Robert D. Hatcher

The Appalachian-Ouachita orogen in the United States

Includes 14 chapters on the Appalachian orogen, 15 of the Ouachita orogen, and a chapter on the connection between them beneath the eastern Gulf Coastal Plain. The Appalachian chapters synthesize the geologic development of the orogen by tectonostratigraphic intervals (pre-orogenic, Taconic, Acadian, Alleghanian, and post-Alleghanian), and also treat Paleozoic paleontologic control, regional geophysics, thermal history of the crystalline terranes, parts of the orogen buried beneath the Atlantic and eastern Gulf coastal plains, regional geomorphology, mineral and energy resources; an integration chapter also is included. The Ouachita chapters cover physical stratigraphy and biostratigraphy of the Paleozoic rocks, structural geology, a synthesis of the subsurface geology beneath the western Gulf Coastal Plain, a review of the mineral and energy resources, regional geophysics, and a tectonic synthesis. Twelve excellent plates provide four-color geologic maps, structural cross sections, tectonic syntheses, and geophysical maps; a black-and-white synthesis of Appalachian mineral deposits, and a reflection seismic cross section.

Developmental Model for the Southern Appalachians

A developmental model is proposed for the southern Appalachians. This model integrates current ideas on structure, sedimentation, metamorphism, and igneous activity with the concept of plate tectonics. Progressive metamorphism culminated about 1,000 m.y. ago (middle Precambrian) and again about 400 m.y. ago (Silurian-Devonian). Later, localized retrogressive metamorphism affected the rocks of the Blue Ridge and Piedmont. Igneous activity occurred during the middle Precambrian (1,000 m.y. ago), prior to, during, and after Paleozoic regional metamorphism, and during the Mesozoic. The Valley and Ridge and Cumberland Plateau structures are treated as areas deformed by thin-skinned tectonics generated during the late Paleozoic, as is the Blue Ridge thrust sheet. The Blue Ridge anticlinorium, Murphy syncline, Chauga belt, mobilized Inner Piedmont, Kings Mountain belt, Charlotte and Carolina slate belts developed during the deformational event accompanying early-mid-Paleozoic regional metamorphism. The Blue Ridge anticlinorium, mobilized Inner Piedmont and Charlotte belts are interpreted as anticlinoria composed of middle Precambrian basement and later Precambrian metasedimentary and metavolcanic rocks mobilized to a greater or lesser degree. The Murphy, Chauga, Kings Mountain, and Carolina slate belts are inferred to be synclinoria composed of younger (late Precambrian to early-mid-Paleozoic) metasedimentary and metavolcanic rocks. The development of the sedimentary, deformational, metamorphic, and intrusive history of the southern Appalachians may be directly related to the Paleozoic history of movement of the North American and African continents as portions of major Paleozoic lithospheric plates. This developmental scheme is divisible into several phases: (1) An early (late Precambrian to mid-Ordovician) phase of continental margin sedimentation, igneous activity, and initial compression occurred. (2) This was followed by an intermediate (late Precambrian to late Devonian) phase of compression producing isoclinal folding, regional metamorphism, and intrusive activity with deposits from a rising tectonic source land. These phenomena are the direct result of westward underflow of a proto-Atlantic plate beneath an eastward-moving proto-North American plate with concomitant development of a subduction zone along the continental margin. (3) Next a later (mid-late Paleozoic) phase of compression and igneous activity accompanied by continued deposition from the rising mountain system occurred. The Brevard Zone, Towaliga, Goat Rock, and Gold Hill faults developed early in phase 3 and experienced renewed movement during formation of the thrusts and folds in the Valley and Ridge, Cumberland Plateau, and the late structures in the Blue Ridge. Compressional stresses that formed these structures were generated during the collision and suturing of Africa with southeastern North America. (4) Finally, there was a tensional phase (Triassic-Jurassic) accompanied by normal faulting, igneous activity, and deposition related to the decoupling of Africa and North America and the formation of the present Atlantic Ocean and continental margin.

Eastern Piedmont fault system: Speculations on its extent

Geologic mapping, interpretation, and field checking of recent aeromagnetic data suggest the existence of a closely associated series of faults and splays extending from Alabama to Virginia, herein termed the Eastern Piedmont fault system. Characteristic magnetic anomalies were found to be associated with known faults and were used to trace them through covered intervals. The fault system extends northeastward from the Goat Rock fault of Alabama and west-central Georgia, crossing the lower Piedmont of South Carolina, passes beneath a segment of the Coastal Plains in the Carolinas, and then flanks the Raleigh belt in North Carolina and continues into Virginia. From east-central Georgia to Virginia, cataclastic rocks along the faults of the system are bounded to the northwest and southeast by rocks of the Carolina slate belt, forming perhaps the most extensive fault system in eastern North America. Its movement history is similar to that of the Brevard fault: an early ductile mylonitic phase, followed by periods of brittle deformation. We interpret the fault system to have been initiated during collapse of the late Precambrian–early Paleozoic Carolina slate belt island arc. The Paleozoic continental suture probably lies farther east, buried beneath the Coastal Plain.

Whole-rock Pb and Sm-Nd isotopic constraints on the growth of southeastern Laurentia during Grenvillian orogenesis

The conventional view that the basement of the southern and central Appalachians represents juvenile Mesoproterozoic crust, the final stage of growth of Laurentia prior to Grenville collision, has recently been challenged. New whole-rock Pb and Sm‑Nd isotopic data are presented from Mesoproterozoic basement in the southern and central Appalachians and the Granite-Rhyolite province, as well as one new U-Pb zircon age from the Granite-Rhyolite province. These data, combined with existing data from Mesoproterozoic terranes throughout southeastern Laurentia, further substantiate recent suggestions that the southern and central Appalachian basement is exotic with respect to Laurentia. Sm-Nd isotopic compositions of most rocks from the southern and central Appalachian basement are consistent with progressive growth through reworking of the adjacent Granite-Rhyolite province. However, Pb isotopic data, including new analyses from important regions not sampled in previous studies, do not correspond with Pb isotopic compositions of any adjacent crust. The most distinct ages and isotopic compositions in the southern and central Appalachian basement come from the Roan Mountain area, eastern Tennessee–western North Carolina. The data set indicates U-Pb zircon ages up to 1.8 Ga for igneous rocks, inherited and detrital zircon ages >2.0 Ga, Sm-Nd depleted mantle model (T DM ) ages >2.0 Ga, and the most elevated 207 Pb/ 204 Pb observed in southeastern Laurentia. The combined U-Pb geochronologic and Sm-Nd and Pb isotopic data preclude derivation of southern and central Appalachian basement from any nearby crustal material and demonstrate that Grenville age crust in southeastern Laurentia is exotic and probably was transferred during collision and assembly of Rodinia. These new data better define the boundary between the exotic southern and central Appalachian basement and adjacent Laurentian Granite-Rhyolite province.

Thrusts and nappes in the North American Appalachian Orogen

Summary The Appalachian Orogen in North America was subjected to three major deformational-thermal events: the Taconic (Ordovician-Silurian), Acadian (late Devonian), and Alleghanian (Permian). Each event involved large-scale horizontal transport of thrust-nappes ranging from tens to hundreds of kilometres in different parts of the orogen. Transport was dominantly westward although major Acadian-generated eastward transport occurred in southern New England. There is a direct relationship between chronological proximity to a thermal peak and numbers of thrusts. Thrusts were produced during the Taconic and Acadian events which pre- and post-dated the thermal peak, as well as being synchronous with it. Transport of the Bay of Islands ophiolites and other large masses along the western margin of the orogen occurred before Taconic metamorphism, but probably only immediately before. Many large thrusts of the Appalachians were active during two or even three of the deformational-thermal events, and more than once within a single event. This is particularly true for the Blue-Ridge-Inner Piedmont mega-nappe, which involves at least 225 km of horizontal transport. Compressional tectonics were probably the dominant process responsible for all thrusts in the Appalachians, except the Taconic klippes. The Alleghanian décollement Valley and Ridge thrusts are overridden by crystalline Alleghanian and older thrusts of the Blue Ridge in the southern Appalachians. The same mechanism must apply to the central Appalachians. Thrusting and formation of crystalline thrust-nappes in the Appalachians and other mountain chains may be an adiabatic process which functions to dissipate much of the thermal energy produced during subduction (both A and B types) and collision events.

Controls on hinge‐parallel extension fracturing in single‐layer tangential‐longitudinal strain folds

The stress history results from a published viscous layer folding solution are used as the basis for a fracture mechanics analysis of the factors that control hinge-parallel extension fracturing in tangential-longitudinal strain folds. The analysis incorporates published results for the change in sedimentary rock mode I fracture toughness at increasing confining stress to examine the relationship between regional strain rate, depth of burial, pore fluid pressure, initial crack size, layer viscosity, and the amount of fold shortening required for the propagation of a bed-perpendicular, hinge-parallel extension fracture. Tangential-longitudinal strain folding of layers can occur at all scales in a foreland thrust system and is the result of the buckling and bending of stratigraphic units during the development of decollement, fault bend, and fault propagation folds. Hinge-parallel extension fractures oriented perpendicular to bedding are a common fracture set observed in tangential-longitudinal strain folds. The fractures propagate as a result of local tensile stresses that develop by the stretching of layers in the outer arc of fold hinges during bending. We considered a range of geologically reasonable boundary conditions to show that at one extreme, fracturing can occur as a result of only minor shortening by folding to the other extreme where a tight fold can form with no associated extension fracturing. For folds formed at shallow depths, where the confining stress on the system is less than the bending stresses in the layer and where the confining stress has not greatly increased the fracture toughness of the rock, hinge-parallel extension fractures can grow under hydrostatic fluid pressure conditions. As depth increases, however, much higher pore fluid pressures are required to cause fracturing under similar strain rates. The observed controls are used to hypothesize how hinge-parallel extension fracturing in fault bend folds can vary spatially and temporally across a thrust belt as a function of strain (thrusting) rate, the amount of bending at thrust ramps, and the depth of folding.

The Wayah granulite-facies metamorphic core, southwestern North Carolina: High-grade culmination of Taconic metamorphism in the southern Blue Ridge

The Wayah granulite-facies metamorphic core, in the eastern Blue Ridge, southwestern North Carolina, contains the metamafic assemblage hornblende-orthopyroxene-clinopyroxene which defines the hornblende (lower) granulite facies. The metamorphic progression from the kyanite-almandine zone into the Wayah granulite core is continuous across the Hayesville fault, a major premetamorphic-peak tectonostratigraphic boundary. This metamorphic transition includes a distinct muscovite-absent (+alkali feldspar) second-sillimanite zone. Retrogression is limited. This entire range in metamorphic grade apparently was produced during a single, prograde, Paleozoic (likely Taconic) metamorphic event. The Hayesville fault crosses the study area mostly within the first-sillimanite zone, northwest of the Wayah granulite core. Both supracrustal sequences juxtaposed along this fault consist predominantly of metasedimentary rocks probably deposited during the Late Proterozoic. Although paragneiss pre-dominates, the southeastern terrane is distinguished by the presence of metamorphosed felsic, mafic, and ultramafic igneous rocks. No evidence supports the presence of Grenville basement within the study area. The second of four deformational events, synchronous with the metamorphic peak, created northeast-southwest- to east-west-trending, upright isoclinal folds which dominate the outcrop pattern. Macroscopic F 2 folds deformed the Hayesville fault. Application of mineralogic thermometry and barometry yields conditions which range from 585 °C, 5.5 kb at the kyanite-sillimanite isograd to 842 °C, 9.8 kb in the hornblende granulite facies. Macroscopic distributions of migmatite and perthite, published experimental data, and barometric calculations suggest that T = ∼669 °C, P = ∼6.52 kb, and X(H 2 O) = ∼0.62 at the second-sillimanite isograd. Results indicate that the Wayah granulite-facies core constitutes the high-grade culmination of Paleozoic metamorphism in the southern Appalachian Blue Ridge.

Pine Mountain terrane, a complex window in the Georgia and Alabama Piedmont; evidence from the eastern termination

The Pine Mountain terrane is exposed in a complex window within the Piedmont of Georgia and Alabama. The eastern end of the terrane is framed by three ductile faults of demonstrably different ages. The polydeformed pre-thermal peak Box Ankle fault is truncated to the south by the younger pre-thermal peak Goat Rock fault, and to the north by the even younger post-thermal peak Towaliga fault. The three faults framing the eastern termination of the window are clearly neither (1) part of the same detachment nor (2) part of the Appalachian detachment.

Cryptic crustal events elucidated through zone imaging and ion microprobe studies of zircon, southern Appalachian Blue Ridge, North Carolina–Georgia

Compositional zoning reveals multistage growth histories and resorption events in zircon from a high-grade terrane in the eastern Blue Ridge of North Carolina and Georgia. These zoning patterns were used to guide high-resolution ion microprobe dating that places important constraints on the evolution of the southern Appalachian crust. Zircons from granulite facies metapelite have unzoned rims that yield concordant U-Pb ages of 495 ± 14 Ma. We interpret this as the time of rim growth, which occurred during peak metamorphism early in the protracted orogenic history of the region. Detrital cores, characterized by truncated euhedral zoning, are of Grenville age (1.04–1.26 Ga). Zircons from the Whiteside and Rabun plutons have well-defined, rounded, inherited cores and euhedral, oscillatory-zoned magmatic rims. Rims of Rabun zircons record magmatic crystallization at 374 ± 4 Ma, whereas Whiteside rims yield a 466 ± 10 Ma crystallization age. Cores from both plutons include 1.1–1.3 Ga and 2.6–2.7 Ga ages. These data indicate that there was no single, voluminous episode of plutonism in this area, that similar material underpinned the region at least from 370 to 470 Ma, and that previously unrecognized Archean basement or Archean basement–derived sedimentary rock was present in the southern Appalachians. Results of this study verify the value of combining zoning and ion microprobe studies: Using conventional U-Pb methods or ion microprobe dating without knowledge of zoning would have made interpreting the events recorded in these zircons and the ages that they yield difficult or impossible.

Graphic correlation of Middle Ordovician graptolite shale, southern Appalachians: An approach for examining the subsidence and migration of a Taconic foreland basin

Graptolite-rich shale in the Valley and Ridge province of Alabama, Georgia, Tennessee, and Virginia records subsidence and migration of the Sevier foreland basin, which formed in response to arc-continent collision during the Middle Ordovician Blountian phase of the Taconic orogeny. Rapid, tectonically induced subsidence of what had been a shallow carbonate platform led to pelagic deposition of graptolite shale in a deep basinal setting. The contact of the shale with the underlying carbonate rocks is a distinct record of initiation of subsidence. Graptolites were collected extensively and systematically from the shale at 50 localities in the southern Appalachians. Correlation with zonal biostratigraphy demonstrated that the shale sequence is diachronous. In localities to the southeast, in Alabama and Georgia, it correlates with the Didymograptus murchisoni Zone. To the northwest, across strike in Alabama, Tennessee, and Virginia, the shale becomes progressively younger and correlates sequentially with the Glyptograptus teretiusculus , Nemagraptus gracilis , and Climacograptus bicornis Zones. This diachronism records the northwestward, cratonward migration of the foreland basin, which was driven by postcollisional plate convergence. Biostratigraphic data from 22 localities were amenable to analysis by graphic correlation. At these localities, graptolites were collected from measured sections, and ranges of all species in the 22 sections were compiled into a CSRS (composite standard reference section) that correlates with the biostratigraphic interval from the G. teretiusculus Zone to the C. bicornis Zone. On the basis of the most reliable radiometric ages correlated into the graptolite zonation, the duration of the composite standard reference section is estimated to be 5 m.y. It is divisible into 89 CSUs (composite standard units), each with an approximate duration of 56 000 yr. Recorrelation of each of the 22 measured sections with the composite standard reference section allowed the stratigraphic base of the shale in each section to be expressed in terms of composite standard units, and such correlations permitted the diachronism of the basal shale contact to be measured with much greater precision than with zonal biostratigraphy. In addition, age differences of the basal shale contact between sections can be expressed in terms of time. With the temporal information provided by the composite standard reference section and original distances between sections determined from a palinspastic reconstruction, the rate of migration of the foreland basin as recorded in the diachronous basal shale contact was determined. Between sections in northeastern Tennessee and adjacent Virginia, calculated rates averaged 13 mm/yr and decreased northwestward from 40 to 9 mm/yr. This cratonward decrease in rate may represent deceleration as foreland basin migration, and the arc-continent collision that drove it, slowed to a halt. The rate of 40 mm/yr may be considered as the maximum rate of plate convergence following arc-continent collision. Precollisional plate convergence rates may have been significantly greater. The rate of 40 mm/yr is, however, well within the range of modern rates of plate convergence. In addition, the deceleration recorded in the Sevier foreland basin is similar to the deceleration of postcollisional plate convergence at the Timor Trough.