Ian David Bastow

Uplift and widespread Late Miocene‐present magmatism in Anatolia are commonly attributed to lithospheric thinning, possibly facilitated by the arrival of hot mantle plume material. There are various lines of evidence for thin present‐day lithosphere, but the spatial extent and degree of lithospheric thinning and the underlying mantle temperatures remain debated. Here we apply a Bayesian thermo‐compositional approach that jointly inverts surface‐wave dispersion, elevation and surface heat flow for temperatures of the lithosphere and underlying mantle to 360 km depth. Our results reveal that thermal lithospheric thickness across the study region is generally thin, 40–85 km, with thicker lithosphere (>70 km) beneath the Central Pontides and northernmost Arabia and exceptionally thin (40–55 km) lithosphere beneath the Eastern Pontides and the region between the Central and Eastern Anatolian Plateaus. Furthermore, we find a pronounced west‐to‐east increase in mantle potential temperature of ∼140C across 35E. The sharp mantle temperature gradient implies that any inflow of hot plume material is largely confined to the east and therefore is unlikely to be a primary driver of Anatolia's westward motion. The pervasively thin but variable lithospheric thickness indicates widespread, yet heterogeneous lithospheric removal, with the influx of hot mantle material potentially enhancing thinning beneath eastern Anatolia.

Lithospheric thin zones, such as recently failed rifts, are generally assumed to be weak spots where magmatism and deformation can concentrate during rifting and large igneous province development. Yet, the Turkana Depression in East Africa, the site of the failed 66-million-year-old Anza Rift, did not experience the widespread flood magmatism seen on the adjacent Ethiopian Plateau, despite being a lithospheric thin spot when the region encountered hot plume material around 45 million years ago. Using data from the 2019-2021 Turkana Rift Arrays Investigating Lithospheric Structure (TRAILS) project and surrounding seismograph networks we jointly invert surface-wave and receiver function data to constrain crustal and upper-mantle seismic structure and evaluate lithospheric thermo-mechanical modification. Evidence for thick lower crustal intrusions, ubiquitous below the uplifted Ethiopian Plateau, is comparatively lacking below the Depression’s failed Anza Rift system, which ongoing East African rifting is circumnavigating, not exploiting. The mantle lithosphere below the Depression has also retained its cool, fast-wavespeed ‘lid’ character, contrasting the Ethiopian Plateau. Volatile depletion during failed Anza rifting probably rendered the thinned lithosphere refractory without later rejuvenation. Subsequent rifting and magmatism thus initiated away from the still-thin Anza Rift, in regions where fertile lithosphere enabled melting and the sufficient lowering of plate yield strength. Areas of thinned lithosphere are thus not necessarily persistent weak zones where significant extension and magmatic provinces will develop. Kounoudis, R., Bastow, I.D., Ebinger, C.J. et al. The importance of past rifting in large igneous province development. Nature 647, 115–120 (2025). https://doi.org/10.1038/s41586-025-09668-7

The East African Rift provides a natural laboratory to study the influence of pre-existing lithospheric thin spots on the development of rifting and hotspot tectonism. Below the Ethiopian Rift and elevated Ethiopian Plateau, extensive magmatic and thermal modification due to Eocene-Oligocene flood basalt magmatism and Miocene-Recent rifting has resulted in slow lithospheric mantle velocities (< 4.1km/s; Dugda et al., 2007, JGR). In contrast, below the previously rifted, lower-lying Turkana Depression to the south, the lithospheric mantle appears relatively unmodified (4.2-4.8 km/s; Kounoudis et al., 2023, EPSL), despite being underlain by hot, mantle plume material. Important in this picture are detailed constraints on the lithosphere-asthenosphere boundary (LAB).Why the Turkana Depression, and particularly the failed Anza Rift terranes, remained resistant to thermal and magmatic modification, is debated. Although the Turkana Depression was a lithospheric thin spot at the onset of plume magmatism, Cenozoic rifting is now circumnavigating, not exploiting, the Anza Rift terranes (Musila et al., 2023, G3). Lithospheric thin spots therefore don't necessarily mark weak zones that are exploited by subsequent rifting and magmatism. One hypothesis for the apparently refractory nature of the Anza lithosphere is that Mesozoic rifting removed easily fusible phases, suppressing subsequent melting and associated strain localisation (Kounoudis et al., 2025, Nature).To test this geodynamic scenario, we calculated teleseismic S-to-p receiver functions and examined lithospheric thickness variations in the Turkana Depression, where the contrast between fast, relatively unmodified lithospheric mantle and slow, partially molten, plume-infiltrated asthenosphere is expected to provide impulsive S-to-p conversions at the LAB. We observe that the least impulsive and shallowest LAB conversions are associated with Miocene-Recent rift zones, and isolated shield volcanoes. Elsewhere, sharper and deeper S-to-p conversions attest to a lithosphere that has resisted thermo-mechanical modification.

At the northern Hikurangi margin, Aotearoa New Zealand, slow slip events (SSEs) recur every 6-24 months to ~30 km depth. While shallow SSEs (0-10 km) are well-studied offshore, the deeper portion (10-30 km) remains poorly understood, limiting insight into SSE initiation. In Woodward et al. 2026 we investigate the relationships between newly resolved SSEs and seismicity. We combine passive seismological, geodetic, geochemical and seismic reflection data to analyse the relationships between seismicity and slow slip events, and the mechanisms that invoke them. Using time-dependent inversions, we resolve two small SSEs (MW 6.2 and 6.4), one of which extends unusually deeply from 15 to 30 km depth. Using data from a dense onshore seismograph network, deployed directly above this deeper portion from December 2017 to October 2018, we construct a catalog of 3,071 high-quality earthquakes with hypocentral uncertainties ≤5 km, located with a 3-D velocity model and our new 1-D model. Focal mechanisms reveal numerous normal-faulting earthquakes, including some within the slab mantle. Seismicity distributions and normal-faulting earthquakes occur along vertically aligned pathways that link the subducting slab mantle to surface seeps, where fluids show mantle-derived signatures. We infer that normal faults form due to slab bending and localized uplift of subducting seamounts, which enhance plate interface roughness, damage the upper plate, and promote fluid migration. Landward of ~100 km from the trench, both surface seeps and normal-faulting mechanisms cease, coinciding with the downdip limit of shallow SSEs. Together, these results suggest that the Hikurangi margin’s rough subducting plate interface exerts strong control on forearc dewatering and SSE genesis.

At the northern Hikurangi margin, Aotearoa New Zealand, slow slip events (SSEs) recur every 6–24 months to 30 km depth. Although shallow SSEs (0–10 km) are well‐studied offshore, the deeper portion (10–30 km) remains poorly understood, limiting insight into SSE initiation. Here we investigate this deeper region and examine relationships between newly resolved SSEs and seismicity. Using time‐dependent inversions, we resolve two small SSEs ( 6.2 and 6.4), including one that extends from 15 to 30 km depth. Using data from a dense onshore seismograph network deployed directly above this deeper portion from December 2017 to October 2018, we construct a catalog of 3,071 high‐quality earthquakes with hypocentral uncertainties 5 km, located using a 3‐D velocity model and a new 1‐D model. Earthquake magnitudes range from −0.84 to 4.40, with a completeness magnitude of 1.7 and a b‐value of 1.06. Focal mechanisms reveal numerous normal‐faulting earthquakes, including some within the slab mantle. Vertically‐aligned seismicity and normal‐faulting earthquakes outline pathways linking the slab mantle to surface seeps of mantle‐derived fluids. We infer that normal faults form due to slab bending and localized uplift of subducting seamounts, which roughen the plate interface, damage the upper plate, and promote fluid migration. Landward of 100 km from the trench, both surface seeps and normal‐faulting mechanisms cease, coinciding with the downdip limit of shallow SSEs. Together, these results suggest that the Hikurangi margin's rough subducting plate interface exerts strong control on forearc dewatering and SSE genesis.

Lithospheric thin zones, such as recently failed rifts, are generally assumed to be weak spots where magmatism and deformation can concentrate during rifting and large igneous province development ^1–3 . Yet, the Turkana Depression in East Africa, the site of the failed 66-million-year-old Anza Rift, did not experience the widespread flood magmatism seen on the adjacent Ethiopian Plateau, despite being a lithospheric thin spot when the region encountered hot plume material around 45 million years ago ⁴ . Here we jointly invert surface-wave and receiver function data to constrain crustal and upper-mantle seismic structure below the Depression to evaluate lithospheric thermo-mechanical modification. Evidence for thick lower crustal intrusions, ubiquitous below the uplifted Ethiopian Plateau ^5,6 , is comparatively lacking below the Depression’s failed Anza Rift system, which ongoing East African rifting is circumnavigating, not exploiting. The mantle lithosphere below the Depression has also retained its cool, fast-wavespeed ‘lid’ character, contrasting the Ethiopian Plateau. Volatile depletion during failed Anza rifting probably rendered the thinned lithosphere refractory without later rejuvenation. Subsequent rifting and magmatism thus initiated away from the still-thin Anza Rift, in regions where fertile lithosphere enabled melting and the sufficient lowering of plate yield strength. Areas of thinned lithosphere are thus not necessarily persistent weak zones where significant extension and magmatic provinces will develop.

High topography (1 km) on the Anatolian Plate is widely attributed to buoyant mantle support, not just the isostatic response of crustal shortening. However, uncertainties in lithospheric structure hinder attempts to discriminate between competing uplift mechanisms. We jointly inverted receiver functions with Rayleigh wave group‐velocity dispersion curves to obtain 1D shear velocity profiles for 575 seismograph stations across Anatolia. Crustal thickness increases from 25 km in the west to 44 km below Eastern Anatolia, with short length‐scale Moho topography in places. Within the Central Anatolian Plateau (CAP), thicker crust, lower residual topography, higher residual Bouguer gravity anomalies, and faster upper‐mantle wavespeeds north of N than to the south suggest that northern CAP lithospheric removal is less‐developed. Processes like slab break‐off, lithospheric delamination and/or dripping are therefore more likely to contribute to the uplift in the south, with additional support from buoyant mantle flow through the Cyprus slab tear. A 5 km Moho step across the Eastern Anatolian Fault (EAF) results partly from dominant crustal shortening on the Anatolian Plate. Further east, no Moho depth contrast exists across the Bitlis suture; Arabia‐Eurasia crustal shortening has thus been accommodated by both plates. An ENE–WSW‐trending band of positive residual topography and negative residual Bouguer anomalies parallels the EAF below the northwestern tip of Arabia. This connects to a zone of peak residual topography and slow uppermost mantle wavespeeds below the magmatically active Eastern Anatolian Plateau. These observations are explained best by NE‐flowing, buoyant mantle plume material from the Afar hotspot to the south.

Theory and geoscientific observations demonstrate that plate stretching, heating, faulting, active and frozen magma intrusions, and extrusive eruptive products are consequences of mantle upwelling mechanism driving continental rifting. Problematic to this picture is the lack of consensus on how, when and where these processes modify the crust’s thermal and mechanical structure. We use data from East Africa’s 300-km wide Turkana Depression to investigate how the superposition of these rift processes and the spatial migration of the active plate boundary through time within one geodynamic setting modify the crust’s structure. Utilizing ambient noise seismic methods and data from the 34 station Turkana Rift Arrays Investigating Lithospheric Structure (TRAILS) seismic network, we invert for Rayleigh and Love tomographic models and overlay results with our local earthquakes crustal splitting results. Preliminary results show that regions that experienced Eocene flood magmatism have localized high Vs of > 3.4 km/s at mid-lower crustal depths implying that flood magmatism is fed by unknown localized centers and/or dike swarms. Quaternary eruptive centers with Vs < 3.4 km/s at mid-lower crustal depths are punctuated and irregularly spaced suggesting that bottom-up mantle upwelling influence their location. Regions with superposed Cretaceous-Paleogene and Miocene-Recent rift phases have persistent low velocities (Vs ≥ 3.8 km/s) to the mid-crust with thinner crust (~ 20 km); the active Miocene-Recent rift structures are oblique to the largely inactive Cretaceous-Paleogene rift structures implying no reactivation of pre-existing structures during modern-day rifting.

Rift initiation within cold, thick, strong lithosphere and the evolving linkage to form a contiguous plate boundary remains debated in part owing to the lack of time–space constraints on kinematics of basement‐involved faults. Different rift sectors initiate diachronously and may eventually link to produce a jigsaw spatial pattern, as in the East African rift, and along the Atlantic Ocean margins. The space–time distribution of earthquakes illuminates the geometry and kinematics of fault zones within the crystalline crust, as well as areas with pressurized magma bodies. We use seismicity and Global Navigation System Satellites (GNSS) data from the Turkana Rift Array Investigating Lithospheric Structure (TRAILS) project in East Africa and a new digital compilation of faults and eruptive centres to evaluate models for the kinematic linkage of two initially separate rift sectors: the Main Ethiopian Rift (MER) and the Eastern rift (ER). The ca. 300 km wide zone of linkage includes failed basins and linkage zones; seismicity outlines active structures. Models of GNSS data indicate that the ca. 250 km‐wide zone of seismically active en echelon basins north of the Turkana Depression is a zone, or block, of distributed strain with small counterclockwise rotation that serves to connect the Main Ethiopian and Eastern rifts. Its western boundary is poorly defined owing to data gaps in South Sudan. Strain across the northern and southern boundaries of this block, and an ca. 50 km‐wide kink in the southern Turkana rift is accommodated by en echelon normal faults linked by short strike‐slip faults in crystalline basement, and relay ramps at the surface. Short segments of obliquely oriented basement structures facilitate across‐rift linkage of faults, but basement shear zones and Mesozoic rift faults are not actively straining. This configuration has existed for at least 2–5 My without the development of localized shear zones or transform faults, documenting the importance of distributed deformation in continental rift tectonics.

The East African Rift System (EARS) provides an opportunity to constrain the relationship between magmatism and plate thinning. During continental rifting, magmatism is often considered a derivative of strain accommodation—as the continental plate thins, decompression melting of the upper mantle occurs. The Turkana Depression preserves among the most extensive Cenozoic magmatic record in the rift. This magmatic record, which comprises distinct basaltic pulses followed by periods of relative magmatic quiescence, is perplexing given the lack of evidence for temporal heterogeneity in the thermo‐chemical state of the upper mantle, the nonexistence of lithospheric delamination related fast‐wave speed anomalies in the upper mantle, and the absence of evidence for sudden, accelerated divergence of Nubia and Somalia. We focus on the Pliocene Gombe Stratoid Series and show how lithospheric thinning may result in pulsed magma generation from a plume‐influenced mantle. By solving the 1D advection‐diffusion equation using rates of plate thinning broadly equivalent to those measured geodetically today we show that despite elevated mantle potential temperature, melt generation may not occur and thereby result in extended intervals of quiescence. By contrast, an increase in the rate of plate thinning can generate magma volumes that are on the order of that estimated for the parental magma of the Gombe Stratoid Series. The coincidence of large‐volume stratiform basalt events within the East African Rift shortly before the development of axial zones of tectonic‐magmatic activity suggests that the plate thinning needed to form these stratiform basalts may herald the onset of the localization of strain.