1,720,984 research outputs found
Holocene opening directions along the axes of Red Sea (Afar) and Main Ethiopian rifts: an overview
No full textFormation of normal faults along the axial zone of the Ethiopian Rift
No full textThe axial zone of the Ethiopian Rift is made up of Quaternary extensional fractures and normal faults. Field analysis was performed to study the mechanism of development of the normal faults. The collected data show that the normal faults (1) are subvertical at surface, (2) have dilation proportional to the throw, and (3) end laterally as extension fractures, that is, tension fractures which gradually decrease in dilation. The minimum measured opening of normal faults is 2 m and the maximum measured dilation of the extension fractures is 4 m. The minimum measured length of normal faults is 800 m and the maximum measured length of the extension fractures is 400 m. The collected data suggest that the normal faults nucleate from wider extension fractures in the axial zone. When the extension fractures reach critical dimensions (length 1/4 ,∼800 m and dilation 1/4 2-4 m, corresponding to a predicted depth of ∼700 m), the shear rupture behavior controls the further propagation of the fractures at depth. This mechanism has close similarities with models previously proposed for fault formation along the oceanic ridge of Iceland. This suggests common rifting processes along diverging plates, independently from the oceanic or continental nature of the lithosphere
The role of pre-existing structures in the origin, propagation and architecture of faults in the Main Ethiopian rift
No full textStructure of Tendaho Graben and Manda Hararo Rift: implications for the evolution of the Red Sea propagator in Central Afar
No full textThe Red Sea and Aden rifts (or propagators) meet in Afar. Here we use remote sensing and field analyses to define the geology and structure of the southern part of the Red Sea propagator, in Central Afar. This consists of the NW-SE trending Tendaho Graben (TG) and the younger and active NW-SE trending Manda Hararo Rift (MHR), partly within TG. Tectonic and volcanic activity within TG developed mostly between ~1.8 to ~0.6 Ma, with a stretching factor ~1.1, an extension rate ~3.6 mm/yr and the fissural eruption of part (~7000 km3/Ma) of the Afar Stratoid sequence (mainly basaltic lava flows and ignimbrites). MHR, before terminating southward, has a ~1.04 and extension rate ~1.2 mm/yr, and is associated with the emission of ~600 km3/Ma of basalts in the last ~0.2 Ma. These data suggest that, after the exceptional amount of magma erupted between ~1.8 to ~0.6 Ma, magmatic and tectonic activity significantly decreased along the southern part of the Red Sea propagator in the last ~0.2 Ma. This decrease coincides with the on-land development and migration of the more active (inferred extension rate in the order of ~10 mm/yr, as proposed in previous studies) Aden propagator, suggesting that spreading in Central Afar mainly occurred along one active propagator at any one time
Quaternary faulting and volcanism in the main Ethiopian Rift
No full textThe Main Ethiopian Rift (MER) is associated with bimodal Quaternary magmatism. Field, remote sensing, and geochronology data are used to examine the relationships between axial acidic volcanoes and basaltic eruptions. Two main Quaternary magmatic episodes are recognizeable in MER: a) basaltic flows followed by ignimbrites and silicic centers in the rift floor (2-1 Ma) and b) axial silicic volcanoes and basalts since ~650 Ka. The first episode consists mainly of basaltic flows related to the Afar Stratoid and outcrops in the central and northern MER. Scattered silicic centers developed subsequently along the rift floor. In the second episode, spatial and temporal correlation between rift localization and silicic centers becomes more evident. The silicic centers are located at the intersection of the WFB with earlier structures, especially E-W faults. With ageing, these centers become faulted and allow basalts to erupt right through the volcanic edifice, suggesting a decrease in the amount of differentiation in the magma chambers, possibly due to their cooling. This style of evolution appears to be characteristic of continental rifts prior to the onset of drifting
Elliptic calderas in the Ethiopian Rift: control of pre-existing structures
No full textThe Ethiopian Rift is characterized by several Quaternary calderas. Remote sensing and field analyses were used to investigate the regional structural control on three calderas (Fantale, Gariboldi, Gedemsa) in the axial part of the rift. These calderas are located along the Wonji Fault Belt (WFB), a zone of Quaternary NNE-SSW normal faults and extensional fractures. The three calderas show E-W elongation and major E-W vent alignments, oblique with regard to the mean NW-SE extension direction. No significant evidence of E-W tectonic structures has been found near the calderas, the only relevant systems being those of the WFB. Conversely, left-lateral E-W-trending faults are present at the rift borders and on the Nubia and Somalia plateaus, implying a predominant pre-rift activity. The E-W fractures were partly reactivated during rifting, possibly controlling the development of the magma chambers. Thus, the E-W elongation of the calderas would be the surface expression of such a control, rather than the result of regional extension. An evolutionary model on the role of different structures on magmatism at different crustal levels within the rift is proposed
- …
