ABSTRACT The Brevard fault zone is one of the largest faults in the Appalachians, extending from Alabama to Virginia. It had a very complex history of movement and reactivation, with three movement episodes: (1) Acadian-Neoacadian (403–345 Ma) movement accompanying the thermal peak of metamorphism and deformation with dextral, southwest-directed emplacement of the Inner Piedmont; (2) ductile dextral reactivation during the early Alleghanian (~280 Ma) under lower-greenschist-facies conditions; and (3) brittle dip-slip reactivation during the late Alleghanian (260 Ma?). The Brevard is comparable to other large faults with polyphase movement in other orogens worldwide, for example, the Periadriatic line in the Alps. Two types of far-traveled, fault-bounded horses have been identified in the Brevard fault zone in the Carolinas: (1) metasedimentary and granitoid horses located along the southeastern margin of the Alleghanian retrogressive ductile dextral Brevard fault zone in North and South Carolina; and (2) limestone/dolostone horses located along the brittle, late Alleghanian Rosman thrust, the contact between Blue Ridge and Brevard fault zone rocks in North and South Carolina. Field, stratigraphic, petrographic, and Sr-isotope data suggest the carbonate horses may be derived from Valley and Ridge carbonates in the Blue Ridge–Piedmont megathrust sheet footwall. The horses of metasedimentary and granitoid rocks occur along faults that cut klippen of the southwest-directed Inner Piedmont Acadian-Neoacadian Alto (Six Mile) allochthon. New laser ablation– inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb zircon analyses from the metasedimentary mylonite component yield a detrital zircon suite dominated by 600 and 500 Ma zircons, and a second zircon population ranging from 2100 to 1300 Ma, with essentially no Grenvillian zircons, suggesting a peri-Gondwanan provenance. The granitoid component has a sensitive high-resolution ion microprobe (SHRIMP) age of 421 ± 14 Ma, similar to the ~430 Ma plutonic suite in northern Virginia and Maryland—a prominent component of the Cat Square terrane detrital zircon suite in the Carolinas. Peri-Gondwanan Neoproterozoic to Cambrian Avalon–Carolina superterrane rocks are nowhere in contact with the Brevard fault zone at present erosion level. While these far-traveled metasedimentary and granitoid horses may have originated several hundred kilometers farther northeast in the central Appalachians, they could alternatively be remnants of Avalon–Carolina superterrane rocks that once formed the tectonic lid of the southwest-directed Neoacadian–early Alleghanian (Late Devonian–early Mississippian) orogenic channel formed during north-to-south zippered accretion of Avalon–Carolina. The remnant fossil subduction zone survives as the central Piedmont suture. Avalon–Carolina terrane rocks would have once covered the Inner Piedmont (and easternmost Blue Ridge) to depths of >20 km, and have since been eroded. Data from these two suites of horses provide additional insights into the mid- to late Paleozoic history and kinematics of the Brevard fault zone, Inner Piedmont, and Avalon–Carolina superterrane. It was six men of Indostan To learning much inclined, Who went to see the Elephant (Though all of them were blind), That each by observation Might satisfy his mind. … And so these men of Indostan Disputed loud and long, Each in his own opinion Exceeding stiff and strong, Though each was partly in the right, And all were in the wrong. —John Godfrey Saxe (1816–1887) “The Blind Men and the Elephant”

The Mars Hill terrane (MHT), a lithologically diverse belt exposed between Roan Mountain, North Carolina–Tennessee, and Asheville, North Carolina, is distinct in age, metamorphic history, and protoliths from the structurally overlying Eastern Blue Ridge and underlying Western Blue Ridge. MHT lithologies include diverse granitic gneisses, abundant mafic and sparse ultramafic bodies, and mildly to strongly aluminous paragneisses. These lithologies experienced metamorphism in the granulite facies and are intimately interspersed on cm to km scale, reflecting both intrusive and tectonic juxtaposition. Previous analyses of zircons by high-resolution ion microprobe verified the presence of Paleoproterozoic orthogneiss (1.8 Ga). New data document a major magmatic event at 1.20 Ga. Inherited and detrital zircons ranging in age from 1.3 to 1.9 Ga (plus a single 2.7 Ga core), ubiquitous Sm-Nd depleted mantle model ages ca. 2.0 Ga, and strongly negative ε Nd during Mesoproterozoic time all attest to the pre-Grenville heritage of this crust that was suggested by previous whole-rock Pb and Rb-Sr isotope studies. A single garnet amphibolite yielded a magmatic age of 0.73 Ga, equivalent to the Bakersville dike swarm, which cuts both the MHT and the adjacent Western Blue Ridge. Zircons from this sample display 0.47 Ga metamorphic rims. Zircons from all other samples have well-developed ca. 1.0 Ga metamorphic rims that date granulite-facies metamorphism. Silica contents of analyzed samples range from 45 to 76 wt%, reflecting the extreme diversity observed in the field and the highly variable protoliths. The MHT contrasts strikingly with basement of the adjacent Eastern and Western Blue Ridge, which comprise relatively homogeneous, 1.1 to 1.2 Ga granitic rocks with initial ε Nd values near 0. It appears to have more in common with distant Paleoproterozoic crustal terranes in the Great Lakes region, the southwestern United States, and South America.

Sedimentary and metasedimentary rocks within the southern Appalachian Blue Ridge and Inner Piedmont contain a valuable record of Late Proterozoic Laurentian margin evolution following the breakup of Rodinia. Paleogeographic reconstructions and increasing amounts of geochronologic and isotopic data limit the derivation of these paragneisses to the Laurentian and/or west Gondwanan craton(s). Southern Appalachian crystalline core paragneiss samples have ε Nd values between –8.5 and –2.0 at the time of deposition and contain abundant 1.1–1.25 Ga zircon cores with Grenville 1.0–1.1 Ga metamorphic rims. Less abundant detrital zircons are pre-Grenvillian: Middle Proterozoic 1.25–1.6 Ga, Early Proterozoic 1.6–2.1 Ga, and Late Archean 2.7–2.9 Ga. Blue Ridge Grenvillian basement has almost identical ε Nd values and displays the same dominant magmatic core and metamorphic rim zircon ages. Based on our data, nonconformable basement-cover relationships, and crustal ages in eastern North America, we contend that the extensive sedimentary packages in the southern Appalachian Blue Ridge and western Inner Piedmont are derived from Laurentia. ε Nd values from Carolina terrane volcanic, plutonic, and volcaniclastic rocks are isotopically less evolved than southern Appalachian paragneisses and Blue Ridge Grenvillian basement, easily separating this composite terrane from the mostly Laurentian terranes to the west. Neoproterozoic and Ordovician, as well as Grenvillian and pre-Grenvillian, zircons in eastern Inner Piedmont paragneisses indicate that these samples were deposited much later and could have been derived entirely from a Panafrican source or possibly a mixture of Panafrican and recycled Laurentian margin assemblages.

A number of Grenvillian basement massifs occur in the southern Appalachian Blue Ridge. The largest are contained in the Blue Ridge anticlinorium, which extends northward from its widest point in western North Carolina to Maryland. The Tallulah Falls dome, Toxaway dome, and Trimont Ridge area contain small internal basement massifs in the eastern and central Blue Ridge of the Carolinas and northeastern Georgia. All are associated with Paleozoic antiformal culminations, but each contains different basement units and contrasting Paleozoic structure. The Tallulah Falls dome is a broad foliation antiform wherein basement rocks (coarse augen 1158 ± 19 Ma Wiley Gneiss [ion microprobe, 207 Pb/ 206 Pb], medium-grained 1156 ± 23 Ma [ 207 Pb/ 206 Pb] and 1126 ± 23 Ma [ 207 Pb/ 206 Pb] Sutton Creek Gneiss, and medium-grained to megacrystic 1129 ± 23 Ma Wolf Creek Gneiss [sensitive high resolution ion microprobe, SHRIMP, 207 Pb/ 206 Pb]) form a ring and spiral pattern on the west, south, and southeast sides of the dome. Basement rocks are preserved in the hinges of isoclinal anticlines whose axial surfaces dip off the flanks of the dome. The Wiley Gneiss was intruded by Sutton Creek Gneiss. The Toxaway dome consists predominantly of coarse, banded 1151 ± 17 Ma and coarse augen 1149 ± 32 Ma (SHRIMP 206 Pb/ 238 U) Toxaway Gneiss folded into a northwest-vergent, gently southwest- and northeast-plunging antiform that contains a boomerang structure of Tallulah Falls Formation metasedimentary rocks in the core near the southwest end. The coarse augen gneiss phase constitutes a larger proportion of the Toxaway Gneiss toward the northeast. Field evidence indicates that the augen phase intruded the banded Toxaway lithology; U/Pb isotopic ages of these lithologies, however, are statistically indistinguishable. The Trimont Ridge massif occurs in an east-west–trending antiform west of Franklin, North Carolina, and consists of felsic gneiss that yielded a 1103 ± 69 Ma SHRIMP 207 Pb/ 206 Pb age. An ε Nd -depleted mantle model age of 1.5–1.6 Ga permits derivation of all of these basement rocks (including most from the western Blue Ridge) from eastern granite-rhyolite province crust, except the Mars Hill terrane rocks, which yield 1.8–2.2-Ga model ages. The small Grenvillian internal massifs were probably rifted from Laurentia during the Neoproterozoic, and became islands in the Iapetus ocean that were later swept onto the eastern margin of Laurentia during Ordovician subduction and arc accretion. These massifs were additionally penetratively deformed and metamorphosed during the Taconian and Neoacadian orogenies.

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New U-Pb and Rb-Sr isotopic data on the suite of the Crossnore Complex, herein referred to as the Crossnore plutonic suite (CPS), indicate that these plutons crystallized between 680 and 710 m.y. ago; the Crossnore Pluton itself may be as young as approximately 650 m.y. Bulk zircon separates from these rocks contain an older xenocrystic component due to contamination of the CPS magmas by older gneisses. During late Precambrian time, the ancestral Atlantic Ocean basin (Iapetus) formed and separated crustal units which are now part of the Caledonian-Appalachian mountains. These new CPS isotopie data indicate that continental rifting, which preceded the actual formation of the Iapetus Ocean, occurred approximately 690 m.y. ago. The Iapetus Ocean formed 690 to 570 m.y. ago.