Publications
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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.
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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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.
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.
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.
Venereau C, Robert M-S, Bastow I, et al., 2019, The role of variable slab dip in driving mantle flow at the eastern edge of the Alaskan subduction margin: insights from SKS shear-wave splitting, Geochemistry, Geophysics, Geosystems, Vol: 20, Pages: 2433-2448, ISSN: 1525-2027
Alaska provides an ideal tectonic setting for investigating the interaction between subduction and asthenospheric flow. Within the span of a few hundred kilometers along strike, the geometry of the subducting Pacific plate varies significantly and terminates in a sharp edge. Furthermore, the region documents a transition from subduction along the Aleutian Arc to strike‐slip faulting along the Pacific Northwest. To better understand mantle interactions within this subduction zone, we conduct an SKS shear‐wave splitting analysis on passive‐source seismic data collected between 2011 and 2018 at 239 broadband seismometers, including those from the Transportable Array. Anisotropic fast directions in the east of our study area parallel the Queen Charlotte and Fairweather transform faults, suggesting that the ongoing development of lithospheric anisotropy dominates the results there. However, our observed delay times (δt = 1–1.5 s) obtained across the study region may also imply an asthenospheric contribution to the splitting pattern. Our splitting observations exhibit slab‐parallel fast directions northwest of the trench and a rotation of fast directions around the northeastern slab edge. These observations suggest the presence of toroidal asthenospheric flow around the edge of the downgoing Pacific plate. We suggest that Wrangell Volcanic Field volcanism might be caused by mantle upwelling associated with this flow. Splitting observations closer to the trench can be explained by fossil anisotropy within the downgoing Pacific‐Yakutat plate combined with entrained subslab mantle. The geometry of the slab, including its variable dip and its abrupt eastern edge, thus plays an important role in governing mantle flow beneath Alaska.
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