Lebenszyklus des Cabo-Verde-Archipels: Wachstum und Denudation von Seebergen und Inseln

The Cabo Verde Archipelago in the Atlantic Ocean exhibits seamounts and islands in different stages of evolution. Mature islands are well-eroded, exposing early stages of volcanism. However, little is known about the submarine realm of the archipelago. To gain a better understanding of the geological and geodynamic evolution of the archipelago, data and observations from seamounts and islands were combined and different phases of magmatic-tectonic evolution investigated. The thesis provides the first geochronological data for the Cabo Verde Seamounts. A pillow lava from the extinct Nova Holanda (Senghor) Seamount in the NE yields a 40Ar-39Ar weighted mean age of 14.872 ± 0.027 Ma (2sigma). Lava samples from the phonolitic Cadamosto Seamount (SW) were 40Ar-39Ar dated, revealing young eruption ages with a combined weighted mean age of 21.14 ± 0.62 ka (2sigma). This volcanic eruption period can be linked to rapid crustal unloading due to sea level lowstands in the Last Glacial Maximum. Combined with petrological observations a long-lived magmatic plumbing system below Cadamosto is revealed. The evolution of the mature Maio Island was investigated. Uplifted mid-ocean ridge basalts exposed on Maio and overlying deep-sea sedimentary sequences contain Early Cretaceous fossil assemblages and record information on the young Atlantic Ocean. Based on field observations, 40Ar-39Ar geochronology and structural geology the Miocene evolution of Maio was refined, confirming a period of intense igneous growth between ~16 and 8.7 Ma. Extensive polymict conglomerates below Late Miocene lavas are re-interpreted as landslide deposits, confining an erosional period to between 8.7 to 6.7 Ma. A Pleistocene large-scale fossil dune located in the E of Maio records recent uplift and changes in the paleo-environmental conditions. The thesis provides important insights to different phases of the magmatic-tectonic evolution of the archipelago and ocean island evolution worldwide

3-D analysis and interpretation of magnetotelluric data from the Aluto-Langano geothermal field, Ethiopia

The Main Ethiopian Rift Valley encompasses a number of volcanoes, which are known to be actively deforming with reoccurring periods of uplift and setting. One of the regions where temporal changes take place is the Aluto volcanic complex. It hosts a productive geothermal field and the only currently operating geothermal power plant of Ethiopia. We carried out magnetotelluric (MT) measurements in early 2012 in order to identify the source of unrest. Broad-band MT data (0.001-1000s) have been acquired at 46 sites covering the expanse of the Aluto volcanic complex with an average site spacing of 1km. Based on this MT data it is possible to map the bulk electrical resistivity of the subsurface down to depths of several kilometres. Resistivity is a crucial geophysical parameter in geothermal exploration as hydrothermal and magmatic reservoirs are typically related to low resistive zones, which can be easily sensed by MT. Thus by mapping the electrical conductivity one can identify and analyse geothermal systems with respect to their temperature, extent and potential for production of energy. 3-D inversions of the observed MT data from Aluto reveal the typical electrical conductivity distribution of a high-enthalpy geothermal system, which is mainly governed by the hydrothermal alteration mineralogy. The recovered 3-D conductivity models provide no evidence for an active deep magmatic system under Aluto. Forward modelling of the tippers rather suggest that occurrence of melt is predominantly at lower crustal depths along an off-axis fault zone a few tens of kilometres west of the central rift axis. The absence of an active magmatic system implies that the deforming source is most likely situated within the shallow hydrothermal system of the Aluto-Langano geothermal fiel

Geomorphology and age constraints of seamounts in the Cabo Verde Archipelago, and their relationship to island ages and geodynamic evolution

The Cabo Verde Archipelago is related to a mantle plume located close to the rotational pole of the African Plate. It consists of islands and seamounts arranged in a horseshoe‐shaped pattern open to the west, thus forming two volcanic chains, each with a weak east‐west age progression. High‐resolution swath bathymetry of 12 Cabo Verde seamounts is used here to assign each seamount to its pre‐shield, shield or post‐shield evolutionary stage, respectively. The eastern seamounts exhibit degraded and partially eroded morphologies, and are mainly in their post‐shield stage. A new 40Ar‐39Ar date for Senghor Seamount at 14.872 ± 0.027 Ma supports old ages for the eastern seamounts. The western seamounts generally exhibit younger volcanic‐edifice‐construction morphologies, showing fresh effusive and explosive volcanics, including rarely observed deep‐water explosive volcanism in the Charles Darwin Volcanic Field. Furthermore, the two previously unknown seamounts Sodade and Tavares in the westernmost termini of both volcanic chains exhibit pristine volcanic morphologies, in agreement with present‐day volcanism and seismic activity recorded from the western seamounts. The islands and seamounts rest on three submarine platforms to the east, northwest and southwest, respectively. Taken together, the seamount and island data suggest a shift in igneous activity from the eastern to the other platforms at about 8–6 Ma. However, the complex evolution pattern for both volcanic chains includes the simultaneous occurrence of pre‐shield or shield edifices at any time, followed by erosional and rejuvenation stages. The new seamount data still demonstrate ongoing westward submarine‐growth in both volcanic chains

Causes of unrest at silicic calderas in the East African Rift: new constraints from InSAR and soil-gas chemistry at Aluto volcano, Ethiopia

This work is a contribution to the Natural Environment Research Council (NERC) funded RiftVolc project (NE/L013932/1, Rift volcanism: past, present, and future). W.H., J.B., T.A.M., and D.M.P. are supported by and contribute to the NERC Centre for the Observation and Modelling of Earthquakes, Volcanoes, and Tectonics (COMET). Envisat data were provided by ESA. ALOS data were provided through ESA third party mission. W.H. funded by NERC studentship, NE/J5000045/1. Additional funding for fieldwork was provided by University College (University of Oxford), the Geological Remote Sensing Group, the Edinburgh Geological Society, and the Leverhulme Trust. Analytical work at the University of New Mexico was supported by the Volcanic and Geothermal Volatiles Lab at the Center for Stable Isotopes and an NSF grant EAR-1113066 to T.P.F.Restless silicic calderas present major geological hazards, and yet many also host significant untapped geothermal resources. In East Africa this poses a major challenge, although the calderas are largely unmonitored their geothermal resources could provide substantial economic benefits to the region. Understanding what causes unrest at these volcanoes is vital for weighing up the opportunities against the potential risks. Here we bring together new field and remote sensing observations to evaluate causes of ground deformation at Aluto, a restless silicic volcano located in the Main Ethiopian Rift (MER). Interferometric Synthetic Aperture Radar (InSAR) data reveal the temporal and spatial characteristics of a ground deformation episode that took place between 2008 and 2010. Deformation time-series reveal pulses of accelerating uplift that transition to gradual long-term subsidence, and analytical models support inflation source depths of ∼5 km. Gases escaping along the major fault zone of Aluto show high CO2 flux, and a clear magmatic carbon signature (CO2–δ13C of −4.2 to −4.5 ‰). This provides compelling evidence that the magmatic and hydrothermal reservoirs of the complex are physically connected. We suggest that a coupled magmatic-hydrothermal system can explain the uplift-subsidence signals. We hypothesize that magmatic fluid injection and/or intrusion in the cap of the magmatic reservoir drives edifice wide inflation while subsequent deflation is related to magmatic degassing and depressurization of the hydrothermal system. These new constraints on the plumbing of Aluto yield important insights into the behaviour of rift volcanic systems and will be crucial for interpreting future patterns of unrest.Publisher PDFPeer reviewe