Search

SUMMARY The Transportable Array in south-central Alaska spans several subduction zone features: backarc, forearc and volcanic arc, making it an ideal tool to study subduction zone anisotropy. Shear wave splitting analysis of 157 local earthquakes of mb ≥ 3.0 that occurred between 2014 and 2019 yields 210 high-quality measurements at 23 stations. Splitting delay times (δt) are generally small (δt ≈ 0.3 s), increasing with distance from the trench. Arc-parallel fast directions, ϕ, are only seen in the forearc, but rotate to arc-perpendicular ϕ in the backarc. Observed ϕ values generally do not parallel teleseismic SKS splitting results, implying that the latter is sensitive primarily to subslab mantle flow, not mantle wedge dynamics. The forearc local-earthquake signal likely originates from anisotropic serpentinite in fractures atop the subducting Pacific Plate, with possible additional signal coming from fractures in the North American crust. Mantle wedge corner flow, potentially with additional arc-perpendicular anisotropy in the subducting slab, explains backarc anisotropy.

Magnetic stripes have long been assumed to be indicative of oceanic crust. However, continental crust heavily intruded by magma can also record magnetic stripes. We re-evaluate the nature of the Cuvier Abyssal Plain (CAP), offshore NW Australia, which hosts magnetic stripes and has previously been defined as oceanic crust. We show that chemical data from a basalt within the CAP, previously described as an enriched mid-ocean ridge basalt, could equally be interpreted to contain evidence of contamination by continental material. We also recognize seaward-dipping reflector sequences in seismic reflection data across the CAP. Borehole data from overlying sedimentary rocks suggests that these seaward-dipping reflectors were emplaced in a shallow water (500 km further offshore NW Australia than currently thought. This would impact plate tectonic reconstructions, as well as heat flow and basin modelling studies. Our work also supports the growing consensus that magnetic stripes cannot, by themselves, be used to determine crustal affinity.

Africa’s Cenozoic tectonism is often attributed to mantle plumes, particularly below East Africa, but their morphology, number, location, and impact on the African lithosphere are debated. The broad slow wavespeed African Superplume, ubiquitous in large-scale tomographic models, originates below South Africa, reaching the surface somewhere below East Africa. However, whether the diverse East African mantle geochemistry is best reconciled with one heterogeneous upwelling, or current tomographic models lack the resolution to image multiple distinct plumes, remains enigmatic. S-wavespeed tomographic images of Africa are legion, but higher-frequency P-wavespeed whole-mantle models possessing complementary diagnostic capabilities are comparatively lacking. This hinders attempts to disentangle the effects of Cenozoic hotspot tectonism and Pan African (and older) tectonic events on the East African lithosphere. Here we develop a continental-scale P-wave tomographic model capable of resolving structure from upper-to-lower mantle depths using a recently-developed technique to extract absolute arrival-times from noisy, temporary African seismograph deployments. Shallow-mantle wavespeeds are δVP ≈–4% below Ethiopia, but less anomalous (δVP ≥–2%) below other volcanic provinces. The heterogeneous African Superplume reaches the upper mantle below the Kenyan plateau. Below Ethiopia/Afar we image a second sub-vertical slow wavespeed anomaly rooted near the core-mantle boundary outside the African LLVP, meaning multiple disparately sourced whole-mantle plumes may influence East African magmatism. In contrast to other African cratons, wavespeeds below Tanzania are only fast to 90–135km depth. When interpreted alongside Lower Eocene on-craton kimberlites, our results support pervasive metasomatic lithospheric modification caused by subduction during the Neoproterozoic Pan-African orogeny.

SUMMARYSince the Mesozoic, central and eastern European tectonics have been dominated by the closure of the Tethyan Ocean as the African and European plates collided. In the Miocene, the edge of the East European Craton and Moesian Platform were reworked in collision during the Carpathian orogeny and lithospheric extension formed the Pannonian Basin. To investigate the mantle deformation signatures associated with this complex collisional-extensional system, we carry out SKS splitting analysis at 123 broad-band seismic stations in the region. We compare our measurements with estimates of lithospheric thickness and recent seismic tomography models to test for correlation with mantle heterogeneities. Reviewing splitting delay times in light of xenolith measurements of anisotropy yields estimates of anisotropic layer thickness. Fast polarization directions are mostly NW–SE oriented across the seismically slow West Carpathians and Pannonian Basin and are independent of geological boundaries, absolute plate motion direction or an expected palaeo-slab roll-back path. Instead, they are systematically orthogonal to maximum stress directions, implying that the indenting Adria Plate, the leading deformational force in Central Europe, reset the upper-mantle mineral fabric in the past 5 Ma beneath the Pannonian Basin, overprinting the anisotropic signature of earlier tectonic events. Towards the east, fast polarization directions are perpendicular to steep gradients of lithospheric thickness and align along the edges of fast seismic anomalies beneath the Precambrian-aged Moesian Platform in the South Carpathians and the East European Craton, supporting the idea that craton roots exert a strong influence on the surrounding mantle flow. Within the Moesian Platform, SKS measurements become more variable with Fresnel zone arguments indicating a shallow fossil lithospheric source of anisotropy likely caused by older tectonic deformation frozen in the Precambrian. In the Southeast Carpathian corner, in the Vrancea Seismic Zone, a lithospheric fragment that sinks into the mantle is sandwiched between two slow anomalies, but smaller SKS delay times reveal weaker anisotropy occurs mainly to the NW side, consistent with asymmetric upwelling adjacent to a slab, slower mantle velocities and recent volcanism.

The northern Main Ethiopian Rift (MER) in East Africa is considered a region of incipient oceanic spreading, with Miocene border faulting now largely abandoned at the expense of magmatic extension in the Wonji Fault Belt (WFB). However, whether magmatic extension began when the Ethiopian lithosphere was still-thick, or heavily stretched, is unknown. The Galema range, a linear Pliocene dike swarm parallel to the eastern margin of the present-day central MER, is an ideal study locale to constrain melting depths, and by inference the thickness of the lithosphere, during early magmatic rifting. To address this issue, we present whole-rock, trace element data on 77 samples of Galema range magmas. We interpret contrasting results between two modeling approaches as evidence for magma ponding subsequent to melt generation. Trace element models of melt generation reveal melting conditions of TP = 1418–1450 °C at 2.9–3.2 GPa, some ~68–100 °C above ambient. In contrast, Si/Mg activity thermobarometry, which probes the point at which these magmas last re-equilibrated with the mantle, yielded broadly similar temperatures (1435–1474 °C) but at lower pressures (2.1–2.6 ± 0.2 GPa: 78–89 km depth); these results are broadly parallel to contemporaneous magmatism on the western rift margin in the Akaki Magmatic Zone. We interpret these results as evidence for magma stalling at a thermo-mechanical boundary to ascent: the lithosphere-asthenosphere boundary. The Ethiopian continental lithosphere has therefore remained relatively thick late into the rifting process, with important potential implications for late-stage decompression melting prior to the onset of seafloor spreading.

The northern Hudson Bay region in Canada comprises several Archean cratonic nuclei, assembled by a number of Paleoproterozoic orogenies including the Trans-Hudson Orogen (THO) and the Rinkian–Nagssugtoqidian Orogen. Recent debate has focused on the extent to which these orogens have modern analogues such as the Himalayan–Karakoram–Tibet Orogen. Further, the structure of the lithospheric mantle beneath the Hudson Strait and southern Baffin Island is potentially indicative of Paleoproterozoic underthrusting of the Superior plate beneath the Churchill collage. Also in question is whether the Laurentian cratonic root is stratified, with a fast, depleted, Archean core underlain by a slower, younger, thermally-accreted layer. Plate-scale process that create structures such as these are expected to manifest as measurable fossil seismic anisotropic fabrics. We investigate these problems via shear wave splitting, and present the most comprehensive study to date of mantle seismic anisotropy in northern Laurentia. Strong evidence is presented for multiple layers of anisotropy beneath Archean zones, consistent with the episodic development model of stratified cratonic keels. We also show that southern Baffin Island is underlain by dipping anisotropic fabric, where underthrusting of the Superior plate beneath the Churchill has previously been interpreted. This provides direct evidence of subduction-related deformation at 1.8 Ga, implying that the THO developed with modern plate-tectonic style interactions.

The thick, seismically fast lithospheric keels underlying continental cores (cratons) are thought to have formed in the Precambrian and resisted subsequent tectonic destruction. A consensus is emerging from a variety of disciplines that keels are vertically stratified, but the processes that led to their development remain uncertain. Eastern Canada is a natural laboratory to study Precambrian lithospheric formation and evolution. It comprises the largest Archean craton in the world, the Superior Craton, surrounded by multiple Proterozoic orogenic belts. To investigate its lithospheric structure, we construct a frequency-dependent anisotropic seismic model of the region using Rayleigh waves from teleseismic earthquakes recorded at broadband seismic stations across eastern Canada. The joint interpretation of phase velocity heterogeneity and azimuthal anisotropy patterns reveals a seismically fast and anisotropically complex Superior Craton. The upper lithosphere records fossilized Archean tectonic deformation: anisotropic patterns align with the orientation of the main tectonic boundaries at periods ≤110 s. This implies that cratonic blocks were strong enough to sustain plate-scale deformation during collision at 2.5 Ga. Cratonic lithosphere with fossil anisotropy partially extends beneath adjacent Proterozoic belts. At periods sensitive to the lower lithosphere, we detect fast, more homogenous, and weakly anisotropic material, documenting postassembly lithospheric growth, possibly in a slow or stagnant convection regime. A heterogeneous, anisotropic transitional zone may also be present at the base of the keel. The detection of multiple lithospheric fabrics at different periods with distinct tectonic origins supports growing evidence that cratonization processes may be episodic and are not exclusively an Archean phenomenon.

Shallow - level sill emplacement can uplift Earth’s surface via forced folding, providing insight into the location and size of potential volcanic eruptions. Linking the structure and dynamics of ground deformation to sill intrusion is thus critical in volcanic hazard assessment. This is challenging, however, because: (1) active intrusions cannot be directly observed, meaning that we rely on transient host rock deformation patterns to model their structure; and (2) where ancient sill - fo ld structure can be observed, magmatism and deformation has long - since ceased. To address this problem, we combine structural and dynamic analyses o f the Alu dome, Ethiopia; a 3.5 - km - long, 346 - m - high, elliptical dome of outward - dipping, tilted lava flows c ross - cut by a series of normal faults. Vents distributed around Alu feed lava flows of different ages that radiate out from or deflect around its periphery. These observations, coupled with the absence of bounding faults or a central vent, implies that Alu is not a horst or a volcano, as previously thought, but is instead a forced fold. Interferometric synthetic aperture radar data captured a dynamic growth phase of Alu during a nearby eruption in A.D. 2008, with periods of uplift and subsidence previously attributed to intrusion of a tabular sill at 1 km depth. To localize volcanism beyond its periphery, we contend that Alu is the first forced fold to be recognized to be developing above an incrementally emplaced saucer - shaped sill, as opposed to a tabular sill or laccolith.

Cratons, the ancient cores of the continents, have survived thermal and mechanical erosion over multiple Wilson cycles, but the ability of their margins to withstand modification during continental convergence is debated. The Proterozoic Grenville orogeny operated for ≥ 300 Myr along the eastern edge of the proto-North American continent Laurentia, whose age varied north-to-south from ∼ 1.5 − 0.25 Gyr at the time of collision. The preserved Grenville Province, west of the Appalachian terranes, has remained largely tectonically quiescent since its formation. Thick, cool, mantle lithosphere that underlies these Proterozoic regions is typically identified by elevated seismic velocities but lithospheric modification by fluid/melt-derived metasomatic enrichment above a subduction zone, can lead to a reduction in V P with little effect on V S and density. Absolute P-wavespeed constraints are therefore a vital complement to existing S-wave tomographic models of North America to investigate craton edge modification mechanisms in the Grenville orogen. New P-wave tomographic imaging of the North American continent, which benefits from recent developments in arrival-time processing of regional network deployments from the Canadian shield, reveals along strike wavespeed variation in the Grenville orogen. In the north, high seismic wavespeeds (to depths of 250 km) extend eastwards, from the Archean core of North America to beneath the Canadian Grenville Province. In contrast, below the southern U.S., high lithospheric wavespeeds are restricted to west of the Grenville Province, in particular at depths less than 150 km. We argue that subduction-derived metasomatism beneath eastern Laurentia modified the southern Grenville, prior to thermal stabilization and perhaps mantle keel formation. Beneath the northern Grenville, the thick, depleted Laurentian lithosphere resisted extensive metasomatism. Along strike age differences in Grenvillian terranes and their resulting metasomatic modification histories suggests that at least 250 Myr is required for Proterozoic lithosphere to gain resistance to modification.

Magnetic stripes have long been used to define the presence and age of oceanic crust. However, continental crust heavily intruded by magma can record magnetic reversals akin to those observed in oceanic crust. We re-evaluate the nature of the Cuvier Abyssal Plain (CAP), offshore NW Australia, which hosts magnetic stripes and has previously been defined as oceanic crust. We use magnetic, 2D seismic reflection, and geochemical data to test whether the CAP structure and composition is consistent with unambiguous oceanic crust. We show chemical data from a basalt within the CAP, previously described as displaying an enriched MORB-like signature, actually contains evidence of contamination by continental material. We also recognise seaward-dipping reflector (SDR) sequences across the CAP. Borehole data from overlying sedimentary rocks suggests these SDRs were emplaced in a shallow-water (

The southern Alaskan margin captures a transition between compression and strike-slip dominated deformation, accretion of the over-thickened Yakutat terrane, termination of Aleutian arc magmatism and the enigmatic Wrangell Volcanic Field. The extent of subduction and mantle structure below this region is uncertain, with important implications for volcanism. We present compressional- and shear-wave mantle velocity models below south-central Alaska that leverage a new seismometer deployment to produce the most complete image of the subducting Pacific-Yakutat plate to date. We image a steeply-dipping slab extending below central Alaska to >400 km depth, which abruptly terminates east of ~145°W. There is no significant slab anomaly beneath the nearby Wrangell volcanoes. A paucity of volcanism is observed above the subducting Yakutat terrane, but the slab structure below 150 km depth and Wadati-Benioff zone here are similar to those along the Aleutian-Alaska arc. Features of the mantle wedge or overlying lithosphere are thus responsible for the volcanic gap.

This work was completed as part of Magee’s Junior Research Fellowship funded by Imperial College London. Muirhead acknowledges support from Fulbright New Zealand and the Ministry of Science and Innovation. The structure of upper crustal magma plumbing systems controls the distribution of volcanism and influences tectonic processes. However, delineating the structure and volume of plumbing systems is difficult because (1) active intrusion networks cannot be directly accessed; (2) field outcrops are commonly limited; and (3) geophysical data imaging the subsurface are restricted in areal extent and resolution. This has led to models involving the vertical transfer of magma via dikes, extending from a melt source to overlying reservoirs and eruption sites, being favored in the volcanic literature. However, while there is a wealth of evidence to support the occurrence of dike-dominated systems, we synthesize field- and seismic reflection-based observations and highlight that extensive lateral magma transport (as much as 4100 km) may occur within mafic sill complexes. Most of these mafic sill complexes occur in sedimentary basins (e.g., the Karoo Basin, South Africa), although some intrude crystalline continental crust (e.g., the Yilgarn craton, Australia), and consist of interconnected sills and inclined sheets. Sill complex emplacement is largely controlled by host-rock lithology and structure and the state of stress. We argue that plumbing systems need not be dominated by dikes and that magma can be transported within widespread sill complexes, promoting the development of volcanoes that do not overlie the melt source. However, the extent to which active volcanic systems and rifted margins are underlain by sill complexes remains poorly constrained, despite important implications for elucidating magmatic processes, melt volumes, and melt sources. Publisher PDF

During continental breakup, the locus of strain shifts from a broad region of border faulting and ductile plate stretching to a narrow zone of magma intrusion in a young ocean basin. Recent studies of volcanic rifts and margins worldwide suggest this shift occurs subaerially, before the onset of seafloor spreading. We test this hypothesis using recently acquired seismic reflection and borehole data from the Danakil Depression, Ethiopia, a unique region of transition between continental rifting and seafloor spreading. Our data, located near Dallol, ~30 km northwest of the Erta'Ale Volcanic Segment, reveal a remarkably thick (>1-km) sequence of young (~100-ka) evaporites in a basin bound by a major (≤400-m-throw), east-dipping normal fault. To generate such a large amount of subsidence in such a relatively short time, we propose that upper-crustal extension in Danakil is currently dominated by faulting, not magmatic intrusion. Given the region's markedly thinned crust (~15-km-thick), relative to elsewhere in Afar where magma-assisted rifting dominates and maintains crustal thickness at ~25 km, mechanical extension in Danakil is likely coupled with ductile extension of the lower-crust and mantle lithosphere. Despite proximity to the voluminous lavas of the active Erta'Ale Volcanic Segment, evidence for igneous material in the upper ~2 km of the 6- to 10-km-wide basin is limited. Late-stage stretching was likely aided by thermal/strain-induced lithospheric weakening following protracted magma-assisted rifting. Basin formation immediately prior to the onset of seafloor spreading may also explain the accumulation of thick marine-seepage-fed evaporite sequences akin to those observed, for example, along the South Atlantic rifted margins.

The transition from mechanical thinning toward focused magmatic intrusion during continental rifting is poorly constrained; the tectonically active Main Ethiopian Rift (MER) provides an ideal study locale to address this issue. The presence of linear magmatic-tectonic belts in the relatively immature central MER may indicate that the transition from mechanical to magmatic rifting is more spatially distributed and temporally protracted than has previously been assumed. Here we examine lava geochemistry and vent distribution of a Pliocene-Quaternary linear magmatic chain along the western margin of the central MER—the Akaki Magmatic Zone. Our results show limited variability in parental magma that evolve in a complex polybaric fractionation system that has not changed significantly over the past 3 Ma. Our results suggest the following: (1) channeling of plume material and the localization of shear- or topography-induced porosity modulates melt intrusion into the continental lithosphere. (2) Pre-existing lithospheric structures may act as catalysts for intrusion of magmas into the lithospheric mantle. (3) The midcrustal to upper crustal strain regime dictates the surface orientation of volcanic vents. Therefore, although linear magmatic belts like those in the central MER may young progressively toward the rift axis and superficially resemble oceanic style magmatism, they actually represent prebreakup magmatism on continental crust. The oldest linear magmatic belts observed seismically and magnetically at the edge of the ocean basins thus may not, as is often assumed, actually mark the onset of seafloor spreading.

Investigations of a variety of continental rifts and margins worldwide have revealed that a considerable volume of melt can intrude into the crust during continental breakup, modifying its composition and thermal structure. However, it is unclear whether the cause of voluminous melt production at volcanic rifts is primarily increased mantle temperature or plate thinning. Also disputed is the extent to which plate stretching or thinning is uniform or varies with depth with the entire continental lithospheric mantle potentially being removed before plate rupture. Here we show that the extensive magmatism during rifting along the southern Red Sea rift in Afar, a unique region of sub-aerial transition from continental to oceanic rifting, is driven by deep melting of hotter-than-normal asthenosphere. Petrogenetic modelling shows that melts are predominantly generated at depths greater than 80 kilometres, implying the existence of a thick upper thermo-mechanical boundary layer in a rift system approaching the point of plate rupture. Numerical modelling of rift development shows that when breakup occurs at the slow extension rates observed in Afar, the survival of a thick plate is an inevitable consequence of conductive cooling of the lithosphere, even when the underlying asthenosphere is hot. Sustained magmatic activity during rifting in Afar thus requires persistently high mantle temperatures, which would allow melting at high pressure beneath the thick plate. If extensive plate thinning does occur during breakup it must do so abruptly at a late stage, immediately before the formation of the new ocean basin.

The Hudson Bay basin in northern Canada conceals one of the major collisional zones of the Canadian Shield, the Trans-Hudson Orogen (THO), which marks the Paleoproterozoic collision between the Archean Superior and Western Churchill cratons at ∼ 1.9 – 1.8 Ga . Improved knowledge of upper mantle structure beneath the region is essential to establish the nature of the THO, specifically whether Himalayan-style plate tectonics operated in Paleoproterozoic times. Detailed seismological constraints on lithospheric architecture are also required to advance our understanding of the mechanism and timing of keel formation. We use surface wave tomography to illuminate new details of the lithospheric architecture of the Hudson Bay region, resolving both seismic wavespeed and azimuthal anisotropy. Phase velocity maps are calculated from fundamental-mode Rayleigh wave dispersion curves, then used to construct a 3D model exploring upper mantle structure to depths of ∼ 300 km . Fast shear wavespeeds suggest a lithospheric thickness varying from ∼ 180 km to almost 280 km beneath the Hudson Bay region. The new study confirms previous inferences that there is no correlation between crustal ages and lithospheric thickness. Patterns of shear wavespeed and azimuthal anisotropy indicate a layered lithosphere. In the uppermost mantle, both the highest velocities and the anisotropic fast directions wrap around the Bay. This structure is likely related to the formation processes of the Paleozoic intracratonic basin. At greater depth ( ∼ 70 – 150 km ) we resolve two high-wavespeed cores separated by a relatively narrow near-vertical lower-velocity curtain. This internal architecture is suggested to result from the terminal phase of a modern-style plate-tectonic collision between the Archean Superior and Churchill cratons during the Trans-Hudson orogeny, entrapping juvenile Proterozoic material. The lower lithosphere ( ≥ 160 km depth) has a relatively homogeneous wavespeed structure across the region, with distinct patterns of anisotropy closely resembling the subsurface geometry of the THO. We interpret this basal layer as juvenile or reworked material accreted to the base of the existing cratonic lithosphere during or soon after the Trans-Hudson orogeny. The formation of the Laurentian keel thus likely occurred in multiple phases, with a basal layer developing in post-Archean times, during the THO.

The structural and stratigraphic evolution of rift basins and passive margins has been widely studied, with many analyses demonstrating that delta systems can provide important records of postrift geodynamic processes. However, the apparent lack of ancient synbreakup delta systems and the paucity of seismic imaging across continent-ocean boundaries mean that the transition from continental rifting to oceanic spreading remains poorly understood. The Early Cretaceous Barrow Group of the North Carnarvon Basin, offshore NW Australia, was a major deltaic system that formed during the latter stages of continental rifting and represents a rich sedimentary archive, documenting uplift, subsidence, and erosion of the margin. We use a regional database of 2-D and 3-D seismic and well data to constrain the internal architecture of the Barrow Group. Our results highlight three major depocenters: the Exmouth and Barrow subbasins and southern Exmouth Plateau. Overcompaction of pre-Cretaceous sedimentary rocks in the South Carnarvon Basin, and pervasive reworking of Permian and Triassic palynomorphs in the offshore Barrow Group, suggests that the onshore South Carnarvon Basin originally contained a thicker sedimentary succession, which was uplifted and eroded prior to breakup. Backstripping of sedimentary successions encountered in wells in the Exmouth Plateau depocenter indicates that anomalously rapid tectonic subsidence (≤0.24 mm yr−1) accommodated Barrow Group deposition, despite evidence for minimal, contemporaneous upper crustal extension. Our results suggest that classic models of uniform extension cannot account for the observations of uplift and subsidence in the North Carnarvon Basin and may indicate a period of depth-dependent extension or dynamic topography preceding breakup.

International audience; The geological record of SE Canada spans more than 2.5Ga, making it a natural laboratory for the study of crustal formation and evolution over time. We estimate the crustal thickness, Poisson's ratio, a proxy for bulk crustal composition, and shear velocity (V-s) structure from receiver functions at a network of seismograph stations recently deployed across the Archean Superior Craton, the Proterozoic Grenville, and the Phanerozoic Appalachian provinces. The bulk seismic crustal properties and shear velocity structure reveal a correlation with tectonic provinces of different ages: the post-Archean crust becomes thicker, faster, more heterogeneous, and more compositionally evolved. This secular variation pattern is consistent with a growing consensus that crustal growth efficiency increased at the end of the Archean. A lack of correlation among elevation, Moho topography, and gravity anomalies within the Proterozoic belt is better explained by buoyant mantle support rather than by compositional variations driven by lower crustal metamorphic reactions. A ubiquitous approximate to 20km thick high-V-s lower crustal layer is imaged beneath the Proterozoic belt. The strong discontinuity at 20km may represent the signature of extensional collapse of an orogenic plateau, accommodated by lateral crustal flow. Wide anorthosite massifs inferred to fractionate from a mafic mantle source are abundant in Proterozoic geology and are underlain by high-V-s lower crust and a gradational Moho. Mafic underplating may have provided a source for these intrusions and could have been an important post-Archean process stimulating mafic crustal growth in a vertical sense.

SUMMARY The Hudson Bay basin is the least studied of four major Phanerozoic intracratonic basins in North America and the mechanism by which it formed remains ambiguous. We investigate the crustal structure of Hudson Bay based on ambient-noise tomography, using 21 months of continuous recordings from 37 broad-band seismograph stations that encircle the Bay. Green's functions that emerge from the cross correlation of these ambient noise recordings are dominated by fundamental-mode Rayleigh waves. In the microseismic period band (5–20 s), these signals are most prominently expressed in certain preferred azimuths indicative of stationary coastal source regions in southern Alaska and Labrador. Seasonal variations are subtle but consistent with more energetic noise sources during winter months, when wave heights in the Pacific and north Atlantic are larger than in the summer. Noise emanating from Hudson Bay does not appear to contribute significantly to the cross correlograms. Group-velocity dispersion curves are obtained by time-frequency analysis of cross-correlation functions. We test and implement a signal-to-noise ratio (SNR) selection method for producing one-sided cross correlograms, which yields better-defined dispersion ridges than the standard two-sided averaging approach. Tomographic maps and cross sections obtained in the 5–40 s period range reveal significantly lower crustal velocities beneath Hudson Bay than in the bounding Archean Superior craton. The lowest mid-crustal velocities correspond to a previously determined region of maximum lithospheric stretching near the centre of the basin. Pseudosections extracted from the tomographic inversions along profiles across Hudson Bay provide the first compelling direct evidence for crustal thinning beneath the basin. Our results are consistent with a recent estimate of 3 km of crustal thinning, but not consistent with a proposed model for basin subsidence triggered by eclogitization of a remnant crustal root.

Whether or not plate tectonic processes operated on a younger, hotter Earth remains ambiguous. Seismic data from new networks in the Hudson Bay region of the Canadian Shield, where the Precambrian geological record spans more than 2 billion years, offer fresh scope to address this problem. Using receiver function analyses we show that the crust of the Rae domain, which exhibits ages of Paleo- to Neoarchean (3.9–2.7 Ga), is likely felsic-to-intermediate in composition (average Vp/Vs < 1.73) and seismically transparent with a sharp Moho. There is little evidence for modern-style plate tectonics, and based on the simplicity and spatial extent of the felsic crust, models favouring vertical tectonic processes such as crustal delamination or plume activity appear better suited to the results. Data from the Hearne domain, which exhibits widespread ~ 2.7 Ga granite-and-greenstone geology, show a more complex crust with higher Vp/Vs ratios, consistent with a greater mafic component. The Trans-Hudson Orogen (THO), proposed to be a Himalayan-scale mountain belt during the Paleoproterozoic, is thought to have formed during the ~ 1.8 Ga collision of the Superior and Churchill plates. Results from the Quebec–Baffin Island segment of the THO appear to map out the first-order shape of the underthrusting Superior plate, with elevated Vp/Vs ratios likely representing the rifted margin of the Superior craton. Consistently thicker crust is observed beneath central and southern Baffin Island (~43 km), coincident with widespread high-grade metamorphic surface geology. These features can be explained by crustal thickening due to stacking of accreted terranes during continent–continent collision, analogous to the present-day Tibetan Plateau, followed by erosion. When reviewed in light of age and compositional constraints from the geological record, our seismic observations point towards secular crustal evolution from non-plate tectonic during the Paleo- to Mesoarchean evolving towards fully-developed modern-style plate tectonics during the Paleoproterozoic.

The geology of Scotland documents a protracted geological history from Precambrian basement formation through the Caledonian Orogeny and to the Tertiary opening of the Atlantic. A temporary deployment of 22 closely spaced broadband seismic stations in Scotland traverses many of the major terrane boundaries in the region and provides a unique opportunity to place spatial and temporal constraints on variations in crustal structure. With teleseismic P -wave receiver functions, we use the H − κ method to determine variations in bulk crustal parameters: crustal thickness ( H ) and V P / V S ratio. Mean crustal thickness is ~ 28 km, varying from ~ 23 km in the NE highlands and increasing to > 30 km near the Highland Boundary Faultin the southern part of the study area. Mean V P / V S values of ~ 1.76 show no significant variation across the study area. An abrupt increase in crustal thickness of ~ 4.5 km NW across the Moine Thrust is not easily linked to Tertiary–Recent tectonic activity. Instead it appears that Scotland's crust has retained features for hundreds of millions of years since it was first formed, despite abundant volcanic activity in Tertiary times. Our work favours the view that the Moho is a compositional boundary rather than a mineralogical one defined by a reaction in P – T space.

In August 2002, there was high seismic activity in Afar concentrated at the plateau margin of the northern Ethiopian rift east of Mekele, near the western part of the Danakil microplate. The spatial and temporal distributions of this seismic activity over four weeks indicate the NNW propagation of the Gulf of Aden rift across the Afar Depression towards the western Ethiopian plateau. Fault plane solutions for six larger earthquakes from the August 2002 sequence are estimated from moment tensor inversion of local broadband waveform data. The results show only normal faulting on NNW trending and NE dipping faults, which agree with tectonics of the area and distribution of aftershocks. No strike-slip component is observed in any of our fault plane solutions or those of other workers including Harvard CMT solutions in the region. Such motion would be indicative of oblique-slip deformation between the Nubian plate and the Danakil microplate consistent with counter-clockwise rotation of the microplate. Hypocentral depths of well-constrained events are 5–7 km, which is the approximate elastic plate thickness in the Main Ethiopian rift, possibly indicating the depth to the brittle–ductile transition zone in this part of the Afar Depression. The shallowness of the depth estimates agree with the macroseismic reports available from a wide area in northern Ethiopia. Potential future shallow crustal deformation may cause significant loss of human life and damage to property in the densely populated highland region around Mekele unless measures are taken in improving building standards. The b-value for this sequence is estimated to be 0.66 using a least squares fit, while it is 0.67 ± 0.16 from a maximum-likelihood approach. This estimated b-value is low or the frequency of occurrence of relatively larger magnitude events is high indicating that it is a highly stressed region as evidenced by the recent increase of the seismicity in the area.

The geological record of the Borborema Province, northeast Brazil, documents tectonic events that characterised the Precambrian formation and Mesozoic breakup of Gondwana. Large-scale shear zones and associated granitic plutons that developed during the Neoproterozoic Brasiliano/Pan-African orogeny, and major sedimentary basins of Mesozoic age, indicate significant deformation across the region. However, whether or not the shear zones resulted from Precambrian terrane accretion, or are simply the result of episodes of subsequent intra-plate deformation is debated. Also poorly understood is the effect of the thick Sao Francisco mantle keel on present-day asthenospheric flow. To address these issues we have performed a teleseismic shear wave splitting study of mantle seismic anisotropy from a new broadband seismograph network centred on the Borborema Province. Shear wave splitting parameters (φ, δt) reveal a lack of plate-scale anisotropic fabrics associated within the continental interior, perhaps supporting models of formation and evolution of the Borborema Province involving minimal deformation of the lithospheric mantle. Delamination of anisotropic lithosphere during the development of Cenozoic volcanism that eroded older fossil lithospheric fabrics is unlikely because the widespread Cenozoic magmatism required to achieve this is absent in the geological record. Instead, the apparently low levels of seismic anisotropy observed in the interior of the Borborema Province may simply reflect depth-dependent anisotropy: nulls/low δt observations may be the subtractive result of orthogonal fast directions in the lithosphere and asthenosphere. Towards the Brazilian coast δt > 1 s, and fast directions are sub-parallel to stretching fabrics formed during the opening of the South Atlantic. This may imply that the mantle lithosphere was deformed but not completely destroyed during Gondwana breakup. However, a more complete backazimuthal coverage of splitting measurements across the region is needed to confirm or refute these hypotheses unambiguously.

The geological record at rifts and margins worldwide often reveals considerable along-strike variations in volumes of extruded and intruded igneous rocks. These variations may be the result of asthenospheric heterogeneity, variations in rate and timing of extension; alternatively, pre-existing plate architecture and/or the evolving kinematics of extension during breakup may exert first order control on magmatism. The Main Ethiopian Rift (MER) in East Africa provides an excellent opportunity to address this dichotomy: it exposes, along-strike, several sectors of asynchronous rift development from continental rifting in the south to incipient oceanic spreading in the north. Here we perform studies of volcanic cone density and rift obliquity along strike in the MER. By synthesizing these new data in light of existing geophysical, geochemical and petrological constraints on magma generation and emplacement, we are able to discriminate between tectonic and mantle geodynamic controls on the geological record of a newly forming magmatic rifted margin. The timing of rift sector development, the three-dimensional focusing of melt, and the ponding of plume material where the rift dramatically narrows, each influence igneous intrusion and volcanism along the MER. However, rifting obliquity plays an important role in localizing intrusion into the crust beneath en-echelon volcanic segments. Along-strike variations in volumes and types of igneous rocks found at rifted margins thus likely carry information about the development of strain during rifting, as well as the physical state of the convecting mantle at the time of breakup.

A consensus is emerging from studies of continental rifts and rifted margins worldwide that significant extension can be accommodated by magma intrusion prior to the development of a new ocean basin. However, the influence of loading from magma intrusion, lava extrusion, and sedimentation on plate flexure and resultant subsidence of the basin is not well understood. We address this issue by using three-dimensional flexural models constrained by geological and geophysical data from the Main Ethiopian Rift and the Afar Depression in East Africa. Model results show that axial mafic intrusions in the crust are able to cause significant downward flexure of the opening rift and that the amount of subsidence increases with decreasing plate strength accompanying progressive plate thinning and heating during continental breakup. This process contributes to the tilting of basaltic flows toward the magma injection axis, forming the typical wedge-shaped seaward-dipping reflector sequences on either side of the eventual rupture site as the new ocean basin forms.

The Afar Depression is a subaerial triple junction between the Nubian, Somalian and Arabian Plates, the only place where the final stages of continental break-up can be observed on-land. In spite of the region being hot and inhospitable, scientists have carried out fundamental work in this unique geological setting over the last few decades. We have long-known that rifting began on large-scale border faults that now bound the Afar Depression but what role magma played in the development of this incipient ocean basin was not clear. However, in recent years, it has been revealed that repeated dike intrusions together with normal faulting accommodate extension producing a landscape dominated by spectacular fresh fault scarps and active volcanic edifices that have been created during episodic tectonic, volcano-tectonic and purely volcanic events. Observations from Ethiopia have fundamentally changed the way we think about continental break-up. The challenge now is to take what we have learned and apply it to the geological record of the rifted margins elsewhere on Earth.

The structure of upper crustal magma plumbing systems controls the distribution of volcanism and influences tectonic processes. However, delineating the structure and volume of plumbing systems is difficult because (1) active intrusion networks cannot be directly accessed; (2) field outcrops are commonly limited; and (3) geophysical data imaging the subsurface are restricted in areal extent and resolution. This has led to models involving the vertical transfer of magma via dikes, extending from a melt source to overlying reservoirs and eruption sites, being favored in the volcanic literature. However, while there is a wealth of evidence to support the occurrence of dike-dominated systems, we synthesize field- and seismic reflection–based observations and highlight that extensive lateral magma transport (as much as 4100 km) may occur within mafic sill complexes. Most of these mafic sill complexes occur in sedimentary basins (e.g., the Karoo Basin, South Africa), although some intrude crystalline continental crust (e.g., the Yilgarn craton, Australia), and consist of interconnected sills and inclined sheets. Sill complex emplacement is largely controlled by host-rock lithology and structure and the state of stress. We argue that plumbing systems need not be dominated by dikes and that magma can be transported within widespread sill complexes, promoting the development of volcanoes that do not overlie the melt source. However, the extent to which active volcanic systems and rifted margins are underlain by sill complexes remains poorly constrained, despite important implications for elucidating magmatic processes, melt volumes, and melt sources.

Despite the importance of continental breakup in plate tectonics, precisely how extensional processes such as brittle faulting, ductile plate stretching, and magma intrusion evolve in space and time during the development of new ocean basins remains poorly understood. The rifting of Arabia from Africa in the Afar depression is an ideal natural laboratory to address this problem since the region exposes subaerially the tectonically active transition from continental rifting to incipient seafloor spreading. We review recent constraints on along-axis variations in rift morphology, crustal and mantle structure, the distribution and style of ongoing faulting, subsurface magmatism and surface volcanism in the Red Sea rift of Afar to understand processes ultimately responsible for the formation of magmatic rifted continental margins. Our synthesis shows that there is a fundamental change in rift morphology from central Afar northward into the Danakil depression, spatially coincident with marked thinning of the crust, an increase in the volume of young basalt flows, and subsidence of the land towards and below sea-level. The variations can be attributed to a northward increase in proportion of extension by ductile plate stretching at the expense of magma intrusion. This is likely in response to a longer history of localised heating and weakening in a narrower rift. Thus, although magma intrusion accommodates strain for a protracted period during rift development, the final stages of breakup are dominated by a phase of plate stretching with a shift from intrusive to extrusive magmatism. This late-stage pulse of decompression melting due to plate thinning may be responsible for the formation of seaward dipping reflector sequences of basalts and sediments, which are ubiquitous at magmatic rifted margins worldwide.

The unique physical and chemical properties of cratonic lithosphere are thought to be key to its long-term survival and its resistance to pervasive modification by tectonic processes. Study of mantle structure in southeast Canada and the northeast US offers an excellent opportunity to address this issue because the region spans 3 billion years of Earth history, including Archean formation of the Superior craton and younger accretion of terranes to eastern Laurentia during the Proterozoic Grenville and Phanerozoic Appalachian orogenies. Trending NW–SE through each of these terranes is the track of the Great Meteor hot spot, which affected the region during the Mesozoic. Here we study mantle seismic velocity structure beneath this region of eastern North America using tomographic inversion of teleseismic P-wave relative arrival-times recorded by a large-aperture seismograph network. There are no large-scale systematic differences between Superior and Grenville mantle wave speed structure, which may suggest that tectonic stabilization of cratons occurred in a similar fashion during the Archean and Proterozoic. Cratonic lithosphere is largely thought to be resistant to modification by hot spot processes, in contrast to younger terranes where lithospheric erosion and significant magmatism are expected. Low velocities beneath the regions affected by the Great Meteor hot spot are broadest beneath the Paleozoic Appalachian terranes, indicating pervasive modification of the lithosphere during magmatism. The zone of modification narrows considerably into the Proterozoic Grenville province before disappearing completely in the Archean Superior craton, where the surface signature of Mesozoic magmatism is limited to kimberlite eruptions.

The crust underlying Hudson Bay, Canada records a long and complex tectonic history. In this study, we investigate this region using tomographic inversion based on continuous ambient noise recordings from 37 broadband seismograph stations that encircle Hudson Bay. The ambient noise data were processed to obtain group-velocity dispersion measurements from 10–35 s period, which were inverted using an algorithm that incorporates the effects of anisotropy. This work is among the first in which ambient noise data have been used to investigate azimuthal anisotropy. The inversion method uses smoothing and damping to regularize the solution; due to the significantly increased number of model parameters relative to the isotropic case, we performed a careful analysis for parameter selection to determine whether “leakage” occurs between isotropic and anisotropic model parameters. We observe a robust pattern of anisotropic fast directions in the mid-crust that are consistent with large-scale tectonic trends based on magnetic-anomaly patterns. In particular, a distinctive double-indentor shape for the Superior craton is clearly expressed in both data sets. This pattern breaks down deeper in the crust, suggesting that some degree of lithospheric decoupling in the lower crust, such as channel flow, occurred during orogenesis. Given regional evidence for vertically coherent deformation in the crust and underlying mantle, we interpret this pattern in the lower crust as a tectonic overprint that post-dates the main phase of Trans-Hudson deformation. At most levels in the crust, we observe a profound change in direction of anisotropic fast direction across an inferred suture beneath Hudson Bay.

Magmatism strongly influences continental rift development, yet the mechanism, distribution, and timescales on which melt is emplaced and erupted through the shallow crust are not well characterized. The Main Ethiopian Rift (MER) has experienced significant volcanism, and the mantle beneath is characterized by high temperatures and partial melt. Despite its magma-rich geological record, only one eruption has been historically recorded, and no dedicated monitoring networks exist. Consequently, the present-day magmatic processes in the region remain poorly documented, and the associated hazards are neglected. We use satellite-based interferometric synthetic aperture radar observations to demonstrate that significant deformation has occurring at four volcanic edifices in the MER (Alutu, Corbetti, Bora, and Haledebi) from 1993 to 2010. This raises the number of volcanoes known to be deforming in East Africa beyond 12, comparable to many subduction arcs despite the smaller number of recorded eruptions. The largest displacements are at Alutu volcano, the site of a geothermal plant, which showed two pulses of rapid inflation (10–15 cm) in 2004 and 2008 separated by gradual subsidence. Our observations indicate a shallow (

In seismically and tectonically active regions, the present-day strain field tends to bias interpretation of the geological record. This is usually reasonable, but in areas such as triple junctions, the orientation of stress and the locus of strain can evolve abruptly in space and time. We present deformation measurements using satellite radar interferometry (InSAR) and seismicity that together capture the intrusion of a ~6 km long, ~1.5 m wide dike into the upper crust of the Ethiopian rift in southern Afar during May 2000. Dike-induced volcano-tectonic seismicity suggests that the intrusion was injected laterally during a period of ~4 days. Seismic moment release accounts for only 5% of the total 1.6 × 1018 Nm geodetic moment, showing that diking accommodates the majority of strain. The intrusion intriguingly strikes at N122°E, perpendicular to the trend of the present-day East African rift. The geometry and age constraints on faulting and volcanic activity in southern Afar, combined with plate reconstructions, suggest that the dike likely intrudes an ~ESE-SE striking magmatic system that localized strain during Oligo-Miocene rifting in the Red Sea and Gulf of Aden. We also identify the southerly extent of the Arabian Plate in Afar during the Oligocene in mantle seismic tomographic images: an abrupt increase in seismic velocity in southern Afar is coincident with a stepped increase of ~20 Myr in the time elapsed since the onset of plate stretching. The anomalous orientation of the May 2000 intrusion implies that African-Arabian tectonics still influences the stress field in southern Afar and is at least partly accommodated by magma intrusion.

[1] Quaternary lavas erupted in zones of tectonomagmatic extension within the Main Ethiopian Rift (MER) preserve details of lithospheric structure in the East African Rift System. Despite observed source heterogeneity, basalts, trachybasalts, and basaltic trachyandesites erupted in the Wonjii Fault Belt (WFB) and the Silti-Debre Zeyit Fault Zone (SDFZ) form coherent fractionation paths dominated by variable removal of observed phenocryst phases. Crustal assimilation is not widespread, though it is observed at the southern end of the WFB where both fault belts merge; farther north, assimilation of cumulate phases related to fractional crystallization of previous magmas is identified. Shallow fractionation conditions (∼1 kbar) within the WFB do not change from north to south. In contrast, lavas erupted within the contemporaneous SDFZ fractionate at various crustal depths. These results indicate a better developed magmatic system beneath the WFB where magmas rose quickly before undergoing more significant fractionation at near surface levels and a less developed system beneath the SDFZ. The distribution of magmatism and extant geophysical data indicate thinned crust and a single rift-centered zone of magmatic activity northeast of 8°30′N, consistent with a transitional lithosphere between continental and oceanic settings. Southwest of 8°30′N, thicker crust and rift-marginal axes of extension suggest lithosphere with continental affinities. The WFB is propagating southward in response to extension within the Red Sea Rift; the northward propagating SDFZ is related to rifting within the East African Rift System. This region records the unification of two rift systems, requiring care in interpreting the MER as simply transitional between continental and oceanic environments.