Tectonics of the Afar triple junction from InSAR and GPS derived strain maps 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 and GPS derived strain maps and seismicity to understand the tectonics of the current Afar triple junction plate boundary zone. Our results show that that the plate spreading motion is accommodated in different ways in the Red Sea Rift after jumping southeastward along the Gulf of Aden Rift. At the Red Sea Rift, extension and shear are coupled with seismicity, occurring both along-rift but also in areas off-rift. In the Gulf of Aden Rift extension and normal faulting occur in the central parts of the rifts while at the rifts tips strike-slip earthquakes are observed. The extensional strains occur over a broad zone encompassing several overlapping rifts. Conversely the strike-slip earthquakes are focused along a narrow EW trending lineament. The pattern suggests that the recent history of magmatic intrusions in the Red Sea Rift still dominates the plate boundary deformation inducing earthquakes even in areas off-rift and with no previous faults mapped. On the other hand, in the Gulf of Aden Rift our strain and seismicity maps are consistent mainly with extensional tectonics occurring over an exceptionally broad zone (over 200 km). We interpret the strike-slip earthquakes observed at the rift tips as the result of shearing at the rifts tips where the extension terminates against continental lithosphere

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

Tectonic model of the Malaŵi Rift, Africa

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, 1986.MICROFICHE COPY AVAILABLE IN ARCHIVES AND LINDGREN.Includes bibliographies.by Cynthia Joan Ebinger.M.S

Seismic analysis of magmatism in the Galápagos Archipelago and East Africa

Thesis (Ph. D.)--University of Rochester. Dept. of Physics and Astronomy, 2016.Magmatism and deformation are consequences of fundamental processes shaping Earth’s ~150 km-thick continental and <125 km-thick oceanic plates. Earthquake seismology encompasses many methods to detect compositional and thermal boundaries from Earth’s surface to the dynamic mantle driving plate tectonics. This work uses three different seismic methods to probe magma migration and storage and tectonism in two intraplate hotspot provinces: the Galápagos and East Africa. First, seismic body-wave tomography is used to image magma within oceanic crust of the largest Galápagos volcano, Sierra Negra. A laterally large, low-velocity region with many smaller, high-magnitude velocity anomalies is imaged at 8-15.5 km depths. No sharp seismic velocity increase is imaged within the resolvable depths, indicating that the thickened crust is at least 16 km deep. The second study involves a spectral analysis of earthquakes induced by the intrusion of thin sheets of magma rising beneath the Afar rift, East Africa. Earthquakes have varying spectral content, some with unusually large amplitude low-frequency content and enhanced surface waves. The analysis showed no clear boundaries between spectral types, suggesting that they are all primarily the result of brittle failure. Deep dike segments (tops > 3 km) induce only high-frequency volcanotectonic earthquakes, while shallower dike segments induce the full range of spectral types. This suggests that low-frequency content is a result of shallow hypocenters, with path and site effects, surface ruptures, and dike fluid interactions all possible secondary causes. In the final study, shear-wave splitting analysis of teleseismic body-wave phases is conducted to evaluate strain and crack fabrics at the base of the continental plate as a consequence of magmatism, mantle flow, and plate stretching in the Western rift, East Africa. On average, fast directions are northeast, consistent with geodynamic models of mantle flow from the African superplume and passive rifting. In the northern study area, splitting directions become complex and rotate northwest. The variational splitting in this region is likely due to mantle flow complexities caused by encounters with deep cratonic roots. Complex flow at craton boundaries may have led to the formation of the magmatic Rungwe Volcanic Province within the largely amagmatic Western rift

Evolution of a volcanic rifted margin: Southern Red Sea, Ethiopia

The process of strain localization as rifting proceeds to continental breakup is readily observed along the Oligocene-Recent southern Red Sea rift, yet much of the Red Sea margin in Ethiopia remains unmapped. Rifting initiated above or near a mantle plume, which is marked by the Eo-Oligocene Ethiopia-Yemen flood basalt province. Objectives of this field, remote sensing, and geochronology study are to establish a structural and stratigraphic framework for the southernmost Red Sea passive margin using new and existing Ar-40/Ar-39 age data along 6 transects. We present new sketch geological maps and cross sections to document the timing of extension in relation to magmatism and its variation along strike. These new data are integrated with plate kinematic, geological, and geophysical data to present a model for evolution of the southern Red Sea margin. Faults commonly marked by eruptive centers initiated between 29 and 26 Ma, coincident with rifting in the Gulf of Aden. The Red Sea rift terminated at 10(circle)N until linkage of the Main Ethiopian rift and southern Red Sea occurred at ca. 11 Ma. Rifting progressed in three distinct stages; each new phase saw a marked change in the style of volcanism and a narrowing of the locus of extension. Stage 1 rhyolites were emplaced from 29 to 26 Ma in basins bounded by a steep border fault system. Between 25 and 20 Ma, strain localized to narrow zones of basaltic fissural eruptions and minor faulting. Stage 2 faults and eruptive centers are located similar to 50 km to the east of the border faults, and they comprise flows spanning at least 16-7 Ma. After ca. 7 Ma, the locus of strain again migrated eastward (Stage 3). Strain in Stage 3 was largely accommodated by dike injection. Plate reconstructions predict high stretching factors (beta similar to 3) in the southern Red Sea, suggesting that Stages 2 and 3 mark the onset of formation of crust transitional between oceanic and continental

Receiver function imaging of lithospheric structure and the onset of melting beneath the Galápagos Archipelago

The Galápagos Archipelago represents an opportunity to investigate the properties of young oceanic lithosphere, the effects of a hotspot anomaly on lithospheric thickness, and melting dynamics in a hotspot-ridge interaction. Here we use data recorded by the SIGNET array and permanent station PAYG on the Islands Santa Cruz and Isabela, respectively. We used P-to-S (Ps) and S-to-P (Sp) receiver functions to constrain crust and mantle structure. A simultaneous deconvolution method was used to constrain 1-D structure and also for the modeling of robust features. A migrated extended multitaper method was used to investigate 3-D structural variations. Ps images a velocity increase with depth at 11±7 km, probably the base of the pre-plume crust, or old Moho. Sp imaging and modeling images a second, deeper velocity increase at 37±7 km depth. A velocity decrease with depth is imaged on average at 75±12 km likely associated with the lithosphere–asthenosphere boundary. This discontinuity is imaged deeper, 82 km, in the southwest and shallower, 66 km, in the northeast near the spreading ridge. Although the trend is consistent with lithospheric thickening with age, the thickness is much larger than predicted by conductive cooling models of 5–10 My oceanic lithosphere. We infer a compositional contribution to velocity variations. Finally, a velocity increase with depth is imaged at ?125 to 145±15 km depth that is likely associated with the onset of melting. The discontinuity is imaged deeper in 3 sectors of the Galápagos platform-ridge region, all coincident with the slowest surface wave shear velocity anomalies in the upper 100 km. One is located in the southwest in a hypothesized plume location. The other two are to the northwest and northeast, possibly illuminating multiple plume diversions related to complex plume–ridge interactions

Dynamics of dike intrusions and 3D velocity structure beneath an incipient seafloor spreading center in Afar, Ethiopia

Thesis (Ph. D.)--University of Rochester. Dept. of Earth and Environmental Sciences, 2012.A rifting episode started in September 2005 with an intrusion of a 60 km-long mega-dike along the Dabbahu-Manda Hararo (DMH) rift segment in Afar, Ethiopia. Between 2005 and 2009 thirteen smaller volume dikes intruded different portions of the rift segment. Out of the 13 dikes, 9 were recorded on a temporary network of 44 three-component broadband stations. The dynamics of the dike intrusions are studied using the detailed analysis of the spatial and temporal distribution of dike-induced earthquakes and their source mechanisms. In addition, a 3D model of seismic velocity structure is determined using local earthquake travel time tomography algorithm. The dike-induced migration patterns of the earthquakes show the dikes were fed from a ~5 km-radius zone at the middle of the DMH segment, and traveled northward and southward along the rift axis. The dikes that propagated north of the mid-segment have higher propagation rates and short migration duration relative to the dikes that propagated south. Faulting and graben formation above the dikes occurs hours after the passage of the dike tip, coincident with the onset of low-frequency earthquakes, and accounts for the large percentage of seismic energy release during an intrusion. The large deficit between total seismic and geodetic moment estimates, and the similarity between total seismic slip and geodetic slip estimates on normal faults above the dikes indicates that dike inflation and most of plate boundary deformation occurs largely aseismically. Local earthquake travel time tomography reveals low velocity zones at depths >13 km beneath the Dabbahu volcanic complex, and a broad zone of low velocity beneath the mid-segment. These regions are interpreted to be the magma source zones at different stages of the rifting cycle along the DMH rift segment. However, the lack of migrating seismicity originating from the Dabbahu volcano suggest that only the magma source zone beneath the Ado'Ale Volcanic Complex is actively feeding the dikes. The DMH rift segment is at the magmatic stage of the tectono-magmatic cycle proposed for slow spreading ridges, where magma intrusion accommodates most of the plate boundary deformation, and tectonic forces are less important