ABSTRACT The southern Appalachian western Blue Ridge preserves a Mesoproterozoic and mid-Paleozoic basement and Neoproterozoic to Ordovician rift-to-drift sequence that is metamorphosed up to sillimanite grade and dissected by northwest-directed thrust faults resulting from several Paleozoic orogenic events. Despite a number of persistent controversies regarding the age of some western Blue Ridge units, and the nature and extent of multiple Paleozoic deformational/metamorphic events, synthesis of several multidisciplinary data sets (detailed geologic mapping, geochronology and thermochronology, stable-isotope chemostratigraphy) suggests that the western Blue Ridge likely records the effects of two discrete orogenic events. The earlier Taconic (470–440 Ma) event involved a progression from open folding and emplacement of the Greenbrier–Rabbit Creek and Dunn Creek thrust sheets as a foreland fold-and-thrust to low-grade hinterland system (D 1A), followed by deep burial (>31 km), pervasive folding of the earlier-formed fault surfaces, and widespread Barrovian metamorphism (D 1B). Because this high-grade (D 1B) metamorphic event is recorded in Ordovician Mineral Bluff Group turbidites, this unit must have been deposited prior to peak orogenesis, possibly as a foreland basin or wedge-top unit in front of and/or above the developing fold-and-thrust belt. The later Alleghanian (325–265 Ma) event involved widespread northwest-directed brittle thrusting and folding related to emplacement of the Great Smoky thrust sheet (D 2 ; hanging wall of the Blue Ridge– Piedmont thrust). Mid-Paleozoic 40 Ar/ 39 Ar muscovite ages from western Blue Ridge samples likely record post-Taconic cooling (hornblende and some muscovite 40 Ar/ 39 Ar ages) and/or Alleghanian thrust-related exhumation and cooling (ca. 325 Ma muscovite 40 Ar/ 39 Ar and 300–270 Ma zircon fission-track ages), as opposed to resulting from a discrete Neoacadian thermal-deformational event. The lack of evidence for a discrete Neoacadian event further implies that all deformation recorded in the Silurian–Mississippian(?) Maggies Mill–Citico Formation must be Alleghanian. We interpret this structurally isolated sequence to have been derived from the footwall of the Great Smoky fault as an orphan slice that was subsequently breached through the Great Smoky hanging wall along the out-of-sequence Maggies Mill thrust.

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.

In the Appalachian orogen, the Neoproterozoic(?)–Lower Cambrian Chilhowee Group represents the initial drift-facies deposits along and across the eastern Laurentian continental margin following rifting. In the Southern Appalachians, this group forms thrust sheets along the west flank of the Talladega–Blue Ridge belt. Where the base is unfaulted, it lies depositionally above Ocoee Supergroup rift-facies rocks or Grenville basement. Regionally, the Chilhowee grades up into the Lower Cambrian Shady Dolomite, the initial deposits of the marginwide Cambrian–Ordovician carbonate bank. Sequences more interior to the orogen, including the Kahatchee Mountain Group (Talladega belt), the Nantahala and Brasstown Formations (western Blue Ridge), and the Hollis Quartzite (Pine Mountain belt), are considered to be correlative with the Chilhowee based upon similarities in lithostratigraphic sequence, sequence stratigraphy, sandstone provenance, and paleocurrent studies. Assuming an autochthonous Pine Mountain window, palinspastic restorations of foreland thrusts suggest that the Chilhowee Group restores essentially astride that window, and Chilhowee-equivalent units in the Talladega–Blue Ridge belts, in turn, restore farther southeast. This places the respective sequences southeastward in the order of increasing thickness and depth to basement from the base of the carbonate bank facies, with units restored farthest southeast having the most distal marine characteristics. Retro-deformation of thrust belt structures and the Pine Mountain cover sequence restores the Kahatchee Mountain Group at least to the subsurface position of the Wiggins-Suwannee suture, the southeastern limit of Laurentian continental crust, indicating that this group's basement was subducted beneath Gondwanan or peri-Gondwanan crust, and that the basement of even more outboard Laurentian sequences (e.g., eastern Blue Ridge) was overridden even farther.

There is no completely accepted definition of a mélange, and the papers in this volume reflect this fact. In our regional assessment, the term mélange is principally used for a technically fragmented and mixed body of rock. A different term, olistostrome, is used for a chaotic and mixed rock body that formed by sedimentary processes such as slumping or gravity sliding. The term olistostromal mélange is used here if sedimentary processes and tectonic deformation were both involved in the fragmentation and mixing. In some cases there is evidence that these were effectively concurrent. Four main belts of Paleozoic mélanges and olistostromes have been recorded in the Appalachians of the northeastern United States and mainland Canada. These include: (1) olistostromes and olistostromal mélanges along Logan’s line and the Taconic allochthons, which are related to thrusting during the Taconian orogeny; (2) mélanges associated with ophiolite fragments along the Baie Verte–Brompton line, which are thought to represent a Taconian suture; (3) mélanges containing ophiolite fragments along the Hurricane Mountain mélange belt (Boone and Boudette, this volume), which are thought to represent a Penobscottian terrane boundary; and (4) Acadian mélanges and olistostromes such as the Silurian Deadman Harbour mélange, an olistostromal mélange that probably formed at the front of an Acadian overthrust. In addition, Precambrian olistostromes have been recognized in southeastern New England and in the Green Head Formation of New Brunswick. These may have originated along normal faults on the rifted continental shelf of Gondwanaland, or they may relate to an earlier Pan-African cycle. These Precambrian olistostromes, therefore, accompany the extensional development of the Iapetus Ocean, whereas the Paleozoic olistostromes and mélanges mark its progressive closure. Five main sets of Paleozoic mélanges and olistostromes have been recorded in the central and southern Appalachians. These are: (1) a composite mélange-olistostrome belt in the Piedmont of Maryland and Virginia that includes olistostromes such as the Sykesville Formation (of unknown age) and mélanges such as those of the Morgan Run Formation (of debated age) and Mine Run Complex, some of which contain possible ophiolite fragments; (2) mélanges in the Blue Ridge Province from Virginia to Alabama, which contain possible ophiolite fragments; (3) Silurian or Early Devonian olistostromes of the Lay Dam Formation in the Talladega slate belt of Alabama; (4) mélanges, including the Falls Lake and Juliette mélanges in the Carolinas and Georgia, that border the Carolina terrane as defined by Secor and others (1983); and (5) broken formations and mélanges along major faults such as the Pulaski and Brevard, which are mostly related to Alleghanian thrusting. In addition, there is good evidence in the Ocoee Supergroup of the Blue Ridge Province for Late Proterozoic olistostromes related to the initiation of grabens prior to the opening of the Iapetus Ocean.

The Chunky Gal Mountain complex is a mélange consisting of largely metasedimentary matrix and abundant fragments and blocks of different rock types, including metasandstones, plagiogranites, amphibolites, and ultramafites. The mélange, as described here, is situated in the Blue Ridge geologic province of North Carolina. It may be part of a much larger mélange terrane, here called the Eastern Blue Ridge mélange, which extends southwestward into Georgia and Alabama, and northeastward to Virginia. The mélange at Chunky Gal Mountain has been generated during successive episodes of thrusting and shearing. It is probably of Acadian and/or Taconian age and represents the effect of either island arc-continent or continent-continent collision.

The southwesternmost exposures of Grenville-age basement in the Appalachian Blue Ridge are present in rootless anticlinoria that lie along the western margin of the Blue Ridge thrust sheet in north-central Georgia. The southernmost of these massifs, the Corbin Gneiss Complex, lies in the core of the Salem Church anticlinorium, while farther to the north, the Fort Mountain Gneiss is exposed in the core of a smaller, unnamed anticlinorium. Both the Corbin Gneiss Complex and the Fort Mountain Gneiss are mantled by thick sequences of predominantly clastic rocks of the Ocoee Supergroup. Those clastic rocks lying nonconformably on basement gneisses, the Pinelog and Parr-Branch Formations, respectively, clearly were derived from the gneisses themselves and are probably lithostratigraphic equivalents of the Snowbird Group, basal member of the Ocoee Supergroup. Conformably overlying these coarse clastics are graphitic phyllites, metaconglomerates, and sandy marbles of the Wilhite Formation. While relict textures related to Grenville-age granulite facies metamorphism still persist locally in the basement gneisses, no evidence of this event is apparent in the cover rocks. However, all aforementioned rocks show evidence of an episode of mid-Paleozoic regional metamorphism that retrograded earlier-formed, higher temperature mineral assemblages in basement rocks. Coincident with Paleozoic metamorphism was development of overturned to recumbent isoclinal folds (F 1) with well-developed axial-planar schistosity. Subsequent deformational events (1) fold earlier structures, (2) deform isograds formed during the Paleozoic metamorphism, and (3) are at least partially responsible for the arcuate trace of the Great Smoky fault 1 in this area.

For many years, the question regarding what happens to the rocks of the Talladega belt in the vicinity of their apparent northeastern terminus near Cartersville has been the subject of controversy. This has coincided with the debate over the age and correlation of metasedimentary rocks that overlie the billion-year-old Corbin gneiss complex to the east of Cartersville. Both of these problems are interrelated, and the resolution of each is dependent on the other. Stratigraphic relationships in the polydeformed rocks exposed in the Salem Church anticlinorium east of Cartersville indicate that the rocks unconformably overlying the Corbin gneiss complex are lithostratigraphic equivalents of the lowermost Ocoee Supergroup. These lithologies can be traced southwestward to the area east of Emerson where the Talladega belt has been presumed to end. Here, it is evident from studying the small- and large-scale structural features that folding has played an important role in the structural and stratigraphic complications that occur. Our mapping suggests that although part of the Ocoee Supergroup does disappear southwest of Cartersville because of folding, other parts of the Ocoee continue on to the southwest and into the Talladega belt. In the Talladega belt of Alabama, rock units such as the Heflin Phyllite, Abel Gap Formation, and Lay Dam Formation are lithologically similar but may be much younger than parts of the lowermost Ocoee Supergroup sequence present in Georgia. Other rock units of the Talladega belt in Alabama also resemble parts of the Ocoee sequence, but they too are not directly relatable to the Ocoee. AH long-range correlations can be considered only speculative until detailed mapping in western Georgia and eastern Alabama is completed. However, there is evidence to suggest that at least part of the Talladega belt is Precambrian in age and was deposited synchronously with the Ocoee Supergroup.

Talladega “Series” rock units in Polk and Paulding Counties have been mapped through the Cartersville area along the southeastern fault boundary of the “Corbin granite complex,” and correlated with rock units of the Great Smoky Group in Cherokee County. The Talladega “Series” in this area is bounded on the northwest by the Emerson (Cartersville) fault, along which the Talladega was thrust over (1) Valley and Ridge rocks ranging in age from Early Cambrian (Chilhowee Group) to Mississippian (Fort Payne Chert), (2) the Ocoee Supergroup, and (3) the “Corbin granite complex.” The Emerson fault also overrode the Great Smoky fault, which separates Ocoee from Chilhowee rocks in the Cartersville area. The thrust fault along the southeastern boundary of the Talladega “Series” is only one of numerous closely spaced imbricate faults within and southeast of the Talladega. The presence of closely spaced thrusts within the Talladega, the association of Hillabee Chlorite Schist-“Pumpkinvine Creek” lithologies with many of these faults, and the fact that detailed mapping has not been completed between the Rockmart-Yorkville area and the Alabama state line emphasize the need for caution in projecting correlations and interpretations across this area.