Magmatic Segments Research Articles

Surface wave tomography across Afar, Ethiopia: Crustal structure at a rift triple-junction zone

[1] The Afar Depression in northeast Africa contains the rift triple-junction between the Nubia, Arabia and Somalia plates. We analyze Rayleigh wave group velocity from 250 regional earthquakes recorded by 40 broadband stations to study the crustal structure across Afar and adjacent plateau regions in northern Ethiopia. The dispersion velocities are inverted to obtain surface wave tomographic maps for periods between 5 and 25 seconds, sensitive to approximately the top 30 km of the lithosphere. The tomographic maps show a significant low dispersion velocity anomaly (>20%) within the upper crust, below the site of recent dyke intrusions (2005–present) in the Dabbahu and Manda-Hararo magmatic segments. Similar low velocity regions are imaged where magma intrusion in the Afar crust has been inferred over the last decade from seismicity or volcanic eruptions. We invert two group velocity curves to compare the S-wave velocity structure of the crust within an active magmatic segment with that of adjacent areas; the active region has a low velocity zone (Vs ∼ 3.2 km/s), between about 6–12 km, which we infer to be due to the presence of partial melt within the lower crust.

Multiple melt injection along a spreading segment at Askja, Iceland

[1] Lower crustal earthquakes (12–25 km depth) have been detected since August 2005 in the Askja volcanic system along the north Iceland rift, in the normally ductile part of the crust. The earthquakes occur in three clusters, which have stable dimensions and locations through time and are interpreted as positions of repeated melt supply from the mantle to the lower crust. Seismic velocity Vp/Vs ratios are consistent with the presence of partial melt in the lower crust at Askja. The spatial separation of the clusters shows that there are multiple positions of melt injection within this one magmatic segment and all three positions are currently active. This pattern of melt supply is more like that observed on fast spreading ridges than slow spreading ridges and is probably a consequence of the increased melt production beneath Iceland compared to the rest of the Mid-Atlantic Ridge. However, the relative number of earthquakes in each cluster shows that two thirds of the melt is supplied to the central volcano Askja (i.e., segment center). During the last major rifting episode shallow lateral melt migration occurred from the magma chamber beneath the volcano. Therefore on long time scales melt supply is probably greater at the segment center, with melt redistribution in the upper crust, even though there are multiple points of lower crustal injection along the segment.

Earthquakes along the East African Rift System: A multiscale, system‐wide perspective

On the basis of a comprehensive data set of precisely determined depths of 121 large to moderate‐sized earthquakes along and near the entire East African Rift System (EARS), there are three distinct patterns in focal depths which seem to correlate with progressive stages in the development of the largest active rift in the world. First, away from both ends of the western, younger branch of the EARS, very large (Mw ≥ 7) earthquakes occurred in the top 15 km of the crust where surficial expressions of rifting are yet to appear. Curiously, there are unusually deep aftershocks reaching down to 35 ± 3 km. Second, under well‐developed but amagmatic rift segments, focal depths show a bimodal distribution, with peaks centered near depths of about 15 ± 5 km and 35 ± 5 km. This pattern is present both under the main axis of the EARS, where rift zone have lengths approaching 1000 km, and beneath rift units 10 times shorter in length. Underside reflections off the Moho indicate that at least part of the second peak in seismicity is due to mantle earthquakes down to 44 ± 4 km, attesting to high differential stress in the mantle lithosphere which is capable of accumulating seismogenic, elastic strain (the “jelly sandwich” rheology). Third, beneath magmatic segments of well‐developed rifts, seismicity is largely confined to the upper 15 km of the crust as observed previously, akin to the pattern along mid‐ocean ridges where plastic flow due to high temperature inhibits accumulation of shear stress deep in the lithosphere.

Recent plumbing system of the Krakatau volcano revealed by teleseismic earthquake distribution

Spatial and temporal analysis of global seismological data 1964–2005 reveals a distinct teleseismic earthquake activity producing a columnar-like formation in the continental wedge between the Krakatau volcano at the surface and the subducting slab of the Indo-Australian plate. These earthquakes occur continuously in time, are in the body-wave (mb) magnitude range 4.5–5.3 and in the depth range 1–100 km. The Krakatau earthquake cluster is vertical and elongated in the azimuth N30°E, suggesting existence of a deep-rooted fault zone cutting the Sunda Strait in the SSW-NNE direction. Possible continuation of the fault zone in the SW direction was activated by an intensive 2002/2003 aftershock sequence, elongated in the azimuth of N55°E. Beneath the Krakatau earthquake cluster, an aseismic gap exists in the Wadati-Benioff zone of the subducting plate at the depths 100–120 km. We interpret this aseismic gap as a consequence of partial melting inhibiting stress concentration necessary to generate stronger earthquakes, whereas the numerous earthquakes observed in the overlying lithospheric wedge beneath the volcano probably reflect magma ascent in the recent plumbing system of the Krakatau volcano. Focal depth of the deepest events (~100 km) of the Krakatau cluster constrains the location of the primary magma generation to greater depths. The ascending magmatic fluids stress fault segments within the Sunda Strait fault zone and change their friction parameters inducing the observed tectonic earthquakes beneath Krakatau.

Formation and stability of magmatic segments in the Main Ethiopian and Afar rifts

As rifting progresses to seafloor spreading, extension within the continental crust commonly is accommodated by a combination of fault slip and dike intrusion. Consistent patterns in the spatial arrangement of long-lived magma intrusion zones in the Ethiopian and Afar rift sectors, East Africa, suggest that the magma intrusions help to localize strain during repeated rifting episodes. Within the broad Main Ethiopian Rift, extensional deformation has localized since ∼3m.y. in narrow magmatic segments, that are oriented oblique to the orientation of the Miocene border faults, but (sub-) orthogonal to the extension direction. Numerical models combined with geophysical and geological observations from East Africa are used to examine the viability of self-sustaining magmatic segmentation. Initiation of the magmatic segments is shown to result from magma injections, which focus strain in narrow elongated zones. During magmatic phases of segment evolution the segments are weak, and extensional stresses localize at the rift tips, promoting along-axis lengthening. During amagmatic phases of extension, the numerical models predict strain localization within the magmatic segments and, to a lesser extent, broadly distributed extension within the rift zone. This promotes segment stability; the segments remain the preferred location for magma intrusion during new magmatic phases. These results are applied to the formation and maintenance of MER segmentation. The Fentale–Dofen segment is currently in a non-magmatic phase of extension; the Dabbahu segment in the Red Sea Rift is currently experiencing a rifting episode and therefore is in a transient magmatic cycle. The observed patterns of instantaneous localized deformation, seismicity, and dike intrusions sometimes propagating beyond the tip of the magmatic segments occur as predicted by the models.

September 2005 Manda Hararo‐Dabbahu rifting event, Afar (Ethiopia): Constraints provided by geodetic data

We provide a new set of complementary geodetic data for the 2005 rifting event of Afar (Ethiopia). Interferometric synthetic aperture radar and subpixel correlations of synthetic aperture radar and SPOT images allow us to deduce 3‐D surface displacement unambiguously. We determine the geometry of the dike and neighboring magma chambers and invert for the distribution of opening of the dike, as well as slip on rift border faults. The volume of the 2005 dike (1.5–2.0 km3) is not balanced by sufficient volume loss at Dabbahu and Gabho volcanoes (0.42 and 0.12 km3, respectively). Taking into account the deflation of a suspected deep midsegment magma chamber simultaneously to dike intrusion produces a smoother opening distribution along the southern segment. Above the dike, faults slipped by an average 3 m, yielding an estimated geodetic moment of 3.5 × 1019 Nm, one order of magnitude larger than the cumulative seismic moment released during the earthquake swarm. Between Dabbahu and Ado'Ale volcanic complexes, significant opening occurred on the western side of the dike. The anomalous location of the dike at this latitude, offset to the east of the axial depression, may explain this phenomenon. A two‐stage intrusion scenario is proposed, whereby rifting in the northern Manda Hararo Rift was triggered by magma upwelling in the Dabbahu area, at the northern extremity of the magmatic segment. Although vigorous dike injection occurred during the September 2005 event, the tectonic stress deficit since the previous rifting episode was not fully released, leading to further intrusions in 2006–2009.

Paleomagnetism of the Fentale-magmatic segment, main Ethiopian Rift: New evidence for counterclockwise block rotation linked to transtensional deformation

We carried out the first ever paleomagnetic investigations in the area around the Fentale volcanoes, Main Ethiopian Rift. A total of 28 paleomagnetic sites were sampled around the two Fentale (Tinish and Tilik) volcanic centers. Lava and pyroclastic flows ranging in age from 2 Ma to 0.1 Ma was sampled for this study. After routine paleomagnetic collection and sample preparation procedures specimens were subjected to progressive alternating fields and thermal demagnetizations. The directional analysis shows either one or two components of Natural Remanent Magnetization to characterize the entire measurements. The low stability components are removed either by alternating fields up to 15 mT or by heating up to 250 °C. The high stability and primary components are isolated above those steps and vector is directed towards the origin, and it represents the Characteristic Remanent Magnetization. The principal component analysis and site means directions calculations yielded normal and reversed polarities together with two anomalous directions. When an overall mean direction is calculated from the individual site mean directions for the Fentale region, D s = 354.3°, I s = 19.3°, N = 27, K = 33, α 95 = 4.9° are obtained. When this result is compared with the mean expected reference dipole geomagnetic field direction ( D = 1.0°, I = 16.4°, α 95 = 2.3°, K = 105.6, N = 32) for an average age of 1.5 Ma, the following differences of Δ D = −6.7° ± 4.3°, Δ I = −2.9° ± 4.3° can be obtained. These results from the region indicate ∼7° counterclockwise block rotations about vertical axis consistent with the transtentional deformations that characterize the Fentale magmatic segment.

Magma productivity and early seafloor spreading rate correlation on the northern Vøring Margin, Norway — Constraints on mantle melting

Continental rifting at the Vøring Margin off mid-Norway was initiated during the earliest Eocene (~ 54 Ma), and large volumes of magmatic rocks were emplaced during and after continental breakup. In 2003, a marine survey collecting ocean bottom seismometer, single-channel reflection, and magnetic data was conducted on the Norwegian Margin to constrain continental breakup and early seafloor spreading processes. The profile described here crosses the northern part of the Vøring Plateau, and the crustal velocity model was constructed through a combination of ray-tracing and forward gravity modeling, the latter corrected for the thermal effects remaining from the seafloor spreading. We found a maximum igneous crustal thickness of 18 km, decreasing to 6.5 km over the first ~ 6 M.y. after continental breakup. Both the volume and the duration of excess magmatism are about twice as large as that of the Møre Margin south of the East Jan Mayen Fracture Zone, which offsets the two margin segments by ~ 170 km. A similar reduction in magmatism occurs to the north over an along-margin distance of ~ 150 km to the Lofoten Margin, but without a margin offset. Both the geochemical data and the mean P-wave velocity indicate that there is active mantle upwelling combined with a moderate temperature increase during the earliest mantle melting at the Vøring Margin. The mean P-wave velocity versus crustal thickness also indicates that there is a transition from convection dominated to temperature dominated magma production ~ 2 M.y. after breakup. The magnetic data were used to derive plate half-spreading rates for the Northern Vøring Margin, which are very similar to that obtained at the Møre Margin. There is a strong correlation between magma productivity and early plate spreading rate, suggesting a common cause. A model for the breakup-related magmatism should be able to explain this correlation, but also the magma production peak at breakup, the along-margin magmatic segmentation, and the active mantle upwelling. Proposed end-member hypotheses comprise elevated upper-mantle temperatures caused by a hot mantle plume, or edge-driven small-scale convection fluxing mantle rocks through the melt zone. Edge-driven convection does not easily explain these observations, but a mantle plume model in which buoyant plume material flows laterally to pond in the rift-topography at the base of the lithosphere close to breakup time is promising: When the continents break apart, the hot and buoyant plume-material can flow up into the rift zone from surrounding areas as the rift transits to drift, and the excess temperature of this material will then cause excess magmatism which dies off as the rift-restricted material is spent. The buoyancy of the plume-material may in addition cause active upwelling which can increase the melting furthermore, and also increase the force on the plate boundaries to enhance plate spreading rate. This conceptual model explains how both excess magmatism and spreading rate will be reduced similarly with time as the plume material is consumed by plate spreading, and thus correlate.

Fault rotation and core complex formation: Significant processes in seafloor formation at slow‐spreading mid‐ocean ridges (Mid‐Atlantic Ridge, 13°–15°N)

The region of the Mid‐Atlantic Ridge (MAR) between the Fifteen‐Twenty and Marathon fracture zones displays the topographic characteristics of prevalent and vigorous tectonic extension. Normal faults show large amounts of rotation, dome‐shaped corrugated detachment surfaces (core complexes) intersect the seafloor at the edge of the inner valley floor, and extinct core complexes cover the seafloor off‐axis. We have identified 45 potential core complexes in this region whose locations are scattered everywhere along two segments (13° and 15°N segments). Steep outward‐facing slopes suggest that the footwalls of many of the normal faults in these two segments have rotated by more than 30°. The rotation occurs very close to the ridge axis (as much as 20° within 5 km of the volcanic axis) and is complete by ∼1 My, producing distinctive linear ridges with roughly symmetrical slopes. This morphology is very different from linear abyssal hill faults formed at the 14°N magmatic segment, which display a smaller amount of rotation (typically <15°). We suggest that the severe rotation of faults is diagnostic of a region undergoing large amounts of tectonic extension on single faults. If faults are long‐lived, a dome‐shaped corrugated surface develops in front of the ridges and lower crustal and upper mantle rocks are exposed to form a core complex. A single ridge segment can have several active core complexes, some less than 25 km apart that are separated by swales. We present two models for multiple core complex formation: a continuous model in which a single detachment surface extends along axis to include all of the core complexes and swales, and a discontinuous model in which local detachment faults form the core complexes and magmatic spreading forms the intervening swales. Either model can explain the observed morphology.

Discontinuous and diachronous evolution of the Main Ethiopian Rift: Implications for development of continental rifts

The Main Ethiopian Rift (MER) is commonly considered the archetypal magma-assisted rift. Tomographic images of upper-mantle upwellings beneath the rift, aligned anisotropy beneath magmatic segments, and pervasive magmatic modification of the crust all indicate the importance of magmatic processes in present-day rift evolution. It has been suggested that this magmatic development is responsible for the straight and continuous path the rift cuts across the Ethiopian Plateau. We compile new evidence indicating that the MER is not as continuous and its development not as simple as previously believed. Significant lithospheric heterogeneities are evident in our compilation of recently acquired seismic, gravity, and geologic data. Numerical models of rift propagation in such heterogeneous lithosphere show that rift propagation may stall at rheological boundaries. We propose that the heterogeneities in the MER caused irregular rift propagation, resulting in a distinct discontinuity visible within the rift lithosphere. This discontinuity in structure spatially correlates to an apparent discontinuity in the age of extension between the northern MER and the central MER, lending support to our hypothesis. Our interpretation leads to a two-phase model of rift propagation in the MER, with initial rift development primarily controlled by lithospheric structure and a later phase during which magmatic processes are dominant. During the initial phase, rift propagation was irregular and at times stalled or was diverted away from the modern rift trend along pre-existing structures. Our model, while acknowledging the importance of magmatic processes in volcanic extensional regions, shows that even in this classic example of magma-assisted rifting, inherited lithospheric structure localized initial extension and controlled rift propagation. This early phase formed the template for future rift development and continental break-up.

Fault growth at a nascent slow-spreading ridge: 2005 Dabbahu rifting episode, Afar

SUMMARY We present a preliminary account of the near-field surface strain associated with a major magmatic rifting episode at a nascent slow spreading ridge in the Afar depression. Between 2005 September 14 and October 4, a volcanic eruption and 163 earthquakes (mb > 3.9), including seismic tremor, occurred within the ∼60-km-long Dabbahu magmatic segment. Results of the early response team demonstrated that ground deformation, derived from satellite radar data (InSAR), together with seismicity, is consistent with dyke-induced deformation along the entire length of the segment. We document the distribution of brittle strain associated with the early part of this rifting cycle to verify the predicted pattern of deformation and constrain a conceptual model for normal fault growth in Afar, with general application to other slow spreading divergent margins. Our field investigations concentrate on the northern half of the segment, which ruptured through to the surface over a length of >30 km and a width of ∼5 km, consistent with the pattern of microseismicity recorded using a network deployed ∼1 month after the initial onset of the rifting episode on September 14. Severe ground shaking during the event was more widespread; fresh rock fall is common across the entire magmatic segment, particularly at the intersections between faults. Recent ground breaks, in the form of reactivated or newly initiated normal faults and fissures, opened with horizontal displacements up to 3 m and vertical displacements locally up to 5 m, but commonly ∼2 m. These structures are generally subvertical and open along pre-existing cooling joints. Fault offset is greater than expected given the magnitude of earthquakes during the episode. The axial relief that developed consequent on fault and fissure initiation and reactivation during the 2005 Dabbahu episode is consistent with that of the entire magmatic segment. We therefore suggest that melt delivery is sufficiently frequent that favourable stress conditions for faulting are primarily achieved during dyke events.

Evolution of the Southwest Indian Ridge from 55°45′E to 62°E: Changes in plate‐boundary geometry since 26 Ma

From 55°45′E to 58°45′E and from 60°30′E to 62°00′E, the ultraslow‐spreading Southwest Indian Ridge (SWIR) consists of magmatic spreading segments separated by oblique amagmatic spreading segments, transform faults, and nontransform discontinuities. Off‐axis magnetic and multibeam bathymetric data permit investigation of the evolution of this part of the SWIR. Individual magmatic segments show varying magnitudes and directions of asymmetric spreading, which requires that the shape of the plate boundary has changed significantly over time. In particular, since 26 Ma the Atlantis II transform fault grew by 90 km to reach 199 km, while a 45‐km‐long transform fault at 56°30′E shrank to become an 11 km offset nontransform discontinuity. Conversely, an oblique amagmatic segment at the center of a first‐order spreading segment shows little change in orientation with time. These changes are consistent with the clockwise rotation of two ∼450‐km‐wide first‐order spreading segments between the Gallieni and Melville transform faults (52–60°E) to become more orthogonal to spreading. We suggest that suborthogonal first‐order spreading segments reflect a stable configuration for mid‐ocean ridges that maximizes upwelling rates in the asthenospheric mantle and results in a hotter and weaker ridge‐axis that can more easily accommodate seafloor spreading.

Deformation distribution and type in the Main Ethiopian Rift (MER): A remote sensing study

The Main Ethiopian Rift (MER) is magmatically segmented and seismically active. The magmatic segments are arranged en echelon (e.g. Ebinger, C.J., Casey, M., 2001. Continental breakup in magmatic provinces: An Ethiopian example. Geology Society of America Bulletin, vol. 12, pp. 527–530). Furthermore they are about 50 km long and up to 20 km wide, orientated about N10°–20°E, and are therefore oblique to the main direction of Mid-Miocene border faults (N35°E). In this paper we propose that these segments not only represent magmatic but also tectonic segmentation. We use the association of the high spectral resolution of Landsat data (principal component analysis and classifications) and the very high spatial resolution of digitized aerial photographs to characterize and quantify the deformation along the rift. Additionally, several digital elevation models of metric resolution have been produced at different locations along the segments in order to quantify and describe the faulting. We define 4 tecto-magmatic segments (Gedemsa, Bosetti, Kone and Fantale), 40–70 km, 10–15 wide, arranged en echelon and N10°–20°E orientated. Each tecto-magmatic segment (TMS) is separated by 2–18 km wide domains exhibiting minor deformation. Furthermore, the TMS overlap in a N–S direction and show similar symmetrical deformation patterns. Within an individual segment deformation varies from its centre, where it is essentially magmatic, to its tips, where brittle deformation is predominant. The largest volumes of recent basalt are erupted in the centres and nearby. In contrast, recent magma injections are absent in the tip domains. It seems probable that dikes connect laterally to, and are fed by, the magmatic centres. Dikes are sub-parallel to the segment axis and therefore orientated normal to the recent extension direction. Within the TMS, aligned cones, atypical fault geometries, en echelon fault segments, and atypical displacement–length ratios all suggest that faulting is dike-induced. Along axis propagation of the TMS generate interactions between younger (Quaternary) intra-rift faults and older (Mid-Miocene) border faults. Thus, at the deformed tips, cross-cutting and long curved faults are produced. The fault interactions in the MER confirm that the border faults have been inactive during the recent stage of rifting. We discuss the implications of oblique rifting with moderate obliquity ( α = 27°, angle between the extension direction and the trend perpendicular to the rift) at slow extension rates on active rifting processes associated with intermediate continental crust. The tecto-magmatic segmentation mirrors crustal segmentation beneath the rift axis, as indicated from 3-D-tomography (Daly, E., Keir, D., Ebinger, C., Stuart, G., Ayele, A., Waltham, D., 2007. Crustal structure of the northern Main Ethiopian Rift from a tomographic inversion of local earthquakes. Geophysical Journal International, in press; Keranen, K., Klemperer, S.L., Gloaguen, R., EAGLE Working Group, 2004. Three-dimensional seismic imaging of a protoridge axis in the Main Ethiopian rift. Geology 32 (11), 949–952). These long and narrow zones (40–70 km long and 10–15 km wide), where deformation is generated principally by diking and is associated with magmatic centres, have similarities with along-axis segmentation at mid ocean ridges. Consequently, deformation distribution and type in the MER can be considered the outcome of a transition between continental and active oceanic rifting.

Gravity analysis of the main Ethiopian rift

An analysis of gravity data collected during the 2003 Ethiopia-Afar Geoscientific Lithospheric Experiment (EAGLE) and previously available data is used in conjunction with EAGLE seismic results to determine a preliminary crustal structure model of the main Ethiopian rift (MER). The Bouguer gravity anomaly is dominated by a regional gravity anomaly that increases in amplitude from the central MER to the southern Afar region, while the residual gravity anomaly indicates gravity maxima occur over magmatic segments that occur within the MER. Gravity modeling using the EAGLE controlled-source axial seismic model as a constraint indicates that the crust thins from ∼40 km over the central MER to ∼30 km over the southern Afar region. However, to model the large amplitude gravity gradient in the Nazreth region, a denser lower crust and less dense upper mantle were needed over the southern Afar region. The denser lower crust is interpreted to be caused by mafic material formed during extensional tectonics, while the less dense upper mantle is indicative of a hotter thermal regime. The presence of a thinned crust and higher density lower crust is characteristic of volcanic rifted margins and implies that the southern Afar region may be developing into a continental margin. The gravity maxima occurring over the magmatic segments were modeled as dense, mafic bodies caused by intrusion into the lower and upper crust with tops between 7 and 10 km. These bodies add constraints to recently obtained three-dimensional seismic tomographic models suggest that the lower to lower–upper crust is in a ductile regime and the upper 7 km of the upper crust is in a brittle regime.

Development of en echelon magmatic segments along oblique spreading ridges

En echelon magmatic segments commonly develop along obliquely spreading oceanic ridges. To clarify some of the dynamic aspects of this plate boundary, we performed a series of thermo-mechanical numerical tests. When extension of oceanic lithosphere becomes oblique, deformation within the axial region localizes into distinct upwelling centers. Temperatures are elevated in the upwelling cells, which are shallow mantle features that form the new plate boundary. The predicted features are similar to the axial volcanic ridges documented at Mohns and Reykjanes Ridges, and we conclude that they become the new loci of extensional deforma tion, upwelling, and magmatic activity. These ridges, suborthogonal to the plate spreading direction, only develop when the axis rift zone is weak. The subsegment length and spacing depend primarily on obliquity and axial width. Predicted crustal thickness along the subsegmented axis varies discernibly; this might explain the morphology and satellite gravity of the flanks of oblique spreading ridges.

Influence of ridge migration and proximity to hot spots on the morphology of slow‐ and intermediate‐spreading centers

Recent observations of ridge bathymetry suggest magmatic segmentation at fast‐ and intermediate‐spreading centers is linked to the migration of the spreading axis over the mantle. At fast‐ and intermediate‐spreading centers, Carbotte et al. (2004) observed that leading segments, those that are offset in the direction of ridge migration, are typically shallower (interpreted to be magmatically robust) relative to trailing segments across first‐ and second‐order discontinuities. The model set forth for this correlation invokes asymmetrical mantle upwelling in response to the absolute motion of the ridge axis and the entrainment of melt from across discontinuities. In this investigation, differences in ridge axis depth across first‐ and second‐order discontinuities are examined within the context of absolute plate motions for portions of the slow‐spreading Mid‐Atlantic Ridge (MAR 22–36°N and 25–35°S latitude), intermediate‐spreading Galápagos Spreading Center (GSC 83–98°W), and intermediate‐spreading South East Indian Ridge (SEIR 77–114°E). Portions of each of these areas display chemical and/or physical anomalies resulting from hot spot–ridge interaction. Along non–hot spot–influenced sections of intermediate‐spreading ridges, leading segments coincide with shallower segments across 72% of the first‐ and second‐order discontinuities (86% of transform faults and 55% of second‐order discontinuities). Depth asymmetries vary with ridge offset length, with maximum asymmetries for ridge offset lengths of 50–100 km. A weaker correlation is observed between ridge migration direction and ridge morphology at the slow‐spreading MAR, where leading segments are shallower across ∼60% of first‐ and second‐order discontinuities. For hot spot–influenced spreading centers, hot spot proximity dominates ridge morphology at intermediate‐spreading centers, but it is not a consistent predictor of axial depth asymmetries at slow‐spreading centers. This spreading‐rate‐dependent influence of ridge migration and hot spot proximity on axial morphology may reflect a more limited entrainment of melt from across slow‐spreading discontinuities due to the predominance of three‐dimensional upwelling and melt focusing to segment centers.

Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode

Seafloor spreading centres show a regular along-axis segmentation thought to be produced by a segmented magma supply in the passively upwelling mantle. On the other hand, continental rifts are segmented by large offset normal faults, and many lack magmatism. It is unclear how, when and where the ubiquitous segmented melt zones are emplaced during the continental rupture process. Between 14 September and 4 October 2005, 163 earthquakes (magnitudes greater than 3.9) and a volcanic eruption occurred within the approximately 60-km-long Dabbahu magmatic segment of the Afar rift, a nascent seafloor spreading centre in stretched continental lithosphere. Here we present a three-dimensional deformation field for the Dabbahu rifting episode derived from satellite radar data, which shows that the entire segment ruptured, making it the largest to have occurred on land in the era of satellite geodesy. Simple elastic modelling shows that the magmatic segment opened by up to 8 m, yet seismic rupture can account for only 8 per cent of the observed deformation. Magma was injected along a dyke between depths of 2 and 9 km, corresponding to a total intrusion volume of approximately 2.5 km3. Much of the magma appears to have originated from shallow chambers beneath Dabbahu and Gabho volcanoes at the northern end of the segment, where an explosive fissural eruption occurred on 26 September 2005. Although comparable in magnitude to the ten year (1975-84) Krafla events in Iceland, seismic data suggest that most of the Dabbahu dyke intrusion occurred in less than a week. Thus, magma intrusion via dyking, rather than segmented normal faulting, maintains and probably initiated the along-axis segmentation along this sector of the Nubia-Arabia plate boundary.

Strain accommodation by magmatism and faulting as rifting proceeds to breakup: Seismicity of the northern Ethiopian rift

The volcanically active Main Ethiopian rift (MER) marks the transition from continental rifting in the East African rift to incipient seafloor spreading in Afar. We use new seismicity data to investigate the distribution of strain and its relationship with magmatism immediately prior to continental breakup. From October 2001 to January 2003, seismicity was recorded by up to 179 broadband instruments that covered a 250 km × 350 km area. A total of 1957 earthquakes were located within the network, a selection of which was used for accurate location with a three‐dimensional velocity model and focal mechanism determination. Border faults are inactive except for a cluster of seismicity at the structurally complex intersection of the MER and the older Red Sea rift, where the Red Sea rift flank is downwarped into the younger MER. Earthquakes are localized to ∼20‐km‐wide, right‐stepping en echelon zones of Quaternary magmatism and faulting, which are underlain by mafic intrusions that rise to 8–10 km subsurface. Seismicity in these “magmatic segments” is characterized by low‐magnitude swarms coincident with Quaternary faults, fissures, and chains of eruptive centers. All but three focal mechanisms show normal dip‐slip motion; the minimum compressive stress is N103°E, perpendicular to Quaternary faults and aligned volcanic cones. The earthquake catalogue is complete above ML 2.1, and the estimated b value is 1.13 ± 0.05. The seismogenic zone lies above the 20‐km‐wide intrusion zones; intrusion may trigger faulting in the upper crust. New and existing data indicate that during continental breakup, intrusion of magma beneath ∼20‐km‐wide magmatic segments accommodates the majority of strain and controls the locus of seismicity and faulting in the upper crust.

Tectonics at the axis of the very slow spreading Southwest Indian Ridge: Insights from TOBI side‐scan sonar imagery

We present the analysis of the deformation in the axial valley of two contrasted regions of the very slow spreading Southwest Indian Ridge based on side‐scan sonar images. Our objective is to investigate how the obliquity is accommodated along the system. We show that the robust magmatic segments have axial valleys and major faults subperpendicular to spreading. The other sections show fault populations with various degrees of obliquity, often arranged in left‐stepping echelons, accommodating part of the strike‐slip deformation. Side‐scan sonar reveals the presence of a corrugated surface near 59°E interpreted to be an incipient detachment fault. We show that the large width of the SWIR oblique sections, and the difference in tectonic style between the robust volcanic segments and the magma‐starved sections, is accounted for by large variations in the thickness of the brittle lithosphere. We suggest that the emplacement of mantle rocks in the surveyed amagmatic ridge sections can occur by alternating conjugate faults. Serpentinization of outcropping peridotites might also play a significant role in the development of faults in thin crust regions and the distribution of deformation in space and time.

Distributed Nubia-Somalia relative motion and dike intrusion in the Main Ethiopian Rift

SUMMARY The Main Ethiopian Rift (MER) in central Ethiopia extended in the rift-normal direction at a mean rate of 4.0 ± 0.9 mm yr −1 (1σ ) during the period 1992‐2003, nearly a factor of two slower than the opening rate estimated from global plate motion inversions. Rift opening near a geodetic array during this period was accommodated by a single dike injection event in 1993, spatially coincident with active magmatic segments, probably triggered by observed seismicity. Following dike injection, the crust in the rift relaxed as a layered medium, with a ∼15-km-thick elastic lid over a viscous half space of 10 18 Pa s. Diking, rather than normal faulting on rift-bounding faults, appears to be the predominant mechanism of extension in the MER, explaining the very low regional rates of moment release. The length scale and temporal behaviour of surface displacements require viscoelastic rheology in the rift.