Recent seismicity of the East African rift system and its implications
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East African Rift
The present state of knowledge of the East African Rift System is described, with particular reference to the Gregory Rift, which is characterised by the widest diversity of volcanic formations; its detailed study over the past ten years has provided the most comprehensive data on the structural and volcanic evolution of any part of the Rift System.
East African Rift
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East African Rift
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Half-graben
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East African Rift
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Abstract The East African Rift System (EARS) provides a unique location for exploring factors influencing the development and maturation of continental rifting. In particular, the geographical relationships between Cenozoic rifts and Pre‐Cambrian lithospheric structures suggest that such preexisting structures exert an influence on early‐stage rift geometry and behavior. This study uses Rayleigh wave phase velocity at periods of 20 to 100 s to study lateral variability in the lithospheric structures of rift segments and preexisting structures in the central and southern EARS. The model is constructed using records of 789 earthquakes, recorded by a composite station array of 235 stations from nonconcurrent seismic networks between 1994 and 2015. In the central EARS, we observe fast velocities beneath the Tanzania Craton, isolated low‐velocity regions along the Western Rift Branch, and low velocities in all resolved portions of the Eastern Rift Branch, consistent with previous regional surface wave studies. South of the Tanzania Craton, we observe linear low‐velocity zones trending both southeast and southwest from the Tanzania Divergence Zone, suggesting a southern bifurcation of the Eastern Rift Branch. In the southern portions of the Western Rift Branch, the Malawi Rift borders fast velocities associated with the Bangweulu Block and Irumide Belt. Anomalously fast velocities in these regions persist to long periods, confirming the existence of cratonic lithosphere inferred from previous studies. Fast velocities observed beneath the Irumide Belt extend across the southernmost portion of the Malawi Rift, suggesting that strong lithosphere in this region may hinder the southern propagation of the rift.
East African Rift
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The seismically and volcanically active East African rift system is a system of faultbounded sedimentary basins and uplifted flanks spanning the African continent from the Horn of Africa to southern Africa. The East African rift transects a broad zone of uplift associated with the African Superplume province, with large tracts of the rift experiencing pre- and syn-rift magmatism. We review the rifting processes creating the broad plateaux, the uplifted rift flanks and fault-bounded rift valleys, outline the complex interplay between tectonically induced vertical crustal movements, climate, erosion, and sedimentation in rift zones, characterize spatial and temporal patterns of rift architecture from rift inception in Botswana to rupture within the Afar rift, and summarize the sedimentary framework of late Cenozoic basins in the East African rift system. Consistent spatial and temporal patterns document the feedbacks between faulting and sedimentation, and enable the development of predictive models for rift basin structure and stratigraphy, including the role of magmatism prior to and during rifting.
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East African Rift
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Uplifted flanks at intracontinental rifts are supported by flexural isostasy, as shown by the pattern of isostatic residual gravity anomalies associated with them. Models for flexural rift flank uplift differ in their kinematic description of extension, with respect to the asymmetry of rifting and the importance of brittle versus elastic upper crustal deformation. In this paper, I test different kinematic models of continental extension by comparing their predictions of rift flank topography and crustal structure with observations from the Baikal rift (SE Siberia). The rift is characterized by prominent flank topography on both sides of lake Baikal. The flanks reach similar elevations but differ in their structure: the tilt of the footwall flank is away from the basin, whereas the basinward part of the hanging wall flank tilts toward the basin center. Fission track data indicate that very little erosion affected the flanks since rifting started; geomorphological and sedimentological observations suggest that prerift relief was minor. Thus, the present‐day topography reflects rift‐related tectonic uplift. Pure‐shear “necking” and pure‐shear/simple‐shear “detachment” models of extension predict the topographic and Bouguer gravity anomaly patterns observed along a profile across the central Baikal rift equally well. They do not permit to discriminate between different scenarios that have been proposed for the central Baikal rift; that is, half‐graben versus full graben development; rifting at a continuous rate since the Oligocene versus a large increase in extension rate since the Pliocene. The models predict that the kinematics of rifting in Baikal are controlled by a midcrustal (20 km) depth of necking and/or a mid to lower crustal (20–30 km) detachment level; best‐fit elastic thicknesses are in the range 30–50 km. These predictions are in agreement with results from coherence studies of Bouguer gravity and topography, as well as with the rheology of the lithosphere underneath Baikal as inferred from heat flow, seismic refraction and seismological observations. In contrast, a “flexural cantilever” model with low (< 10 km) elastic thickness predicts topographic patterns which are very different from those observed, for a wide range of rifting scenarios. Significant (> 3 km) erosion of the footwall flank is required to fit the topography if a flexural cantilever model is applied; this is incompatible with the fission track data. Thus, the kinematics of extension at deep and narrow intra‐continental rifts such as Baikal appear to be controlled by a strong elastic lithosphere and require significant brittle deformation of the upper crust, as suggested by dynamic models for continental rifting.
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Abstract Close relationships between deformation and volcanism are well documented in relatively late evolutionary stages of continental rifting, whereas these are poorly constrained in less mature rifting stages. To investigate the control of inherited structures on faulting and volcanism, we present a statistical analysis of volcanic features, faults and pre‐rift fabric in the Tanzania Divergence, where volcanic features occur extensively in in‐rift and off‐rift areas. Our results show that in mature rift sectors (Natron), magma uprising is mostly controlled by fractures/faults responding to the far‐field stress, whereas the distribution of volcanism during initial rifting (Eyasi) is controlled by inherited structures oblique to the regional extension direction. Off‐rift sectors show a marked control of pre‐rift structures on magma emplacement, which may not respond to the regional stress field. Thus, the use of off‐rift magmatic features as stress indicators should take into account the role of pre‐existing structures.
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