Andean Hydrothermal and Structural System Dynamics: Insights from 3D Magnetotelluric Inverse Modelling

In an active volcanic arc, magmatically sourced fluids are channelled through the brittle crust by structural features inherent in the lithological setting. This interaction is observed in the Andean volcanic mountain chain, where volcanoes, geothermal springs and major mineral deposits are spatially coherent with first-order NNE oriented thrust fault systems, and convergent-margin oblique WNW striking Andean Transverse Faults (ATF). The volcanic and hydrothermal activity at Tinguiririca and Planchón-Peteroa volcanoes demonstrate this relationship, as both volcanic complexes and their spatially associated thermal springs show strike alignment to the outcropping NNE oriented El Fierro thrust fault system. This study aims to constrain the 3D architecture of this fault system in the proximity of the volcanoes and its interaction with volcanically sourced hydrothermal fluids from a combined magnetotelluric (MT) and seismic field study. Data from a 24 station broadband magnetotelluric survey were interpreted using 3D inversion. Over 700 seismic hypocentres from a 12 station coeval seismic survey are also presented in support of the final 3D conductivity model. The combined results show a correlation of conductivity anomalies with seismic clusters in the top 10 km of the crust, including a distinct seismogenic WNW oriented feature that occurs at an abrupt electrical conductivity contrast, which is most apparent at a 6 km depth. It is concluded that this discrete feature is an Andean Transverse Fault (ATF), and that the conductors are signatures of either geothermal fluid reservoirs or fluid saturated lithologies at depth. The associated fluids are channelled parallel to the margin-oblique ATF plane and cause fault reactivation due to increased pore fluid pressure acting on the fault plane. Seismicity induced by this mechanism is limited to the east of the El Fierro fault system, as fluids are compartmentalized along the footwall due to the low permeability fault core that prevent cross-fault fluid migration. This study thus contributes novel insight into how WNW oriented AFT systems interact with local volcanic, structural and hydrothermal systems

Introduction to Special Section: Integrated Geophysical Imaging

This special section illustrates the value of integration with nonseismic geophysical methods, namely potential fields (gravity and magnetics) and electric and electromagnetic techniques. The primary objective is to overcome the overall underappreciation of these methods as exploration tools. We provide their brief overview and present nine case studies illustrating how the integrative approach to geophysical data analysis influences the overall result and reduces the uncertainty of the derived solution

Porosity and fluid budget of a water-rich megathrust revealed with electromagnetic data at the Middle America Trench

Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 17 (2016): 4495–4516, doi:10.1002/2016GC006556.At convergent margins, the distribution of fluids released from the downgoing slab modulates the state of stress and seismic coupling at the megathrust plate interface. However, existing geophysical data are unable to quantify the porosity along this interface. Here we use controlled-source electromagnetic data collected across the Middle America Trench offshore Nicaragua to image the electrical conductivity structure of the outer fore arc. Our results detect a highly conductive channel, inferred to be the region around the décollement, showing the entire section of water-rich seafloor sediments underthrust with the subducting lithosphere. We use an empirical model of the electrical conductivity of porous media to quantify the channel porosity. Our estimates are consistent with sediment compaction studies, showing a rapid decay of 65%–10% porosity from the trench to 25 km landward. We constrain the channel thickness and use the porosity estimates to determine the water budget, which represents the fraction taken up by fluid. The porosity and water budget estimates show significant lateral variations that we attribute to changes in subducted sediment thickness caused by outer rise bending faults. Between 18 and 23 km from the trench, the conductive channel broadens greatly to 1.5–2 km thick, possibly due to concentrated blind faults or sediment underplating, which suggests a sudden change in hydrogeologic structure at the plate interface. The impact of the anomalous conductor on the seismic coupling and mechanical properties of the megathrust is potentially related to the discrepancy in estimated fault slip between seismic and tsunami source inversions for the 1992 Nicaragua tsunami earthquake.National Science Foundation Grant Numbers: OCE-0841114 , OCE-0840894; Scripps Institution of Oceanography2017-05-1

Three-dimensional inversion of transient-electromagnetic data: A comparative study

Inversion of transient-electromagnetic (TEM) data arising from galvanic types of sources is approached by two different methods. Both methods reconstruct the subsurface three-dimensional (3D) electrical conductivity properties directly in the time-domain. A principal difference is given by the scale of the inversion problems to be solved. The first approach represents a small-scale 3D inversion and is based upon well-known tools. It uses a stabilized unconstrained least-squares inversion algorithm in combination with an existing 3D forward modeling solver and is customized to invert for 3D earth models with a limited model complexity. The limitation to only as many model unknowns as typical for classical least-squares problems involves arbitrary and rather unconventional types of model parameters. The inversion scheme has mainly been developed for the purpose of refining a priori known 3D underground structures by means of an inversion. Therefore, a priori information is an important requirement to design a model such that its limited degrees of freedom describe the structures of interest. The inversion is successfully applied to data from a long-offset TEM survey at the active volcano Merapi in Central Java (Indonesia). Despite the restriction of a low model complexity, the scheme offers some versatility as it can be adapted easily to various kinds of model structures. The interpretation of the resistivity images obtained by the inversion have substantially advanced the structural knowledge about the volcano. The second part of this work presents a theoretically more elaborate scheme. It employs imaging techniques originally developed for seismic wavefields. Large-scale 3D problems arising from the inversion for finely parameterized and arbitrarily complicated earth models are addressed by the method. The algorithm uses a conjugate-gradient search for the minimum of an error functional, where the gradient information is obtained via migration or backpropagation of the differences between the data observations and predictions back into the model in reverse time. Treatment for electric field and time derivative of the magnetic field data is given for the specification of the cost functional gradients. The inversion algorithm is successfully applied to a synthetic TEM data set over a conductive anomaly embedded in a half-space. The example involves a total number of more than 376000 model unknowns. The realization of migration techniques for diffusive EM fields involves the backpropagation of a residual field. The residual field excitation originates from the actual receiver positions and is continued during the simulated time range of the measurements. An explicit finite-difference time-stepping scheme is developed in advance of the imaging scheme in order to accomplish both the forward simulation and backpropagation of 3D EM fields. The solution uses a staggered grid and a modified version of the DuFort-Frankel stabilization method and is capable of simulating non-causal fields due to galvanic types of sources. Its parallel implementation allows for reasonable computation times, which are inherently high for explicit time-stepping schemes

Geophysical and geological characterization of fault-controlled geothermal systems: The Vallès Basin case of study

[eng] Geothermal energy is a renewable source of energy that harnesses heat from the Earth's interior. Temperature increases with depth, defining the geothermal gradient, which can be variable depending on the geological context. The geological setting of western Europe favors a relatively high geothermal gradient that could be exploited to generate electricity or for its direct use, for example, for its application in industry, greenhouses, or heating systems. In each of these cases, geothermal could favor the community's energy independence and reduce the use of polluting energy sources. To appropriately exploit areas with a significant geothermal gradient, it is essential to know the origin of the temperature anomaly and the system's functioning. In this context, developing appropriate exploration methodologies and techniques is essential for its adequate and efficient use. This thesis develops a methodology focused on a geothermal system type characterized by being located in highly fractured zones. These fractures connect the surface with great depths, allowing the rapid ascent of deep fluids at high temperatures without giving them time to cool down. Specifically, this thesis applies this methodology to a study case located in the Vallès Basin, close to Barcelona city (NE Iberian Peninsula), where some localities, such as La Garriga and Caldes de Montbui towns, have thermal hot springs (60ºC and 70ºC, respectively). In particular, the methodology applied to study the Vallès Basin geothermal fractured system, is focused on two main cores, geophysical and geological techniques. Geophysical methods allow the characterization of the subsurface physical properties, reaching great depths without having to drill. For example, if the physical characteristics of the subsurface have enough contrast, they could allow distinguishing between different types of rocks, fractured zones, or if there is any fluid circulation. However, the geophysical results have to be complemented with other geoscientific studies in order to make a proper interpretation. In this case, this thesis includes a characterization of the area's geology, fracturing, and hydrology. Finally, the integration of the applied techniques has allowed the understanding of the origin and system's functioning, which is presented in the form of a 3D conceptual model, geological model, and temperature model. This innovative methodology, which integrates different geoscientific techniques at different scales, combining traditional techniques with novel digital tools, has facilitated the characterization of a geothermal system controlled by geological structures. Therefore, it is established as a methodical option to characterize systems of similar origin.[cat] La Geotèrmia és una font renovable d'energia que aprofita la temperatura de l'interior de la Terra. El grau en què aquesta temperatura augmenta en profunditat, ve definint pel gradient geotèrmic, el qual pot ser variable segons el context geològic. La geologia de la regió oest del continent europeu afavoreix un gradient geotèrmic relativament alt que podria ser aprofitat per generar electricitat o per a ús directe, com és el cas d'aplicacions en indústria, hivernacles o sistemes de calefacció. En qualsevol cas, la geotèrmia podria afavorir la independència energètica i una disminució en l’ús de fonts d’energia contaminants. Per a un aprofitament d'aquestes zones amb un gradient geotèrmic significatiu, és essencial conèixer-ne l'origen i el funcionament. En aquest context, és basic desenvolupar metodologies d'exploració que siguin adequades i eficients. Aquesta tesis desenvolupa una metodologia aplicada a un exemple de sistema geotèrmic caracteritzat per estar ubicat en una zona molt fracturada. Aquestes fractures connecten la superfície amb grans profunditats, permetent l'ascens ràpid de fluids profunds que es troben a temperatures altes, sense que els doni temps a refredar-se. Concretament, aquesta zona d'estudi es situa a la Conca del Vallès (NE Península Ibèrica), on algunes localitats com La Garriga i Caldes de Montbui, tenen surgències d'aigua termal (60ºC i 70ºC, respectivament). Concretament, la metodologia aplicada es basa en dues parts principals: l'exploració geofísica i la geològica. Els mètodes geofísics ens permeten conèixer les propietats físiques del subsol arribant a grans profunditats sense haver de fer perforacions. Si les característiques físiques del terreny presenten un contrast suficient, poden permetre, per exemple, distingir entre tipus de roques, zones fracturades, o si hi ha circulació d'algun fluid. Tot i així, els resultats geofísics s'han de complementar amb altres estudis geocientífics per una correcta interpretació dels resultats. En aquest cas, aquesta tesis inclou una caracterització de la geologia, la fracturació i la hidrologia de la zona. La integració final de totes les dades ha permès entendre l'origen i el funcionament d'aquest sistema, resultat del qual es presenta en forma d'un model 3D conceptual, geològic i de temperatures. Aquesta metodologia innovadora, que integra diferents tècniques geocientífiques a escala diferent, ha combinat tècniques tradicionals amb eines digitals noves, facilitant la caracterització d'un sistema geotèrmic controlat per estructures geològiques. Per tant, s’estableix com una opció metòdica a seguir per a la caracterització de sistemes d’origen similar.[spa] La Geotermia es una fuente renovable de energía que aprovecha el calor del interior de la Tierra. La temperatura del interior de la Tierra aumenta con la profundidad, y este aumento, definido como gradiente geotérmico, puede ser variable según el contexto geológico. El contexto geológico del oeste del continente europeo favorece un gradiente geotérmico relativamente alto que podría ser aprovechado para generar electricidad o para su uso directo, como es el caso de aplicaciones en industria, invernaderos o sistemas de calefacción. En cualquier caso, la geotermia podría favorecer la independencia energética y una disminución del uso de fuentes de energía contaminantes. Para un apropiado aprovechamiento de estas zonas con un gradiente geotérmico significativo, es esencial conocer su origen y funcionamiento. En este contexto, es necesario un avance en el desarrollo de metodologías de exploración que sean adecuadas y eficientes. Esta tesis desarrolla una metodología aplicada a un tipo de sistema geotérmico caracterizado por estar ubicado en zonas muy fracturadas. Estas fracturas conectan la superficie con grandes profundidades, permitiendo el ascenso rápido de fluidos profundos que se encuentran a altas temperaturas sin que les dé tiempo a enfriarse. Geográficamente, esta zona de estudio se encuentra en la Cuenca del Vallès, cerca de Barcelona (NE Península Ibérica), donde algunas localidades como La Garriga y Caldes de Montbui, tienen surgencias de agua termal (60ºC y 70ºC, respectivamente). Concretamente, esta metodología se puede separar en dos partes principales, la exploración geofísica y la geológica. Los métodos geofísicos nos permiten conocer las propiedades físicas del subsuelo, llegando a grandes profundidades, sin tener que hacer perforaciones. Si las características físicas del terreno presentan un contraste suficiente, nos pueden permitir, por ejemplo, distinguir entre tipos de rocas, zonas fracturadas, o si hay circulación de algún fluido. Aun así, los resultados geofísicos tienen que complementarse con otros estudios geocientíficos para poder hacer una apropiada interpretación. Esta tesis incluye una caracterización de la geología, la fracturación y la hidrología de la zona, cuya integración final ha permitido entender el origen y funcionamiento de este sistema. Los resultados finales se presentan en forma de un modelo 3D conceptual, geológico y de temperaturas. Esta metodología innovadora integra distintas técnicas geocientíficas a distinta escala, combinando técnicas tradicionales con herramientas digitales novedosas, facilitando la caracterización de un sistema geotérmico controlado por estructuras geológicas. Por lo tanto, se establece como una opción metódica a seguir para la caracterización de sistemas de origen similar

Geophysical and geological characterization of fault-controlled geothermal systems: The Vallès Basin case of study

[eng] Geothermal energy is a renewable source of energy that harnesses heat from the Earth's interior. Temperature increases with depth, defining the geothermal gradient, which can be variable depending on the geological context. The geological setting of western Europe favors a relatively high geothermal gradient that could be exploited to generate electricity or for its direct use, for example, for its application in industry, greenhouses, or heating systems. In each of these cases, geothermal could favor the community's energy independence and reduce the use of polluting energy sources. To appropriately exploit areas with a significant geothermal gradient, it is essential to know the origin of the temperature anomaly and the system's functioning. In this context, developing appropriate exploration methodologies and techniques is essential for its adequate and efficient use. This thesis develops a methodology focused on a geothermal system type characterized by being located in highly fractured zones. These fractures connect the surface with great depths, allowing the rapid ascent of deep fluids at high temperatures without giving them time to cool down. Specifically, this thesis applies this methodology to a study case located in the Vallès Basin, close to Barcelona city (NE Iberian Peninsula), where some localities, such as La Garriga and Caldes de Montbui towns, have thermal hot springs (60ºC and 70ºC, respectively). In particular, the methodology applied to study the Vallès Basin geothermal fractured system, is focused on two main cores, geophysical and geological techniques. Geophysical methods allow the characterization of the subsurface physical properties, reaching great depths without having to drill. For example, if the physical characteristics of the subsurface have enough contrast, they could allow distinguishing between different types of rocks, fractured zones, or if there is any fluid circulation. However, the geophysical results have to be complemented with other geoscientific studies in order to make a proper interpretation. In this case, this thesis includes a characterization of the area's geology, fracturing, and hydrology. Finally, the integration of the applied techniques has allowed the understanding of the origin and system's functioning, which is presented in the form of a 3D conceptual model, geological model, and temperature model. This innovative methodology, which integrates different geoscientific techniques at different scales, combining traditional techniques with novel digital tools, has facilitated the characterization of a geothermal system controlled by geological structures. Therefore, it is established as a methodical option to characterize systems of similar origin.[cat] La Geotèrmia és una font renovable d'energia que aprofita la temperatura de l'interior de la Terra. El grau en què aquesta temperatura augmenta en profunditat, ve definint pel gradient geotèrmic, el qual pot ser variable segons el context geològic. La geologia de la regió oest del continent europeu afavoreix un gradient geotèrmic relativament alt que podria ser aprofitat per generar electricitat o per a ús directe, com és el cas d'aplicacions en indústria, hivernacles o sistemes de calefacció. En qualsevol cas, la geotèrmia podria afavorir la independència energètica i una disminució en l’ús de fonts d’energia contaminants. Per a un aprofitament d'aquestes zones amb un gradient geotèrmic significatiu, és essencial conèixer-ne l'origen i el funcionament. En aquest context, és basic desenvolupar metodologies d'exploració que siguin adequades i eficients. Aquesta tesis desenvolupa una metodologia aplicada a un exemple de sistema geotèrmic caracteritzat per estar ubicat en una zona molt fracturada. Aquestes fractures connecten la superfície amb grans profunditats, permetent l'ascens ràpid de fluids profunds que es troben a temperatures altes, sense que els doni temps a refredar-se. Concretament, aquesta zona d'estudi es situa a la Conca del Vallès (NE Península Ibèrica), on algunes localitats com La Garriga i Caldes de Montbui, tenen surgències d'aigua termal (60ºC i 70ºC, respectivament). Concretament, la metodologia aplicada es basa en dues parts principals: l'exploració geofísica i la geològica. Els mètodes geofísics ens permeten conèixer les propietats físiques del subsol arribant a grans profunditats sense haver de fer perforacions. Si les característiques físiques del terreny presenten un contrast suficient, poden permetre, per exemple, distingir entre tipus de roques, zones fracturades, o si hi ha circulació d'algun fluid. Tot i així, els resultats geofísics s'han de complementar amb altres estudis geocientífics per una correcta interpretació dels resultats. En aquest cas, aquesta tesis inclou una caracterització de la geologia, la fracturació i la hidrologia de la zona. La integració final de totes les dades ha permès entendre l'origen i el funcionament d'aquest sistema, resultat del qual es presenta en forma d'un model 3D conceptual, geològic i de temperatures. Aquesta metodologia innovadora, que integra diferents tècniques geocientífiques a escala diferent, ha combinat tècniques tradicionals amb eines digitals noves, facilitant la caracterització d'un sistema geotèrmic controlat per estructures geològiques. Per tant, s’estableix com una opció metòdica a seguir per a la caracterització de sistemes d’origen similar

Source-independent full wavefield converted-phase elastic migration velocity analysis

Converted phase (CP) elastic seismic signals are comparable in amplitude to the primary signals recorded at large offsets and have the potential to be used in seismic imaging and velocity analysis. We present an approach for CP elastic wave equation velocity analysis that does not use source information and is applicable to surface-seismic, microseismic, teleseismic and vertical seismic profile (VSP) studies. Our approach is based on the cross-correlation between reflected or transmitted PP and CP PS (and/or SS and CP SP) waves propagated backward in time, and is formulated as an optimization problem with a differential semblance criterion objective function for the simultaneous update of both P- and S-wave velocity models. The merit of this approach is that it is fully data-driven, uses full waveform information, and requires only one elastic backward propagation to form an image rather than the two (one forward and one backward) propagations needed for standard reverse-time migration. Moreover, as the method does not require forward propagation, it does not suffer from migration operator source aliasing when a small number of shots are used. We present a derivation of the method and test it with a synthetic model and field micro-seismic data

Time-lapse walkaway vertical seismic profile monitoring for CO2 injection at the SACROC enhanced oil recovery field: A case study

Geologic carbon storage involves large-scale injections of carbon dioxide into underground geologic formations. Changes in reservoir properties resulting from CO2CO2 injection and migration can be characterized using monitoring methods with time-lapse seismic data. To achieve economical monitoring, vertical seismic profile (VSP) data are often acquired to survey the local injection area. We investigated the capability of walkaway VSP monitoring for CO2CO2 injection into an enhanced oil recovery field at SACROC, West Texas. VSP data sets were acquired in 2008 and 2009, and CO2CO2 injection took place after the first data acquisition. Because the receivers were located above the injection zone, only reflection data contain the information from the reservoir. Qualitative comparison between reverse-time migration images at different times revealed vertical shifts of the reflectors’ center, indicating the presence of velocity changes. We examined two methods to quantify the changes in velocity: standard full-waveform inversion (FWI) and image-domain wavefield tomography (IDWT). FWI directly inverts seismic waveforms for velocity models. IDWT inverts for the time-lapse velocity changes by matching the baseline and time-lapse migration images. We found that, for the constrained geometry of VSP surveys, the IDWT result was significantly more consistent with a localized change in velocity as expected from a few months of CO2CO2 injection. A synthetic example was used to verify the result from the field data. By contrast, FWI failed to provide quantitative information about the volumetric velocity changes because of the survey geometry and data frequency content

Magnetotelluric study of the Mérida Andes and surrounding basins, Venezuela

The Caribbean and South American tectonic plates bound the north-eastwards expulsion of the North Andean Block in western Venezuela. This complicated geodynamic setting resulted in the formation of major strike-slip fault systems and sizeable mountain chains. The 100 km wide Mérida Andes extend from the Colombian/Venezuelan border to the Coastal Cordillera. To the north and south, the Mérida Andes are bound by hydrocarbon-rich sedimentary basins. Knowledge of lithospheric structures, related to the formation of the Mérida Andes, is limited though, due to a lack of deep geophysical data. This thesis presents the results of the first broadband magnetotelluric profile crossing the Mérida Andes and the Maracaibo and Barinas-Apure foreland basins spanning a distance of 240 km. The MT dataset consists of 72 stations installed during March and April 2015 with a minimum recording period of 3 days per station. Geoelectrical strike and dimensionality analyses are consistent with one- or two- dimensional subsurface structures for the sedimentary basins yet also indicate a strong three- dimensional setting for the Mérida Andes. Even more significantly, these analyses showed the presence of off-profile features that influenced the data considerably, particularly at long periods. Therefore, a combination of 2D and 3D modelling was necessary for analysing the geoelectrical structures associated with this dataset. Off-profile structures can significantly affect the outcome of a 2D inversion. Thus, the systematic examination of the influence of 3D structures on 2D inversions was necessary to support the obtained result. Synthetic data sets derived from 3D modelling allowed identification and quantification of spurious off-profile features as well as smoothing artefacts due to limited areal station coverage of data collected along a profile. In general, structures in the 2D inversion are affected by the projection and rotation of the data resulting in sub-horizontal anomalies to reproduce the oblique extent of the fault systems and sedimentary basins. Moreover, a profile distributed dataset can limit the lateral resolution of a 3D inversion considerably. Hence, the effect of data distribution on a 3D inversion was carefully studied to determine the areas of the models that can be confidently explained by the data. To this end, several synthetic datasets were derived from 3D models with varying levels of complexity. The analysis of the synthetic datasets allows determination of the lateral resolution of the 3D models and identification of spurious shallow and deep features considered artefacts related to off-profile features. Furthermore, the inversion of synthetic models provided support to the geological interpretation of the recovered anomalies for the 2D and 3D modelling. The 2D and 3D inversion models were similar above the sedimentary basins and showed marked differences above the Mérida Andes, due to the 3D nature of this section. The inversion models show electrically conductive basins with depths of 2 to 5 km for the Barinas-Apure and 2 to 9 km for the Maracaibo basins. Many resistive bodies within the Maracaibo basin could be related to active deformation causing juxtaposition of older geological formations and younger basin sediments. A conductive zone under the Maracaibo Basin correlates spatially with the location of a Bouguer anomaly low and seem to describe the SE tilt of the Maracaibo Triangular Block under the Mérida Andes. This conductive zone is limited towards the mountain by the north-western thrust system, whose fault plane may function as a detachment surface reaching depth larger than 30 km in the 3D inversion models. The most prominent fault systems of the area, the Boconó and Valera Faults, cross-cut the Mérida Andes in NE-SW direction along its strike with a length 400 km and N-S direction at its centre with a length 60 km, respectively. Both faults are associated with sub-vertical zones of high electrical conductivity and sensitivity tests suggest that the Valera fault reach depths of up to 12 km. The Boconó fault can be considered a crustal structure with a depth up to 35 km. The observed anomalies seem to show a deep connection of the fault planes, possibly related to the formation of the fault systems in a transpressive regime. Conductive anomalies to the south of the Boconó Fault seem to represent a considerable back thrust structure well constrained between 3 and 10 km depth. The high conductivity of these structures is possibly related to weathering water from the surface and the accumulation of clay minerals in the fault gauges. However, fluids related to the flat and shallow subduction of the Caribbean Plate in north-western Venezuela could better explain the low resistivity of the deep structures (> 15 km). A sizeable conductor at 50 km depth, which appears consistently in the 2D sections, could be identified as an inversion artefact caused by a conductor east of the profile. The 3D inversion places this structure 10 km to the east at 15 km deep. This model also shows depth connection (12 km depth) of the anomalies related to the Valera and Boconó faults with the off-profile conductor. The observed anomalies in the 2D and 3D inversion related to these conductors were tested and reproduced employing synthetic datasets, leading to the speculation that the high conductivity associated with the off-profile conductor may be related to the detachment of the Trujillo Block. The models obtained confirm the shape and distribution of the known geological structures related to the complicated geodynamic settings responsible for the formation of the Mérida Andes. These results partially support the "floating orogen hypothesis" developed to explain the geodynamic evolution of western Venezuela, and they highlight the relevance of the Trujillo Block in this process. However, they also show that features of known structures such as the Boconó fault system maximum depth, the back-thrusting in the Mérida Andes, and the relevance of the escape of the Trujillo Block in the tectonic processes need to be adjusted to the current knowledge.Die Karibische und Südamerikanische tektonischen Platten begrenzten die nordöstliche Abschiebung des Nord-Anden-Blocks im Westen Venezuelas. Diese komplizierte geodynamische Umgebung führte zur Bildung großer Blattverschiebungen-Verwerfungssysteme und beträchtlicher Gebirgsketten. Die 100 km breiten Mérida-Andes erstrecken sich von der Grenze zwischen Kolumbien und Venezuela bis zum Coastal Cordillera. Im Norden und Süden sind die Mérida-Anden von kohlenwasserstoffreichen Sedimentbecken umgeben. Das Wissen über lithosphärische Strukturen im Zusammenhang mit der Bildung der Mérida-Anden ist jedoch aufgrund des Mangels an tiefen geophysikalischen Daten begrenzt. Diese Arbeit präsentiert die Ergebnisse des ersten breitbandigen magnetotellurischen Profils, das die Mérida-Anden und die Vorlandbecken Maracaibo und Barinas-Apure über eine Entfernung von 240 km quert. Der MT-Datensatz besteht aus 72 Stationen, die im März und April 2015 mit einer Mindestaufzeichnungsdauer von 3 Tagen pro Station installiert wurden. Geoelektrische Streich- und Dimensionalitätsanalysen stimmen mit ein- oder zweidimensionalen Untergrundstrukturen für die Sedimentbecken überein, weisen jedoch auch auf eine starke dreidimensionale Strukturen in der Umgebung der Mérida-Andes hin.. Noch wichtiger ist, dass diese Analysen das Vorhandensein von Merkmalen außerhalb des Profils zeigten, die die Daten insbesondere für lange Perioden erheblich beeinflussen. Daher war eine Kombination aus 2D- und 3D-Modellierung erforderlich, um Störungssysteme und Sedimentbecken quer zum Profil zu reproduzieren. Off-Profile-Strukturen können das Ergebnis einer 2D-Inversion erheblich beeinflussen. Daher war die systematische Untersuchung des Einflusses von 3D-Strukturen auf 2D-Inversionen erforderlich, um das erhaltene Ergebnis zu verifizieren. Synthetische Datensätze, die aus der 3D-Modellierung abgeleitet wurden, ermöglichten die Identifizierung und Quantifizierung von störenden Strukturen außerhalb des Profils sowie die Glättung von Artefakten aufgrund der begrenzten Stationsüberdeckung der entlang eines Profils gesammelten Daten. Im Allgemeinen werden Strukturen in der 2D-Inversion durch die Projektion und Rotation der Daten beeinflusst, was zu flach stehenden Anomalien führt, um die schräge Ausdehnung der Verwerfungssysteme und Sedimentbecken zu reproduzieren. Darüber hinaus kann ein Datensatz entlang eines Profils die laterale Auflösung einer 3D-Inversion erheblich einschränken. Daher wurde die Auswirkung der Datenverteilung auf eine 3D-Inversion sorgfältig untersucht, um die Bereiche der Modelle zu bestimmen, die durch die Daten sicher erklärt werden können. Zu diesem Zweck wurden mehrere synthetische Datensätze aus 3D-Modellen mit unterschiedlicher Komplexität abgeleitet. Die Analyse der synthetischen Datensätze ermöglicht die Bestimmung der lateralen Auflösung der 3D-Modelle und die Identifizierung von störenden oberflächennahen und tiefen Merkmalen, die als Artefakte im Zusammenhang mit Strukturen außerhalb des Profils betrachtet werden. Darüber hinaus unterstützte die Inversion synthetischer Modelle die geologische Interpretation der reproduzierten Anomalien für die 2D- und 3D-Modellierung. Die 2D- und 3D-Inversionsmodelle stimmen über den Sedimentbecken überein. Aufgrund der 3D Strukturen über den MA ergaben sich jedoch deutliche unterschiede. Die Inversionsmodelle zeigen elektrisch leitende Becken mit Tiefen von 2 bis 5 km für das Barinas-Apure und 2 bis 9 km für das Maracaibo-Becken. Viele Gebiete höheren Widerstands im Maracaibo-Becken könnten mit einer aktiven Deformation zusammenhängen, die ein Nebeneinander älterer geologischer Formationen und jüngerer Beckensedimente verursacht. Eine besserleitende Zone unter dem Maracaibo-Becken korreliert räumlich mit der Lage einer Bouguer-Anomalie und scheint die SE-Neigung des Maracaibo-Dreiecksblocks unter den Mérida-Anden zu markieren. Diese leitende Zone ist in Richtung des Gebirges durch das nordwestliche Schubsystem begrenzt, dessen Störungsebene als Ablösefläche fungieren kann, die in den 3D-Inversionsmodellen eine Tiefe von mehr als 30 km erreicht. Die bekanntesten Störungssysteme des Gebiets, die Verwerfungen Boconó und Valera, kreuzen die Mérida-Anden in Nordost-Südwest-Richtung entlang ihres Streichens mit einer Länge von 400 km und die N-S-Richtung in ihrer Mitte mit einer Länge von 60 km. Beide Störungen sind durch steil stehende Zonen hoher elektrischer Leitfähigkeit verbunden. Sensitivitätsstudien legen nahe, dass die Valera-Störung Tiefen von bis zu 12 km erreicht. Die Boconó-Verwerfung kann als Krustenstruktur mit einer Tiefe von bis zu 35 km angesehen werden. Die beobachteten Anomalien scheinen eine tiefe Verbindung der Verwerfungsebenen zu zeigen, möglicherweise im Zusammenhang mit der Bildung der Verwerfungssysteme in einem transpressiven Regime. Leitfähige Anomalien südlich der Boconó-Verwerfung scheinen eine beträchtliche Rückschubstruktur darzustellen, die zwischen 3 und 10 km Tiefe gut lokalisiert ist. Die hohe Leitfähigkeit dieser Strukturen hängt möglicherweise mitWasser aus Verwitterungsprozessen nahe der Erdoberfläche und der Ansammlung von Tonmineralien in den Störungszonen zusammen. Alte Fluide im Zusammenhang mit flach stehenden und oberflächenahen Subduktion der Karibikplatte im Nordwesten Venezuelas könnten jedoch den geringen spezifischen Widerstand der tiefen Strukturen (> 15 km) besser erklären. Ein beträchtlicher Leiter in einer Tiefe von 50 km, der in den 2D-Schnitten konsistent erscheint, konnte als Inversionsartefakt identifiziert werden, der durch einen Leiter östlich des Profils verursacht wird. Durch die 3D-Inversion wird diese Struktur 10 km östlich in 15 km Tiefe platziert. Dieses Modell zeigt auch die Tiefenverbindung (12 km Tiefe) der Anomalien im Zusammenhang mit den Störungen von Valera und Boconó mit dem Leiter außerhalb des Profils. Die beobachteten Anomalien in der 2D- und 3D-Inversion in Bezug auf diese Leiter wurden unter Verwendung synthetischer Datensätze getestet und reproduziert. Daher kann man annehmen das die mit dem Leiter abseits des Profils verbundene Leitfähigkeit mit der Ablösung des Trujilo Blocks zusammenhängt. Die erhaltenen Modelle bestätigen die Form und Verteilung der bekannten geologischen Strukturen im Zusammenhang mit dem komplizierten geodynamischen Millieu, welches für die Bildung der Mérida-Andes verantwortlich ist. Diese Ergebnisse stützen teilweise die "schwimmende Orogenhypothese", die entwickelt wurde, um die geodynamische Entwicklung West-Venezuelas zu erklären, und sie unterstreichen die Relevanz des Trujillo-Blocks in diesem Prozess. Sie zeigen jedoch auch, dass Merkmale bekannter Strukturen wie die maximale Tiefe des Boconó-Verwerfungssystems, das Zurückschieben in den Mérida-Anden und die Relevanz des Entweichens des Trujillo-Blocks in den tektonischen Prozessen an den aktuellen Kenntnisstand angepasst werden müssen

Seismotectonics of the Andean Southern Volcanic Zone: insights from crustal seismicity

The interaction of faults and fluids in the crust plays a fundamental role on fluid migration as well as on the location, frequency and style of earthquakes. Therefore, understanding these interactions at all scales is key for assessing a diverse range of topics from geohazards, the location and exploitation of mineral and energy resources and for understanding tectonic processes. The area covering the upper Tinguiririca and Teno river valleys, in the Southern Volcanic Zone of the Chilean Andes provides an excellent natural laboratory for studying this. It has undergone recent volcanic and seismic activity, with clear spatial correlations between outcropping faults and arc-sourced fluids outflow (e.g., volcanic features, hot springs and hydrothermally altered areas). To study this, seismicity data was collected locally by 12 seismometers deployed between 2017 and 2018 together with a coeval magnetotelluric survey in the study area. In this thesis I present a seismicity data analysis complemented by the study of the resistivity architecture of the area studied that reveal the relationship of faults and hydrothermal fluids stored in the upper crust. Through location and relative relocation of earthquakes, I show that both the regional El Fierro Fault System and other previously unknown Andean Transverse Faults act as barriers to lateral fluid flow and thus control the distribution of fluid accumulations. Temporal variations of the seismicity rate, the seismic b-value and rupture style of seismicity clustered around the Teno river valley, denote fluctuations of the local differential stress that can be related to pore fluid pressure changes within nearby hydrothermal fluids reservoirs. The interaction of fluids on nearby faults is further evidenced by apparent stress field rotations coetaneous with the changes in rate, b-value and rupture style constrained from the seismicity dataset. These results suggest I have captured part of a long-lasting seismic swarm linked to changes in natural hydrothermal reservoirs. The fault-fluid interactive configuration found in the Tinguiririca - Teno area represents an optimum scenario for the development of ore porphyry deposits, being of high interest to understand the geomechanical and hydrogeological processes that favour the generation of such mineral deposits