115 results found
Chen F, Davies DR, Goes S, et al., 2022, How Slab Age and Width Combine to Dictate the Dynamics and Evolution of Subduction Systems: A 3-D Spherical Study, GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, Vol: 23
Goes S, Yu C, ballmer M, et al., 2022, Compositional heterogeneity in the mantle transition zone, Nature Reviews Earth & Environment, Vol: 3, Pages: 533-550, ISSN: 2662-138X
Earth’s mantle transition zone (MTZ) is characterized by several sharp increases in seismic wave speed between ~300 km and ~850 km depth. These seismic discontinuities are generally attributed to solid-state phase transitions that lead to density and viscosity increases, which could cause a barrier to convection by segregating thermally and chemically heterogeneous material. This Review discusses insights into the role of MTZ compositional heterogeneity in mantle convection, derived from the joint constraints of MTZ discontinuity-reflected, discontinuity-refracted and discontinuity-transmitted seismic waves and thermodynamic and convection models. Growing seismic data sets and advances in analysis techniques show that the topography of these discontinuities mainly reflects variations in mantle temperature and, hence, present-day mantle flow. However, the discordant behaviour of the 410 km and 660 km discontinuities shows that the thermal structure is not vertically coherent across the MTZ in many areas, indicating that the MTZ delays the convective transport of cold material from above and hot material from below. Variable reflectivity of the MTZ discontinuity provides evidence of lateral and vertical heterogeneity in major element chemistry and volatile content. Seismic results are consistent with whole-mantle mechanical mixing of tectonic plates, with segregated material accumulating in the MTZ over multiple mantle convection cycles.
Bie L, Hicks S, Rietbrock A, et al., 2022, Imaging slab-transported fluids and their deep dehydration from seismic velocity tomography in the Lesser Antilles subduction zone, Earth and Planetary Science Letters, Vol: 586, Pages: 117535-117535, ISSN: 0012-821X
Volatiles play a pivotal role in subduction zone evolution, yet their pathways remain poorly constrained. Studying the Lesser Antilles subduction zone can yield new constraints, where old oceanic lithosphere formed by slow-spreading subducts slowly. Here we use local earthquakes recorded by the temporary VoiLA (Volatile recycling in the Lesser Antilles) deployment of ocean-bottom seismometers in the fore- and back-arc to characterize the 3-D seismic structure of the north-central Lesser Antilles subduction zone. Along the slab top, mapped based on seismicity, we find low Vp extending to 130–150 km depth, deeper than expected for magmatic oceanic crust. The slab's most prominent, elevated Vp/Vs anomalies are beneath the fore- and back-arc offshore Guadeloupe and Dominica, where two subducted fracture zones lie with the obliquely subducting boundary between Proto-Caribbean and Equatorial Atlantic lithosphere. These structures, therefore, enhance hydration of the oceanic lithosphere as it forms and evolves and the subsequent dehydration of mantle serpentinite when subducted. Above the slab, we image the asthenosphere wedge as a high Vp/Vs and moderate Vp feature, indicating slab-dehydrated fluids rising through the overlying cold boundary layer that might induce melting further to the west. Our results provide new evidence for the impact of spatially-variable oceanic plate formation processes on slab dehydration and mantle wedge volatile transfer that ultimately impact volcanic processes at the surface, such as the relatively high magmatic output observed on the north-central islands in the Lesser Antilles.
Suchoy L, Goes S, Chen F, et al., 2022, How aseismic ridges modify the dynamics of free subduction: a 3-D numerical investigation, Frontiers in Earth Science, Vol: 10, ISSN: 2296-6463
The subduction of positively buoyant features has been implicated in the development of flat and shallow dipping slabs, the formation of cusps in trench geometry, and the cessation of associated arc magmatism. However, how such buoyant anomalies influence subduction dynamics to produce these different tectonic expressions remains debated. In this paper, using a series of multi-material 3-D simulations of free subduction, we investigate how linear buoyant ridges modify subduction dynamics, in particular downgoing plate velocities, trench motions and slab morphology. We examine the sensitivity of results to downgoing plate age (affecting buoyancy and strength), ridge buoyancy and ridge location along the trench, finding that buoyant ridges can locally change slab sinking and trench retreat rates, in turn modifying the evolution of slab morphology at depth and trench shape at the surface. In all cases examined, trench retreat is reduced, or switches to trench advance, where the ridge subducts. These effects depend strongly on downgoing plate age: on young, weak plates, the change in trench shape is more localised than on old, strong plates. Slab shallowing at the ridge only occurs for young plates, while the stronger and more negatively buoyant older plates pull down the ridge at a steeper angle than the rest of the slab. On old plates, ridges located near regions of trench stagnation or advance, which typically develop in wide slabs, have a stronger effect on trench and slab shape. The combined effects of buoyant feature location, subducting plate age and overriding plate properties can result in a range of responses: from mainly trench deformation, through local slab shallowing, to the formation of a flat slab, a variation in expressions also observed on Earth.
Okuwaki R, Hicks SP, Craig TJ, et al., 2021, Illuminating a contorted slab with a complex intraslab rupture evolution during the 2021 Mw 7.3 East Cape, New Zealand earthquake, Geophysical Research Letters, Vol: 48, Pages: 1-13, ISSN: 0094-8276
The state-of-stress within subducting oceanic plates controls rupture processes of deep intraslab earthquakes. However, little is known about how the large-scale plate geometry and the stress regime relate to the physical nature of the deep intraslab earthquakes. Here we find, by using globally and locally observed seismic records, that the moment magnitude 7.3 2021 East Cape, New Zealand earthquake was driven by a combination of shallow trench-normal extension and unexpectedly, deep trench-parallel compression. We find multiple rupture episodes comprising a mixture of reverse, strike-slip, and normal faulting. Reverse faulting due to the trench-parallel compression is unexpected given the apparent subduction direction, so we require a differential buoyancy-driven stress rotation, which contorts the slab near the edge of the Hikurangi plateau. Our finding highlights that buoyant features in subducting plates may cause diverse rupture behavior of intraslab earthquakes due to the resulting heterogeneous stress state within slabs.
Hicks S, Goes S, Whittaker A, et al., 2021, Multivariate statistical appraisal of regional susceptibility to induced seismicity: application to the Permian Basin, SW United States, Journal of Geophysical Research. Solid Earth, Vol: 126, ISSN: 2169-9356
Induced earthquake sequences are typically interpreted through causal triggering mechanisms. However, studies of causality rarely consider large regions and why some regions experiencing similar anthropogenic activities remain largely aseismic. Therefore, it can be difficult to forecast seismic hazard at a regional scale. In contrast, multivariate statistical methods allow us to find the combinations of factors that correlate best with seismicity, which can help form the basis of hypotheses that can be subsequently tested with physical models. Whilst strong correlations do not necessarily equate to causality, such a statistical approach is particularly important for large regions with newly emergent seismicity comprising multiple distinct clusters and multi-faceted industrial operations. Recent induced seismicity in the Permian Basin provides an excellent test-bed for multivariate statistical analyses because the main causal industrial and geological factors driving earthquakes in the region remain highly debated. Here, we use logistic regression to retrospectively predict the spatial variation of seismicity across the western Permian Basin. We reproduce the broad distribution of seismicity using a combination of both industrial and geological factors. Our model shows that the proximity to neotectonic faults west of the Delaware Basin is the most important factor that contributes to induced seismicity. The second-most important factor is salt-water disposal at shallow depths, with hydraulic fracturing playing a less dominant role. The higher tectonic stressing, together with a poor correlation between seismicity and large-volume deep salt-water disposal wells indicates a very different mechanism of induced seismicity compared to that in Oklahoma.
Braszus B, Goes S, Allen R, et al., 2021, Subduction history of the Caribbean from upper-mantle seismic imaging and plate reconstruction, Nature Communications, Vol: 12, Pages: 1-14, ISSN: 2041-1723
The margins of the Caribbean and associated hazards and resources have been shaped by a poorly understood history of subduction. Using new data, we improve teleseismic P-wave imaging of the eastern Caribbean upper mantle and compare identified subducted-plate fragments with trench locations predicted from plate reconstruction. This shows that material at 700–1200 km depth below South America derives from 90–115 Myr old westward subduction, initiated prior to Caribbean Large-Igneous-Province volcanism. At shallower depths, an accumulation of subducted material is attributed to Great Arc of the Caribbean subduction as it evolved over the past 70 Ma. We interpret gaps in these subducted-plate anomalies as: a plate window and tear along the subducted Proto-Caribbean ridge; tearing along subducted fracture zones, and subduction of a volatile-rich boundary between Proto-Caribbean and Atlantic domains. Phases of back-arc spreading and arc jumps correlate with changes in age, and hence buoyancy, of the subducting plate.
Schlaphorst D, Harmon N, Kendall JM, et al., 2021, Variation in Upper Plate Crustal and Lithospheric Mantle Structure in the Greater and Lesser Antilles From Ambient Noise Tomography, GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, Vol: 22
Harmon N, Rychert CA, Goes S, et al., 2021, Widespread hydration of the back arc and the link to variable hydration of the incoming plate in the lesser Antilles from rayleigh wave imaging, Geochemistry, Geophysics, Geosystems, Vol: 22, Pages: 1-27, ISSN: 1525-2027
Subduction zone dynamics are important for a better understanding of natural hazards, plate tectonics, and the evolution of the planet. Despite this, the factors dictating the location and style of volcanism are not well-known. Here we present Rayleigh Wave imaging of the Lesser Antilles subduction zone using the ocean bottom and land seismic data collected as a part of the VoiLA experiment. This region is an important global endmember that represents a slow (<19 mm/yr) convergence rate of old (80–120 Ma), Atlantic lithosphere formed at a slow spreading ridge. We image the fast slab, the fast-overriding plate and the slow mantle wedge across the entire arc. We find slow velocity anomalies (∼4.1 km/s) in the mantle wedge directly beneath the arc with local minima beneath Dominica/Martinique, Montserrat and the Grenadines. We observe that slow velocities in the wedge extend 200 km into the back arc west of Martinique. The slowest mantle wedge velocity anomaly is more muted than several global wedges, likely reflecting the lower temperatures and less partial melt predicted for the Antilles. Subducted fracture zones and plate boundaries are a potential source of hydration, since they are located near the anomalies, although not directly beneath them. To match our observations, geodynamic models with a broadly hydrated mantle wedge are required, which can be achieved via deep hydration of the slab, and fluid release further into the back arc. In addition, 3-D flow and melt migration or ponding are required to explain the shape and location of our anomalies.
Suchoy L, Goes S, Maunder B, et al., 2021, Effects of basal drag on subduction dynamics from 2D numerical models, Solid Earth, Vol: 12, Pages: 79-93, ISSN: 1869-9510
Subducting slabs are an important driver of plate motions, yet the force balance governing subduction dynamics remains incompletely understood. Basal drag has been proposed to be a minor contributor to subduction forcing, because of the lack of correlation between plate size and velocity in observed and reconstructed plate motions. Furthermore, in single subduction system models, low basal drag, associated with a low ratio of asthenospheric to lithospheric viscosity, leads to subduction behaviour most consistent with the observation that trench migration velocities are generally low compared to convergence velocities. By contrast, analytical calculations and global mantle flow models indicate basal drag can be substantial. In this study, we revisit this problem by examining the drag at the base of the lithosphere, for a single subduction system, in 2D models with a free trench and composite non-linear rheology. We compare the behaviour of short and long plates for a range of asthenospheric and lithospheric rheologies. We reproduce results from previous modelling studies, including low ratios of trench over plate motions. However, we also find that any combination of asthenosphere and lithosphere viscosity that produces Earth-like subduction behaviour leads to a correlation of velocities with plate size, due to the role of basal drag. By examining Cenozoic plate motion reconstructions, we find that slab age and plate size are positively correlated: higher slab pull for older plates tends to be offset by higher basal drag below these larger plates. This, in part, explains the lack of plate velocity-size correlation in observations, despite the important role of basal drag in the subduction force-balance.
Altoe I, Eeken T, Goes S, et al., 2020, Thermo-compositional structure of the north-eastern Canadian Shield from Rayleigh wave dispersion analysis as a record of its tectonic history, Earth and Planetary Science Letters, Vol: 547, ISSN: 0012-821X
The thermal and compositional structure of lithospheric keels underlying cratons, which are stable continental cores formed during the Precambrian, is still an enigma. Mapping lithospheric temperatures and compositional heterogeneities is essential to better understand geodynamic processes that control craton formation and evolution. Here we investigate the northeastern part of North America which comprises the Superior Craton, the largest Archean craton in the world, and surrounding Proterozoic belts. We model Rayleigh-wave dispersion curves from a previous study, which were regionalised based on cluster analysis. Next, we perform a grid search for sub-crustal thermal and compositional structures that are consistent with the average dispersion curve for each cluster. We apply constraints on crustal structure and use thermodynamic methods to map thermo-compositional structures into seismic velocity. In agreement with previous studies, most regions require concentrations of metasomatic minerals over certain depth intervals to fit the seismic profiles. Our results further require vertical as well as lateral variations in compositional and thermal structures, which appear to reflect different stages of formation and modification of the lithosphere below the region, with distinct structures found under Archean cores, Archean/Paleoproterozoic collision belts, mid-late Proterozoic collision belts, and zones affected by rifting.
Thermal structure of the lithosphere exerts a primary control on its strength and density and thereby its dynamic evolution as the outer thermal and mechanic boundary layer of the convecting mantle. This contribution focuses on continental lithosphere. We review constraints on thermal conductivity and heat production, geophysical and geochemical/petrological constraints on thermal structure of the continental lithosphere, as well as steady-state and non-steady state 1D thermal models and their applicability. Commonly used geotherm families that assume that crustal heat production contributes an approximately constant fraction of 25–40% to surface heat flow reproduce the global spread of temperatures and thermal thicknesses of the lithosphere below continents. However, we find that global variations in seismic thickness of continental lithosphere and seismically estimated variations in Moho temperature below the US are more compatible with models where upper crustal heat production is 2–3 times higher than lower crustal heat production (consistent with rock estimates) and the contribution of effective crustal heat production to thermal structure (i.e. estimated by describing thermal structure with steady-state geotherms) varies systematically from 40 to 60% in tectonically stable low surface heat flow regions to 20% or lower in higher heat flow tectonically active regions. The low effective heat production in tectonically active regions is likely partly the expression of a non-steady thermal state and advective heat transport.
Cooper GF, Macpherson CG, Blundy JD, et al., 2020, Variable water input controls evolution of the Lesser Antilles volcanic arc (vol 87, pg 931, 2020), Nature, Vol: 584, Pages: E36-E36, ISSN: 0028-0836
Kounoudis R, Bastow I, Ogden C, et al., 2020, Seismic tomographic imaging of the Eastern Mediterranean Mantle: Implications for terminal-stage subduction, the uplift of Anatolia, and the development of the North Anatolian Fault, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 21, ISSN: 1525-2027
The Eastern Mediterranean captures the eastwest transition from active subduction of Earth'soldest oceanic lithosphere to continental collision, making it an ideal location to study terminalstagesubduction. Asthenospheric or subductionrelated processes are the main candidates for the region's ∼2kmuplift and Miocene volcanism; however, their relative importance is debated. To address these issues, wepresent new P and S wave relative arrivaltime tomographic models that reveal fast anomalies associatedwith an intact Aegean slab in the west, progressing to a fragmented, partially continental, Cyprean slabbelow central Anatolia. We resolve a gap between the Aegean and Cyprean slabs, and a horizontal tear in theCyprean slab below the Central Anatolian Volcanic Province. Below eastern Anatolia, the completelydetached “Bitlis” slab is characterized by fast wave speeds at ∼500 km depth. Assuming slab sinkingrates mirror ArabiaAnatolia convergence rates, the Bitlis slab's location indicates an Oligocene (∼26 Ma)breakoff. Results further reveal a strong velocity contrast across the North Anatolian Fault likelyrepresenting a 40–60 km decrease in lithospheric thickness from the Precambrian lithosphere north of thefault to a thinned Anatolian lithosphere in the south. Slow uppermostmantle wave speeds below activevolcanoes in eastern Anatolia, and ratios of P to S wave relative traveltimes, indicate a thin lithosphere andmelt contributions. Positive central and eastern Anatolian residual topography requires additional supportfrom hot/buoyant asthenosphere to maintain the 1–2 km elevation in addition to an almost absentlithospheric mantle. Smallscale fast velocity structures in the shallow mantle above the Bitlis slab maytherefore be drips of Anatolian lithospheric mantle.
Eeken T, Goes S, Petrescu L, et al., 2020, Lateral variations in thermochemical structure of the Eastern Canadian Shield, Journal of Geophysical Research, Vol: 125, Pages: 1-19, ISSN: 0148-0227
The origin of 3D seismic heterogeneity in Precambrian lithosphere has been enigmatic, because temperature variations in old stable shields are expected to be small and seismic sensitivity to major‐element compositional variations is limited. Previous studies indicate that metasomatic alteration may significantly affect average 1‐D structure below shields. Here, we perform a grid search for 3‐D thermo‐chemical structure, including variations in alteration, to model published Rayleigh‐wave phase velocities between 20 and 160 s for the eastern part of the Archean Superior and Canadian Proterozoic Grenville Provinces. We find that, consistent with constraints from surface heatflow and xenoliths, the lithosphere is coolest (Moho heatflow 12‐17 mW/m2) and the thermal boundary layer thickest (>250 km) in the northeastern Superior and warmest in the southeastern Grenville (Moho heatflow 20‐25 mW/m2, thermal boundary thickness 160‐200 km). Compositionally, the phase velocities for most of the Superior within our study region require little alteration, but in a few regions, fast velocities need to overlie slower velocities. These can be modelled with an eclogite layer in the mid lithosphere, consistent with active‐seismic and xenolith evidence for remnants of subducted Archean crust. The phase velocities from the Grenville Province require significant metasomatic modification to explain the relatively low velocities of the shallow lithosphere, and the required intensity of alteration is highest in parts of the Grenville associated with arc accretion. Thus, composition of the northeastern Canadian Shield appears to reflect different stages and styles of craton assembly.
Cooper GF, Macpherson CG, Blundy JD, et al., 2020, Variable water input controls evolution of the Lesser Antilles volcanic arc, Nature, Vol: 582, Pages: 525-529, ISSN: 0028-0836
Oceanic lithosphere carries volatiles, notably water, into the mantle through subduction at convergent plate boundaries. This subducted water exercises control on the production of magma, earthquakes, formation of continental crust and mineral resources. Identifying different potential fluid sources (sediments, crust and mantle lithosphere) and tracing fluids from their release to the surface has proved challenging1. Atlantic subduction zones are a valuable endmember when studying this deep water cycle because hydration in Atlantic lithosphere, produced by slow spreading, is expected to be highly non-uniform2. Here, as part of a multi-disciplinary project in the Lesser Antilles volcanic arc3, we studied boron trace element and isotopic fingerprints of melt inclusions. These reveal that serpentine—that is, hydrated mantle rather than crust or sediments—is a dominant supplier of subducted water to the central arc. This serpentine is most likely to reside in a set of major fracture zones subducted beneath the central arc over approximately the past ten million years. The current dehydration of these fracture zones coincides with the current locations of the highest rates of earthquakes and prominent low shear velocities, whereas the preceding history of dehydration is consistent with the locations of higher volcanic productivity and thicker arc crust. These combined geochemical and geophysical data indicate that the structure and hydration of the subducted plate are directly connected to the evolution of the arc and its associated seismic and volcanic hazards.
Davy R, Collier JS, Henstock TJ, et al., 2020, Wide‐angle seismic imaging of two modes of crustal accretion in mature Atlantic Ocean crust, Journal of Geophysical Research: Solid Earth, Vol: 125, Pages: 1-21, ISSN: 2169-9313
We present a high‐resolution 2‐D P‐wave velocity model from a 225 km long active seismic profile, collected over ~60‐75 Ma central Atlantic crust. The profile crosses five ridge segments separated by a transform and three non‐transform offsets. All ridge discontinuities share similar primary characteristics, independent of the offset. We identify two types of crustal segment. The first displays a classic two‐layer velocity structure with a high gradient layer 2 (~0.9 s‐1) above a lower gradient layer 3 (0.2 s‐1). Here PmP coincides with the 7.5 km s‐1 contour and velocity increases to >7.8 km s‐1 within 1 km below. We interpret these segments as magmatically‐robust, with PmP representing a petrological boundary between crust and mantle. The second has a reduced contrast in velocity gradient between upper and lower crust, and PmP shallower than the 7.5 km s‐1 contour. We interpret these segments as tectonically dominated, with PmP representing a serpentinized (alteration) front. Whilst velocity depth profiles fit within previous envelopes for slow‐spreading crust, our results suggest that such generalizations give a misleading impression of uniformity. We estimate that the two crustal styles are present in equal proportions on the floor of the Atlantic. Within two tectonically dominated segments we make the first wide‐angle seismic identifications of buried oceanic core complexes in mature (> 20 Ma) Atlantic Ocean crust. They have a ~20 km wide “domal” morphology with shallow basement and increased upper‐crustal velocities. We interpret their mid‐crustal seismic velocity inversions as alteration and rock‐type assemblage contrasts across crustal‐scale detachment faults.
Maunder B, Prytulak J, Goes S, et al., 2020, Rapid subduction initiation and magmatism in the Western Pacific driven by internal vertical forces, Nature Communications, Vol: 11, ISSN: 2041-1723
Plate tectonics requires the formation of plate boundaries. Particularly important is the enigmatic initiation of subduction: the sliding of one plate below the other, and the primary driver of plate tectonics. A continuous, in situ record of subduction initiation was recovered by the International Ocean Discovery Program Expedition 352, which drilled a segment of the fore-arc of the Izu-Bonin-Mariana subduction system, revealing a distinct magmatic progression with a rapid timescale (approximately 1 million years). Here, using numerical models, we demonstrate that these observations cannot be produced by previously proposed horizontal external forcing. Instead a geodynamic evolution that is dominated by internal, vertical forces produces both the temporal and spatial distribution of magmatic products, and progresses to self-sustained subduction. Such a primarily internally driven initiation event is necessarily whole-plate scale and the rock sequence generated (also found along the Tethyan margin) may be considered as a smoking gun for this type of event.
Bie L, Rietbrock A, Hicks S, et al., 2020, Along‐arc heterogeneity in local seismicity across the lesser antilles subduction zone from a dense ocean‐bottom seismometer network, Seismological Research Letters, Vol: 91, Pages: 237-247, ISSN: 0895-0695
The Lesser Antilles arc is only one of two subduction zones where slow‐spreading Atlantic lithosphere is consumed. Slow‐spreading may result in the Atlantic lithosphere being more pervasively and heterogeneously hydrated than fast‐spreading Pacific lithosphere, thus affecting the flux of fluids into the deep mantle. Understanding the distribution of seismicity can help unravel the effect of fluids on geodynamic and seismogenic processes. However, a detailed view of local seismicity across the whole Lesser Antilles subduction zone is lacking. Using a temporary ocean‐bottom seismic network we invert for hypocenters and 1D velocity model. A systematic search yields a 27 km thick crust, reflecting average arc and back‐arc structures. We find abundant intraslab seismicity beneath Martinique and Dominica, which may relate to the subducted Marathon and/or Mercurius Fracture Zones. Pervasive seismicity in the cold mantle wedge corner and thrust seismicity deep on the subducting plate interface suggest an unusually wide megathrust seismogenic zone reaching ∼65km∼65 km depth. Our results provide an excellent framework for future understanding of regional seismic hazard in eastern Caribbean and the volatile cycling beneath the Lesser Antilles arc.
Civiero C, Armitage J, Goes S, et al., 2019, The seismic signature of upper-mantle plumes: application to the northern East African Rift, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 20, Pages: 6106-6122, ISSN: 1525-2027
Several seismic and numerical studies proposed that below some hotspots upper‐mantle plumelets rise from a thermal boundary layer below 660 km depth, fed by a deeper plume source. We recently found tomographic evidence of multiple upper‐mantle upwellings, spaced by several 100 km, rising through the transition zone below the northern East African Rift. To better test this interpretation, we run 3D numerical simulations of mantle convection for Newtonian and non‐Newtonian rheologies, for both thermal instabilities rising from a lower boundary layer, and the destabilisation of a thermal anomaly placed at the base of the box (700‐800 km depth). The thermal structures are converted to seismic velocities using a thermo‐dynamic approach. Resolution tests are then conducted for the same P‐ and S‐data distribution and inversion parameters as our travel‐time tomography. The Rayleigh Taylor models predict simultaneous plumelets in different stages of evolution rising from a hot layer located below the transition zone, resulting in seismic structure that looks more complex than the simple vertical cylinders that are often anticipated. From the wide selection of models tested we find that the destabilisation of a 200°C, 100 km thick thermal anomaly with a non‐Newtonian rheology, most closely matches the magnitude, the spatial and temporal distribution of the anomalies below the rift. Finally, we find that for reasonable upper‐mantle viscosities, the synthetic plume structures are similar in scale and shape to the actual low‐velocity anomalies, providing further support for the existence of upper‐mantle plumelets below the northern East African Rift.
Allen R, Collier J, Stewart A, et al., 2019, The role of arc migration in the development of the Lesser Antilles: a new tectonic model for the Cenozoic evolution of the eastern Caribbean, Geology, Vol: 47, Pages: 891-895, ISSN: 0091-7613
Continental arc systems often show evidence of large-scale migration both towards and away from the incoming plate. In oceanic arc systems however, whilst slab roll-back and the associated processes of back-arc spreading and arc migration towards the incoming plate are commonplace, arc migration away from the incoming plate is rarely observed. We present a new compilation of marine magnetic anomaly and seismic data in order to propose a new tectonic model for the eastern Caribbean region that includes arc migration in both directions. We synthesise new evidence to show two phases of back-arc spreading and eastward arc migration towards the incoming Atlantic. A third and final phase of arc migration to the west subdivided the earlier back-arc basin on either side of the present-day Lesser Antilles Arc. This is the first example of regional multi-directional arc migration in an intra-oceanic setting and has implications for along-arc structural and geochemical variations. The back and forth arc migrations are probably due to the constraints the neighbouring American plates impose on this isolated subduction system rather than variations in subducting slab buoyancy.
Halpaap F, Rondenay S, Perrin A, et al., 2019, Earthquakes track subduction fluids from slab source to mantle wedge sink, Science Advances, Vol: 5, ISSN: 2375-2548
Subducting plates release fluids as they plunge into Earth’s mantle and occasionally rupture to produce intraslab earthquakes. It is debated whether fluids and earthquakes are directly related. By combining seismic observations and geodynamic models from western Greece, and comparing across other subduction zones, we find that earthquakes effectively track the flow of fluids from their slab source at >80 km depth to their sink at shallow (<40 km) depth. Between source and sink, the fluids flow updip under a sealed plate interface, facilitating intraslab earthquakes. In some locations, the seal breaks and fluids escape through vents into the mantle wedge, thereby reducing the fluid supply and seismicity updip in the slab. The vents themselves may represent nucleation sites for larger damaging earthquakes.
Goes S, Collier J, Blundy J, et al., 2019, Project VoiLA: Volatile Recycling in the Lesser Antilles, Eos, Vol: 100, ISSN: 2324-9250
Deep water cycle studies have largely focused on subduction of lithosphere formed at fast spreading ridges. However, oceanic plates are more likely to become hydrated as spreading rate decreases.
Beghein C, Xing Z, Goes S, 2019, Thermal nature and resolution of the lithosphere–asthenosphere boundary under the Pacific from surface waves, Geophysical Journal International, Vol: 216, Pages: 1441-1465, ISSN: 0956-540X
It is strongly debated whether the interface between the lithosphere and underlying asthenosphere is a temperature-dependent rheological transition, as expected in a thermal convection system, or additionally affected by the presence of melts and/or fluids. Previous surface wave studies of Pacific oceanic lithosphere have found that shear velocity and azimuthal anisotropy vary with seafloor crustal age as expected for a thermal control; however radial anisotropy does not. Various thermomechanical models have been proposed to explain this disparate behaviour. Nonetheless, it is unclear how robust the surface wave constraints are, and this is what we test in this study. We apply a Bayesian model space search approach to three published Pacific surface wave dispersion data sets, two phase-velocity and one combined phase- and group-velocity set, and determine various proxies for the depth of the lithosphere–asthenosphere boundary (LAB) and their uncertainties based on the velocity and radial anisotropy model distributions obtained. In their overall character and pattern with age, the velocity models from different data sets are consistent with each other, although they differ in their values of LAB depths. Uncertainties are substantial (as much as 20 km on LAB depths) and the addition of group-velocity data does not reduce them. Radial anisotropy structures differ even in pattern and display no obvious age dependence. However, given the uncertainties, we cannot exclude that radial anisotropy, azimuthal anisotropy and velocity models actually reflect compatible, age-dependent, LAB depth estimates. The velocity LAB trends are most like those expected for half-space cooling, because velocity differences persist at old ages, below the depth of common plate cooling models. Any direct signature of sub-ridge melt would be too small-scale to be resolved by these data. However, the velocity-increasing effects of dehydration and depletion due to melting below the rid
Perrin A, Goes S, Prytulak J, et al., 2018, Mantle wedge temperatures and their potential relation to volcanic arc location, Earth and Planetary Science Letters, Vol: 501, Pages: 67-77, ISSN: 0012-821X
The mechanisms underpinning the formation of a focused volcanic arc above subduction zones are debated. Suggestions include controls by: (i) where the subducting plate releases water, lowering the solidus in the overlying mantle wedge; (ii) the location where the mantle wedge melts to the highest degree; and (iii) a limit on melt formation and migration imposed by the cool shallow corner of the wedge. Here, we evaluate these three proposed mechanisms using a set of kinematically-driven 2D thermo-mechanical mantle-wedge models in which subduction velocity, slab dip and age, overriding-plate thickness and the depth of decoupling between the two plates are systematically varied. All mechanisms predict, on the basis of model geometry, that the arc-trench distance, D, decreases strongly with increasing dip, consistent with the negative D-dip correlations found in global subduction data. Model trends of sub-arc slab depth, H, with dip are positive if H is wedge-temperature controlled and overriding-plate thickness does not exceed the decoupling depth by more than 50 km, and negative if H is slab-temperature controlled. Observed global H-dip trends are overall positive. With increasing overriding plate thickness, the position of maximum melting shifts to smaller H and D, while the position of the trenchward limit of the melt zone, controlled by the wedge's cold corner, shifts to larger H and D, similar to the trend in the data for oceanic subduction zones. Thus, the limit imposed by the wedge corner on melting and melt migration seems to exert the first-order control on arc position.
Maguire R, Ritsema J, Goes S, 2018, Evidence of subduction-related thermal and compositional heterogeneity below the United States from transition zone receiver functions, Geophysical Research Letters, Vol: 45, Pages: 8913-8922, ISSN: 0094-8276
The subduction of the Farallon Plate has altered the temperature and composition of the mantle transition zone (MTZ) beneath the United States. We investigate MTZ structure by mapping P‐to‐S conversions at mineralogical phase changes using USArray waveform data and theoretical seismic profiles based on experimental constraints of phase transition properties as a function of temperature and composition. The width of the MTZ varies by about 35 km over the study region, corresponding to a temperature variation of more than 300 K. The MTZ is coldest and thickest beneath the eastern United States where high shear velocity anomalies are tomographically resolved. We detect intermittent P‐to‐S conversions at depths of 520 km and 730 km. The conversions at 730‐km depth are coherent beneath the southeastern United States and are consistent with basalt enrichment of about 50%, possibly due to the emplacement of a fragment of an oceanic plateau (i.e., the Hess conjugate).
Agrusta R, van Hunen J, Goes S, 2018, Strong plates enhance mantle mixing in early Earth, Nature Communications, Vol: 9, Pages: 1-10, ISSN: 2041-1723
In the present-day Earth, some subducting plates (slabs) are flattening above the upper–lower mantle boundary at ~670 km depth, whereas others go through, indicating a mode between layered and whole-mantle convection. Previous models predicted that in a few hundred degree hotter early Earth, convection was likely more layered due to dominant slab stagnation. In self-consistent numerical models where slabs have a plate-like rheology, strong slabs and mobile plate boundaries favour stagnation for old and penetration for young slabs, as observed today. Here we show that such models predict slabs would have penetrated into the lower mantle more easily in a hotter Earth, when a weaker asthenosphere and decreased plate density and strength resulted in subduction almost without trench retreat. Thus, heat and material transport in the Earth’s mantle was more (rather than less) efficient in the past, which better matches the thermal evolution of the Earth.
Eeken T, Goes S, Pedersen H, et al., 2018, Seismic evidence for depth-dependent metasomatism in cratons, Earth and Planetary Science Letters, Vol: 491, Pages: 148-159, ISSN: 0012-821X
The long-term stability of cratons has been attributed to low temperatures and depletion iniron and water, which decrease density and increase viscosity. However, steady-state thermalmodels based on heat flow and xenolith constraints systematically overpredict the seismicvelocity-depth gradients in cratonic lithospheric mantle. Here we invert for the 1-D thermalstructure and a depth distribution of metasomatic minerals that fit average Rayleigh-wavedispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the ProterozoicBaltic Shield below Finland. To match the seismic profiles, we need a significant amount ofhydrous and/or carbonate minerals in the shallow lithospheric mantle, starting between theMoho and 70 km depth and extending down to at least 100-150 km. The metasomaticcomponent can consist of 0.5-1 wt% water bound in amphibole, antigorite and chlorite, ~0.2wt% water plus potassium to form phlogopite, or ~5 wt% CO2 plus Ca for carbonate, or acombination of these. Lithospheric temperatures that fit the seismic data are consistent withheat flow constraints, but most are lower than those inferred from xenolithgeothermobarometry. The dispersion data require differences in Moho heat flux betweenindividual cratons, and sublithospheric mantle temperatures that are 100-200°C less beneathYilgarn, Slave and Finland than beneath Kaapvaal. Significant upward-increasingmetasomatism by water and CO2-rich fluids is not only a plausible mechanism to explain theaverage seismic structure of cratonic lithosphere but such metasomatism may also lead to theformation of mid-lithospheric discontinuities and would contribute to the positive chemicalbuoyancy of cratonic roots.
Yu C, Day E, De Hoop M, et al., 2018, Compositional heterogeneity near the base of the mantle transition zone beneath Hawaii, Nature Communications, Vol: 9, Pages: 1-9, ISSN: 2041-1723
Global seismic discontinuities near 410 and 660 km depth in Earth’s mantle are expressions of solid-state phase transitions. These transitions modulate thermal and material fluxes across the mantle and variations in their depth are often attributed to temperature anomalies. Here we use novel seismic array analysis of SS waves reflecting off the 410 and 660 below the Hawaiian hotspot. We find amplitude–distance trends in reflectivity that imply lateral variations in wavespeed and density contrasts across 660 for which thermodynamic modeling precludes a thermal origin. No such variations are found along the 410. The inferred 660 contrasts can be explained by mantle composition varying from average (pyrolitic) mantle beneath Hawaii to a mixture with more melt-depleted harzburgite southeast of the hotspot. Such compositional segregation was predicted, from petrological and numerical convection studies, to occur near hot deep mantle upwellings like the one often invoked to cause volcanic activity on Hawaii.
Roberts GG, Lodhia B, Fraser A, et al., 2018, Continental margin subsidence from shallow mantle convection: example from West Africa, Earth and Planetary Science Letters, Vol: 481, Pages: 350-361, ISSN: 0012-821X
Spatial and temporal evolution of the uppermost convecting mantle plays an important role in determining histories of magmatism, uplift, subsidence, erosion and deposition of sedimentary rock. Tomographic studies and mantle flow models suggest that changes in lithospheric thickness can focus convection and destabilize plates. Geologic observations that constrain the processes responsible for onset and evolution of shallow mantle convection are sparse. We integrate seismic, well, gravity, magmatic and tomographic information to determine the history of Neogene-Recent (<23 Ma) upper mantle convection from the Cape Verde swell to West Africa. Residual ocean-age depths of +2 km and oceanic heat flow anomalies of +16 ± 4 mW m−2 are centered on Cape Verde. Residual depths decrease eastward to zero at the fringe of the Mauritania basin. Backstripped wells and mapped seismic data show that 0.4–0.8 km of water-loaded subsidence occurred in a ∼500 × 500 km region centered on the Mauritania basin during the last 23 Ma. Conversion of shear wave velocities into temperature and simple isostatic calculations indicate that asthenospheric temperatures determine bathymetry from Cape Verde to West Africa. Calculated average excess temperatures beneath Cape Verde are View the MathML source °C providing ∼103 m of support. Beneath the Mauritania basin average excess temperatures are View the MathML source °C drawing down the lithosphere by ∼102 to 103 m. Up- and downwelling mantle has generated a bathymetric gradient of ∼1/300 at a wavelength of ∼103 km during the last ∼23 Ma. Our results suggest that asthenospheric flow away from upwelling mantle can generate downwelling beneath continental margins.
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