The tectonic history of southern Africa includes Archean craton formation, multiple episodes of subduction and rifting and some of the world's most significant magmatic events. Lithospheric models based on seismological and magnetotelluric data show highly heterogeneous crustal structure, significant anomalies in the lithospheric mantle and strong variations of the depth to the lithosphere-asthenosphere boundary. While some of the spatial patterns agree between different geophysical methods, there are also significant differences in the geometry and location of many structures. I perform a joint inversion of magnetotelluric and satellite gravity data to reconcile these apparent discrepancies. Resistivity and density are coupled through a newly developed Variation of Information constraint which strives to establish a one-to-one relationship between the two quantities. This allows the resistivity-density relationship to evolve data driven during the inversion. The final combined resistivity-density model shows detailed lithospheric and sub-lithospheric structure below the Kaapvaal Craton and adjacent mobile belts. In addition, the retrieved parameter relationship exhibits several branches indicating strong variations in composition. Compared to results from a similar inversion in the western United States, the inversion indicates significantly less fluid related low resistivity anomalies and a dominance of high-density low-resistivity structures. This corroborates earlier ideas that fluids are difficult to contain within in the Earth over long time scales. I will discuss the implications for the tectonic evolution of the region and for the interpretation of low resistivity anomalies world-wide.

How to cite: Moorkamp, M.: An integrated resistivity-density model of Southern Africa, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6068, https://doi.org/10.5194/egusphere-egu23-6068, 2023.

Minimum-structure, or Occam’s, style of inversion deals with the fundamental non-uniqueness of the inverse problem by finding the simplest Earth model that reproduces the observations. As an additional consequence of this approach, minimum-structure inversion is also reliable and robust. Because of these reasons, it has been extensively utilized in mineral and petroleum exploration problems, and lithospheric studies. The method has been adapted and extended in many ways to obtain more reliable and realistic models of the Earth’s subsurface. Joint inversion of geophysical data-sets is one of the most important extensions of minimum-structure inversion. This method can reduce the non-uniqueness of the inverse problem by combing two, or more, different geophysical data-sets in a single inverse problem. Different geophysical methods have different sensitivity to different physical properties, hence, it is hoped that the null space for one type of data can be spanned by the other.

Joint inversion algorithms can be divided into two main categories, structural-based and petrophysical-based joint inversion methods, depending on the coupling measure used between the physical property models. We have adopted the fuzzy c-mean (FCM) clustering technique which is a petrophysical-based method to jointly invert MT and gravity data-sets. The optimization of this method is not as challenging as for structural-based approaches. We have also performed constrained FCM clustering for independent MT and gravity inversions to compare the constructed models of this method with the joint inversion, and independent MT and gravity inversions. The FCM clustering method makes effective use of statistical petrophysical data which may exist in complex geological structures, or can be anticipated, to encourage the inverted physical property values to move towards the a priori petrophysical data as target clusters.

The capabilities of the joint and constrained FCM clustering inversion are evaluated on synthetic and real examples. The constructed density and conductivity models from the joint inversion have a more plausible representation of the true model’s geometry and have a reasonable range of the recovered physical property values compared to the independent constrained FCM clustering technique and independent unconstrained MT and gravity inversions.

Keywords: Fuzzy c-mean clustering, Gravity, Joint inverse problem, MT, Unstructured tetrahedral mesh

How to cite: Kangazian, M. and Farquharson, C.: Fuzzy c-mean clustering joint inversion of magnetotelluric (MT) and gravity data-sets using unstructured tetrahedral meshes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10193, https://doi.org/10.5194/egusphere-egu23-10193, 2023.

The western part of the Anatolian peninsula is defined with an N-S extensional regime and resulting graben systems, referred as the Aegean Extensional Province (AEP). The transition of this extensional regime in the west to E-W compressional regime in the east is bounded by the Isparta Angle, which is a reverse v-shaped major structure developed due to nappe emplacements and related clockwise and counter-clockwise rotations.

The study area is located at the eastern end of the Curuksu Graben and covers the western part of the Acigol Graben. Curuksu Graben is part of the Denizli Horst-Graben system and located between the junction point of three major grabens of the AEP in the west and Acigol Graben in the east. The development of the Graben systems in the AEP, including Curuksu and Acigol grabens, is resulted from two extensional periods interrupted by a compressional phase creating a disconformity between deformed ancient and undeformed neotectonic graben fills in the region. Denizli Horst-Graben System consists of incipient grabens and the modern graben (Curuksu Graben) which is developed with the introduction of the neotectonic regime with the latest Pliocene. This episodic development caused an inward development of normal faults. The compressional regime created many strike-slip and reverse faults in the region while many of the currently active normal faults with strike-slip components are resulted from the present day NNE-SSW extensional phase, including seismically active margin-bounding major faults with seismic hazard potential.

In this study, we have reassessed the data from 300 Magnetotelluric stations which were previously collected by a private contractor for geothermal research purposes. We have investigated the main properties of the data with Phase Tensor analysis and inverted it using ModEM software to reveal 3D subsurface conductivity structure of the area. Our analysis shows that the Phase Tensor ellipses for the highest frequencies indicate rather 1D behavior in compliance with the undeformed, nearly horizontal bedding of the neotectonic graben fills. The strike directions obtained through induction vectors and Phase Tensor ellipses reflects the dominant effect of the major faults and conductive basin fills. The recovered model from the 3D inversion results show the depth of the basin fills and conductivity anomalies due to normal faults controlling the basin development.

How to cite: Karcioglu, G., Turkoglu, E., and Avsar, U.: Resistivity structure of Denizli Çürüksu & Acıgöl Graben connection: Preliminary results from 3D inversion of magnetotelluric data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-347, https://doi.org/10.5194/egusphere-egu23-347, 2023.

We collected long-period magnetotelluric (LMT) data on the 64 stations by using a remotely controlled measurement system in northwestern Anatolia, Türkiye. The data was collected along four nearly parallel 300-km-long lines. In our previous project, we already collected broadband magnetotelluric(MT) data on 358 stations along those four lines. These lines are crossing main tectonic traverses such as North Anatolia Fault Zones, and Intra-Pontid Suture zones.  The remotely controlled MT measurement system provides us to control data quality during measurement and we can change the station location if the data quality is not good in the current stations.  This ensures the good data quality of all MT sites. After the time series analysis and main data processing procedure such as phase tensor decomposition and static shift correction, we interpreted each line's data set by using a two-dimensional inversion algorithm. We also inverted LMT data by using a three-dimensional inversion algorithm. The three-dimensional resistivity model also showed us to main tectonic units as two-dimensional resistivity models. Additionally, crust lithosphere relations were also revealed. We obtained upper and lower crust boundaries by using magnetic data and crust al depth by using gravity data. Those results also validated our resistivity models obtained from MT data inversion. We are going to give preliminary interpretation results of the lithosphere structure of northwest Anatolia in this presentation.

Acknowledgement:  This study is supported by TUBITAK (The Scientific and Technological Research Council of Turkey) Project with Grant Number 119Y197. We thanks TUBITAK.

How to cite: Candansayar, M. E., Demirci, İ., Gündoğdu, N. Y., Oskay, M. D., and Erdoğan, E.: Revealing Lithospheric structure of Northwestern Anatolia by using 3D inversion of Long Period Magnetotelluric Data collected remotely controlled measurement system: Preliminary results, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16800, https://doi.org/10.5194/egusphere-egu23-16800, 2023.

The South China Block (SCB) is located at the junction of the Pacific, Eurasian, and Tethys plates. Their interaction led to large-scale multi-stage mineralization in the SCB during the Mesozoic. Several regional ore-concentration areas, such as the Middle-Lower Yangtze metallogenic belt (MLYMB), the Wuyishan metallogenic belt (WYMB), the Nanling metallogenic belt (NLMB), and the Qinzhou-Hangzhou Metallogenic Belt (QHMB) were formed during this process. However, the mineral types of these metallogenic belts are different. To study the deep mechanisms of the different metallogenic types developed in the same tectonic background at almost the same period, the magnetotelluric sounding (MT) data from 691 sites located mainly within the eastern SCB (Fig.1) are employed to obtain a regional lithospheric 3-D resistivity model.

According to this model, Large-scale low-resistivity bodies extend from the crust to the upper mantle beneath the MLYMB and the QHMB, which are interpreted as channels of upper mantle upwelling. While the upper- to mid-crust beneath the WYMB and the NLMB is characterized by high resistivity with small-scale low-resistivity anomalies, indicating upwelling mantle materials having invaded the crust on a small scale. Large-scale upper mantle low-resistivity anomalies extend along its strike direction beneath the MLYMB and the QHMB. It could be concluded from the resistivity model that deep low-resistivity anomalies and mantle upwelling channels mainly controlled almost all the Mesozoic deposits(Fig.3). However, the scales of low-resistivity anomalies and upper-crust ore-controlling structures are different. Significantly, the upper-mantle low-resistivity anomalies beneath the eastern SCB show a spatial distribution that is gradually shallowing from south to north, probably indicating that the asthenospheric materials are upwelling from south to north, corresponding with the changing progressively of magma and metallogenic activities. We propose that the lithospheric delamination and asthenospheric upwelling caused by the far-field effects of the paleo-Pacific Plate subduction are the source of solid magmatic activities and related metallogeny (Fig.3).

* This work was jointly supported by the China Geological Survey project (DD20160082 and DD20190012) and the SINOPROBE project.

Fig. 1 Distribution of MT stations within the study area. The metallogenic belts after Yan et al. (2021).