SKS splitting beneath continental rift zones
Stephen S. GaoPaul M. DavisHuansui LiuP. D. SlackAndrew W. RigorYu.A. ZorinВ. В. МордвиноваV. M. KozhevnikovN. A. Logatchev
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Abstract:
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.Keywords:
Rift zone
East African Rift
Half-graben
Rift valley
East African Rift
Half-graben
Doming
Horst and graben
Rift zone
Mantle plume
Rift valley
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The seismicity of East Africa has been investigated using the Durham University seismometer army at Kaptagat. Events recorded by this station originate from five main seismic regions. Three of the five regions, the Kavirondo Rift, the Siria Fault and the area from Entebbe to the Ruwenzori Mountains, form east-west bands of activity. The activity In the remaining regions, the Western Rift, In particular the Ruwenzori Mountains, and the Gregory Rift shows a north-south distribution. Earthquakes from the Western Rift are associated with the boundary faults. In the Kavirondo Rift the events are associated with the eastern end of the graben and have been used to Infer an easterly extension of the faults. Within the central Gregory Rift the shocks are associated with the axis of the rift. Where the Gregory Rift passes Into the North Tanzania Divergence and the Turkwel Depression the events are distributed across the width of the Rift. From the slope, b, of the cummulative frequency-magnitude curve It Is suggested that the Western Rift can be divided Into a northern and southern section. The former section Is the younger and Is tectonically similar to the Kavirondo graben. The Eyasi and Gregory Rifts have shown the same value for 'b' and seem to be tectonically similar. However, they differ In the upper cut off limit on the magnitude of the earthquakes. The pattern of seismicity In the Gregory Rift and the travel time, compared to normal shield structure, across the rift have been Interpreted to give a model for the crystal structure within the rift. The model consists of a continuous velocity Increase with depth Interrupted by a drop In velocity. The anomalous material which the model represents also has a low Q value of approximately 100.
East African Rift
Half-graben
Rift valley
Rift zone
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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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East African Rift
Rift valley
Rift zone
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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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