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The Pampean flat slab in central Chile and Argentina is characterized by the inland migration and subsequent cessation of arc volcanism since the mid‐Miocene. Slab flattening also affects the distribution and number of intermediate‐depth earthquakes and the evolution of the overlying continental thermal structure. In this study, we combine thermal‐mechanical models with petrological models to examine temporal changes in pressure, temperature, and composition during flat‐slab subduction and estimate water carrying capacity, predicted melt distributions and predicted changes in melt composition. Model results indicate that the present‐day flattened Nazca plate carries water to ∼700 km inland from the trench and could cause flux melting if the material above the slab remains fertile. Observed slab seismicity matches areas where hydrated materials have ∼>3 wt% H2O in the oceanic crust and mantle lithosphere. Seismicity increases as slab water carrying capacity decreases (slab dehydration). As P‐T conditions and compositions of the rock trapped above the slab change during slab flattening, flux melting switches from a peridotite‐dominated early phase to a combined mid‐ocean ridge basalt/eclogite and peridotite melting at ∼8 Ma. The results provide broad consistency with known earthquake distributions, seismic velocities, and observed temporal and spatial changes in volcanic patterns above the Pampean flat slab and point toward the role of melt depletion in the decrease and ultimate cessation of arc volcanism in this region. Key Points: We estimate the water carrying capacities, melt distributions, and composition during the Pampean flat‐slab subductionThe predicted hydrated areas match the observed slab seismicity in the Pampean regionFlux melting above the flat slab is predicted to migrate inland, providing constraints on the causes of the spatiotemporal changes in magmatism [ABSTRACT FROM AUTHOR]

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 ∼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. Plain Language Summary: 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. Key Points: There is a present‐day geophysical anomaly in the upper mantle co‐located with unusually young volcanism in the Central AppalachiansWe synthesize constraints from geophysics, petrology/geochemistry, and geomorphology to constrain possible models for lithospheric lossWe favor one or more Rayleigh‐Taylor lithospheric instabilities, perhaps in combination with shear‐driven upwelling [ABSTRACT FROM AUTHOR]

Seismic tomography of the crust is an essential tool for studying the three‐dimensional structure of magmatic plumbing systems feeding active volcanoes, but it is often limited in resolution by the absence of deep local seismicity. Teleseismic receiver functions can be used to illuminate local structural variations, but typically do not account for the effects of three‐dimensional velocity heterogeneities. Here we harness the complementary strengths of both techniques by processing Ps‐P delay times derived from teleseismic receiver functions in a tomographic S wave inversion. Using our inversion technique, we produce the first tomographic crustal velocity model beneath Cleveland Volcano, identifying a vertically extensive high VP/VS anomaly beneath the volcano that likely signifies a middle‐to‐lower crustal magma reservoir. The observation is the first of its kind in the central Aleutians, illustrating the potential of our technique to advance our understanding of crustal magmatic systems without broad seismic networks or distributed local seismicity. Plain Language Summary: Detailed models of magma pathways and storage regions beneath volcanoes are critical for our understanding of the dynamics of volcanic systems and their long‐term hazard to society, but there is often a large gap in these images in the lower crust. With this study, we fill that gap at Cleveland Volcano in the central Aleutians, Alaska, by using seismic waves generated at the crust‐mantle interface. These waves traverse the entire crust before reaching seismic stations on the volcano, allowing us to infer relative variations in seismic velocity in the crust. With this technique, we produce an image of a low velocity anomaly directly beneath the volcano that reaches from the volcano through the entire crustal column that represents an extensive network of stored magma, rock, and/or volatiles. This network is likely what supplies Cleveland Volcano's persistent activity. Our new technique has the potential to help fill similar imaging gaps beneath other volcanoes. Key Points: We use a new technique, crustal Ps‐P tomography, to image crustal velocity structure beneath Cleveland VolcanoWe image a vertically extensive magma reservoir localized beneath the volcanoPs‐P tomography improves lower crustal imaging beneath sparsely instrumented volcanoes [ABSTRACT FROM AUTHOR]

Processes related to eruptions at arc volcanoes are linked by structures that transect the entire crust. Imaging the mid- to lower-crustal portions (here, ~5–15 km and >15 km respectively) of these magmatic systems where intermediate storage may occur has been a longstanding challenge. Tomography, local seismic source studies, geodetic, and geochemical constraints, are typically most sensitive to shallow (<5 km) storage and/or have insufficient resolution at these depths. Geophysical methods are even further limited at frequently-erupting volcanoes where well-developed trans-crustal magmatic systems are likely to exist, due to a lack of deep seismicity. Here we show direct evidence for mid-crustal magma storage beneath the frequently erupting Cleveland volcano, Alaska, using a novel application of seismic receiver functions. We use P-s scattered waves from the Moho as virtual sources to investigate S-wave velocities between the Moho and the surface. Our forward modeling approach allows us to provide direct constraints on the geometry of low velocity regions beneath volcanoes despite having a comparatively sparse seismic network. Our results show clear evidence of mid-crustal magma storage beneath the depths of located volcanic seismicity. Future work using similar approaches will enable an unprecedented comparative examination of magmatic systems beneath sparsely instrumented volcanoes globally. [ABSTRACT FROM AUTHOR]

Current end-member models for the geodynamic evolution of orogenic plateaus predict ( a) slow and steady rise during crustal shortening and ablative subduction (i.e., continuous removal) of the lower lithosphere or ( b) rapid surface uplift following shortening, which is associated with punctuated removal of dense lower lithosphere and/or lower crustal flow. This review integrates results from recent studies of the modern lithospheric structure, geologic evolution, and surface uplift history of the Central Andean Plateau to evaluate the geodynamic processes involved in forming it. Comparison of the timing, magnitude, and distribution of shortening and surface uplift, in combination with other geologic evidence, highlights the pulsed nature of plateau growth. We discuss specific regions and time periods that show evidence for end-member geodynamic processes, including middle-late Miocene surface uplift of the southern Eastern Cordillera and Altiplano associated with shortening and ablative subduction, latest Oligocene-early Miocene and late Miocene-early Pliocene punctuated removal of dense lower lithosphere in the Eastern Cordillera and Altiplano, and late Miocene-early Pliocene crustal flow in the central and northern Altiplano. [ABSTRACT FROM AUTHOR]

Flat-slab subduction occurs when the descending plate becomes horizontal at some depth before resuming its descent into the mantle. It is often proposed as a mechanism for the uplifting of deep crustal rocks ('thick-skinned' deformation) far from plate boundaries, and for causing unusual patterns of volcanism, as far back as the Proterozoic eon. For example, the formation of the expansive Rocky Mountains and the subsequent voluminous volcanism across much of the western USA has been attributed to a broad region of flat-slab subduction beneath North America that occurred during the Laramide orogeny (80-55 million years ago). Here we study the largest modern flat slab, located in Peru, to better understand the processes controlling the formation and extent of flat slabs. We present new data that indicate that the subducting Nazca Ridge is necessary for the development and continued support of the horizontal plate at a depth of about 90 kilometres. By combining constraints from Rayleigh wave phase velocities with improved earthquake locations, we find that the flat slab is shallowest along the ridge, while to the northwest of the ridge, the slab is sagging, tearing, and re-initiating normal subduction. On the basis of our observations, we propose a conceptual model for the temporal evolution of the Peruvian flat slab in which the flat slab forms because of the combined effects of trench retreat along the Peruvian plate boundary, suction, and ridge subduction. We find that while the ridge is necessary but not sufficient for the formation of the flat slab, its removal is sufficient for the flat slab to fail. This provides new constraints on our understanding of the processes controlling the beginning and end of the Laramide orogeny and other putative episodes of flat-slab subduction. [ABSTRACT FROM AUTHOR]

The Central Andes of southern Peru, Bolivia, Argentina and Chile (between 12°S and 42°S) comprise the largest orogenic plateau in the world associated with abundant arc volcanism, the Central Andean Plateau, as well as multiple segments of flat-slab subduction making this part of the Earth a unique place to study various aspects of active plate tectonics. The goal of this continental-scale ambient noise tomography study is to incorporate broad-band seismic data from 20 seismic networks deployed incrementally in the Central Andes from 1994 May to 2012 August, to image the vertically polarized shear wave velocity (Vsv) structure of the South American Cordillera. Using dispersion measurements calculated from the cross-correlation of 330 broad-band seismic stations, we construct Rayleigh wave phase velocity maps in the period range of 8–40 s and invert these for the shear wave velocity (Vsv) structure of the Andean crust. We provide a dispersion misfit map as well as uncertainty envelopes for our Vsv model and observe striking first-order correlations with our shallow results (∼5 km) and the morphotectonic provinces as well as subtler geological features indicating our results are robust. Our results reveal for the first time the full extent of the mid-crustal Andean low-velocity zone that we tentatively interpret as the signature of a very large volume Neogene batholith. This study demonstrates the efficacy of integrating seismic data from numerous regional broad-band seismic networks to approximate the high-resolution coverage previously only available though larger networks such as the EarthScope USArray Transportable Array in the United States. [ABSTRACT FROM AUTHOR]