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Geology Licenciate thesis Stockholm 2016 Department of Geological Sciences Stockholm University SE-106 91 Stockholm Constraining the Uplift History of the Al Hajar Mountains, Oman Reuben Hansman
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Geology

Licenciate thesis

Stockholm 2016

Department of Geological SciencesStockholm UniversitySE-106 91 Stockholm

Constraining the Uplift History of the

Al Hajar Mountains, Oman

Reuben Hansman

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Cover page photograph: Camels and limestone beds in Wadi Mistal, Oman.

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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.

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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.

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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

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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.

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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).

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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 sediments are deposited in front of the advancing thrust sheets. (c) By 60 Ma ophiolite emplacement has finished, and exhumation of the high-pressure rocks forms the Jabal Akhdar and Saih Hatat anticlines. Coeval with uplift is erosion of the anticlines and deposition of the lower Hadhramaut Group. (d) By 45 Ma erosion of the culminations cease, regional subsidence occurs, and the upper Hadhramaut Group blankets the area.

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2.1. Plate Reconstruction

To relate the uplift of the Al Hajar Mountains with the Arabian–Eurasian Plate

boundary, we require a plate reconstruction until at least 45 Ma. Currently the

Arabian Plate is moving northwards towards the Eurasian Plate at 2 cm yr-1, see

Figure 2 (DeMets et al., 2010; McClusky et al., 2003). Before the Zagros collision

began, Neotethys separated the Arabian continental crust from the continental crust

of Eurasia (Fig. 4). Based on plate reconstructions (McQuarrie, 2003), the Neotethys

was about 1400 km wide at 60 Ma (Fig. 4a), and as the oceanic crust was subducted

underneath the Eurasian Plate the ocean was reduced to about 600 km width by

35 Ma (Fig. 4b). Oceanic crust is still subducting under Eurasia at the Makran

subduction zone, at the northeast corner of the Arabian Plate (Fig. 2).

Figure 4. (a) 60 Ma and (b) 35 Ma plate reconstructions from (Hansman et al., 2016), based on McQuarrie

(2003) and McQuarrie and Van Hinsbergen (2013).

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Ages of the sediments in the Makran accretionary prism (Arthurton et al., 1982;

White, 1982) and arc magmatism (Berberian and Berberian, 2013) indicate that

Makran subduction was well underway by at least 80 Ma, but probably started much

earlier. The Zagros Mountains have been forming since 30 Ma at the earliest

(Fakhari et al., 2008; Gavillot et al., 2010; Mouthereau et al., 2012), and major uplift

in the Zagros Mountains developed by ca. 12 Ma (Gavillot et al., 2010; Morley et al.,

2009; Mouthereau, 2011). Therefore, if the Zagros collision did generate the uplift

further south in the Al Hajar Mountains, the Zagros collision should have occurred

first. This means the Al Hajar Mountains need to be younger than 12 Ma if they are

to be related to the Zagros collision.

3. Aims

1. To constrain the cooling history of the Al Hajar Mountains by using low-

temperature thermochronology techniques which include apatite fission track

as well as apatite and zircon (U-Th)/He dating.

2. To infer the uplift history of the Al Hajar Mountains by interpreting the

cooling history. This is achieved by a study of the stratigraphy from the

literature and questions answered are:

a. which sediments are pre- syn- or post-tectonic;

b. clast composition and provenance, and any reworking of older rocks

which can indicate erosion due to local surface uplift;

c. presence of unconformities, which indicates a sedimentary hiatus or

erosion from surface uplift.

3. To relate the uplift history of the Al Hajar Mountains to the Arabian–

Eurasian Plate Boundary and understand if the formation of the mountains

was coeval with the Zagros collision.

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4. Methods

A three week field season was carried out in Oman during January 2015, which

covered the central Al Hajar Mountains. Samples, that potentially contained apatite

and zircon, were collected for analysis by low-temperature thermochronology

techniques. Samples were then crushed and mineral separation was carried out

following standard procedures (Donelick et al., 2005). The apatite separates were

taken to the University of Arizona where fission track and (U-Th)/He dating was

completed over seven weeks. During this time I was taught these methods. The

zircon separates were sent to the University of Tübingen, where (U-Th)/He dating

was carried out. Samples that were successfully dated include 15 apatite fission track

ages, 10 apatite (U-Th)/He ages, and 4 zircon (U-Th)/He ages.

4.1. Apatite Fission Track

Apatite is a uranium bearing mineral (Donelick et al., 2005), and damage to the

crystal lattice of an apatite grain occurs during spontaneous fission decay of 238U.

This is a naturally occurring ‘rare’ event, when a 238U atom splits into two fragments

causing damage to the crystal (Reiners and Brandon, 2006). This damage results in a

fission track (Fig. 5a). A newly formed track is typically 16 µm long (Gleadow et al.,

1986b). If the crystal is at a high enough temperature, then the fission tracks that

form will be annealed. This is when tracks ‘heal’ and progressively shorten (Green et

al., 1986, 1985). Eventually, if the mineral remains at a high temperature for long

enough, the tracks will be completely removed (Fleisher et al., 1975; Green et al.,

1986). The temperature at which the tracks will be annealed is the closure

temperature, and for apatite this is between 120ºC to 90ºC (Ketcham et al., 1999).

Because the spontaneous fission decay of 238U occurs at a known constant rate

(Steiger and Jäger, 1977), the tracks produced in a crystal is a function of time. The

tracks retained in a crystal is a function of temperature. Therefore, the number of

tracks counted in a sample dates the time at which that sample cooled below about

90ºC. In addition, the track lengths are measured and are a relative measure of the

cooling rate. A sample with many short track lengths indicates slow cooling from

120ºC through to 90ºC, as the tracks have had time to anneal. A sample with only

long tracks indicates rapid cooling to below 90ºC (Gleadow et al., 1986a).

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4.2. (U-Th)/He Dating

Helium is produced through alpha decay (α-decay). This is when a parent nuclide,

such as U or Th, decays to a stable Pb daughter product and in the process releases

an α-particle of 2 neutrons and 2 protons, which is a helium nucleus (Rutherford and

Royds, 1908). This decay rate is a known constant and therefore is a function of time

(Hourigan et al., 2005). Crystals, such as apatite and zircon, only retain helium

below their closure temperature. If the crystal temperature is greater than the closure

temperature, the helium will diffuse out of the grain. Therefore, the amount of

helium contained within a crystal (Fig. 5b) is used to calculate the age at which the

crystal cooled below its closure temperature. This can be measured with a

quadrupole mass spectrometer. For apatite, the closure temperature is between 80ºC

to 55ºC (Farley, 2000) and for zircon, between 200ºC to 160ºC (Reiners et al., 2004).

Figure 5. Light microscope photographs of apatite grains. (a) Polished and etched surface of an apatite grain used for the fission track method. (b) An example of a suitable euhedral apatite grain, used for the apatite (U-Th)/He dating method.

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4.3. Cooling Ages and Uplift

The results from using these low-temperature thermochronology techniques indicate

the age at which a rock sample cooled through the various thermochronometer

closure temperatures. These temperatures can then be used to infer depth, based on

assumptions of past surface temperatures and geothermal gradients (Reiners, 2007).

The average geothermal gradient for continental crust is 25ºC km-1 (Chapman,

1986), and the surface temperature in Oman is also 25ºC (Seeb Climate Normals

1961–1990). Using the average closure temperature for each thermochronometer, a

depth can be calculated (Fig. 6). If we use an average closure temperature of 60ºC

for apatite (U-Th)/He, this would translate to about 1 km depth. For apatite fission

track an average closure temperature of 110ºC would indicate about 3 km, and for

Figure 6. An example of a steady state thermal model used to understand what the low-temperature thermochronometer cooling ages mean, after Reiners and Brandon (2006). In this model the surface uplift has reached equilibrium and does not change because rock exhumation and erosion are both equal. Also, enough time has passed to stabilise the isotherms which mimic the topography. In this case the ages indicate the time at which the sample passed through the relevant thermochronometer’s closure temperature. The closure temperature is also used to infer the depth, which is dependent on the surface temperature and the geothermal gradient. Cooling in this model is achieved by erosion. AHe: apatite (U-Th)/He dating. AFT: apatite fission track dating. ZHe: is zircon (U-Th)/He dating.

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zircon (U-Th)/He about 6 km depth with a 180ºC closure temperature. Therefore,

one sample collected at the surface can give three ages detailing the exhumation

from 6 km to 1 km. An example of this, in a simple steady state orogeny, is shown in

Figure 6.

Exhumation is the vertical displacement of a package of rock relative to the Earth’s

surface, and this movement is caused by tectonic and/or surficial processes (England

and Molnar, 1990; Ring et al., 1999). A result of exhumation is cooling, because a

package of rock loses heat as it moves towards the Earth’s surface. When surface

uplift occurs due to crustal thickening, a steep, less stable terrain develops. As a

result, surficial processes become more active on the landscape, for example erosion

by rivers and landslides. This removes rock material and results in exhumation and

cooling of the rocks below (Reiners and Brandon, 2006). Therefore, the cooling ages

from the low-temperature thermochronometers can be used to infer the surface uplift

of a rock sample.

5. Manuscript Results and Conclusions

The results of the low-temperature thermochronometry ages from the central Al

Hajar Mountains constrain the timing of two cooling events. The first event began in

the Late Cretaceous (ca. 79 Ma) and ended by the early Eocene (ca. 50 Ma). This

cooling occurred after the Semail Ophiolite and Hawasina Complex had been

emplaced, during a rifting episode, and is related to the exhumation of the high-

pressure rocks through normal faulting (Fig. 3c). Therefore, the first cooling event is

inferred to have been caused by footwall exhumation (Searle et al., 2004).

The second cooling event began in the mid-Eocene (ca. 45 Ma) and ended in the

mid-Miocene (ca. 15 Ma). This event is measured as cooling from 180 ± 20ºC to

surface temperatures (ca. 25ºC). Based on the geological context, including an

Oligocene (ca. 35 Ma) regional unconformity and structural data, this cooling is

inferred to have been generated by erosional exhumation due to surface uplift.

Therefore, we interpret that this cooling event occurred when the high topography of

the Al Hajar Mountains formed. Peak exhumation was about 9 km to 5 km, which

was eroded leaving behind 3 km of elevation.

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Our data also indicate that the second cooling event was more dramatic in the

western central mountains compared to the east. Our study is the first to report this

differential cooling, and it is indicative of a blind west-dipping thrust proposed by

Mount et al.(1998) based on the geometry of the mountains. This thrust generated

more uplift and erosion in the hanging wall, and less uplift occurred in the east

within the footwall.

Our results constrain the timing at which the Al Hajar Mountains formed. While the

mountains are near the converging Arabian–Eurasian Plate boundary and the

continental Zagros collision zone, our results indicate that the uplift is unrelated to

this plate boundary. This is because the Al Hajar Mountains began to uplift at 45 Ma,

when the Neotethys still separated the Arabian and Eurasian continents. These

continents did not collide until about 30 Ma (Fakhari et al., 2008; Gavillot et al.,

2010; Mouthereau et al., 2012). Therefore, the Zagros collision zone is unrelated to

the uplift in the Al Hajar Mountains.

6. Future Work

Currently low-temperature thermochronology studies have only been carried out at

the western and eastern ends as well as within the central mountains. There remains

a 200 km spatial gap in these data between the central mountains and the western

end. All previous researchers have discussed their local study areas but have not

investigated the 700 km long mountain range as a whole. Therefore, additional

sampling for low-temperature thermochronometry analysis will be carried out to

cover the gap, and a comprehensible cooling and uplift history will be interpreted

across the entire Al Hajar Mountain range.

We have also collected samples from faults within the central mountains, in order to

date them. Two methods are to be used. One method is K-Ar dating of illite from

fault gouge. Illite clay mineralisation occurs during near-surface faulting and

therefore dates the age at which the fault was active. The other method is U-Pb

dating of calcite veins associated with faulting and fault slickenfibres. We also

measured the fault kinematics, which indicates if the fault sample formed from a

shortening or an extensional event. These data will then be related to the low-

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temperature thermochronology data and will help constrain the evolution of the

mountains more robustly.

Another method we will use is a palaeomagnetic analysis of the Cenozoic sediments.

These measurements will tell us how much the sediments have rotated since they

were first deposited, and will help us understand when the curvature of the Al Hajar

Mountains occurred. If these sediments are also rotated, as observed in the Ophiolite,

then the curvature is a young feature.

In addition, to better infer the surface uplift from the cooling history, a 3D structural

model of the mountains will be created. This model will be based on a series of

balanced 2D cross-sections created from geological maps, seismic profiles and

structural data collected during fieldwork.

Acknowledgements

I would like to thank the following organisations for their support, as this project

would not have been possible without them; the Bolin Centre for Climate Research,

RA6 and RA1; the Royal Swedish Academy of Sciences; Stiftelsen Lars Hiertas

Minne; and the Swedish Foundation for International Cooperation in Research and

Higher Education.

To my supervisor Prof Uwe Ring, thank you.

I am incredibly grateful to Stuart Thomson from the University of Arizona, who

helped prepare my samples and taught me the AFT dating method. Much

appreciation for the help received from Peter Reiners, Erin Abel, and Uttam

Chowdhury from the University of Arizona as well as Konstanze Stübner from the

University of Tübingen, with the (U-Th)/He dating method.

Thanks to Bas den Brok for introducing me to the geology of Oman, and for his

help. I also thank Mohammed Al-Wardi from Sultan Qaboos University for his

useful comments. Cheers to Alexander Lewerentz for the translation. I am also

thankful to the following people who helped me along the way; Per-Olof Persson,

Dan Zetterberg, Runa Jacobsson, Krister Junghahn, Cora Wohlgemuth-Ueberwasser,

my parents Rachael and Harry Hansman, and my friends on the 4th floor.

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