Rift obliquity in the Northern Volcanic Zone in Iceland using UAV-based structural data
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Abstract:
Rift obliquity is known to be very frequent at divergent plate boundaries and shear components in the opening of fissures have been detected through seismicity in the monitoring of recent rifting events. However, the field structural evidence of obliquity in active rifts for Holocene events is still poorly studied, despite its relevance at both the lithospheric and upper-crust scales. We performed extensive high resolution UAV surveys on Holocene terrains in four areas along the active rift in the Northern Volcanic Zone of Iceland, to map structures and morphotectonic features in detail. We did a kinematic analysis and found an increase of the obliquity from N to S. We further discuss the origins of this obliquity and suggest that the combination of inherited structures reactivation, magmatic intrusions, and topography influence on the propagation of both fractures and magma intrusions can have an impact on the mismatch between rift opening direction and the far-field strain. We conclude that obliquity can be inferred for Holocene rifting events from surface structural data. It is thus fundamental to take into account the existing structural pattern and its (re)activation history in active rift zones to avoid missing out on possible preferential structure reactivation, magma propagation ways, and eruption sites during the monitoring of ongoing rifting events.Keywords:
Rift zone
East African Rift
The rheological properties of the lithosphere in the East African Rift System are estimated from regional heat flow and seismic constraints. Heat flow data are used to infer average, maximum, and minimum geotherms for the Eastern Rift, the Western Rift, and the surrounding shield (having surface heat flow of 106±51, 68±47, and 53±19mW m−2, respectively). Combining the geotherms with brittle and ductile deformation laws for a lithosphere of appropriate structure and composition yields rheological profiles, thickness of brittle layers, and thickness and total strength of the lithosphere. The thickness of the uppermost brittle layer varies from 10 ± 2 km in the Eastern Rift, to 186+−9km in the Western Rift, and 26+ km in the shield; seismicity is confined to the brittle layers. The rheological thickness of the lithosphere in Eastern Rift (23+8−9km), Western Rift, and shield is approximately in the the ratio 1:2.5:5, and matches the elastic flexural thickness. The total resistance to deformation is one order of magnitude larger in the shield than in the rifted regions (∼1012N m−1), where the whole lithosphere is probably in a state of failure.
East African Rift
Brittleness
Rift zone
Lithospheric flexure
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Abstract Although the East African rift system formed in cratonic lithosphere above a large‐scale mantle upwelling, some sectors have voluminous magmatism, while others have isolated, small‐volume eruptive centers. We conduct teleseismic shear wave splitting analyses on data from 5 lake‐bottom seismometers and 67 land stations in the Tanganyika‐Rukwa‐Malawi rift zone, including the Rungwe Volcanic Province (RVP), and from 5 seismometers in the Kivu rift and Virunga Volcanic Province, to evaluate rift‐perpendicular strain, rift‐parallel melt intrusion, and regional flow models for seismic anisotropy patterns beneath the largely amagmatic Western rift. Observations from 684 SKS and 305 SKKS phases reveal consistent patterns. Within the Malawi rift south of the RVP, fast splitting directions are oriented northeast with average delays of ~1 s. Directions rotate to N‐S and NNW north of the volcanic province within the reactivated Mesozoic Rukwa and southern Tanganyika rifts. Delay times are largest (~1.25 s) within the Virunga Volcanic Province. Our work combined with earlier studies shows that SKS‐splitting is rift parallel within Western rift magmatic provinces, with a larger percentage of null measurements than in amagmatic areas. The spatial variations in direction and amount of splitting from our results and those of earlier Western rift studies suggest that mantle flow is deflected by the deeply rooted cratons. The resulting flow complexity, and likely stagnation beneath the Rungwe province, may explain the ca. 17 Myr of localized magmatism in the weakly stretched RVP, and it argues against interpretations of a uniform anisotropic layer caused by large‐scale asthenospheric flow or passive rifting.
East African Rift
Shear wave splitting
Seismic anisotropy
Rift zone
Seismometer
Volcanic belt
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Epicenter
East African Rift
Rift zone
Focal mechanism
Stress field
Rift valley
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Crustal stress pattern provide important information for the understanding of regional tectonics and for the modelling of seismic hazard. Especially for small rifts (e.g. Upper Rhine Graben) and beside larger rift structures (e.g. Baikal Rift, East African Rift System) only limited information on the stress orientations is available. We refine existing stress models by using new focal mechanisms combined with existing solutions to perform a formal stress inversion. We review the first-order stress pattern given by previous models for the Upper Rhine Graben, the Baikal Rift, and the East African Rift System. Due to the new focal mechanisms we resolve second-order features in areas of high data density. The resulting stress orientations show dominant extensional stress regimes along the Baikal and East African Rift but strike-slip regimes in the Upper Rhine Graben and the interior of the Amurian plate.
East African Rift
Half-graben
Rift zone
Stress field
Extensional tectonics
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East African Rift
Half-graben
Echelon formation
Rift zone
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The Asal‐Ghoubbet (AG) Rift has sustained a major volcano‐tectonic rifting episode in 1978 and has been subsequently monitored with continuous geodetic and seismological surveys. It is therefore an ideal place to study the transient magmato‐tectonic processes that operate after a rifting episode. We examine the space‐time evolution of ∼2500 Md ≤ 2.8 earthquakes recorded in the rift from 1979 to 2001. We focus on the relationships between this seismic activity and both the three‐dimensional structure of the rift and its postrifting behavior depicted from geodesy. The results highlight the major role of the central magmatic system (Fieale‐Shark Bay) on the structure, seismic activity, and overall behavior of the rift. From 1978 to 1986, the rift opens at a fast rate, yet mainly aseismically; the opening is magmatically driven and accommodated. Since 1986, when the opening rate abruptly decreased, the seismicity is concentrated in the central part of the rift and reveals pulses of activity of the central volcanic system. These pulses result from that magmatic zone undergoing alternating stretching and inflating episodes. Thus, while the plate‐driven induced stresses have been rebuilding in the rift since 1987, the rift opening is still essentially accommodated in the axial magmatic zone. The AG Rift has thus sustained a postrifting unsteady opening over more than 23 years following its stretching episode. That transient opening has essentially occurred aseismically, and most tectonic faults remain relaxed and locked.
Rift zone
East African Rift
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We present measurements of SKS splitting at 28 digital seismic stations and 35 analog stations in the Baikal rift zone, Siberia, and adjacent areas, and at 17 stations in the East African Rift in Kenya and compare them with previous measurements from the Rio Grande Rift of North America. Fast directions in the inner region of the Baikal rift zone are distributed in two orthogonal directions, NE and NW, approximately parallel and perpendicular to the NE strike of the rift. In the adjacent Siberian platform and northern Mongolian fold belt, only the rift‐orthogonal fast direction is observed. In southcentral Mongolia, the dominant fast direction changes to rift‐parallel again, although a small number of measurements are still rift‐orthogonal. For the axial zones of the East African and Rio Grande Rifts, fast directions are oriented on average NNE, that is, rotated clockwise from the N‐S trending rift. All three rifts are underlain by low‐velocity upper mantle as determined from teleseismic tomography. Rift‐related mantle flow provides a plausible interpretation for the rift‐orthogonal fast directions. The rift‐parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small‐scale mantle convection with rift‐parallel flow. The agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift‐parallel fast directions are caused by oriented magmatic cracks.
Rift zone
East African Rift
Half-graben
Rift valley
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Tectonic stress field in rift systems a comparison of Rhinegraben, Baikal Rift and East African Rift
Crustal stress pattern provide important information for the understanding of regional tectonics and for the modelling of seismic hazard. Especially for small rifts (e.g. Upper Rhine Graben) and beside larger rift structures (e.g. Baikal Rift, East African Rift System) only limited information on the stress orientations is available. We refine existing stress models by using new focal mechanisms combined with existing solutions to perform a formal stress inversion. We review the first-order stress pattern given by previous models for the Upper Rhine Graben, the Baikal Rift, and the East African Rift System. Due to the new focal mechanisms we resolve second-order features in areas of high data density. The resulting stress orientations show dominant extensional stress regimes along the Baikal and East African Rift but strike-slip regimes in the Upper Rhine Graben and the interior of the Amurian plate.
East African Rift
Half-graben
Rift zone
Stress field
Extensional tectonics
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East African Rift
Half-graben
Rift zone
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Citations (75)