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Abstract Low‐velocity anomalies in the upper mantle beneath eastern North America, including the Northern Appalachian Anomaly (NAA), the Central Appalachian Anomaly (CAA), and the weaker Southern Coastal Anomaly (SCA), have been characterized by many continent‐scale and regional seismic studies. Different models have been proposed to explain their existence beneath the passive margin of eastern North America, variously invoking the past passage of hot spot tracks, modern upwelling due to edge‐driven convection, or other processes. Depending on the nature and origin of these anomalies, they may influence, and/or be influenced by, the mantle transition zone (MTZ) structure beneath them. Previous receiver function studies have identified an overall thinner MTZ beneath the eastern margin of the US than beneath the continental interior. In this study, we resolve the MTZ geometry beneath these low‐velocity anomalies in unprecedented detail using the scattered wavefield migration technique. We find substantially thinned MTZ beneath the NAA and the CAA, and a moderately thinned MTZ beneath the SCA. In all cases, the thinning is achieved via a minor depression of the 410‐km discontinuity and a major uplift of the 660‐km discontinuity, which suggests the presence of a series of MTZ‐penetrating deep upwellings beneath eastern North America. The upwellings beneath eastern North America and a similar style upwelling beneath Bermuda may initiate from ponded thermally buoyant materials below the MTZ fed by hot return flows from the descending Farallon slab in the deep mantle. 

Abstract The Indo‐Burman subduction zone represents a global endmember for extreme sediment accretion and is a region characterized by ambiguous tectonic structure. The recent collection of broadband seismic data across the Indo‐Burman accretionary margin as part of the Bangladesh‐India‐Myanmar Array (BIMA) experiment provides an opportunity to investigate the subsurface velocity structure across the incoming plate of an endmember subduction system. We construct a three‐dimensional model for seismic shear velocity using a joint inversion of surface‐ and scattered‐wave constraints. Rayleigh‐wave phase velocities measured from ambient‐noise (12–25 s) and teleseismic earthquakes (20–80 s) constrain absolute shear velocities, while we constrain the locations of and relative contrasts across significant discontinuities in the subsurface using observations from scattered‐wave imaging. From the resulting inversion, we observe two model classes that characterize the evolution of consolidation within the markedly slow uppermost sediments and metasediments along a predominantly southwest‐to‐northeast trend. We interpret variations in deeper seismic structure under two proposed scenarios: (a) a Moho of ∼21–26 km depth underlying a package of metasediments and a thinned basement component, with a slow mantle lithosphere (4.2 km/s) that may contain retained melt from the onset of India‐Antarctica seafloor spreading; or (b) a Moho of ∼51–59 km depth underlying a package of metasediments, basement, and a thick slug of mafic material, which may correspond to significant Kerguelen‐plume‐related underplating. By combining constraints from highly resolved phase‐velocity estimates and scattered‐wave images, we successfully characterize the lateral transitions across the Indo‐Burman forearc margin. 

Abstract Along‐strike variation of the Laurentian rifted margin and the Appalachian orogen has long been recognized in the geologic record. We investigated the manifestation of this along‐strike variation at depth by generating scattered wavefield migration profiles from four dense seismic arrays deployed across the Appalachian orogen at different latitudes. All profiles exhibit a similar crustal thickness decrease of 15–20 km from the Mesoproterozoic Grenville Province to the Paleozoic Appalachian accreted terranes, but the Moho architecture differs dramatically along strike. The profiles beneath the central and southern Appalachians show a smoothly varying Moho geometry; in contrast, there is an abrupt Moho depth offset beneath the New England Appalachians. This contrast in Moho geometry may result from variations in the Laurentian rifted margin architecture, changes in Taconic orogeny subduction polarity, and greater crustal shortening during the Acadian‐Neoacadian orogeny in southern New England and the Alleghanian orogeny in the central and southern Appalachians. A first‐order along‐strike transition in the behavior of Appalachian orogenic processes is located between the central and New England Appalachians. 

Abstract The crust and upper mantle beneath the New England Appalachians exhibit a large offset of the Moho across the boundary between Laurentia and accreted terranes and several dipping discontinuities, which reflect Paleozoic or younger tectonic movements. We apply scattered wavefield migration to the SEISConn array deployed across northern Connecticut and obtain insights not previously available from receiver function studies. We resolve a doubled Moho at a previously imaged Moho offset, which may reflect westward thrusting of rifted Grenville crust. The migration image suggests laterally variable velocity contrasts across the Moho, perhaps reflecting mafic underplating during continental rifting. A west‐dipping feature in the lithospheric mantle is further constrained to have a slab‐like geometry, representing a relict slab subducted during an Appalachian orogenic event. Localized low seismic velocities in the upper mantle beneath the eastern portion of the array may indicate that the Northern Appalachian Anomaly extends relatively far to the south. 

Abstract Southern New England exhibits diverse geologic features resulting from past tectonic events. These include Proterozoic and early Paleozoic Laurentian units in the west, several Gondwana‐derived terranes that accreted during the Paleozoic in the east, and the Mesozoic Hartford Basin in the central part of the region. The Seismic Experiment for Imaging Structure beneath Connecticut (SEISConn) project involved the deployment of a dense array of 15 broadband seismometers across northern Connecticut to investigate the architecture of lithospheric structures beneath this region and interpret how they were created and modified by past tectonic events in the context of surface geology. We carried out P‐to‐S receiver function analysis on SEISConn data, including both single‐station analysis and common conversion point (CCP) stacking. Our images show that the westernmost part of Connecticut has a much deeper Moho than central and eastern Connecticut. The lateral transition is a well‐defined, ∼15 km step‐like offset of the Moho over a ∼20 km horizontal distance. The Moho step appears near the surface boundary between the Laurentian margin and the Gondwana‐derived Moretown terrane. Possible models for its formation include Ordovician underthrusting of Laurentia and/or modification by younger tectonic events. Other prominent features include a strong positive velocity gradient (PVG) beneath the Hartford basin corresponding to the bottom of the sedimentary units, several west‐dipping PVGs in the crust and mantle lithosphere that may correspond to relict slabs or shear zones from past subduction episodes, and a negative velocity gradient (NVG) that may correspond to the base of the lithosphere. 

Abstract Mount St. Helens (MSH) is anomalously 35–50 km trenchward of the main Cascade arc. To elucidate the source of this anomalous forearc volcanism, the teleseismic‐scattered wavefield is used to image beneath MSH with a dense broadband seismic array. Two‐dimensional migration shows the subducting Juan de Fuca crust to at least 80‐km depth, with its surface only 68 ± 2 km deep beneath MSH. Migration and three‐dimensional stacking reveal a clear upper‐plate Moho east of MSH that disappears west of it. This disappearance is a result of both hydration of the mantle wedge and a westward change in overlying crust. Migration images also show that the subducting plate continues without break along strike. Combined with low temperatures inferred for the mantle wedge, this geometry greatly limits possible source regions for mantle melts that contribute to MSH magmas and requires lateral migration over large distances.