Allen J. Dennis

The Carolina Zone: overview of Neoproterozoic to Early Paleozoic peri-Gondwanan terranes along the eastern Flank of the southern Appalachians

Abstract 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.

On resolving shear direction in foliated rocks deformed by simple shear

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.

The Carolina terrane in northwestern South Carolina, U.S.A.: Late Precambrian‐Cambrian deformation and metamorphism in a peri‐Gondwanan oceanic arc

The Carolina terrane comprises an exotic volcanic island arc in the hinterland of the Southern Appalachian orogen. The western portions of the Carolina terrane consist of zoned mafic-ultramafic plutonic complexes intruded into a pile of basalts and basaltic andesites. This sequence of rocks has been interpreted to represent an episode of intra-arc rifting prior to regional metamorphism and deformation. New U-Pb zircon ages from the Mean Crossroads complex in northwestern South Carolina along the central Piedmont suture confirm relative ages inferred from detailed mapping. Two foliated metadiorites yield U-Pb dates of 579±4 and 571±16 Ma, interpreted to be crystallization ages. A foliated metaquartz diorite yields a U-Pb date of 538±5 Ma interpreted to be a crystallization age. An undeformed, unmetamorpbosed diorite intruding these metamorphosed zoned complex intrusive rocks yields an age of ≈ 535 Ma. Hence we believe that intra-arc rifting and regional metamorphism and foliation development both occurred between =580 and 535 Ma. While petrographic and Ar-Ar studies support subsequent middle to late Paleozoic regional metamorphic overprint(s), or at least static recrystallization and/or uplift through hornblende-biotite-muscovite blocking temperatures for Ar, the 535 Ma, undeformed, unmetamorphosed intrusion suggests late Precambrian-Early Cambrian regional metamorphism and deformation was the event responsible for the regional metamorphic fabric in this area of the Piedmont. These observations contradict the idea that this metamorphism and fabric development are related to presumed early Paleozoic accretion of the Carolina arc to Laurentia as well as correlations with Middle Ordovician fabric elements in the eastern Piedmont. Instead, this fabric is interpreted to record a changing plate kinematics (e.g., migration of triple junction or change in relative motion vectors) during semicontinuous, diachronous development of an island arc on the Gondwanide margin.

The Carolina Terrane in northwestern South Carolina: Insights into the development of an evolving island arc

We present petrologic and whole rock geochemical data from metaigneous rocks in the northwestern Carolina terrane in South Carolina. Metaplutonic rocks form zoned intrusive complexes ~10 km in diameter that range from dioritic rims to clinopyroxenite-hornblendite-serpentinite cores. The complexes intrude metavolcanic rocks that include ankaramites and pyroxene porphyries as well as more common laminated amphibolites and greenstones. This rock association is not restricted to this area, but is also recognized in other parts of the western Carolina terrane including central Georgia, along the Georgia–South Carolina border, in north-central South Carolina, and central North Carolina. Whole rock data indicate a wide range in SiO2 content, fractionation trends that are transitional between tholeiitic and calc-alkaline, enrichments in large ion lithophile elements (LILE) and depletions in high field strength elements (HFSE), and Ti/V ratios between 10–20. Regional metamorphism and deformation accompanied arc magmatism ca. 570–535 Ma. We interpret the association to indicate rifting of a pre-existing volcanic arc. Deep seated fracturing accompanying arc-rifting is interpreted to be the mechanism by which highly magnesian magmas ascend to high crustal levels without appreciable fractionation. The maficultramafic zoned intrusive complexes and associated volcanic rocks formed at the same time or slightly after the Persimmon Fork Formation in the well-characterized eastern portions of the Carolina terrane. Strong lithologic and temporal similarities between the Cadomian orogeny (Brittany, France) and the Carolina terrane suggest that Cadomia may be a better candidate for comparison with Carolina than the Avalon terrane of New England or Maritime Canada. Dennis, A. J., and Shervais, J. W., 1996, The Carolina terrane in northwestern South Carolina: Insights into the development of an evolving island arc, in Nance, R. D., and Thompson, M. D., eds., Avalonian and Related Peri-Gondwanan Terranes of the Circum–North Atlantic: Boulder, Colorado, Geological Society of America Special Paper 304. 237 SP304-15.QXD 5•16•96 10:18 am Page 237

Arc rifting of the Carolina terrane in northwestern South Carolina

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 are 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 intea-arc rifting which allowed primitive arc magmas to ascend and erupt without significant crystal fractionation or lithospheric assimilation. This interpretation may help resolve some stratigraphie problems in the eastern part of the Carolina terrane (Carolina slate belt).

Is the central Piedmont suture a low-angle normal fault?

In the crystalline southern Appalachians, the Carolina are terrane is in fault contact with the Piedmont terrane along a seismically reflective surface dipping toward the hinterland and called the central Piedmont suture. The central Piedmont suture may be interpreted as a thrust, but existing data also support a Silurian-Devonian, normal-slip origin: (1) There are lower-grade rocks in the hanging wall than in the footwall. (2) A normal-fault solution allows simultaneous metamorphism of the Piedmont terrane and Carolina terrane, prior to their juxtaposition along the central Piedmont suture. (3) Mineral ages in the Piedmont terrane are older in the west than in the east, consistent with an eastward-progressive unroofing. (4) Along the western edge of the Carolina terrane, a linear belt of Devonian subalkalic to alkalic granitoids and gabbro-norites with low initial 87Sr/86Sr ratios may represent mantle-derived magmas along the axis of rifting that are contemporary with major crustal extension. The westernmost Piedmont terrane includes the Chauga belt. The Chauga belt comprises metavolcanic and metaplutonic units similar in rock type and age to those of the western Carolina terrane. Chauga belt rocks are interpreted to be the westernmost exposures of the Carolina terrane, translated west on the lower plate by extension. The Piedmont and Carolina terranes may thus compose a single lithotectonic element. The Piedmont terrane would represent the basement on which the arc was constructed; the terrane was uplifted during extension along a major low-angle normal fault, recognized today as the central Piedmont suture.

Petrology and geochemistry of Neoproterozoic volcanic arc terranes beneath the Atlantic Coastal Plain, Savannah River Site, South Carolina

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