Thermal and mechanical development of the East African Rift System

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1988The deep basins, uplifted flanks, and volcanoes of the Western and Kenya rift systems have developed along the western and eastern margins of the 1300 km-wide East African plateau. Structural patterns deduced from field, Landsat, and geophysical studies in the Western rift reveal a series of asymmetric basins bounded by approximately 100 km-long segments of the border fault system. These basins are linked by oblique-slip and strike-slip faults cross-cutting the rift valley. Faults bounding the Kenya and Western rift valleys delineate two north-south-trending, 40-75 km wide zones of crustal extension, and little or no crustal thinning has occurred beneath the uplifted flanks or the central plateau. In the Western rift, volcanism in Late Miocene time began prior to or concurrent with basinal subsidence, followed by rift flank uplift. Individual extensional basins developed diachronously, and basinal propagation may give rise to the along-axis segmentation of the rift valley. The coherence between gravity and topography data indicates that the mechanical lithosphere beneath the two rift valleys has been weakened relative to the central plateau and adjacent cratonic regions. Gravity and topography data at wavelengths corresponding to the overcompensated East African plateau can be explained by density variations within the upper mantle that are dynamically maintained

Upper mantle anisotropy of Southeast Arabia passive margin [Gulf of aden northern conjugate margin], Oman

International audienceIn this study, we used data recorded by two consecutive passive broadband deployments on the Gulf of Aden northern margin, Dhofar region, Sultanate of Oman. The objective of these deployments is to map the young eastern Gulf of Aden passive continental margin crust and upper mantle structure and rheology. In this study, we use shear-wave splitting analysis to map lateral variations of upper mantle anisotropy beneath the study area. In this study, we found splitting magnitudes to vary between 0.33 and 1.0 s delay times, averaging about 0.6 s for a total of 17 stations from both deployment periods. Results show distinct abrupt lateral anisotropy variation along the study area. Three anisotropy zones are identified: a western zone dominated by NW-SE anisotropy orientations, an eastern zone dominat- ed with NE-SW anisotropy orientations, and central zone with mixed anisotropy orientations similar to the east and west zones. We interpret these shorter wavelength anisotropy zones to possibly represent fossil lithospheric mantle anisotropy. We postulate that the central anisotropy zone may be representing a Proterozoic suture zone that separates two terranes to the east and west of it. The anisotropy zones west and east were being used indicative of different terranes with different upper mantle anisotropy signatures

Formation and stability of magmatic segments in the Main Ethiopian and Afar rifts

As rifting progresses to seafloor spreading, extension within the continental crust commonly is accommodated by a combination of fault slip and dike intrusion. Consistent patterns in the spatial arrangement of long-lived magma intrusion zones in the Ethiopian and Afar rift sectors, East Africa, suggest that the magma intrusions help to localize strain during repeated rifting episodes. Within the broad Main Ethiopian Rift, extensional deformation has localized since 3 m.y. in narrow magmatic segments, that are oriented oblique to the orientation of the Miocene border faults, but (sub-) orthogonal to the extension direction. Numerical models combined with geophysical and geological observations from East Africa are used to examine the viability of self-sustaining magmatic segmentation. Initiation of the magmatic segments is shown to result from magma injections, which focus strain in narrow elongated zones. During magmatic phases of segment evolution the segments are weak, and extensional stresses localize at the rift tips, promoting along-axis lengthening. During amagmatic phases of extension, the numerical models predict strain localization within the magmatic segments and, to a lesser extent, broadly distributed extension within the rift zone. This promotes segment stability; the segments remain the preferred location for magma intrusion during new magmatic phases. These results are applied to the formation and maintenance of MER segmentation. The Fentale–Dofen segment is currently in a non-magmatic phase of extension; the Dabbahu segment in the Red Sea Rift is currently experiencing a rifting episode and therefore is in a transient magmatic cycle. The observed patterns of instantaneous localized deformation, seismicity, and dike intrusions sometimes propagating beyond the tip of the magmatic segments occur as predicted by the models

Evidence for focused magmatic accretion at segment centers from lateral dike injections captured beneath the Red Sea rift in Afar

Continental breakup occurs through repeated episodes of mechanical stretching and dike injection within discrete, narrow rift segments. However, the time and length scales of the dike intrusions, along with the source regions of melt within continental and oceanic rifts, are poorly constrained. We present measurements of spatial and temporal variability in deformation from the currently active 60-km-long Dabbahu segment of the Red Sea rift in Afar, using satellite radar, global positioning system, and seismicity data sets, that capture emplacement of two ~10-km-long, ~1–2-m-wide dike intrusions in June and July 2006. Our observations show that the majority of strain is accommodated by dikes that propagate laterally over ~4–5 h time scales along the rift axis and are sourced from a reservoir in the middle to lower crust, or upper mantle, beneath the center of the rift segment. New intrusions during the ongoing rifting episode in Afar show that the injection of lateral dikes fed from magma reservoirs beneath rift segment centers is a key component in creating and maintaining regular along-axis rift segmentation during the final stages of continental breakup. Our observations also provide evidence that the focused magmatic accretion at segment centers observed in slow-spreading mid-ocean ridges occurs prior to the onset of seafloor spreading

ICDP workshop on the Lake Tanganyika Scientific Drilling Project: A late Miocene-present record of climate, rifting, and ecosystem evolution from the world\u27s oldest tropical lake

The Neogene and Quaternary are characterized by enormous changes in global climate and environments, including global cooling and the establishment of northern high-latitude glaciers. These changes reshaped global ecosystems, including the emergence of tropical dry forests and savannahs that are found in Africa today, which in turn may have influenced the evolution of humans and their ancestors. However, despite decades of research we lack long, continuous, well-resolved records of tropical climate, ecosystem changes, and surface processes necessary to understand their interactions and influences on evolutionary processes. Lake Tanganyika, Africa, contains the most continuous, long continental climate record from the mid-Miocene (∼ 10 Ma) to the present anywhere in the tropics and has long been recognized as a top-priority site for scientific drilling. The lake is surrounded by the Miombo woodlands, part of the largest dry tropical biome on Earth. Lake Tanganyika also harbors incredibly diverse endemic biota and an entirely unexplored deep microbial biosphere, and it provides textbook examples of rift segmentation, fault behavior, and associated surface processes. To evaluate the interdisciplinary scientific opportunities that an ICDP drilling program at Lake Tanganyika could offer, more than 70 scientists representing 12 countries and a variety of scientific disciplines met in Dar es Salaam, Tanzania, in June 2019. The team developed key research objectives in basin evolution, source-to-sink sedimentology, organismal evolution, geomicrobiology, paleoclimatology, paleolimnology, terrestrial paleoecology, paleoanthropology, and geochronology to be addressed through scientific drilling on Lake Tanganyika. They also identified drilling targets and strategies, logistical challenges, and education and capacity building programs to be carried out through the project. Participants concluded that a drilling program at Lake Tanganyika would produce the first continuous Miocene-present record from the tropics, transforming our understanding of global environmental change, the environmental context of human origins in Africa, and providing a detailed window into the dynamics, tempo and mode of biological diversification and adaptive radiations

ICDP workshop on the Lake Tanganyika Scientific Drilling Project: a late Miocene–present record of climate, rifting, and ecosystem evolution from the world's oldest tropical lake

The Neogene and Quaternary are characterized by enormous changes in global climate and environments, including global cooling and the establishment of northern high-latitude glaciers. These changes reshaped global ecosystems, including the emergence of tropical dry forests and savannahs that are found in Africa today, which in turn may have influenced the evolution of humans and their ancestors. However, despite decades of research we lack long, continuous, well-resolved records of tropical climate, ecosystem changes, and surface processes necessary to understand their interactions and influences on evolutionary processes. Lake Tanganyika, Africa, contains the most continuous, long continental climate record from the mid-Miocene (∼ 10 Ma) to the present anywhere in the tropics and has long been recognized as a top-priority site for scientific drilling. The lake is surrounded by the Miombo woodlands, part of the largest dry tropical biome on Earth. Lake Tanganyika also harbors incredibly diverse endemic biota and an entirely unexplored deep microbial biosphere, and it provides textbook examples of rift segmentation, fault behavior, and associated surface processes. To evaluate the interdisciplinary scientific opportunities that an ICDP drilling program at Lake Tanganyika could offer, more than 70 scientists representing 12 countries and a variety of scientific disciplines met in Dar es Salaam, Tanzania, in June 2019. The team developed key research objectives in basin evolution, source-to-sink sedimentology, organismal evolution, geomicrobiology, paleoclimatology, paleolimnology, terrestrial paleoecology, paleoanthropology, and geochronology to be addressed through scientific drilling on Lake Tanganyika. They also identified drilling targets and strategies, logistical challenges, and education and capacity building programs to be carried out through the project. Participants concluded that a drilling program at Lake Tanganyika would produce the first continuous Miocene-present record from the tropics, transforming our understanding of global environmental change, the environmental context of human origins in Africa, and providing a detailed window into the dynamics, tempo and mode of biological diversification and adaptive radiations.</p

Postvaccination Symptoms After a Fourth Dose of mRNA SARS-CoV-2 Vaccination in Patients With Inflammatory Bowel Disease

Overall, 1933 participants with IBD completed at least 1 questionnaire regarding postvaccination symptoms after administration of each of the 4 mRNA SARS-CoV-2 vaccine doses. Symptoms after a fourth mRNA vaccine are less frequent and generally milder than after previous doses

Geophysical constraints on the dynamics of spreading centres from rifting episodes on land

Most of the Earth's crust is created along 60,000 km of mid-ocean ridge system. Here, tectonic plates spread apart and, in doing so, gradually build up stress. This stress is released during rifting episodes, when bursts of magmatic activity lead to the injection of vertical sheets of magma — termed dykes — into the crust. Only 2% of the global mid-ocean ridge system is above sea level, so making direct observations of the rifting process is difficult. However, geodetic and seismic observations exist from spreading centres in Afar (East Africa) and Iceland that are exposed at the land surface. Rifting episodes are rare, but the few that have been well observed at these sites have operated with remarkably similar mechanisms. Specifically, magma is supplied to the crust in an intermittent manner, and is stored at multiple positions and depths. It then laterally intrudes in dykes within the brittle upper crust. Depending on the availability of magma, multiple magma centres can interact during one rifting episode. If we are to forecast large eruptions at spreading centres, rifting-cycle models will need to fully incorporate realistic crust and mantle properties, as well as the dynamic transport of magma