4.1: Veins are common structures in rocks and occur in different geological settings ranging from continental to oceanic crustal environments. They form by mineral precipitation from a fluid phase within a dilatational site or due to displacive mineral growth. Veins may form under a range of temperatures and pressures and precipitate from fluids of different origin. As a result, vein structures and microtextures as well as mineralogical, elemental, and isotopic compositions vary depending on the geological environment and local to regional physicochemical conditions. Therefore, veins are ubiquitous structures in the geological record and represent an insightful geological material and tool to answer diverse research questions. In previous studies, vein microtextures and geochemical compositions of vein minerals have been widely used to reconstruct kinematic histories of rocks and to quantify the physicochemical conditions under which veins formed, respectively. Thus, this session invites contributions from structural geology using veins as stress and strain indicators, geochemical studies investigating elemental and isotopic vein mineral compositions such as fluid-rock interactions, laboratory as well as numerical experiments simulating natural vein microtextures, and applied geosciences dealing with ore mineralization hosted in veins.
4.2: We invite contributions from the fields of tectonics, structural geology, and crystalline geology. Regional and process-oriented studies from all kinds of active or fossil tectonic settings are welcome – rifting, ocean spreading, subduction, collision, transform, as well as intra-plate deformation. Studies dealing with the development of methods related to the deformation of crust and lithosphere from the micro-scale to plate scale are also invited.
9:00am - 9:15am
Tracing wedge-internal deformation by means of strontium isotope systematics of vein carbonate
1Institut für Geologie, Leibniz Universität Hannover, Germany; 2Institut für Geologie, Universität Bern, Switzerland
Radiogenic strontium isotopes (87Sr/86Sr) of vein carbonates play a crucial role in the tectono-metamorphic study of fold-and-thrust belts and accretionary wedges and have been used to document fluid sources and fluxes, for example, along major fault zones. Moreover, the 87Sr/86Sr ratios of vein carbonates can trace the diagenetic to metamorphic evolution of pore fluids entrapped in accreted sediments. Here we present 87Sr/86Sr ratios of vein carbonates from the paleo-accretionary complex of the central European Alps (Glarus Alps, Switzerland) that formed during early stages of continental collision. We show that the vein carbonates trace the Sr isotopic evolution of pore fluids from an initial seawater-like signature towards the isotopic composition of the host rock. This relation allows us to constrain the diagenetic to low-grade metamorphic conditions of deformation events, including imbricate thrusting, folding, cleavage development, stratal disruption and tectonic transport of thrust slices, bedding-parallel shearing, and extensional vein-formation. Taken together, the strontium isotope systematics of vein carbonate provides new insights into the prograde to early retrograde tectonic evolution of Alpine accretionary complex and helps to understand aspects that are not sufficiently clear from traditional cross-cutting relationships.
9:15am - 9:30am
Closely-spaced carbonate replacement veins: the influence of external stress on focused fluid flow during carbonation of peridotite
1Tectonics and Geodynamics, RWTH Aachen University, Germany; 2University of Texas at Austin, Bureau of Economic Geology, TX, USA
The reaction of serpentinized peridotites with CO2-bearing fluids to listvenite (quartz-carbonate rocks) requires massive fluid flux and maintained permeability despite volume increase. Here we investigate listvenites and serpentinites samples from Hole BT1B of the Oman Drilling Project to improve our understanding of the mechanisms and feedbacks of fracturing and vein formation during peridotite carbonation. The samples are characterized by a high abundance of magnesite veins which are often bundled into closely-spaced, parallel sets. Relative cross-cutting relationships suggest that these veins are among the earliest structures related to carbonation of serpentinite. These veins often show some features that are typical for antitaxial veins such as growth from a median line outwards. Their bisymmetric chemical zonation of variable Ca and Fe contents, a systematic distribution of SiO2 and Fe-oxide inclusions in these zones, and cross-cutting relations with Fe-oxides and Cr-spinel suggest that they are micro-scale reaction fronts recording the replacement of serpentine by carbonate. Local dolomite precipitation and voids along the vein – wall rock interface suggest that the veins acted as a preferred fluid pathway also after the first fracturing formed the central parts of the zoned magnesite veins. The close spacing and (sub)parallel alignment of the veins points to preferential fracturing of the weaker wall rock, in line with the interpretation that the veins formed in a serpentine matrix. The zoned magnesite veins therefore record an early stage of fluid infiltration during listvenite formation, at which focused fluid flow was controlled to large parts by external tectonic stress.
9:30am - 9:45am
2D finite-element modelling of the interaction between poroelastic effects and viscoelastic relaxation during the seismic cycle
1Institut für Geologie, Leibniz Universität Hannover, Germany; 2GFZ Potsdam, Germany
The analysis of Coulomb stress changes has become an important tool for seismic hazard evaluation because such stress changes may trigger or delay next earthquakes. Processes that can cause significant Coulomb stress changes include coseismic slip, earthquake-induced poroelastic effects and transient postseismic processes such as viscoelastic relaxation. In this study, we use 2D finite-element models for intracontinental normal and thrust faults to investigate the spatial and temporal evolution and the interaction of pore fluid pressure changes and postseismic viscoelastic relaxation. In different experiments, we vary (1) the permeability of the upper or lower crust and (2) the viscosity of the lower crust or lithospheric mantle, while keeping the other parameters constant. The results show that the highest pore pressure changes occur within a distance of ~ 1 km around the lower fault tip. In the postseismic phase, the pore pressure relaxes depending on the permeability in the upper crust until the pore pressure reaches the initial pressure of the preseismic phase. For high permeabilities in the upper crust, postseismic velocities within a few kilometers around the fault reach around 120 mm/a and decrease rapidly with time, whereas for low permeabilities velocities remain lower over the years after the earthquake. Models with low viscosity of the lower crust show that postseismic viscoelastic relaxation and poroelastic effects overlap in the early postseismic phase and decrease gradually within a few years after the earthquake. Higher viscosities lead to lower velocities, that last for decades on scales of several tens of kilometers.
9:45am - 10:00am
SpannEnD - The crustal stress state of Germany
1TU Darmstadt, Germany; 2KIT, Germany; 3GeoForschungsZentrum (GFZ), Germany
Information about the recent stress state of the upper crust is important for understanding tectonic processes and for the use of the underground in general. A currently important topic, the search for a radioactive waste deposit, illustrates this relevance, as the crustal stress state is decisive for the short and long-term safety of a possible repository. For example, the integrity of the host rock due to the activation or reactivation of faults and associated fluid pathways during.
However, the level of knowledge of the upper crustal stress field in Germany is quite low. The World Stress Map (WSM) and a new stress magnitude database give some insights, but only spatially unevenly distributed, often incomplete and rarely of good quality. Therefore, we present the first 3D geomechanical model of Germany that allows a comprehensive prediction of the complete stress tensor. The model covers an area of 1250 x 1000 km2 and contains 20 units with individual rock properties (Young's modulus, Poisson's ratio and density). It is calibrated against the datasets of the WSM and the magnitude database. Our results are in good agreement with the orientation of the maximum horizontal stress and show a good fit regarding the magnitudes of the minimum and maximum horizontal stress.
10:00am - 10:15am
The Zagros Mountain Front Flexure in Kurdistan Region of Iraq: Structural style and Late Pleistocene-Holocene Fault Slip Rates Derived from Structural Modeling and Luminescence Dating of River Terraces
1University of Jena, Germany; 2Salahaddin University-Erbil, Kurdistan Region of Iraq; 3University of Freiburg, Germany
The Zagros Mountain Front Flexure (MFF) makes a prominent topographic and structural step along the Zagros Fold-Thrust Belt that accommodates a significant amount of shortening between the Eurasian and Arabian plates. Here, the structural style below the MFF in the Kurdistan Region of Iraq was reconstructed using balanced cross-sections and forward modeling, and Late Pleistocene-Holocene fault-slip rates were calculated across several structures using luminescence dating of river terraces along the Greater Zab River. A balanced and retro-deformable cross-section for the NW Zagros reveals that reverse displacement on a basement fault underlying the MFF, along with fault-related folding above the Triassic detachment, is indispensable to explain the observed structural relief. The uplift rates of river terraces, obtained from their elevation and ages, indicate ongoing slip on faults when integrated with the kinematics of fault-related folds for the structures. The basement fault underlying the MFF accommodates 1.46±0.60 mm a-1 of slip, while a more external basement fault further to the SW is accommodating less than 0.41±0.16 mm a-1. Horizontal slip rates from detachment folding above the Triassic detachment in two anticlines (Sarta and Safin) within the Zagros Foothills are 0.40±0.10 and 1.24±0.36 mm a-1, respectively. Balanced cross-section, distribution of river terraces, and regional topography indicate that basement thrusting, and ductile thickening of the crust are restricted to the NE parts of the belt, and the deformation is limited mainly to folding and thrusting of the sedimentary cover above a Triassic basal detachment there in the SW parts.
10:15am - 10:30am
Slip tendency analysis for 60 3D faults in Germany and adjacent areas
1Karlsruhe Institute of Technology, Germany; 2Technical University Darmstadt, Germany; 3German Research Centre for Geosciences, Germany
Tectonic faults are of great importance for many underground applications such as hydrocarbon extraction, geothermal operations or nuclear waste repositories. In particular, the fault reactivation potential is crucial in regards of safety and efficiency of these applications. Major influences on the reactivation potential are the contemporary tectonic stress field and changes to it due to anthropogenic activities. One measure of the reactivation potential of faults is the ratio of resolved shear stresses to normal stresses on the fault surface, the slip tendency. The components of the stress tensor required for slip tendency analysis have been provided by the 3D geomechanical numerical model of Germany and its adjacent regions of the SpannEnD project. The derived stresses are mapped onto selected faults in order to calculate their slip tendency.
As only a finite number of 3D fault geometries could be generated, criteria for the selection of faults relevant to the scope of the SpannEnD project were identified. Their application led to the selection of 60 faults in the model area. For the selected faults simplified geometries were created (fault set 1). For a subset of the selected faults, vertical profiles and seismic sections could be used to generate semi-realistic 3D fault geometries (fault set 2). Slip tendency calculations using the stress tensor from the SpannEnD model were performed for both 3D fault sets and allow for an assessment of the fault reactivation potential which can be compared with the distribution of seismicity.