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The Piedmont of South Carolina and Georgia is a complex mosaic of exotic terranes of uncertain provenance. Farther south and east, these terranes form the basement beneath several kilometers of Cretaceous and Cenozoic sedimentary rocks, commonly referred to as the Atlantic Coastal Plain. The distribution and geologic history of this hidden crystalline basement can be inferred only on the basis of limited exposures at the margins of the Coastal Plain onlap, aeromagnetic lineaments that define basement trends in the subsurface, and core data from wells that penetrate basement. During the past 40 years, basement cores aggregating more than 6 miles (10,000 m) have been recovered from 57 deep wells at the Department of Energy's Savannah River Site. These cores provide the only known samples of basement terranes that lie southeast of the Fall Line in central South Carolina. Cores from the 57 deep wells, along with structural trends defined by aeromagnetic lineaments, allow us to define four distinct units within the basement beneath the Coastal Plain: (1) the Crackerneck Metavolcanic Complex (greenstones and felsic tuffs, all metamorphosed under greenschist-facies conditions), (2) the Deep Rock Metaigneous Complex (mafic to felsic volcanic and plutonic rocks metamorphosed under tower amphibolite-facies conditions), (3) the Pen Branch Metaigneous Complex (amphibolites, garnet amphibolites, garnet-biotite schists, and gneiss), and (4) the Triassic Dunbarton Basin Group, a sedimentary unit that fills a northeast-trending graben beneath younger sedimentary rocks of the Atlantic Coastal Plain. All of the metaplutonic and metavolcanic rocks have calc-alkaline fractionation trends, consistent with formation in subduction-related are terranes at convergent margins. Zircon U-Pb crystallization ages of ca. 626 Ma to 619 Ma, however, show that the Deep Rock and Pen Branch complexes do not correlate with the younger Carolina terrane (570-535 Ma) or Suwannee terrane (ca. 550 Ma). The Deep Rock and Pen Branch Metaigneous Complexes may he a continuation of Proterozoic basement that forms the older infrastructure of the Carolina are. The contact between the Crackerneck Metavolcanic Complex (= Persimmon Fork Formation?) and the Deep Rock and Pen Branch Metaigneous Complexes thus may be equivalent to the angular unconformity between the Uwharrie Formation and the Virgilina sequence. On the basis of their compositions and ages, we tentatively correlate these rocks with the Hyco Formation in southern Virginia and central North Carolina. The Hyco Formation constitutes the infrastructure of the Carolina terrane in Virginia and North Carolina, where it was affected by the ca. 600 Ma 'Virgilina' orogeny. The rocks of the Deep Rock and Pen Branch Metaigneous Complexes may have formed the arc infrastructure of the Carolina Slate belt in South Carolina, detached by later tectonic events, or may have formed the Late Proterozoic are infrastructure at another location in the arc that has been moved into its current location by transcurrent motions. Limited age and isotopic data suggest that none of these rocks correlate with the Suwannee terrane of North Florida and southern Georgia. Keywords: Neoproterozoic, peri-Gondwana, arc volcanism, Carolina terrane, geochemistry, petrology.

The Carolina Zone is an amalgamation of mainly Neoproterozoic--Early Paleozoic metaigneous-dominated terranes that are clustered along the eastern flank of the southern Appalachians. These terranes are distinguished from other divisions of the orogen by a commonality in gross geologic content and by their close spatial association. They are considered exotic relative to Laurentia on the basis of stratigraphic and tectonic evolution, paleontology, and position in the orogen analogous to that of exotic terranes in the northern Appalachians. They are probably peri-Gondwanan in origin. Within this first-order identity, the terranes exhibit remarkable heterogeneity, with respect to deposition, magmatism, and tectonothermal overprint. The depositional--magmatic history of the zone is viewed in three broad stages, including: (I) pre-600 Ma, (II) ca. 590-560 Ma, and (III) younger than ca. 550 Ma. Although each stage records significant felsic volcanism, there are few compelling stratigraphic linkages between terranes. Stage III plutonism may form a link between the two largest terranes in the zone. The isotopic evolution of the zone reflects the stratigraphic heterogeneity; isotopically juvenile magmatism in some terranes is coeval with more crustally evolved magmatism in others. The tectonothermal history of the zone is heterogeneous, producing a patchwork of suprastructural and infrastructural elements of different ages. Major tectonothermal events responsible for this pattern span the Neoproterozoic--earliest Cambrian, the Late Ordovician--Silurian, and the late Paleozoic. Evidence for regionally extensive events in the zone is sparse and such a fundamental concept as its time of accretion to Laurentia is speculative. The central Piedmont shear zone, a late Paleozoic ductile thrust that defines the western limit of the Carolina Zone, marks the final emplacement of the zone against Laurentian elements. Keywords: Carolina zone; Carolina terrane; Southern Appalachians; Neoproterozoic arcs; peri-Gondwanan terranes

Recent mapping and whole-rock geochemistry studies demonstrate that mafic metavolcanic rocks found along the boundary between the exotic Carolina terrane and the Inner Piedmont formed in a subduction-related volcanic arc and do not represent the Iapetan suture. Mafic metavolcanic rocks are spatially and genetically related to zoned mafic-ultramafic intrusive complexes. These rocks are similar to those found in other ancient and modern volcanic island arcs where ankaramites and picrites are well known, and they are locally associated with zoned complexes, e.g., Sierra Foothills-Klamath Mountains of the western U.S. Cordillera. We propose that prior to accretion to Laurentia in the early to middle Paleozoic, the Carolina arc terrane underwent an episode of intra-arc rifting which allowed primitive arc magmas to ascend and erupt without significant crystal fractionation or lithospheric assimilation. This interpretation may help resolve some stratigraphic problems in the eastern part of the Carolina terrane (Carolina slate belt).

We present a three-dimensional model for the formation of crenulations in ductile shear zones, based on compatibility conditions, infinitesimal-displacement equations, and our own field observations. In many shear zones, at least two and probably three mesoscopic slip systems are active. Crenulation slip is interpreted to compensate for the displacement component of foliation slip normal to the shear zone wall. In zones in which foliation slip is an important mode of deformation, foliation and crenulation slip vectors do not necessarily lie in the same plane, nor are they necessarily perpendicular to crenulation axes. Solutions for the orientation of the crenulation plane, crenulation slip vector, and magnitude of the crenulation slip shear strain belong to one of two possible solution sets, given the orientation of slipping foliation, the foliation slip vector, and the magnitude of slip on foliation. Hence, shear sense may be reliably interpreted from composite planar fabrics in ductile shear zones, but more specifically, shear direction cannot. These results have broad implications in the interpretation of the kinematic significance of mineral lineations oblique to crenulation axes, and in the deduction of shear strain from angular relations between planar fabric elements.