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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.

Causes of intraplate seismicity remain a great unsolved problem, in contrast with plate-boundary seismicity. Modern seismicity records frequent seismic activity in plate-boundary seismic zones, but in fault zones where seismic activity is not frequent, plate boundary or intraplate, resolution of prehistoric earthquake activity is critical for estimating earthquake recurrence interval and maximum expected magnitude. Thus, documenting prehistoric earthquakes is crucial for assessing earthquake hazard posed to infrastructure, including nuclear reactors and large dams. The ∼400 km long eastern Tennessee seismic zone (ETSZ), United States, is the third most active seismic zone east of the Rocky Mountains in North America, although the largest recorded ETSZ earthquake is only Mw 4.8. Ironically, it is the least studied major eastern U.S. seismic zone. Recent ETSZ field surveys revealed an 80 km long, 060°-trending corridor containing northeast-striking Quaternary thrust, strike slip, and normal faults with displacements ≥1 m. It partially overlaps a parallel trend of seismicity that extends 30 km farther southwest, suggesting this active faulting zone may extend ∼110 km within part of the ETSZ. Near Dandridge, Tennessee, a thrust fault in French Broad River alluvium records two earthquakes in the last 40,000 yr. About 50 km southwest near Alcoa, Tennessee, a thrust fault cuts Little River alluvium and records two earthquakes between 15,000 and 10,000 yr ago. About 30 km farther southwest at Vonore, Tennessee, a thrust fault displaces bedrock ≥2 m over colluvium, and alluvium is normal faulted >2 m. This corridor, just west of the Blue Ridge escarpment, overlies a steep gradient in midcrustal S-wave velocities, consistent with a basement fault at hypocentral depths. The corridor faults may be connected to a basement fault or localized coseismic faults above a blind basement fault. Our current data suggest at least two Mw≥6.5 surface rupturing events in the last 40,000 yr.

Specifying the extent and location of rifted, crystalline Precambrian crust in the eastern United States is important for seismic hazard evaluation and for models that relate upper-mantle structure to ancient tectonic features and ongoing tectonism. As currently depicted in the National Seismic Hazard Maps (NSHM), the western limit of Iapetan rifted crust is beneath the Appalachian plateau physiographic province, west of the Valley and Ridge province. New estimates of crustal thickness using EarthScope Transportable Array and other data do not support the presence of rifted crust beneath the Blue Ridge, Valley and Ridge, and Appalachian plateau physiographic provinces. Crustal thicknesses exceed 45 km throughout most of this region. The crust thins to the southeast beneath the southeastern part of the Piedmont physiographic province and is only 36 km thick near the edge of the Atlantic coastal plain. We suggest that the western limit of Iapetan rift-extended crust is east of the Blue Ridge province and is associated with the prominent Appalachian gravity gradient. This location coincides with palinspastic reconstructions based on geologic data for the Iapetan rifted margin. Recognition of thick crust beneath the Blue Ridge and Valley and Ridge provinces, unextended by Iapetan rifting, will support more robust modeling of the effects of mantle structure (such as delamination and abrupt changes in lithospheric thickness) on ongoing tectonism and earthquake activity in the eastern United States and will provide more accurate seismotectonic zonation in the NSHM.

Detailed geologic mapping, U-Pb zircon geochronology and whole-rock geochemical analyses were conducted to test the hypothesis that the southwestern extent of the Cat Square terrane continues from the northern Inner Piedmont (western Carolinas) into central Georgia. Geologic mapping revealed the Jackson Lake fault, a ∼15 m-thick, steeply dipping sillimanite-grade fault zone that truncates lithologically distinct granitoids and metasedimentary units, and roughly corresponds with a prominent aeromagnetic lineament hypothesized to represent the southern continuation of the terrane-bounding Brindle Creek fault. Results of U-Pb SHRIMP geochronology indicate Late Ordovician to Silurian granitoids (444–439 Ma) occur exclusively northwest of the fault, whereas Devonian (404–371 Ma) granitoids only occur southeast of the fault. The relatively undeformed Indian Springs granodiorite (three individual bodies dated 317–298 Ma) crosscuts the fault and occurs on both sides, which indicates the Jackson Lake fault is a pre-Alleghanian structure. However, detrital zircon signatures from samples southeast of the Jackson Lake fault reveal dominant Grenville provenance, in contrast to Cat Square terrane detrital zircon samples from the northern Inner Piedmont, which include peri-Gondwanan (600–500 Ma) and a prominent Ordovician-Silurian (∼430 Ma) signature. We interpret the rocks southeast of the Jackson Lake fault to represent the southwestern extension of the Cat Square terrane primarily based on the partitioning of granitoid ages and lithologic distinctions similar to the northern Inner Piedmont. Data suggest Cat Square terrane metasedimentary rocks were initially deposited in a remnant ocean basin setting and developed into an accretionary prism in front of the approaching Carolina superterrane, ultimately overridden by it in Late Devonian to Early Mississippian time. Burial to >20 km resulted in migmatization of lower plate rocks, forming an infrastructure beneath the Carolina superterrane suprastructure. Provenance patterns support ∼250 km of Devonian dextral translation of the composite Inner Piedmont, which places the northern portion of the Inner Piedmont adjacent to a suite of ∼430 Ma plutons in the Virginia Blue Ridge during deposition. The megascopic thrust-nappe structural style of the northern Inner Piedmont, combined with southwest-directed lateral extrusion at mid-crustal depths, may reconcile differences in timing of metamorphism between the Carolina and central Georgia Inner Piedmont and structural contrasts between the Brindle Creek and Jackson Lake faults.

The eastern Tennessee seismic zone (ETSZ) is the second‐most active seismic zone in the eastern United States, but it has not generated an earthquake larger than M w 4.8 in historic time. Earthquakes are sourced deep in autochthonous basement, and there are no known faults originating at this depth that break the surface. As a result, until recently, there has been virtually no fieldwork to identify Quaternary paleoseismic features in the ETSZ. We present new results from paleoseismic investigations of coseismic features that indicate the ETSZ generated M w ≥6 earthquakes during the late Quaternary. Detailed geologic mapping and trenching near Dandridge, Tennessee, record a northeast‐trending zone of seismically generated features. Optically stimulated luminescence ages delimit timing for the formation of paleoseismic features crosscutting Quaternary alluvium and alluvium‐filled fissures, including a thrust fault with ∼1 m displacement. Collectively, this zone of faults and fissures provides significant evidence that the ETSZ has produced at least three large earthquakes during the late Pleistocene and at least one that exceeded M w 6.

The spatial and temporal distributions of igneous rocks provide significant limits on the geodynamic and thermal record associated with the growth of continents through different tectonic processes. In the central Appalachian orogen, both published and new ion microprobe U/Pb zircon ages coupled with geochemical data from igneous rocks provide a window into the thermal and temporal evolution of the mid-Ordovician Taconic orogeny, as well as younger subduction, delamination and accretionary events. New Ion microprobe U/Pb ages of zircons screened for inheritance and lead loss, yield five discrete groupings of igneous activity: (1) pre-collision arc stage: 470 to 489 Ma (2) syntectonic: 459 to 472 Ma (3) arc magmatism after change in subduction polarity: 441 to 459 Ma (4) delamination induced extension related plutons: 423 to 438 Ma and (5) Neo-Acadian plutons related to collision of Avalon-Carolina superterrane with Laurentian elements: 362 to 381 Ma. Geochemical signature of mafic and felsic rocks of all age groups are dominated by arc-type attributes for primitive mantle normalized signatures for depletion in Nb, P, and Ti, and enrichment in Pb. Differences in alkalinity and a well-developed positive gadolinium anomaly for Group III rocks is interpreted to be the result of remelting of high to intermediate potassic basalts and their plutonic equivalents during the development of a late Ordovician calc-alkaline arc draped over an earlier (Group I) low-K tholeiitic arc assemblage. We utilize these magmatic events, including their geochemical signatures to suggest a tectonic model which includes: (1) arc-continent collision followed by change in subduction polarity, and (2) development of a second arc followed by slab delamination resulting in extension, as well as post-closure shortening related to docking of peri-Gondwanan tracts. The proposed model enables a more robust coupling of the opening and closing of the Iapetus ocean basin with the preserved igneous record, but we recognize that lack of detailed mapping over the area of study, accompanied by modern geochemical/geochronologic data to place igneous rocks in a regional deformation and metamorphic framework, precludes a complete endorsement of our preferred model.

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

The magnetic anomaly map of North America serves as a useful base from which to attempt palinspastic reconstruction of terranes accreted during the Elzevirian orogeny (1250–1200 Ma); the Shawinigan (1200–1150 Ma), Ottawan (1080–1020 Ma), and Rigolet (1020–1000 Ma) phases of the Grenvillian orogeny; and post-Grenvillian magmatism (760–600 Ma) and deformation prior to Iapetan rifting at 565 Ma. Accreted terranes had unique histories prior to amalgamation and share common tectonic events afterwards. Comparisons with magnetic signatures of the Paleozoic craton–craton suture, sutures of accreted terranes, and the Jurassic rifted-margin for the southern-central Appalachians provide a basis for discriminating among alternative Grenvillian sutures beneath the Appalachian orogen. The Elzevirian suture is partially preserved beneath the Appalachians where it separates the Reading Prong terrane from Laurentia (i.e., Adirondacks and composite-arc terrane and Canadian Grenville Province). The Shawinigan suture is partially preserved in the Llano area (Texas), but separated the now-fragmented and allochthonous Amazonian (as indicated from Pb-isotope data) blocks of the outboard Blue Ridge terrane from the Reading Prong terrane in the Appalachians. Isolated blocks of the Sauratown Mountains terrane are interpreted as outboard of the Blue Ridge terrane, but were also accreted during the Shawinigan phase. Within present-day Laurentia, the only fragment of a terrane believed to have been accreted during the main Ottawan phase is the Mars Hill terrane (North Carolina–Tennessee). This suggests that the outboard Ottawan suture may have served as the locus of Iapetan rifting along much of Laurentia. The Rigolet phase (1020–1000 Ma) is characterized by widespread “Basin and Range” type extension (NW–SE) associated with sinistral or dextral movement on the NY-AL lineament, mobilization of core-complexes (Adirondack Highlands), and AMCG magmatism along the outboard flank of the extensional region. Following the Rigolet phase, the Appalachian region continued to be characterized by NW–SE extension during the passage of a possible hotspot along a NE-track (760–600 Ma) across the Blue Ridge and other terranes, and during initial Iapetan rifting (565 Ma). The palinspastic rifted-margin of Laurentia crosses many of these terranes and sutures as well as the possible region of Rigolet extension and the possible hotspot track, thus providing many potential piercing points within the Grenville orogen for comparison with Paleozoic terranes like the Precordillera in South America.

The classic work of Peach et al. (1892, 1907) is first set of quality geologic maps depicting the geometry of the Moine and related crystalline thrusts, and the foreland fold-thrust belt (FFTB) beneath. Modern detailed geologic maps (e.g., M. P. Coward, unpublished) have facilitated more detailed analysis of the Moine thrust zone. C. W. Hayes (1891) was mapping at the same time in the Georgia Appalachians and thinking about the consequences of retrodeformation of thrusts. High quality geologic maps in the Northwest Highlands and in other FFTBs worldwide, along with modern digital seismic reflection data, permit almost quantitative resolution of 2D thrust geometry. These yield the common shortened wedge, thin-skinned, ramp-flat, listric configurations characteristic of thrust belts that propagated into a wedge-shaped platform sequence. FFTBs worldwide are commonly driven toward continental interiors by either large crystalline thrust sheets generated during continent-continent collision, composite terrane or arc-continent collision, or ophiolite obduction. Retrodeformation and attempts to balance portions of linear FFTBs is more readily attainable than in curved segments. Plane strain can reasonably be assumed in linear FFTBs, making 2D sections amenable to line and area balancing. These have a good chance of meeting Elliott’s (1983) minimal viability criterion. Sections in strongly to moderately curved FFTBs may individually appear restorable, but retrodeformation of 2D serial sections produces overlap of their hinterland ends, making the incompatibility problem obvious. Moreover, outward radiating displacement vectors suggest there should be along-strike stretching that increases toward the outer portions of arcuate FFTBs. Evidence other than across-strike joints and veins is frequently lacking to support a stretching mechanism. Balancing of curved FFTB segments thus becomes a 3D material balancing problem (Laubscher, 1988), with major components of nonplane strain. Various 2–D solutions have been proposed, but they generally fail. Retrodeformation of 2–D cross sections through plastic thrust sheets in the internides of orogens presents even greater challenges. The obvious solution is to employ Laubscher’s 3–D material balancing principles in both FFTBs and orogenic interiors. It may be that our principles of restroration are incorrect. Computer software exists that attempts 3–D balancing, but currently is limited because of difficulties of writing codes that can incorporate multiple rheologies (more than Coulomb) and varied material properties. Sophisticated finite-element codes that accommodate more than one rheology and a host of parameters are currently limited to 2–D. Current reality is the recognition that 3–D material balancing is needed, but we still are limited to hand methods and obtaining enough field data. Possible solutions: (1) construct balanced 2–D cross sections through short, acrossand along-strike segments of FFTBs, and (2) construct and analyze displacement vector (particle trajectory) maps around curved segments of FFTBs, but finding enough quality displacement vectors that yield the regional vector field (slip lines) to permit 3–D balancing is difficult. Populations of usable structures are limited and unevenly distributed. Determination of regional displacement vectors remains the key starting point, however, because they track finite strain and provide paths for retrodeformation of curved segments of orogens.

Basement is constituted of rocks which belong to a previous orogenic cycle which have been reactivated and incorporated into a younger cycle. Basement massifs may be classified according to their relative position in an orogen as external or internal massifs. They may also be categorized according to their role in deformation, as thrust-related, fold-related and composite massifs. All Appalachian external massifs were transported following their removal from the overridden edge of the ancient North American continental margin. Most of the internal massifs are also probably transported, but several (Pine Mountain and Sauratown Mountains) may be present as windows exposing parautochthonous basement beneath the main thrust sheet. The latter reside immediately west of the low (west) to high (east) gravity gradient which probably outlines the old edge of Grenvillian crust. Reactivated crustal material generated during early Palaeozoic orogeny plays the same mechanical role in reactivation as basement from the previous Grenville cycle. The domes of the Bronson Hill anticlinorium cored with Ordovician or older gneisses illustrate this behaviour. Basement (Grenville) massifs are distributed throughout the Appalachians as a belt of external massifs (Blue Ridge, Reading Prong, Hudson and Berkshire Highlands, Green Mountains, and Long Range Mountains) along the western edge of the crystalline metamorphic core. Additionally, internal massifs are also present (Pine Mountain belt, Tallulah Falls and Toxaway domes, Sauratown Mountains anticlinorium, State Farm gneiss dome, Baltimore Gneiss domes, Mine Ridge anticline, and Chain Lakes massif). Basement internal massifs probably served to localize thrusts by causing them to ramp over and around the massifs. Their antiformal shape may in part be as much related to thrust mechanics as to folding.