Search for: All records

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. 

The southeastern New England Avalon Terrane (AT) accreted to the southeastern margin of the Nashoba Terrane (NT) at the onset of the Acadian orogeny (latest Silurian to Devonian). The NT represents the trailing edge of Ganderia. Rocks of the NT have previously been interpreted as having been extruded to the southeast over the AT as part of a channel flow zone (CFZ). Based on fold symmetries, it was inferred that only the top and center of this zone are located in the NT. Bedrock and structural mapping were carried out in the AT adjacent to the NT to test whether the bottom of the CFZ may be located in the AT. Data were collected from migmatitic biotite gneiss, mylonite, foliated quartzite, and gneiss. Structural data were divided into NE and SW domains. In the NE domain, foliations dip predominantly NW, and lineations plunge NE and SW. Migmatitic and gneissic rocks are absent in the SW domain, and orientations of mylonite zones and foliations in quartzite vary. Compared to the NE domain, rocks in the SW domain are strongly faulted and intruded by Ediacaran and late Silurian/Devonian granitic and gabbroic plutons. The presence of migmatite and consistency in structural orientations in the NE domain, and the general resemblance of structures to those in the NT make the NE domain a likely candidate to represent the bottom of the CFZ. U-Pb zircon data of the migmatitic biotite gneiss yielded a detrital zircon signature typical for Avalonia, with predominantly Mesoproterozoic and minor Paleoproterozoic and Tonian populations. Furthermore, zircon overgrowths are ~585 Ma, which suggests that high-grade metamorphism and partial melting occurred in the Ediacaran, i.e., not during the Acadian orogeny. Hence, the migmatitic biotite gneiss in the AT terrane does not represent the bottom of the CFZ. We believe that the Bloody Bluff Fault along the Nashoba-Avalon terrane boundary may have cut off the bottom of the CFZ. Our analysis is complemented by and provides context for high-resolution seismic imaging of the crust enabled by the ongoing GENESIS deployment of broadband seismometers across the NT. Preliminary results from GENESIS suggest a transition in crustal structure across the boundary between NT and AT, consistent with geological observations. 

Continental lithosphere is deformed, destroyed, or otherwise modified in several ways. Processes that modify the lithosphere include subduction, terrane accretion, orogenesis, rifting, volcanism/magmatism, lithospheric loss or delamination, small-scale or edge-driven convection, and plume-lithosphere interaction. The eastern North American margin (ENAM) provides an exceptional locale to study this broad suite of processes, having undergone multiple complete Wilson cycles of supercontinent formation and dispersal, along with ∼200 Ma of postrift evolution. Moreover, recent data collection efforts associated with EarthScope, GeoPRISMS, and related projects have led to a wealth of new observations in eastern North America. Here I highlight recent advances in our understanding of the structure of the continental lithosphere beneath eastern North America and the processes that have modified it through geologic time, with a focus on recent geophysical imaging that has illuminated the lithosphere in unprecedented detail.▪Eastern North America experienced a range of processes that deform, destroy, or modify continental lithosphere, providing new insights into how lithosphere evolves through time.▪Subduction and terrane accretion, continental rifting, and postrift evolution have all played a role in shaping lithospheric structure beneath eastern North America.▪Relict structures from past tectonic events are well-preserved in ENAM lithosphere; however, lithospheric modification that postdates the breakup of Pangea has also been significant. 

Abstract Previous geophysical studies in the New England Appalachians identified a ∼15 km offset in crustal thickness near the surface boundary between Laurentia and the accreted terranes. Here, we investigate crustal structure using data from a denser array: New England Seismic Transects experiment, which deployed stations spaced ∼10 km apart across the Laurentia‐Moretown terrane suture in northwestern Massachusetts. We used receiver function (RF) analysis to detectPtoSVconverted waves and identified multiple interfaces beneath the transect. We also implemented a harmonic decomposition analysis to identify features at or near the Moho with dipping and/or anisotropic character. Beneath the Laurentian margin, the Ps converted phase from the Moho arrives almost 5.5 s after the initialPwave, whereas beneath the Appalachian terranes, the pulse arrives at 3.5 s, corresponding to ∼48 and ∼31 km depth, respectively. The character of the RF traces beneath stations in the middle of our array suggests a complex transitional zone with dipping and/or anisotropic boundaries extending at least ∼30 km. This extension is measured in our profiles and perpendicular to the suture. We propose one possible crustal geometry model that is consistent with our observations and results from previous studies. 

Constraints on the thickness, transitional boundaries, and composition of Earth's crust are pivotal in studying its formation and evolution. We use data from 132 seismic installations throughout the northeastern US to explore how tectonic events, such as orogenesis and rifting, have altered the crust of the northeastern US and southeastern Canada, and to distinguish between Laurentia and the Appalachian terranes. We include data from seismic installations from the NEST and SEISConn experiments, spanning the Laurentia–Appalachian boundary, and present estimates of crustal thickness,V_p/V_s, and thickness of the transition between crustal and mantle rocks using Ps receiver functions. We find some first-order differences between Laurentia and Appalachian terranes, with Laurentia exhibiting thicker crust (c.39 v.c.33 km) and a broader crust–mantle transition thickness (c.3 v. <1.5 km). AverageV_p/V_svalues are similar between Laurentia (c.1.77) and Appalachian terranes (c.1.74); however, we identify anomalousV_p/V_sin a few regions, including highV_p/V_saround the Adirondack Mountains and lowV_p/V_sin southern New England. The southern New England region is also anomalous in terms of its systematically thinner crust and sharper crust–mantle transition, which may be a consequence of the formation and collapse of the Acadian altiplano during the mid-to-late Paleozoic. 

Abstract Seismic signals, whether caused by earthquakes, other natural phenomena, or artificial noise sources, have specific characteristics in the time and frequency domains that contain crucial information reflecting their source. The analysis of seismic time series is an essential part of every seismology-oriented study program. Enabling students to work with data collected from their own campus, including signals from both anthropogenic and natural seismic sources, can provide vivid, practical examples to make abstract concepts communicated in classes more concrete and relevant. Data from research-grade broadband seismometers enable us to record time series of vibrations at a broad range of frequencies; however, these sensors are costly and are often deployed in remote places. Participation in the Raspberry Shake citizen science network enables seismology educators to record seismic signals on our own campuses and use these recordings in our classrooms and for public outreach. Yale University installed a Raspberry Shake three-component, low-cost seismometer in the Earth and Planetary Sciences department building in Summer 2022, enabling the detection of local, regional, and teleseismic earthquakes, microseismic noise, and anthropogenic noise sources from building construction, an explosive event in a steam tunnel, and general building use. Here, we discuss and illustrate the use of data from our Raspberry Shake in outreach and education activities at Yale. In particular, we highlight a series of ObsPy-based exercises that will be used in courses taught in our department, including our upper-level Introduction to Seismology course and our undergraduate classes on Natural Disasters and Forensic Geoscience. 

Many regions of the Earth's mantle are seismically anisotropic, including portions of the lowermost mantle, which may indicate deformation due to convective flow. The splitting of ScS phases, which reflect once off the core-mantle boundary (CMB), is commonly measured to identify lowermost mantle anisotropy, although some challenges exist. Here, we use global wavefield simulations to evaluate commonly used approaches to inferring a lowermost mantle contribution to ScS splitting. We show that due to effects of the CMB reflection, only the epicentral distance range between 60° and 70° is appropriate for ScS splitting measurements. For this distance range, splitting is diagnostic of deep mantle anisotropy if no upper mantle anisotropy is present; however, if ScS is also split due to upper mantle anisotropy, the reliable diagnosis of deep mantle anisotropy is challenging. Moreover, even in the case of a homogeneously anisotropic deep mantle region sampled from a single azimuth by multiple ScS waves with different source polarizations (in absence of upper mantle anisotropy), different apparent fast directions are produced. We suggest that ScS splitting should only be measured at null stations and conduct such an analysis worldwide. Our results indicate that seismic anisotropy is globally widespread in the deep mantle.