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Continental rift systems open up unique possibilities to study the geodynamic system of our planet: geodynamic localization processes are imprinted in the morphology of the rift by governing the time-dependent activity of faults, the topographic evolution of the rift or by controlling whether a rift is symmetric or asymmetric. Since lithospheric necking localizes strain towards the rift centre, deformation structures of previous rift phases are often well preserved and passive margins, the end product of continental rifting, retain key information about the tectonic history from rift inception to continental rupture.
Current understanding of continental rift evolution is based on combining observations from active rifts with data collected at rifted margins. Connecting these isolated data sets is often accomplished in a conceptual way and leaves room for subjective interpretation. Geodynamic forward models, however, have the potential to link individual data sets in a quantitative manner, using additional constraints from rock mechanics and rheology, which allows to transcend previous conceptual models of rift evolution. By quantifying geodynamic processes within continental rifts, numerical modelling allows key insight to tectonic processes that operate also in other plate boundary settings, such as mid ocean ridges, collisional mountain chains or subduction zones.
In this thesis, I combine numerical, plate-tectonic, analytical, and analogue modelling approaches, whereas numerical thermomechanical modelling constitutes the primary tool. This method advanced rapidly during the last two decades owing to dedicated software development and the availability of massively parallel computer facilities. Nevertheless, only recently the geodynamical modelling community was able to capture 3D lithospheric-scale rift dynamics from onset of extension to final continental rupture.
The first chapter of this thesis provides a broad introduction to continental rifting, a summary of the applied rift modelling methods and a short overview of previews studies. The following chapters, which constitute the main part of this thesis feature studies on plate boundary dynamics in two and three dimension followed by global scale analyses (Fig. 1).
Chapter II focuses on 2D geodynamic modelling of rifted margin formation. It highlights the formation of wide areas of hyperextended crustal slivers via rift migration as a key process that affected many rifted margins worldwide. This chapter also contains a study of rift velocity evolution, showing that rift strength loss and extension velocity are linked through a dynamic feed-back. This process results in abrupt accelerations of the involved plates during rifting illustrating for the first time that rift dynamics plays a role in changing global-scale plate motions. Since rift velocity affects key processes like faulting, melting and lower crustal flow, this study also implies that the slow-fast velocity evolution should be imprinted in rifted margin structures.
Chapter III relies on 3D Cartesian rift models in order to investigate various aspects of rift obliquity. Oblique rifting occurs if the extension direction is not orthogonal to the rift trend. Using 3D lithospheric-scale models from rift initialisation to breakup I could isolate a characteristic evolution of dominant fault orientations. Further work in Chapter III addresses the impact of rift obliquity on the strength of the rift system. We illustrate that oblique rifting is mechanically preferred over orthogonal rifting, because the brittle yielding requires a lower tectonic force. This mechanism elucidates rift competition during South Atlantic rifting, where the more oblique Equatorial Atlantic Rift proceeded to breakup while the simultaneously active but less oblique West African rift system became a failed rift. Finally this Chapter also investigates the impact of a previous rift phase on current tectonic activity in the linkage area of the Kenyan with Ethiopian rift. We show that the along strike changes in rift style are not caused by changes in crustal rheology. Instead the rift linkage pattern in this area can be explained when accounting for the thinned crust and lithosphere of a Mesozoic rift event.
Chapter IV investigates rifting from the global perspective. A first study extends the oblique rift topic of the previous chapter to global scale by investigating the frequency of oblique rifting during the last 230 million years. We find that approximately 70% of all ocean-forming rift segments involved an oblique component of extension where obliquities exceed 20°. This highlights the relevance of 3D approaches in modelling, surveying, and interpretation of many rifted margins. In a final study, we propose a link between continental rift activity, diffuse CO2 degassing and Mesozoic/Cenozoic climate changes. We used recent CO2 flux measurements in continental rifts to estimate worldwide rift-related CO2 release, which we based on the global extent of rifts through time. The first-order correlation to paleo-atmospheric CO2 proxy data suggests that rifts constitute a major element of the global carbon cycle.
Earthquake faults interact with each other in many different ways and hence earthquakes cannot be treated as individual independent events. Although earthquake interactions generally lead to a complex evolution of the crustal stress field, it does not necessarily mean that the earthquake occurrence becomes random and completely unpredictable. In particular, the interplay between earthquakes can rather explain the occurrence of pronounced characteristics such as periods of accelerated and depressed seismicity (seismic quiescence) as well as spatiotemporal earthquake clustering (swarms and aftershock sequences). Ignoring the time-dependence of the process by looking at time-averaged values – as largely done in standard procedures of seismic hazard assessment – can thus lead to erroneous estimations not only of the activity level of future earthquakes but also of their spatial distribution. Therefore, it exists an urgent need for applicable time-dependent models. In my work, I aimed at better understanding and characterization of the earthquake interactions in order to improve seismic hazard estimations. For this purpose, I studied seismicity patterns on spatial scales ranging from hydraulic fracture experiments (meter to kilometer) to fault system size (hundreds of kilometers), while the temporal scale of interest varied from the immediate aftershock activity (minutes to months) to seismic cycles (tens to thousands of years). My studies revealed a number of new characteristics of fluid-induced and stress-triggered earthquake clustering as well as precursory phenomena in earthquake cycles. Data analysis of earthquake and deformation data were accompanied by statistical and physics-based model simulations which allow a better understanding of the role of structural heterogeneities, stress changes, afterslip and fluid flow. Finally, new strategies and methods have been developed and tested which help to improve seismic hazard estimations by taking the time-dependence of the earthquake process appropriately into account.
This habilitation thesis includes seven case studies that examine climate variability during the past 3.5 million years from different temporal and spatial perspectives. The main geographical focus is on the climatic events of the of the African and Asian monsoonal system, the North Atlantic as well as the Arctic Ocean. The results of this study are based on marine and terrestrial climate archives obtained by sedimentological and geochemical methods, and subsequently analyzed by various statistical methods.
The results herein presented results provide a picture of the climatic background conditions of past cold and warm periods, the sensitivity of past climatic climate phases in relation to changes in the atmospheric carbon dioxide content, and the tight linkage between the low and high latitude climate system. Based on the results, it is concluded that a warm background climate state strongly influenced and/or partially reversed the linear relationships between individual climate processes that are valid today. Also, the driving force of the low latitudes for climate variability of the high latitudes is emphasized in the present work, which is contrary to the conventional view that the global climate change of the past 3.5 million years was predominantly controlled by the high latitude climate variability. Furthermore, it is found that on long geologic time scales (>1000 years to millions of years), solar irradiance variability due to changes in the Earth-Sun-Moon System may have increased the sensitivity of low and high latitudes to Influenced changes in atmospheric carbon dioxide.
Taken together, these findings provide new insights into the sensitivity of past climate phases and provide new background conditions for numerical models, that predict future climate change.
Quantitative thermodynamic and geochemical modeling is today applied in a variety of geological environments from the petrogenesis of igneous rocks to the oceanic realm. Thermodynamic calculations are used, for example, to get better insight into lithosphere dynamics, to constrain melting processes in crust and mantle as well as to study fluid-rock interaction. The development of thermodynamic databases and computer programs to calculate equilibrium phase diagrams have greatly advanced our ability to model geodynamic processes from subduction to orogenesis. However, a well-known problem is that despite its broad application the use and interpretation of thermodynamic models applied to natural rocks is far from straightforward. For example, chemical disequilibrium and/or unknown rock properties, such as fluid activities, complicate the application of equilibrium thermodynamics.
One major aspect of the publications presented in this Habilitationsschrift are new approaches to unravel dynamic and chemical histories of rocks that include applications to chemically open system behaviour. This approach is especially important in rocks that are affected by element fractionation due to fractional crystallisation and fluid loss during dehydration reactions. Furthermore, chemically open system behaviour has also to be considered for studying fluid-rock interaction processes and for extracting information from compositionally zoned metamorphic minerals. In this Habilitationsschrift several publications are presented where I incorporate such open system behaviour in the forward models by incrementing the calculations and considering changing reacting rock compositions during metamorphism. I apply thermodynamic forward modelling incorporating the effects of element fractionation in a variety of geodynamic and geochemical applications in order to better understand lithosphere dynamics and mass transfer in solid rocks.
In three of the presented publications I combine thermodynamic forward models with trace element calculations in order to enlarge the application of geochemical numerical forward modeling. In these publications a combination of thermodynamic and trace element forward modeling is used to study and quantify processes in metamorphic petrology at spatial scales from µm to km. In the thermodynamic forward models I utilize Gibbs energy minimization to quantify mineralogical changes along a reaction path of a chemically open fluid/rock system. These results are combined with mass balanced trace element calculations to determine the trace element distribution between rock and melt/fluid during the metamorphic evolution. Thus, effects of mineral reactions, fluid-rock interaction and element transport in metamorphic rocks on the trace element and isotopic composition of minerals, rocks and percolating fluids or melts can be predicted.
One of the included publications shows that trace element growth zonations in metamorphic garnet porphyroblasts can be used to get crucial information about the reaction path of the investigated sample. In order to interpret the major and trace element distribution and zoning patterns in terms of the reaction history of the samples, we combined thermodynamic forward models with mass-balance rare earth element calculations. Such combined thermodynamic and mass-balance calculations of the rare earth element distribution among the modelled stable phases yielded characteristic zonation patterns in garnet that closely resemble those in the natural samples. We can show in that paper that garnet growth and trace element incorporation occurred in near thermodynamic equilibrium with matrix phases during subduction and that the rare earth element patterns in garnet exhibit distinct enrichment zones that fingerprint the minerals involved in the garnet-forming reactions.
In two of the presented publications I illustrate the capacities of combined thermodynamic-geochemical modeling based on examples relevant to mass transfer in subduction zones. The first example focuses on fluid-rock interaction in and around a blueschist-facies shear zone in felsic gneisses, where fluid-induced mineral reactions and their effects on boron (B) concentrations and isotopic compositions in white mica are modeled. In the second example, fluid release from a subducted slab and associated transport of B and variations in B concentrations and isotopic compositions in liberated fluids and residual rocks are modeled. I show that, combined with experimental data on elemental partitioning and isotopic fractionation, thermodynamic forward modeling unfolds enormous capacities that are far from exhausted.
In my publications presented in this Habilitationsschrift I compare the modeled results to geochemical data of natural minerals and rocks and demonstrate that the combination of thermodynamic and geochemical models enables quantification of metamorphic processes and insights into element cycling that would have been unattainable so far.
Thus, the contributions to the science community presented in this Habilitatonsschrift concern the fields of petrology, geochemistry, geochronology but also ore geology that all use thermodynamic and geochemical models to solve various problems related to geo-materials.
The habilitation deals with the numerical analysis of the recurrence properties of geological and climatic processes. The recurrence of states of dynamical processes can be analysed with recurrence plots and various recurrence quantification options. In the present work, the meaning of the structures and information contained in recurrence plots are examined and described. New developments have led to extensions that can be used to describe the recurring patterns in both space and time. Other important developments include recurrence plot-based approaches to identify abrupt changes in the system's dynamics, to detect and investigate external influences on the dynamics of a system, the couplings between different systems, as well as a combination of recurrence plots with the methodology of complex networks. Typical problems in geoscientific data analysis, such as irregular sampling and uncertainties, are tackled by specific modifications and additions. The development of a significance test allows the statistical evaluation of quantitative recurrence analysis, especially for the identification of dynamical transitions. Finally, an overview of typical pitfalls that can occur when applying recurrence-based methods is given and guidelines on how to avoid such pitfalls are discussed. In addition to the methodological aspects, the application potential especially for geoscientific research questions is discussed, such as the identification and analysis of transitions in past climates, the study of the influence of external factors to ecological or climatic systems, or the analysis of landuse dynamics based on remote sensing data.
Alfred Wegeners ideas on continental drift were doubted for several decades until the discovery of polarization changes at the Atlantic seafloor and the seismic catalogs imaging oceanic subduction underneath the continental crust (Wadati-Benioff Zone). It took another 20 years until plate motion could be directly observed and quantified by using space geodesy. Since then, it is unthinkable to do neotectonic research without the use of satellite-based methods.
Thanks to a tremendeous increase of instrumental observations in space and time over the last decades we significantly increased our knowledge on the complexity of the seismic cycle, that is, the interplay of tectonic stress build up and release. Our classical assumption, earthquakes were the only significant phenomena of strain release previously accumulated in a linear fashion, is outdated. We now know that this concept is actually decorated with a wide range of slow and fast processes such as triggered slip, afterslip, post-seismic and visco-elastic relaxation of the lower crust, dynamic pore-pressure changes in the elastic crust, aseismic creep, slow slip events and seismic swarms. On the basis of eleven peer-reviewed papers studies I here present the diversity of crustal deformation processes. Based on time-series analyses of radar imagery and satellited-based positioning data I quantify tectonic surface deformation and use numerical and analytical models and independent geologic and seismologic data to better understand the underlying crustal processes.
The main part of my work focuses on the deformation observed in the Pamir, the Hindu Kush and the Tian Shan that together build the highly active continental collision zone between Northwest-India and Eurasia. Centered around the Sarez earthquake that ruptured the center of the Pamir in 2015 I present diverse examples of crustal deformation phenomena. Driver of the deformation is the Indian indenter, bulldozing into the Pamir, compressing the orogen that then collapses westward into the Tajik depression. A second natural observatory of mine to study tectonic deformation is the oceanic subduction zone in Chile that repeatedly hosts large earthquakes of magnitude 8 and more. These are best to study post-seismic relaxation processes and coupling of large earthquake.
My findings nicely illustrate how complex fashion and how much the different deformation phenomena are coupled in space and time. My publications contribute to the awareness that the classical concept of the seismic cycle needs to be revised, which, in turn, has a large influence in the classical, probabilistic seismic hazard assessment that primarily relies on statistically solid recurrence times.
The ecohydrological transfers, interactions and degradation arising from high-intensity storm events
(2015)
The habilitation thesis presented here includes results from several studies dealing with fluid-rock interactions and rock deformation processes in active fault zones. The focus in all of these studies is on the influence of clay minerals on the geochemical and the hydro-mechanical behavior of the fault rocks. The research was conducted on rock cores and cuttings from four scientific drilling projects at the San Andreas Fault (USA), the Nankai Trough subduction zone and the Japan Trench subduction zone (Japan), as well as the Alpine Fault in New Zealand. These ICDP (International Continental Scientific Drilling Program) and IODP (International Ocean Discovery Program) funded projects were all conducted with the aim to monitor and better understand earthquakes.
Chapter 1 contains a short introduction to the topic with basic principles and objectives regarding the research approach. Chapter 2 describes the state of the art in clay mineral and fault zone science, gives a short description of the individual drilling projects and their locations on which the research was based, and summarizes the most important analytical methods used. Chapter 3 comprises ten peer-reviewed publications that are connected thematically and methodologically. The papers were published in the years 2006-2015, and additional related publications including myself as co-author are given in the literature list. The ten publications address different questions concerning the formation of clay minerals and processes of fluid-rock interaction in active fault zones. Six papers contain results from the SAFOD drilling project, USA (San Andreas Fault Observatory at Depth), with the main focus on fluid-rock interaction processes in fault rocks and the formation and location of clay minerals. Three publications report on research from the NanTroSEIZE drilling project (Nankai Trough Seismogenic Zone Experiment) and the JFAST drilling project (Japan Trench Fast Drilling Project). Both projects are situated in Japan. Here, the swelling behavior of smectite clay minerals in relation to changing environmental conditions (e.g. temperature and/or humidity) was investigated. The last publication included here concerns a study from the DFDP project (Deep Fault Drilling Project) in New Zealand, where I investigated the deformation of clay minerals on the context of the hydro-mechanical behavior of the fault zone rocks. I was first author in nine of the publications and in charge of the project preparation, measurements and data analyses, and the completion of the manuscript. As co-author on the other publication I was responsible for electronmicroscopy analyses (SEM and TEM) and their interpretation.
The key results from the publications in Chapter 3 are discussed in Chapter 4 with additional considerations from more recent papers. Following the major theses in Chapter 5, Chapter 6 highlights a future research project in clay mineralogy research at the GFZ. An appendix includes more detailed descriptions of the laboratory equipment and lists of all publications, conference contributions and teaching courses and modules.