidence for any significant volcaniclastic detritus in the Carboniferous turbidites, indicating that volcanic arc sources were minimal. Nd isotopes in fluvio-deltaic strata of the Ouachita-Appalachian foreland and continental interior, that is, Arkoma, Illinois, and Black Warrior basins (ϵ Nd = −7 to −10), imply that continental margin pathways and interior basins received the same detritus as the Ouachita trough by Pennsylvanian time. These data are consistent with a composite Carboniferous Ouachita submarine fan complex built down the axis of a remnant ocean basin from varied mature/immature delivery systems tapping dominantly Appalachian fold-thrust belt sources to the east (Graham et al., 1975). Carboniferous turbidites from the Marathon fold belt (west Texas), which are isotopically similar (ϵ Nd = −8 to −11) to Ouachita turbidites, may have been ultimately derived from similar sources; however, they probably do not represent merely distal turbidites of a Ouachita fan complex. It is suggested that dominantly Appalachian-derived detritus, augmented by uplifted plutonic and fold-thrust belt sources south of the Marathon basin, was swept up into subduction complexes on the north side of the approaching arc and recycled along the collision zone.

Restoration of cross-sections through unfaulted, variably strained strata

Data on the dip of bedding and the state of strain at specific locations within a cross-section constrain the restoration of unfaulted, two-dimensional, variably strained strata. There are two major categories for restoration of such strata. The first is one-step restoration, which involves retro-deforming the strain ellipses into their corresponding unit circles while simultaneously removing dips from associated beds. This procedure is appropriate if strain indicators record the transformation of the strata from the undeformed state to present deformation. The second category pertains to situations in which strain indicators record the transformation of the present deformed state from an intermediate, deformed state. The second category requires multiple step restoration. For the one-step restoration procedure presented in this paper, the first task is to determine the transformation constants that describe development of observed strain and bedding for each location. The next task is to determine the retro-deformation constants which relate points in their present deformed state to their locations in the undeformed state. Retro-deformation constants are related to the spatial derivatives of the Cartesian retro-deformation displacements functions, U∗(x,y) and V∗(x,y) (for movements in the x and y directions, respectively). The spatial derivatives of these functions at locations of observed strain and the displacements of selected points about these sites are the basis for constructing continuous, retro-deformation displacements functions for the deformed section as a whole. The retro-deformation displacements functions are then invoked to restore the cross-section. This procedure is illustrated with an example cross-section from the Appalachian Fold Belt, and the example shows that the section may be satisfactorily restored in this way. It is just as likely, however, that the strain indicators for the example section are a measure of the transformation of the cross-section from a prior, deformed state to the present deformation. Accordingly, multiple-step restoration is required, first to an intermediate, less deformed state and then to the undeformed state. Two sets of retro-deformation displacement functions are then needed, i.e. one set for transformation to an intermediate state followed by a second to the undeformed state. This procedure was done for the Appalachian fold-thrust belt example, restoring first the present deformed section to an intermediate state derived from the regional fold history. In turn, intermediate state points were transformed to an undeformed state assuming knowledge of an undeformed stratal thickness and assuming that cross-sectional area has been preserved. This procedure is technically different to that used in the one-step procedure, but conceptually similar. The objective, once again, is to discover appropriate retro-deformation displacements functions.

Microstructures, deformation mechanisms and strain patterns in a vertical profile, inner appalachian fold-thrust belt, Alabama

A core from the American Anniston No. 1 well, drilled in the Pell City thrust sheet, Calhoun County, Alabama, provides an excellent scientific opportunity to study in a vertical profile the deformation mechanisms and strain patterns associated with large-scale structures. The core contains stratigraphic units from Cambrian to Mississippian in age and two major thrust faults. A detailed structural analysis revealed two deformation episodes, a pre-orogenic deformation and Alleghanian orogenic deformation. In the pre-orogenic deformation, synsedimentary folds and growth faults are the characteristic early structures, and normal faults formed in lithifield rocks later. During Alleghanian deformation, earlier fractures are overprinted by later S 1 penetrative structures. In limestone, shale and siltstone within thrust sheets, S 1 is solution cleavage. Dolomites in the thrust sheets were deformed by fracturing, and no penetrative cleavages formed. In major fault zones, S 1 mylonitic foliation formed in limestone, shale and siltstone. Fault-related dolomites were deformed cataclastically and no S surfaces formed. Strain magnitude increases towards major thrust faults in both the hanging walls and footwalls. For the typical fault configuration in the core with limestone-dolomite in the hanging wall and shale-siltstone in the footwall, strain is mainly caused by pressure solution and cataclasis in the hanging wall and by plastic deformation in the footwall.

Low-temperature deformation of sandstone, southern Appalachian fold-thrust belt

Sandstone samples of Lower Pennsylvanian and Upper Mississippian units from a transect across the external southern Appalachian fold-thrust belt show variations in the relative abundances of the thin section-scale deformation mechanisms, including grain-boundary recrystallization, grain-boundary pressure solution, microfracturing, transgranular faulting, and cataclasis. The transect extends from the Sequatchie anticline in the northwest, crosses the Birmingham anticlinorium and the imbricate thrust belt, and ends in the Coosa deformed belt in the southeast. Thermal maturation data correlate to vitrinite reflectances of 1.0-2.0, the catagenic metamorphic regime. The deformation-mechanism abundances correlate with both position along the transect and proximity to thrust faults. Within a thrust sheet, 2%-11% of the grain boundaries are affected by recrystallization and 25%-29% by pressure solution in the samples from the external (northwestern) part of the transect, and 10%-14% by recrystallization and 47%-66% by pressure solution in the samples from the internal (southeastern) part of the transect. Regardless of location on the transect, fault-related damage zones are characterized by higher percentages (16%-48%) of grain-boundary recrystallization and 1%-19% transgranular deformation. The deformation-mechanism abundances thus prove to be sensitive indicators of regional structural position and deformation history.

Structure of the Kingston orocline in the Appalachian fold-thrust belt, New York

The trend of the post-Taconic Appalachian fold-thrust belt changes at the city of Kingston in the New York recess. To the southwest of Kingston, fold-thrust structures trend northeast-southwest, whereas to the north of Kingston, fold-thrust structures trend due north to north-northeast. This paper discusses the structural geometry and origin of this change in trend. Detailed mapping (1:9,600) suggests that the fold-thrust belt in the vicinity of Kingston can be divided into three structural domains (northern, central, and southern). In the northern domain, megascopic folds are open, significant faults do not intersect the present-day ground surface, Ordovician strata below the Taconic unconformity are not extensively thrust over post-unconformity strata, and shortening is less than 5%. In the southern domain, shortening across the fold-thrust belt is 30%, numerous faults intersect the ground surface, and Ordovician strata are significantly thrust over postunconformity strata. In the central domain, there are numerous accommodation faults, there are locally two noncoaxial-spaced cleavages, and strike-slip lineations overprint dip-slip lineations on many slip surfaces. To explain the contrasts between domains, we suggest that the bend at Kingston is an intersection orocline created when northeast-southwest-trending structures of the Kittatiny-Shawangunk segment of the Appalachian fold-thrust belt overprinted and reoriented older north-south-trending structures of the Hudson Valley fold-thrust belt. Calcite twin strain-gauge measurements indicate that there was less than 2% penetrative tangential stretching accompanying the development of this orocline.

Acadian dextral transpression and synorogenic sedimentary successions in the Appalachians

The successive Seboomook-Littleton (northern Appalachians) and Catskill-Pocono (central Appalachians) clastic wedges suggest oblique convergence and southwestward migration of Acadian orogeny beginning in Early Devonian and continuing into Early Mississippian. Wrench-fault movement in Maritime Canada coincided with deposition of all but the earliest components of the Catskill-Pocono clastic wedge and continued into the Pennsylvanian. Contrasts between a thin, Lower to Middle Devonian shallow-shelf facies in the Alabama Appalachian fold-thrust belt and a time-equivalent, thick, shallowing-upward sedimentary to volcanic succession in the adjacent Talladega slate belt are interpreted to reflect a wrench-fault basin. A wrench-fault setting for Devonian rocks in Alabama integrated with manifestations of oblique convergence during the Acadian orogeny in the central and northern Appalachians can be accommodated in dextral transpression along the entire length of the Acadian Appalachian orogen.

Paleozoic Synsedimentary Structural Deformation in Appalachian Fold-Thrust Belt of Alabama and West Georgia: ABSTRACT

The Appalachian fold-thrust belt consists of allochthonous Paleozoic sedimentary cover rocks transported by late Paleozoic Alleghenian thrust faults. Local variations in stratigraphy in Alabama and west Georgia indicate episodic, synsedimentary, structural movement during much of the Paleozoic. To provide a base map for interpreting prethrust stratigraphy, a new palinspastic map was constructed from a series of balanced and restored cross sections. Local thickness variations suggest that synsedimentary deformation was associated with high-angle basement faults, and some basement faults are confirmed by seismic data. Two major phases of synsedimentary basement fault movement account for most of the local thickness variation: (1) Early and Middle Cambrian, and (2) Early Mississippian to Early Pennsylvanian. The Upper Cambrian-Lower Ordovician Knox Group is a regionally pervasive carbonate unit that reflects tectonic quiescence. The interval from Middle Ordovician to Lower Mississippian is characterized by thin, laterally variable, shallow marine units. The section includes several unconformities that locally coalesce, thus completely removing this succession. Different expressions of facies and unconformities locally suggest both synsedimentary and postdepositional structural basins. Minor reactivation of basement faults evidently occurred during the middle Paleozoic. Relative locations of early synsedimentary structures and later Alleghenian thin-skinned structures suggest both strike-parallel and cross-strike basement faults more » that locally correspond to frontal and lateral decollement ramps, respectively. Thus, Paleozoic basement-related, thick-skinned, synsedimentary structures probably controlled the location and geometry of some decollement ramps generated during Alleghenian thrusting. « less