title: Modelling continental rift dynamics
title: Modellierung kontinentaler Riftdynamik
Creators: name: Brune, Sascha
Publisher: name: Universität Potsdam
Year: 2019

description: 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.
description: Kontinentale Grabensysteme eröffnen einzigartige Einsichten in das geodynamische System unseres Planeten: Geodynamische Lokalisierungs-prozesse prägen die Morphologie von Riftsystemen, indem sie die zeitabhängige Aktivität von Störungen, die topographische Entwicklung des Rifts oder dessen Symmetrieentwicklung kontrollieren. Da die Verformung oft in Richtung des Riftzentrums lokalisiert, sind die Deformationsstrukturen früherer Riftphasen meist gut erhalten und passive Ränder, die Endprodukte kontinentalen Riftings, beinhalten wichtige Informationen über die tektonische Geschichte vom Riftbeginn bis zum kontinentalen Zerbrechen. Unser gegenwärtiges Verständnis der Riftentwicklung basiert auf der Kombination von Beobachtungen in aktiven Rifts mit Informationen, die an passiven Kontinental-rändern gesammelt wurden. Die Einbindung dieser isolierten Datensätze erfolgt oft konzeptionell und lässt Raum für subjektive Interpretationen. Geodynamische Vorwärtsmodelle haben jedoch das Potenzial, einzelne Datensätze quantitativ zu verknüpfen, wobei zusätzliche Informationen aus der Gesteinsmechanik und Rheologie verwendet werden, die es ermöglichen, frühere konzeptionelle Riftmodelle weiter zu entwickeln. In dieser Arbeit kombiniere ich numerische, plattentektonische, analytische und analoge Modellierungsansätze, wobei die numerische thermomechanische Modellierung das primäre Werkzeug darstellt. Diese Methode hat sich in den letzten zwei Jahrzehnten aufgrund dedizierter Softwareentwicklung und der Verfügbarkeit von massiv parallelisierten Supercomputern sehr schnell entwickelt. Dennoch gelang es der geodynamischen Modellierungsgemeinschaft erst vor kurzem, die dreidimensionale lithosphärenskalige Riftentwicklung vom Beginn der Dehnung bis zum endgültigen Zerbrechen eines Kontinents zu erfassen. Meine Habilitationsschrift beinhaltet eine Einführung in kontinentale Rifttektonik, eine Zusammenfassung der angewendeten Modellierungsmethoden und einen kurzen Überblick über Vorstudien. In dem Hauptteil dieser Arbeit werden Untersuchungen auf Plattenrandskala in zwei und drei Dimensionen durchgeführt, gefolgt von globalen Analysen der Riftentwicklung. Dabei beschreibe ich die Bildung extrem ausgedehnter kontinentaler Kruste, die an vielen gerifteten Kontinentalrändern beobachtet wurde, die Rückkopplung zwischen Riftdynamik und Plattenbewegungen, der Wirkung von Riftschrägheit und tektonischer Vererbung auf die Riftarchitektur sowie den Einfluss von Riftsystemen auf CO2-Entgasung in Bezug auf die paläoklimatische Entwicklung der Erde seit dem Zerbrechen Pangäas.