The Walker Top Granite (here formally named) is a peraluminous megacrystic granite that occurs in the Cat Square terrane, Inner Piedmont, part of the southern Appalachian Acadian-Neoacadian deformational and metamorphic core. The granite occurs as disconnected concordant to semi-concordant plutons in migmatitic, sillimanite zone rocks of the Brindle Creek thrust sheet. Locally garnet-bearing, the Walker Top Granite contains blocky alkali feldspar megacrysts 1–10 cm long in a groundmass of muscovite-biotite-quartz-plagioclase-alkali feldspar and accessory to trace zircon, titanite, epidote, sillimanite (xenocrysts), and apatite. It varies from granite to granodiorite and contains several xenoliths of biotite gneiss, amphibolite, quartzite, and in one location encloses charnockite (here formally named Vale Charnockite). New sensitive high-resolution ion microprobe U-Pb zircon magmatic crystallization ages obtained from the plutons of the Walker Top Granite are: 407 ± 1 Ma in the Brushy Mountains; 366 ± 2 Ma in the South Mountains; and 358 ± 5 Ma in the Vale–Cat Square area. An age of 366 ± 3 Ma was obtained from the Vale Charnockite at its type locality. Major-, trace-element, and isotopic chemistry indicates that Walker Top is a high-K, peraluminous granite, plotting as volcanic arc or syn-collisional on tectonic discrimination diagrams and suggests that it represents deep-seated anatectic magma with S- to I-type affinity. The alkali calcic, ferroan Vale Charnockite likely formed by deep crustal melting, and similar geochemical and trace-element compositions suggest a similar tectonic origin as Walker Top Granite. The discontinuous nature of the Walker Top Granite plutons precludes it intruded as a volcanic arc. Instead, the peraluminous nature, common xenoliths of surrounding country rock, and geochemical and isotopic signatures suggest it formed by partial melting of Cat Square and Tugaloo terrane rocks. Following emplacement and crystallization, Walker Top plutons were deformed into elliptical to linear shapes—SW-directed sheath folds—enveloped by partially melted, pelitic and quart-zofeldspathic rocks. Collectively, Walker Top and other plutons helped weaken the crust and facilitate lateral crustal flow in a SW-directed, tectonically driven orogenic channel during the Acadian-Neoacadian event. A comparison with the northern Appalachians recognizes a similar temporal magmatic and deformational history during the Acadian and Neoacadian orogenies, although while the Walker Top Granite intruded the lower plate during eastward subduction beneath the peri-Gondwanan Carolina superterrane, the northern Appalachian plutons intruded the upper plate during subduction of the Avalon superterrane westward beneath Laurentia. We hypothesize that a transform fault, located near the southern end of the New York promontory, accommodated oppositely directed lateral plate motion and different subduction polarity between the Carolina and Avalon superterranes during the Acadian and Neoacadian orogenies.

The Appalachian Inner Piedmont comprises the deformational and meta morphic core of the middle Paleozoic Acadian-Neoacadian (410–350 Ma) orogen and is one of the largest continuous exposures of sillimanite zone rocks in the world. It extends from southern Virginia (Milton terrane—Inner Piedmont east of the Sauratown Mountains window) through the Carolinas, Georgia, and Alabama to the Coastal Plain overlap (Fig. 1). While the flanks of the terrane are lower grade, most of its internal parts are sillimanite to sillimanite II zone migmatite, revealing a history of peak metamorphism and deformation in the middle crust. Accompanying this assemblage of metasedimentary and metavolcanic rocks are numerous deep-seated felsic plutons that were intruded during all three Paleozoic orogenies that affected this part of the Appalachians. The western flank of the Inner Piedmont is dominated by Ordovician–Silurian plutons intruded during the Taconic orogeny (e.g., the extensive Henderson Gneiss [ca. 450 Ma]; Moecher et al., 2011; Huebner et al., 2017), but some plutons related to the Alleghanian orogeny also occur here and may be less deformed (e.g., the Stone Mountain [337 ± 3 Ma] and Elberton Granites [302 ± 3 Ma]; Mueller et al., 2011). Only small bodies of Mesoproterozoic basement have been recognized in the western Inner Piedmont (Merschat, 2009; Merschat et al., 2012; Huebner and Hatcher, 2017; Huebner et al., 2017), and the western Inner Piedmont contains a distinctly Laurentian detrital zircon suite (Bream, 2003; Bream et al., 2004). No Mesoproterozoic basement bodies have been mapped in the eastern Inner Piedmont, Cat Square terrane, and no plutons older than 407 Ma are known in this segment.

The central and eastern Inner Piedmont is composed of the Cat Square terrane, an assemblage of metasedimentary and metavolcanic(?) rocks that are separated from the western Inner Piedmont by the Brindle Creek fault (Giorgis, 1999). The Cat Square terrane was recognized as a separate tectonostratigraphic terrane based on its unique detrital zircon assemblage that reveals sediment derived from both the Laurentian assemblages to the west and the exotic Carolina superterrane to the east (Bream, 2003; Bream et al., 2004).

This study examines the field relationships, petrology, geochemistry, and geochronology of the deep-seated megacrystic Walker Top Granite (here formally named) in the eastern Inner Piedmont, Cat Square terrane. Megacrysts—large, centimeterscale (here, up to 10 cm) euhedral crystals—of alkali feldspar give the granite a distinct and characteristic porphyritic texture. Over the course of nearly two decades of geologic mapping in central-western North Carolina, different plutons and bodies of megacrystic Walker Top Granite were identified, mapped in detail, and studied, culminating in several areas of detailed study of the geochemistry, geochronology, petrology, and structural and kinematic relationships between the Walker Top Granite and surrounding high-grade schist and gneiss (Table 1). These areas include: (1) the South Mountains, North Carolina, the here-defined type area of the Walker Top Granite (for additional description, see Giorgis et al., 2002); (2) the Brushy Mountains, North Carolina; and (3) Cat Square and Vale, North Carolina (Figs. 1 and 2), where megacrystic granite contains a charnockite xenolith. From these areas, three samples of Walker Top Granite and the charnockite were collected for U-Pb geochronology.

The age and petrogenesis of the Walker Top Granite have important tectonic implications for Acadian and Neoacadian orogenies in the southern Appalachians and the development of the melt-weakened crust of the Inner Piedmont, which accommodated lateral ductile flow of the orogen (e.g., Hatcher and Merschat, 2006). Despite temporal and limited lithostratigraphic correlation between the Acadian and Neoacadian orogenic events in the southern and northern Appalachians (e.g., Merschat and Hatcher, 2007), there are fundamental geometric and kinematic differences between the polarity of subduction in the northern and southern segments. We consider the chronology and petrogenetic formation of the Walker Top Granite to hypothesize about possible plate configurations that would reconcile these fundamental differences.

The Inner Piedmont was recognized decades ago as a zone of high-grade rocks that extend from the Carolinas to Alabama (Crickmay, 1952; geologic maps of Alabama, Georgia, and North Carolina), but King (1955) recognized it as a distinct unit of high-grade gneisses of unknown internal structure separate from the Blue Ridge to the west and the Carolina superterrane to the east. Griffin (1969, 1971, 1974) began working in the Inner Piedmont during the 1960s and first hypothesized that it consists of a stack of map-scale westward-vergent recumbent isoclinal fold nappes analogous to the dominant structures in the Pennine Alps. He also proposed that these structures fit a “stockwork” model with an infrastructure and suprastructure separated by an abscherung zone, analogous to Haller’s (1956) model for the East Greenland Caledonides. Griffin recognized and named several nappes in the South Carolina Inner Piedmont and concluded that they are west-vergent structures that comprise the migmatitic core of the Inner Piedmont—the infrastructure—whereas the now-eroded Carolina superterrane comprises the suprastructure emplaced above the mobile migmatitic core. Griffin suggested the highly complex Brevard fault zone comprises part of the abscherung zone, which must also be preserved along the southeastern flank of the Inner Piedmont where it contacts the Carolina superterrane (Charlotte and Carolina slate belts). It was identified there as the central Piedmont suture by Hatcher and Zietz (1980). The shear zone character of the boundary was recognized by Horton (1981). The Kings Mountain belt (Horton et al., 1981; Horton, 2008; Dennis, 2014) may be a remnant abscherung zone along the central Piedmont suture. Bentley and Neathery (1970) recognized similar structures in the Inner Piedmont in Alabama, and they also concluded the structures are west-vergent. The nappe hypothesis has been supported by more recent geologic mapping in the Inner Piedmont in Georgia (e.g., Hooper, 1986; Higgins et al., 1988; Hopson and Hatcher, 1988; Huebner, 2013; Rehrer, 2014) and in the Carolinas (e.g., Hatcher and Acker, 1984; McConnell, 1989; Liu, 1991; Davis, 1993; Giorgis, 1999; Hatcher, 2000, 2001b; Bier, 2001; Hatcher and Liu, 2001; Hatcher et al., 2001; Merschat, 2003; Wilson, 2006; Byars, 2010). These studies helped define a polyphase deformation history for the Inner Piedmont. A thorough description of the deformations is provided in Merschat et al. (2005) and Hatcher and Merschat (2006); the deformations include: D1 early foliations in boudins and intrafolia folds; D2 penetrative high-grade fabrics S2, L2, and F2, and primary nappe and fault formation; D3 associated with tertiary foliation, folds, and lineations, locally well developed; and D4–D5 related to Alleghanian deformation along the Brevard fault zone and crenulations to broad open folds.

Modern structural geological criteria, including mineral lineations and shear-sense indicators, however, have determined that the transport direction of the nappes and mid-crustal flow varies within the Inner Piedmont but is dominantly southwest-directed (e.g., Hatcher, 2001a, his fig. 3; Hatcher, 2002, his fig. 10; Merschat et al., 2005, their fig. 2; Hatcher and Merschat, 2006, their fig. 3). A curved lineation pattern is interpreted to result from a process that involved massive, middle-crustal flow, with a component of simple shear, in a tectonically forced, southwest-directed orogenic channel (Hatcher and Merschat, 2006). The tectonic driving force was hypothesized to be the Devonian–Early Mississippian collision of Carolina superterrane with the previously accreted Taconian Laurentian sourced tectonostratigraphic terranes and the Laurentian continent to the west (Merschat et al., 2005; Hatcher et al., 2007; Merschat and Hatcher, 2007; Hatcher, 2010; Merschat et al., 2010).

Detailed geologic mapping and secondary ionization mass spectrometry (SIMS) detrital zircon geochronology revealed that the Inner Piedmont contains a terrane boundary that separates sequences to the west that can be tied by both stratigraphy and detrital zircon geochronology to a peri-Laurentian source from rocks to the east of the boundary that have a mixed Laurentian and peri-Gondwanan detrital zircon source (Bream, 2003; Bream et al., 2004; Merschat et al., 2010). The boundary, the Brindle Creek fault, was first recognized by Giorgis (1999) and has been traced throughout the Carolinas and Georgia Inner Piedmont where it truncates against the Towaliga fault on the northwest limb of the Pine Mountain window (Fig. 1; Huebner et al., 2014, 2017). Kinematic shear sense of the Brindle Creek fault has been determined to be dextral with the east side of the fault directed toward the southwest, while the northwest boundary of the western Inner Piedmont, the Brevard fault zone, also has a dextral movement sense (Bobyarchick, 1984; Merschat et al., 2005, 2010; Hatcher and Merschat 2006; Hatcher et al., 2017; Huebner et al., 2017).

The structural style within the Inner Piedmont is delineated partly by the mesoscopic structures (e.g., fieldwork of Griffin and others referenced above) and also by the geometries of plutons in the western Inner Piedmont, particularly in the South and Brushy Mountains in North Carolina. Plutons in the westernmost Inner Piedmont in the Carolinas and northeastern Georgia have a very linear outcrop pattern (Fig. 2). In contrast, plutons in the central and eastern parts of the Inner Piedmont, including many in the Cat Square terrane, have a more irregular to tabular shape and locally round to elliptical shape (Fig. 2). This is true regardless of age, including plutons of the same group (e.g., the Walker Top Granite plutons) despite all being deep seated.

Prior to modern U-Pb geochronologic studies, granitic magmatism in the Inner Piedmont was largely considered to be Cambrian to Ordovician (Goldsmith et al., 1988; Fullagar et al., 1997; Nelson et al., 1998). In the North Carolina Inner Piedmont, Goldsmith et al. (1988) mapped several disconnected, irregular to elliptical-shaped plutons of Toluca Granite, a leucocratic medium-grained granite with subordinate porphyritic granite and granodiorite and a porphyritic to gneissic Sandy Mush granite (Figs. 1 and 2). The Cherryville Granite, located in the eastern Inner Piedmont near the central Piedmont suture (355 ± 2 Ma; Mapes, 2002), is a medium- to coarse-grained, weakly foliated, two-mica granite. Also mapped by Goldsmith et al. (1988) was a megacrystic granite that was interpreted to be the less deformed protolith of the Henderson Gneiss, a ca. 450 Ma granitic augen gneiss (Moecher et al., 2011; Huebner, 2013; Huebner et al., 2017), which forms a large elliptical body, and several smaller linear satellite bodies in the western Inner Piedmont (Fig. 1). These less deformed bodies previously thought to be protoliths of the Henderson Gneiss are what we recognize as the Walker Top Granite. A charnockite xenolith was identified in a Walker Top Granite pluton in a road cut near Vale, North Carolina (Goldsmith et al., 1988). Originally considered possible Mesoproterozoic basement in the Inner Piedmont, Kish (1997) obtained discordant U-Pb zircon ages ranging from 357 to 348 Ma and suggested the middle Paleozoic age is related to the high grade of metamorphism in the Inner Piedmont. SIMS U-Pb data (presented herein), however, revealed a Devonian age for this xenolith and a clearer picture of Paleozoic magmatism in the Inner Piedmont: (1) Ordovician to Silurian magmatism occurred in the Tugaloo terrane; and (2) Devonian to Mississippian magmatism was partitioned into the Cat Square terrane and the Carolina superterrane to the east (e.g., Mapes, 2002; Bream, 2003; Huebner and Hatcher, 2017; Huebner et al., 2017). This temporal and spatial variation of plutonism further refined the differences between the Tugaloo and Cat Square terranes (e.g., Hatcher et al., 2007).

Detailed geologic mapping delineated bodies of Walker Top Granite, as well as their macro- and meso-scale structure and field relationships. Representative samples of the Walker Top Granite and Vale Charnockite were collected for thin section, whole-rock geochemistry, and U-Pb geochronology (Mapes, 2002; Gatewood, 2007; Byars, 2010). Whole-rock geochemical samples were cut into thin slabs on a trim saw, rinsed with isopropyl alcohol and dried, broken into 1.0 cm × 0.5 cm fragments, mixed for homogeneity, and crushed into a fine powder using an alumina ceramic mill and shatterbox. Approximately 30 g of each sample were separated and sent to Activation Laboratories in Ancaster, Ontario, for whole-rock geochemical analysis. Major elements and Ba, Be, Sr, V, and Y were determined using inductively coupled plasma (ICP) emission spectroscopy employing lithium metaborate and tetraborate fusion (FUS-ICP). Total digestion (TD-ICP) methods were used to determine Ag, Cd, Cu, Ni, Pb, S, and Zn. Trace and rare-earth elements (REEs) were determined by fusion mass spectrometry (FUS-MS) methods and instrumental neutron activation analysis (INAA) (Table S11). Data plots were constructed using Igpet2018, and CIPW norms were calculated using the CIPW application.

Zircon separation and processing were carried out at Vanderbilt University and the Stanford University–U.S. Geological Survey Micro Analysis Center (SUMAC). Initial crushing of the samples to gravel-sized fragments and smaller was done by hand. Samples were further crushed and sieved to <500 μm. Density sorting was completed using a water table followed by heavy liquids to separate out less dense minerals. Magnetic minerals were removed using a Frantz magnetic separator. Twenty to 30 zircon grains were handpicked under a binocular microscope for analysis. At SUMAC, these grains were mounted in epoxy and polished to the average grain center. Cathodoluminescence (CL) images were collected using a Jeol 5600 LV scanning electron microscope equipped with a Hamamatsu photomultiplier. CL images with reflected light images, also collected at SUMAC, were used to assess shape, zoning, morphology, and structural integrity (presence or absence of fractures and inclusions) of grains and guide beam placement prior to analysis. The sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) was operated under standard operating conditions (Bacon et al., 2000). Uranium- Pb- Th, trace-, and REE analyses were performed in sequential analytical sessions using a 15–20 μm spot and methods described by Mazdab and Wooden (2006). Standard R33 (ca. 419 Ma; Black et al., 2004), provided by SUMAC, was measured every fifth analysis. All data (Table 2) were reduced using the computer program SQUID v. 1.02 (Ludwig, 2001). Age calculations and geochronologic plots were produced with Isoplot v. 3.0 (Ludwig, 2003) and IsoplotR (Vermeesch, 2018). Concordia and weighted-average plots at 2σ error were made using 204Pb-corrected 206Pb/238U ages unless otherwise indicated. Metamorphic rim ages were identified in CL images and were not used to determine crystallization ages.

The type area of the Walker Top Granite, originally described in Giorgis et al. (2002), is located on the northwest-facing slopes of Walker Top and Burkemont Mountains in the southwestern quarter of the Morganton South 7.5-minute quadrangle, North Carolina. The best outcrops occur below the ridge crest as small cliffs on the steep, northwest-facing slopes of Walker Top and Burkemont Mountains (35.646° N, 81.72° W) and road cuts along Burkemont Mountain Road near Walker Top Mountain (35.649° N, 81.707° W to 35.641° N, 81.723° W).

In the literature, the name Walker Top Granite has been applied to similar megacrystic granites in the northern Cat Square terrane (north of 35 °N) in North Carolina and parts of South Carolina (Merschat et al., 2005; Hatcher and Merschat, 2006; Hatcher et al., 2007; Merschat and Hatcher, 2007; Huebner et al., 2017). Other megacrystic granite bodies occur in parts of eastern Inner Piedmont and Cat Square terrane in North Carolina (Sandy Mush granite; Goldsmith et al., 1988), South Carolina (Anderson Mill, Gray Court, and Pelham granites; Nelson et al., 1998; Mapes, 2002), and Georgia (High Falls Granite; Huebner et al., 2017) (Figs. 1 and 2). Although these units are texturally and chemically similar (Mapes, 2002; Huebner et al., 2017), they are not yet correlated with the Walker Top Granite, and additional studies are needed to confirm any relations.

The Walker Top Granite occurs in migmatitic, sillimanite zone rocks of the Cat Square terrane in the Brindle Creek thrust sheet. The disconnected bodies are largely tabular and concordant to semi-concordant with regional structures and foliation (likely transposed); some are more elongate; others are more elliptical (Goldsmith et al., 1988; Merschat et al., 2005). The various plutons of Walker Top Granite exhibit typical characteristics of deepseated, catazonal plutons (Table 1; Buddington, 1959; Raymond, 1995). This study examines three plutons from central-western North Carolina in the Bushy Mountains (north), South Mountains and type area (south), and near Vale and Cat Square crossroads (Vale–Cat Square pluton) where detailed geologic mapping was paired with petrologic and geochronologic studies (Figs. 1 and 2).

The Walker Top Granite is a medium-gray, garnet-bearing, foliated megacrystic granite to granodiorite and locally a mylonitic porphyroclastic gneiss (Figs. 35). It is characterized by 1–10-cm-long alkali feldspar megacrysts with common Carlsbad twins and myrmekite rims. The simple Carlsbad twins common of the alkali feldspar megacrysts indicate a magmatic origin (Vernon, 2004); however, subsequent solid-state deformation has converted the megacrysts to microcline (Figs. 3E and 5B). Plagioclase phenocrysts are reported from a few samples (Mapes, 2002). The alkali feldspar megacrysts and mantled myrmekite rims are aligned parallel to subparallel to the foliation and are locally porphyroclastic (Figs. 3 and 4). Goldsmith et al. (1988) suggested that the alignment of megacrysts was, at least locally, a magmatic flow foliation; however, our field observations and microstructural fabric data have not confirmed this suggestion. The megacrysts are set in a darker-gray, medium- to coarse-grained matrix of muscovite, biotite, quartz, alkali feldspar, and plagioclase; this matrix is interpreted to be a primary magmatic assemblage, although some muscovite and other phases may invariably result from post-crystallization solid-state deformation. The abundance of mm-sized biotite in the matrix commonly produces a schistose texture. Accessory minerals include zircon, titanite, epidote, sillimanite (xenocrysts), and apatite. Although some epidote may be igneous, most appears to be secondary. Modally, the Walker Top Granite varies from granite to granodiorite in the IUGS classification of granitoids (Giorgis et al., 2002; Gatewood, 2007; Byars, 2010). The alkali feldspar megacrysts are commonly poikilitic with inclusions of biotite, quartz, plagioclase, and garnet (Fig. 3B). Garnet, which is locally common (Fig. 5A), ranges in diameter from 1 to 11 mm and occurs as skeletal or partially resorbed crystals in the matrix and as inclusions in megacrysts, indicating an igneous origin and early crystallization history (Fig. 3B).

Xenoliths of amphibolite, metagabbro, migmatitic metagraywacke, quartzite, charnockite, and aluminous schist are present in the Walker Top Granite. Quartzite xenoliths occur in Walker Top Granite in the type area along Burkemont Road (Hatcher, 2002); however, migmatitic metagraywacke (biotite gneiss to schist) and amphibolite are more common in many outcrops throughout different bodies of the granite. An enclave of orthopyroxene-bearing metagabbro was mapped in the South Mountains, North Carolina (Giorgis, 1999). Charnockite, mapped by Goldsmith et al. (1988), is limited to its type locality, herein designated as the roadcut outcrop ~100 m west of Vale Post Office, North Carolina, on North Carolina State Road 1113 (Reepsville Road), 35.540278° N, 81.398056° W. Informally named by Byars (2010) because of its location in Vale, North Carolina, it is here formally proposed as Vale Charnockite and replaces the informal name Cat Square charnockite of Kish (1997). The main occurrence of charnockite is in the western portion of the outcrop, a large, lenticular xenolith that is 3.7 m long and has a variable thickness ranging from 0.7 m to 1.1 m (Fig. 5E). Along the length of the outcrop, several other smaller pods up to 1 m in length can be found with contact relationships similar to the larger xenolith.

The Vale Charnockite and Walker Top Granite can be difficult to distinguish in outcrop due to parallel foliations, textural similarities (moderately foliated, megacrystic, inequigranular gneiss) (Fig. 5E), and appearance of weathered surfaces. On closer inspection, fresh Vale Charnockite has a dark, brown-green color with semitranslucent, brownish-olive green alkali-feldspar megacrysts mantled by thin myrmekite rims (Fig. 5C). Fine-grained, subhedral to anhedral phenocrysts of orthopyroxene help define the foliation with lesser amounts of biotite compared with the biotite-rich, pyroxene-absent Walker Top Granite (Figs. 5B and 5D). The contact relationship between the charnockite and Walker Top Granite varies from sharp to gradational (Fig. 5E). At the gradational contact, foliation, grain size, and grain shape are concordant across a color change from whitish-gray to translucent brown-green feldspars in the charnockite. Compositionally, there is a transition from orthopyroxene-clinopyroxene-hornblende-biotite–bearing mafic segregations in the charnockite to dominantly biotite in mafic foliation bands of the Walker Top Granite.

The Vale Charnockite consists of alkali-feldspar megacrysts surrounded by a matrix of plagioclase (An44) and minor quartz. Mafic foliation bands are composed mainly of biotite and orthopyroxene with minor amounts of fine-grained clinopyroxene, hornblende, ilmenite, and garnet (Fig. 5D). Accessory minerals include epidote, apatite, pyrite, titanite, and zircon. Orthopyroxenes break down to biotite and hornblende, indicative of small amounts of water present in the melt, late crystallization, or post-crystallization hydration during retrograde metamorphism (Byars, 2010).

Geochemistry

Major- and trace-element geochemistry of samples from the Brushy Mountains, South Mountains, and Vale–Cat Square plutons of the Walker Top Granite are similar. Compositionally, the Walker Top samples are granite to granodiorite (Fig. 6A) and contain 65–75% SiO2 (Table S1, see footnote 1). Most major elements (Al2O3, MgO, FeO, TiO2, CaO, and P2O5) show an inverse (decreasing) trend with increasing SiO2 (Fig. 7). The concentrations of Na2O and K2O vary from 0.9%–1.7% and 3.4%– 7.2%, respectively; there is no strong trend between SiO2 and the alkalis (Fig. 7). The Vale Charnockite (B9CH) and the Walker Top granodioritic sample from the South Mountains pluton (WT-1) have the least SiO2, 59% and 62%, respectively (Figs. 6 and 7). All samples are peraluminous—as expected based on observed magmatic garnet (Fig. 4B) and muscovite—except the metaluminous charnockite, and all samples broadly plot along calc-alkalic and alkalic-calc trends (Fig. 6D).

Chondrite-normalized (Sun and McDonough, 1989) REE patterns are moderately depleted in heavy REE, although Walker Top samples from the Vale–Cat Square pluton, B9WT, reveal a slight increase in some heavy REE (Fig. 8). Walker Top samples from the Brushy Mountains pluton exhibit the most depletion of heavy REE. All samples display negative Eu anomalies. Normalized to primitive mantle (Sun and McDonough, 1989), samples contain minor to moderate depletion in Ba, Nb, Ta, Sr, P, and Ti, and rather pronounced positive anomalies in Pb and Zr. These patterns are similar to other megacrystic granites of the eastern Inner Piedmont Cat Square terrane from North Carolina to Georgia (Fig. 8; Mapes, 2002; Huebner et al., 2017).

Four samples of megacrystic Walker Top Granite were collected for U-Pb geochronology from different spatially disconnected plutons separated by distances of 30–70 km. These samples were collected from the areas of detailed mapping and study in the Cat Square terrane and include: (1) Icy Knob near the type area on Walker Top Mountain in the South Mountains; (2) Russell Gap, in the Moravian Falls 7.5-minute quadrangle, Brushy Mountains, North Carolina; and (3) megacrystic granite and enclosed Vale Charnockite xenolith at Vale, North Carolina. Results of U-Pb zircon geochronology are reported in Table 2.

South Mountains, Icy Knob (IK)

Megacrystic Walker Top Granite from Icy Knob, near the type area in the South Mountains, contains mostly acicular, doubly terminated zircons. The zircons are 200–550 μm long with aspect ratios between 2:1 and 6:1. Internal zoning is normally concentric oscillatory zoning, although some grains contain rounded zones that mark areas that have been partially resorbed after formation of the original zircon (Fig. 9). Excluding one analysis with significant Pb-loss, 206Pb/238U ages range from 388 to 344 Ma (Fig. 9). A weighted-mean age for the data is 366 ± 2 Ma (mean square of weighted deviates [MSWD] = 3.13, 2 sigma error); the age is interpreted to be the age of magmatic crystallization.

Brushy Mountains, Russell Gap (MV-19-WT)

Tabular-shaped, sill-like bodies of Walker Top Granite are continuous along strike in the Brushy Mountains for >30 km (Fig. 2) and outline map-scale sheath folds near the base of the Brindle Creek thrust sheet (Merschat et al., 2005). The Walker Top Granite is penetratively deformed and is locally mylonitic. The sample locality (MV-19-WT) in Russell Gap is from a roadcut ~4 km from the intersection of NC18 and Russell Gap Road (Fig. 2).

Zircons from Walker Top Granite in Russell Gap are characteristically large (200–300 μm). In CL, the grains have oscillatory zoned cores interpreted to be magmatic and dark rims (20–50 μm) interpreted to be metamorphic (Fig. 10A). Except for a few analyses of the metamorphic rims, an effort was made to analyze oscillatory zoned regions of zircons from this sample.

Twenty-four zircons were analyzed, and 206Pb/238U ages range from 442 to 374 Ma; these zircons include 18 analyses of cores and six analyses of rims based on CL imaging. Eight of these zircons yield very high U concentrations, including three anomalously old ages (Fig. 10B). SHRIMP U-Pb analysis of high-U (>2500 ppm) zircons commonly results in older 206Pb/238U ages due to matrix effects associated with radiation damage (e.g., White and Ireland, 2012). These eight analyses are not considered further. Of the remaining 16 analyses, 14 yield a weighted average of 407 ± 1 Ma (MSWD 1.22; Fig. 10C). This age probably represents the magmatic crystallization age of Walker Top Granite in the Brushy Mountains, based on zircon morphology and textures (Fig. 10A). The remaining two analyses are interpreted to be metamorphic and yielded 206Pb/238U ages of 374 ± 5 and 368 ± 5 Ma, although both are reversely discordant.

Walker Top Granite (B9WT) and Vale Charnockite (B9CH), Vale, North Carolina

Two elongate elliptical bodies of Walker Top Granite occur east of the South Mountains near Cat Square, North Carolina—one truncates against the Brindle Creek fault, which frames the Newton window, and the other body trends NNW-SSE (Fig. 2). These bodies are interpreted to be on the flanks of a map-scale F2 sheath fold (Byars, 2010) and suggest the original geometry of the Walker Top Granite was a thick, tabular pluton. The Vale Charnockite occurs (Byars, 2010) as several xenoliths within the Vale–Cat Square pluton of the Walker Top Granite at its type locality. Samples of both the Walker Top Granite and charnockite from the larger xenolith in the western part of the roadcut were collected for U-Pb geochronology.

Walker Top Granite zircons range from 200 to 675 μm long and 75–280 μm wide and have aspect ratios between 2:1 and 6:1. The grains are commonly euhedral to subhedral, acicular, and doubly terminated where not broken. CL imaging reveals normally concentric to modified oscillatory zoning in the cores of the zircons with minor metamorphic overgrowths (Fig. 11A). Metamorphic rims typically embay and truncate zoning in zircon cores. Some grains display convoluted zoning: this may be the product of recrystallization transgressing into the grain driven by dissolution or regrowth along fractures.

Vale Charnockite zircons morphologically resemble those of the Walker Top Granite (B9WT). Grains range from 175–550 μm long, 75–175 μm wide, and have aspect ratios between 2:1 and 5:1. Reflected light images reveal that inclusions are more abundant in zircons from the charnockite than the Walker Top Granite zircons. Most zircons have normally concentric to modified oscillatory zoning with dark overgrowths in CL (Fig. 11B). Dark CL zones locally embay and truncate zoning in bright CL zircon cores, suggesting they are metamorphic. Grains that are not zoned are fairly uniform in color and may have been dissolved and reprecipitated during metamorphism. Shapes range from euhedral to subhedral, acicular to stubby, and are commonly doubly terminated.

All zircons analyzed from the Walker Top Granite (B9WT) and Vale Charnockite (B9CH) yielded U-Pb ages ranging from Late Devonian to Early Mississippian. No inherited cores were observed or measured. Some rims returned older ages than the zircon interior. This could result from subsequent recrystallization of zircon along a fracture that penetrated the zircon core, as observed in CL images, or Pb loss or U enrichment in parts of the grains. Where the cores and rims were measured on the same grains, they typically overlap at 2σ and suggest the metamorphic event closely followed crystallization.

The data from all Walker Top zircons are concordant within 2σ error, mainly clustering between 365 and 350 Ma (Fig. 11B). A coherent group of ten Walker Top Granite zircons, interpreted to be magmatic cores, produced 206Pb/238U crystallization ages ranging from 366 to 348 Ma. Three points were not used because of elevated 204Pb, or because a younger age was analyzed in the core than the rim. A weighted mean for these seven grains gave an age of 358 ± 5 Ma (MSWD = 0.84; Fig. 11E). When all ten analyses are taken into consideration, a weighted mean yielded an age of 355 ± 5.2 Ma (MSWD = 1.2). Metamorphic rims, determined from zircon morphology, produced 206Pb/238U ages of ca. 351 Ma and ca. 342 Ma. Vale Charnockite zircons produced concordant 206Pb/238U ages with crystallization ages ranging from 373 to 361 Ma from a coherent group of nine analyses. Metamorphic rims yielded ages of ca. 361 Ma, ca. 359 Ma, and ca. 355 Ma. Analyses with elevated amounts of 204Pb and a reverse of core-rim ages were excluded. A weighted mean of the nine crystallization ages gave an age of 366 ± 3 Ma (MSWD = 2.17; Fig. 11F). Data for all 14 analyses yield a weighted mean of 365 ± 3 Ma (MSWD = 4.8). Concordant data, within 2σ error, cluster between 370 and 360 (Fig. 11D). Based on observed grain shape and morphology, these ages are interpreted as the magmatic crystallization ages of the Walker Top Granite and Vale Charnockite. The few zircons described above that appeared to be wholly metamorphic in origin were not analyzed.

Some tectonic models for the Walker Top Granite have suggested it is the product of syncollisional events and partial melting of Cat Square terrane metasediments (Hatcher and Merschat, 2006; Merschat and Hatcher, 2007; Huebner et al. 2017), but other models suggest the megacrystic granites are related to extension (e.g., Dennis, 2007). The age of the Walker Top Granite bodies dated herein clearly falls within the Acadian and Neoacadian orogenies and temporally overlaps peak thermal conditions and deformational events in the Inner Piedmont (e.g., Merschat et al., 2005, 2017; Hatcher and Merschat, 2006). Melt-weakened middle crust with disseminated syn-tectonic granite bodies is a salient component for orogenic channel flow (Beaumont et al., 2004) and is one of the characteristics of the Inner Piedmont that led Hatcher and Merschat (2006) to suggest it was a tectonically forced orogenic channel. A holistic view of the Devonian to Mississippian Acadian-Neoacadian Appalachian orogen draws on far-field connections with the magmatism, accretion of peri-Gondwanan terranes, and polarity of subduction.

Structural and Tectonic Evolution of the Walker Top Granite

The peraluminous nature of the Walker Top Granite has been suggested as evidence of partial melting of the surrounding metasedimentary and associated rocks of the Cat Square terrane (Hatcher and Merschat, 2006; Merschat and Hatcher, 2007). The megacrystic granites generally overlap with compositional fields defined by Patiño Douce (1999) to represent melting of different sedimentary protoliths (Fig. 12E). This is further supported by the abundant xenoliths of biotite gneiss and/or metagraywacke, quartzite, amphibolite, and the trace occurrence of xenocrystic sillimanite in Walker Top Granite samples (Figs. 3D and 5E; Gatewood, 2007; Byars, 2010). Trace-element data suggest that Walker Top Granite formed in a collisional tectonic environment. The majority of samples plot as volcanic-arc and syn-collisional granites in standard discrimination diagrams (Figs. 12A12D). The Vale Charnockite plots as a within-plate granite in all diagrams; however, it has REE and trace-element patterns that are similar to those of Walker Top samples (Fig. 11). The gradational to diffuse contact zones and nearly identical ages of the charnockite and enclosing Walker Top Granite (Fig. 5E) suggest they likely formed in a similar tectonic environment.

Estimates for zircon saturation temperatures for Paleozoic eastern Inner Piedmont plutons ranged from 810 to 950 °C (Mapes, 2002; Miller et al., 2003; Byars, 2010). Zircon saturation temperatures from the Vale–Cat Square and South Mountains plutons of the Walker Top Granite are 802 °C (Byars, 2010) and 824 °C (Mapes, 2002; Miller et al., 2003), respectively. The Vale Charnockite has anomalously high Zr content (978 ppm; Table S1, see footnote 1), yielding a high zircon saturation temperature of 839 °C (Byars, 2010). These temperatures represent minimum dehydration melting conditions in the eastern Inner Piedmont, Cat Square terrane and regionally match the high-metamorphic grade of the terrane; however, they are still within the normal conditions of hydrous melts.

Charnockites are interpreted to form in four different tectonic environments: (1) rift-related, A-type, ferroan magmatism associated with Precambrian anorthosite-mangerite-charnockite-granite (AMCG) suites; (2) deeply eroded magmatic arcs (Cordilleran-type) with magnesian calcic to calc-alkalic, metaluminous granitoids; (3) rare magnesian, alkali-calcic to alkalic magmatism (Caledonian-type) that formed by delamination of a thickened continental crust; and (4) deep crustal melting with participation of a significant crustal component to produce weakly to moderately peraluminous magmas (Frost and Frost, 2008). The Vale Charnockite is ferroan, alkalic-calcic, weakly metaluminous, and plots as a within-plate, A-type granite (Figs. 6D6F), but it is not associated with AMCG suites nor other parts of a deeply eroded magmatic arc. Therefore, we interpret the Vale Charnockite to be the product of deep crustal melting, although evidence of input from Cat Square terrane metasedimentary rocks is limited to isotopic compositions (e.g., Byars, 2010) and metaluminous composition of the charnockite (Fig. 6B). The charnockite magma would have been anhydrous with higher Fe number and crystallized under higher temperatures to explain the presence of pyroxene. After partial or full crystallization of the Vale Charnockite, it was incorporated into the Walker Top Granite magma as it moved through the crust to final emplacement levels. The later pulses of typical Walker Top magma began as more hydrous melts with a lower Fe number at shallower mid-crustal levels.

Isotopically, the Walker Top Granite has similar initial 87Sr/86Sr, 143Nd/144Nd, and depleted mantle model ages (TDM) to other granitoids from the Inner Piedmont, and the values suggest recycling of Paleoproterozoic and Mesoproterozoic crust to form these granites (Fullagar et al., 1997; Mapes, 2002; Byars, 2010). Initial 87Sr/86Sr (0.70431–0.70925) and initial 143Nd/144Nd (0.51194–0.512094) ratios are similar to other Inner Piedmont granites (Fullagar et al., 1997; Bream et al., 2004; Byars, 2010). Initial εNd values from Walker Top Granite and Vale Charnockite samples range from −7.02 to −4.37, and model ages range from 1.3 to 1.09 Ga (Table 1; Mapes, 2002; Byars, 2010). Bream et al. (2004) reported similar initial Sm-Nd isotopic ratios, εNd values (−2.6 and −6.8), and TDM ages (1.64 Ga and 1.19 Ga) from the Cat Square terrane metasedimentary rocks. Depleted mantle model ages of the Walker Top Granite clearly contrast with juvenile depleted mantle model ages for Neoproterozoic arc-related rocks of the Carolina superterrane (TDM = 1.0–0.6 Ga; Fullagar et al., 1997) and were not a source for Walker Top Granite magmas.

The trace-element data, combined with isotopic and field relationships, suggest that the Walker Top Granite is a syn-collisional intrusion that formed by melting of significant proportions of metasedimentary rocks of the Cat Square terrane. The numerous separate or disconnected bodies do not favor that the Walker Top Granite intruded as part of a volcanic arc but, instead, support partial melting of metasedimentary and other rocks during Acadian-Neoacadian deformation. Isotopic data, similar 87Sr/86Sr, 143Nd/144Nd, and TDM indicate the recycling of Paleoprotero zoic and Mesoproterozoic crust and lack of evidence of a juvenile source, as might be expected in a volcanic arc setting or if the subduction zone was located beneath the Cat Square terrane or the Laurentian margin. The evolved isotopic signature overlaps with metasedimentary rocks of the Cat Square and Tugaloo terranes and Grenville basement rocks, suggesting melting of these rocks may represent a possible source.

The High Falls Granite (406–372 Ma) in the Cat Square terrane in the central Georgia Inner Piedmont is texturally, compositionally, and temporally similar to the Walker Top Granite (Huebner et al., 2017). Despite the span of magmatic ages from the Walker Top Granite, other megacrystic granites in the Inner Piedmont in the Carolinas, and the High Falls Granite, there does not appear to be a systematic change in chemistry of the granites to suggest significant changes in the tectonic process by which they formed (e.g., volcanic arc followed by crustal anatexis). We suggest these granites may be the product of anatexis of crust that was subducted beneath the Carolina superterrane, followed by intrusion into the Cat Square terrane (e.g., Huebner et al., 2017). The Vale Charnockite indicates that, at least locally, higher temperatures or conditions were reached to produce anhydrous magmas.

The Walker Top Granite places important limits on the deformational and thermal history of the Cat Square terrane and Inner Piedmont. Merschat et al. (2005) recognized that the Walker Top Granite postdates D1 and predates D2 and D3 deformations, which in part may be related to the range of ages from 407 Ma to 358 Ma. Merschat et al. (2005) and Gatewood (2007) mapped the Walker Top Granite (ca. 407 Ma) in the Brushy Mountains in a SW-directed sheath fold formed during the dominant deformation, D2. Mineral lineations, L2, and fold axes, F2, are parallel to the SW-directed sheath fold axes. The Brindle Creek fault truncates the northwest limb of the sheath fold (Gatewood, 2007; Merschat et al., 2012). Similar structural relationships are recognized throughout the Inner Piedmont. Bier et al. (2002) concluded the Walker Top is exposed in F2 sheath folds in the South Mountains. Near Cat Square, North Carolina, Byars (2010) mapped the Walker Top Granite (ca. 358 Ma) as limbs in a NW-vergent sheath fold with a southeast limb excised by the Brindle Creek fault surrounding the Newton window. D2 structures are syn- to post-intrusion of the Walker Top Granite, and the Vale–Cat Square pluton is truncated along the Brindle Creek fault, delimiting D2 deformation to be ca. 358 Ma or younger (Fig. 13).

The Inner Piedmont was affected by a protracted middle Paleozoic thermal event during which time the Walker Top Granite and other granites were intruded (Hatcher and Merschat, 2006; Merschat et al., 2017). Metamorphic zircon rims from 395 to 345 Ma suggest middle-to-upper amphibolite-facies metamorphism occurred in the Inner Piedmont from Late Devonian to Early Mississippian times and correspond to the Acadian and Neoacadian orogenies (Huebner et al., 2017; Merschat et al., 2017). D1 and D2 structures are associated with high-grade mineral assemblages containing sillimanite and migmatite formed during these events (Fig. 13).

Ages from the Walker Top Granite in the Brushy Mountains (ca. 407 Ma) and the South Mountains and Vale–Cat Square area, North Carolina (ca. 360 Ma), do not overlap even when errors are considered (Figs. 911). Although Pb-loss and/or complete recrystallization during metamorphism could produce such a discrepancy in ages, the data from each individual sample are internally consistent. Furthermore, CL images display oscillatory zoned magmatic cores, and SHRIMP-RG spots used to determine crystallization ages were from these textural zones. Discordant or high U or Pb analyses and analyses of metamorphic rims were excluded from age determination from all samples. We suggest that these data indicate a protracted period of heating and partial melting that produced magma generation via similar processes over 40 m.y. in the Inner Piedmont. This occurred as the Carolina superterrane obliquely collided diachronously from north to south with Laurentian terranes to the west, closing the small Rheic ocean (Merschat and Hatcher, 2007).

Channel-Flow Model and the Inner Piedmont

A critical element of orogenic channel flow is the existence of a lower viscosity crustal layer between higher viscosity upper and lower crustal layers (Beaumont et al., 2004; Godin et al., 2006). This is translated into a partially melted (migmatitic), melt-weakened middle crust that is able to flow laterally, similar to the processes thought to have occurred in the Himalayas (Fig. 14A). The amount of partial melt necessary to reduce the effective viscosity of rocks is small, ~7% melt (Rosenberg and Handy, 2005). The Walker Top Granite accounts for 2.4% of the area of Cat Square terrane in North Carolina, essentially the migmatitic, northern portion of the Inner Piedmont (Fig. 2; Table 3). This assumes that all Walker Top Granite plutons were melts at the same time (within 2 sigma error); the Brushy Mountain pluton (18.8 km2) does not overlap with ages of the South Mountains and Vale–Cat Square plutons but only reduces the total percent area of the Walker Top Granite by 0.2%. Including other Devonian and Early Mississippian granites—Toluca, Cherryville, and Sandy Mush—the cumulative percent area of granitic melt increases to 12% in the northern Cat Square terrane (Table 3). However, the limited crystallization ages for the various plutons do not all overlap (at 2 sigma error; Fig. 13), and the volume fraction of melt present in the Cat Square terrane at any time during the Acadian-Neoacadian is likely less than 12%. Furthermore, the estimate of the area of granite, especially in this shallow-dipping terrane with low present-day topographic relief, may overestimate the volume of granite (melt) present. Not included in this estimate is the abundant migmatite present in pelitic schists and metagraywacke lithologies in the Cat Square terrane and Inner Piedmont (e.g., Hatcher and Merschat, 2006). Regardless, these limited estimates suggest that parts of the Inner Piedmont contained significant amounts of syntectonic granite, partial melt, and migmatite to flow laterally over an appropriate duration of time (~40 m.y.; e.g., Beaumont et al., 2004; Godin et al., 2006). This would have occurred during peak deformation and metamorphism, D2 (Fig. 13).

Flow in a partially melted channel is described as either Couette or Poiseuille flow, both of which are probably not fully realized in natural partially melted crust (e.g., Beaumont et al., 2004; Godin et al., 2006). As such, the region between the upper and lower crustal layers is a shear zone several kilometers to several tens of kilometers thick involving penetrative simple-shear deformation and laminar flow throughout the channel (Godin et al., 2006). Structures formed within the channel are products of the high internal shear strain in the channel. Hatcher and Merschat (2006) suggested that the Inner Piedmont represents a tectonically forced—rather than Himalayan, gravity-driven—orogenic channel that functioned during the Acadian and Neoacadian orogenies. The geometries of the Walker Top Granite and other plutons may be used as markers for bulk strain within the channel.

Pluton shape can thus be a qualitative measure of regional strain in different parts of the Inner Piedmont, assuming all had an initial quasiellipsoidal or tabular (sill) shape and became highly strained (west) and less strained (east) during the deformational event that accompanied peak Acadian-Neoacadian metamorphism in the Inner Piedmont (Fig. 14). Examples of different map geometries of Walker Top plutons are readily available in the western and eastern Inner Piedmont Cat Square terrane with the Walker Top Granite and Toluca Granite plutons (Fig. 2; Merschat et al., 2005; Hatcher and Merschat, 2006). Walker Top plutons in the western half of the Cat Square terrane have an elongate shape, but the plutons are more elliptical to equidimensional in the eastern Cat Square terrane (Fig. 2). The linear outcrop patterns are interpreted as highly dextrally strained and transposed sheath folds, whereas the more easterly outcrop patterns represent either moderately strained sheath folds or less deformed elliptical plutons (e.g., Merschat et al., 2005, their fig. 6). The Walker Top Granite occurs here in variously strained exposures from megacrystic granite to ultramylonite, suggesting progressive deformation and strain gradients occur throughout these outcrop belts (Fig. 4). In the western Inner Piedmont (west of the Cat Square terrane), the Henderson Gneiss (ca. 450 Ma; Moecher et al., 2011; Huebner et al., 2017) occurs in the near-horizontal axial zones of map-scale sheath folds (Hatcher, 2002), but in the eastern Inner Piedmont, Walker Top and Toluca plutons have either elliptical shape or, in one case, an anvil-shaped sheath fold (Merschat et al., 2005; their Figs. 11 and 13). The various possible initial pluton shapes may be readily sheared into possible map geometries (Fig. 14D) but require shear strains >8, which is just below the threshold of experimental shear strains necessary to form sheath folds (Cobbold and Quinquis, 1980). The tabular and sheath fold geometries of the Walker Top and Toluca Granites (Merschat et al., 2005) are likely the products of melting, intrusion, and high shear strain in a melt-weakened crustal channel.

Connections along the Orogen

The Acadian and Neoacadian orogenies have traditionally been thought to be a New England and northward event (e.g., Robinson et al., 1998). The growing evidence, however, of Late Devonian to Early Mississippian magmatic and tectonothermal events provides confirmation of the existence of both the Acadian and Neoacadian orogenies in the southern Appalachians (e.g., Merschat et al., 2017). The palinspastic kinematic reconstructions of the Inner Piedmont to the Pennsylvania embayment have provided possible connections with parts of the central and northern Appalachians (Merschat et al., 2005; Merschat and Hatcher, 2007; Huebner and Hatcher, 2017; Huebner et al., 2017). The ca. 407 Ma ages obtained for the Brushy Mountains pluton of the Walker Top Granite and the High Falls Granite are temporally coeval with oldest Acadian magmatism in the Bronson Hill anticlinorium associated with the Kinsman and Bethlehem Granodiorites (Kohn et al., 1992, and references therein; Merschat et al., 2015). The polarity of subduction, however, is interpreted to be in opposite directions. In New England, the Avalon terrane was subducted beneath the Laurentian margin and the Bronson Hill anticlinorium (e.g., Robinson et al., 1998; Van Staal and Barr, 2012). The Kinsman and Bethlehem Granodiorites were intruded as sheets into the deforming Acadian orogen, often along the base of nappes (Spear, 1992; Dorais, 2003; Wintsch et al., 2014). The collision of Avalonia with the northern Appalachians also involved a significant component of dextral transpression (e.g., Waldron et al., 2015). In the southern Appalachians, we suggested the Carolina superterrane overrode and subducted the Inner Piedmont, including the Cat Square terrane (Merschat et al., 2005; Hatcher and Merschat, 2006; Hatcher et al., 2007; Huebner and Hatcher, 2017; Huebner et al., 2017). Partial confirmation of this hypothesis was provided by recognition of horses of Carolina superterrane rocks in the Brevard fault zone (Hatcher et al., 2017).

To resolve this enigma in subduction polarity, we hypothesize that a middle Paleozoic transform fault existed near the New York promontory to accommodate the different plate tectonic settings and kinematics between the Avalon (northern Appalachians) and Carolina (southern Appalachians) superterranes (Fig. 15). The transform fault would also account for the different structural architecture and opposite vergence of Acadian and Neoacadian structures in the Inner Piedmont and New England orogens (for example, compare Merschat et al., 2005, and Wintsch et al., 2014). Such a transform was first implied by Phinney (1986) from interpretation of crustal seismic-reflection data in southern New England. Kuiper (2016) also hypothesized a triple-junction with a transform fault in southern New England accommodating motion on the Norumbega fault during the Devonian but does not resolve opposite directions of subduction in the northern and southern Appalachians during the Acadian and Neoacadian.

Our postulated transform would be located along the south flank of the New York promontory, parallel to the proposed location of one of the transform faults related to the Neoproterozoic to Cambrian opening of the Iapetus ocean (Thomas, 2006), raising the possibility of structural inheritance in this part of the orogen. Evidence supporting this hypothesis rests primarily with the consanguinity between the southern Appalachian Cat Square terrane detrital zircons and those in one or more New England tectonostratigraphic terranes (e.g., Merschat et al., 2010). Moreover, the paired Bouger gravity anomalies, which trend NE-SW in the Virginia promontory, trend WSW-ENE through the New York promontory (Thomas, 1983) and parallel the orientation of Thomas’ proposed Iapetan rift-related transform. Additionally, the contrast in subduction polarity likely influenced the magmatic history from the Virginia Piedmont into New York from 430 to 420 Ma. The Maryland Piedmont contains several Late Silurian plutons, 430–420 Ma, that may have formed during a period of mantle delamination or extension (Group IV of Sinha et al., 2012). This period of mantle rearrangement may also correspond to the beginning of eastward subduction beneath the Carolina superterrane and reactivation of the transform accommodating the relative motion between the Carolina (south) and Avalon (north) superterranes. This is also consistent with new ages from components of the Petersburg Granite. This “batholith” was previously considered to be an Alleghanian granite in the eastern Virginia Piedmont, but new data demonstrate it is a composite pluton that includes a foliated granitic gneiss dated at 416 ± 4 Ma (Carter et al., 2019; McAleer et al., 2020). The Silurian–Devonian infrastructure could be early intrusions into the upper plate of the east-subducting Acadian orogen. In the New Jersey and New York Hudson Highlands, the Cortlandt and Beemerville complexes define a nearly E-W belt of mantle-derived alkalic–calc-alkali plutons, dikes, and diatremes (Fig. 15; Ratcliffe 1981; Eby, 2004). The Cortlandt complex intruded folded and cleaved rocks of the Hudson Highlands and rocks as young as the Ordovician Martinsburg Formation. A tectonic setting for the mantle-derived Cortlandt and Beemerville complexes is not clearly identified (Ratcliffe, 1981). Eby (2004) obtained a titanite fission-track age of 420 ± 6 Ma from the Beemerville complex, which he interpreted as the age of emplacement. The reactivation of an NW-SE–trending transform fault along the New York promontory may have produced the necessary extension that facilitated the intrusion of the mantle-derived Cortlandt and Beemerville complexes into middle- and upper-crustal rocks of the Hudson Highlands at ca. 420 Ma.

The change in subduction polarity between the southern and northern Appalachians may have been caused by the ancient subcontinent of peri-Gondwanan terranes—Avalonia and Carolina superterranes—interacting with the New York promontory. In the northern Appalachians, west-directed subduction of Avalon beneath Laurentia developed a continental arc and eventually an orogenic plateau with the thickest crust in southern New England (Hillenbrand and Williams, 2021; Hillenbrand et al., 2021). The New York promontory may have caused the polarity to reverse to the east beneath the Carolina superterrane, producing arc-related magmatism of the Concord-Salisbury Plutonic Suite (e.g., Huebner and Hatcher, 2017). The transition from B-subduction to A-subduction, and the difficulty of subducting continental crust, may have led to overthickened crust and increased partial melting in the Inner Piedmont and intrusion of the Walker Top and coeval Toluca and High Falls Granites. Eventually the development of a tectonically forced orogenic channel was facilitated by the syntectonic anatectic granites and migmatite.

  1. The Walker Top Granite documents deep-seated plutonism in the eastern Inner Piedmont, Cat Square terrane, from 407 to 358 Ma. Intrusion of the Walker Top Granite accompanied mid-crustal deformation associated with the Acadian and Neoacadian orogenies in the southern and central Appalachians. New SHRIMP U-Pb zircon magmatic crystallization ages obtained from the Walker Top Granite are: 407 ± 1 Ma in the Brushy Mountains; 366 ± 2 Ma in the South Mountains; and 358 ± 5 Ma in the Vale–Cat Square area. An age of 366 ± 3 Ma was obtained from the Vale Charnockite at its type locality.

  2. The Walker Top Granite is peraluminous with high K and SiO2. Trace-element data suggest volcanic-arc to syn-collisional plutonism, not related to extension. Whole-rock geochemistry and similar initial Sr/Sr, Nd/Nd, and TDM values to Cat Square terrane rocks suggest the Walker Top Granite may have formed by anatexis of surrounding Cat Square terrane rocks. Furthermore, the numerous separate or disconnected bodies do not favor the Walker Top Granite being intruded into a volcanic arc but instead support partial melting of metasedimentary rocks during Acadian-Neoacadian deformation.

  3. The Vale Charnockite (366 Ma) formed from deep-crustal melting resulting in an anhydrous magma that crystallized early and then was incorporated into more typical hydrous Walker Top magmas as it moved to shallower mid-crustal levels.

  4. Syntectonic intrusion of the Walker Top Granite into the partially melted high-grade rocks of the Inner Piedmont likely facilitated the lateral flow of the Inner Piedmont as part of a melt-weakened orogenic channel. Cumulative estimates of the areal extent of Devonian to Early Mississippian syn-tectonic granites may have approached 2%–12% of the migmatitic portion of the northern Inner Piedmont.

  5. We hypothesize that a northwest-trending transform fault existed near the New York promontory during the Acadian and Neoacadian orogenies; this fault separated the Avalon and Carolina superterranes to accommodate their different plate tectonic dynamics and east- and west-directed subduction during the Acadian and Neoacadian orogenies.

  6. Northern and southern segments of the Acadian-Neoacadian orogens require westward and eastward subduction, respectively. We suggest that the flip in polarity of subduction occurred by interaction with the New York promontory; southward, the peri-Gondwanan terrane subducted the Laurentian margin, but to the north, it subducted Avalon. The difficulty of subducting continental crust helped to thicken crust beneath the Carolina superterrane in the southern Appalachians and to increase partial melting and intrusion of the Walker Top Granite, which cumulatively reduced the effective viscosity and facilitated lateral flow of the Inner Piedmont westward and then southwestward from beneath the Carolina superterrane.

1Supplemental Material. Whole-rock major- and trace-element geochemical data from the Walker Top Granite and other megacrystic granites from the Inner Piedmont. Please visit https://doi.org/10.1130/GEOS.S.21625838 to access the supplemental material, and contact editing@geosociety.org with any questions.
Science Editor: Shanaka de Silva
Guest Associate Editor: Guilherme Gualda

The authors express our gratitude to Calvin F. Miller for his superb career and pioneering research into the magmatic and tectonic history of the southern Appalachians, and isotope geochronology, particularly using the SHRIMP-RG. While using the SHRIMP-RG, Calvin often amazed students with his ability to closely calculate the U-Pb age of an analysis from isotopic ratios, one of many things to do to pass time when using the SHRIMP. Calvin also provided a thorough and constructive review of an earlier version of this manuscript. The manuscript benefitted from two anonymous journal reviewers and reviews by Ryan J. McAleer (USGS), Mark W. Carter (USGS), and Nancy Stamm (USGS). Geologic mapping was supported by National Cooperative Geologic Mapping Program EDMAP grants 98HQAG2026, 99HQAG0026, 00HQAG003, 01HQPA0004, 03HQAG0021, and 04HQAG0026 to RDH; geochronology and geochemistry was supported by National Science Foundation grant EAR-9814800 to RDH and Calvin F. Miller, and University of Tennessee Science Alliance Center for Excellence research fund to RDH. Matthew T. Huebner is acknowledged and greatly appreciated for help with compiling geochemistry of Inner Piedmont granites, and for discussion of the tectonic origin of plutons in the Inner Piedmont. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Gold Open Access: This paper is published under the terms of the CC-BY-NC license.