TY - JOUR A1 - Araya Vargas, Jaime Andrés A1 - Meqbel, Naser M. A1 - Ritter, Oliver A1 - Brasse, H. A1 - Weckmann, Ute A1 - Yanez, Gonzalo A1 - Godoy, B. T1 - Fluid Distribution in the Central Andes Subduction Zone Imaged With Magnetotellurics JF - Journal of geophysical research : Solid earth N2 - We present a model of the electrical resistivity structure of the lithosphere in the Central Andes between 20 degrees and 24 degrees S from 3-D inversion of 56 long-period magnetotelluric sites. Our model shows a complex resistivity structure with significant variability parallel and perpendicular to the trench direction. The continental forearc is characterized mainly by high electrical resistivity (>1,000m), suggesting overall low volumes of fluids. However, low resistivity zones (LRZs, <5m) were found in the continental forearc below areas where major trench-parallel faults systems intersect NW-SE transverse faults. Forearc LRZs indicate circulation and accumulation of fluids in highly permeable fault zones. The continental crust along the arc shows three distinctive resistivity domains, which coincide with segmentation in the distribution of volcanoes. The northern domain (20 degrees-20.5 degrees S) is characterized by resistivities >1,000m and the absence of active volcanism, suggesting the presence of a low-permeability block in the continental crust. The central domain (20.5 degrees-23 degrees S) exhibits a number of LRZs at varying depths, indicating different levels of a magmatic plumbing system. The southern domain (23 degrees-24 degrees S) is characterized by resistivities >1,000m, suggesting the absence of large magma reservoirs below the volcanic chain at crustal depths. Magma reservoirs located below the base of the crust or in the backarc may fed active volcanism in the southern domain. In the subcontinental mantle, the model exhibits LRZs in the forearc mantle wedge and above clusters of intermediate-depth seismicity, likely related to fluids produced by serpentinization of the mantle and eclogitization of the slab, respectively. KW - Subduction Zone KW - Central Andes KW - Magnetotellurics KW - Seismotectonic segmentation KW - Fluid processes Y1 - 2019 U6 - https://doi.org/10.1029/2018JB016933 SN - 2169-9313 SN - 2169-9356 VL - 124 IS - 4 SP - 4017 EP - 4034 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Becken, Michael A1 - Ritter, Oliver A1 - Bedrosian, Paul A. A1 - Weckmann, Ute T1 - Correlation between deep fluids, tremor and creep along the central San Andreas fault JF - Nature : the international weekly journal of science N2 - The seismicity pattern along the San Andreas fault near Parkfield and Cholame, California, varies distinctly over a length of only fifty kilometres. Within the brittle crust, the presence of frictionally weak minerals, fault-weakening high fluid pressures and chemical weakening are considered possible causes of an anomalously weak fault northwest of Parkfield(1-4). Non-volcanic tremor from lower-crustal and upper-mantle depths(5-7) is most pronounced about thirty kilometres southeast of Parkfield and is thought to be associated with high pore-fluid pressures at depth(8). Here we present geophysical evidence of fluids migrating into the creeping section of the San Andreas fault that seem to originate in the region of the uppermost mantle that also stimulates tremor, and evidence that along-strike variations in tremor activity and amplitude are related to strength variations in the lower crust and upper mantle. Interconnected fluids can explain a deep zone of anomalously low electrical resistivity that has been imaged by magnetotelluric data southwest of the Parkfield-Cholame segment. Near Cholame, where fluids seem to be trapped below a high-resistivity cap, tremor concentrates adjacent to the inferred fluids within a mechanically strong zone of high resistivity. By contrast, sub-vertical zones of low resistivity breach the entire crust near the drill hole of the San Andreas Fault Observatory at Depth, northwest of Parkfield, and imply pathways for deep fluids into the eastern fault block, coincident with a mechanically weak crust and the lower tremor amplitudes in the lower crust. Fluid influx to the fault system is consistent with hypotheses of fault-weakening high fluid pressures in the brittle crust. Y1 - 2011 U6 - https://doi.org/10.1038/nature10609 SN - 0028-0836 VL - 480 IS - 7375 SP - 87 EP - U248 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Quesnel, Yoann A1 - Weckmann, Ute A1 - Ritter, Oliver A1 - Stankiewicz, Jacek A1 - Lesur, Vincent A1 - Mandea, Mioara A1 - Langlais, Benoit A1 - Sotin, Christophe A1 - Galdéano, Armand T1 - Simple models for the Beattie Magnetic Anomaly in South Africa N2 - The origin of the approximately 1000 km-long Beattie Magnetic Anomaly (BMA) in South Africa remains unclear and contentious. Key issues include the width, depth and magnetization of its source. In this study, we use uniformly magnetized spheres, prisms and cylinders to provide the simplest possible models which predict the 1 km-altitude aeromagnetic measurements along a profile across the BMA. The source parameters are adjusted by forward modeling. In case of a sphere, an inversion technique is applied to refine the parameters. Our results Suggest that two similarly magnetized and adjacent sources. With a vertical offset, can explain the observed magnetic anomaly. The best fitting model corresponds to two highly-magnetized (>5 A m(-1)) sheet-like prisms, extending from 9 to 12 kill depth, and from 13 to 18 kill depth, respectively, and with a total width reaching 80 km. Other less-preferred models show thicker and deeper magnetized volumes. Associated magnetizations seem to be mostly induced, although a weak remanent component is required to improve the fit. We also compare our results With the interpretation of independent magnetotelluric and seismic experiments along the same profile. It suggests that the geological sources for the BMA are mostly located in the middle crust and may be displaced by a shear zone or a fault. Contrary to previous models suggesting a serpentinized sliver of paleo-oceanic crust within the Natal-Namaqua Mobile Belt, we propose that granulite-facies mid-crustal rocks within this belt may cause the BMA. Y1 - 2009 UR - http://www.sciencedirect.com/science/journal/00401951 U6 - https://doi.org/10.1016/j.tecto.2008.11.027 SN - 0040-1951 ER - TY - JOUR A1 - Stankiewicz, Jacek A1 - Munoz, G. A1 - Ritter, Oliver A1 - Bedrosian, Paul A. A1 - Ryberg, Trond A1 - Weckmann, Ute A1 - Weber, Michael H. T1 - Shallow lithological structure across the Dead Sea Transform derived from geophysical experiments JF - Geochemistry, geophysics, geosystems N2 - In the framework of the DEad SEa Rift Transect (DESERT) project a 150 km magnetotelluric profile consisting of 154 sites was carried out across the Dead Sea Transform. The resistivity model presented shows conductive structures in the western section of the study area terminating abruptly at the Arava Fault. For a more detailed analysis we performed a joint interpretation of the resistivity model with a P wave velocity model from a partially coincident seismic experiment. The technique used is a statistical correlation of resistivity and velocity values in parameter space. Regions of high probability of a coexisting pair of values for the two parameters are mapped back into the spatial domain, illustrating the geographical location of lithological classes. In this study, four regions of enhanced probability have been identified, and are remapped as four lithological classes. This technique confirms the Arava Fault marks the boundary of a highly conductive lithological class down to a depth of similar to 3 km. That the fault acts as an impermeable barrier to fluid flow is unusual for large fault zone, which often exhibit a fault zone characterized by high conductivity and low seismic velocity. At greater depths it is possible to resolve the Precambrian basement into two classes characterized by vastly different resistivity values but similar seismic velocities. The boundary between these classes is approximately coincident with the Al Quweira Fault, with higher resistivities observed east of the fault. This is interpreted as evidence for the original deformation along the DST originally taking place at the Al Quweira Fault, before being shifted to the Arava Fault. KW - magnetotellurics KW - seismic tomography Y1 - 2011 U6 - https://doi.org/10.1029/2011GC003678 SN - 1525-2027 VL - 12 IS - 3-4 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Streich, Rita A1 - Becken, Michael A1 - Ritter, Oliver T1 - Imaging of CO2 storage sites, geothermal reservoirs, and gas shales using controlled-source magnetotellurics : modeling studies N2 - To balance the steady decrease of conventional hydrocarbon resources, increased utilization of unconventional and new energy resources, such as shale gas and geothermal energy, is required. Also, the geological sequestration of carbon dioxide is being considered as a technology that may temporarily mitigate the effects of CO2 emission. Sites suitable for shale gas production, geothermal exploration, or CO2 sequestration are commonly characterized by electrical resistivities distinctly different from those of the surrounding rocks. Therefore, electromagnetic methods can be viable tools to help identify target sites suitable for exploration, and to monitor reservoirs during energy production or CO2 injection. Among the wide variety of electromagnetic methods available, controlled-source magnetotelluric (CSMT) may be particularly suitable because of (i) its ability to resolve both electrically resistive and conductive structures, (ii) controlled sources offering noise control and thus facilitating surveys in populated regions, and (iii) the potential of penetration throughout the depth range accessible by drilling. Nevertheless, CSMT has not yet been widely employed because of logistical challenges of field operations and the requirement of complex and highly computer-intensive data processing. With these difficulties gradually being mitigated by recent technological developments, CSMT may now be reconsidered as an exploration tool. Here, we investigate by 1D and 3D numerical simulations the feasibility of detecting gas shales and identifying sites eligible for geothermal exploration or CO2 sequestration from CSMT data. We consider surface-to-surface, borehole-to-surface, and cross-hole configurations of the sources and receivers. Results and conclusions on the detectability of the targets of interest are presented for various exploration and monitoring scenarios, which are roughly representative of the geological setting of the North German Basin. Y1 - 2010 UR - http://www.sciencedirect.com/science/journal/00092819 U6 - https://doi.org/10.1016/j.chemer.2010.05.004 SN - 0009-2819 ER - TY - JOUR A1 - Streich, Rita A1 - Becken, Michael A1 - Ritter, Oliver T1 - 2.5D controlled-source EM modeling with general 3D source geometries JF - Geophysics N2 - Most 2.5D controlled-source electromagnetic (CSEM) modeling algorithms presented to date explicitly consider only sources that are point dipoles oriented parallel or perpendicular to the direction of constant conductivity. This makes simulations of complex source geometries expensive, requiring separate evaluations of many point dipole fields, and thus limits the practical applicability of such schemes for simulating and interpreting field data. We present a novel 2.5D CSEM modeling scheme that overcomes this limitation and permits efficient simulations of sources with general shape and orientation by evaluating fields for the entire source at once. We accommodate general sources by using a secondary field approach, in which primary fields are computed for the general source and a 1D background conductivity model. To carry out the required Fourier transforms between space and wavenumber domain using the same fast cosine and sine transform filters as in conventional algorithms, we split the primary and secondary fields into their symmetric and antisymmetric parts. For complex 3D source geometries, this approach is significantly more efficient than previous 2.5D algorithms. Our finite-difference algorithm also includes novel approaches for divergence correction at low frequencies and EM field interpolation across conductivity discontinuities. We describe the modeling scheme and demonstrate its accuracy and efficiency by comparisons of 2.5D-simulated data with 1D and 3D results. Y1 - 2011 U6 - https://doi.org/10.1190/GEO2011-0111.1 SN - 0016-8033 VL - 76 IS - 6 SP - F387 EP - F393 PB - Society of Exploration Geophysicists CY - Tulsa ER -