Publications
103 results found
Ebinger C, Reiss M, Bastow I, et al., 2024, Shallow sources of upper mantle seismic anisotropy in East Africa, Earth and Planetary Science Letters, Vol: 625, ISSN: 0012-821X
The East African rift overlies one or more mantle upwellings and it traverses heterogeneous Archaean-Paleozoic lithosphere rifted in Mesozoic and Cenozoic time. We re-analyze XKS shear wave splitting at publicly available stations to evaluate models for rifting above mantle plumes. We use consistent criteria to compare and contrast both splitting direction and strength, infilling critical gaps with new data from the Turkana Depression and North Tanzania Divergence sectors of the East African rift system. Our results show large spatial variations in the amount of splitting (0.1–2.5 s), with fast axes predominantly sub-parallel to the orientation of Cenozoic rifts underlain by thinned lithosphere with and without surface magmatism. The amount of splitting increases with lithospheric thinning and magmatic modification. Nowhere are fast axes perpendicular to the rift, arguing against the development of extensional strain fabrics. Thick cratons are characterized by small amounts of splitting (≤0.5 s) with a variety of orientations that may characterize mantle plume flow. Splitting rotates to rift parallel and increases in strength over short distances into rift zones, implying a shallow depth range for the anisotropy in some places. The shallow source and correlation between splitting direction and the shape of upper mantle thin zones suggests that the combination of channel flow and oriented melt pockets contribute > 1 s to the observed splitting delays. Enhanced flow, metasomatism, and melt intrusion at the lithosphere-asthenosphere boundary suggest that fluid infiltration to the base of the lithosphere may facilitate rifting of cratonic lithosphere.
Kounoudis R, Bastow I, Ebinger C, et al., 2023, The development of rifting and magmatism in the multiply rifted Turkana Depression, East Africa: evidence from surface-wave analysis of crustal and uppermost mantle structure, Earth and Planetary Science Letters, Vol: 621, ISSN: 0012-821X
The Turkana Depression separates the uplifted Ethiopian and East African Plateaus. It was the site, in Mesozoic times, of a failed episode of NE–SW-oriented rifting (the Anza Rift), but now hosts E–W-oriented Nubia–Somalia separation at the junction between the Main Ethiopian Rift in the north and the Eastern Rift to the south. However, the time-integrated effect of these rifting phases on crustal and lithospheric mantle architecture and thermal structure is poorly understood. Utilising data from new seismograph networks in the Turkana Depression and northern Tanzania Craton, we produce a detailed anisotropic crustal and uppermost mantle shear-wave velocity model of the region. Within the Tanzania Craton, slightly lower uppermost mantle wavespeeds (4.4–4.5 km/s) compared to neighbouring regions, and coincident rift-parallel crustal anisotropy, imply the Nyanza Rift developed in relatively weak mobile belt lithosphere between two refractory Archean blocks. At upper-crustal (≲10 km) depths in the Turkana Depression, the slowest velocities (≲3.2 km/s) are attributed to thick Mesozoic-age sedimentary basins. Nowhere within the Depression is the mid-to-lower crust or lithospheric mantle associated with wavespeeds as slow, or seismic anisotropy as strong, as that observed below the melt-rich central and northern Main Ethiopian Rift (MER) and Ethiopian Plateau further north. High upper mantle wavespeeds (≳4.5 km/s), coinciding with the broadening of MER-rifting into southern Ethiopia, confirm the presence of refractory Proterozoic lithosphere acting as a rheological boundary to rift development. Thinned crustal zones associated with failed Mesozoic Anza rifting are also underlain by fast wavespeed (>4.5 km/s) mantle lithosphere, implying this area has resisted significant thermomechanical modification from Miocene-Recent extension and magmatism. Pre-existing crustal thin zones do not, therefore, necessarily represent zones of plate-weakness where
Musila M, Ebinger C, Bastow I, et al., 2023, Active deformation constraints on the Nubia-Somalia plate boundary through heterogenous lithosphere of the Turkana depression, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 24, ISSN: 1525-2027
The role of lithospheric heterogeneities, presence or absence of melt, local and regional stresses, and gravitational potential energy in strain localization in continental rifts remains debated. We use new seismic and geodetic data to identify the location and orientation of the modern Nubia-Somalia plate boundary in the 300-km-wide zone between the southern Main Ethiopian Rift (MER) and Eastern Rift (ER) across the Mesozoic Anza rift in the Turkana Depression. This region exhibits lithospheric heterogeneity, 45 Ma-Recent magmatism, and more than 1,500 m of base-level elevation change, enabling the assessment of strain localization mechanisms. We relocate 1716 earthquakes using a new 1-D velocity model. Using a new local magnitude scaling with station corrections, we find 1 ≤ ML ≤ 4.5, and a b-value of 1.22 ± 0.06. We present 59 first motion and 3 full moment tensor inversions, and invert for opening directions. We use complementary geodetic displacement vectors and strain rates to describe the geodetic strain field. Our seismic and geodetic strain zones demonstrate that only a small part of the 300 km-wide region is currently active; low elevation and high-elevation regions are active, as are areas with and without Holocene magmatism. Variations in the active plate boundary's location, orientation and strain rate appear to correspond to lithospheric heterogeneities. In the MER-ER linkage zone, a belt of seismically fast mantle lithosphere generally lacking Recent magmatism is coincident with diffuse crustal deformation, whereas seismically slow mantle lithosphere and Recent magmatism are characterized by localized crustal strain; lithospheric heterogeneity drives strain localization.
Boyce A, Kounoudis R, Bastow I, et al., 2023, Mantle wavespeed and discontinuity structure below East Africa: implications for Cenozoic Hotspot tectonism and the development of the Turkana Depression, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 24, Pages: 1-18, ISSN: 1525-2027
Ethiopia’s Cenozoic flood basalt magmatism, uplift, and rifting have been attributed to one or more mantle plumes. The Nubian plate, however, has drifted ∼500 km north since initial magmatism at ∼45 Ma, having developed above mantle that now underlies the northern Tanzania craton and the low-lying Turkana Depression. Unfortunately, our knowledge of mantle wavespeed structure and mantle transition zone (MTZ) topogra phy below these regions is poorest, due to a historical lack of seismograph stations. The same data gap means we lack constraints on lithospheric structure in and around the NW–SE trending Mesozoic Anza rift. We exploit data from new seismograph networks in the Turkana Depression and neighboring northern Uganda to develop AFRP22, a new African absolute P-wavespeed tomographic model that resolves whole mantle structure along the entire East African rift system. We also map MTZ thickness using Ps receiver functions. East Africa’s thinnest MTZ (∼25 km thinning) underlies the northwest Turkana Depression. AFRP22 reveals a co-located, previously-unrecognized, slow wavespeed plume tail, extending from the MTZ, deep into the lower mantle. This plume may thus have contributed, along with the African Superplume, to the development of the 45–30 Ma flood basalt province that preceded extension. Pervasive sub-lithospheric slow wavespeeds imply that Turkana’s present-day low elevation is explained best by Mesozoic and Cenozoic age crustal thinning. At ∼100 km depth, AFRP22 illuminates a fast wavespeed SE Ethiopian plateau. In addition to governing the northernmost limit of Mesozoic Anza rifting, therefractory nature of this lithospheric block likely minimized Cenozoic flood basalt magmatism there.
Hurman G, Derek K, Jonathan B, et al., 2023, Quantitative analysis of faulting in the Danakil Depression Rift of Afar: the importance of faulting in the final stages of magma-rich rifting, Tectonics, Vol: 42, Pages: 1-27, ISSN: 0278-7407
Magmatic intrusion and faulting both accommodate crustal extension in magma-rich rifts. However, quantitative constraints on the contribution of faulting to total extension and along-rift variations of faulting during the final stages of break-up are lacking. We targeted the Danakil Depression (Afar, Ethiopia) to conduct a quantitative, high-resolution study of fault activity and interaction in a magma-rich rift near break-up. Quantitative analysis of >500 rift axis faults, identified using remote sensing data (satellite imagery, DEMs), shows an increase in fault density, length and connectivity away from magmatic segments. Kinematic and earthquake focal mechanism data demonstrate a transition from transtensional opening in the northern and central sub-regions of the rift to oblique opening in the southern Giulietti Plain and Tat-Ali sub-regions. Oblique opening is attributed to the along-axis step between the Erta-Ale and Harak sub-regions. Integration of seismic reflection and borehole data with the mapped faults shows that extension is primarily accommodated by magmatism within the rift center, with faulting more significant towards the ends of the rift. ∼30% of crustal extension is accommodated by axial faulting in areas of low magmatism, highlighting the importance of faulting even in the final stages of magma-rich rifting. Comparing our findings with spreading ridge morphology and structure, relevant due to the rift maturity and extensive magmatism, we conclude that the Danakil Depression is in a transitional stage between continental rifting and seafloor spreading. Spatial changes in the importance of faulting and magmatism in accommodating extension, alongside rift morphology, resemble the relationships observed along spreading ridges.
Rooney T, Eric B, Bastow I, et al., 2023, Magmatism during the continent – Ocean transition, Earth and Planetary Science Letters, Vol: 614, Pages: 1-13, ISSN: 0012-821X
As continents break apart, the dominant mechanism of extension transitions from faulting and lithospheric stretching to magma intrusion and oceanic crust formation in a new ocean basin. A common feature of this evolution preserved at magmatic rifted margins worldwide are voluminous lava flows that erupted close to sea level during the final stages of development of the continent-ocean transition (COT). The mechanisms responsible for the generation of the melts that contribute to these voluminous flows, the so-called seaward dipping reflectors (SDR), and their significance in the context of COT development, are relatively poorly understood; they lie deep below post-rift strata along submarine rifted margins where they cannot be studied directly. Extensive coring of the Afar Stratoid Series - an a really-extensive sequence of Pliocene-aged basalts and intercalated sediments that lie atop the developing COT in the sub-aerial Afar Depression, northern Ethiopia - offers fresh scope to address this issue. We present a numericalmodel simulating the formation of enriched metasomes within the continental lithospheric mantle by the passage of magmas resembling modern axial basalts. Thermal destabilization of the metasome, caused by plate stretching, initiates melt formation within the metasome. These melts,when mixed with a depleted lithospheric mantle component, closely match the range of compositions of the Afar Stratoid Series lavas in this study. Metasomatic re-enrichment andsubsequent melting of the lithospheric mantle during the COT may contribute to further plate thinning. These results demonstrate a novel mechanism by which large-volume flows may be erupted during the COT.
Bastow I, Ogden C, Merry T, et al., 2023, Broadband seismological analyses in the Eastern Mediterranean: implications for late-stage subduction, plateau uplift and the development of the North Anatolian Fault
<jats:p>The eastern Mediterranean hosts extensional, strike-slip, and collision tectonics above a set of fragmenting subducting slabs. Widespread Miocene-Recent volcanism and ~2km uplift has been attributed to mantle processes such as delamination, dripping and/or slab tearing/break-off. We investigate this region using broadband seismology: mantle tomographic imaging (Kounoudis et al., 2020), SKS splitting analysis of seismic anisotropy (Merry et al., 2021), and receiver function study of crustal structure (Ogden & Bastow, 2021). Anisotropy and crustal structure are more spatially variable than recognised previously, but variations correspond well with tomographically-imaged mantle structure. Moho depth correlates poorly with elevation, suggesting crustal thickness variations alone do not explain Anatolian topography: a mantle contribution, particularly in central and eastern Anatolia, is needed too. Lithospheric anisotropy beneath the North Anatolian Fault reveals a mantle shear zone deforming coherently with the surface, while backazimuthal variations in splitting parameters indicate fault-related lithospheric deformation. Anisotropic fast directions are either fault-parallel or intermediate between the principle extensional strain rate axis and fault strike, diagnostic of a relatively low-strained transcurrent mantle shear zone.</jats:p>
Ogden C, Bastow I, Ebinger C, et al., 2023, The development of multiple phases of superposed rifting in the Turkana Depression, East Africa: evidence from receiver functions, Earth and Planetary Science Letters, Vol: 609, Pages: 1-13, ISSN: 0012-821X
The Turkana Depression in Eastern Africa separates the elevated plateaus of East Africa to the south andEthiopia-Yemen to the north. It remains unclear whether the Depression lacks dynamic mantle support,or if the entire East Africa region is dynamically supported and the Depression compensated isostaticallyby thinned crust. Also poorly understood is how Miocene-Recent extension has developed across theDepression, connecting spatially separated magmatic rift zones in Ethiopia and Kenya. Receiver functionanalysis is used to constrain Moho depth and bulk-crustal V P /V S ratio below new seismograph networksin the Depression, and on the northern Tanzania craton. Crustal thickness is ∼40 km below northernUganda and 30–35 km below southern Ethiopia, but 20–30 km below most of the Depression, wheremass-balance calculations reveal low elevations can be explained adequately by crustal thinning alone.Despite the fact that magmatism has occurred for 45 Ma across the Depression, more than 15 Ma beforeEast African Rift (EAR) extension initiated, bulk crustal V P /V S across southern Ethiopia and the TurkanaDepression (∼1.74) is similar to that observed in areas unaffected by Cenozoic rifting and magmatism.Evidence for voluminous lower crustal intrusions and/or melt, widespread below the Ethiopian rift andEthiopian plateau to the north, is therefore lacking. These observations, when reviewed in light ofhigh stretching factors (β ≤ 2.11), suggest Cenozoic extension has been dominated until recently byfaulting and plate stretching, rather than magma intrusion, which is likely an incipient process, operatingdirectly below seismically-active Lake Turkana. Early-stage EAR basins to the west of Lake Turkana, withassociated stretching factors of β ≈ 2, formed in crust only moderately thinned during earlier riftingepisodes. Conversely, ∼23 km-thick crust beneath the Kino Sogo Fault Belt (KSFB) has small offset faultsand thin sedimentary strata
Ogden CS, Bastow ID, Ebinger C, et al., 2023, The development of multiple phases of superposed rifting in the Turkana Depression, East Africa: Evidence from receiver functions, EARTH AND PLANETARY SCIENCE LETTERS, Vol: 609, ISSN: 0012-821X
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Pugh S, Boyce A, Bastow I, et al., 2023, Multigenetic origin of the X-discontinuity below continents: insights from African receiver functions, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 24, Pages: 1-22, ISSN: 1525-2027
Constraints on chemical heterogeneities in the upper mantle may be derived from studying the seismically observable impedance contrasts that they produce. Away from subduction zones, several causal mechanisms are possible to explain the intermittently observed X-discontinuity (X) at 230–350 km depth: the coesite-stishovite phase transition, the enstatite to clinoenstatite phase transition, and/or carbonated silicate melting, all requiring a local enrichment of basalt. Africa hosts a broad range of terranes, from Precambrian cores to Cenozoic hotspots with or without lowermost mantle origins. With the absence of subduction below the margins of the African plate for >0.5 Ga, Africa presents an ideal study locale to explore the origins of the X. Traditional receiver function (RF) approaches used to map seismic discontinuities, such as common conversion-point stacking, ignore slowness information crucial for discriminating converted upper mantle phases from surface multiples. By manually assessing depth and slowness stacks for 1° radius overlapping bins, normalized vote mapping of RF stacks is used to robustly assess the spatial distribution of converted upper mantle phases. The X is mapped beneath Africa at 233–340 km depth, revealing patches of heterogeneity proximal to mantle upwellings in Afar, Canaries, Cape Verde, East Africa, Hoggar, and Réunion with further observations beneath Cameroon, Madagascar, and Morocco. There is a lack of an X beneath southern Africa and strikingly, the magmatic eastern rift branch of the southern East African Rift. With no relationships existing between depth and amplitudes of observed X and estimated mantle temperatures, multiple causal mechanisms are required across a range of continental geodynamic settings.
Boyce A, Liddell M, Pugh S, et al., 2023, A new P-wave tomographic model (cap22) for North America: implications for the subduction and cratonic metasomatic modification history of western Canada and Alaska, Journal of Geophysical Research: Solid Earth, Vol: 128, Pages: 1-34, ISSN: 2169-9313
Our understanding of the present-day state and evolution of the Canadian and Alaskan mantle is hindered by a lack of absolute P-wavespeed constraints that provide complementary sensitivity to composition in conjunction with existing S-wavespeed models. Consequently, cratonic modification, orogenic history of western North America and complexities within the Alaskan Proto-Pacific subduction system remain enigmatic. One challenge concerns the difficulties in extracting absolute arrival-time measurements from often-noisy data recorded by temporary seismograph networks required to fill gaps in continental and global databases. Using the Absolute Arrival-time Recovery Method (AARM), we extract >180,000 new absolute arrival-time residuals from seismograph stations across Canada and Alaska and combine these data with USArray and global arrival-time data from the contiguous US and Alaska. We develop a new absolute P-wavespeed tomographic model, CAP22, spanning North America that significantly improves resolution in Canada and Alaska over previous models. Slow wavespeeds below the Canadian Cordillera sharply abut fast wavespeeds of the continental interior at the Rocky Mountain Trench in southwest Canada. Slow wavespeeds below the Mackenzie Mountains continue farther inland in northwest Canada, indicating Proterozoic-Archean metasomatism of the Slave craton. Inherited tectonic lineaments colocated with this north-south wavespeed boundary suggest that both the crust and mantle may control Cordilleran orogenic processes. In Alaska, fast upper mantle wavespeeds below the Wrangell Volcanic Field favor a conventional subduction related mechanism for volcanism. Finally, seismic evidence for the subducted Kula and Yukon slabs indicate tectonic reconstructions of western North America may require revision.
Magee C, Reeve MT, Jackson CA-L, et al., 2023, Reply to Alves et al. (2022) discussion on "Stratigraphic record of continental breakup, offshore NW Australia" by Reeve et al. (2022), Basin Research, Vol: 35, Pages: 483-486, ISSN: 0950-091X
Leah H, Fagereng A, Bastow I, et al., 2022, The northern Hikurangi margin three-dimensional plate interface in New Zealand remains rough 100 km from the trench, Geology, Vol: 50, Pages: 1256-1260, ISSN: 0091-7613
At the northern Hikurangi margin (North Island, New Zealand), shallow slow slip events (SSEs) frequently accommodate subduction-interface plate motion from landward of the trench to <20 km depth. SSEs may be spatially related to geometrical interface heterogeneity, though kilometer-scale plate-interface roughness imaged by active-source seismic methods is only constrained offshore at <12 km depth. Onshore constraints are comparatively lacking, but we mapped the Hikurangi margin plate interface using receiver functions from data collected by a dense 22 × 10 km array of 49 broadband seismometers. The plate interface manifests as a positive-amplitude conversion (velocity increase with depth) dipping west from 10 to 17 km depth. This interface corroborates relocated earthquake hypocenters, seismic velocity models, and downdip extrapolation of depth-converted two-dimensional active-source lines. Our mapped plate interface has kilometer-amplitude roughness we interpret as oceanic volcanics or seamounts, and is 1–4 km shallower than the regional-scale plate-interface model used in geodetic inversions. Slip during SSEs may thus have different magnitudes and/or distributions than previously thought. We show interface roughness also leads to shear-strength variability, where slip may nucleate in locally weak areas and propagate across areas of low shear-strength gradient. Heterogeneous shear strength throughout the depth range of the northern Hikurangi margin may govern the nature of plate deformation, including the localization of both slow slip and hazardous earthquakes.
Reeve MT, Magee C, Jackson CA-L, et al., 2022, Stratigraphic record of continental breakup, offshore NW Australia, BASIN RESEARCH, Vol: 34, Pages: 1220-1243, ISSN: 0950-091X
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Ogden C, Bastow I, 2022, The crustal structure of the Anatolian Plate from receiver functions and implications for the uplift of the Central and Eastern Anatolian plateaus, Geophysical Journal International, Vol: 229, Pages: 1041-1062, ISSN: 0956-540X
Understanding the crustal structure of the Anatolian Plate has important implications for its formation and evolution, including the extent to which its high elevation is maintained isostatically. However, the numerous teleseismic receiver function studies from which Anatolian Moho depths have been obtained return results that differ by ≤21 km at some seismograph stations. To address this issue, we determine Moho depth and bulk crustal VP/VS ratio (κ) at 582 broadband seismograph stations, including ∼100 for which H-κ results have not been reported previously. We use a modified H-κ stacking method in which a final solution is selected from a suite of up to 1000 repeat H-κ measurements, 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 data set, and the extent to which the results are clustered tightly, are used to determine station quality. By refining Moho depth constraints, including identifying 182 stations, analysed previously, where H-κ stacking yields unreliable results (particularly in Eastern Anatolia and the rapidly-uplifting Taurides), our new crustal model (ANATOLIA-HK21) provides fresh insight into Anatolian crustal structure and topography. Changes in Moho depth within the Anatolian Plate occur on a shorter length-scale than has sometimes previously been assumed. For example, crustal thickness decreases abruptly from >40 km in the northern Kirsehir block to <32 km beneath the Central Anatolian Volcanic Province and Tuz Golu basin. Moho depth increases from 30-35 km on the Arabian Plate to 35-40 km across the East Anatolian Fault into Anatolia, in support of structural geological observations that Arabia-Anatolia crustal shortening was accommodated primarily on the Anatolian, not Arabian, Plate. However, there are no consistent changes in Moho depth across the North Anatol
Bastow I, Merry T, Kounoudis R, et al., 2022, Mantle origins of topography, volcanism and the North Anatolian Fault in Anatolia: constraints from seismic tomography, seismic anisotropy and crustal structure
<jats:p>&lt;p&gt;The eastern Mediterranean hosts, within the span of a few hundred kilometres, extensional, strike-slip, and collision tectonics above a set of fragmenting subducting slabs. Widespread Miocene-Recent volcanism and ~2km uplift has been attributed to mantle processes such as delamination, dripping and/or slab tearing/break-off. We investigate this complex region using a variety of broadband seismological techniques, with new P- and S-wave tomographic images in Kounoudis et al. (2020), seismic anisotropy constrained via an updated dataset of SKS shear-wave splitting observations in Merry et al. (2021), and crustal structure imaged by quality-controlled H-&amp;#954; stacking of receiver functions in Ogden &amp; Bastow (2021). Overall, seismic anisotropy and crustal structure are more spatially variable than previously recognised, and such variations correspond well with variations in mantle structure shown by the tomography.&amp;#160;In general, Moho depth is poorly correlated with elevation, suggesting crustal thickness variations do not fully explain topographic differences, and residual topography calculations indicate the requirement for a mantle contribution to Anatolian Plateau uplift. Evidence for such a contribution exists in central Anatolia, where an imaged horizontal tear in the Cyprus slab spatially corresponds with volcanism, a residual topographic high, and a region of reduced splitting delay times and nulls, all consistent with upwelling of asthenospheric material through the tear. Anisotropic fast directions are consistent with flow through the imaged gap between the Cyprus and Aegean slabs, again correlating roughly with both volcanism and high residual topography. Slow uppermost&amp;#8208;mantle wave speeds below active volcanoes in eastern Anatolia, and ratios of P-to-S wave relative traveltimes, indicate a thin lithosphere and melt contributions. Elsewhere, there is more evidence f
Chiasera B, Rooney T, Bastow I, et al., 2021, Magmatic rifting in the Main Ethiopian Rift began in thick continental lithosphere; the case of the Galema range, Lithos, Vol: 406-407, Pages: 1-16, ISSN: 0024-4937
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. TheGalema 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.
Merry T, Bastow I, Kounoudis R, et al., 2021, The influence of the North Anatolian Fault and a fragmenting slab architecture on upper mantle seismic anisotropy in the eastern Mediterranean, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 22, Pages: 1-26, ISSN: 1525-2027
The eastern Mediterranean hosts, within the span of a few hundred kilometers, extensional, strike-slip, and collision tectonics above a set of fragmenting subducting slabs. Slab roll-back, toroidal flow, and lithospheric dripping/delamination processes are also believed to be operating. Associated asthenospheric flow and lithospheric de formation are expected to manifest as seismic anisotropy, measurable via study of SKS shear wave splitting. Surprisingly, previous SKS splitting investigations have resolved only long wavelength patterns of anisotropy in the region, interpreting them as large scale asthenospheric flow; moreover, no anisotropic signature has been associated with the North Anatolian Fault (NAF), unlike other major strike-slip plate boundaries world wide. We present a 29-year record of SKS splitting observations, revealing hitherto unrecognized short-length-scale variations in anisotropy, and backazimuthal variations of splitting parameters that attest to multi-layered anisotropy. Lithospheric anisotropy beneath the NAF exhibits fast directions either fault-parallel or intermediate between the principle extensional strain rate axis and fault strike, diagnostic of a relatively low strained transcurrent mantle shear zone. Elsewhere, anisotropy is consistent with as thenospheric flow through tomographically-imaged slab gaps, and driven by Hellenic trench retreat. Evidence for westward flow of asthenosphere driving Anatolian plate motion is lacking. Shorter splitting delay times and nulls in central Anatolia suggest weaker azimuthal anisotropy in the asthenosphere, supporting models that invoke ver tical mantle flow patterns (lithospheric dripping/asthenospheric upwelling). Thus, we conclude that the signal of mantle anisotropy more closely reflects the lithospheric de formation, complex slab architecture and geodynamic diversity of the region than pre36 viously recognized.
Kounoudis R, Bastow I, Ebinger C, et al., 2021, Body-wave tomographic imaging of the Turkana Depression: Implications for rift development and plume-lithosphere interactions, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 22, Pages: 1-27, ISSN: 1525-2027
The Turkana Depression, a topographically-subdued, broadly-rifted zone between the elevated East African and Ethiopian plateaus, disrupts the N–S, fault-bounded rift basin morphology that characterizes most of the East African Rift. The unusual breadth of the Turkana Depression leaves unanswered questions about the initiation and evolution of rifting between the Main Ethiopian and Eastern rifts. Hypotheses explaining the unusually broad, low-lying area include superposed Mesozoic and Cenozoic rifting and a lack of mantle lithospheric thinning and dynamic support. To address these issues, we have carried out the first body-wave tomographic study of the Depression’s upper mantle. Seismically-derived temperatures at 100 km depth exceed petrological estimates, suggesting the presence of mantle melt, although not as voluminous as the Main Ethiopian Rift, contributes to velocity anomalies. A NW–SE-trending high wavespeed band in southern Ethiopia at urn:x-wiley:15252027:media:ggge22580:ggge22580-math-0001200 km depth is interpreted as refractory Proterozoic lithosphere which has likely influenced the localization of both Mesozoic and Cenozoic rifting. At urn:x-wiley:15252027:media:ggge22580:ggge22580-math-0002100 km depth below the central Depression, a single localized low wavespeed zone is lacking. Only in the northernmost Eastern Rift and southern Lake Turkana is there evidence for focused low wavespeeds resembling the Main Ethiopian Rift, that bifurcate below the Depression and broaden approaching southern Ethiopia further north. These low wavespeeds may be attributed to melt-intruded mantle lithosphere or ponded asthenospheric material below lithospheric thin-spots induced by the region's multiple rifting phases. Low wavespeeds persist to the mantle transition zone suggesting the Depression may not lack mantle dynamic support in comparison to the two plateaus.
Ogden C, Keir D, Bastow I, et al., 2021, Seismicity and crustal structure of the southern main Ethiopian rift: new evidence from Lake Abaya, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 22, Pages: 1-17, ISSN: 1525-2027
The Main Ethiopian Rift (MER) has developed during the 18 Ma-Recent separation of the Nubian and Somalian plates. Extension in its central and northern sectors is associated with seismic activity and active magma intrusion, primarily within the rift, where shallow (urn:x-wiley:15252027:media:ggge22586:ggge22586-math-00015 km) seismicity along magmatic centers is commonly caused by fluid flow through open fractures in hydrothermal systems. However, the extent to which similar magmatic rifting persists into the southern MER is unknown. Using data from a temporary network of five seismograph stations, we analyze patterns of seismicity and crustal structure in the Abaya region of the southern MER. Magnitudes range from 0.9 to 4.0; earthquake depths are 0–30 km. urn:x-wiley:15252027:media:ggge22586:ggge22586-math-0002 ratios of urn:x-wiley:15252027:media:ggge22586:ggge22586-math-00031.69, estimated from Wadati diagram analysis, corroborate bulk-crustal urn:x-wiley:15252027:media:ggge22586:ggge22586-math-0004 ratios determined via teleseismic P-to-S receiver function H-urn:x-wiley:15252027:media:ggge22586:ggge22586-math-0005 stacking and reveal a relative lack of mafic intrusion compared to the MER rift sectors to the north. There is a clear association of seismicity with the western border fault system of the MER everywhere in our study area, but earthquake depths are shallow near Duguna volcano, implying a shallowed geothermal gradient associated with rift valley silicic magmatism. This part of the MER is thus interpreted best as a young magmatic system that locally impacts the geothermal gradient but that has not yet significantly modified continental crustal composition via rift-axial magmatic rifting.
Bastow I, 2021, Ethiopia from top to bottom: using seismology to understand how tectonic plates rise, split, then fall, Transactions of the Leicester Literary & Philosophical Society
Reeve MT, Magee C, Bastow ID, et al., 2021, Nature of the cuvier abyssal plain crust, offshore NW Australia, Journal of the Geological Society, Vol: 178, Pages: 1-17, ISSN: 0016-7649
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 (<200 m depth) or subaerial environment. Our results indicate that the CAP may not be unambiguous oceanic crust, but may instead consist of a spectrum of heavily intruded continental crust through to fully oceanic crust. If the CAP represents such a continent–ocean transition zone, then the adjacent unambiguous oceanic crust would be located >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.
Boyce A, Bastow I, Cottaar S, et al., 2021, AFRP20: New P-wavespeed model for the African mantle reveals two whole-mantle plumes below East Africa and Neoproterozoic modification of the Tanzania craton, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 22, ISSN: 1525-2027
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 90–135 km 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.
Reeve M, Magee C, Bastow I, et al., 2021, Nature of the Cuvier Abyssal Plain crust, offshore NW Australia
Karlowska E, Bastow I, Rondenay S, et al., 2020, The development of seismic anisotropy below south-central Alaska: Evidence from local earthquake shear-wave splitting, Geophysical Journal International, Vol: 225, Pages: 548-554, ISSN: 0956-540X
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.
Wojcicka N, Collins G, Bastow I, et al., 2020, The seismic moment and seismic efficiency of small impacts on Mars, Journal of Geophysical Research: Planets, Vol: 125, Pages: 1-20, ISSN: 2169-9097
Since landing in late 2018, the InSight lander has been recording seismic signals on the surface of Mars. Despite nominal pre-landing estimates of 1–3 meteorite impacts detected per Earth year, none have yet been identified seismically. To inform revised detectability estimates, we simulated numerically a suite of small impacts onto Martian regolith and characterized their seismic source properties. For the impactor size and velocity range most relevant for InSight, crater diameters are 1-30 m. We found that in this range scalar seismic moment is 106−1010Nm and increases almost linearly with impact momentum. The ratio of horizontal to vertical seismic moment tensor components is∼1, implying an almost isotropic P-wave source, for vertical impacts. Seismic efficiencies are ∼10−6, dependent on the target crushing strength and impact velocity. Our predictions of relatively low seismic efficiency and seismic moment suggest that meteorite impact de-tectability on Mars is lower than previously assumed. Detection chances are best for impacts forming craters of diameter>10m.
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.
Petrescu L, Graham S, Houseman G, et al., 2020, Upper mantle deformation signatures of craton-orogen interaction in the Carpathian-Pannonian region from SKS anisotropy analysis, Geophysical Journal International, Vol: 220, Pages: 2105-2118, ISSN: 0956-540X
Since 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 Southe
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., 2019, A multicomponent Isabella anomaly: Resolving the physical state of the Sierra Nevada upper mantle from Vp/Vs anisotropy tomography, Geosphere, Vol: 15, Pages: 2018-2042, 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
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