Cryptic Orogeny uplift of the Al Hajar Mountains at an alleged passive margin Reuben Johannes Hansman Academic dissertation for the Degree of Doctor of Philosophy in Geology at Stockholm University to be publicly defended on Friday 7 September 2018 at 10.00 in De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14. Abstract Mountains evolve and grow because of the large forces that occur from the collision of tectonic plates. Plate boundaries change and move through time, and regions that were once stable, shallow-marine environments can be dragged into subduction zones and get transformed into vast mountain ranges. The Al Hajar Mountains in Oman consist of carbonate rocks which show that during most of the Mesozoic (c. 268 Ma – 95 Ma) they had not yet formed but were flat and below sea level. Following this, in the Late Cretaceous (c. 95 Ma), a major tectonic event caused oceanic crust to be obducted onto this Mesozoic carbonate platform. Then after obduction a shallow marine environment resumed, and Paleogene sedimentary rocks were deposited. Currently, the central mountains are located on the Arabian Plate and are 200 km away from the convergent plate boundary with Eurasia. Here, Arabia is being subducted. Further towards the northwest Arabia and Eurasia are colliding, forming the Zagros Mountains which initiated no earlier than the Oligocene (c. 30 Ma). At this time the mountains were even further away from the plate boundary. The problem with the Al Hajar Mountains is that they record a collision, but are not in a collisional zone. To better understand the formation of the Al Hajar Mountains, a multidiscipline approach was used to investigate the timing at which they developed. This included applying low- temperature thermochronology, U-Pb dating of brittle structures, and balanced cross-sections. Results indicate that the orogeny began in the late Eocene and had concluded by the early Miocene (40 Ma – 15 Ma). Therefore, the uplift of the Al Hajar Mountains is not related to either the older Late Cretaceous ophiolite obduction or the younger Zagros collision, and a new tectonic model is proposed. This research shows that the Cenozoic tectonic history of northern Oman is more cryptic than what has been formerly presented. Keywords: uplift, mountains, structural, low-temperature, thermochronology, dating, fission-track, (U-Th)/He, U-Pb, calcite, structure-from-motion, photogrammetry, UAV, trishear, fault-propagation, Hafit, Al Hajar Mountains, United Arab Emirates, Oman. Stockholm 2018 http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-157506 ISBN 978-91-7797-338-6 ISBN 978-91-7797-339-3 Department of Geological Sciences Stockholm University, 106 91 Stockholm CRYPTIC OROGENY Reuben Johannes Hansman Cryptic Orogeny uplift of the Al Hajar Mountains at an alleged passive margin Reuben Johannes Hansman ©Reuben Johannes Hansman, Stockholm University 2018 ISBN print 978-91-7797-338-6 ISBN PDF 978-91-7797-339-3 Cover: Photograph of the Al Hajar Mountains, Oman (colour correction by Mehran Hussain). Printed in Sweden by Universitetsservice US-AB, Stockholm 2018 Distributor: Department of Geological Sciences, Stockholm University To my friend Graeme Blackburn 1984 – 2018 tetelestai Sammanfattning Bergskedjor bildas till följd av stora de stora krafter som inträffar vid en kontinentkollision. Kontinentala plattgränser förändras och flyttas över tiden och tidigare grundhavsområden kan omvandlas till långsträckta bergskedjor. Al Hajar-bergen i Oman består av karbonatbergarter som deponerades som horisontella lager under havsytan under mesozoikum (c. 268 – 95 Ma), vilket visar att bergskedjan måste ha bildats snare än så. Under senare delen av kritaperioden (c. 95 Ma) orsakade en storskalig tektonisk händelse upplyftning av oceanjordskorpa ovanpå denna mesozoiska karbonatplatform. Efter denna händelse återgick området till deponering av sediment i grundhavsmiljö, vilket givit upphov till paleogena sedimentära bergarter. I dagsläget befinner sig bergskedjans centrala delar inom den Arabiska kontinentalplattan, cirka 200 km från dess konvergenta plattgräns mot Eurasien under vilken den subduceras. Längre mot nordost sker istället aktiv kollision mellan dessa plattor, vilket givit upphov till bildandet av Zagrosbergen. Denna bergskedjeveckning initierades som tidigast under oligocen (c. 30 Ma). Vid denna tidpunkt befann sig Al Hajar-bergen ännu längre från plattgränsen jämfört idag. Den olösta gåtan om Al Hajar-bergen är alltså hur de kan uppvisa spår efter en kontinentkollision utan att befinna sig i en kollisionszon. En kombination av flera olika metoder användes för att undersöka tidpunkten för dess bildande och därmed öka förståelsen för hur de har bildats. Dessa metoder inkluderar lågtemperaturtermokronologi, U-Pb-datering av spröda strukturer samt studier av geologiska profiler. Resultaten indikerar att orogenesen inleddes i senare delen av eocen och var avslutad i den tidigare delen av miocen (40 – 15 Ma). Därmed är bildandet av Al Hajar-bergen varken relaterad till den äldre senkretaceiska ofiolitupplyftningen eller den yngre zagroskollisionen, varför en ny tektonisk modell måste föreslås. Detta visar att den Omans tektoniska historia under kenozoikum är mer komplex än vad tidigare rapporterats. i List of papers and authors’ contributions Paper I Hansman, R. J., Ring, U., Thomson, S. N., den Brok, B., & Stübner, K. (2017). Late Eocene uplift of the Al Hajar Mountains, Oman, supported by stratigraphy and low-temperature thermochronology. Tectonics, 36, 3081–3109. https://doi.org/ 10.1002/2017TC004672 Paper II Hansman, R. J., Albert, R., Gerdes, A., & Ring, U. (2018). Absolute ages of multiple generations of brittle structures by U-Pb dating of calcite. Geology; 46 (3), 207–210. doi: https://doi.org/10.1130/G39822.1 Paper III Hansman, R. J., & Ring, U. (manuscript). Oligocene–Miocene trishear fault- propagation folding of the Jabal Hafit Anticline, supported by a three-dimensional geological model; and assessing structure-from-motion (SfM) photogrammetry of unmanned aerial vehicle (UAV) photographs for mapping. Reuben Hansman carried out three field seasons (one for each paper) with the assistance of Uwe Ring. Hansman wrote the papers and created the figures and tables. Paper I: Reuben Hansman carried out apatite and zircon mineral separation. For fission- track dating Reuben Hansman counted the tracks, and Stuart Thomson mounted, polished and etched the grains and coordinated the irradiation. For (U–Th)/He dating Hansman picked and packed the apatite grains and carried out the He-degassing and quadrupole mass spectrometer analysis. Uttam Chowdhury and Erin Able carried out the isotope dilution and solution high-resolution inductively coupled plasma-mass spectrometry. Zircon grains were sent to Konstanze Stübner who carried out the zircon (U–Th)/He dating. All co-authors helped with revisions. Paper II: Reuben Hansman interpreted the data, Peter Späthe made thick-sections, Richard Albert and Axel Gerdes carried out the U–Pb dating, and all co-authors helped with revisions. Paper III: Reuben Hansman created the three-dimensional geological model, and the UAV– SfM mapping. Uwe Ring assisted with mapping and helped revise the paper. iii Acknowledgements I am incredibly grateful to my supervisor Uwe Ring, who allowed me to go in my own direction but also guided me when I needed it. Also, the preparation, analysis, and interpretation of my samples would have been impossible without the help of Stuart Thomson, Richard Albert, Axel Gerdes, Peter Reiners, Erin Abel, Uttam Chowdhury, Konstanze Stübner, and Per-Olof Persson for which I am thankful. Additionally, I appreciate the time that Victoria Pease and Hemin Koyi gave to commenting on my licentiate thesis which helped to clarify and improve Paper I. I value the discussions I had with Bas den Brok, Alasdair Skelton, and Iain Pitcairn regarding my thesis and geology in general. I would also like to thank, Dan Zetterberg, Eve Arnold, Runa Jacobsson, Krister Junghahn, Malin Andersson, and Viktoria Arwinge for assisting me through the finer details of completing a PhD at Stockholm University. I equally thank Hagen Bender, Alexandre Peillod, Clifford Patten, Barbara Kleine, Alexan- der Lewerentz (additional thanks for the Sammanfattning translation), Emelie Axelsson, Xi- aojing Zhang, Elin Tollefsen, Fitsum Girum, and Ahmad Boskabadi for running the race with me. Also, cheers to Remi Vachon, Henrik Linnros and Hermes Pantazidis. Finally, but more importantly, I cannot thank my parents enough for their continual support. I also acknowledge and appreciate the funding from the Swedish Foundation for Inter- national Cooperation in Research and Higher Education (IB2015-6002), the Bolin Centre for Climate Research (RA1 and RA6), the Royal Swedish Academy of Sciences (GS2015-0002 and GS2017-0028), the Stiftelsen Lars Hiertas Minne (FO2015-0130 and FO2016-0159), the K & A Wallenberg Foundation 2016 Travel Grant, the Stiftelsen Anna-Greta och Holger Crafoords fond and Stockholm University. v Contents Sammanfattning i List of papers and authors’ contributions iii Acknowledgements v Summary–Kappa 1 1 Introduction...................................... 1 2 Thesis Aims...................................... 4 3 Methodology ..................................... 4 3.1 Paper I: Low-Temperature Thermochronology............... 5 3.2 Paper II: U-Pb Calcite Dating ........................ 10 3.3 Paper III: Three-Dimensional Geological Modelling............ 12 4 Summary of Key Results............................... 14