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The eastern margin of North America has been shaped by a series of tectonic events including the Paleozoic Appalachian Orogeny and the breakup of Pangea during the Mesozoic. For the past similar to 200 Ma, eastern North America has been a passive continental margin; however, there is evidence in the Central Appalachian Mountains for post-rifting modification of lithospheric structure. This evidence includes two co-located pulses of magmatism that post-date the rifting event (at 152 and 47 Ma) along with low seismic velocities, high seismic attenuation, and high electrical conductivity in the upper mantle. Here, we synthesize and evaluate constraints on the lithospheric evolution of the Central Appalachian Mountains. These include tomographic imaging of seismic velocities, seismic and electrical conductivity imaging along the Mid-Atlantic Geophysical Integrative Collaboration array, gravity and heat flow measurements, geochemical and petrological examination of Jurassic and Eocene magmatic rocks, and estimates of erosion rates from geomorphological data. We discuss and evaluate a set of possible mechanisms for lithospheric loss and intraplate volcanism beneath the region. Taken together, recent observations provide compelling evidence for lithospheric loss beneath the Central Appalachians; while they cannot uniquely identify the processes associated with this loss, they narrow the range of plausible models, with important implications for our understanding of intraplate volcanism and the evolution of continental lithosphere. Our preferred models invoke a combination of (perhaps episodic) lithospheric loss via Rayleigh-Taylor instabilities and subsequent small-scale mantle flow in combination with shear-driven upwelling that maintains the region of thin lithosphere and causes partial melting in the asthenosphere. U.S. National Science Foundation EarthScope programNational Science Foundation (NSF)NSF - Directorate for Geosciences (GEO) [EAR-1460257, EAR-1249412, EAR-1249438, EAR-1250988, EAR-1251538, EAR-1251515]; United States National Science FoundationNational Science Foundation (NSF) [EAR-1261681]; U.S. National Science Foundation GeoPRISMS program [EAR-1460257, EAR-1249412, EAR-1249438, EAR-1250988, EAR-1251538, EAR-1251515] Published version This work benefited from discussions at the EarthScope Synthesis Workshop on the Evolution of the Southern Appalachian Lithosphere, held at Brown University in March 2017. The authors acknowledge support from the U.S. National Science Foundation EarthScope and GeoPRISMS programs via grants EAR-1460257 (R. L. Evans), EAR-1249412 (E. Gazel), EAR-1249438 (E. A. Johnson), EAR-1250988 (S. D. King), EAR-1251538 (E. Kirby), and EAR-1251515 (M. D. Long). The authors thank Randy Keller for making the PACES gravity data available. The collection and dissemination of most of the geophysical data and models discussed in this study were facilitated by the Incorporated Research Institutions for Seismology (IRIS). The facilities of the IRIS Consortium are supported by the United States National Science Foundation under Cooperative Agreement EAR-1261681. The authors thank two anonymous reviewers whose valuable comments have helped us to improve the quality of the article. For the past 200 million years, the east coast of North America has been situated in the middle of a tectonic plate. Contrary to the expectations for this setting, a region of the Central Appalachian Mountains centered near the boundary between the U.S. states of Virginia and West Virginia exhibits atypical properties. The unusual observations include volcanic activity in the geologic past far away from a plate boundary, elevated rates of erosion associated with high topography in the Central Appalachians, and anomalous structure in the upper mantle that has been detected using geophysical methods. This article describes, synthesizes, and compares a suite of observations that show that this part of the Central Appalachians is unusual compared to other so-called passive continental margins. We discuss a range of different models that might describe how the lithosphere, or the rigid part of the crust and upper mantle that defines the tectonic plate, has evolved through time beneath our study region. We show that the lithosphere today is thin, and that past episodes of lithospheric loss involving a portion of dense lithosphere "dripping" into the mantle under the force of gravity may provide a good explanation for the observations.

Intraplate magmatic provinces found away from plate boundaries provide direct sampling of the composition and heterogeneity of the Earth's mantle. The chemical heterogeneities that have been observed in the mantle are usually attributed to recycling during subduction1-3, which allows for the addition of volatiles and incompatible elements into the mantle. Although many intraplate volcanoes sample deep-mantle reservoirs-possibly at the core-mantle boundary4-not all intraplate volcanoes are deep-rooted5, and reservoirs in other, shallower boundary layers are likely to participate in magma generation. Here we present evidence that suggests Bermuda sampled a previously unknown mantle domain, characterized by silica-undersaturated melts that are substantially enriched in incompatible elements and volatiles, and a unique, extreme isotopic signature. To our knowledge, Bermuda records the most radiogenic 206Pb/204Pb isotopes that have been documented in an ocean basin (with 206Pb/204Pb ratios of 19.9-21.7) using high-precision methods. Together with low 207Pb/204Pb ratios (15.5-15.6) and relatively invariant Sr, Nd, and Hf isotopes, the data suggest that this source must be less than 650 million years old. We therefore interpret the Bermuda source as a previously unknown, transient mantle reservoir that resulted from the recycling and storage of incompatible elements and volatiles6-8 in the transition zone (between the upper and lower mantle), aided by the fractionation of lead in a mineral that is stable only in this boundary layer, such as K-hollandite9,10. We suggest that recent recycling into the transition zone, related to subduction events during the formation of Pangea, is the reason why this reservoir has only been found in the Atlantic Ocean. Our geodynamic models suggest that this boundary layer was sampled by disturbances related to mantle flow. Seismic studies and diamond inclusions6,7 have shown that recycled materials can be stored in the transition zone11. For the first time, to our knowledge, we show geochemical evidence that this storage is key to the generation of extreme isotopic domains that were previously thought to be related only to deep recycling.

In the Caledonides of northern Scotland temperatures of metamorphism (Tm) and deformation (Td) progressively increase structurally up section in the Moine thrust sheet at the foreland edge of the Scandian (mid Silurian) orogenic wedge. However, the thermal history of the structurally overlying, more hinterland positioned thrust sheets is less well known. This study focuses on determining Td and Tm for both the central/upper part of the Moine thrust sheet and the lower part of the overlying Sgurr Beag thrust sheets in the middle of the Northern Highlands Terrane. Preserved microstructures and quartz c-axis fabric opening angles in the Moine and Sgurr Beag thrust sheets imply Td of 460 °C to 605 °C ± 50 °C. Thermobarometry and pseudosection-based P-T constraints, indicate Tm of ∼550–680 °C at 4.8–7.2 kbar in the Moine thrust sheet and Tm of ∼620 °C at 5.6–7.7 kbar in the Sgurr Beag thrust sheet. Together, Td and Tm in the Moine and Sgurr Beag thrust sheets indicate that deformation continued after peak metamorphic conditions in the Sgurr Beag thrust sheet. Monazite and xenotime petrochronology show that Tm, and possibly Td, record Precambrian metamorphism. Peak metamorphism is associated with the Knoydartian orogenic event (840-720 Ma), with possible reworking during Scandian thrusting (430-425 Ma).

Bermuda’s explosive past has changed how geologists think about the processes that make volcanoes. The chemical composition of the lavas analyzed in this study indicate that Bermuda ‘tapped’ a geologically young, volatile rich layer in the mantle, unlike anything previously known.

Accessory phase geochronology records several phases of metamorphism that affected Moine metasedimentary rocks in the Northern Highlands Terrane of Scotland. Within the Sgurr Beag nappe, monazite and xenotime record initial metamorphism at c. 750 Ma, with later resetting at c. 600 Ma and c. 425 Ma. Monazite, xenotime, titanite and zircon also cryptically record c. 750 Ma and c. 600 Ma metamorphism in the Naver and Ben Hope nappes prior to the dominant 435-415 Ma Scandian orogeny. Textural, geochemical, and isotopic data indicate that c. 600 Ma and c. 425 Ma isotopic resetting in the Sgurr Beag nappe was caused by fluid-mediated dissolution and reprecipitation of monazite and xenotime. The compositions of fluid-altered monazite fall into two distinct trends, with high and low Y+HREE apparently reflecting the distinct timing and temperature conditions of dissolution and reprecipitation. Monazite-xenotime thermometry is applied to constrain the temperatures of fluid alteration. High Y+HREE, low Th/U monazite is related to high temperature (∼650°C) alteration at c. 600 Ma and low Y+HREE, high Th/U monazite is related to lower temperature (∼400°C) reactivity at c. 425 Ma. These data show that the composition of reprecipitated monazite is sensitive to metamorphic conditions during fluid alteration and is therefore useful in understanding the evolution of polyphase metamorphic terranes. These data suggest that a widespread metamorphic or metasomatic event affected northern Scotland at c. 600 Ma, likely related to higher heat flux and magmatic activity during the break-up of Rodinia. Additionally, the now adjacent Sgurr Beag and Moine nappes appear to have contrasting metamorphic records at least until Caledonian orogenesis, which suggests that final regional scale motion on the Sgurr Beag thrust must be of Caledonian age.

Understanding the evolution of passive margins requires knowledge of temporal and chemical constraints on magmatism following the transition from super-continent to rifting, to post-rifting evolution. The Eastern North American Margin (ENAM) is an ideal study location as several magmatic pulses occurred in the 200 My following rifting. In particular, the Virginia-West Virginia region of the ENAM has experienced two post-rift magmatic pulses at ∼152 Ma and 47 Ma, and thus provides a unique opportunity to study the long-term magmatic evolution of passive margins. Here we present a comprehensive set of geochemical data that includes new 40Ar/39Ar ages, major and trace-element compositions, and analysis of radiogenic isotopes to further constrain their magmatic history. The Late Jurassic volcanics are bi-modal, from basanites to phonolites, while the Eocene volcanics range from picrobasalt to rhyolite. Modeling suggests that the felsic volcanics from both the Late Jurassic and Eocene events are consistent with fractional crystallization. Sr-Nd-Pb systematics for the Late Jurassic event suggests HIMU and EMII components in the magma source that we interpret as upper mantle components rather than crustal interaction. Lithospheric delamination is the best hypothesis for magmatism in Virginia/West Virginia, due to tectonic instabilities that are remnant from the long-term evolution of this margin, resulting in a “passive-aggressive” margin that records multiple magmatic events long after rifting ended. This article is protected by copyright. All rights reserved.

The complex geodynamic evolution of Aeolian Arc in the southern Tyrrhenian Sea resulted in melts with some of the most pronounced along the arc geochemical variation in incompatible trace elements and radiogenic isotopes worldwide, likely reflecting variations in arc magma source components. Here we elucidate the effects of subducted components on magma sources along different sections of the Aeolian Arc by evaluating systematics of elements depleted in the upper mantle but enriched in the subducting slab, focusing on a new set of B, Be, As, and Li measurements. Based on our new results, we suggest that both hydrous fluids and silicate melts were involved in element transport from the subducting slab to the mantle wedge. Hydrous fluids strongly influence the chemical composition of lavas in the central arc (Salina) while a melt component from subducted sediments probably plays a key role in metasomatic reactions in the mantle wedge below the peripheral islands (Stromboli). We also noted similarities in subducting components between the Aeolian Archipelago, the Phlegrean Fields, and other volcanic arcs/arc segments around the world (e.g., Sunda, Cascades, Mexican Volcanic Belt). We suggest that the presence of melt components in all these locations resulted from an increase in the mantle wedge temperature by inflow of hot asthenospheric material from tears/windows in the slab or from around the edges of the sinking slab.

Intraplate magmatic provinces found away from plate boundaries provide direct sampling of the composition and heterogeneity of the Earth's mantle. The chemical heterogeneities that have been observed in the mantle are usually attributed to recycling during subduction

Tungsten is strongly incompatible during magmatic processes and is fluid mobile in subduction zones. Here we show that W isotope fractionation in arc lavas provide a powerful new tool for tracing slab dehydration and melting in subduction zones. Geochemically well characterized, representative arc-lavas from three subduction zones were chosen for this study to evaluate W isotope fractionation under different sub-arc conditions. Arc-lavas from SW Japan are produced by subducting a young, hot slab, and lavas from the volcanic front and rear arc of the Sangihe and Izu arcs are produced during subduction of a cold slab. The heaviest W isotope compositions ( δ 184 W ∼ 0.06 ‰ ) are observed in fluid-rich samples from the volcanic fronts of the Sangihe and Izu arcs. With increasing distance from the volcanic front, more melt-rich samples are characterized by progressively lighter W isotope compositions. Enriched alkali basalts from SW Japan, thought to be the product of mantle melting at a slab tear, and adjacent shoshonites have the lightest W isotope compositions ( δ 184 W ∼ 0 ‰ ). The correlation of W isotope fractionation with various indices of fluid release (e.g., Ce/Pb, Ba/Th) suggests that the heavy W isotope signatures record fluid recycling near the volcanic front due to dehydration of the subducted slab. Upon release of the heavy W, the residual slab preferentially retains isotopically light W, which is released during subsequent melting of drier lithologies in hot subduction zones, such as SW Japan. These data suggest that W isotopes can be used as a tracer of slab dehydration, potentially helping to determine the onset of cold subduction zone magmatism and hence, modern-style plate tectonics.

The rifted eastern North American margin (ENAM) provides important clues to the long-term evolution of continental margins. An Eocene volcanic swarm exposed in the Appalachian Valley and Ridge Province of Virginia and West Virginia (USA) contains the youngest known igneous rocks in the ENAM. These magmas provide the only window into the most recent deep processes contributing to the postrift evolution of this margin. Here we present new 40 Ar/ 39 Ar ages, geochemical data, and radiogenic isotopes that constrain the melting conditions and the timing of emplacement. Modeling of the melting conditions on primitive basalts yielded an average temperature and pressure of 1412 ± 25 °C and 2.32 ± 0.31 GPa, corresponding to a mantle potential temperature of ∼1410 °C, suggesting melting conditions slightly higher than average mantle temperatures beneath mid-ocean ridges. When compared with magmas from Atlantic hotspots, the Eocene ENAM samples share isotopic signatures with the Azores and Cape Verde. This similarity suggests the possibility of a large-scale dissemination of similar sources in the upper mantle left over from the opening of the Atlantic Ocean. Asthenosphere upwelling related to localized lithospheric delamination is a possible process that can explain the intraplate signature of these magmas that lack evidence of a thermal anomaly. This process can also explain the Cenozoic dynamic topography and evidence of rejuvenation of the central Appalachians.