Abstract The Ethiopia‐Yemen flood basalts are spatially zoned with progressively lower TiO 2 lavas from near the Afar depression toward the margins. The timing and rate of emplacement of low TiO 2 (LT) lavas are poorly known compared with the ultra‐high TiO 2 (HT2) lavas. We measured two high‐precision 40 Ar/ 39 Ar ages of 29.63 ± 0.14 and 30.02 ± 0.22 Ma (2σ) from basalts of the 2‐km‐thick LT lava sequence at the Afar plume head margin. Using our eruption age model constructed from our and previous 40 Ar/ 39 Ar ages with the paleomagnetic directions, we estimate that the LT lava eruption continued over Chrons C12r‐C12n‐C11r. The eruption of the plume head margin started earlier than the plume head axis emplacement in C12n. Also, the eruption rate was low at the margin, high at the axis. We estimate that the LT lavas are induced by the edge‐driven convection, the result of a plume‐lithosphere interaction, not a plume head.
Maximum entropy regularization of the geomagnetic core field inverse problem M. A. Lewis and Y. Ben-Zion: Examination of scaling between proposed early signals in P waveforms and
Abstract In this study, we draw on a unique combination of well‐resolved fault‐slip data and earthquake focal mechanisms to constrain spatial variations in style of faulting in the obliquely extending Main Ethiopian Rift, East Africa. These data show that both boundary and internal faults – oblique and orthogonal to the plate divergence (PD) respectively – exhibit almost pure dip‐slip motion, and indicate significant local deflection in orientation of the extension direction at rift margins. Scaled analogue models closely replicate the multidisciplinary observations from the rift and suggest that the process is controlled by the presence of a deep‐seated, pre‐existing weakness – oblique to the direction of PD – that is able to cause a local rotation in the orientation of the extension direction at rift margins. Minor counterclockwise block rotations are required to accommodate the difference in slip direction along the different fault systems, as supported by existing and new palaeomagnetic data from the rift.
A new detailed palaeomagnetic study of Tertiary volcanics, including extensive K‐Ar and 40Ar/39Ar dating, helps constrain the deformation mechanisms related to the opening processes of the Afar depression (Ethiopia and Djibouti). Much of the Afar depression is bounded by 30 Myr old flood basalts and floored by the ca 2 Myr old Stratoid basalts, and evidence for pre‐2 Ma deformation processes is accessible only on its borders. K‐Ar and 40Ar/39Ar dating of several mineral phases from rhyolitic samples from the Ali Sabieh block shows indistinguishable ages around 20 Myr. These ages can be linked to separation of this block in relation to continental breakup. Different amounts of rotation are found to the north and south of the Holhol fault zone, which cuts across the northern part of the Ali Sabieh block. The southern domain did not record any rotation for the last 8 Myr, whereas the northern domain experienced approximately 12 ± 9° of clockwise rotation. We propose to link this rotation to the counter‐clockwise rotation observed in the Danakil block since 7 Ma. This provides new constraints on the early phases of rifting and opening of the southern Afar depression in connection with the propagation of the Aden ridge. A kinematic model of propagation and transfer of extension within southern Afar is proposed, with particular emphasis on the previously poorly‐known period from 10 to 4 Ma.
The Afar Depression is a unique place on Earth where active rift processes can be directly observed. It is believed to be close to continental breakup. The Afar hotspot has a strong influence on the geology of the Depression. Despite the strong geological interest in the region, difficult field access slowed scientific discoveries. During the last two decades, new projects and studies resulted in a better characterization of the region. New field data and global advances in understanding rift processes call for an integrative and holistic review of the tectonostratigraphic evolution of the Afar Depression. This study compiles new geological maps and reviews the stratigraphy and the geological history of the Afar Depression and the Afro-Arabian Rift System. A new kinematic evolution model and integrative paleogeological maps are proposed. Results show that geological events are diachronous throughout the region. We consider the Afar Rift to be distinct from the Red Sea Rift, both being separated by the Arrata Microplate. The Afar Rift is propagating northwards and forms a relay structure with the Red Sea Rift, linked to the counter-clockwise rotation of the Danakil Block since the Mid- to Late Miocene. The Afar Depression can be segmented into two distinct domains, Central Afar and the Danakil Depression. Central Afar experienced significant extension, protracted and extensive magmatism and magma-compensated thinning. It is believed to be strongly influenced by the Afar hotspot. In comparison, the Danakil Depression is younger and went through less extension and less magmatic activity until Recent (~0.6 Ma) times. The absence of magma-compensated thinning allowed the development of an evolved stage of continental breakup. The tectonostratigraphic evolution of the Afar Depression with distinct rifting styles shows the complexity of continental break-up.
We investigate the relationship between rift propagation and volcanism in the Afar Depression in the last 4 Myr. Potassium‐argon and thermoluminescence dating allow detailed reconstruction of the temporal evolution of volcanism. Volcanic activity is almost continuous since 3.5 Ma, with intervals characterized by more intense activity, especially around 2 Ma. Spatial distribution of ages reveals that Stratoid Series volcanism migrated northward along a 200‐km trend between 3 and 1 Ma, at about 10 cm/yr, linked to northward propagation of the Gulf of Aden Ridge, after it had cut across the Danakil horst at 4 Ma. Our work underlines the role of rhyolitic volcanism in initiation of rifting. Acid volcanoes, initially formed near the axes of extensional zones, have been subsequently dissected and are presently located on both sides of active rift segments. These lavas were the first to be erupted in areas of low extensional strain and were followed by basaltic lavas as extension increased. Differentiated volcanoes acted as zones of local weakness and guided localization of fractures, then leading to fissural magmatism. This regional‐scale, composite style of rifting, including volcanic and tectonic components, can be compared to the large‐scale continental breakup process itself. Deformation occurs through propagation of faults and fissures under a regional stress field. These become localized because of weakening of the crust (or lithosphere) due to emplacement of magmas, under the influence of a plume in the large‐scale case, or of silicic centers linked to magma chambers in the regional‐scale case.
'%3e%3ccircle r='120' fill='%230f47af'/%3e%3cg id='a'%3e%3cpath fill='%23fcdd09' d='m0-96-4.206 12.944 17.348 53.39h-23.13l-2.599 8h74.163l11.011-8H21.553z'/%3e%3cpath stroke='%23fcdd09' stroke-width='4' d='m25.863-35.597 30.564-42.069'/%3e%3c/g%3e%3cuse xlink:href='%23a' transform='rotate(72)'/%3e%3cuse xlink:href='%23a' transform='rotate(144)'/%3e%3cuse xlink:href='%23a' transform='rotate(216)'/%3e%3cuse xlink:href='%23a' transform='rotate(288)'/%3e%3c/g%3e%3c/svg%3e)
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)