Ian D. Bastow

Lower crustal earthquakes near the Ethiopian rift induced by magmatic processes

Lower crustal earthquakes are commonly observed in continental rifts at depths where temperatures should be too high for brittle failure to occur. Here we present accurately located earthquakes in central Ethiopia, covering an incipient oceanic plate boundary in the Main Ethiopian Rift. Seismicity is evaluated using the combination of exceptionally well resolved seismic structure of the crust and upper mantle, electromagnetic properties of the crust, rock geochemistry, and geological data. The combined data sets provide evidence that lower crustal earthquakes are focused in mafic lower crust containing pockets of the largest fraction of partial melt. The pattern of seismicity and distribution of crustal melt also correlates closely with presence of partial melt in the upper mantle, suggesting lower crustal earthquakes are induced by ongoing crustal modification through magma emplacement that is driven by partial melting of the mantle. Our results show that magmatic processes control not only the distribution of shallow seismicity and volcanic activity along the axis of the rift valley but also anomalous earthquakes in the lower crust away from these zones of localized strain.

Elevated mantle temperature beneath East Africa

The causes of magmatism at magmatic rifted margins and large igneous provinces (LIPs) are uncertain because the condition of the mantle that underlay them during formation can no longer be directly observed. Therefore, whether the mantle was characterized by elevated potential temperatures ( T P ), small-scale convection, or anomalously fertile composition is debated. East Africa is an ideal area in which to address this problem because it contains both the young African-Arabian LIP and the tectonically and magmatically active East African Rift system. Here we present mantle T P estimates for 53 primitive magmas from throughout the region to reveal that thermal anomalies currently peak in Djibouti (140 °C above ambient upper mantle). Slightly warmer conditions accompanied the Oligocene African-Arabian LIP, when the T P anomaly was 170 °C. These values are toward the low end of the global temperature range of LIPs, despite the markedly slow seismic velocity mantle that underlies the region. Mantle seismic velocity anomalies in East Africa cannot, therefore, as is often assumed, be attributed simply to elevated mantle temperatures. We conclude that CO 2 -assisted melt production in the African superplume contributes to the markedly slow seismic velocities below East Africa.

Pulses of deformation reveal frequently recurring shallow magmatic activity beneath the Main Ethiopian Rift

Magmatism strongly influences continental rift development, yet the mechanism, distribution, and timescales on which melt is emplaced and erupted through the shallow crust are not well characterized. The Main Ethiopian Rift (MER) has experienced significant volcanism, and the mantle beneath is characterized by high temperatures and partial melt. Despite its magma-rich geological record, only one eruption has been historically recorded, and no dedicated monitoring networks exist. Consequently, the present-day magmatic processes in the region remain poorly documented, and the associated hazards are neglected. We use satellite-based interferometric synthetic aperture radar observations to demonstrate that significant deformation has occurring at four volcanic edifices in the MER (Alutu, Corbetti, Bora, and Haledebi) from 1993 to 2010. This raises the number of volcanoes known to be deforming in East Africa beyond 12, comparable to many subduction arcs despite the smaller number of recorded eruptions. The largest displacements are at Alutu volcano, the site of a geothermal plant, which showed two pulses of rapid inflation (10–15 cm) in 2004 and 2008 separated by gradual subsidence. Our observations indicate a shallow (<10 km), frequently replenished zone of magma storage associated with volcanic edifices and add to the growing body of observations that indicate shallow magmatic processes operating on a decadal timescale are ubiquitous throughout the East African Rift. In the absence of detailed historical records of volcanic activity, satellite-based observations of monitoring parameters, such as deformation, could play an important role in assessing volcanic hazard.

Lateral magma flow in mafic sill complexes

The structure of upper crustal magma plumbing systems controls the distribution of volcanism and influences tectonic processes. However, delineating the structure and volume of plumbing systems is difficult because (1) active intrusion networks cannot be directly accessed; (2) field outcrops are commonly limited; and (3) geophysical data imaging the subsurface are restricted in areal extent and resolution. This has led to models involving the vertical transfer of magma via dikes, extending from a melt source to overlying reservoirs and eruption sites, being favored in the volcanic literature. However, while there is a wealth of evidence to support the occurrence of dike-dominated systems, we synthesize field- and seismic reflection–based observations and highlight that extensive lateral magma transport (as much as 4100 km) may occur within mafic sill complexes. Most of these mafic sill complexes occur in sedimentary basins (e.g., the Karoo Basin, South Africa), although some intrude crystalline continental crust (e.g., the Yilgarn craton, Australia), and consist of interconnected sills and inclined sheets. Sill complex emplacement is largely controlled by host-rock lithology and structure and the state of stress. We argue that plumbing systems need not be dominated by dikes and that magma can be transported within widespread sill complexes, promoting the development of volcanoes that do not overlie the melt source. However, the extent to which active volcanic systems and rifted margins are underlain by sill complexes remains poorly constrained, despite important implications for elucidating magmatic processes, melt volumes, and melt sources.

Mantle upwelling and initiation of rift segmentation beneath the Afar Depression

The Afar Depression, at the northern end of the East African Rift, is the only place on land where the transition from a plume-induced continental breakup to seafloor spreading is active today. New images of seismic velocity structure, based on exceptional new data sets, show that the mantle plume that initiated rifting in Africa is absent beneath Afar today. The images are dominated by a major low-velocity feature at ∼75 km depth closely mimicking the abrupt changes in rift axis orientation seen at the surface. This is likely associated with passive upwelling beneath the rift. Additional focused low-velocity anomalies show that small diapiric upwellings are present beneath major off-axis volcanoes. These multiple melting sources can explain the wide range of geochemical signatures seen in Afar. These images suggest that passive upwelling beneath Afar marks the initiation of rift segmentation as continental breakup progresses to seafloor spreading.

The magma-assisted removal of Arabia in Afar: Evidence from dike injection in the Ethiopian rift captured using InSAR and seismicity

In seismically and tectonically active regions, the present-day strain field tends to bias interpretation of the geological record. This is usually reasonable, but in areas such as triple junctions, the orientation of stress and the locus of strain can evolve abruptly in space and time. We present deformation measurements using satellite radar interferometry (InSAR) and seismicity that together capture the intrusion of a ~6 km long, ~1.5 m wide dike into the upper crust of the Ethiopian rift in southern Afar during May 2000. Dike-induced volcano-tectonic seismicity suggests that the intrusion was injected laterally during a period of ~4 days. Seismic moment release accounts for only 5% of the total 1.6 × 1018 Nm geodetic moment, showing that diking accommodates the majority of strain. The intrusion intriguingly strikes at N122°E, perpendicular to the trend of the present-day East African rift. The geometry and age constraints on faulting and volcanic activity in southern Afar, combined with plate reconstructions, suggest that the dike likely intrudes an ~ESE-SE striking magmatic system that localized strain during Oligo-Miocene rifting in the Red Sea and Gulf of Aden. We also identify the southerly extent of the Arabian Plate in Afar during the Oligocene in mantle seismic tomographic images: an abrupt increase in seismic velocity in southern Afar is coincident with a stepped increase of ~20 Myr in the time elapsed since the onset of plate stretching. The anomalous orientation of the May 2000 intrusion implies that African-Arabian tectonics still influences the stress field in southern Afar and is at least partly accommodated by magma intrusion.

Precambrian plate tectonics: Seismic evidence from northern Hudson Bay, Canada

The Canadian Shield is one of the largest exposures of Precambrian rocks on Earth. It is a mosaic of several Archean terranes that were brought together during a series of Paleoproterozoic orogens culminating in the so-called Trans-Hudson orogen, which is thought to have been similar to the Himalayan orogen in scale and nature. The tectonic evolution and lithospheric subdivisions of this region are poorly understood, but new seismic networks in northern Hudson Bay provide fresh opportunity to place constraints on the Precambrian processes that formed and shaped it. We show, via a study of seismic anisotropy, that the lithosphere of the northern Hudson Bay region retains a strong signature of Archean–Paleoproterozoic tectonics. We map the boundary between the upper (Churchill) and lower (Superior) plates that collided ca. 1.8 Ga and identify back azimuth–dependent splitting parameters (φ, δ t ) on Baffin Island that indicate complex anisotropy (e.g., dipping fabric) beneath the region. Our results support the view that significant lithospheric deformation occurred during the Paleoproterozoic and that modern-day plate tectonic processes were thus in operation by at least ca. 1.8 Ga.

The origin of along-rift variations in faulting and magmatism in the Ethiopian Rift

The geological record at rifts and margins worldwide often reveals considerable along-strike variations in volumes of extruded and intruded igneous rocks. These variations may be the result of asthenospheric heterogeneity, variations in rate and timing of extension; alternatively, pre-existing plate architecture and/or the evolving kinematics of extension during breakup may exert first order control on magmatism. The Main Ethiopian Rift (MER) in East Africa provides an excellent opportunity to address this dichotomy: it exposes, along-strike, several sectors of asynchronous rift development from continental rifting in the south to incipient oceanic spreading in the north. Here we perform studies of volcanic cone density and rift obliquity along strike in the MER. By synthesizing these new data in light of existing geophysical, geochemical and petrological constraints on magma generation and emplacement, we are able to discriminate between tectonic and mantle geodynamic controls on the geological record of a newly forming magmatic rifted margin. The timing of rift sector development, the three-dimensional focusing of melt, and the ponding of plume material where the rift dramatically narrows, each influence igneous intrusion and volcanism along the MER. However, rifting obliquity plays an important role in localizing intrusion into the crust beneath en-echelon volcanic segments. Along-strike variations in volumes and types of igneous rocks found at rifted margins thus likely carry information about the development of strain during rifting, as well as the physical state of the convecting mantle at the time of breakup.

Towards a better understanding of African topography: a review of passive-source seismic studies of the African crust and upper mantle

Abstract Explaining the cause and support of Africas varied topography remains a fundamental question for our understanding of the long-term evolution of the continent. As geodynamical modelling becomes more frequently used to investigate this problem, it is important to understand the seismological results that can be incorporated into these models. Crustal thickness estimates are crucial for calculating components of topography that are isostatically compensated. Variations in seismic velocity help constrain variations in subsurface temperature and density and thus buoyancy; measurements of anisotropy can also be used to determine the contribution of the mantle flow field to dynamic topography. In this light, we review the results of passive seismic studies across Africa. At the continental scale there are significant differences in crustal models, meaning large uncertainties in corrections for isostatic topography. In east Africa, multiple seismic experiments have provided firm constraints on crustal and mantle structure. Tomographic images illuminate a broad (c. 500 km wide) low-velocity region in the upper mantle, with possible connection to the African Superplume in the lower mantle. These observations, alongside the variations in radial anisotropy, strongly suggest that the mantle flow field contributes significantly to the uplift of the region. Beneath southern Africa, low velocities are observed near the base of the continental lithosphere; the depth to transition zone discontinuities however suggests that they are not linked to the superplume beneath. It is thus less clear what role the sublithospheric mantle plays in supporting the regions high topography. Many of Africas secondary topographic features (e.g. Atlas, Hoggar, Bie Dome) are underlain by slow velocities at depths of 100–150 km and are adjacent to rapid changes in lithospheric thickness. Whether these variations in lithospheric structure promote small-scale convection or simply guide the larger-scale mantle flow field remains ambiguous.

P‐wave tomography of eastern North America: Evidence for mantle evolution from Archean to Phanerozoic, and modification during subsequent hot spot tectonism

The unique physical and chemical properties of cratonic lithosphere are thought to be key to its long-term survival and its resistance to pervasive modification by tectonic processes. Study of mantle structure in southeast Canada and the northeast US offers an excellent opportunity to address this issue because the region spans 3 billion years of Earth history, including Archean formation of the Superior craton and younger accretion of terranes to eastern Laurentia during the Proterozoic Grenville and Phanerozoic Appalachian orogenies. Trending NW–SE through each of these terranes is the track of the Great Meteor hot spot, which affected the region during the Mesozoic. Here we study mantle seismic velocity structure beneath this region of eastern North America using tomographic inversion of teleseismic P-wave relative arrival-times recorded by a large-aperture seismograph network. There are no large-scale systematic differences between Superior and Grenville mantle wave speed structure, which may suggest that tectonic stabilization of cratons occurred in a similar fashion during the Archean and Proterozoic. Cratonic lithosphere is largely thought to be resistant to modification by hot spot processes, in contrast to younger terranes where lithospheric erosion and significant magmatism are expected. Low velocities beneath the regions affected by the Great Meteor hot spot are broadest beneath the Paleozoic Appalachian terranes, indicating pervasive modification of the lithosphere during magmatism. The zone of modification narrows considerably into the Proterozoic Grenville province before disappearing completely in the Archean Superior craton, where the surface signature of Mesozoic magmatism is limited to kimberlite eruptions.