Licenciate thesis Geology Constraining the Uplift History of the Al Hajar Mountains, Oman Reuben Hansman Stockholm 2016 Department of Geological Sciences Stockholm University SE-106 91 Stockholm Cover page photograph: Camels and limestone beds in Wadi Mistal, Oman. Abstract Mountain building is the result of large compressional forces in the Earth’s crust where two tectonic plates collide. This is why mountains only form at plate boundaries, of which the Al Hajar Mountains in Oman and the United Arab Emirates is thought to be an example of. These mountains have formed near the Arabian– Eurasian convergent plate boundary where continental collision began by 30 Ma at the earliest. However, the time at which the Al Hajar Mountains developed is less well constrained. Therefore, the timing of both the growth of the mountains, and the Arabian–Eurasian collision, needs to be understood first to be able to identify a correlation. Following this a causal link can be determined. Here we show, using apatite fission track and apatite and zircon (U-Th)/He dating, as well as stratigraphic constraints, that the Al Hajar Mountains were uplifted from 45 Ma to 15 Ma. We found that the mountains developed 33 Myr to 10 Myr earlier than the Arabian– Eurasian plate collision. Furthermore, the plate collision is ongoing, but the Al Hajar Mountains are tectonically quiescent. Our results indicate that the uplift of the Al Hajar Mountains cannot be correlated in time to the Arabian–Eurasian collision. Therefore the Al Hajar Mountains are not the result of this converging plate boundary. i Abstrakt Bergskedjeveckning sker till följd av de stora kompressionskrafter som uppstår när två tektoniska plattor kolliderar. Därav kan bergskedjor endast bildas vid tektoniska plattgränser, vilket Al Hajarbergen i Oman och Förenade Arabemiraten anses vara ett exempel på. Denna bergskedja har bildats nära den konvergenta eurasiska- arabiska plattgränsen, där kontinentkollision kan ha inträffat tidigast 30 Ma. Tidpunkten för bildande av Al Hajarbergen är däremot mindre välkänd och för att undersöka ett eventuellt samband mellan dess bildande och den eurasiska-arabiska kontinentkollisionen krävs datering av båda dessa händelser. Med hjälp av fission track-datering av apatit, (U-Th)/He-datering av apatit och zirkon samt stratigrafisk analys, fastslås i denna avhandling att upplyftning av Al Hajarbergen inträffade mellan 45 Ma och 15 Ma. Därmed har denna bergskedja bildats åtminstone 15 Mår före den eurasiska-arabiska kontinentkollisionen. Dessutom är denna kontinentkollision aktivt än i dag, medan Al Hajarbergen är tektoniskt inaktiva. Resultat ur denna undersökning indikerar att tidpunkten för Al Hajarbergens upplyftning inte korrelerar med den eurasiska-arabiska kontinentkollisionen, därmed kan dessa inte ha bildats i denna konvergenta plattgräns. ii Contents Abstract ............................................................................................................. i 1. Introduction ................................................................................................ 1 2. Geological Setting ..................................................................................... 3 2.1. Plate Reconstruction ......................................................................................... 5 3. Aims ............................................................................................................ 6 4. Methods ...................................................................................................... 7 4.1. Apatite Fission Track ....................................................................................... 7 4.2. (U-Th)/He Dating ............................................................................................. 8 4.3. Cooling Ages and Uplift ................................................................................... 9 5. Manuscript Results and Conclusions ..................................................... 10 6. Future Work .............................................................................................. 11 Acknowledgements ........................................................................................ 12 References ....................................................................................................... 13 Manuscript ....................................................................................................... 18 iii 1. Introduction Long linear mountain ranges are geological features unique to Earth, and are not seen on any of the other planets in our solar system. Planets, such as Mars, do have mountains but they are thought to have been formed by plume volcanism or impact craters. These generate lone peaks or rims, but not linear ranges (Breuer and Spohn, 2003). Earth is unique because of its plate tectonics which form linear mountain ranges at converging plate boundaries (Condie, 2013; McKenzie and Parker, 1967; Simkin et al., 1989). There are several boundary types. However, the boundary that creates the most dramatic mountain ranges are where two continents collide (Fig. 1a). An example of this are the Himalayas, where the Indian continental plate is colliding with the Eurasian continental plate (Dewey et al., 1989; Larson et al., 1999). The result of this collision is the formation of the highest peak on Earth, with an elevation of 8848 m. This massive uplift occurred on the overriding plate which has undergone crustal thickening by thrusting (de Sigoyer et al., 2000; Mattauer, 1986; Murphy and Yin, 2003). Mountains also form at oceanic–continental plate boundaries where a plate, comprised of oceanic crust, subducts beneath a continental plate (Fig. 1b). This is observed in the Andes in South America, which is the longest mountain range on Earth, with a peak of 6961 m. Here the oceanic Nazca Plate is subducting under the continental South American Plate (Isacks, 1988; Jordán et al., 1983). Figure 1. Schematic drawing of two types of convergent plate boundaries modified from Simkin et al. (1989). (a) Continental–continental collision, and (b) oceanic–continental subduction. In both scenarios mountains can develop in the overriding plate by thrusting and crustal thickening. The Al Hajar Mountains are located where uplift does not typically occur. 1 At continental–continental and oceanic–continental plate boundaries, the uplift and mountain building occurs in the overriding plate (Cawood et al., 2009; Sobolev and Babeyko, 2005). Mountains usually do not form in the downgoing plate. However, one mountain range, the Al Hajar Mountains in Oman and the United Arab Emirates has developed on the downgoing slab (Fig. 1b). This arcuate mountain range has a peak of 3009 m and is currently 200 km away from the Makran subduction zone (Fig. 2), an oceanic–continental convergent plate boundary between the Arabian Plate and the Eurasian Plate (Kopp et al., 2000). This plate boundary transitions westwards from the Makran zone into the Zagros continental-continental collision zone (Mouthereau, 2011; Regard et al., 2010; Snyder and Barazangi, 1986), which is even further from the Al Hajar Mountains. Both the Zagros and Makran zones have mountains forming in their overriding plates (McCall and Kidd, 1982; Mouthereau et al., 2012). Therefore, we want to understand if the formation of the Al Hajar Mountains is related to this convergent plate boundary or not. To answer this, we first need to know when the Al Hajar Mountains began to grow and if they are still active. Figure 2. Simplified map of the current Arabian Tectonic Plate configuration, from (Hansman et al., 2016), modified from Stern and Johnson (2010), and plate motion from DeMets et al. (2010). 2 2. Geological Setting The geology of the Al Hajar Mountains can be categorised into four major tectono- stratigraphic groups (Glennie et al., 1974; Mount et al., 1998). The first group includes the pre-Permian basement rocks of the Huqf and Haima Supergroups (Fig. 3a Group 1). The second group is the mid-Permian to mid-Cretaceous Hajar Supergroup, a sequence of continental shelf carbonates, deposited unconformably on top of the basement rocks (Fig. 3a Group 2). The third group are allochthonous rocks, which have been transported in a series of nappes over at least 300 km laterally from the northeast and emplaced on top of the Hajar Supergroup during the Late Cretaceous (Searle and Cox, 1999). This group is comprised of the Permian to mid-Cretaceous Hawasina Complex continental slope-rise sediments, and the Semail Ophiolite, an assemblage of Cretaceous oceanic lithosphere (Fig. 3a–b Group 3). The fourth, and last group, are Late Cretaceous to Miocene terrestrial and shallow marine sediments (Fig. 3b–c Group 4), which cover the older groups (Nolan et al., 1990). At 45 Ma, the rocks that would form the Al Hajar Mountains had been completely submerged (Fig. 3d) in a shallow marine environment (Mann et al., 1990; Nolan et al., 1990). Therefore, based on the sedimentary record, the high topography must have developed after 45 Ma. Figure 3 (next page). Schematic diagrams illustrating the geological history of the rocks of the Al Hajar Mountains, modified from Carbon (1996), Cowan et al. (2014), Searle et al. (2004), and Searle (2007). (a) Formation of the Semail oceanic crust at 95 Ma, and deposition of the Hawasina Complex deep marine sediments as well as the Hajar Supergroup carbonates. (b) At 80 Ma, ophiolite emplacement is ongoing and high-pressure metamorphism in the downgoing plate occurs. The Aruma Group