3-D Magnetotelluric Image of Offshore Magmatism at the Walvis Ridge and Rift Basin

Highlights • We report on marine 3D Magnetotelluric study on Walvis Ridge • Derived 3D electrical resistivity model shows a large scale resistive zone, which we link to crustal extension due to local uplift. It might indicate the location where the hot-spot impinged on the crust prior to rifting • Smaller scale resistive region is attributed to magma ascent during rifting • Rift basin is identified by low resistivity region The Namibian continental margin marks the starting point of the Tristan da Cunha hotspot trail, the Walvis Ridge. This section of the volcanic southwestern African margin is therefore ideal to study the interaction of hotspot volcanism and rifting, which occurred in the late Jurassic/early Cretaceous. Offshore magnetotelluric data image electromagnetically the landfall of Walvis Ridge. Two large-scale high resistivity anomalies in the 3-D resistivity model indicate old magmatic intrusions related to hot-spot volcanism and rifting. The large-scale resistivity anomalies correlate with seismically identified lower crustal high velocity anomalies attributed to magmatic underplating along 2-D offshore seismic profiles. One of the high resistivity anomalies (above 500 Ωm) has three arms of approximately 100 km width and 300 km to 400 km length at 120 degree angles in the lower crust. One of the arms stretches underneath Walvis Ridge. The shape is suggestive of crustal extension due to local uplift. It might indicate the location where the hot-spot impinged on the crust prior to rifting. A second, smaller anomaly of 50 km width underneath the continent ocean boundary may be attributed to magma ascent during rifting. We attribute a low resistivity anomaly east of the continent ocean boundary and south of Walvis Ridge to the presence of a rift basin that formed prior to the rifting

Deep weathering in the semi-arid Coastal Cordillera, Chile

The weathering front is the boundary beneath Earth’s surface where pristine rock is converted into weathered rock. It is the base of the “critical zone”, in which the lithosphere, biosphere, and atmosphere interact. Typically, this front is located no more than 20 m deep in granitoid rock in humid climate zones. Its depth and the degree of rock weathering are commonly linked to oxygen transport and fluid flow. By drilling into fractured igneous rock in the semi-arid climate zone of the Coastal Cordillera in Chile we found multiple weathering fronts of which the deepest is 76 m beneath the surface. Rock is weathered to varying degrees, contains core stones, and strongly altered zones featuring intensive iron oxidation and high porosity. Geophysical borehole measurements and chemical weathering indicators reveal more intense weathering where fracturing is extensive, and porosity is higher than in bedrock. Only the top 10 m feature a continuous weathering gradient towards the surface. We suggest that tectonic preconditioning by fracturing provided transport pathways for oxygen to greater depths, inducing porosity by oxidation. Porosity was preserved throughout the weathering process, as secondary minerals were barely formed due to the low fluid flow

A new method to image crustal conductivity structures using magnetotelluric data from Namibia

Titel, Inhalts-, Abbildungs- und Tabellenverzeichnis, Nomenklatur 1\. Einführung und Motivation 1 2\. Zum Verständnis komplizierter Leitfähigkeitsverteilungen 5 2.1. Elektromagnetische Tiefensondierung 5 2.2. Eigenschaften des Impedanztensors 8 3\. Geologische Einordnung des Messgebiets 14 3.1. Die Damara Orogenese 15 3.2. Die Öffnung des Südatlantik 17 3.3. Leitfähigkeitsverteilungen im Süden Afrikas 17 4\. 3D Effekte in den Daten aus Namibia 20 4.1. Datengewinnung 20 4.2. Processing der MT Daten 23 4.3. Besonderheiten der Übertragungsfunktionen aus Namibia 25 4.4. Dimensionalitätsuntersuchung des Messgebiets 30 4.5. Rotationswinkel und Tensordekomposition 32 4.6. Die magnetischen Übertragungsfunktionen 34 4.7. Vergleich der Namibia-Daten mit bekannten Datensätzen 37 5\. Abbildungsmethoden 40 5.1. Eggers' Eigenstate Analysis 42 5.2. LaTorraca Singular Value Decomposition 44 5.3. Propagation Number Analysis (PNA) 46 5.4. Vergleich der Abbildungsmethoden 54 5.5. Verwendung von Invarianten 57 6\. Anwendung der Abbildungsmethoden 58 6.1. Die Abbildung eines 2D Untergrundes durch die Ellipsen 58 6.2. Anwendung der Darstellungsmethoden auf die Daten aus Namibia 64 6.3. Diskussion 73 7\. Modellierung der 3D Effekte 75 7.1. Die Ringstruktur im Norden des Messgebiets 76 7.2. Modellierung der 3D Effekte im zentralen Bereich 88 7.3. Die Leitfähigkeitsverteilung südlich der WF/OL 94 7.4. Diskussion 96 8\. Zusammenfassung 99 9\. Summary 103 A. Anhang 107 A.1. Hat der scheinbare spezifische Widerstand Tensoreigenschaften? 107 A.2. Ist die Ausbreitungszahl ein Tensor? 108 A.3. Gesteine sedimentären Ursprungs 110 A.4. Aufspaltung einer Leitfähigkeitsverteilung durch die PNA 111 A.5. Übertragungsfunktionen 114 Literaturverzeichnis 124 Danksagung 130 Lebenslauf 132Im Rahmen eines geowissenschaftlichen Projekts in Namibia wurden Magnetotellurikdaten an 107 Stationen in einem breiten Periodenbereich von 0 .001s-1000s aufgezeichnet. In der vorliegenden Arbeit konzentriere ich mich auf 60 Stationen in der Umgebung der Waterberg Fault / Omaruru Lineament (WF/OL), einer großen tektono-stratigraphischen Zonengrenze im Damara Gürtel in Namibia. Der Großteil der Stationen verteilt sich auf zwei 18 km lange Profile, die mit einem Stationsabstand von 500 m, beziehungsweise 2000 m vermessen wurden. Mit diesen beiden dichten Profile sowie 20 Stationen östlich und westlich davon ergibt sich eine gute Stationsüberdeckung, um die WF/OL detailliert zu untersuchen. Die MT Ergebnisse an einigen Stationen in der Nähe der Fault sind stark von 3D Effekten beeinflusst: Wir beobachten hohe Skewwerte, Phasen über 90° und eine starke Korrelation zwischen parallelen elektrischen und magnetischen Feldkomponenten für lange Perioden. Die Interpretation solch eines komplizierten Datensatzes gestaltet sich als schwierig und sehr zeitaufwändig. Um andererseits die gemessenen Daten direkt, also ohne a priori Information, abzubilden, ist eine Transformation des komplexen Impedanztensors in eine physikalisch aussagekräftige Größe nötig. Durch die geläufige Berechnung von scheinbaren spezifischen Widerständen verliert diese Größe ihre Tensoreigenschaften. Aus diesem Grund habe ich ein Verfahren weiterentwickelt, um einen scheinbaren Widerstandstensor zu erhalten. Dabei wird die MT Admittanzgleichung mit den Maxwell-Gleichungen kombiniert, und man erhält einen tensoriellen Ausdruck für die Ausbreitungszahl. Diese Größe lässt sich anschließend in zwei Tensoren, den scheinbaren Permittivitätstensor und dem scheinbaren Leitfähigkeitstensor, aufspalten. Aus letzterem kann ein scheinbarer Widerstandstensor berechnet werden. Diese Methode bezeichne ich als "Propagation Number Analysis" (PNA). Durch eine Anwendung der PNA auf MT Daten erhalten wir ein Abbild der Leitfähigkeitsverteilung in Form von Widerstandsellipsen. Darüber hinaus lässt sich die PNA mit der Eggers' Eigenwert Analyse und der LaTorraca Singular Value Decomposition vergleichen. Alle im Rahmen dieser Arbeit vorgestellten Abbildungsmethoden werden sowohl auf synthetische als auch auf gemessene Daten aus Namibia angewendet. Die mit Hilfe der PNA erhaltenen Widerstandsellipsen zeigen, dass es sich bei der WF/OL nicht um eine schmale Störung, sondern um eine breite Störungszone handelt. Die meisten beobachteten 3D-Effekte lassen sich durch eine Kombination von (i) einer oberflächennahen gut leitenden Ringstruktur und (ii) einer anisotropen Zone in der oberen und einer anisotropen Schicht in der unteren Kruste erklären. Die anisotrope Zone parallel zur WF/OL ist etwa 10km breit, und Modellstudien ergeben, dass sie sich mindestens in eine Tiefe von 14 km erstreckt.Within the framework of an integrated geophysical and geological project in Namibia we recorded MT and GDS data in a broad frequency range between 0.001s and 1000s at 107 sites. In my thesis I concentrate on a subset of sites across the Waterberg Fault / Omaruru Lineament (WF/OL), a major tectono-stratigraphic zone boundary in the Central Zone of the Damara Belt in Namibia. Most of the sites are aligned along two parallel 18 km long profiles with a site spacing of 500 m and 2 km, respectively. These dense profiles together with 20 additional sites to the East and West provide a good spatial coverage to study the WF/OL in detail. The MT results at some sites in the vicinity of the Fault are strongly affected by 3D effects - we observe high skews, phases over 90° and a strong correlation of parallel electric and magnetic field components for long periods. The interpretation of such a complicated magnetotelluric data set with 3D forward modelling is difficult and very time consuming. To image the measured data directly - without a priori information - requires the transformation of the complex impedance tensor in a physically meaningful quantity. Normal apparent resistivities calculated from the impedance tensor do not retain the tensor properties. Therefore I developed a method to derive an apparent resistivity tensor. Combining the MT admittance equation with Maxwell's equations we yield a tensorial equation for the propagation number. This quantity is subsequently separated into two tensors - an apparent permittivity and an apparent conductivity tensor. From the latter an apparent resistivity tensor can be computed. This method is called Propagation Number Analysis (PNA). Applying PNA to MT data, we obtain an image of the conductivity distribution in form of resistivity ellipses. The PNA is also compared with two already known methods for imaging conductivity anomalies: Eggers' Eigenstate Analysis and LaTorraca's Singular Value Decomposition. All imaging methods presented are applied to synthetic data and measured MT data from Namibia. The resistivity ellipses obtained by the PNA indicate that the WF/OL is not a narrow fault, but a wider fault zone extended to the North. Most of the observed 3D effects are explained by the combination of (i) a shallow conductive ring structure and (ii) an anisotropic zone in the upper crust and a lower crust which itself is also anisotropic. The anisotropic zone is approximately 10km wide and modelling studies suggest that the fault zone might continue down to at least 14 km

Entwicklung eines Verfahrens zur Abbildung krustaler Leitfähigkeitsstrukturen anhand von Magnetotellurikdaten aus Namibia

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3D imaging of the subsurface electrical resistivity structure in West Bohemia/Upper Palatinate covering mofettes and Quaternary volcanic structures by using Magnetotellurics

The region of West Bohemia and Upper Palatinate belongs to the West Bohemian Massif. The study area is situated at the junction of three different Variscan tectonic units and hosts the ENE-WSW trending Ohře Rift as well as many different fault systems. The entire region is characterized by ongoing magmatic processes in the intra-continental lithospheric mantle expressed by a series of phenomena, including e.g. the occurrence of repeated earthquake swarms and massive degassing of mantle derived CO2 in form of mineral springs and mofettes. Ongoing active tectonics is mainly manifested by Cenozoic volcanism represented by different Quaternary volcanic structures. All these phenomena make the Ohře Rift a unique target area for European intra-continental geo-scientific research. With magnetotelluric (MT) measurements we image the subsurface distribution of the electrical resistivity and map possible fluid pathways. Two-dimensional (2D) inversion results by Muñoz et al. (2018) reveal a conductive channel in the vicinity of the earthquake swarm region that extends from the lower crust to the surface forming a pathway for fluids into the region of the mofettes. A second conductive channel is present in the south of their model; however, their 2D inversions allow ambiguous interpretations of this feature. Therefore, we conducted a large 3D MT field experiment extending the study area towards the south. The 3D inversion result matches well with the known geology imaging different fluid/magma reservoirs at crust-mantle depth and mapping possible fluid pathways from the reservoirs to the surface feeding known mofettes and spas. A comparison of 3D and 2D inversion results suggests that the 2D inversion results are considerably characterized by 3D and off-profile structures. In this context, the new results advocate for the swarm earthquakes being located in the resistive host rock surrounding the conductive channels; a finding in line with observations e.g. at the San Andreas Fault, California

Magnetotelluric impedances of marine measurements conducted during Maria S. Merian cruise MSM17/1 and MSM17/2 in 2011, Walvis Ridge, Namibian Margin

The magnetotelluric impedance tensor elements and errors were calculated with the software BIRRP (Chave &Thomson, 2004; doi:10.1111/j.1365-246X.2004.02203.x). Magnetotelluric measurements were conducted from December 2010 to January 2011 at the Walvis Ridge, offshore Namibia. Coordinates of the MT stations are included in the EDI files and cover an area at ~8°E - 12°E, ~18°S-22°S. Station deployment and recovery was conducted on the two cruises MSM17/1 and MSM17/2 on German research vessel Maria S. Merian as part of the SPP1375 priority program SAMPLE. The SAMPLE project focused on the processes linked to continental margin formation

Introduction to the special issue on “Cost-effective and innovative mineral exploration solutions”

Accepted Author ManuscriptApplied Geophysics and Petrophysic