Abstract

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 development1,2,3. 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 ago4. 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 Plateau5,6, 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.

Main

Ancient continental flood-basalt provinces and magmatic rifted margins mark some of Earth’s most voluminous magmatic events7. Often associated with the presence of mantle plumes, their development is expected to alter plate thickness and thermo-mechanical structure significantly8. However, as post-large igneous province (LIP) cooling has re-defined the thickness and structure of the plates, the extent of this modification, including its development through time, can only be inferred from theoretical models, or from the geological record preserved at ancient LIPs. Consequently, the influence of pre-existing variations in plate thickness and volatile content during LIP formation and subsequent rift development remain poorly understood. Plume magmatism is commonly expected to exploit zones of thinned lithosphere1,2,3. These thin zones are also often assumed to be rheologically weaker, promoting deformation and magmatism by enabling strain localization and adiabatic decompression of underlying plume material that subsequently rises through the weakened lithosphere9,10.

East Africa (Fig. 1a) offers a unique opportunity to examine plate modification during plume–lithosphere interaction as it is host to the world’s youngest continental flood-basalt province—the Ethiopian Traps—whose main phase saw 1–2 km of flood basalts erupt in the Oligocene epoch (31–29 million years ago (Ma)), concomitant, and spatially coincident, with the development of broad-scale Ethiopian Plateau uplift and onset of extension11,12. Below the Ethiopian Plateau, voluminous intrusive magmatism is observed as an 8–12 km-thick lower-crustal-intrusion layer5. Deeper still, the lithospheric mantle has seismic wavespeeds that are barely faster than those of the convecting slow-wavespeed asthenosphere below6,13 (Fig. 2b), consistent with the view that the plateau lithosphere has been heavily modified by heating and magma intrusion in the Cenozoic era.

Fig. 1: Rifting and magmatism in East Africa.

a, Mesozoic (Anza) and Cenozoic (East African) rift systems. The dashed box shows the Turkana Depression’s Oligocene–recent and Cretaceous–Palaeogene rift basins and faults17,50. The red triangles are Holocene–recent eruptive centres. The shaded grey regions and the magenta outline show Cenozoic magmatism and the Ethiopian flood-basalt province, respectively19. The yellow dashed line indicates the location of a reference station within the Mozambique Belt used for comparison26. b, Seismograph stations in the Turkana Depression. TRAILS, Turkana Rift Arrays Investigating Lithospheric Structure; GEOFON, GEOFOrschungsNetz Seismic Network.

Fig. 2: How heating and magmatic intrusions alter lithospheric structure.

a,b, Schematic contrasting regions of minor (a) and major (b) magmatic modification, including their crustal and uppermost-mantle seismological characteristics.

Immediately to the south of the flood-basalt-capped Ethiopian Plateau lies the Turkana Depression, which also overlies plume-influenced mantle14,15,16. The Depression was the site of the failed Anza Rift system in the Mesozoic era (primarily Late Cretaceous; about 100–66 Ma)17, which stretched the crust by a factor of ≤2.11 (ref. 18). This rendered parts of the Depression a lithospheric thin spot during LIP development (Fig. 1a), as the mantle lithosphere would have only partially thermally equilibrated before plume arrival in the Eocene epoch ≤20 Myr later: models estimate 30–60 Myr must elapse before the net effect of permanently thinned crust and re-thickening mantle lithosphere sufficiently restores plate strength to suppress further extension and magmatism10. Yet, voluminous early Eocene flood-basalt magmatism (45–32 Ma)16,19 and subsequent Oligocene–recent East African rifting20,21 only developed west of Lake Turkana near the craton edge (Fig. 1b). East of the lake, where the main Anza depocentres lie20, Eocene-age flood-basalt volcanism and Miocene–recent rifting and magmatism are comparatively lacking; Pliocene–recent magmatism comprises mainly isolated shield volcanoes22.

Analysis of broadband seismic data across the Depression reveals bulk-crustal VP/VS (compressional-to-shear wavespeed) ratios that are markedly lower (about 1.74)18 than below the plateau (VP/VS > 1.8)23, implying that the relative lack of widespread mafic magmatism compared with the north persists at crustal depths. These observations collectively challenge the notion that thin lithosphere (for example, the Anza Rift) is inherently weak and enhances magmatic volumes through extension-related melt generation and drainage of plume material2,9; a hypothesis often invoked to explain notable along-strike variations in many magmatic rifted margins worldwide24,25. The Depression thus offers a unique opportunity to examine plume–lithosphere interactions and rift development in a region of previously rifted and still-thin lithosphere. Key to understanding better the extent to which Eocene–recent magmatism has impacted the Depression’s failed rift zones is an improved knowledge of its lithospheric seismic and thermal structure.

Utilizing data from seismograph deployments in the Turkana Depression and surrounding regions (Fig. 1b), we constrain absolute one-dimensional shear-wave velocities below a network of 38 seismograph stations via joint inversion26 of fundamental-mode Rayleigh-wave group velocities (4–100-second period)27,28 and P-to-S receiver functions18. We pay close attention to the Moho, whose architecture will vary according to the volume of lower crustal intrusions. The lithosphere–asthenosphere system is also a major focus—specifically, whether or not a fast-wavespeed mantle lithospheric lid is readily discernible from hot, plume-affected asthenosphere (Fig. 2). Thermodynamic conversion of shear velocities to temperatures permits a thermal, as well as seismological, means of defining the transition from conductive to convective mantle and allows us to assess variations in mantle potential temperature across the region. Our study demonstrates that lithospheric heterogeneity from previous tectonic events (for example, past rifting)—beyond variations in plate thickness—strongly influences where rifting and LIPs subsequently develop, and may be akin to processes that once shaped ancient continental LIPs and magmatic rifted margins globally.

Plate wavespeed and thermal structure

Figure 3 shows the shear-wave velocity profiles derived from the joint inversion procedure26 (Methods), grouped into domains characterized by similar velocity structures at crustal and upper-mantle depths: the Somalian Plate (Fig. 3a), the broadly rifted region of southern Ethiopia (Fig. 3b,c), the failed Mesozoic Anza Rift (Fig. 3d) and the Late-Oligocene–recent rift zones west of Lake Turkana (Fig. 3e).

Fig. 3: Seismic and thermal profiles across southern Ethiopia and the Turkana Depression.

a–e, One-dimensional shear-wave velocity profiles grouped into domains of similar velocity: Somalian Plate (a), southwest Ethiopia (b), southeast Ethiopia (c), east Turkana (d), and west Turkana (e). The shading indicates the maximum velocity spread in each region. Seismically derived Moho and LAB depths, and their plausible ranges, are shown in grey and yellow, respectively. Average shear-wave profiles, and associated Moho depth ranges, for the northwest Ethiopian Plateau (dark blue line)6 and Mozambique Belt (dark green line)26 are shown in a. The solid black bar is the 8–12-km-thick lower-crustal-intrusion layer from the EAGLE (Ethiopia-Afar Geoscientific Lithospheric Experiment) wide-angle seismic profile on the Ethiopian Plateau5. The dashed red line is the H–κ stacking (H, crustal thickness; κ, VP/VS ratio) derived Moho depths18. The dashed vertical line is the normal mantle velocity (4.48 km s−1). f,g, Temperature profiles derived from c and d, respectively, with their associated LAB depths (purple dashed line) and best-fitting mantle adiabat at asthenospheric depths (black dashed line) shown. The pink lines are the wet and dry solidi41.

Source Data

The Moho is identified as the base of the steepest positive velocity gradient, where typical lowermost-crustal and uppermost-mantle velocities reside (3.8–4.2 km s−1; Fig. 3). Defining a lithosphere–asthenosphere boundary (LAB) depth seismically is more challenging. The lithosphere remains mechanically strong to depths of about 30 km above the base of the conductive lithosphere29. Forward modelling suggests that the base of this predominantly conductive layer corresponds to a transition from a strong negative to a mildly positive velocity gradient30. Pinpointing this transition unambiguously on seismic profiles is difficult, so researchers typically estimate seismic plate thickness using the somewhat-shallower depth of the strongest negative velocity gradient. We thus assume this strategy to define a minimum LAB-depth estimate (Fig. 3).

Thermodynamic conversions of mantle shear-wave velocities to temperatures (Methods) yield temperature profiles (Fig. 3) from which a thermal proxy for LAB depth can be defined where the geotherm transitions from a conductive gradient in the uppermost mantle to an adiabatic gradient in the convective asthenosphere30. The temperature in the latter is no longer governed by conductive cooling and is instead unambiguously in convective mantle30,31. Therefore, the thermal LAB is generally deeper than its seismically derived counterpart. At asthenospheric depths, the best-fitting adiabat provides a good indication of mantle potential temperature (Fig. 3).

Moho depth and architecture

An approximately 10-km Moho step marks the transition from thicker Ethiopian Plateau crust (about 35 km) to thinned, previously rifted, Turkana Depression crust (20–25 km; Fig. 3a–e). Crustal wavespeeds (3.2–3.8 km s−1) resemble Mozambique Belt terranes to the south (Fig. 3b–d) that have been un-modified by Cenozoic hotspot tectonism26. Unlike the Ethiopian Rift and Ethiopian Plateau (Fig. 3a), the Depression crust shows slower wavespeeds than expected for cooled gabbroic intrusions, but not slow enough to suggest high temperatures and/or melt. Our results thus corroborate previous receiver function analysis across the Depression18, which reveals low bulk-crustal VP/VS ratios (about 1.74) and thin crust (<25 km) as evidence for relatively melt-poor, mechanically stretched, crust.

In addition to modifying wavespeeds, active melt and/or cooled mafic intrusions in the mid-to-lower crust are expected to fundamentally alter Moho architecture. In the heart of the Ethiopian Plateau, a substantial 8–12-km-thick lower-crustal-intrusion layer, imaged in controlled-source seismic experiments5, is manifest in one-dimensional velocity profiles as a gradual transition from crust to mantle velocities6 (Fig. 3a). In contrast, below most of the Depression, the Moho is a relatively sharp wavespeed discontinuity (<4 km; Fig. 3b–d), reminiscent of melt-poor Mozambique Belt terranes to the south26. Only west of Lake Turkana is a more gradational Moho found (Fig. 3e). In the north, this coincides with the Lotikipi Plain, an area of late Eocene/early Oligocene flood-basalt magmatism32. In the south, the Lokichar Basin contains igneous basin infill, dykes and sills33,34 linked to eastwards-migrating Oligocene–recent rifting from the Tanzania craton edge to its current location below Lake Turkana. Despite localized flood-basalt magmatism and Miocene–recent rifting, the Moho architecture west of the lake has been modified less than that below the Ethiopian Rift and Ethiopian Plateau (Fig. 3a). Magma-compensated rifting dominates over faulting and stretching in only the most recent (<1 Ma) extensional phase below Lake Turkana21,35. Recent magma-assisted rifting and/or earlier Eocene–Oligocene flood-basalt magmatism have contributed only a 2-km-thick lower-crustal-intrusion layer, imaged by the KRISP (Kenya Rift International Seismic Project) Lake Turkana wide-angle refraction profile36, contrasting the 8–12-km-thick layer below the Ethiopian Plateau5.

Detectability of the mantle lithosphere

Melt and elevated temperatures associated with magmatic rifting and flood-basalt magmatism are expected to obscure the transition from fast-wavespeed, conductive mantle lithosphere to slow-wavespeed, convecting asthenosphere. Below the heavily melt-influenced central/northern Main Ethiopian Rift and Ethiopian Plateau, a characteristic fast-wavespeed lid is absent in one-dimensional velocity profiles6 (Fig. 3a); neither is a LAB ubiquitous in S-to-P receiver function studies, particularly below the Main Ethiopian Rift37. However, immediately to the south, southernmost Ethiopia and the Turkana Depression show a discernible, high-wavespeed mantle lithospheric lid (VS > 4.4 km s−1) atop hot, slow-wavespeed (<4.3 km s−1) asthenosphere (Fig. 3b–d), indicating seismic and thermal lithospheric thicknesses of 65–81 km and 76–95 km, respectively, across the region (Extended Data Table 1).

A distinct lithospheric lid architecture is most prominent below the Proterozoic terranes of southern Ethiopia and the Mesozoic Anza Rift (Fig. 3c,d), where lithospheric mantle wavespeeds resemble normal mantle (about 4.48 km s−1). Evidence for widespread heating of the mantle lithosphere in these regions is therefore lacking, consistent with the view that Cenozoic magmatism has not heavily infiltrated or markedly modified the lithosphere to the same extent as the elevated Ethiopian Plateau. This is consistent with mantle anisotropy analysis38 that attributes weak anisotropy in the Depression to a paucity of melt-filled fractures in the lithosphere. Only below isolated regions of Eocene flood-basalt magmatism and Oligocene–recent rift zones west of Lake Turkana (Fig. 3e) do slower mantle wavespeeds indicate thermo-mechanical modification, albeit to a much lesser degree than the Main Ethiopian Rift and Ethiopian Plateau. Here, present-day plate thicknesses are smaller (55–60 km; Fig. 3e) compared with the rest of the Depression.

Elevated asthenospheric temperatures are ubiquitous below the Depression (Fig. 3f,g): the best-fitting mantle adiabats signify mantle potential temperatures of 1,400–1,450 °C, some 50–100 °C above ambient mantle, an observation corroborated by petrological studies of 10 Ma–recent lavas erupted near Lake Turkana39,40. All asthenospheric temperatures are close to, but not above, the dry solidus41 (Fig. 3), consistent with geochemical studies that suggest Miocene–recent magmatism below the Depression (Fig. 1b) has been pulsed, not continuous22. A slow-wavespeed (about 10% slower than normal mantle), hot asthenosphere, also corroborates the view that approximately 600 m of mantle-derived uplift is required to explain the Depression’s higher-than-expected elevation given its thin crust18. Below the Ethiopian Plateau6 and Somalian Plate, slow wavespeeds (<4.3 km s−1; Fig. 3) indicate that relatively low-density (and thus less negatively buoyant) lithospheric mantle probably contributes to their overall uplift.

Next we discuss why two adjacent regions—the Ethiopian Plateau and the Turkana Depression—that share the same geodynamic ‘plume’ setting are so profoundly different in their lithospheric structure. This contrast is particularly pronounced below the Depression’s failed Anza Rift which, despite marking a lithospheric thin spot before plume arrival, shows a surprising lack of magmatic modification.

LIPs and plate thermo-mechanical structure

One hypothesis for the differences in modification is that the Depression was not underlain by a plume-affected mantle for as long as the Ethiopian Plateau. Plate reconstructions reveal that the plateau lay atop the thinnest—and therefore hottest—mantle transition zone at 30 Ma, with the Depression some approximately 500 km south of its present-day location14. However, this hypothesis is difficult to reconcile with the presence of some of the earliest, albeit isolated, Eocene (45–32 Ma) flood-basalt magmatism in parts of the Depression (that is, southwest Ethiopia/northwest of Lake Turkana)4,16. Mantle potential temperature estimates derived from Oligocene-age lavas were akin to those below the Ethiopian Plateau (+150 °C)40, indicating that the Depression was also underlain by hot mantle at the time. While the relative timing of plume impingement may in part explain the low-volume nature of Eocene magmatism and relatively minor levels of plate modification observed to the west of Lake Turkana (Fig. 3e), it fails to account for the lack of modification to the east, where the lithosphere was thin at the time: 3–9-km-thick Mesozoic-age extensional sedimentary basins mark the failed Anza Rift20. An alternative hypothesis arises from a recent modelling study42 that asserts that Turkana Depression lithosphere, thinned and weakened by previous rifting, inhibits melt ascent and promotes melt retention within the plate, whereas stronger, more elastic lithosphere proposed to exist below the Ethiopian Plateau readily allows magma extrusion. However, contrary to model predictions, our seismological observations find a melt-modified lithosphere associated with melt retention is absent beneath the Depression’s Anza Rift but ubiquitous below the plateau (Figs. 3 and 4).

Fig. 4: The importance of past rifting on lithospheric strength and LIP development.

Schematic illustrating the key seismic and thermal observations and interpretations arising from this study. Two contrasting areas of magmatic modification above a hot, plume-influenced, mantle are shown. Thinned but relatively metasome-poor lithosphere below the failed Anza Rift is more resistant to thermo-mechanical modification than previously un-rifted regions (for example, west of Lake Turkana and the Ethiopian Plateau). EAR, East African Rift.

The common assumption2,3 that lithospheric thin spots (for example, the Anza Rift) mark weak zones that focus deformation and facilitate the ascent of plume-related melt during the development of a flood-basalt province, does not hold true. We propose that heating and low-degree melting during short-lived magmatism associated with the late stages of Anza rifting17,20 would have removed easily fusible phases (for example, volatiles) from the lithosphere43, suppressing its potential for future thermo-mechanical modification (Fig. 4). Furthermore, partial thermal re-equilibration in the ≤20-Myr period before plume arrival and distant from any subduction zone could have rendered Anza’s newly formed lowermost mantle lithosphere relatively metasome poor. Quantifying the extent of magmatism during Anza rifting is challenging because borehole and seismic reflection data are sparse17,20; however, peridotite xenoliths from Marsabit Volcano (within the Anza Rift) document a history of melt extraction and depletion in incompatible element and volatile phases linked to the development of the Anza Rift, and lack evidence for metasomatic re-fertilization44,45. Petrological analysis of Pliocene–recent magmatism east of Lake Turkana indicates that it is predominantly asthenosphere-derived and relatively lacking in easily fusible lithospheric phases22 compared with Miocene lavas west of the lake43. This is probably owing to the more depleted nature of the Anza lithosphere: melt erupts, feeding isolated shield volcanoes, without causing widespread, seismically discernible, thermal and magmatic modification18,27 (Fig. 3).

Our results provide direct evidence of a fast-wavespeed and, therefore, refractory mantle lithosphere beneath the Mesozoic Anza Rift, highlighting the region’s resistance to Cenozoic plume-related modification and rifting. The Oligocene–recent geological record20 and analysis of present-day seismicity and geodetic data21 have demonstrated that East African rifting has circumnavigated, not exploited, the failed Anza Rift terranes18,21,27; extension instead developed to the west of Lake Turkana. Here, easily fusible phases (for example, metasomes; Fig. 4) in lithospheric mantle assembled during the Pan-African orogeny43,46, and steep thermal gradients at the edge of the Tanzania craton could have i