Tectonism and metamorphism along a southern Appalachian transect across the Blue Ridge and Piedmont, USA

2021 ◽  
pp. 143-180
Author(s):  
Harold Stowell* ◽  
Elizabeth Bollen* ◽  
Matthew P. McKay* ◽  
J. Ryan Thigpen* ◽  
Hannah F. Dickson* ◽  
...  
Keyword(s):  
Crustal Thickening ◽  
Garnet Growth ◽  
Blue Ridge ◽  
Tectonic History ◽  
Southern Appalachian ◽  
The World ◽  
Higher Temperature ◽  
Orogenic Belts ◽  
Temperature Time ◽  
Sillimanite Schist

ABSTRACT The Appalachian Mountains expose one of the most-studied orogenic belts in the world. However, metamorphic pressure-temperature-time (P-T-t) paths for reconstructing the tectonic history are largely lacking for the southernmost end of the orogen. In this contribution, we describe select field locations in a rough transect across the orogen from Ducktown, Tennessee, to Goldville, Alabama. Metamorphic rocks from nine locations are described and analyzed in order to construct quantitative P-T-t paths, utilizing isochemical phase diagram sections and garnet Sm-Nd ages. P-T-t paths and garnet Sm-Nd ages for migmatitic garnet sillimanite schist document high-grade 460–411 Ma metamorphism extending south from Winding Stair Gap to Standing Indian in the Blue Ridge of North Carolina. In the Alabama Blue Ridge, Wedowee Group rocks were metamorphosed at biotite to staurolite zone, with only local areas of higher-temperature metamorphism. The Wedowee Group is flanked by higher-grade rocks of the Ashland Supergroup and Emuckfaw Group to the northwest and southeast, respectively. Garnet ages between ca. 357 and 319 Ma indicate that garnet growth was Neoacadian to early Alleghanian in the Blue Ridge of Alabama. The P-T-t paths for these rocks are compatible with crustal thickening during garnet growth.

Assembly and Dispersal of Supercontinents

2004 ◽  
Author(s):  
John J. W. Rogers ◽  
M. Santosh
Keyword(s):  
Crustal Thickening ◽  
Rock Deformation ◽  
Lateral Compression ◽  
Small Individual ◽  
Magmatic Intrusion ◽  
Magmatic Rocks ◽  
Higher Temperature ◽  
Temperature And Pressure ◽  
Almost All ◽  
Orogenic Belts

Supercontinents are assemblies that contain all, or nearly all, of the earth’s continental blocks. The concept arose with the recognition of Gondwana in the late 1800s (chapters 1 and 8), and it has been greatly expanded since then. In this chapter we build on the ideas developed in chapters 2 through 5 to discuss the origin and dispersal of supercontinents. The first section considers various mechanisms for the accretion of supercontinents, and the second section considers the reasons for their assembly. The last two sections consider evidence that former supercontinents have broken up and the reasons for their dispersal. We emphasize that the processes of accretion and dispersal overlap, with rifting of some parts of a supercontinent occurring at the same times as suturing in other areas. This overlapping produces a time when the supercontinent has its largest coherent area, which we refer to as the time of “maximum packing.” All supercontinents contain the same types of terranes that occur in individual continents (chapter 4). All models of assembly recognize that some terranes accreted as small individual blocks and some as continental-sized masses that contained several individual blocks that had been previously sutured together. Differences between models involve the area of the supercontinent that consisted of previously sutured large blocks and the area formed by accretion of small individual blocks. Resolution of this problem requires an understanding of the nature of orogenic belts developed during assembly, and we discuss this issue first. All orogenic belts have many similarities. They all underwent lateral compression that led to rock deformation and crustal thickening. Thickening pushed some rocks down into realms of higher temperature and pressure, causing metamorphism, and magmatic intrusion locally raised temperatures even higher in some areas. Almost all orogens contain magmatic rocks from various sources, including rocks partially melted within the orogen and magmas from subducted lithosphere and mantle below the deformed belt. Despite these similarities, different orogens contain features that enable us to distinguish different environments of formation.

scholarly journals Linking metamorphism, magma generation, and synorogenic sedimentation to crustal thickening during Southern Appalachian mountain building, USA

Lithosphere ◽  
2019 ◽  
Vol 11 (5) ◽  
pp. 722-749
Author(s):  
H.H. Stowell ◽  
J.J. Schwartz ◽  
S.B. Ingram ◽  
J. Madden ◽  
C. Jernigan ◽  
...  
Keyword(s):  
Regional Metamorphism ◽  
Foreland Basin ◽  
Progressive Increase ◽  
Crustal Thickening ◽  
Garnet Growth ◽  
Metamorphic Rocks ◽  
Blue Ridge ◽  
Magma Generation ◽  
The North ◽  
North American Craton

Abstract The nature of metamorphism, magma compositions, the spatial distribution of plutons, and foreland sediments reflect, in part, the character and thickness of continental crust. We utilized metamorphic pressure-temperature-time (P-T-t) paths, garnet Sm-Nd ages, zircon U-Pb ages, and pluton compositions to estimate paleocrustal thickness and temporal changes in crustal magma sources in the Blue Ridge of the southernmost Appalachians. Garnet Sm-Nd ages for amphibolite-facies metamorphic rocks range from 331 ± 4 to 320 ± 3 Ma. Low- and high-Sr/Y plutons that intruded these metamorphic rocks have zircon U-Pb ages of 390 ± 1 to 365 ± 1 Ma and 349 ± 2 to 335 ± 1 Ma, respectively. Therefore, garnet growth began during regional metamorphism synchronous with or shortly after intrusion of the youngest high-Sr/Y trondhjemite plutons. Phase diagram sections and thermobarometry indicate that garnet growth initiated at ∼5.8 kbar and 540 °C and grew during temperature increases of 60–100 °C and pressure increases of 2–3 kbar. The older, low-Sr/Y magmas are inferred to have been sourced in the crust at depths <∼30 km, insufficient for garnet to be stable. However, the younger, high-Sr/Y magmas are inferred to have been sourced at >30 km depths where garnet was stable. Hafnium isotopic compositions for all the plutons, but one, exhibit a range from negative initial εHf(i) to weakly positive initial εHf(i), indicating incomplete mixing of dominantly crustal sources. Our data require minimum crustal thicknesses of ∼33 km at 331 Ma; however, Alleghanian crustal thicknesses must have locally reached 39 km, based on crustal reconstruction adding the Alleghanian thrust sheet beneath the eastern Blue Ridge. We infer the presence of hot, tectonically thickened crust during intrusion of the early Alleghanian high-Sr/Y plutons and conclude that garnet growth and plutonism reflect a progressive increase in crustal thickness and depth of magma generation. The crustal thickening was synchronous with deposition of Mississippian to early Pennsylvanian sediments in the foreland basin of the Appalachian orogen between 350 and 320 Ma. This crustal thickening may have preceded emplacement of the Alleghanian thrust sheets onto the North American craton.

scholarly journals Episodic construction of the early Andean Cordillera unravelled by zircon petrochronology

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
José Joaquín Jara ◽  
Fernando Barra ◽  
Martin Reich ◽  
Mathieu Leisen ◽  
Rurik Romero ◽  
...  
Keyword(s):  
Continental Lithosphere ◽  
Complex Interplay ◽  
Continental Arcs ◽  
The Andes ◽  
Andean Cordillera ◽  
Continental Arc ◽  
The World ◽  
Oceanic Plates ◽  
Orogenic Belts ◽  
Continental Growth

AbstractThe subduction of oceanic plates beneath continental lithosphere is responsible for continental growth and recycling of oceanic crust, promoting the formation of Cordilleran arcs. However, the processes that control the evolution of these Cordilleran orogenic belts, particularly during their early stages of formation, have not been fully investigated. Here we use a multi-proxy geochemical approach, based on zircon petrochronology and whole-rock analyses, to assess the early evolution of the Andes, one of the most remarkable continental arcs in the world. Our results show that magmatism in the early Andean Cordillera occurred over a period of ~120 million years with six distinct plutonic episodes between 215 and 94 Ma. Each episode is the result of a complex interplay between mantle, crust, slab and sediment contributions that can be traced using zircon chemistry. Overall, the magmatism evolved in response to changes in the tectonic configuration, from transtensional/extensional conditions (215–145 Ma) to a transtensional regime (138–94 Ma). We conclude that an external (tectonic) forcing model with mantle-derived inputs is responsible for the episodic plutonism in this extensional continental arc. This study highlights the use of zircon petrochronology in assessing the multimillion-year crustal scale evolution of Cordilleran arcs.

Status of Ordovician and Silurian Radiolarian Studies in North America

1995 ◽  
Vol 8 ◽  
pp. 19-30 ◽  
Author(s):  
P. J. Noble ◽  
J. C. Aitchison
Keyword(s):  
North America ◽  
Middle Devonian ◽  
Pacific Rim ◽  
The Pacific ◽  
The World ◽  
Radiolarian Biostratigraphy ◽  
Orogenic Belts

Polycystine radiolaria that produce siliceous tests are known to range from Cambrian to Holocene. They have proven to be enormously useful in providing age control for siliceous marine sequences of Middle Devonian and younger ages, particularly for cherts and shales that are commonly devoid of other biostratigraphically useful fossils. The utility of radiolarian biostratigraphy became widely recognized in the 1970s and 1980s when it was applied in dating deformed marine siliceous sequences in orogenic belts around the world, most notably in Cordilleran North America and other areas along the Pacific rim (e.g., Jones and Murchey, 1986; Aitchison and Murchey, 1992; Ichikawa et al., 1990).

Petrogenesis of leucogranites in collisional orogens

2019 ◽  
Vol 491 (1) ◽  
pp. 179-207 ◽  
Author(s):  
Peter I. Nabelek
Keyword(s):  
Source Rocks ◽  
Crustal Thickening ◽  
Heat Sources ◽  
Active Deformation ◽  
Radiogenic Heat ◽  
Source Regions ◽  
Low Concentrations ◽  
Shear Heating ◽  
Orogenic Belts ◽  
Collisional Orogens

AbstractLeucogranites are a characteristic feature of collisional orogens. Their generation is intimately related to crustal thickening and the active deformation and metamorphism of metapelites. Data from Proterozoic to present day orogenic belts show that collisional leucogranites (CLGs) are peraluminous, with muscovite, biotite and tourmaline as characteristic minerals. Isotopic ratios uniquely identify the metapelitic sequences in which CLGs occur as sources. Organic material in pelitic sources results in fO2 in CLGs that is usually below the fayalite–magnetite–quartz buffer. Most CLGs form under vapour-poor conditions with melting involving a peritectic breakdown of muscovite. The low concentrations of Mg, Fe and Ti that characterize CLGs are largely related to biotite–melt equilibria in the source rocks. Concentrations of Zr, Th and rare earth elements are lower than expected from zircon and monazite saturation models because these minerals often remain enclosed in residual biotite during melting. Melting involving muscovite may limit the temperatures achieved in the source regions. A lack of nearby mantle heat sources in thick collisional orogens has led to thermal models for the generation of CLGs that involve flux melting, or large amounts of radiogenic heat generation, or decompression melting or shear heating, the last one emphasizing the link of leucogranites and their sources to crustal-scale shear zone systems.

Late Triassic orogenic assembly of the Tibetan Plateau: constraints from magmatism and metamorphism in the east Lhasa terrane

2021 ◽  
Author(s):  
Yanfei Chen ◽  
Zeming Zhang ◽  
Richard M Palin ◽  
Zuolin Tian ◽  
Hua Xiang ◽  
...  
Keyword(s):  
Crustal Thickening ◽  
Late Triassic ◽  
The Tibetan Plateau ◽  
Plate Convergence ◽  
Lhasa Terrane ◽  
The North ◽  
Tectonic History ◽  
Peak Metamorphism ◽  
Tethys Ocean ◽  
Lower Crustal

Abstract The early Mesozoic evolution of the Lhasa terrane, which represents a major component of the Himalayan-Tibetan orogen, remains highly controversial. In particular, geological units and events documented either side of the eastern Himalayan syntaxis (EHS) are poorly correlated. Here, we report new petrological, geochemical and geochronological data for co-genetic peraluminous S-type granites and metamorphic rocks (gneiss and schist) from the Motuo–Bomi–Chayu region of the eastern Lhasa terrane, located on the eastern flank of the EHS. Zircon U–Pb dating indicates that these units record both Late Triassic magmatic (216–206 Ma) and metamorphic (209–198 Ma) episodes. The granites were derived from a Paleoproterozoic crustal source with negative zircon εHf(t) values (–5.5 to –16.6) and TDM2 model ages of 1.51–1.99 Ga, and are interpreted to have formed by crustal anatexis of nearby metasediments during collisional orogeny and crustal thickening. The gneisses and schists experienced similar upper amphibolite-facies peak metamorphism and associated partial melting, followed by decompressional cooling and retrograde metamorphism. These rocks were buried to lower-crustal depths and then exhumated to the surface in a collisional orogenic setting during plate convergence. From comparison of these data to other metamorphic belts with similar grades and ages, and association of coeval granitic magmatism widespread in the central-east Lhasa terrane, we propose that the studied co-genetic magmatism and metamorphism in the Motuo–Bomi–Chayu region records Late Triassic accretion of the North Lhasa and South Lhasa terranes, which represents the first evidence of the Paleo-Tethys ocean (PTO) closure in this part of Asia. These data provide new constraints on the spatial and temporal evolution of the Paleo-Tethyan Wilson Cycle and provide a ‘missing link’ to correlate the geology and tectonic history of the Lhasa terrane continental crust on either side of the EHS.

Late Quaternary Landscape Evolution at Flat Laurel Gap, Blue Ridge Mountains, North Carolina

1988 ◽  
Vol 30 (1) ◽  
pp. 7-11 ◽  
Author(s):  
David S. Shafer
Keyword(s):  
North Carolina ◽  
Landscape Evolution ◽  
Late Quaternary ◽  
Appalachian Mountains ◽  
Blue Ridge ◽  
Blue Ridge Mountains ◽  
Southern Appalachian ◽  
Organic Sediment ◽  
Absolute Age ◽  
Age Determinations

Analysis of colluvial, fluvial, and bog sediments at Flat Laurel Gap (1500 m) in the Blue Ridge Mountains of North Carolina provides a record of late Quaternary landscape evolution. Thermoluminescence (TL) analysis provides the first absolute-age determinations available for presumed periglacial deposits in the southern Appalachian Mountains. The Pleistocene/Holocene transition, dated between 11,900 and 10,100 yr B.P., represents a period of climatic amelioration and a change from colluvial to alluvial processes. A TL date of 7400 ± 1000 yr B.P. for matrix within a block-stream indicates possible early Holocene reworking of Pleistocene periglacial colluvium. Organic sediment deposition in a bog that began about 3400 yr B.P. increased in rate from 0.02 to 0.09 cm/yr with the onset of logging and land clearance about 1880 A.D.

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