ALBERT

Description: Despite the amount of research focused on the Alpine orogen, significant unknowns remain regarding the thermal field and long term lithospheric strength in the region. Previous published interpretations of these features primarily concern a limited number of 2D cross sections, and those that represent the region in 3D typically do not conform to measured data such as wellbore or seismic measurements. However, in the light of recently published higher resolution region specific 3D geophysical models, that conform to secondary data measurements, the generation of a more up to date revision of the thermal field and long term lithospheric yield strength is made possible, in order to shed light on open questions of the state of the orogen. The study area of this work focuses on a region of 660 km x 620 km covering the vast majority of the Alps and their forelands, with the Central and Eastern Alps and the northern foreland being the best covered regions.

Description: The long‐term strength of the lithosphere is controled by two different modes of deformation: a brittle‐like, effective pressure‐sensitive behavior at shallow crustal depth, which gradually transits to a thermally activated ductile flow rheology with increasing depth. All applications dealing with long‐term tectonics therefore share the necessity to describe in a consistent way the multiphysics coupling among the different deformation mechanisms controlling the bulk behavior of the lithosphere. We describe an efficient numerical implementation of a consistent visco‐elasto‐plastic rheology suitable to describe the first‐order aspects of continental rock masses. Different from typical long‐term geodynamics numerical frameworks, we explicitly account for both volumetric and deviatoric response of lithospheric rocks to applied loads. Plastic correction to a viscoelastic stress state is introduced via a non‐associative Drucker‐Prager model, without resorting to the assumption of a plastic limiter. The transient behavior of crustal and lithospheric rocks is accounted for by an overstress (rate‐dependent) viscoplastic rheology, which additionally helps solving for numerical issues related to plastic strain accumulation even in the absence of energetic feedbacks. When applied to the study of the dynamics of plume‐lithosphere interactions, our implementation is able to reproduce a surface topography with complex multiharmonic wavelength patterns in agreement with observations. In the final chapter, we discuss main limitations of the current rheological description when applied to the study of transient semi‐brittle rock behavior. These aspects are tackled in a companion paper, where a thermodynamically consistent formulation extending the current numerical description is presented.

Description: We develop a fully coupled hydro-mechanical model to simulate fault slip due to fluid injection. We consider the interaction between a hydraulic fracture and pre-existing faults as well as the fluid exchange between the fracture/fault and the porous matrix. In order to consider a pressure diffusion mechanism, we set a relatively high permeability around the stimulated path. Our parametric study shows that a couple of factors affect the fault activation and its slip behavior such as fault properties, friction properties and injection scenario. We observe that pore pressure diffusion induces poroelastic stress change, which are able to produce shut-in events with a time and space lag. This mechanism also affects the slip behavior during injection in particular when the surrounding permeability is high (e.g., up to 1e-13 m/s), and provides a new insight into understanding the occurrence of stronger seismic events after shut-in compared to the injection phase. In addition, we show that small perturbations may trigger large seismic fault slip which highlights the key role of the initial fault stress state. The results have profound implications for deep fluid injection related engineering as well as for soft cyclic injection strategies aiming to mitigate the risk of large earthquakes.

Description: The North Patagonian Massif (NPM) area in Argentina includes a plateau of 1200 m a.s.l. (meters above sea level) average height, which is 500–700 m higher than its surrounding areas. The plateau shows no evidence of internal deformation, while the surrounding basins have been deformed during Cenozoic orogenic events. Previous works suggested that the plateau formation was caused by a lithospheric uplift event during the Paleogene. However, the causative processes responsible for the plateau origin and its current state remain speculative. To address some of these questions, we carried out 3D lithospheric-scale steady-state and transient thermal simulations of the NPM and its surroundings, as based on an existing 3D geological model of the area. Our results are indicative of a thicker and warmer lithosphere below the NPM plateau compared with its surroundings, suggesting that the plateau is still isostatically buoyant and thus explaining its present-day elevation. The transient thermal simulations agree with a heating event in the mantle during the Paleogene as the causative process leading to lithospheric uplift in the region and indicate that the thermo-mechanical effects of such an event would still be influencing the plateau evolution today. Although the elevation related to the heating would not be enough to reach the present plateau topography, we discuss other mechanisms, also connected with the mantle heating, that may have caused the observed relief. Lithosphere cooling in the plateau is ongoing, being delayed by the presence of a thick crust enriched in radiogenic minerals as compared to its sides, resulting in a thermal configuration that has yet to reach thermodynamic equilibrium.

Description: Transient processes play a major role in geophysical applications. In this paper, we quantify the significant influence arising from transient processes for conductive heat transfer problems for sedimentary basin systems. We demonstrate how the thermal properties are affected when changing the system from a stationary to a non-stationary (transient) state and what impact time-dependent boundary conditions (as derived from paleoclimate information) have on the system's overall response. Furthermore, we emphasize the importance of the time-stepping approach adopted to numerically solve for the transient case and the overall simulation duration since both factors exert a direct influence on the sensitivities of the thermal properties. We employ global sensitivity analyses to quantify not only the impact arising from the thermal properties but also their parameter correlations. Furthermore, we showcase how the results of such sensitivity analysis can be used to gain further insights into the complex Central European Basin System in central and northern Europe. This computationally very demanding workflow becomes feasible through the construction of high-precision surrogate models based on the reduced basis (RB) method.

Description: In Geosciences, we face the challenge of characterizing uncertainties to provide reliable predictions of the earth surface to allow, for instance, a sustainable and renewable energy management. In order, to address the uncertainties we need a good understanding of our geological models and their associated subsurface processes. Therefore, the essential pre-step for uncertainty analyses are sensitivity studies. Sensitivity studies aim at determining the most influencing model parameters. Hence, we require them to significantly reduce the parameter space to avoid unfeasibly large compute times. We distinguish two types of sensitivity analyses: local and global studies. In contrast, to the local sensitivity study, the global one accounts for parameter correlations. That is the reason, why we employ in this work a global sensitivity study. Unfortunately, global sensitivity studies have the disadvantage that they are computationally extremely demanding. Hence, they are prohibitive even for state-of-the-art finite element simulations. For this reason, we construct a surrogate model by employing the reduced basis method. The reduced basis method is a model order reduction technique that aims at significantly reducing the spatial and temporal degrees of freedom of, for instance, finite element solves. In contrast to other surrogate models, we obtain a surrogate model that preserves the physics and is not restricted to the observation space. As we will show, the reduced basis method leads to a speed-up of five to six orders of magnitude with respect to our original problem while retaining an accuracy higher than the measurement accuracy. In this work, we elaborate on the advantages of global sensitivity studies in comparison to local ones. We use several case studies, from large-scale European sedimentary basins to demonstrate how the global sensitivity studies are used to learn about the influence of transient, such as paleoclimate effects, and stationary effects. We also demonstrate how the results can be used in further analyses, such as deterministic and stochastic model calibrations. Furthermore, we show how we can use the analyses to learn about the subsurface processes and to identify model short comes.

Description: The Upper Rhine Graben (URG), as a part of the wider European Cenozoic Rift System, is a tectonically active area that has been extensively investigated for its geothermal energy potential. In this study, we carry out a first investigation of the present-day thermo-mechanical stability of the area as based on a detailed 3D geological and thermal model. The overall goal is, therefore, to assess how the lithospheric strength varies within the URG in response to the natural tectonic setting as well as the internal thermal configuration, and how those variations can be related to the recorded seismicity. The results from the modeling indicate that there is a spatial correlation between the predictions for the graben-wide rheological configuration with both the deep thermal field and the configuration of the crystalline crust. We find that the regional characteristics of the long-term strength of the lithosphere match the spatial distribution of seismicity, indicating that the mechanical stability of the area is primarily controlled by resolved strength variations. By cross-plotting the modeled strength distribution with available seismicity catalogs, our results suggest that seismicity in the graben area is shallower and of lower intensity due to a hotter and weaker crust compared to its surrounding areas. In contrast, seismic energy release appears to occur at deeper levels and being of larger magnitudes east of the graben and in the adjacent Lower Rhine Graben to the north. These results demonstrate the relevance of a proper quantification of the lithospheric rheological configuration and its spatial variability in response to its tectonic inheritance as an asset to interpret the pattern and distribution of seismicity observed in the area.

Description: Temperature exerts a first order control on rock strength, principally via thermally activated creep deformation and on the distribution at depth of the brittle-ductile transition zone. The latter can be regarded as the lower bound to the seismogenic zone, thereby controlling the spatial distribution of seismicity within a lithospheric plate. As such, models of the crustal thermal field are important to understand the localisation of seismicity. Here we relate results from 3D simulations of the steady state thermal field of the Alpine orogen and its forelands to the distribution of seismicity in this seismically active area of Central Europe. The model takes into account how the crustal heterogeneity of the region effects thermal properties and is validated with a dataset of wellbore temperatures. We find that the Adriatic crust appears more mafic, through its radiogenic heat values (1.30E-06 W/m3) and maximum temperature of seismicity (600 °C), than the European crust (1.3–2.6E-06 W/m3 and 450 °C). We also show that at depths of 〈10 km the thermal field is largely controlled by sedimentary blanketing or topographic effects, whilst the deeper temperature field is primarily controlled by the LAB topology and the distribution and parameterization of radiogenic heat sources within the upper crust.

Description: The brittle‐ductile transition is a domain of finite extent characterized by high differential stress where both brittle and ductile deformation are likely to occur. Understanding its depth location, extent, and stability through time is of relevance for diverse applications including subduction dynamics, mantle‐surface interactions, and, more recently, proper targeting of high‐enthalpy unconventional geothermal resources, where local thermal conditions may activate ductile creep at shallower depths than expected. In this contribution, we describe a thermodynamically consistent physical framework and its numerical implementation, therefore extending the formulation of the companion paper Jacquey and Cacace (2020, https://doi.org/10.1029/2019JB018474) to model thermo‐hydro‐mechanical coupled processes responsible for the occurrence of transitional semi‐brittle, semi‐ductile behavior in porous rocks. We make use of a damage rheology to account for the macroscopic effects of microstructural processes leading to brittle‐like material weakening and of a rate‐dependent plastic model to account for ductile material behavior. Our formulation additionally considers the role of porosity and its evolution during loading in controlling the volumetric mechanical response of a stressed rock. By means of dedicated applications, we discuss how our damage poro‐visco‐elasto‐viscoplastic rheology can effectively reconcile the style of localized deformation under different confining pressure conditions as well as the bulk macroscopic material response as recorded by laboratory experiments under full triaxial conditions.