Volume iv

William A. Thomas , Robert D. HatcherJr, in Encyclopedia of Geology (2nd Edition), 2021

Alleghanian High-Grade and Plutonic Rocks

Coeval with or slightly younger than Alleghanian deposition of sediments in internal basins similar the Narragansett basin in Rhode Island, amphibolite-facies metamorphism, polyphase deformation, and plutonism are recorded in an antiformal chugalug along the Coastal Plain overlap from Virginia to Alabama, variously called the Goochland terrane-Raleigh belt, Kiokee belt, and Pine Mount terrane. An Alleghanian thermal issue is also recorded across much of the Tugaloo and True cat Square terranes in the southern Appalachians ( Dennis and Wright, 1997; Merschat et al., 2005).

Southern Appalachian Alleghanian plutons are largely peraluminous, S-blazon granites with ages ranging from 330 to 300   Ma (Sinha and Zeitz, 1982; Samson et al., 1995; Coler et al., 2000), and a suite of Alleghanian gabbros with ages ranging from 302 to 311   Ma in the Carolina superterrane in the Carolinas (Huebner and Hatcher, 2017). These plutons are also in the Kiokee belt and Goochland terrrane/Raleigh belt metamorphic core, extending from Virginia to South Carolina, and Carolina superterrane, into the Inner Piedmont in the Atlanta region and in the eastern Blueish Ridge as the Rabun, Circular Mountain, and Looking Glass granites (Mueller et al., 2011; Stahr, 2007).

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The Appalachian and Black Warrior Basins: Foreland Basins in the Eastern United States

Frank R. Ettensohn , ... Will Gilliam , in The Sedimentary Basins of the United States and Canada (Second Edition), 2019

Alleghanian Mountains Postorogenic Plummet and Extension

Past the cease of Permian time, the Alleghanian orogen was probably a high-continuing, Himalayan-type mountain range with an added orogenic thickening of 3–10  km based on structural and thermal modeling (Levine, 1986; Friedman, 1994; Faill, 1998). The mountains may take included a broad altiplano beyond much of the southern and cardinal Appalachians (Rodgers, 1987) that would have been like to the modern primal Andes, a 300   km-broad range with a mean pinnacle of about iv   km (Beaumont et al., 1987; Slingerland and Furlong, 1989; Faill, 1998). Preserved sediment in the foreland, moreover, seems to reflect sediments of older recycled orogenic provenance, largely from earlier Alleghanian tectonism to the s (Faill, 1998; Thomas et al., 2004). In the north-fundamental Appalachians, nonetheless, the presence of units representing relief-sensitive estuarine and lacustrine environments as far east as northeastern Pennsylvania suggests that latest Alleghanian deformation was postclastic wedge and post-Early Permian in historic period. Even though parts of the Appalachian Basin itself must take experienced major faulting and folding afterward this time, deformation may not accept been readily visible, equally the bowl almost certainly became the repository for much of the detritus eroded from the new Alleghanian highlands. This repository most probable took the form of a vast west-dipping regional alluvial-fan circuitous during Permian and early Mesozoic time, nothing of which is now preserved (Faill, 1998). In fact, based on isotopic evidence, Dallmeyer (1989) concluded that the sometime orogen experienced major uplift during Late Permian to Early on Triassic time, and that by Late Triassic fourth dimension (~   220   Ma) parts of the old orogen in New England had been exhumed to nearly the present erosional level. Some of the resulting sediments may have been carried thousands of kilometers westward across the continental interior (Kehn et al., 1981; Archer and Greb, 1995; Ettensohn, 2004), and Dickinson and Gehrels (2003) and Gehrels et al. (2011) accept shown that Appalachian sediments were transported as far as the western continental margin based on the ages of detrital zircons from Permian and Jurassic sandstones of the Colorado Plateau. Eroded sediments were also transported e, as prove indicates that rocks in southern New England had been cached by more than 5   km of sediments in Early Jurassic time (Roden-Tice and Wintsch, 2002). Moreover, in places on the American due east coast, widespread early Mesozoic alluvial fans must have completely enveloped older Paleozoic rocks, and what are preserved today of these fans are mere remnants defenseless in synrift basins that were agile as tardily equally Cretaceous fourth dimension (Roden-Tice and Wintsch, 2002).

The breakup of Pangea in the fundamental Atlantic region was part of major plate reorganization that had begun by Pennsylvanian time on the due north and northeastern margins of former Laurussia and proceeded southward through Cretaceous fourth dimension (Ziegler, 1989; Ettensohn, 1997; Miall and Balkwill, Chapter fifteen). In the Atlantic region, breakup occurred both during the final phase of Pangean assembly and during the initial phases of Pangean dispersal along the tectonothermally thickened and highly elevated Alleghanian orogen (Manspeizer, 1994). Manspeizer (1994) has suggested that the breakup reflected continued northwest convergence in the lower crust concomitant with major uplift and southeasterly extension in the upper chaff; as elevations increased, so did the vertical stress needed to drive orogenic collapse and extension. Extension may have also been abetted by the tectonic extrusion of onetime microcontinents or Grenvillian/Amazonian crustal fragments along dextral megashears (Manspeizer, 1994), which as they passed into the orogen seem to reflect reactivated shear zones that had previously accommodated Late Precambrian rifting and Paleozoic terrane accession.

Whalen et al. (2015) suggested another interpretation based on the presence of the Appalachian expanse in the Central Atlantic Magmatic Province (Camp), a large igneous province within and on both sides of the Atlantic Ocean, related to the breakup of the supercontinent Pangea. Whalen et al. (2015) postulated that subsequently subduction stalled, possibly due to impact against an older, inherited Precambrian rift bowl (Benoit et al., 2014), the apartment Rheic slab, which underplated Laurussia, began to tear below the inherited indenter. The tear propagated into a full-calibration delamination event, producing mantle upwelling, pall melting, and injection of mafic magmas into crustal zones of weakness. Equally a result, near 40 offshore and onshore Late Triassic-Early Jurassic synrift basins (Fig. 2) formed on mostly accreted terranes in the former orogen along depression-angle detachment surfaces that had been former Alleghanian thrust or strike-slip faults (Manspeizer, 1988, 1994; Manspeizer et al., 1989). The basins that are presently preserved throughout the Appalachian area are all lateral grabens formed on listric faults, but the main centric rift basin is probably preserved under the continental shelf equally the Due east Coast Magnetic Bibelot.

Beyond near of the Atlantic surface area, basin sedimentation in half-graben basins began in the Eye Triassic (Ladinian, ~   240   Ma) and concluded by Early on Jurassic (Sinemurian, ~   195   Ma) fourth dimension (Fig. two), during a period of nigh 45   Ma (Olsen et al., 1989) when body of water-floor spreading began. Further rifting and intrusion patently continued for another 30   Ma into Middle Jurassic (Callovian, ~   165   Ma) time (Olsen et al., 1989). The western onshore basins were mostly high-relief, high-distance fluvial-lacustrine basins, whereas several of the offshore basins were evidently low-relief, sea-level evaporite basins proximal to the future spreading center (Manspeizer, 1988; Manspeizer et al., 1989). The onshore basins contain a diverseness of terrestrial facies, including alluvial-fan and fluvial conglomerates and arkoses, sandstones and redbeds, every bit well as lacustrine mudstones, siltstones, and local coals arranged into uncomplicated transgressive-regressive cycles, probably driven by climate changes (Van Houten, 1969, 1980; Olsen, 1984, 1986). Many of the basins likewise exhibit Lower Jurassic flood basalts and related hypabyssal sills and dikes, which are parts of a larger menstruum of spreading-related postorogenic igneous activity that persisted from Early on Triassic into Eocene fourth dimension (Manspeizer et al., 1989). Onshore basin sediments, included in the Upper Triassic-Lower Jurassic Newark Supergroup, accumulated to thicknesses of 3–ix   km and reflect more subhumid conditions in southern basins closer to the equator and more than barren weather in northern basins, although basin-related topography and later proximity to linear seas must have periodically forced unique climatic conditions (Manspeizer, 1994). Inasmuch as basin sediments overlie older Paleozoic crust and occur in postorogenic, by and large intermontane, basins developed on top of an inactive suture belt (Ingersoll and Busby, 1995), the Newark basins, in the broadest sense, are Alleghanian successor basins. Although orogen collapse and rifting had begun earlier across other parts of the Appalachian-Caledonian suture, forth which the Iapetus-Rheic oceans had previously closed, Late Triassic rifting was largely the result of supercontinental inheritance and effectively ended the largely Paleozoic Appalachian Wilson bike and initiated the current Atlantic wheel. By Eye Jurassic time, spreading had generated an embryonic Atlantic Ocean, while in the Appalachian Bowl, the regional Alleghany alluvial fan was already undergoing deep autopsy.

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Book 5

Charlotte Southward. Miller , Viktoria Baranyi , in Encyclopedia of Geology (Second Edition), 2021

Triassic Paleogeography

At the onset of the Triassic, the continents were assembled into a single supercontinent chosen Pangaea (Fig. 1 ). The Pangaean supercontinent began to class at the cease of the Carboniferous, when Laurasia and Gondwana collided during the Variscan orogeny along the Alleghanian–Variscan–Ural mountain chains, but finally reached its maximum extent with the addition of Southward East asia, parts of Communist china, Kazakhstan and Siberia. Pangaea extended from c. 85°N to 90°S, near centering on the equator. The remainder of the Globe comprised of an enormous ocean called Panthalassa, roofing c. 70% of the Earth's surface, with a westward extending limb cutting into the Pangaean continent, called the Tethys Sea (Fig. 1). The Tethys Body of water was underlain by oceanic crust sea floor and was restricted latitudinally to effectually 30°N–S, within the tropical-subtropical belt. No large continent rearrangement is documented across the Permian-Triassic transition and into Early on Triassic except for the gradual northward drift and counter-clockwise rotation of the entire Pangaean continent. Pangaea existed for c. 100   Ma before information technology began to break up in the Belatedly Triassic/Early Jurassic. Rifting began forth the northwest African and North American margins, along pre-existing structures. This resulted in the formation of a series of rift basins. This rifting of the Pangaean supercontinent ultimately led to the opening of the cardinal Atlantic forth the slow spreading Central Atlantic ridge. The cause of the rifting of Pangaea remains highly debated with some authors suggesting that the rifting started prior to the extrusion of the Central Atlantic Magmatic Province lavas, while others advise that lava extrusion coincided with the interruption-upwards of Pangaea, but was non the cause.

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The Appalachian Orogen☆

Robert D. HatcherJr., in Reference Module in Earth Systems and Ecology Sciences, 2014

Discussion: Developmental History

The Appalachian orogen was synthetic between the breakup of the supercontinent Rodinia, and the end of the Appalachian Wilson cycle with formation of supercontinent Pangea at the stop of the Paleozoic. Passive-margin development followed rifting of Rodinia, which resulted in the greatest inundation of the interior of Laurentia in geologic fourth dimension from the Belatedly Cambrian and Early Ordovician ( Figure 4 ). Interior Laurentia, possibly along with parts of northwestern Africa and other continents, was and so uplifted by lithospheric processes, producing the widespread 'post–Knox-Beekmantown' unconformity, recognizable from eastern Laurentia to as far west as Utah, and likewise in northwestern Africa (Morocco) and perhaps in Republic of estonia and Siberia (Hatcher and Repetski, 2007). The eastern margin of Laurentia was once again inundated past shallow seas at the beginning of the Middle Ordovician, and passive margin deposition resumed, but the renewed passive margin was brusque-lived: water depths increased and clastic sediment began to get in from the east. The Taconic orogeny had begun in the interior of the chain, and ophiolites began to load the outer platform in the northern Appalachians (Stevens, 1970; Williams, 1979). In the southern and central Appalachians, nonetheless, the foredeep began to develop, but the sediment limerick reflects an eastern nonvolcanic, nonophiolitic source, which contrasts with the source of sediment farther north that contains ophiolitic droppings (Hiscott, 1984). Judging from clast compositions of Middle Ordovician conglomerate in the southern Appalachians that include all lithologies of the early Middle Ordovician and older rifted margin and platform sequences, along with some basement clasts (Kellberg and Grant, 1956), uplift of the platform east of the foredeep occurred throughout the Centre and Tardily Ordovician without a volcanic source, initially (incorrectly) suggesting that the uplift was a forebulge and the foredeep was a forebulge basin (Hatcher et al., 2004). Despite the source, the 3   km accumulated thickness in the Sevier basin is too swell for it to exist considered a forebulge bowl. Deposition in the key Appalachian foredeep began in the Eye Ordovician Darriwilian, and sediments reached their greatest thickness (~   five   km) in the Martinsburg basin during the Belatedly Ordovician. Middle Ordovician foreland basins are all diachronous, becoming filled during the late Arenig (Floian) and Llanvirn (Dapingian) in the southern and northern Appalachians, only continued-to-be filled during the Caradocian (Sandbian) in the key Appalachians (Shanmugam and Lash, 1982).

Taconian foreland deposition reflects volcanic arc evolution and closing of the Iapetus bounding main and its margins. The resolution of the nature of the arc systems varies markedly along the orogen because of the variability of the intensity of transposition and metamorphic class. A complex arc system was congenital off Newfoundland during the Ordovician that involved both west- and e-dipping subduction zones, with accession of the Dashwoods block (van Staal et al., 2007) ( Figure 5 ). A parallel history may accept taken place farther south, but information technology is obscured by Taconian and later on polydeformation, and medium- to high-grade metamorphism. The best-preserved ophiolites in the orogen were obducted onto the Laurentian margin in Newfoundland during the Taconian event, forth with accretion of several arc terranes (e.grand., Stevens, 1970). The only other recognizable ophiolites in the orogen occur in New Brunswick and Québec (St. Julien and Hubert, 1975; van Staal et al., 1998, 2008), only these consist of incomplete sections. Ordovician blueschist has been recognized in northern Vermont and southern Québec (Laird, 1988), and in New Brunswick, but the New Brunswick occurrence appears to take a Late Ordovician historic period and emplacement related to the ~   425   Ma Silurian Salinic orogeny (van Staal et al., 2008). Further south, either no ophiolites or blueschists are preserved on the Laurentian margin or they are located in the internides where they are polydeformed and metamorphosed to amphibolite facies or college grade assemblages, thereby obscuring their origin (Robinson et al., 1998), although several mafic-ultramafic complexes in the southern Appalachians possess whole-stone and mineral geochemical characteristics that advise normal mid-bounding main-ridge basalt (Due north-MORB) to arc provenance and ophiolite emplacement (Hatcher et al., 1984; Swanson et al., 2005).

Taconic allochthons that emplaced deep-water, offshore facies onto the Laurentian margin occur from New York n (Zen, 1967, 1972; St. Julien and Hubert, 1975; Rowley et al., 1979) ( Effigy 1 ). No Taconic allochthons occur farther due south, but several nappes were thrust into the Martinsburg basin during the Center to Tardily Ordovician and were originally interpreted as a Taconic allochthon, called the 'Hamburg klippe' (Stose, 1946; Lash and Drake, 1984; Drake et al., 1989). Contempo detailed paleontologic work, however, has revealed that the Cocalico Formation of variegated sandstone and shale that originally was thought to comprise the Hamburg klippe contains olistoliths of older units, but was also thrust into the Martinsburg basin during the Heart to Late Ordovician (Ganis and Wise, 2008; Wise and Ganis, 2009). Wise and Ganis (2009) suggested this grouping of parautochthonous to allochthonous rocks exist chosen the 'Hamburg complex.' This occurred close to the fourth dimension of accretion of several smaller more than outboard terranes in the central Appalachians, including the Westminster, Potomac, and Philadelphia terranes (Faill, 1997).

Taconian deformation produced the foreland fold-and-thrust belt from Vermont northward to Newfoundland (Rodgers, 1971; St. Julien and Hubert, 1975). Logan's line thrust in Québec, and the Champlain and Hinesburg thrusts further south in Vermont, are the prominent faults in the Taconian foreland in New England. Devonian rocks are plain-featured in the foreland further south in the Hudson Valley of New York, and Marshak (1986) suggested that these thrusts formed during the Acadian orogeny. There are, nevertheless, no age constraints on the youngest possible timing of deformation, and then these thrusts could easily have formed during the Alleghanian orogeny, and constitute the northeastern continuation of the southern-central Appalachian foreland fold-and-thrust belt.

Taconian deformation becomes intense and polyphase into the interior of the Appalachian orogen in New England and is inconsistent with arc accretion between 496 and 428   Ma (Stanley and Ratcliffe, 1985; Karabinos, 2006). A suite of external and more than internal (Heart Proterozoic) basement massifs (Hudson–Housatonic Highlands, Berkshire Mountains, Green Mountains, and Lincoln massif, and the internal Chester-Athens dome) occurs inboard from the foreland from southeastern New York to northern Vermont, and appears to correspond rifted fragments of Grenvillian crust that were covered with Neoproterozoic to Cambrian sedimentary and pocket-sized volcanic rocks, and and so thrust onto the Laurentian margin below the accreting arcs during the Taconian result (Stanley and Ratcliffe, 1985) ( Figure 5 ).

The 'Taconian suture' consists of a major mistake and terrane boundary that is traceable throughout the orogen. Information technology is called the Baie Verte–Brompton Line from Newfoundland to New England, and consists of faults with a multifariousness of names: Whitcomb Summit, Cameron's Line, and several in the central Appalachians, including the Martic Line, then the Gossan Lead–Burnsville–Chattahoochee–The netherlands Mount–Hollins Line mistake. The Hayesville fault forms the western boundary of two smaller terranes (Cartoogechaye and Cowrock) located due west of the Chattahoochee error in the southern Appalachians. These faults divide the rifted margin metasedimentary and rift-related volcanic rocks (with the exception of the 471   Ma arc-related Hillabee Greenstone in Alabama and western Georgia; McClellan et al., 2007) from dismembered ophiolites, just due south of Québec deformation at high-form metamorphic atmospheric condition prevents structural or stratigraphic resolution of their provenance. Despite this, numerous geochemical studies over the past 25 years accept consistently suggested that the mafic and ultramafic rocks accept a not-continental origin. Detrital zircons, even so, accept proven that the metasedimentary rocks are distal Laurentian sediments that were likely deposited in an oceanic setting, and all of these terranes have a Laurentian provenance. Taconian metamorphism reached upper-amphibolites-facies, or higher, weather in New England (Robinson et al., 1998) and in the southern Appalachian Blueish Ridge (Force, 1976; Eckert et al., 1989; Moecher et al., 2004). Timing of Taconian metamorphism ranges from 460 to 455   Ma in the southern Appalachians to 480–455   Ma in New England, to 495–455   Ma further northward (Robinson et al., 1998; Moecher et al., 2004; van Staal et al., 2004). Timing of deformation and metamorphism in the internides is roughly coeval with the timing of foreland bowl development.

A few Ordovician (~   460   Ma) granitoid plutons occur in the southern Blue Ridge (e.g., Hatcher et al., 2007b), but many more occur in the Inner Piedmont (Miller et al., 2000; Bream, 2003; McClellan et al., 2007), and 470–458   Ma plutons and volcanic rocks occur in the Milton–Chopawamsic–Potomac terrane (eastward.g., Coler et al., 2000). These continue into the central Appalachian Piedmont of Virginia to Maryland, with the addition of several Silurian (Salinian?) granitoid plutons (Wilson, 2001; Sinha et al., 2012), and reappear in western New England as part of the Bronson Colina arc; nearly have an arc geochemistry (Stanley and Ratcliffe, 1985; Robinson et al., 1998).

Late Ordovician to Early Silurian molasse spread across most of the fundamental and part of the southern Appalachians post-obit the Taconic orogeny, and similar deposits formed in the north, truncating tilted Ordovician rocks in New Bailiwick of jersey and New York (the classic Taconic unconformity) (Rodgers, 1971). Early Silurian felsic volcanic rocks were extruded in key-western Newfoundland, suggesting that rifting followed the orogeny (van Staal et al., 1998). There also is evidence for compression in the northern and central Appalachians, which has been called the Salinic orogeny (van Staal et al., 1998, 2004; Aleinikoff et al., 2007). A suite of Tardily Ordovician to Early Silurian granitoid plutons also intruded the Virginia to Maryland central Appalachians (Wilson, 2001; Sinha et al., 2012).

Acadian and Neoacadian Orogenies

Clastic degradation began during the Late Ordovician (?) and Silurian in internal New England and the Canadian Maritimes (Robinson et al., 1998), and in the remnant Rheic ocean off the central Appalachians (Merschat and Hatcher, 2007). Parallel deposition of clastic sediments on the cardinal Appalachian foreland began in the Eye Devonian (Chemung and Catskill Groups) ( Figure 6 ). Deposition continued through most of the rest of the Devonian, but the clastic wedge became diachronous and younger equally it propagated southwestward in the foreland, forming the all-encompassing organic-rich shales (Marcellus, Ohio, Chattanooga) that coarsen to the due east (Ettensohn, 2004). Paralleling foreland deposition, deformation and metamorphism occurred in the interior of the orogen, reaching granulite-facies assemblages in southern New England and at least sillimanite in the southern Appalachians. This is the Acadian orogeny in New England, New Brunswick, and Nova Scotia, and points south (Barr et al., 1998; Robinson et al., 1998), and the Neoacadian orogeny from New England s. The Acadian orogeny began in the Tardily Devonian (~   410   Ma), and the Neoacadian ended in the early Mississippian (~   345   Ma) (Osberg et al., 1989; Robinson et al., 1998; Merschat et al., 2005; Hatcher et al., 2007b; Merschat and Hatcher, 2007).

Figure 6. Possible relationships during Late Devonian to early Mississippian time (~   360   Ma) among Laurentia, already formed Taconic crust, and Carolina-Gander superterranes that collided transpressionally (southwestdirected) with the Laurentian–Taconian assemblage and subducted these elements beneath them, producing high-class metamorphism and uplift in southern New England and progressively subducting more of the Laurentian–Taconian terranes, together with sediments deposited in the remnant Rheic body of water, southwestward reaching burial depths of eighteen–20   km in ane–four   chiliad.y. (depending on dip of the subduction zone). The event was a tectonically forced, southwestward escaping, orogenic channel of partially melted Cat Square sediments and Laurentian Taconian crust (Hatcher and Merschat, 2006). Configuration of diachronously prograding deltas on the platform (from Ettensohn, 2004) correlate directly with the northeast-to-southwest transpressionally zippered closing of the remnant Rheic ocean.

 (Figure modified from Merschat and Hatcher, 2007.) Reproduced from Hatcher, 2010.

The Acadian orogeny was clearly the dominant event in the New England and next Canadian Appalachians, producing polyphase deformation and high-grade metamorphism in southern New England, and arable plutonism (Robinson et al., 1998). This event is likely related to collision of Avalon (and Ganderia?) superterrane with Laurentia (Skehan and Rast, 1990). Wintsch et al. (2003) suggested that tectonic wedging was involved with collisional emplacement of the Avalon terrane in southern New England, producing much of the circuitous deformation in Connecticut. Much of this deformation and metamorphism is now thought to be Alleghanian, based on modern geochronologic data that demonstrate involvement of Permian rocks in the deformation ( Walsh et al., 2007).

The Acadian and Neoacadian orogenies are the product of zippered north-to-south endmost of the Rheic bounding main as peri-Gondwanan superterranes, Avalon and Carolina, and perchance Gander, collided with Laurentia and the early Paleozoic terranes accreted during the Taconian outcome (Rast and Skehan, 1983; Barr et al., 1998; Merschat et al., 2005; Hatcher and Merschat, 2006; Hatcher et al., 2007a; Merschat and Hatcher, 2007). The Cat Square remnant ocean accumulated detrital zircons from both Laurentian and peri-Gondwanan sources, and zircons as young as 430   Ma (Bream, 2003), indicating deposition occurred during the Silurian and Devonian (Merschat and Hatcher, 2007). From the fundamental Appalachians southward, Carolina subducted terranes to the west beneath it later closing the Cat Square remnant ocean bowl, producing anatectic melting by 407   Ma (Gatewood, 2007) and wholesale migmatization of Cat Square and Tugaloo terrane assemblages, and generating plutons in these terranes, as well as in the Carolina superterrane. Neoacadian plutons are not as abundant in the Carolina superterrane, but they are present in the Carolinas and Georgia as the Salisbury plutonic suite and associated mafic plutons in the Carolinas (Butler and Fullagar, 1978; McSween et al., 1991; Esawi, 2004), and the likely Devonian mafic plutonic suite in key Georgia (e.g., Hooper and Hatcher, 1989). In contrast, Acadian plutons are arable, fifty-fifty ascendant, in New England and the Canadian Maritimes. The reason for this difference may exist that final emplacement of Avalon was by westward subduction beneath New England (Phinney, 1986; Robinson et al., 1998), only was e in the southern and central Appalachians. Because of the transpressive nature of standoff leaving only a remnant ocean, the amount of ocean crust that was available to be subducted was minimal, thus limiting the ability to generate suprasubduction-zone plutons. In one case the limited amount of ocean crust was subducted, continental crust began to exist subducted and the ability to generate plutons was close off, analogous to the subduction of modern Australian continental crust beneath Indonesia, shutting off arc volcanism (e.g., Hamilton, 1979). The high-grade western Carolina superterrane contains a 360–350   Ma metamorphic overprint and numerous younger (Devonian to Carboniferous) granitoids and gabbros (Dallmeyer et al., 1986; McSween et al., 1991; Hibbard et al., 2002) probably related to the centre to late Paleozoic docking of the Carolina superterrane. The Smith River allochthon ( Figure 2 ) has been identified every bit a peri-Gondwanan terrane and outlier of the Carolina superterrane based on monazite chemic ages (Hibbard et al., 2003), but detrital zircon data (Carter et al., 2006; Merschat and Hatcher, 2007; Merschat, 2009) do not support this conclusion. The greenschist-grade and lower-grade central and eastern Carolina superterrane consists more often than not of volcanic and volcaniclastic rocks interrupted by the Alleghanian Kiokee–Raleigh chugalug metamorphic core. Southeast of this metamorphic core are more depression-grade volcanic and volcaniclastic rocks that have been called dissimilar terranes past Hibbard et al. (2002), although the Carolina superterrane was subdivided differently by Horton et al. (1989). Some of the Hibbard et al. (2002) terranes vary only in metamorphic class and do non contrast in central history or stratigraphy.

Evidence supporting mid-Paleozoic accretion of Carolina superterrane consists of: (i) deposition of True cat Foursquare terrane sediments that occurred subsequent to 430   Ma (Bream, 2003), almost coeval with deposition of the Silurian–Devonian sediments in New England and the Canadian Maritimes; (2) parallel diachronous timing of metamorphism and deformation in the Tugaloo and Cat Square terranes in the southern Appalachian internides with diachronous, southward-younging clastic wedge sedimentation in the foreland, get-go in the north with the Devonian Catskill sediments, and ending in the southward with the latest Devonian–Mississippian Chattanooga Shale and coarser equivalents to the due east (eastward.g., Ettensohn, 2004; Merschat and Hatcher, 2007); (3) both mafic and felsic magmatism in the overriding Carolina superterrane in North Carolina, Southward Carolina, and Georgia (Butler and Fullagar, 1978; McSween et al., 1984; Hooper and Hatcher, 1989; Esawi, 2004); and (4) a mid-Paleozoic 360   Ma thermal overprint recorded in 40Ar/39Ar plateau ages in the western Carolina superterrane (Dallmeyer et al., 1986). Moreover, criticism leveled at the conclusion based on arable data of the mid-Paleozoic docking of Carolina superterrane (Hibbard et al., 2007a) is partly based on paleomagnetic data that place Carolina superterrane at similar paleolatitude with Laurentia in the Ordovician (Vick et al., 1987; Noel et al., 1988), but these data practise not bespeak paleolongitude, permitting as much every bit 180° of uncertainty in the location of Carolina.

Hibbard (2000) and Hibbard et al. (2012a, 2012b) ended that Carolina was accreted to Laurentia during the Ordovician based on dated Ordovician deformation inside the Carolina superterrane along the Gold Hill-Silver Hill error organization in the Carolinas, and 40Ar/39Ar data. These faults take pocket-sized displacements and their Ordovician deformation in the Carolina superterrane could have occurred while Carolina was far from Laurentia well before mid-Paleozoic accretion, every bit the overwhelming structural, detrital zircon, and other geochronologic data bespeak. Moreover, the probability-frequency plot of Ar data (their Figure 8) contains the greatest frequency of ages in the middle Paleozoic clustered around 370   Ma, consistent with metamorphic ages determined from monazite and zircon rims (Dennis and Wright, 1997; Bream 2003; Merschat, 2009). Their few Ordovician ages could be related to localized uplift related to internal deformation in the Carolina superterrane prior to accretion.

Alleghanian orogeny

Clastic sediments derived from the interior of the orogen began in the late Mississippian in the southern and central Appalachian foreland, and in basins in the interior of the orogen from New England to Newfoundland ( Figure 1 ). These interior basins are considered stepover rhomb grabens related to big dextral faults (Cobequid–Chedabucto, Bellisle, Cabot), products of the initial stages of collision of Gondwana with Laurentia, and the closing of the Theic sea, which was followed by the terminal associates of Pangea (Bradley, 1982) ( Figure 7 ). Coeval with or slightly younger than deposition, amphibolites-facies metamorphism, polyphase deformation, and plutonism are recorded in southern New England and in an antiformal belt along the Coastal Evidently overlap from Virginia to Alabama, variously called the Goochland terrane–Raleigh belt, Kiokee chugalug, and Pine Mount terrane ( Figure 1 ). An Alleghanian thermal upshot is also recorded across much of the Tugaloo and Cat Square terranes in the south ( Dennis and Wright, 1997; Merschat et al., 2005), and is much more widespread beyond southern New England than was previously thought (Wintsch et al., 2003; Walsh et al., 2007).

Figure seven. Zipper closing of Theic ocean to class the Alleghanian orogen (continents are shown on Robinson projection; reconstruction modified from Ziegler, 1990). Red lines and symbols indicate feature is active in the fourth dimension interval shown. (A) Initial contact between Gondwana and Laurentia occurred in belatedly Early Carboniferous (tardily Mississippian), producing initially sinistral faulting in New England followed immediately by dextral motion and pull-apart basins, then shedding of clastic sediments onto the continent, and Lackawanna-phase deformation. (B) S movement and rotation of Gondwana with respect to Laurentia in early Late Carboniferous (early on Pennsylvanian) produced dextral motion throughout orogen, waning of Lackawanna phase deformation, and greater dispersal of sediments onto the Laurentian foreland. (C) Connected clockwise rotation of Gondwana with respect to Laurentia during the Belatedly Carboniferous closed the Theic ocean southward, bringing Gondwana into head-on collision with Laurentia, and producing the first move on the Blue Ridge–Piedmont megathrust canvass. (D) Early Permian caput-on collision of Gondwana with Laurentia produced major transport on Blue Ridge–Piedmont megathrust sheet that collection foreland fold-thrust chugalug deformation in the Valley and Ridge and Plateau alee of it.

 Reproduced from Hatcher, 2002.

The basic outline of the southern and cardinal Appalachians is dominated by Alleghanian structures that inherited the Neoproterozoic shape of the continental margin (Thomas, 2006). Foreland deformation is Alleghanian, and timing of faulting in the domain of large strike-slip faults in the interior from the Brevard fault eastward is predominantly Alleghanian, with the exception of the Neoacadian Central Piedmont suture, but it besides was locally reactivated during the Alleghanian ( Effigy 3 ). Farther due north, the Alleghanian record is confined to the interior of the chain, with several plutons and extensive metamorphism in New England (including the large Sebago batholith in Maine) (Wintsch and Sutter, 1986; Walsh et al., 2007), and several major dextral faults on the eastern side from southeastern New England to Nova Scotia, which reappear in interior western Newfoundland ( Figure 2 ). The large, by and large stepover basins containing Carboniferous and Permian sediments are direct related to these faults (Bradley, 1982; Mosher, 1983). LeFort and Van der Voo (1981) and LeFort (1984) suggested that the Reguibat Promontory in Westward Africa collided with Laurentia here earlier collision of the primary African continent in order to explain the narrow, strongly curved segment of the Pennsylvania salient in the central Appalachians ( Effigy 7 ). They concluded that collision of the promontory produced an escape tectonics scenario, where dextral faults facilitated southward escape of crustal blocks and sinistral faults carried blocks n out of the collision zone. The movement sense of the array of Alleghanian faults south of the projected collision zone, including the Brevard, parts of the central Piedmont suture, and all of the eastern Piedmont fault arrangement ( Figure one ), is conspicuously dextral, only the motion sense of faults north of the standoff zone, including the Clinton-Newbury, Bloody Barefaced, and all of those farther north, is also dextral (eastward.g., Bothner and Hussey, 1999; Goldstein and Hepburn, 1999). Based on the ages of stratigraphic sequences and fault kinematics, I take proposed that the standoff involved both rotation and transpression, and that standoff began at the northeastern end of the Appalachians and airtight the Theic sea s similar closing a zipper. In this scenario, Gondwana would take rotated into head-on standoff with southeastern Laurentia in Late Carboniferous to Permian time, producing the Blue Ridge–Piedmont megathrust sheet that pushed foreland deformation in front end of information technology from southern New York to Alabama (eastward.g., Hatcher, 2002; Hatcher et al., 2007c). The zipper tectonics scenario fits more of the data related to the tectonostratigraphic and kinematic endmost of the Theic ocean and concluding assembly of Pangea than others. The Alleghanian orogeny in Laurentia, and the equivalent Variscan components in Europe (including the Uralian orogeny; Pushkov, 1997), joined all existing continents to create supercontinent Pangea. This process took some 490   m.y. from the time of initial rifting of Rodinia to the terminal assembly of Pangea. Appalachian history began with the Neoproterozoic breakup of supercontinent Rodinia, and the rifting and progressive separation of southeastern Laurentia from Gondwana, opening the Iapetus ocean. Once rifting began, a continuous record of the transition of rifted margin to stable platform depositional conditions existed on the Laurentian margin. The Appalachian orogen may be unique among orogenic belts in that three major orogenies affected the entire orogen during its development, ane of which involved arc accession (Taconian), while the other two involved large terrane (Acadian–Neoacadian) and continent-continent collision (Alleghanian) that completed the Paleozoic Wilson cycle, forming supercontinent Pangea. These three orogenies produced widespread penetrative deformation, metamorphism to at least amphibolite facies, volcanism, suites of felsic and mafic plutons, and diachronous clastic wedges throughout the orogen (Taconian) or restricted to the southern and fundamental Appalachian foreland (Acadian-Neoacadian and Alleghanian). These clastic wedges clearly rails diachronous events taking place deep inside the orogen or along paleo-bounding main margins.

Actually, the uniqueness of the Appalachian events is paralleled by similar almost coeval events in the Variscan and Uralian orogens. Like Ordovician, mid-Paleozoic, and Carboniferous–Permian events occurred in the Variscan of Iberia, in western central Europe, and in the Urals (Pushkov, 1997; Martínez Catalán et al., 2002, 2007; Matte, 2002). Matte (2002) presented an interesting comparison of the plate-tectonic processes that occurred near at the same fourth dimension in all of these orogens, illustrating their parallel development.

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Triassic☆

South.M. Lucas , M.J. Orchard , in Reference Module in Earth Systems and Environmental Sciences, 2013

Palaeogeography

At the onset of the Triassic, the earth's continents were assembled into a unmarried supercontinent called Pangaea ( Figure 3 ). The residuum of the globe comprised a single vast ocean called Panthalassa, with a westward-extending arm chosen Tethys. This followed the Tardily Palaeozoic associates of the continents when Laurentia, Asia, and Gondwana collided along the Alleghanian–Variscan–Ural mount chains. The nearly hemispheric Pangaean supercontinent was encircled past subduction zones that dipped beneath the continents while the Panthalassan and Tethyan plates carried isle arcs and oceanic plateaus that were destined to become accreted to the continental margins.

Figure 3. Triassic Pangaea, showing major tectonic elements.

 After Lucas (2000) The epicontinental Triassic, an overview. Zentralblatt für Geologie und Paläontologie Teil I 7–viii: 475–496.

The supercontinent drifted northward and rotated clockwise throughout the Triassic, and then there was considerable latitudinal spread to the landmass, which was nearly symmetrical near the equator ( Figure 3 ). However, no sooner had the supercontinent been assembled than pregnant fragmentation began. Thus, Gondwana and Laurasia began to separate in Tardily Triassic fourth dimension with the onset of rifting in the Gulf of Mexico bowl. It was not until the Early Jurassic, though, that significant marine sedimentation took place in the nascent Atlantic Sea basin.

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NORTH AMERICA | Southern and Cardinal Appalachians

R.D. HatcherJr, in Encyclopedia of Geology, 2005

Pine Mountain Terrane

The Pine Mountain Terrane in west-central Georgia and eastern Alabama is a complex window exposing i.1-Gy-old Grenvillian basement and a thin encompass sequence of Early on Palaeozoic(?) schist, quartzite–dolomite, and a college schist. The window is framed by the dextral Towaliga and Caprine animal Stone and Dean Creek faults to the n and south, respectively, but is airtight by the Box Ankle Thrust at the east end. All of these faults yield Alleghanian ages, but the Box Talocrural joint Fault was emplaced at sillimanite-grade pressure and temperature (P–T) conditions, whereas the Towaliga and Caprine animal Rock(?) faults moved during garnet-class conditions, and the Dean Creek Fault moved under chlorite-course conditions (the same as the Modoc, farther east). Detrital zircons from the cover-sequence quartzite inside the window yield the usual authorization of ages from i.i  Ga, but a significant component of zircons from one.9–2.2   Ga occur there, suggesting a Gondwanan provenance for this sequence and basement. If this is a rifted cake of Gondwanan basement, the emplacement kinematics would be very complex, because of the Belatedly Palaeozoic ages of all of the faults framing the window. This basement block and its encompass take traditionally been considered Laurentian and the cover sequence is thought to exist metamorphosed platform rocks. It remains a suspect terrane.

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The Appalachian Orogenic Belt

Joseph A. DiPietro , in Landscape Evolution in the United States, 2013

Carolina Superterrane

The Carolina superterrane is the merely well-defined peri-Gondwana microcontinent exposed in the Southern Appalachians. With the exception of internal massifs such as the Goochland terrane and the Pine Mountain window, it covers all of Southern Appalachia east of the Central Piedmont Shear zone. The superterrane is an amalgamation of equally many as fifteen smaller terranes, many with variable depositional-tectonic histories. Overall, the rocks consist of upper Precambrian to lower Ordovician volcanic arc, accretionary prism, and ocean basin assemblages with Gondwana fossils. Given that Carolina amalgamated primarily from oceanic rock, the microcontinent may take been an oceanic plateau rather than a true fragment of Gondwana. A stiff metamorphism occurred in the Belatedly Precambrian-Early on Cambrian (617 to 530 Ma) associated with folding, faulting, and plutonic intrusion, and this is the ascendant metamorphism in several areas. On the basis of radiometric dating and cross-cutting relationships we can propose that much of the deformation and metamorphism occurred over a short interval betwixt 557 and 535 Ma. This was the Virgilinan orogeny and it is responsible for final associates of the Carolina superterrane. Assembly apparently occurred far from the Laurentian mainland while Carolina was fastened to or near the larger Gondwana continent. Afterward orogenic events affected various parts of the Carolina superterane, equally discussed below.

The Carolina superterrane is divisible into belts of low-grade and high-grade metamorphism. Three belts from westward to east are the loftier-grade Charlotte Chugalug, the low-grade Carolina Slate Belt, and a chugalug of mixed high- and depression-course stone along the eastern margin that includes the Raleigh and Kiokee Belts (Figure 23.4). The eastern belt is partially hidden beneath younger rock of the Littoral Apparently. The contact betwixt the Charlotte Belt and the Carolina Slate Belt is depositional along part of its length and a fault along other parts. Both belts are separated from the Raleigh and Kiokee Belts past several faults collectively referred to equally the Eastern Piedmont mistake system. Internal deformation inside each belt is circuitous and variable. High-grade rocks include gneiss, schist, amphibolite, and minor ophiolite (Mocksville and Burks Mountain complexes, Raleigh and Lake Murray gneisses, Battlefield and Blacksburg formations) with Virgilinan, Salinic, and Alleghanian intrusions. Depression-grade rocks consist of silicic volcanic, pyroclastic, and volcanoclastic rocks, basalt, sandstone, and mudstone (Virgilina, Albemarle, South Carolina, and Cary sequences) with Alleghanian intrusions.

The Virgilinan orogeny and assembly of the Carolina superterrane was followed in the Middle Cambrian-Early on Ordovician past deposition of clastic rock along a passive continental margin (Asbill Swimming germination). Late Ordovician-Early on Silurian twoscoreAr/39Ar ages on micas in depression-grade rocks, and on hornblende in high-grade rocks, advise that at least part of the Carolina superterrane was afflicted by Taconic-Salinic metamorphism. On this basis, information technology has been argued that the previously assembled Carolina superterrane was accreted to Laurentia past the close of the Taconic orogeny.

The timing of accretion, however, is controversial and is i of several unsolved mysteries of the Appalachians. In that location is besides evidence in the Carolina superterrane for minor right-lateral strike-slip faulting, deformation, and metamorphism during Middle-Upper Devonian Acadian-Neoacadian orogeny (391 to 358 Ma). On this basis, and on the ground of stiff Neoacadian metamorphism in the underlying Inner Piedmont zones including the True cat Square terrane, it has been suggested that the Carolina superterrane was accreted to Laurentia during Neoacadian orogeny. The thought, as mentioned previously, is that the Carolina superterrane overthrust the Inner Piedmont during collision, causing metamorphism in the underlying rocks.

Still others take argued for an Alleghany collision. This hypothesis is based on the presence of Allegheny-age strike-sideslip faulting, thrust faulting, folding, metamorphism, and plutonism across all of Southern Appalachia, including the Carolina superterrane. The Alleghany orogeny was the first major event to have affected both Carolina and the Inner Piedmont. Orogeny could have resulted from Carolina-Laurentia collision or, alternatively, from final collision of Gondwana against an already accreted Carolina terrane.

Any standoff scenario must business relationship for the presence of Neoacadian and Alleghanian thrust and strike-slip faults across Southern Appalachia. These faults, maybe more than anything else, have altered or subconscious before collision-related structures, in large office creating the controversy. The Central Piedmont Shear zone itself is an Alleghanian (circa 330 Ma) thrust fault that has carried the Carolina zone more than 20 miles above the Inner Piedmont, thereby burial the original suture zone that once separated the two terranes. The Eastern Piedmont Error system was agile every bit a strike-slip fault during Alleghany orogeny.

The presence of strike-skid faults, and the absence of an Acadian foredeep, permit the proffer that the Carolina zone, and maybe many of the Inner Piedmont terranes, collided initially northward of their present location and then sometime later were transported southward along Neoacadian and Alleghanian strike-slip faults. Two variants to the preceding collision scenarios accept into account both strike-sideslip faulting and the presence of the Cat Square terrane, whose circa 430 Ma detrital zircons and circa 380 Ma intrusions imply that it existed as a depositional bowl between 430 and 380 Ma. The first interprets the Cat Foursquare terrane as a pull-apart basin associated with strike-sideslip faulting. The 2d interprets the Cat Square equally a remnant bounding main basin that closed similar a zipper from northward to south during collision of Carolina.

In the first scenario, Carolina collides with Laurentia past the shut of the Taconic orogeny well to the north of its nowadays location, maybe in the vicinity of New Jersey and Delaware. Post-obit collision, the Carolina terrane was rafted away from Laurentia and transported southward thereby opening a narrow pull-apart body of water basin now recognized as the Cat Square terrane that was perhaps similar to the Gulf of California. Carolina then recollided with Laurentia in the Southern Appalachians probably during Neoacadian orogeny. The Cat Foursquare was squeezed, metamorphosed, and plain-featured during Neoacadian orogeny.

In the 2d scenario, Carolina is oriented oblique to Laurentia such that the northern half of Carolina collides sometime after 430 Ma, leaving a pocket-sized ocean basin (the Cat Foursquare terrane) between Laurentia and the southern half of Carolina. This collision also is interpreted to accept occurred in the Fundamental Appalachians. Between 430 and 380 Ma, Carolina rotated clockwise into Laurentia, thereby closing the Cat Foursquare sea basin like a zipper resulting in deformation and metamorphism in the Inner Piedmont. The terranes were later shuffled and transported southward along strike-skid faults. The major divergence in the ii interpretations is that the first involves full collision during Taconic (or maybe early Silinian) orogeny and subsequent opening of a rift basin, whereas the second involves oblique collision entirely during Acadian-Neoacadian orogeny beginning in the north and progressing due south via clockwise rotation.

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Tectonism, Climate, and Geomorphology Spatial and Temporal Perspectives☆

A.R. Orme , in Reference Module in Earth Systems and Ecology Sciences, 2014

Plate Interiors

The interiors of continental plates comprise Precambrian cratons, Phanerozoic cover rocks, and denuded orogens, each a record of a continent′s more afar past ( Table 1 ). Laurentia, North America′s cratonic nucleus, sutured to the Rodinia supercontinent during the Grenvillian orogeny around 1100   Ma (million years before present) (Hoffman, 1989). Separating from Rodinia later on ~   800   Ma, Laurentia′s active eastern (Appalachian) margin experienced several Paleozoic orogenies, including the Caledonian event (~   440–400 Ma) during standoff with Baltica and the Alleghanian event (330–260  Ma) during collision with northwest Africa (Gondwana) equally Pangea assembled. Due south America′s cratonic nucleus included the big Amazonian craton whose rocks were welded in several orogenies before joining Rodinia around k   Ma (Cordani et al., 2000). On separating from Rodinia, these cratons were involved in the Brasiliano–Panafrican orogeny (~   650–550   Ma) that culminated in the assembly of western Gondwana and were covered by Phanerozoic embrace rocks that remain little deformed over wide areas today ( Figure 4 ). The Fennoscandian Shield, Baltica′s exposed craton, comprises Precambrian crystalline rocks and orogens whose later interest in the Caledonian orogeny provides Scandinavia's mountainous courage, although piddling disturbed Neoproterozoic sedimentary rocks survive beneath the Gulf of Bothnia (Lidmar-Bergström and Näslund, 2005). These quintessential plate interiors were amidst many like terranes that formed the late Paleozoic nucleus of Pangea and, on the latter′due south rupture, were unremarkably exposed along newly rifted margins.

Figure 4. Tectonic provinces of South America.

 Reproduced from Orme, A.R., 2007b. Tectonism, climate, and mural change. In: Veblen, T.T., Immature, Grand.R., Orme, A.R. (Eds.), The Concrete Geography of South America. Oxford University Press, New York, NY, pp. 23–44.

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Carbonate Reservoirs

Clyde H. Moore , William J. Wade , in Developments in Sedimentology, 2013

Lower Ordovician Upper Knox Dolomite of the Southern Appalachians, USA

The Lower Ordovician, Upper Knox Grouping and its temporal equivalents, the Arbuckle Grouping in Oklahoma and Arkansas, and the Ellenburger Group in Texas were deposited on a wide and gently sloping platform, several hundred kilometers wide and over 5000   km long, extending from w Texas to Newfoundland. The shallow water carbonates of this platform were extensively dolomitized during and shortly later deposition past marine and modified marine waters controlled past high-frequency sea-level changes (Montañez and Read, 1992).

This huge platform was broken up during the Alleghenian/Ouachita orogeny in the Pennsylvanian–Permian. Today, the Upper Knox group is exposed in the fold and thrust belt of the Valley and Ridge Province of the central and southern Appalachians (Fig. 10.22), while the Arbuckle and Ellenburger produce prodigious quantities of gas in the subsurface of Texas and Oklahoma.

Figure 10.22. Location map showing locations of measured sections and cores within the Valley and Ridge Province, key and southern Appalachians. Major thrust faults are shown every bit heavy black lines: K   =   Knoxville; Southward   =   Saltville; CC   =   Copper Creek; BR   =   Blue Ridge; P   =   Pulaski; Northward   =   Narrows; H   =   Clinchport-Honaker; STC   =   St. Clair; LNM   =   Fiddling Northward Mountain. Shaded area marks location of Mascot-Jefferson Mississippi Valley Type mining commune, east Tennessee.

 From Montañez (1994). AAPG©1994 reprinted by permission of AAPG whose permission is required for further use.

The Upper Knox Group in the Appalachians ranges in thickness from 200 to 1200   k. Thick Knox sequences were deposited in several depocenters separated by positive features such as the Virginia Arch (Fig. x.23) (Montañez and Read, 1992). Today, the Knox forms portions of multiple thrust fault blocks that range vertically from surface outcrops to depths in excess of 1700   m. Figure ten.24 shows a general burial bend for the Knox Group in Tennessee and Virginia. It is estimated that the Knox was buried to depths in excess of 5   km during Alleghenian tectonism and experienced temperatures near 150   °C. Tectonic shortening of the Knox carbonate platform ranges from 25% to lx%, depending on the location (Montañez, 1994).

Effigy 10.23. Upper Knox isopach map on palinspastic base showing locations of selected measured sections and cores. Y   =   Immature Mine core, Jefferson-Mascot District, TN; Tl   =   ARCO core TNC-1, Green Co., TN; T3   =   ARCO core TNC-3, Hamblen Co., TN; T5   =   ARCO cadre TNC-v, Hawkins Co., TN; AB   =   Avens Bridge, Washington Co., VA; V1   =   ARCO core VAC-l, Tazewell Co., VA; V4   =   ARCO cadre VAC-iv, Alleghany Co., VA; W   =   ARCO core WVC-l core, Monroe Co., WV; H   =   Hot Springs core, Alleghany Co., VA.

 From Montañez and Read (1992). Used with permission of GSA.

Effigy ten.24. Generalized burying history plot synthetic for the stratigraphic section on the Saltville thrust sheet (Figure 10.22). Dashed lines signal inferred burial depth by depositional and tectonic overburden, and subsequent uplift.

 Adapted from Mussman et al. (1988). From Montañez (1994). AAPG©1994 reprinted by permission of AAPG whose permission is required for further use.

Figure 10.25 illustrates the paragenetic sequence of the Upper Knox Grouping in east Tennessee. The well-nigh striking aspects of this sequence are that regional burial replacement dolomitization and dolomite cementation events seem to span almost of the Paleozoic burying history of the Knox, punctuated by regional dissolution events. Figure 10.26 shows a schematic of the relationships betwixt burial replacement dolomites and later dolomite cements, dissolution events, and hydrocarbon migration.

Figure 10.25. Paragenetic sequence inferred for Upper Knox carbonates in the study area. Overlapping phases are listed according to their outset appearance. Confined bespeak relative depth/time ranges of the different diagenetic events. The transition between shallow burial and intermediate-to-deep burial is interpreted every bit the onset of stylolitization; the transition from burying to uplift is divers by the initiation of the retrograde thermal history. No temporal significance is unsaid for the time span betwixt Ordovician and Pennsylvanian/Permian periods.

 From Montañez (1994). AAPG©1994 reprinted by permission of AAPG whose permission is required for further use.

Figure ten.26. Schematic dolomite crystal showing paragenetic relationships of dolomite zones 2–6 to noncarbonate diagenetic minerals and dissolution surfaces (DS).

 From Montañez (1994). AAPG©1994 reprinted past permission of AAPG whose permission is required for further utilize.

Figure ten.27 shows a cross-plot of δ xviiiO versus 87Sr/86Sr for Knox dolomites. Early, syndepositional dolomite shows elevated 87Sr/86Sr ratios relative to Ordovician marine waters, suggesting that these have been recrystallized, probably by the fluids responsible for the zones 2 and 3 replacement dolomites. All other dolomite zones show elevated 87Sr/86Sr ratios and progressively lighter δ eighteenO values, suggesting equilibration with waters at elevated temperatures that take interacted extensively with siliciclastics (basinal fluids) (Montañez, 1994).

Figure 10.27. Crossplot of dolomite δ 18O and 87Sr/86Sr values. Inside zone 2/R field, crosses   =   late diagenetic replacement dolomites, filled squares   =   zone two dolomite cement. Marine field from Lohmann and Walker (1989), Popp et al. (1986), Burke et al. (1982), and Gao and Land (199l).

 From Montañez (1994). AAPG©1994 reprinted by permission of AAPG whose permission is required for further employ.

While there is niggling incertitude that the elevated temperatures and pressures encountered in the subsurface tend to favor dolomitization (Hardie, 1987; Morse and Mackenzie, 1990), the volume of water required for these deep burial (ane–2.v   km) dolomitization events is enormous. Several studies accept concluded that compactional dewatering cannot provide the volume of water or deliver the solutes necessary for regional dolomitization (Machel, 2004; Montañez, 1994). Montañez (1994) concludes that compressional loading associated with the Alleghanian/Ouachita orogeny in the Carboniferous was responsible for the voluminous and rapid motion of dolomitizing and dissolutional fluids into and through the Knox carbonates ( Fig. 10.28). This interpretation is supported past the thermal history of the fluids responsible for various diagenetic phases every bit estimated from isotopic compositions and two-phase fluid inclusions (Fig. 10.29). These data suggest an early prograde (increasing temperature) and later retrograde (decreasing temperature) thermal history that mirrors the tectonic loading (prograde) and subsequent uplift (retrograde) of the platform expected during an orogenic event. MVT mineralization and hydrocarbon migration occurred near the end of the loading stage, when temperatures would take been at their maximum.

Figure 10.28. Schematic diagram illustrating fluid flow pathways from Alleghenian highlands westward through the thrusted Paleozoic department. Stipple pattern   =   upper Knox carbonates; cross-hatched design   =   decollement sediments. Structure cross section for southwestern Virginia after Woodward (1987). R   =   Richlands thrust fault; STC   =   St. Clair thrust error; North   =   Narrows thrust fault; S   =   Saltville thrust fault; P   =   Pulaski thrust error; LM   =   Fiddling North Mountain; F   =   Fries thrust fault. Magnitude of estimated tectonic relief based on estimated overburden thickness and fluid inclusion homogenization temperatures. Arrows announce interpreted movement of fluids through Knox carbonates and along thrust planes. Catamenia is cross-formational at the elevated recharge expanse of the basin and strongly focused and channeled along the deeply cached Knox carbonate intervals and thrust planes cratonward.

 From Montañez (1994). AAPG©1994 reprinted past permission of AAPG whose permission is required for further use.

Figure 10.29. Summary diagram of the estimated precipitation temperatures for late diagenetic minerals in Knox carbonates. Dashed lines represent temperature ranges derived indirectly from the oxygen isotopic values of zoned dolomites. Solid lines represent temperature ranges obtained from fluid inclusion homogenization temperatures. Calculated means indicated by symbols: dots   =   this study; triangles   =   from Kesler et al. (1989) and Haynes et al. (1989); X   =   from Caless (1983).

 From Montañez (1994).

How did these fluids movement through the Knox and what impact did they have on porosity and permeability of the Knox carbonates? Montañez (1997) recognized that superior porosities and permeabilities in Knox carbonates appear to occur preferentially in TSTs of low-frequency sea-level cycles (Fig. ten.30). Low rates of accommodation infinite generation during HSTs resulted in pervasive dolomitization, small crystal sizes, and therefore low permeabilities. Dolomitization in the TST was incomplete and consisted of larger, isolated crystals in a matrix of pellet or ooid grainstone, packstone, or wackestone. Upon burying, when the first pulses of hot dolomitizing fluids began to move through the Knox, the TSTs had higher porosities and permeabilities and acted as preferential fluid conduits. In the HSTs, minor crystal sizes and depression permeabilities caused rapid nucleation of dolomite cements, rapid recrystallization, and degradation of remaining porosity. Meanwhile, isolated nucleation sites resulted in fibroid replacement dolomite crystals in the TSTs, while balance calcite was dissolved during the dolomitizing process, leading to enhanced porosities. Montañez (1997) suggests that these conduits remained as the major pathways for after diagenetic events such as MVT mineralization and hydrocarbon (re)migration.

Figure 10.30. Measured porosity of dolomitized subtidal and tidal-flat samples and limestone samples plotted by their occurrence in stacks of regressive or transgressive cycles. Plot shows that dolomitized transgressive bicycle facies have porosity values that either equal or significantly exceed the range of porosity values of dolomitized regressive cycle facies. Limestone facies have the lowest measured porosity and smallest range in values.

 From Montañez (1997). Used with permission of SEPM.

The tectonically driven late diagenetic history of Upper Knox carbonates strongly parallels the models of late dolomitization documented in the Ordovician Ellenberger in Texas (Kupecz and State, 1991) and Arbuckle in Oklahoma (State et al., 1991).

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