Caldera resurgence during the 2018 eruption of Sierra Negra volcano, Galápagos Islands.

Recent large basaltic eruptions began after only minor surface uplift and seismicity, and resulted in caldera subsidence. In contrast, some eruptions at Galápagos Island volcanoes are preceded by prolonged, large amplitude uplift and elevated seismicity. These systems also display long-term intra-caldera uplift, or resurgence. However, a scarcity of observations has obscured the mechanisms underpinning such behaviour. Here we combine a unique multiparametric dataset to show how the 2018 eruption of Sierra Negra contributed to caldera resurgence. Magma supply to a shallow reservoir drove 6.5 m of pre-eruptive uplift and seismicity over thirteen years, including an Mw5.4 earthquake that triggered the eruption. Although co-eruptive magma withdrawal resulted in 8.5 m of subsidence, net uplift of the inner-caldera on a trapdoor fault resulted in 1.5 m of permanent resurgence. These observations reveal the importance of intra-caldera faulting in affecting resurgence, and the mechanisms of eruption in the absence of well-developed rift systems

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.© Author(s) 2020. This open access article is distributed under the Creative Commons Attribution 4.0 License

The Afar triple junction accommodation zone from InSAR derived strain and seismicity

Strain and seismicity show us the mode by which deformation is accommodated in rifting continents. Here we present a combined analysis of InSAR derived strain maps and seismicity of the Afar triple junction from 2006 to 2010. Our analysis shows that that the plate spreading motion is accommodated in different modes. A dogbone-shaped seismicity and strain distribution dominates the northern Red Sea branch of the triple junction, likely as a result of repeated dike intrusions 2005-2010. East of the triple junction, in the Gulf of Aden branch the strain and seismicity distribution appears decoupled. The strain focuses across the central part of several overlapping rifts, while the seismicity mainly occurs at the rift tips. Conversely, the Main Ethiopian Rift branch shows a narrow and elongated zone of both high strain and seismicity. The pattern suggests that the recent history of magmatic intrusions in the northern branch and mainly tectonic extension in the other branches creates a diverse triple junction accommodation zone

All scales must be considered to understand rifts

SCOPUS: no.jinfo:eu-repo/semantics/publishe

The origin of strike-slip tectonics in continental rifts

Although continental rifts are zones of lithospheric extension, strike-slip tectonics is also accommodated within rifts and its origin remains controversial. Here we present a combined analysis of recent seismicity, InSAR and GPS derived strain maps to reveal that the plate motion in Afar is accommodated primarily by extensional tectonics in all rift arms and lacks evidences of regional scale bookshelf tectonics. However in the rifts of central Afar we identify crustal extension and normal faulting in the central part of the rifts but strike-slip earthquakes at the rift tips. We investigate if strike-slip can be the result of Coulomb stress changes induced by recent dyking but models do not explain these earthquakes. Instead we explain strike-slips as shearing at the tips of a broad zone of spreading where extension terminates against unstretched lithosphere. Our results demonstrate that plate spreading can develop both strike-slip and extensional tectonics in the same rifts

Crustal structure at a young continental rift: a receiver function study from the Tanganyika Rift

The southern Tanganyika Rift, within the Western rift, Africa, has earthquakes to depths of 37 km, yet few constraints exist on crustal thickness, or of early stage rifting processes in apparently amagmatic rift sectors. The aim of the TANGA14 experiment was to constrain bulk crustal properties to test whether magmatic processes modify the lithosphere in areas of deep seismicity, and the degree of lithospheric thinning. We use eleven broadband seismometers to implement receiver function analysis using H-κ stacking, a method sensitive to crustal thickness and VP/VS ratio, to determine bulk crustal properties. Analyses include extensive error analysis through bootstrap, variance and phase-weighted stacking. Results show the Archean Tanzanian Craton and Bangweulu Block are characterized by VP/VS ratios of 1.75-1.77, implying a felsic bulk composition. Crust beneath the fault bounded basins has high VP/VS (>1.9). Anorthosite bodies and surface sediments within the region may contribute to localized high VP/VS. However, elevated VP/VS values within fault-bounded extensional basins where elevated heat flow, hydrothermal vent sites, and deep earthquakes are observed suggest that magma may be intruding the lower crust beneath the southern Tanganyika Rift. Crustal thicknesses on/near the relatively un-extended Tanzanian craton and Bangweulu Block are 41.6-42.0 km. This contrasts with the Tanganyika Rift where crustal thicknesses are 31.6 km to 39 km from north to south. Our results provide evidence for ~20% crustal thinning localized to fault-bounded basins. Taken together, they suggest a previously unrecognized role of magma intrusion in early-stage continental rifting in the Western rift, Africa

State of stress and stress rotations: Quantifying the role of surface topography and subsurface density contrasts in magmatic rift zones (Eastern Rift, Africa)

In rift settings, the crustal stress field is dominated by extension, which leads to rift-parallel topography and basin alignments. However in some continental rift systems, some observables of the orientation of principal stresses show substantial deviations from these patterns. Such stress field rotations are currently poorly understood and could reflect the critical role of rift magmatism in the creation of topography, the plate state-of-stress, and volcanic and tectonic processes. Yet the role of magma intrusions, crustal thinning, and rift basin and flank topography on rift zone stress field rotations remain poorly quantified. The seismically- and volcanically-active Magadi-Natron-Manyara region of the East African Rift shows a 60 degrees local stress field rotation with respect to regional extension. Here, we test the hypothesis that such rotation is due to the cumulative effects of surface and subsurface loads (lateral subsurface density contrasts). We use analytical and calibrated numerical models of magmatic rift zones to simulate lithospheric deformation in the presence of magma bodies, crustal thinning, and topography to quantify their effect on intrusions and fault kinematics in a rift setting. Our 3D static models of a weakly extended rift suggest that surface topography influences shallow stress localization, whereas subsurface density contrasts play a larger role in lower crustal stress localization. Both patterns suggest a preferred region for melt storage beneath the rift valley. We show that the interaction between topography, crustal thinning, extension, and a pressurized magma reservoir could generate principal stress orientations consistent with the local stress rotation observed from earthquake focal mechanisms. Our results demonstrate how rift topography and the geometry of crustal thinning can guide magmatism and strain localization, highlighting the need for a three-dimensional treatment of rift kinematics

Low-Frequency hybrid earthquakes near a magma chamber in Afar: quantifying path effects

Areas of active volcanism contain elaborate velocity structures that complicate interpretations of earthquake source mechanisms. We examine the spectral characteristics of 805 earthquakes that immediately followed a large volume basaltic dike intrusion and associated silicic flank eruption of Dabbahu volcano in the Afar Depression as recorded on near-source seismometers. We use these results to quantify the contribution of scattering and attenuation to the observed spectra of low-frequency hybrid and volcano-tectonic earthquake clusters from beneath Dabbahu volcano and around the dike zone. We find strong variations in the signal amplitude and frequency content of earthquakes recorded at stations separated by as little as 2 km, caused by preferential attenuation of high frequencies depending on the vantage point. These observations imply that there are large impedance contrasts near the cooling, solidifying, and recently intruded dike. We estimate the intrinsic absorption attenuation coefficient, QI, and inverse scattering length, g0, averaged over a 300-sq-km area beneath Dabbahu. Our results are consistent with the highest attenuation coefficients from studies of volcanic provinces in Italy (QI-1 ? 0.02, g0?0.1 km-1 for a signal at 2 Hz). The magnitude of these two parameters indicates there are large impedance contrasts present in the area due to the recent intrusion of magma and associated fracturing