The Inner Piedmont is a large, composite, sillimanite-grade terrane that extends from near the Virginia–North Carolina border to central Alabama and consists of the eastern Tugaloo and Cat Square terranes. It is bound to the west by the Brevard fault zone and to the east by the central Piedmont suture. It is the core of the Neoacadian (360–350 Ma) orogen in the southern Appalachians and records Late Devonian–Mississippian closure and high-grade metamorphism (sillimanite I and II) of Siluro-Devonian sediments deposited in the remnant Rheic ocean basin. The Cat Square terrane is bounded by the younger-over-older Brindle Creek fault to the west and the central Piedmont suture to the east. It consists of a unique sequence of Siluro-Devonian metapsammite and pelitic schist that was intruded by Devonian anatectic granitoids (Toluca Granite, ∼378 Ma, and Walker Top Granite, ∼366 or ∼407 Ma). Rare mafic and ultramafic rocks occur in the eastern Cat Square terrane. Minimum sediment thickness is estimated at 4 km (13,000 ft). Detrital zircons indicate that Cat Square terrane rocks have a maximum age of ∼430 Ma, with both Laurentian (2.8, 1.8, 1.4, 1.1 Ga) and peri-Gondwanan (600, 500 Ma) affinities. Deposition on oceanic crust explains the existence of several mafic and ultramafic bodies and the absence of continental basement in the Cat Square terrane. The Cat Square terrane petrotectonic assemblage represents a Siluro-Devonian remnant ocean basin between Laurentia and the approaching Carolina superterrane. Metapsammite and pelitic schist may represent turbidites shed from approaching tectonic highlands on both flanks of the closing ocean. Palinspastic restoration of the Inner Piedmont constrains the location of the Cat Square basin to the Pennsylvania embayment, and links the mid-Devonian to Mississippian deformation in the Neoacadian core to the SW-migrating pulses of the diachronous Acadian-Neoacadian clastic wedge. Location and SW migration of the clastic wedge in concert with structural patterns in the Inner Piedmont support a transpressive NW-directed collision of the Carolina superterrane with the New York promontory and zippering the basin shut from NE to SW.

We present a newly compiled geologic map of the Pine Mountain window based on available 1:24,000 (and smaller) scale geologic maps; this map provides an improved basis to reconcile long-standing issues regarding tectonic evolution. We integrate sensitive high-resolution ion microprobe (SHRIMP) single-grain U-Pb ages of igneous, metamorphic, and detrital zircons from Grenville basement rocks, associated metasedimentary units, and cover rocks to help clarify the pre-Appalachian history and to better delimit the distribution of Laurentian versus peri-Gondwanan and Gondwanan units along the southeast flank of the window. U-Pb results indicate that some units, which earlier had been correlated with Neoproterozoic to Early Cambrian Laurentian rift deposits of the Ocoee Supergroup (i.e., Sparks-Halawaka Schist), actually are supracrustal rocks deposited prior to ~1100 Ma that were intruded and metamorphosed during the Ottawan phase of the Grenville orogeny. Zircons from the Phelps Creek Gneiss are 425 ± 7 Ma and overlap in time with plutons that intruded rocks of the Carolina superterrane during the Silurian (i.e., the Concord-Salisbury suite). The host units to the Phelps Creek Gneiss had also previously been interpreted as Sparks-Halawaka Schist, but field relations combine with the Silurian intrusive age to suggest that they rather belong to the peri-Gondwanan Carolina superterrane, helping to refine the position of the Central Piedmont suture in its most southern exposures. Results suggest that the Pine Mountain window is not framed by a single fault, but by Alleghanian faults of different timing, rheology, and kinematics, some of which were reactivated while others were not. The new map and U-Pb dates reveal that the southwesternmost exposures of the Central Piedmont suture are located farther northwest, so the width of the Pine Mountain window narrows from 22 km wide in central Georgia to only 5 km in Alabama. At its narrowest, the flanks of the Pine Mountain window are marked by two relatively thin normal faults (the Towaliga and Shiloh faults, northwest and southeast, respectively) that have excised the wider, earlier-formed mylonite zones. All of the Alleghanian faults are cut by later high-angle, normal and left- and right-slip brittle faults (Mesozoic?), which also influenced the present configuration of the window.

A new tectonic map of the southern and central Appalachians incorporates modern field and structural-stratigraphic, geochronologic (mostly sensitive high-resolution ion microprobe–reverse geometry, SHRIMP–RG, and Sm-Nd), geochemical, and geophysical data to identify crustal boundaries and blocks. Major tectonic units include the ∼735 Ma Laurentian failed rift, ∼565 Ma rifted margin sedimentary-volcanic assemblage deposited on Grenvillian and pre-Grenvillian crust, the Laurentian platform, and a series of distal Laurentian terranes (Cowrock, Cartoogechaye, Tugaloo-Chopawamsic-Potomac) accreted to Laurentia during the Taconian (Ordovician) or Neoacadian (Late Devonian–early Mississippian) orogenies. The Dahlonega gold belt consists of more proximal metasandstone and pelitic schist; it also contains Ordovician arc volcanic rocks, and a mixed detrital zircon suite of Laurentian and Gondwanan, or Penokean, affinity. The newly recognized Cat Square terrane contains Laurentian, Avalonian, and 430 Ma detrital zircons, and is considered a remnant ocean basin that closed during Acadian-Neoacadian accretion of the Carolina superterrane. The Pine Mountain terrane (southernmost exposed Appalachians) consists of Grenvillian basement and a cover sequence bearing Gondwanan or Penokean detrital zircons. The Carolina superterrane contains numerous peri-Gondwanan terranes that were deformed, metamorphosed, and amalgamated prior to 530 Ma, then accreted to Laurentia during the Neoacadian along the central Piedmont suture. The Raleigh-Goochland terrane contains blocks of Laurentian basement and cover that moved SW (dextrally) out of the collision zone to the north as the Theic ocean closed north to south during the early Alleghanian orogeny. This event also produced the Kiokee-Raleigh belt high-grade metamorphic core in the eastern Piedmont, and includes faults of the Pine Mountain window. The latter is framed by Alleghanian thrust and dextral faults formed at different crustal depths (times?). Subsurface components of the southern and central Appalachians are recognized in potential field and limited drill data. The Carolina superterrane extends beneath the Coastal Plain—possibly eastward to the East Coast magnetic anomaly. South of the Carolina superterrane and north of the Wiggins suture is the Brunswick (Charleston) terrane, another peri-Gondwanan terrane. The east–west Alleghanian Wiggins suture with the Suwannee terrane is recognizable to the south beneath Georgia and Alabama in potential field data, truncating all Appalachian structures and older crustal blocks west of the Appalachians. South of the suture, African basement and cover lie in the eastern Florida subsurface, while to the west are other Gondwanan or peri-Gondwanan components that may have originally connected with Yucatan.

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”

Abstract The Appalachian Inner Piedmont (IP) extends along orogenic strike some 700 km from North Carolina to Alabama. Its physical attributes contrast with those of other Appalachian tectonic elements: gentle dip of dominant foliation; imbricate stack of fold nappes; dominant sillimanite-grade metamorphism and near ubiquitous migmatization; heterogeneous, non-plane deformation; and earlier S-foliations transposed to C-foliations southeast of the mid-Palaeozoic Brevard fault zone forming a 10–20 km wide amphibolite-facies shear zone along the western flank of the IP. The IP contains west-and SW-directed thrust sheets and mineral stretching lineation, sheath folds on all scales, and other indicators that define a curved crustal flow pattern throughout the belt. Field and modern geochronologic data confirm that the IP is not exotic. It contains a Laurentian component (eastern Tugaloo terrane) and an internal terrane (Cat Square) that contains both Laurentian and Gondwanan detrital zircons, separated by the Brindle Creek fault. Cat Square terrane rocks likely accumulated in a Devonian remnant ocean that closed beginning c . 400 Ma. The complex but consistently asymmetric, NW- to west- to SW-directed flow pattern throughout the IP reflects confinement beneath a > 15 km thick overburden produced during subduction of Cat Square and Laurentian components beneath the approaching Carolina superterrane along the Central Piedmont suture. Oblique NE-to-SW transpressive subduction to > 15 km depth initiated partial melting, forcing escape from the collision zone in an along-strike orogenic channel. The IP detached from rocks to the west of the mid-Palaeozoic Brevard fault zone as the collision zone tightened and the IP mass flowed c . 200 km southwestward in the channel. The top of the channel is preserved at the NE end of the IP, and the base (Brevard fault zone) is preserved to the west and SW. As an exhumed orogenic channel, the curved IP flow paths may provide insight for middle to lower crustal deformation and flow in modern orogens.