John W. Shervais: Research Projects

Island Arcs and Accreted Terranes

The formation of island arcs and their accretion to continental margins is the most fundamental process forming continental crust since the Archean. I am currently studying the formation and evolution of accreted arc terranes in two main areas, the Carolina arc terrane in the southern Appalachians, and the Kohistan arc terrane of NW Pakistan. I have also been involved in projects examining arc rocks in other parts of the world, including the Sierra Nevada Foothills of California and the Greater Caucusus Mountains of southern Russia.
 

Island Arcs and Accreted Terranes,Project #1: The Carolina Terrane and a related Neoproterozoic Arc Complex Beneath the Atlantic Coastal Plain, South Carolina: The Savannah River Site Terranes

The hinterland of the Southern Appalachians, which lies SE of Grenville basement exposed in the Blue Ridge province, comprises a complex mosaic of exotic terranes of uncertain provenance. These terranes include (from NW to SE) the Inner Piedmont, the Carolina terrane (including the Carolina slate belt), and the Raliegh belt. Further to the SE, crystalline basement of the Laurentian margin is largely concealed beneath several kilometers of Mesozoic 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.

Over the last 35 years more than 8000 meters of basement core has been recovered from 26 deep wells at the Department of Energy's Savannah River Site. This core provides the only known exposure of basement terranes that lie SE of the Carolina terrane in central South Carolina, beneath Cretaceous and Tertiary sediments the Atlantic Coastal Plain. Core from these 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, (2) the Deep Rock metaigneous complex, (3) the Pen Branch metaigneous complex, and (4) the Triassic Dunbarton basin series.

The Crackerneck Metavolcanic Complex underlies the NW quarter of the site. It is dominated by intercalated mafic greenstones, felsic tuffs, and lapilli tuffs, all metamorphosed under lowermost greenschist to subgreenschist facies conditions. The Deep Rock metaigneous complex consists of two units: the Deep Rock metavolcanic suite and the DRB1 metaplutonic suite. The Deep Rock Metavolcanic Complex comprises mafic to felsic metavolcanic rocks that have been metamorphosed under middle to upper amphibolite facies conditions. The DRB1 metaplutonic suite includes hornblende diorites, hornblende quartz diorites, and tonalites. Rocks of the Deep Rock metaigneous complex were metamorphosed under upper greenschist to lower amphiblolite facies conditions. The Pen Branch metaigneous complex also consists of two units: the Pen Branch Metavolcanic suite (amphibolites, garnet amphibolites, and garnet-biotite schists, all metamorphosed under upper amphibolite to lowermost granulite facies conditions), and the PBF Metaplutonic suite. The PBF Metaplutonic suite were originally granodiorites metamorphosed under upper amphibolite to lower granulite facies conditions, but many metaplutonic rocks of this suite have undergone extensive hydrothermal alteration under greenschist facies conditions, during which potassic fluids infiltrating along fractures replaced calcic feldspar with K-feldspar, causing severe depletion of CaO and Sr, addition of K2O and SiO2, and coloring the rocks bright pink.

All of the metaplutonic and metavolcanic rocks have calc-alkaline fractionation trends, consistent with formation in active, subduction-related arc terranes. Reported crystallization ages of Å619 Ma (Deep Rock Metaigneous complex) to Å626 Ma (Pen Branch Metaigneous complex), however, show that these rocks do not correlate with accreted arc rocks that lie closer to the Grenville margin (Carolina terrane) because the latter are too young (Å535 to 570 Ma). The Crackerneck Metavolcanic complex may however, correlate with rocks of the Carolina Slate belt (Persimmon Fork formation). These ages may indicate that rocks of Deep Rock and Pen Branch metaigneous complexes are a continuation of Proterozoic basement which lies beneath, and is the older infrastructure of, the Carolina arc. This may also indicate that the contact between Crackerneck Metavolcanic Complex (ÅPersimmon Fork Formation) and Deep Rock/Pen Branch metaigneous complexes is equivalent to the angular unconformity between the Uwharrie Formation and the Virgilina sequence. Based on their compositions and ages, we tentatively correlate these rocks with the Hyco Formation in southern Virginia and central North Carolina. The Hyco Formation forms the infractructure of the Carolina terrane in Virginia and North Carolina, where it was affected by the circa 600 Ma "Virgilina" orogeny. The Deep Rock/Pen Branch arc may represent the infrastructure of the Carolina slate belt in South Carolina, detached by later tectonic events, or it may represent late Proterozoic arc infrastructure from another location in the arc that has been moved into its current location by transcurrent motions.

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Masters Theses

Island Arcs and Accreted Terranes, Project #2: Pre-Alleghenian Eclogites and granulites of the Carolina Terrane

We have recently discovered "medium temperature" eclogite within the central part of the Carolina Arc terrane, an exotic "Avalonian" terrane that originally formed adjacent to Gondwana, and was accreted to Laurentia during the Alleghanian orogeny. These relict garnet-pyroxene assemblages record a previously unrecognized episode of high pressure/medium temperature metamorphism within the infrastructure of the Carolina arc terrane that has broad implications for the evolution of the southern Appalachians, and for models of metamorphism and uplift in accreted arc terranes in general. New electron microprobe analyses and high-resolution x-ray maps of mineral phases within one of these eclogites record three distinct stages of metamorphism: This project is a detailed study of the distribution, origin, age, and P-T-t history of these eclogites/granulites and associated high grade rocks in the central Piedmont of South Carolina. We hope to illuminate the role of pre-Alleghanian collisions in the evolution of the Carolina terrane, and provide insights into the formation and exhumation of high pressure rocks in other collisional orogens.

Our work on these eclogites and their enclosing gneisses includes: (1) Detailed mapping (1:24,000) to determine the distribution of eclogite facies rocks and their enclosing gneisses selected quadrangles, (2) Calculation of paleo P-T-t paths based on chemical zonations in the primary mineral phases, retrograde mineral compositions, and on mineral inclusions in garnet; search for possible ultra-high P mineral assemblages in the inclusion suites; Calculation of paleo P-T-t paths for the enclosing gneisses, and search for relict high P assemblages in the enclosing gneisses; (3) Determination of the protoliths of the eclogites and their enclosing gneisses by whole rock major and trace element geochemistry, combined with their distribution and age relationships. (4) Determination of the age of eclogite formation and retrograde metamorphism using Sm-Nd isochrons and U-Pb on monazite.

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Recent tectonic models for the development of the western Trans-Himalaya suggest that the Kohistan-Ladakh terrane, which began as an intra-oceanic island arc in the late Jurassic, became a continental Andean-type arc on the southern margin of the Asiatic plate after the closure of a small back arc basin along the Main Karakoram Thrust in the mid Cretaceous (Å100 Ma). Continued northward subduction of the Tethyan lithosphere led to development of a "successor arc" in the late Cretaceous to late Paleocene, built upon the accreted Mesozoic arc. Initial closure of the Neo-Tethys ocean occurred circa 50 to 55 Ma, followed by underthrusting of Indo-Pakistan beneath Asia along the Main Mantle Thrust.

The Dir-Utror volcanics represent a continental margin arc assembled along the southern border of Asia after collision of the Mesozoic Kohistan island arc and its subsequent amalgamation to the mainland. Detailed geologic mapping shows that in the region around Dir the Dir-Utror volcanic series is dominated by mafic to intermediate composition rocks derived from LREE-enriched mantle beneath the arc. The high proportion of high-MgO basalts (12% areally) is similar to that observed in the Aleutian arc. The scarcity of more evolved felsic volcanics (dacite, rhyolite) can be explained by the nature of the underlying crust, which consists of accreted intra-oceanic arc volcanic and plutonic rocks, and is mafic relative to normal continental margins.

Most felsic volcanics (rhyolites, dacites) have REE systematics that are consistent with the hypothesis that they formed by fractional crystallization of more mafic basaltic andesites. Magma-mixing of low-MgO basalt with rhyolite or dacite does not seem to be important in this volcanic series, although this process appears to be common in the southern Andes . Some andesites may have formed as crustal melts, based on their high LILE contents, high La/Lu, and deep negative Eu anomalies. The REE pattern for one of these andesites crosses the chondrite-normalized patterns of dacites and rhyolites, showing that these rocks cannot be related by fractional crystallization, assimilation, or magma-mixing. The REE systematics of these andesites are compatible with an origin by crustal anatexis, leaving a refractory residue mineralogically similar to high pressure mafic and ultramafic granulites of the Jijal complex.

The northern margin of the Indian plate in NW Pakistan was deformed and metamorphosed to amphibolite facies conditions during its collision with Asia and the Kohistan arc terrane. The locus of this collision is the lower Swat region, just south of the confluence of the Swat and Indus Rivers. Rocks assigned here to the Lower Swat terrane include basement gneiss (the Swat Gneisses), a detached "cover sequence" of metasediments (the Alpurai Group), and the Manglaur thrust zone, a schüppenzone composed of imbricated slices of Swat Gneisses and Alpurai Group metasediments that underlies the more coherent units. Many of the tectonic contacts within and between these units have been intruded by sills of syntectonic tourmaline granite. Counter-clockwise rotation of India after its collision with Asia resulted in west-vergent thrusting, duplex formation, and doming of the earlier thrust sheets.

Petrography, microprobe-generated X-ray maps of chemical zoning in garnets, and garnet zoning profiles all indicate a multi-stage garnet growth history in paragneisses of Lower Swat terrane. Thermobarometry calculations indicate that garnet cores formed at lower temperatures, followed by progressively higher temperatures and pressures for subsequent garnet generations. These P-T estimates show that paragneisses of the Lower Swat terrane developed during two stages of prograde metamorphic growth under amphibolite to upper amphibolite facies conditions. The second phase of garnet growth was followed by retrograde metamorphism to greenschist facies conditions.

Correlation of the structural-tectonic history with the P-T estimates and garnet growth history suggest that during the initial collision of India and Asia around 55-45 Ma, the Lower Swat terrane was subducted underneath the Kohistan island arc terrane to a depth of Å30-35 km, forming the first generation of garnets. Counterclockwise rotation of India at Å45 Ma caused a temporary hiatus in subduction and resulted partial exhumation of the Lower Swat terrane, and partial resorption of the G1 garnets. Continued convergence with Asia, along with counterclockwise rotation of India, caused renewed subduction of India under the Kohistan arc terrane and resulted in widespread intrusion of tourmaline leucogranite along all active tectonic boundaries, and west-vergent thrusting and imbrication in the Indian plate margin. The second generation garnets are the result of this event. The extreme margins of G2 garnet and the widespread retrograde mineral assemblages result from rapid uplift and unroofing of the lower Swat terrane during extensional unroofing of the orogen along high-angle normal faults.
 
 

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    Doctoral Dissertations