Continental rift formation and transition to oceanic seafloor spreading: a case study of the Afar triple junction De vorming van continentale riftzones en de transitie naar mid- oceanische spreidingsruggen: een studie van de Afar driehoek (met een samenvatting in het Nederlands) Proefschrift ter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de rector magnificus, prof.dr. G.J. van der Zwaan, ingevolge het besluit van het college voor promoties in het openbaar te verdedigen op vrijdag 30 juni 2017 des middags te 12.45 uur door Alessio Lavecchia geboren op 14 mei 1985 te Terlizzi, Italië Promotor: Prof. dr. S. Cloetingh Copromotoren: Dr. F. Beekman Dr. S. Clark This thesis was partly accomplished with financial support from Simula School of Research and Innovation, Fornebu, Norway. "O frati," dissi, "che per cento milia perigli siete giunti a l’occidente, a questa tanto picciola vigilia d’i nostri sensi ch’è del rimanente non vogliate negar l’esperïenza, di retro al sol, del mondo sanza gente. Considerate la vostra semenza: fatti non foste a viver come bruti, ma per seguir virtute e canoscenza". “O brothers!” I began, “woe to the west 'Through perils without number now have we reach'd; 'To this the short remaining watch, that yet 'Our senses have to wake, refuse not proof 'Of the unpeopled world, following the track 'Of Phoebus. Call to mind from whence ye sprang: 'Ye were not form'd to live the life of brutes, 'But virtue to pursue and knowledge high.” Dante, Divina Commedia, Canto XXVI, Inferno Translated by Rev. H. Cary, 1819 Members of the Reading Committee: Prof. dr. R. Buck Earth Institute, Columbia University, Palisades, NY Prof. dr. G. Corti Università degli studi di Firenze, Italy Prof. dr. D.Dingwell Ludwig-Maximilians Universität München, Germany Prof. dr. T. Gerya Department of Earth Sciences, ETH Zürich, Switzerland Dr. C. Thieulot Faculty of Geosciences, Utrecht University, Netherlands The research presented in this thesis was carried out at Simula Research Laboratory, Fornebu, Norway, and the Tectonics Research Group, Department of Earth Sciences, Faculty of Geosciences, Utrecht University, the Netherlands. This is Utrecht Studies in Earth Sciences volume 131 ISBN: 978-90-6266-472-6 © Alessio Lavecchia, 2017 Printed in the Netherlands by Ridderprint (www.ridderprint.nl), typeset in LibreOffice Writer by the author. Cover: Cross-section showing viscosity field in one of the numerical experiments described in this research study (see Figure 5.4). All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system without prior written permission from the author. Chapter 3 reproduced with kind permission of John Wiley and Sons. Chapter 4 and 5 reproduced with kind permission of Elsevier. Table of Contents Acknowledgements..................................................................................1 Summary..................................................................................................3 Samenvatting...........................................................................................6 1. Introduction..........................................................................................7 1.1 Continental rifts: general aspects..................................................8 1.2 Magmatic and amagmatic riftings, active and passive..................9 1.3 Mantle plumes, relationships with rifting areas..........................11 1.4 Scope of the thesis.......................................................................12 1.5 Outline of the thesis....................................................................14 2. Afar Rift, geological and tectonic setting..........................................15 2.1 General overview........................................................................15 2.2 Neoproterozoic to Eocene: before the African superplume........17 2.3 Oligocene to Pliocene: continental rifting and beginning of oceanic spreading...........................................................................................18 2.4 Quaternary: last stages of the Afar evolution..............................20 2.5 A kinematic reconstruction of plate movement in the Afar area.20 2.6 Mechanics of Afar rifting and spreading: one or more plumes?.21 3. Thermal perturbation, mineral assemblages,and rheology variations induced by dyke emplacement in the crust...............................................................23 3.1 Introduction.................................................................................23 3.2 Characteristics of the crustal model............................................25 3.2.1 Geometrical and thermal features........................................25 3.2.2 Metamorphic features..........................................................29 3.2.3 Rheological features............................................................31 3.3 Modelling results and sensitivity................................................33 3.3.1 Modelling results.................................................................33 3.3.2 Parameter sensitivity...........................................................39 3.4 Discussion...................................................................................40 3.5 Conclusions.................................................................................43 4. Thermo-rheological aspects of crustal evolution during continental breakup and melt intrusion: The Main Ethiopian Rift, East Africa.....................44 4.1 Introduction.................................................................................44 4.2 Tectonic setting...........................................................................45 4.3 Model description.......................................................................47 4.3.1 Geometrical features............................................................47 4.3.2 Thermal and petrogenetic features......................................50 4.3.3 Rheological features............................................................53 4.4 Results.........................................................................................55 4.5 Discussion...................................................................................60 4.6 Conclusions.................................................................................64 5 Lithosphere erosion and continental breakup: interaction of extension, plume upwelling and melting...........................................................................66 5.1 Introduction.................................................................................66 5.2 Model setting...............................................................................68 5.2.1 Model geometry and governing equations..........................68 5.2.2 Rheology.............................................................................71 5.2.3 Density model......................................................................72 5.2.4 Partial melting.....................................................................72 5.3 Results.........................................................................................74 5.4 Discussion...................................................................................85 5.5 Conclusions.................................................................................87 6. Conclusions and synthesis.................................................................89 6.1 Outlook and suggestions for future research...............................93 References..............................................................................................94 Acknowledgements This thesis is the final result of dedication, efforts and both professional and personal growth. During the last four years I have been through experience that helped me to mature, in so many different ways that I could have never imagined. This shaped me in a very different person from who I was when I left Italy, and it would have never happened without the help and support of all the people that crossed their path with mine. First, I need to express my gratitude to my supervisors Fred Beekman, Stuart Clark, and Sierd Cloetingh, for offering me the possibility of pursuing my research in two institutes of high scientific profile like Simula Research Laboratory and Utrecht University, and working on a very challenging but inspiring project. In addition, special thanks go to Cedric Thieulot, for giving me the opportunity to work with him. His help has been fundamental for finalize my thesis, and I learned a lot from our work together. They have been people that marked my professional and personal growth, and it was a privilege to work with them. I really hope that we will have further opportunities to work together in the future. My appreciation goes to Bjørn Rasmussen, for promoting the collaboration between Simula Research Laboratory and Utrecht University, giving me the opportunity to work with such high- profile people and making this PhD successful. I would like to commend the members of my reading committee, Prof. Roger Buck, Prof. Giacomo Corti, Prof. Donald Dingwell, Prof. Taras Gerya and Dr. Cedric Thieulot. Their feedback greatly contributed in improving the quality of my thesis. I would also like to thank professors Alfredo Caggianelli and Giorgio Ranalli, not only for the fruitful