3D multi-scale finite element analysis of the present-day crustal state of stress and the recent kinematic behaviour of the northern and central Upper Rhine Graben

Cloetingh, S.A.P.L. [Promotor]Wenzel, F. [Promotor]Beekman, F. [Copromotor]Connoly, P.T. [Copromotor

Characterization of fracture initiation in non-cylindrical buckle folds using 3D finite element analysis

The spatial distribution of fracture sets associated with buckle folds has been well documented in field studies. There are difficulties, however, in placing accurate constraints on the timing of the initiation of individual fracture sets during the deformation history of the fold under in-situ conditions. This study investigates specific conditions that give rise to the initiation of various fracture sets in the hinge and limb of a pericline, based on an analysis of the effective stress evolution during the processes of buckling and erosional unloading. A 3D finite element modeling approach is used to simulate the effective stress evolution in single-layer folds with a Maxwell viscoelastic rheology, while including the influence of overburden stress, pore pressure, and a geologic strain rate. Several material properties and geometric features are varied to test their influence on fracture initiation. The modeling results show that fracturing is most heavily influenced by permeability, initial overburden thickness, and erosional unloading. Further analysis reveals that six fracture sets, which are observed in natural buckle folds, are also observed in the modeling results: outer arc tensile fractures, outer arc normal faults, and inner arc thrusts, all of which strike parallel to the fold axis, are determined to be common fracture sets; outer arc tensile fractures that strike perpendicular to the fold axis, and thrusts in the limb that strike roughly parallel to the fold axis are determined to be less common fracture sets that require low permeability (\u3c 10-21 m2) folding layers in order to initiate; vertical conjugate shear fractures in the outer arc, where the fold axis bisects the acute angle between fracture planes, are determined to be a rare fracture set. Most importantly, the timing of initiation is determined for each set, thus providing the most difficult information to obtain from a field study on fold related fractures --Abstract, page iii

Geomechanical-numerical modeling of the crustal stress state of Germany

The stress state in the earth crust is an important quantity for many scientific and technical questions, e.g., seismic hazard assessment, borehole stability and underground storage. However, the level of knowledge about the recent stress field in Germany is still limited. There are basically two larger data sets available: (1) the World Stress Map (WSM) providing mainly information about the orientation of the maximum horizontal stress (SHmax) as well as the stress regime and (2) a stress magnitude database of Germany providing magnitude information about the individual components of the absolute stress tensor. However, the data are sparse, pointwise and unequally distributed. Therefore, a continuous prediction of the recent crustal stress state of Germany by linear interpolation between these data points is not suitable, in particular due to vertical and lateral inhomogeneities, e.g., mechanical properties, or faults leading to stress variations within in the crust. The presented cumulative dissertation is part of the SpannEnD project that aims to enhance the state of knowledge of the stress field in Germany. This dissertation contains three manuscripts: two dealing with the continuous prediction of the recent crustal stress state of Germany by large scale (1000 x 1250 x 100 km3) geomechanical-numerical models and one with the analysis of slip tendency (TS) of faults in Germany using results of one of these models. The two geomechanical-numerical models contain units describing the present geological conditions, which are parameterized with individual rock properties. Linear elasticity is assumed and the Finite Element Method (FEM) is used to solve the equilibrium of forces. The models enable a continuous prediction of the absolute stress state based on continuum mechanics within the upper lithosphere for the entire area of Germany for the first time. The first model presented in this cumulative dissertation contains seven units: a sedimentary unit, four laterally overlapping units of the upper crust, the lower crust and parts of the lithospheric mantle. It is calibrated against magnitudes of the minimum horizontal stress (Shmin) and compared with orientations of SHmax of the WSM and some additional data. The results show an overall good fit to the orientation of SHmax with a mean of the absolute deviations of 15.6° and a median of 5.6° and to the Shmin magnitudes with a mean of the absolute differences of 3.3 MPa used for calibration. However, the SHmax magnitudes show some larger differences especially too low values within the upper part of the model. The second model is an improved version of the first model with focus on a higher stratigraphic resolution of the sedimentary unit, containing 22 units. In combination with an 18-time higher mesh resolution and an additional calibration with SHmax magnitudes the results show an overall good fit to the magnitudes of all principal stresses (Shmin, SHmax and the vertical stress (SV)) and the WSM orientation data. This is indicated by absolute differences of 0.0 MPa for SV, 4.6 MPa for Shmin and 6.4 MPa for SHmax and by a median of 0.3° and absolute differences of 11.9° for the SHmax orientations within the central part. The third manuscript shows a possible application for the results of a large-scale geomechanical-numerical model. It is a TS analysis of faults for Germany using the results of the first model of this study and three different sets of faults with increasing complexities. The analysis show a good spatial agreement between the calculated TS and earthquakes within the study area. However, the fit between the depth of earthquake occurrence and the highest predicted TS show some discrepancies. In general, the study shows the influence of the fault geometry, the fault orientation in relation to the stress field and the crucial influence of the pore pressure. The results of this cumulative dissertation are a step towards a better understanding of the recent stress field of Germany. By two geomechanical models providing - for the first time the 3D stress tensor - and results, which are in good agreement with different calibration and comparison data sets. However, there are still some local and few general deviations that need to be further investigated, for example, with smaller more complex models, for which the stress field of this study can be used as an initial stress state

Impact of geogenic degassing on C-isotopic composition of dissolved carbon in karst systems of Greece

The Earth C-cycle is complex, where endogenic and exogenic sources are interconnected, operating in a multiple spatial and temporal scale (Lee et al., 2019). Non-volcanic CO2 degassing from active tectonic structures is one of the less defined components of this cycle (Frondini et al., 2019). Carbon mass-balance (Chiodini et al., 2000) is a useful tool to quantify the geogenic carbon output from regional karst hydrosystems. This approach has been demonstrated for central Italy and may be valid also for Greece, due to the similar geodynamic settings. Deep degassing in Greece has been ascertained mainly at hydrothermal and volcanic areas, but the impact of geogenic CO2 released by active tectonic areas has not yet been quantified. The main aim of this research is to investigate the possible deep degassing through the big karst aquifers of Greece. Since 2016, 156 karst springs were sampled along most of the Greek territory. To discriminate the sources of carbon, the analysis of the isotopic composition of carbon was carried out. δ13CTDIC values vary from -16.61 to -0.91‰ and can be subdivided into two groups characterized by (a) low δ13CTDIC, and (b) intermediate to high δ13CTDIC with a threshold value of -6.55‰. The composition of the first group can be related to the mixing of organic-derived CO2 and the dissolution of marine carbonates. Springs of the second group, mostly located close to Quaternary volcanic areas, are linked to possible carbon input from deep sources

Impact of Etna’s volcanic emission on major ions and trace elements composition of the atmospheric deposition

Mt. Etna, on the eastern coast of Sicily (Italy), is one of the most active volcanoes on the planet and it is widely recognized as a big source of volcanic gases (e.g., CO2 and SO2), halogens, and a lot of trace elements, to the atmosphere in the Mediterranean region. Especially during eruptive periods, Etna’s emissions can be dispersed over long distances and cover wide areas. A group of trace elements has been recently brought to attention for their possible environmental and human health impacts, the Technology-critical elements. The current knowledge about their geochemical cycles is still scarce, nevertheless, recent studies (Brugnone et al., 2020) evidenced a contribution from the volcanic activity for some of them (Te, Tl, and REE). In 2021, in the framework of the research project “Pianeta Dinamico”, by INGV, a network of 10 bulk collectors was implemented to collect, monthly, atmospheric deposition samples. Four of these collectors are located on the flanks of Mt. Etna, other two are in the urban area of Catania and three are in the industrial area of Priolo, all most of the time downwind of the main craters. The last one, close to Cesarò (Nebrodi Regional Park), represents the regional background. The research aims to produce a database on major ions and trace element compositions of the bulk deposition and here we report the values of the main physical-chemical parameters and the deposition fluxes of major ions and trace elements from the first year of research. The pH ranged from 3.1 to 7.7, with a mean value of 5.6, in samples from the Etna area, while it ranged between 5.2 and 7.6, with a mean value of 6.4, in samples from the other study areas. The EC showed values ranging from 5 to 1032 μS cm-1, with a mean value of 65 μS cm-1. The most abundant ions were Cl- and SO42- for anions, Na+ and Ca+ for cations, whose mean deposition fluxes, considering all sampling sites, were 16.6, 6.8, 8.4, and 6.0 mg m-2 d, respectively. The highest deposition fluxes of volcanic refractory elements, such as Al, Fe, and Ti, were measured in the Etna’s sites, with mean values of 948, 464, and 34.3 μg m-2 d-1, respectively, higher than those detected in the other sampling sites, further away from the volcanic source (26.2, 12.4, 0.5 μg m-2 d-1, respectively). The same trend was also observed for volatile elements of prevailing volcanic origin, such as Tl (0.49 μg m-2 d-1), Te (0.07 μg m-2 d-1), As (0.95 μg m-2 d-1), Se (1.92 μg m-2 d-1), and Cd (0.39 μg m-2 d-1). Our preliminary results show that, close to a volcanic area, volcanic emissions must be considered among the major contributors of ions and trace elements to the atmosphere. Their deposition may significantly impact the pedosphere, hydrosphere, and biosphere and directly or indirectly human health

EVOLUTION OF THE SUBCONTINENTAL LITHOSPHERE DURING MESOZOIC TETHYAN RIFTING: CONSTRAINTS FROM THE EXTERNAL LIGURIAN MANTLE SECTION (NORTHERN APENNINE, ITALY)

Our study is focussed on mantle bodies from the External Ligurian ophiolites, within the Monte Gavi and Monte Sant'Agostino areas. Here, two distinct pyroxenite-bearing mantle sections were recognized, mainly based on their plagioclase-facies evolution. The Monte Gavi mantle section is nearly undeformed and records reactive melt infiltration under plagioclase-facies conditions. This process involved both peridotites (clinopyroxene-poor lherzolites) and enclosed spinel pyroxenite layers, and occurred at 0.7–0.8 GPa. In the Monte Gavi peridotites and pyroxenites, the spinel-facies clinopyroxene was replaced by Ca-rich plagioclase and new orthopyroxene, typically associated with secondary clinopyroxene. The reactive melt migration caused increase of TiO2 contents in relict clinopyroxene and spinel, with the latter also recording a Cr2O3 increase. In the Monte Gavi peridotites and pyroxenites, geothermometers based on slowly diffusing elements (REE and Y) record high temperature conditions (1200-1250 °C) related to the melt infiltration event, followed by subsolidus cooling until ca. 900°C. The Monte Sant'Agostino mantle section is characterized by widespread ductile shearing with no evidence of melt infiltration. The deformation recorded by the Monte Sant'Agostino peridotites (clinopyroxene-rich lherzolites) occurred at 750–800 °C and 0.3–0.6 GPa, leading to protomylonitic to ultramylonitic textures with extreme grain size reduction (10–50 μm). Compared to the peridotites, the enclosed pyroxenite layers gave higher temperature-pressure estimates for the plagioclase-facies re-equilibration (870–930 °C and 0.8–0.9 GPa). We propose that the earlier plagioclase crystallization in the pyroxenites enhanced strain localization and formation of mylonite shear zones in the entire mantle section. We subdivide the subcontinental mantle section from the External Ligurian ophiolites into three distinct domains, developed in response to the rifting evolution that ultimately formed a Middle Jurassic ocean-continent transition: (1) a spinel tectonite domain, characterized by subsolidus static formation of plagioclase, i.e. the Suvero mantle section (Hidas et al., 2020), (2) a plagioclase mylonite domain experiencing melt-absent deformation and (3) a nearly undeformed domain that underwent reactive melt infiltration under plagioclase-facies conditions, exemplified by the the Monte Sant'Agostino and the Monte Gavi mantle sections, respectively. We relate mantle domains (1) and (2) to a rifting-driven uplift in the late Triassic accommodated by large-scale shear zones consisting of anhydrous plagioclase mylonites. Hidas K., Borghini G., Tommasi A., Zanetti A. & Rampone E. 2021. Interplay between melt infiltration and deformation in the deep lithospheric mantle (External Liguride ophiolite, North Italy). Lithos 380-381, 105855