75 results found
Ogden C, Bastow I, Gilligan A, et al., 2019, A reappraisal of the H-κ stacking technique: implications for global crustal structure, Geophysical Journal International, Vol: 219, Pages: 1491-1513, ISSN: 0956-540X
H-κ stacking is used routinely to infer crustal thickness and bulk-crustal VP/VS ratio from teleseismic receiver functions. The method assumes that the largest amplitude P-to-S conversions beneath the seismograph station are generated at the Moho. This is reasonable where the crust is simple and the Moho marks a relatively abrupt transition from crust to mantle, but not if the crust-mantle transition is gradational and/or complex intra-crustal structure exists. We demonstrate via synthetic seismogram analysis that H-κ results can be strongly dependent on the choice of stacking parameters (the relative weights assigned to the Moho P-to-S conversion and its subsequent reverberations, the choice of linear or phase-weighted stacking, input crustal P-wave velocity) and associated data parameters (receiver function frequency content and the sample of receiver functions analyzed). To address this parameter sensitivity issue, we develop an H-κ approach in which cluster analysis selects a final solution from 1000 individual H-κ results, each calculated using randomly-selected receiver functions, and H-κ input parameters. Ten quality control criteria that variously assess the final numerical result, the receiver function dataset, and the extent to which the results are tightly clustered, are used to assess the reliability of H-κ stacking at a station. Analysis of synthetic datasets indicates H-κ works reliably when the Moho is sharp and intra-crustal structure is lacking but is less successful when the Moho is gradational. Limiting the frequency content of receiver functions can improve the H-κ solutions in such settings, provided intra-crustal structure is simple. In cratonic Canada, India and Australia, H-κ solutions generally cluster tightly, indicative of simple crust and a sharp Moho. In contrast, on the Ethiopian plateau, where Paleogene flood-basalts overlie marine sediments, H-κ results are unstable and erron
Bernardino M, Jones C, Levandowski W, et al., A multicomponent Isabella anomaly: Resolving the physical state of the Sierra Nevada upper mantle from Vp/Vs anisotropy tomography, Geosphere, ISSN: 1553-040X
The Isabella anomaly, a prominent upper-mantle high-speed P-wave anomaly located within the southern Great Valley and southwestern foothills of the Sierra Nevada, has been interpreted either as foundering sub-Sierranlithosphere or as remnant oceanic lithosphere. We used Vp/Vs anisotropytomography to distinguish among the probable origins of the Isabella anomaly.S waveforms were rotated into the Sierran SKSFast and SKSSlow directionsdetermined from SKS-splitting studies. Teleseismic P-, SFast-, SSlow-, SKSFast-, andSKSSlow-wave arrival times were then inverted to obtain three-dimensional(3-D) perturbations in Vp, Vp/VsMean, and percent azimuthal anisotropy usingthree surface wave 3-D starting models and one one-dimensional (1-D) model.We observed the highest Vp/Vs anomalies associated with slower velocitiesin regions marked by young volcanism, with the largest of these anomaliesbeing the Mono anomaly under the Long Valley region, which extends todepths of at least 75 km. Peak Vp/Vs perturbations of +4% were found at 40km depth. The low velocities and high Vp/Vs values of this anomaly couldbe related to partial melt.The high wave speeds of the Isabella anomaly coincide with low Vp/Vsvalues with peak perturbations of −2%, yet they do not covary spatially. TheP-wave inversion imaged the Isabella anomaly as a unimodal eastward-plungingbody. However, the volume of that Isabella anomaly contains three separatebodies as defined by varying Vp/Vs values. High speeds, regionally averageVp/Vs values (higher than the other two anomalies), and lower anisotropycharacterize the core of the Isabella anomaly. The western and shallowestpart has high wave speeds and a lower Vp/Vs values than the surroundingmantle. The eastern and deepest part of the anomaly also contains highspeeds and lower Vp/Vs values but exhibits higher anisotropy. We consideredcombinations of varying temperature, Mg content (melt depletion), or modalgarnet to reproduce our observations. Our results suggest t
Boyce A, Bastow I, Golos E, et al., 2019, Variable modification of continental lithosphere during the Proterozoic Grenville Orogeny: evidence from teleseismic P-wave tomography, Earth and Planetary Science Letters, Vol: 525, ISSN: 0012-821X
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 ≥300Myr along the eastern edge of the proto-North American continent Laurentia, whose age varied north-to-south from ∼1.5−0.25Gyr 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 VP with little effect on VS 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 250km) 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 150km. 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
Venereau C, Robert M-S, Bastow I, et al., 2019, The role of variable slab dip in driving mantle flow at the eastern edge of the Alaskan subduction margin: insights from SKS shear-wave splitting, Geochemistry, Geophysics, Geosystems, Vol: 20, Pages: 2433-2448, ISSN: 1525-2027
Alaska provides an ideal tectonic setting for investigating the interaction between subduction and asthenospheric flow. Within the span of a few hundred kilometers along strike, the geometry of the subducting Pacific plate varies significantly and terminates in a sharp edge. Furthermore, the region documents a transition from subduction along the Aleutian Arc to strike‐slip faulting along the Pacific Northwest. To better understand mantle interactions within this subduction zone, we conduct an SKS shear‐wave splitting analysis on passive‐source seismic data collected between 2011 and 2018 at 239 broadband seismometers, including those from the Transportable Array. Anisotropic fast directions in the east of our study area parallel the Queen Charlotte and Fairweather transform faults, suggesting that the ongoing development of lithospheric anisotropy dominates the results there. However, our observed delay times (δt = 1–1.5 s) obtained across the study region may also imply an asthenospheric contribution to the splitting pattern. Our splitting observations exhibit slab‐parallel fast directions northwest of the trench and a rotation of fast directions around the northeastern slab edge. These observations suggest the presence of toroidal asthenospheric flow around the edge of the downgoing Pacific plate. We suggest that Wrangell Volcanic Field volcanism might be caused by mantle upwelling associated with this flow. Splitting observations closer to the trench can be explained by fossil anisotropy within the downgoing Pacific‐Yakutat plate combined with entrained subslab mantle. The geometry of the slab, including its variable dip and its abrupt eastern edge, thus plays an important role in governing mantle flow beneath Alaska.
Martin-Short R, Allen R, Bastow I, et al., 2018, Seismic imaging of the Alaska Subduction Zone: implications for slab geometry and volcanism, Geochemistry, Geophysics, Geosystems, Vol: 19, Pages: 4541-4560, ISSN: 1525-2027
Alaska has been a site of subduction and terrane accretion since the mid‐Jurassic. The area features abundant seismicity, active volcanism, rapid uplift, and broad intraplate deformation, all associated with subduction of the Pacific plate beneath North America. The juxtaposition of a slab edge with subducted, overthickened crust of the Yakutat terrane beneath central Alaska is associated with many enigmatic volcanic features. The causes of the Denali Volcanic Gap, a 400‐km‐long zone of volcanic quiescence west of the slab edge, are debated. Furthermore, the Wrangell Volcanic Field, southeast of the volcanic gap, also has an unexplained relationship with subduction. To address these issues, we present a joint ambient noise, earthquake‐based surface wave, and P‐S receiver function tomography model of Alaska, along with a teleseismic S wave velocity model. We compare the crust and mantle structure between the volcanic and nonvolcanic regions, across the eastern edge of the slab and between models. Low crustal velocities correspond to sedimentary basins, and several terrane boundaries are marked by changes in Moho depth. The continental lithosphere directly beneath the Denali Volcanic Gap is thicker than in the adjacent volcanic region. We suggest that shallow subduction here has cooled the mantle wedge, allowing the formation of thick lithosphere by the prevention of hot asthenosphere from reaching depths where it can interact with fluids released from the slab and promote volcanism. There is no evidence for subducted material east of the edge of the Yakutat terrane, implying the Wrangell Volcanic Field formed directly above a slab edge.
Bastow IA, Booth AD, Corti G, et al., 2018, The development of late-stage continental breakup: seismic reflection and borehole evidence from the Danakil Depression, Ethiopia, Tectonics, Vol: 37, Pages: 2848-2862, ISSN: 0278-7407
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 sub‐aerially, 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, ~30km northwest of the Erta'Ale Volcanic Segment (EAVS), reveal a remarkably‐thick (>1km) sequence of young (~100ka) evaporites in a basin bound by a major (≤400m 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 ~25km, 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 EAVS, evidence for igneous material in the upper ~2km of the 6–10‐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.
Liddell MV, Bastow I, Rawlinson N, et al., 2018, Precambrian Plate Tectonics in Northern Hudson Bay: Evidence FromPandSWave Seismic Tomography and Analysis of Source Side Effects in Relative Arrival-Time Data Sets, Journal of Geophysical Research: Solid Earth, Vol: 123, Pages: 5690-5709, ISSN: 2169-9313
Corti G, Molin P, Sembroni A, et al., 2018, Control of pre-rift lithospheric structure on the architecture and evolution of continental rifts: insights from the main Ethiopian rift, East Africa, Tectonics, Vol: 37, Pages: 477-496, ISSN: 0278-7407
We investigate the along‐axis variations in architecture, segmentation, and evolution of the Main Ethiopian Rift (MER), East Africa, and relate these characteristics to the regional geology, lithospheric structure, and surface processes. We first illustrate significant along‐axis variations in basin architecture through analysis of simplified geological cross sections in different rift sectors. We then integrate this information with a new analysis of Ethiopian topography and hydrography to illustrate how rift architecture (basin symmetry/asymmetry) is reflected in the margin topography and has been likely amplified by a positive feedback between tectonics (flexural uplift) and surface processes (fluvial erosion and unloading). This analysis shows that ~70% of the 500 km long MER is asymmetric, with most of the asymmetric rift sectors being characterized by a master fault system on the eastern margin. We finally relate rift architecture and segmentation to the regional geology and geophysical constraints on the lithosphere. We provide strong evidence that rift architecture is controlled by the contrasting nature of the lithosphere beneath the homogeneous, strong Somalian Plateau and the weaker, more heterogeneous Ethiopian Plateau, differences originating from the presence of pre‐rift zones of weakness on the Ethiopian Plateau and likely amplified by surface processes. The data provided by this integrated analysis suggest that asymmetric rifts may directly progress to focused axial tectonic‐magmatic activity, without transitioning into a symmetric rifting stage. These observations have important implications for the asymmetry of continental rifts and conjugate passive margins worldwide.
Ebinger C, Keir D, Bastow ID, et al., 2017, Crustal structure of active deformation zones in Africa: Implications for global crustal processes, Tectonics, Vol: 36, Pages: 3298-3332, ISSN: 0278-7407
The Cenozoic East African rift (EAR), Cameroon Volcanic Line (CVL), and Atlas Mountains formed on the slow-moving African continent, which last experienced orogeny during the Pan-African. We synthesize primarily geophysical data to evaluate the role of magmatism in shaping Africa's crust. In young magmatic rift zones, melt and volatiles migrate from the asthenosphere to gas-rich magma reservoirs at the Moho, altering crustal composition and reducing strength. Within the southernmost Eastern rift, the crust comprises ~20% new magmatic material ponded in the lower crust and intruded as sills and dikes at shallower depths. In the Main Ethiopian Rift, intrusions comprise 30% of the crust below axial zones of dike-dominated extension. In the incipient rupture zones of the Afar rift, magma intrusions fed from crustal magma chambers beneath segment centers create new columns of mafic crust, as along slow-spreading ridges. Our comparisons suggest that transitional crust, including seaward dipping sequences, is created as progressively smaller screens of continental crust are heated and weakened by magma intrusion into 15–20 km thick crust. In the 30 Ma Recent CVL, which lacks a hot spot age progression, extensional forces are small, inhibiting the creation and rise of magma into the crust. In the Atlas orogen, localized magmatism follows the strike of the Atlas Mountains from the Canary Islands hot spot toward the Alboran Sea. CVL and Atlas magmatism has had minimal impact on crustal structure. Our syntheses show that magma and volatiles are migrating from the asthenosphere through the plates, modifying rheology, and contributing significantly to global carbon and water fluxes.
Liddell M, Bastow ID, Darbyshire FA, et al., 2017, The Formation of Laurentia: Evidence From Shear Wave Splitting, Earth and Planetary Science Letters, Vol: 479, Pages: 170-178, ISSN: 0012-821X
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.
Boyce A, Bastow ID, Rondenay S, et al., 2017, From Relative to Absolute Teleseismic Travel Times: The Absolute Arrival‐Time Recovery Method (AARM), Bulletin of the Seismological Society of America, Vol: 107, Pages: 2511-2520, ISSN: 0037-1106
Dense, short‐term deployments of seismograph networks are frequently used to study upper‐mantle structure. However, recordings of variably emergent teleseismic waveforms are often of lower signal‐to‐noise ratio (SNR) than those recorded at permanent observatory sites. Therefore, waveform coherency across a network is frequently utilized to calculate relative arrival times between recorded traces, but these measurements cannot easily be combined or reported directly to global absolute arrival‐time databases. These datasets are thus a valuable but untapped resource with which to fill spatial gaps in global absolute‐wavespeed tomographic models.We developed an absolute arrival‐time recovery method (AARM) to retrieve absolute time picks from relative‐arrival‐time datasets, working synchronously with filtered and unfiltered data. We also include a relative estimate of uncertainty for potential use in data weighting during subsequent tomographic inversion. Filtered waveforms are first aligned via multichannel cross correlation. These time shifts are applied to unfiltered waveforms to generate a phase‐weighted stack. Cross correlation with the primary stack or the SNR of each trace is used to weight a second‐higher SNR stack. The first arrival on the final stack is picked manually to recover absolute arrival times for the aligned waveforms.We test AARM on a recently published dataset from southeast Canada ( ∼10,000∼10,000 picks). When compared with the available equivalent earthquake–station pairs on the International Seismological Centre (ISC) database, ∼83%∼83% of AARM picks agree to within ±0.5 s±0.5 s . Tests using synthetic P‐wave data indicate that AARM produces absolute arrival‐time picks to accuracies of better than 0.25 s, akin to uncertainties in ISC bulletins.
Darbyshire FA, Bastow ID, Petrescu L, et al., 2017, A tale of two orogens: Crustal processes in the Proterozoic Trans-Hudson and Grenville Orogens, eastern Canada, Tectonics, Vol: 36, Pages: 1633-1659, ISSN: 0278-7407
The Precambrian core of North America was assembled in the Proterozoic by a series of collisions between Archean cratons. Among the orogenic belts, two stand out due to their significant spatial extent. The Paleoproterozoic Trans-Hudson Orogen (THO) and Mesoproterozoic Grenville Orogen extend for thousands of kilometers along strike and hundreds of kilometers across strike. Both have been compared to the present-day Himalayan-Karakoram-Tibetan Orogen (HKTO). Over the last 20–30 years, active and passive source seismic studies have contributed a wealth of information about the present-day crustal structure and composition of the two orogens in Canada. The Proterozoic orogenic crust is generally thicker than that of neighboring Archean terranes, with a more variable Moho character, ranging from relatively sharp to highly diffuse. Both orogens have a prominent high-velocity lower crustal layer, consistent with long-term preservation of a partially eclogitized root at the base of the crust and similar to that inferred beneath the western HKTO. Crustal structure in the northern THO strongly resembles the lower crustal structure of the HKTO, suggesting that Moho depths may have reached 60–70 km when the orogen was active. A prominent midcrustal discontinuity beneath the central Grenville Province and changes in the patterns of seismic anisotropy in the THO crust beneath Hudson Bay provide geophysical evidence that lower crustal flow likely played a role in the evolution of both orogens, similar to that inferred beneath the present-day HKTO. The seismic evidence from Canada supports the notion of tectonic uniformitarianism, at least as far back as the Paleoproterozoic.
Petrescu L, Darbyshire FA, Bastow ID, et al., 2017, Seismic anisotropy of Precambrian lithosphere: insights from Rayleigh wave tomography of the eastern Superior craton, Journal of Geophysical Research. Solid Earth, Vol: 122, Pages: 3754-3775, ISSN: 2169-9356
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.
Magee C, Bastow ID, van Wyk de Vries B, et al., 2017, Structure and dynamics of surface uplift induced by incremental sill emplacement, Geology, Vol: 45, Pages: 431-434, ISSN: 1943-2682
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 isthus 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-fold structure can be observed, magmatism and deformation has long-since ceased. To address this problem, we combine structural and dynamic analyses of the Alu dome, Ethiopia; a 3.5-km-long, 346-m-high, elliptical dome of outward-dipping, tilted lava flows cross-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 Aluis not a horst or a volcano, as previously thought, but is instead a forced fold. Interferometric synthetic aperture radardata 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.
Amy Gilligan and researchers from the UK, Canada and the US take a deep look at the North American craton.
Green DN, Bastow ID, Dashwood B, et al., 2016, Characterizing Broadband Seismic Noise in Central London, Seismological Research Letters, Vol: 88, ISSN: 0895-0695
Recordings made at five broadband seismometers, deployed in central London during the summer of2015, reveal the wideband nature (periods, T, of between 0.01 and 100 s) of anthropogenic noise ina busy urban environment. Temporal variations of power spectral density measurements suggesttransportation sources generate the majority of the noise wavefield across the entire wideband, exceptat the secondary microseismic peak (2< T <6 s). The effect of road traffic is greatest at short periods(T <0.4 s) where acceleration noise powers are ∼20dB larger than the New High Noise Model; atT =0.1 s daytime root-mean-square acceleration amplitudes are 1000 times higher in central Londonthan at an observatory station in Eskdalemuir, Scotland. Overground railways generate observablesignals both at short periods (T <0.3 s), which are recorded in close proximity to the tracks, and atvery long periods (T >20 s) which are recorded across the city. We record a unique set of signals 30mabove a subway (London Underground) tunnel interpreted as a short-period dynamic component, aquasi-static response to the train moving underneath the instrument, and a very long period (T>30 s)response to air movement around the tunnel network. A low-velocity clay and sand overburden tensof metres thick is shown to amplify the horizontal component wavefield at T ∼1 s, consistent withproperties of the London subsurface derived from engineering investigations. We provide tabulatedmedian power spectral density values for all stations, to facilitate comparison with any future urbanseismic deployments.
Martin-Short R, Allen R, Bastow ID, 2016, Subduction geometry beneath south-central Alaska and its relationship to volcanism, Geophysical Research Letters, Vol: 43, Pages: 9509-9517, ISSN: 1944-8007
The southern Alaskan margin captures a transition between compression and strike-slip-dominated deformation, accretion of the overthickened Yakutat terrane, termination of Aleutian arcmagmatism, and the enigmatic Wrangell Volcanic Field. The extent of subduction and mantle structurebelow this region is uncertain, with important implications for volcanism. We present compressional andshear wave mantle velocity models below south central Alaska that leverage a new seismometer deploymentto produce the most complete image of the subducting Paciﬁc-Yakutat plate to date. We image a steeplydipping 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 isobserved above the subducting Yakutat terrane, but the slab structure below 150 km depth andWadati-Benioff zone here are similar to those along the Aleutian-Alaska arc. Features of the mantle wedge oroverlying lithosphere are thus responsible for the volcanic gap.
Reeve MT, Jackson CA-L, Bell RE, et al., 2016, The Stratigraphic Record of Pre-breakup Geodynamics: Evidence from the Barrow Delta, offshore Northwest Australia, Tectonics, Vol: 35, Pages: 1935-1968, ISSN: 1944-9194
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 post-rift geodynamic processes. However, the apparent lack of ancient syn-breakup delta systems and the paucity of seismic imaging across continent-ocean boundaries means 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 2D and 3D seismic and well data to constrain the internal architecture of the Barrow Group. Our results highlight three major depocentres: the Exmouth and Barrow sub-basins, and southern Exmouth Plateau. Over-compaction 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 depocentre indicate 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.
Gilligan A, Bastow ID, Darbyshire FA, 2016, Seismological structure of the 1.8 Ga Trans-Hudson Orogen of North America, Geochemistry Geophysics Geosystems, Vol: 17, Pages: 2421-2433, ISSN: 1525-2027
Precambrian tectonic processes are debated: what was the nature and scale of orogenic events on the younger, hotter, and more ductile Earth? Northern Hudson Bay records the Paleoproterozoic collision between the Western Churchill and Superior plates—the ∼1.8 Ga Trans-Hudson Orogeny (THO)—and is an ideal locality to study Precambrian tectonic structure. Integrated field, geochronological, and thermobarometric studies suggest that the THO was comparable to the present-day Himalayan-Karakoram-Tibet Orogen (HKTO). However, detailed understanding of the deep crustal architecture of the THO, and how it compares to that of the evolving HKTO, is lacking. The joint inversion of receiver functions and surface wave data provides new Moho depth estimates and shear velocity models for the crust and uppermost mantle of the THO. Most of the Archean crust is relatively thin (∼39 km) and structurally simple, with a sharp Moho; upper-crustal wave speed variations are attributed to postformation events. However, the Quebec-Baffin segment of the THO has a deeper Moho (∼45 km) and a more complex crustal structure. Observations show some similarity to recent models, computed using the same methods, of the HKTO crust. Based on Moho character, present-day crustal thickness, and metamorphic grade, we support the view that southern Baffin Island experienced thickening during the THO of a similar magnitude and width to present-day Tibet. Fast seismic velocities at >10 km below southern Baffin Island may be the result of partial eclogitization of the lower crust during the THO, as is currently thought to be happening in Tibet.
Boyce A, Bastow ID, Darbyshire FA, et al., 2016, Subduction beneath Laurentia modified the eastern North American cratonic edge: evidence from P wave and S wave tomography, Journal of Geophysical Research. Solid Earth, Vol: 121, ISSN: 2169-9356
The cratonic cores of the continents are remarkably stable and long-lived features. Their ability to resist destructive tectonic processes is associated with their thick (∼250 km), cold, chemically depleted, buoyant lithospheric keels that isolate the cratons from the convecting mantle. The formation mechanism and tectonic stability of cratonic keels remains under debate. To address this issue, we use P wave and S wave relative arrival-time tomography to constrain upper mantle structure beneath southeast Canada and the northeast USA, a region spanning three quarters of Earth's geological history. Our models show three distinct, broad zones: Seismic wave speeds increase systematically from the Phanerozoic coastal domains, through the Proterozoic Grenville Province, and to the Archean Superior craton in central Québec. We also recover the NW-SE trending track of the Great Meteor hot spot that crosscuts the major tectonic domains. The decrease in seismic wave speed from Archean to Proterozoic domains across the Grenville Front is consistent with predictions from models of two-stage keel formation, supporting the idea that keel growth may not have been restricted to Archean times. However, while crustal structure studies suggest that Archean Superior material underlies Grenvillian age rocks up to ∼300 km SE of the Grenville Front, our tomographic models show a near-vertical boundary in mantle wave speed directly beneath the Grenville Front. We interpret this as evidence for subduction-driven metasomatic enrichment of the Laurentian cratonic margin, prior to keel stabilization. Variable chemical depletion levels across Archean-Proterozoic boundaries worldwide may thus be better explained by metasomatic enrichment than inherently less depleted Proterozoic composition at formation.
Gilligan A, Bastow ID, Watson E, et al., 2016, Lithospheric deformation in the Canadian Appalachians: evidence from shear wave splitting, Geophysical Journal International, Vol: 206, Pages: 1273-1280, ISSN: 1365-246X
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.
Petrescu L, Bastow I, Darbyshire F, et al., Three billion years of crustal evolution in eastern Canada: constraints from receiver functions, Journal of Geophysical Research. Solid Earth, Vol: 121
The geological record of SE Canada spans more than 2.5Ga, making it anatural laboratory for the study of crustal formation and evolution over time.We estimate the crustal thickness, Poisson’s ratio, a proxy for bulk crustalcomposition, and shear velocity (Vs) 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.
Bastow ID, Darbyshire FA, Forte AM, et al., 2015, Variability and origin of seismic anisotropy across eastern Canada: evidence from shear-wave splitting measurements, Journal of Geophysical Research. Solid Earth, Vol: 120, Pages: 8404-8421, ISSN: 2169-9313
Measurements of seismic anisotropy in continental regions are frequently interpreted with respect to past tectonic processes, preserved in the lithosphere as “fossil” fabrics. Models of the present-day sublithospheric flow (often using absolute plate motion as a proxy) are also used to explain the observations. Discriminating between these different sources of seismic anisotropy is particularly challenging beneath shields, whose thick (≥200 km) lithospheric roots may record a protracted history of deformation and strongly influence underlying mantle flow. Eastern Canada, where the geological record spans ∼3 Ga of Earth history, is an ideal region to address this issue. We use shear wave splitting measurements of core phases such as SKS to define upper mantle anisotropy using the orientation of the fast-polarization direction ϕ and delay time δt between fast and slow shear wave arrivals. Comparison with structural trends in surface geology and aeromagnetic data helps to determine the contribution of fossil lithospheric fabrics to the anisotropy. We also assess the influence of sublithospheric mantle flow via flow directions derived from global geodynamic models. Fast-polarization orientations are generally ENE-WSW to ESE-WNW across the region, but significant lateral variability in splitting parameters on a ≤100 km scale implies a lithospheric contribution to the results. Correlations with structural geologic and magnetic trends are not ubiquitous, however, nor are correlations with geodynamically predicted mantle flow directions. We therefore consider that the splitting parameters likely record a combination of the present-day mantle flow and older lithospheric fabrics. Consideration of both sources of anisotropy is critical in shield regions when interpreting splitting observations.
Martin-Short R, Allen R, Bastow ID, et al., 2015, Mantle flow geometry from ridge to trench beneath the Gorda-Juan de Fuca plate system, Nature Geoscience, Vol: 8, Pages: 965-968, ISSN: 1752-0908
Tectonic plates are underlain by a low-viscosity mantle layer, the asthenosphere. Asthenospheric flow may be induced by the overriding plate or by deeper mantle convection1. Shear strain due to this flow can be inferred using the directional dependence of seismic wave speeds—seismic anisotropy. However, isolation of asthenospheric signals is challenging; most seismometers are located on continents, whose complex structure influences the seismic waves en route to the surface. The Cascadia Initiative, an offshore seismometer deployment in the US Pacific Northwest, offers the opportunity to analyse seismic data recorded on simpler oceanic lithosphere2. Here we use measurements of seismic anisotropy across the Juan de Fuca and Gorda plates to reconstruct patterns of asthenospheric mantle shear flow from the Juan de Fuca mid-ocean ridge to the Cascadia subduction zone trench. We find that the direction of fastest seismic wave motion rotates with increasing distance from the mid-ocean ridge to become aligned with the direction of motion of the Juan de Fuca Plate, implying that this plate influences mantle flow. In contrast, asthenospheric mantle flow beneath the Gorda Plate does not align with Gorda Plate motion and instead aligns with the neighbouring Pacific Plate motion. These results show that asthenospheric flow beneath the small, slow-moving Gorda Plate is controlled largely by advection due to the much larger, faster-moving Pacific Plate.
Thompson DA, Kendall JM, Helffrich GR, et al., 2015, CAN‐HK: An a Priori Crustal Model for the Canadian Shield, Seismological Research Letters, Vol: 86, Pages: 1374-1382, ISSN: 0895-0695
Bastow ID, Julia J, do Nascimento A, et al., 2015, Upper mantle anisotropy of the Borborema Province, NE Brazil: Implications for intra-plate deformation and sub-cratonic asthenospheric flow, Tectonophysics, Vol: 657, Pages: 81-93, ISSN: 0040-1951
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 São 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
Lamontagne M, Lavoie D, Ma S, et al., 2015, Monitoring the Earthquake Activity in an Area with Shale Gas Potential in Southeastern New Brunswick, Canada, Seismological Research Letters, Vol: 86, Pages: 1068-1077, ISSN: 0895-0695
Corti G, Agostini A, Keir D, et al., 2015, Magma-induced axial subsidence during final-stage rifting: Implications for the development of seaward-dipping reflectors, Geosphere, Vol: 11, Pages: 563-571, ISSN: 1553-040X
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 Depres- sion in East Africa. Model results show that axial mafic intrusions in the crust are able to cause significant downward flexure of the open- ing 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.
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