Detrital Thermochronology to Constrain Meso-Cenozoic Intramontane Basin Evolution in the Northern Tien Shan (Central Asia)

FACULTY OF SCIENCES Master of Science in geology

Coupling basement and detrital thermochronology to constrain Meso-Cenozoic intramontane basin evolution in the northern Tien Shan (Central Asia)

Simon Nachtergaele

Academic year 2015–2016

Master’s dissertation submitted in partial fulfillment of the requirements for the degree of Master in Science in Geology

Promotor: Prof. Dr. Johan De Grave Tutor: Elien De Pelsmaeker Jury: Prof. Dr. Marc De Batist, Dr. Damien Delvaux

“Not everything that counts can be counted, and not everything that can be counted counts.“

(Albert Einstein)

Acknowledgements First of all I want to thank my thesis promoter Prof. Dr. Johan De Grave for the creation of this thesis subject and giving me the possibility to join the team on the Kyrgyz field. He also corrected my strange grammar constructions and last but not least for the time that he invested in me, while he was very busy with teaching and research. Research on geochronology is time-consuming and the interpretation of geochronological data is often complex. I experienced geochronology in Central-Asia as a challenging and therefore attractive research subject.

The next person that I want to thank is research assistant Elien De Pelsmaeker. Despite the heavy teaching load, she always was available for answering my questions. She also calmed me down when a lot of samples appeared to be worthless in november/december 2015. Also during the ‘zircon U/Pb disappointment’, we stayed calm and decided to focuse more on apatite fission track analysis. Our team work in the Tien Shan mountains during the summer of 2015 was an unforgettable experience.

I am very grateful to Ann-Eline Debeer because she helped me out when ±10 AFT samples appeared to be not useful. Thank you for doing the separations of the ‘KB’ basement samples of the 2015 field campaign. Jan Jurceka is also thanked for making the thin-sections.

The next person I want to thank is Dr. Fedor Zhimulev. He was of great help during the field work and is a great field geologist in my opinion. His translations of Russian literature about the Kyrgyz Tien

Shan were of great value. The discussions about AFT, sedimentology and the activity of the Talas

Fergana Fault were interesting.

Prof. Dr. Marc Jolivet also was also an important member of the field work team. I am very grateful for the sedimentary logs that I received from him. It is also because of him that we could have a workshop on thermochronological modelling with QTQt. This workshop was given by Dr. Kerry Gallagher and he succeeded to bring an attractive and interesting workshop about Bayesian statistics.

During the field work, Vlad and Elena Batalev perfectly assisted us in the remote mountains of

Kyrgyzstan. Driver Vladimir is also a thanked for the stories about the hidden treasures on the bottom of the Issyk Kul Lake and especially for his great driving skills.

Prof. Dr. Stijn Glorie did apatite fission track analysis on a few samples in Kyrgyzstan and I am grateful that I can publish his results in this thesis.

Gerben Van Ranst is also thanked for answering my questions on Adobe Illustrator. The discussions on geochronology and sample preparation were fruitful.

The other professors and assistants deserve a lot of respect, because I really enjoyed the lessons and practicals. The nice atmosphere among students and staff members keeps the students motivated. The field trips to the Alps, the Boulonnais region and many other regions were the most instructive days in which we learned a lot.

My girlfriend Ama always supported me – in easy and difficult days – during these five year. Even though she was volunteering in Nicaragua for a half of a year, I had the feeling that she was close to me.

Last but not least, my family (especially my mom) deserves to be thanked for supporting me and paying my studies. Not every boy of 18 years gets the opportunity to study for five years.

Спасибо за кумыса

Simon Nachtergaele 30/05/2016

TABLE OF CONTENT

ACKNOWLEDGEMENTS I

INTRODUCTION 1

CHAPTER 1: THE APATITE FISSION TRACK METHOD 3 1.1 Fission tracks: definition and formation 3 1.2 Fission track revelation and identification 4 1.3 Principles of the apatite fission track method 4 1.3.1 Thermal neutron fluence 5 1.3.2 The fundamental age equation 6 1.4 Calibration with age standards (zeta-calibration) 6 1.4.1 Age standards 7 1.5 The thermal stability of fission tracks 8 1.5.1 Kinetic parameters influencing annealing behaviour 8 1.6 Fission tracks and thermochronology 9 1.6.1 Track length distribution and its geological significance 9 1.6.2 Closure temperature, cooling ages and the apatite partial annealing zone 10 1.7 Geological interpretation of apatite fission track age data 12 1.7.1 Cooling rate 12 1.7.2 Denudation, exhumation and uplift 12 1.7.3 Horizontal and vertical sampling profiles 12 1.8 Multi-method approach: AFT and zircon (U-Th)/He 13 1.9 Thermochronological modelling 14 1.9.1 Introduction to QTQt 15 1.9.2 Modelling with QTQt 16 1.9.3 RadialPlotter and DensityPlotter 19 CHAPTER 2: DETRITAL THERMOCHRONOLOGY 21 2.1 Introduction to detrital thermochronology 21 2.2 Erosion and sediment generation estimations 24 2.2.1 Quantification of erosion rates with thermochronology 25 2.3 Thermal maturity estimation 28 2.3.1 Thermal maturity estimations in sedimentary basins based on organic material 28 2.3.2 Thermal maturity estimations in sedimentary basins based on low-temperature thermochronology 28 CHAPTER 3: FROM SAMPLING TO AFT ANALYSIS 30 3.1 Report of field work 30 3.2 AFT sample preparation 31 3.2.1 Heavy mineral separation 31 3.2.2 Heavy mineral selection 31 3.2.3 Mounting procedure 32 3.2.4 Irradiation at nuclear reactor BR1 33 3.3 Zeta-factor age calibration 34 3.3.1 AFT counting procedure 34 3.3.2 AFT length measurements 35 3.3.3 AFT-analysis: calibration by glass dosimeters 35 3.3.4 AFT-analysis: calibration by age standards 37

III

CHAPTER 4: GEOLOGICAL CONTEXT 38 4.1 Introduction to the Central Asian Orogenic Belt 38 4.2 Precambrian and Palaeozoic evolution of the CAOB 38 4.2.1 Precambrian geodynamical evolution 38 4.2.2 Palaeozoic evolution: assembly of the Kyrgyz Tien Shan 39 4.3 Mesozoic evolution 42 4.3.1 Late Triassic – Early Jurassic event 43 4.3.2 Middle-Jurassic peneplanation 44 4.3.3 Late-Jurassic – Cretaceous cooling 44 4.3.3.1 A short note on AFT ages in fault zones 47 4.3.4 Mesozoic intramontane basins in the Kyrgyz Tien Shan 49 4.3.4.1 East of TFF: Kavak and Issyk Kul basin 50 4.3.4.2 West of the TFF: Tash Kumyr 51 4.3.4.3 West of the TFF: East Fergana basin 52 4.4 Mesozoic volcanism in the Tien Shan region 54 4.4.1 Early-Jurassic volcanism 54 4.4.2 Mantle plume activity in the CAOB? 54 4.5 Cenozoic evolution 57 4.6 Evolution of the Talas-Fergana Fault (TFF) 59 4.6.1 Late Palaeozoic and Mesozoic activity 59 4.6.2 Cenozoic reactivation 60 4.7 Jurassic and Cretaceous climate 61 CHAPTER 5: RESULTS 62 5.1 Sample overview 62 5.2 Sedimentary logs 64 5.2.1 Paleosol occurence 66 5.3 Apatite fission track (AFT) data on basement rocks 67 5.3.1 Track length measurements 69 5.3.2 Thermal history modelling results with QTQt 72 5.4 AFT data on detrital rocks 76 5.4.1 Track length distributions 80 5.5 Zircon(U-Th)/He data 83 CHAPTER 6: DISCUSSION 85 6.1 Basement AFT and zircon (U-Th)/He data 85 6.1.1 Thermal history model compilation 87 6.1.2 Mesozoic basement cooling in the NTS 88 6.1.3 Mesozoic activity of the TFF based on basement AFT and zircon (U-Th)/He data 90 6.2 Detrital apatite fission track thermochronology 91 6.2.1 East Fergana basin samples (west of TFF) 91 6.2.1.1 KS13-22 91 6.2.1.2 KS 13-19 and KS 13-20 91 6.2.2 Lower-Jurassic samples of Minkush (east of TFF) 92 6.2.3 Issyk Kul sections (east of TFF) 93 6.2.3.1 SK 46 93 6.2.3.2 SK 47 94 6.2.3.3 KS 136, KS 137, KS 138 and KS 139 95 6.3 Geodynamic events and thermochronology in the Kyrgyz Tien Shan 96

CHAPTER 7: CONCLUSION 99

REFERENCES I

APPENDICES: XIII A. Geological Map of Central Asia and Adjacent areas (version 2008): xiii B. International Chronostratigraphic chart (version 2016) xv C. Complete AFT dataset of basement samples of the Kyrgyz Tien Shan xvi

Introduction The 2000km long Talas-Fergana fault (TFF) represents the largest strike-slip fault in Central Asia. The

TFF is mainly located in Kyrgyzstan and divides the country in two distinct domains that are displaced over a large distance. The TFF is one of the ancient Palaeozoic inherited faults of the ancestral Tien

Shan mountain belt that is reactivated today and makes of Kyrgyzstan one of the most mountainous countries in the world. The Tien Shan has mountain peaks up to 7400m and is located in the countries of Kazakhstan, Kyrgyzstan and China. It represents an ideal study area for intracontinental deformation in the Central Asian Orogenic Belt (CAOB). Tectonic movements of the TFF are recorded as early as the

Late Palaeozoic, continued during the Mesozoic and are reactivated in the Late Cenozoic (Bande et al.,

2015) as response to ongoing India-Eurasian convergence. The TFF is known to be active during the

Holocene and is expected to generate a strong earthquake (M>7) in the next 50 years (Korzhenkov et al 2014).

Ancient tectonic and exhumation events can be constrained by Apatite Fission Track (AFT) dating. The first objective of this thesis includes AFT dating of granite (basement) samples collected in mountain ranges around the TFF. These Palaeozoic granites were emplaced at great crustal depths during the

Palaeozoic and are now exposed at the surface because of Meso- and Cenozoic tectonic events. AFT ages expressing pulses of exhumation contain a record of Meso- and Cenozoic activity of the TFF.

The second objective of this thesis is to study the burial and reactivation history of sedimentary basin along or crosscut by the TFF. To meet this second objectives detrital AFT thermochronology is used.

After burial by kilometres of sediments, the AFT system is potentially (partially) reset. The AFT age will then be younger than the stratigraphic age of the sediment formation. If the AFT system is completely reset, it potentially records posterior exhumation, similar to a basement sample. In the case that the

AFT system is not reset, ancient tectonic events at the time of deposition can be pinpointed. The detrital grains hence archive exhumation of the source area. With detrital thermochronology it is possible to record AFT signals of exhumed basement rocks that are nowadays completely eroded, but were important at the time of sedimentation. Additionally, indications on the source to sink history of the basin can be revealed.

Several granite basement and detrital samples of the Northern Tien Shan (NTS), Middle Tien Shan

(MTS) and South Tien Shan (STS) were sampled in Kyrgyzstan during a field campaign in the summer of 2015. The Mesozoic sandstones of the Issyk Kul basin, Kavak basin and east Ferghana basin are linked to detailed sedimentary logs based on field observations by sedimentologists. Additional detrital AFT samples of the field campaign of 2013 are also analyzed in the frame of this thesis.

Unpublished basement AFT and zircon (U-Th)/He data of the NTS, MTS and STS collected by Prof. Dr.

Introduction

De Grave and Prof. Dr. Stijn Glorie represents the final part of this thesis. Finally, the time-temperature history of both the detrital and the basement samples is visualized by various state-of-the-art modelling programs.

Chapter one discusses the AFT method and its applications to geology, followed by a brief discussion on thermal history modelling programs. Chapter two provides an introduction to the interpretation of detrital AFT thermochronology data and its applications for hydrocarbon research and thermal history studies. The next chapter discusses the field campaign of 2015, the sample preparation techniques and the zeta-factor calibration. Chapter four gives an overview of the geological context of the Tien Shan mountains in Kyrgyzstan and the assembly of the CAOB. The results of both the detrital and basement

AFT analysis and the thermal history modelling are shown in chapter five and will be discussed in chapter six. The results of the sedimentary logs made during the field campaign of 2015 are also shown in chapter five. Chapter six gives a summary of this thesis and evaluates the different tectonic pulses observed in the Kyrgyz Tien Shan. The conclusions on the activity of the TFF during the

Mesozoic en Cenozoic will be drawn in chapter seven. The list of references is added after the conclusion and is followed by the appendices.

Chapter 1: The apatite fission track method

1.1 Fission tracks: definition and formation The Apatite Fission Track (AFT) method is based upon radioactive decay of 238U. Most of the 238U nuclides decay according to alpha-decay to 206Pb, while only a small amount decays through spontaneous fission. When a fissioning 238U disintegrates it produces two fission fragments which are repulsed from each other with high velocity. This process damages the crystal lattice in which the 238U is embedded as trace element for example. This destruction of the crystal lattice on a submicroscopical scale is called ‘fission track’. The ion explosion spike model (Figure 1) is the globally assumed model which describes how the two charged fission fragments induce ionization along their path through the mineral lattice (e.g. apatite) (Fleischer et al., 1975).

Figure 1: schematic cartoon of the ion explosion spike model, after Fleischer et al. (1975). The fission track is formed when a 238U nucleus fissions and two fission fragments cause a linear track of permanent damage in the apatite lattice.

Other isotopes 234U, 235U and 232Th are assumed to be negligible with respect to natural accumulation of fission tracks (e.g. Wagner and Van den haute, 1992). Fission tracks are submicroscopic, however, after the fission tracks are revealed with a suitable etchant, the fission tracks are widened and can be observed under a normal optical microscope with high (1000x) magnification.

Commonly used minerals for fission track analysis are zircon, titanite (sphene), monazite and especially apatite. Apatite is an accessory phosphate mineral Ca5(PO4)3(F,Cl,OH) of which the fluorine end member or fluorapatite is the most common. Uranium replaces Calcium in the crystal lattice and often has a concentration of 1-100 ppm in apatite. Intrusives, such as granites and granodiorites, and volcanic rocks are the main source of apatite on Earth. Siliciclastic, sedimentary rocks can also contain detrital apatite and can therefore also be used for AFT dating.

Chapter 1: The apatite fission track method

1.2 Fission track revelation and identification As mentioned, before the latent fission tracks can be observed under the microscope, the fission tracks have to be revealed by an appropriate method. The chemical etchant technique reveals all lattice defects on a mineral surface, including the fission tracks. These appear as random orientated needle- shaped structures. The geometry of the etched FT is controlled by chemical composition, the type of etchant and the orientation of the apatite crystal (Sobel and Seward, 2010). Nitric acid (HNO3) is the appropriate chemical etchant used for track revelation in apatite. In this work we use a 5.5M HNO3 solution at 25°C for 20s (e.g. Donelick et al., 1999). Before the chemical etching step can start, the internal surface of the apatite minerals are revealed. This is done by grinding and subsequently high- quality polishing of the apatite crystals so that the fission tracks can be identified on a polished surface.

Occasionally, internal tracks are etched in the apatite crystal. These so-called confined tracks are not crosscut by the polished surface, but are reached through cleavage planes or surface tracks, called

TINCLEs (Track IN CLEavage) and TINTs (Track IN Track) respectively (Figure 2). TINCLEs are commonly not considered because of a resistance to shortening of the tracks or annealing (Jonckheere and

Wagner, 2000). TINTs are preferentially used for track length measurements because the track- shortening or annealing process is well-documented (Carlson et al. 1999, Donelick et al., 2005,

Gleadow et al 1986b).

1.3 Principles of the apatite fission track method Details on the AFT method can be found back in Wagner and Van den haute (1992), De Grave (2003) and Jonckheere and Ratschbacher (2015). The fundamental age equation will be discussed in section

1.3.2 and consists of radioactive decay constants, track densities (ρs and ρi), a production factor Q, the 235U/238U ratio in nature, a factor σ and a neutron fluence estimation φ. This thesis uses the external detector method which mounts external detectors on the etched internal surfaces of the apatite crystals (Figure 2). The main advantage of this method is that single-grain ages for each apatite crystal can be obtained.

Chapter 1: The apatite fission track method

Figure 2: sketch of an etched apatite crystal which is covered by an external detector. Apatite crystals are mounted in epoxy, grinded, polished, etched and covered with U-free muscovite mica before irradiation. Etching of the external detector with a chemical etchant reveals the fission tracks in the external detector that are caused by thermal neutron stimulation (figure after De Grave, 2003).

The amount of fission tracks (Ns and Ni) is counted with a high magnification microscope after which the fission track densities (ρs and ρi) can be obtained via a calibrated grid. Spontaneous fission tracks are revealed in a high quality polished internal apatite surface prior to the thermal irradiation step.

After this, the mounts are covered with an external detector. The external detector method is the most widely used method in fission track geochronology and is also used in this thesis. The (U-free) overlying external detector quantifies the Uranium concentration of each apatite mineral. The external detector accumulates the so-called induced fission tracks caused by the thermal neutrons which stimulate fission of the 235U atoms in the apatite crystal. Low Uranium Goodfellow muscovite mica is used as an external detector because it has perfect cleavage planes which ensure the perfect fitting to the apatite mount. The revelation of the induced fission tracks is executed with a chemical etchant (HF) after the thermal irradiation. The external detector only accumulates fission tracks by one side of the apatite crystal and therefore a geometric ratio factor (G=0.5) has to be applied (for further information see section 1.3.2).

1.3.1 Thermal neutron fluence

The thermal neutrons irradiation which induces fission of the 235U isotope is typically performed in a nuclear reactor. The neutron spectrum is rather wide and consists not only of low energy thermal neutrons, but also has high energy “fast neutrons” and intermediate “epithermal neutrons”. The epithermal and fast neutrons are capable of inducing the unwanted fission of the 238U and 232Th atoms.

Therefore, the energy spectrum in the irradiation channel should be monitored, causing only thermal neutrons to reach the irradiation package.

Chapter 1: The apatite fission track method

1.3.2 The fundamental age equation

The derivation of the fundamental age equation is explained in Wagner and Van den haute (1992) and will not be discussed here. The fission track age can be determined with the following equation:

(1.1)

With: t = fission track age in years

238 =the alpha decay constant for U, equal to according to Jaffey et al (1971) and Steiger and Jäger (1977)

= the fission decay constant, equal to according to Galliker et al. (1970)

ρs = spontaneous fission track density

ρi = induced fission track density

Q = produce factor

G = geometric ratio (0.5 in the used 2π geometry in the external detector method, because the spontaneous fission tracks are accumulated inside the apatite crystal (4π) and the induced fission tracks are formed by the 235U on a half spherical geometry (2π))

I = the present ratio in nature (I= according to Cowan and Adler (1976))

σ = 570.8b, according to Wagner and Van den haute (1992)

φth = thermal neutron fluence of the nuclear reactor

1.4 Calibration with age standards (zeta-calibration) The zeta factor method does not need the precise measurement of the thermal neutron fluence (Green and Hurford, 1984) and is also not dependant on an exact 238U fission decay-constant (Bigazzi, 1981).

By co-irradiating fission track age standards (i.e. apatites with known and accepted age), fission track samples and Uranium-doped glass monitors, the zeta (ζ) calibration factor can be determined. This zeta factor is a calibrated factor that is independent of the irradiation. Several co-irradiated glasses with a precisely known U-concentration accumulate induced fission tracks in the attached external detector. These U-doped glass dosimeters are spread over the irradiation package and describe the thermal neutron fluence φth. The derived glass-calibration curve is used for a calculation of the ρd value for each co-irradiated sample with the formula 1.2 and a proportionality constant B expressed in

Chapter 1: The apatite fission track method neutrons per track, depending on the used glass monitor, the etching parameters and observation conditions (Wagner and Van den haute,1992).

(1.2)

The zeta-factor calibration factor emphasizes multiple factors and is defined as:

(1.3)

By substitution of 1.3 in 1.2, the following equation is obtained:

(1.4)

Or written otherwise:

(1.5)

The unit of the ζ calibration factor is a*cm² and incorporates the parameters Q, σ, B and λf. The zeta calibration factor is strictly individual for each researcher because it depends on the fission track identification, experimental set-up, etching conditions, glass dosimeter and observation conditions

(Wagner and Van den haute, 1992). Due to small variations in calculated zeta factors for the same age standard, a weighted average ζ factor is calculated based on several measurements of the same age standard, called the Sample Weighted Mean Zeta (SWMZ). The Overall Weighted Mean Zeta (OWMZ) is a value that combines the different SWMZ values for different age standards and is often abbreviated in papers as the zeta-value associated to a specific researcher and optical set-up.

The unknown apatite fission track age for a sample can now be calculated with the following equation:

(1.6)

Once the determination of the personal zeta-factor is completed, the only unknowns in equation 1.6 are the track density ratios ρs/ρi of the sample and the interpolated ρd value on the glass calibration curve.

1.4.1 Age standards

Two apatite age standards are recognized by the Fission Track Working Group of the IUGS

Subcommission on Geochronology (Hurford, 1990a and 1990b). The Fish Canyon Tuff is dated with the

40Ar/39Ar method on biotite and yielded an age of 27.8±0.2Ma (Hurford and Hammerschmidt, 1985). A more recent age determination for the Fish Canyon tuff is 27.57± 0.36Ma (Lamphere et al., 2001). A

Chapter 1: The apatite fission track method recent and high-precision study by Philips and Matchan (2013) on sanidine minerals obtained an

40Ar/39Ar age of 28.01 ± 0.04Ma for the Fish Canyon Tuff. Sanidine has the lowest crystallization temperature and is therefore a well-suited mineral for the estimation of the AFT age of the apatite crystals. For this thesis, the value of Hurford and Hammerschmidt (1985) is used, because of an easy comparison to zeta factors of other researchers in the laboratory of Ghent. The second age standard is

Durango apatite, with an age of 31.4±0.5Ma (Green, 1985). A more recent age determination by

McDowell et al. (2005) confirms this age (31.44±0.18Ma).

1.5 The thermal stability of fission tracks Through geological history, apatite fission tracks are constantly created and shortened because of the restoration of lattice defects. Fission tracks are sensitive to high temperatures and are gradually shortened and eventually removed if the temperature is elevated for a sufficient residence time. This track shortening process or ‘annealing’ occurs in the annealing zone which is situated between the isothermals of 60°C and 120°C, depending on the kinetic parameters of the apatite crystal. The isothermals of 60°C and 120°C are located at 2km and 4km depth respectively, in the case of a normal geothermal gradient of 30°C/km. The zone in the earth’s crust where annealing occurs in fission tracks is called the Partial Annealing Zone (PAZ). Below the PAZ in the Total Annealing Zone (TAZ), fission track accumulation is prevailed by a faster annealing process. In the Total Retention Zone (TRZ), located above the PAZ, is fission track accumulation the dominant process.

Multiple isothermal and isochronal laboratory experiments have been carried out in order to unravel the annealing behaviour of fission tracks in apatite (Green et al., 1985, Wagner and Reimer, 1972).

These experiments led to Arrhenius diagrams with a logarithmic time axis and reciprocal absolute temperature axis. The linear behaviour between these two physical quantities are eventually extrapolated to the geological time scale (Green et al., 1985).

1.5.1 Kinetic parameters influencing annealing behaviour

The chemical composition of apatite has an important effect on the annealing behaviour. Fluorapatite or F--rich apatite is found to be less resistant to annealing than chlorine apatite (Green et al., 1985).

Another important kinetic parameter is the etch pit diameter (Dpar) (Figure 3), which represents the distance between the two tips along the c-axis of the hexagonal etch pits that are visible on a c-axis parallel apatite surface (Figure 3a). The Dpar value depends on the etch rate, which is depending on the material and its chemical composition. The Dpar value has to be determined on samples which are etched with the standard conditions (5.5M HNO3, 21°C, 20s). Otherwise, a calibration is necessary (Sobel and Seward, 2010).

Chapter 1: The apatite fission track method

Figure 3: (A) Etched apatite crystal with the characteristic hexagonal etch pits on the surface which indicate the c-axis direction of the apatite crystal. The fission tracks intersecting the crystal surface are not shown. (B) Zoom on a characteristic etch pit with its Dpar value. (C) Microscopic image with the characteristic etch pits. The aligned randomly orientated lines are polishing defects. (figure of Seward and Sobel, 2010)

For apatites from crystalline rocks it is advised to measure etch pit diameter on the grain samples as a whole. For sedimentary or detrital samples – possibly consisting of more than one kinetic population – it is strongly recommended to determine the single grain average Dpar value. A mean Dpar value needs to be established after at least four Dpar measurements on each grain (Donelick et al., 2005). A more descriptive and detailed investigation on the annealing kinetics is done by Carlson et al (1999).

1.6 Fission tracks and thermochronology

1.6.1 Track length distribution and its geological significance

As much fission track lengths as possible are preferentially measured in order to obtain a truth-based track length distribution histogram. This histogram displays in a graphical way the frequency distribution of fission track length measurements. Further on, they are of great importance for the determination of the thermal history of a sample and the meaning of the apparent fission track age.

The length (l0) of a freshly-induced fission track is 16.35µm (Green et al., 1986), but almost never measured because of annealing through geological time.

The shape of the length distribution histogram, the mean track length and the standard deviation are basic requirements for the thermal history characterization. Length measurement information is representative for the time-temperature history that the sample experienced. Gleadow et al. (1986a) and Gleadow et al. (1986b) studied the characteristics of synthetic, volcanic, basement samples and reheated basement samples. A brief overview is given in Table 1 and Figure 4:

Chapter 1: The apatite fission track method

Table 1: typical length distribution patterns. The mean length distribution lm and the standard deviation σ are basic requirements of the track length distribution. The corresponding track length histograms are displayed in Figure 4.

Type of sample Description length distribution lm (µm) σ (µm)

Freshly induced Narrow, symmetrical 16.3 0.9 Undisturbed volcanic Narrow, symmetrical 14.0 – 15.7 0.8 – 1.3 Undisturbed basement Negatively skewed 12.5 – 13.5 1.3 – 1.7 samples Thermal overprinted Mixed distribution, not symmetrical <11.5 Around 2.0 Sedimentary rocks Mixed distribution <13µm >2.0

Figure 4: Track length distribution histograms. (A) Freshly induced tracks, (B) undisturbed volcanic, (C) undisturbed basement, (D) thermal overprinted and (E) sedimentary rock track length distribution. Figure after Gleadow et al. (1986b).

1.6.2 Closure temperature, cooling ages and the apatite partial annealing zone

The accumulation of spontaneous fission tracks depends on the ambient rock temperature. At higher temperatures (>120°C for apatite), fission tracks anneal rapidly no tracks accumulate in the crystal lattice. When the temperature is low enough (~60°C for apatite) fission track annealing is very low and effectively, all tracks are retained. Between these two temperature tresholds, there is a temperature zone in which both fission track shortening annealing occurs but no longer complete annealing will take place. This zone is called the partial retention zone or partial annealing zone (PAZ). In the case of a normal geothermal gradient of 25-30°C/km, the partial retention zone of 60°C to 120°C is located between depths of ~2-4 km. The Total Annealing Zone (TAZ) is located below the 120°C isotherm and the Total Retention Zone (TRZ) is located above the 60°C isotherm. The apatite fission track age system is considered closed at a temperature of 100±20°C (Wagner and Van den haute, 1992). This closure temperature dictates the moment when the apatite fission track clock starts ticking. Therefore, an apatite fission track age is a cooling age; it records the time since the apatite-bearing rock passed below the aforementioned isothermal threshold. This effective closure temperature actually depends on the specific chemical composition of the apatite.

Chapter 1: The apatite fission track method

Depending on the PAZ residence time and possible re-heating of the sample, the interpretation of an

AFT cooling age is different, illustrated by Figure 5:

Figure 5: Four potential time-temperature paths, defined by the residence time in the Partial Annealing Zone (figure after Jonckheere, 1995)

(1) Fast cooling: fast cooling through the Apatite Partial Annealing Zone (APAZ) can represent for

example a very rapid cooled volcanic apatite. Fast cooling results in a undisturbed volcanic

type AFT length distribution (lm= >14.0µm).

(2) Slow cooling: slow cooling through the APAZ causes the tracks to be more annealed. The

apparent AFT age will indicate the passage through the closure temperature isotherm

(TC=±100°C). The mean track length lm will typically be in between 12.5 and 13.5µm.

(3) Weak thermal overprinting: The sample is overprinted by a weak thermal event, which results

in a mixed age without any geological significance. A mixed or bimodal length distribution is

typical for this time-temperature history path. A typical cause of this thermal overprint is burial

by sediments, tectonic reheating, circulation of hot fluids or meteorite impact (Gleadow et al.,

1986b).

(4) Strong thermal overprinting: A strong thermal overprint is capable of bringing the apatite

bearing rock back in the total annealing zone, causing the apparent AFT age to be a

meaningful cooling age.

Chapter 1: The apatite fission track method

1.7 Geological interpretation of apatite fission track age data

1.7.1 Cooling rate

The cooling rate uc of an apatite sample with a fission track age can be calculated with the following equation:

(1.8)

With:

uc = cooling rate (°C/Ma)

Tc = closure temperature (°C)

To = ambient surface temperature (°C)

tAFT = AFT age (Ma)

1.7.2 Denudation, exhumation and uplift

Tectonic uplift is one of the main causes of rock cooling, but denudation of the uplifted rock mass is necessary to remove the overlying rock mass and inducing cooling of the rocks. This is the reason why

AFT cooling ages are bound to both tectonic forces that cause uplift and uplift-induced denudation.

Denudation includes both the removal of surface material by climatic and tectonic erosion events.

These two factors are often linked to each other.

1.7.3 Horizontal and vertical sampling profiles

Sampling for AFT studies can be done by following different strategies: (1) a horizontal profile that is perpendicular to the structural features such as for example faults or (2) a vertical profile that has multiple samples spread over a short horizontal distance. Samples with a great elevation difference of approximately 200m are preferentially taken. From the age-elevation profile it is possible to derive an estimate of a cooling event or detect the fossil partial retention zones and total retention zones (Figure

6).

In these vertical profiles, samples with the highest AFT age lie at the top of the lithostratigraphic column, while the AFT age of the samples decreases with depth until the total annealing zone is reached around 120°C at approximately 4km depth. If the age-elevation data are plotted, a linear trend of increasing age with elevation is the expected trend. The angle of the curve with the x-axis represents an often exaggerated estimate of the denudation rate (Figure 6a). The extrapolation of this line to the x-axis estimates the timing of the cooling event. A more rapid cooling event is visible in Figure 6b. A

Chapter 1: The apatite fission track method characteristic partial annealing zone profile can also be recognized in Figure 6c. A break in slope is a typical feature of a transition from the fossil total retention to the partial annealing zone. Typically, the samples with the lowest elevation have a trend towards the point of zero million years, as visible in

Figure 6c.

Figure 6: age elevation profiles (modified after De Grave 2003)

1.8 Multi-method approach: AFT and zircon (U-Th)/He A more and more used technique applied during the last decades is the combination of thermochronometers that have higher or lower closure temperatures compared to the AFT method.

Two samples of this thesis followed this approach and were analyzed with both the AFT and zircon (U-

Th)/He (zircon Helium) method. This method is based on the disintegration of parent isotopes through alpha-decay. The 238U, 232Th and 147Sm parent isotopes decay with time and produce daughter isotopes of 4He. These 4He particles are able to diffuse through the crystal lattice with a diffusion rate depending on the physical properties of the mineral (e.g. zircon, apatite, titanite). The 4He particles are immobile below the closure temperature of this system and are trapped in the crystal lattice. The measurement of the isotope concentration of 238U, 232Th, 147Sm and 4He enables the calculation of a zircon (U-Th)/He age:

(1.9)

With: 4He, 238U, 235U, 232Th and 147Sm representing the measured concentrations of these isotopes and

λ238, λ235, λ232 and λ147 the decay constants of the nuclear system. The coefficients before each term refer to the number of alpha-particles produced in each full decay cycle.

The presence of inclusions (e.g. monazite) can produce dauther isotopes that interfere with the isotope system of the zircon system. These inclusions produce a certain amount of 4He that is not produced by the zircon and disturbs the radiometric age system. It is thus important to pick inclusion-free zircon crystals under the optical microscope.

However, 100% preservation of the alpha-particles in minerals is never obtained. Alpha-particles produced by the decay of unstable isotopes is disturbed by a diffusion process called “alpha-ejection”.

Chapter 1: The apatite fission track method

Alpha-ejection represents the loss of 4He isotopes on the edges of the mineral through diffusion. The

4He in the center of the mineral is completely preserved, while on the edges of the crystal some of the

4He particles diffused through the crystal lattice. Hence, a correction factor - depending on the geometry of the mineral - needs to be applied in order to reconstruct the original concentration of 4He

(Farley, 2002; Ehlers and Farley, 2003; Reiners, 2005). The difference between the corrected and uncorrected age will typically be low for great crystals. Smaller crystals suffer more from loss of alpha- particles through alpha-ejection, meaning that the corrected age will be much older than the uncorrected age.

Four euhedral, inclusion-free zircon grains for each sample were cleaned and wrapped seperately in

Pt-tubes. These Pt-tubes were heated and degassed, after which the 4He concentration was measured by isotope-dilution mass-spectrometry. This is followed by the determination of the U, Th and Sm concentration with isotope dilution ICP-MS. Analysis of these aliquots was done at the John de Laeter

Center for Isotope research at the Curtin University in Perth (Australia). Applied analytical procedures can be found back in Evans et al. (2005).

Diffusion of 4He particles is a temperature-dependant process, enabling the researcher to use it for thermochronological purposes. At high temperatures – depending on the mineral – 4He is completely expelled from the analyzed mineral, while at low temperatures it is completely trapped in the crystal lattice. The partial retention zone is located in between these two zones and describes the zone in which the alpha particles are partly rejected and partly retained. The closure temperature of the zircon helium system is located around the ~170-190°C isotherm (Reiners et al., 2004). Hence, a zircon

Helium age will typically be older than an AFT age, which has a closure temperature of 100±20°C.

Sometimes it is necessary to discard one zircon helium age if mineral inclusions (e.g. monazite) were present in the zircons. These inclusion-rich zircon will typically yield an anormal age that differs a lot from the other ages obtained on inclusion-free zircons.

1.9 Thermochronological modelling Thermochronological modelling or thermal history modelling attempts to reconstruct the temperature history of a sample through geological time. The apparent AFT age and the track length distribution are basic input requirements from which a thermal history is suggested. Several software have been developed during the past decades (e.g. Monte Trax, AFTSolve and HeFTy), from which the most recent (QTQt, RadialPlotter, DensityPlotter) will be used in this thesis.

Chapter 1: The apatite fission track method

1.9.1 Introduction to QTQt

QTQt stands for Quantitative Thermochronology with Qt multiple sample thermal history modelling

(Gallagher, 2012). The software is based on (probability-based) Bayesian statistics and the Monte Carlo

Markov Chain sampling algorithm. This algorithm samples random in the model space, while the

Markov chain evaluates the next step in the model space. The inverse modelling approach starts from the data and suggests a best-fitting model. This estimated thermal history model tries to approach the true thermal history as close as possible. Several synthetic and real examples of AFT thermal history models and more detailed information can be read in Gallagher (2012).

Almost all kinds of thermochronological data from various dating methods can be used in QTQt.

Apatite and zircon fission track data, zircon and apatite helium data, 40Ar/39Ar data, vitrinite reflectance data and zircon U/Pb data can be used simultaneously. Another advantage is the use of the Bayesian statistics and the Monte Carlo Markov Chain, defining more than one possible time-temperature path and the probability that the sample followed the path. Moreover, the output of QTQt has the tendency to simplify the best fitting time-temperature histories to less complex time-temperature histories.

As input parameters for thermal history modelling based on AFT data with QTQt, the basic required input parameters are the number of spontaneous tracks (Ns) and induced tracks (Ni) (Figure 7). Fission track lengths are also required for the thermal history of the sample in the Partial Annealing Zone

(PAZ). For the annealing behavior, the multi-kinetic annealing model of Ketcham et al. (1999) and

- Ketcham (2007) are used. Additionally, c-axis angle and compositional data (for example Dpar or Cl content) can be used as input parameters to improve the thermal history model characteristics (Figure

8).

Figure 8: window of QTQt which requires the track length Figure 7: input window of QTQt data information and other possible information

Chapter 1: The apatite fission track method

1.9.2 Modelling with QTQt

After input, the amount of burn-in and post burn-in iterations is chosen by the user. In the burn-in stage, the likelihood (or data fitting parameter) increases and the model improves, as visualized by the red line on Figure 9. During the post burn-in stage, the likelihood generally has a horizontal trend, meaning that there general trend of an increase in likelihood is finished. The noise in the curve describes the attempts to fit a new time-temperature history curve to the data.

Figure 9: The likelihood function is displayed in red. The burn-in phase stops at approximately iteration 200 and the post- burn-in phase takes over. During the burn-in phase the likelihood increases, while the post burn-in phase is generally horizontal.

QTQt samples the time-temperature model space with an adjustable step size. This step size or

‘proposal move’ can be chosen for both time and temperature (Figure 10). Every new step proposes a new time-temperature history curve of which the likelihood value determines if it is rejected or accepted. This process is illustrated by the acceptance rates, which are ideally between 0.1 and 0.5. If the acceptance rates are too high, the sampling in the model space is too conservative and the proposal move needs to be increased. On the contrary, if the acceptance rates are too low, the proposal move has to be decreased. If the birth and death acceptance rates (i.e. when models are retained or discarded) are more or less the same, the modelling has reached a stable situation.

Several calculation steps with approximately 104 to 105 iterations and changes in the proposal moves are necessary to have a satisfactory data-fitting model for each sample. The modelling is successful if the likelihood curve and posterior curves remain stable, the acceptance rates are around 0.3 and the birth and death ratios are in balance. The red color on the time-temperature plot defines the most probable time-temperature path, while the blue color indicates a less probable time-temperature path.

The last step is to let the model calculate for 105 to 106 iterations, obtaining a high resolution time- temperature history diagram (Figure 11).

Chapter 1: The apatite fission track method

Figure 10: Monte Carlo Markov Chain parameters of the final run of sample TF-06.

Figure 11: TF-23: possible time-temperature paths after 106 iterations. The vertical temperature axis can be assumed as the depth in which the rock sample is situated. The horizontal axis describes the time in and is expressed in millions of years. The probability that a rock sample was situated at a particular time-temperature point is expressed by a color scale which expresses the probability value. The brown complex curve is the maximum likelihood model that is the best data-fitting, but is usually too complex. The green curve is the maximum posterior model, which is normally too simple. The black curve in between the two other black curves expresses the expected model.

Chapter 1: The apatite fission track method

The strength of QTQt is that it calculates a variety of time-temperature curves (maximum posterior, maximum likelihood, …). The ‘maximum likelihood’ model (brown) fits best the fission track length data

(Figure 12), but is often too complex (Figure 11). The ‘maximum posterior’ model (orange) has less time-temperature points and is typically a more simple curve, because it is the result of the likelihood function multiplied with the prior distribution. The prior distribution prevents the maximum posterior model to become too complex. The ‘maximum mode’ model (white) is obtained from the distribution of all sampled models. The ‘expected’ model (black) is a weighted mean model and is the preferred model to use for interpretation. It contains features of all the other models and also displays a credible interval with 95% probability range. The credible interval is a typical Bayesian statistics term which differs from the confidence interval used in frequentist statistics.

Figure 12: TF-23 maximum mode curve fitting the track length distribution of a AFT sample. The vertical axis expresses the number of tracks, while the fission track lengths are expressed by the horizontal axis. Sedimentary AFT samples can also be modeled with QTQt, but additional parameters have to be added. If the stratigraphic age is known precisely, it can be used to define the time-temperature history at the time of deposition. The option ‘allow for predepositional history’ is another possibility and examines pre-depositional time/temperature points. This option also samples the stratigraphic age of the detrital sample. Another helpful tool for sediments is the simultaneous modelling of multiple vertical samples, which is especially helpful when modelling sedimentary successions from drilling wells and drill cores in sedimentary basins.

Chapter 1: The apatite fission track method

1.9.3 RadialPlotter and DensityPlotter

The RadialPlotter software developed by Vermeesch (2009) is a Java based application used for single- grain age mixtures of AFT and zircon U/Pb datasets. The software displays the age data as dispersed dots based on the age and its uncertainty. The more closer the point is situated to the age axis, the smaller the uncertainty on the single grain age. It is therefore a graphical way to display the uncertainty on each grain and creates an extra dimension compared to an age histogram plot. This enables the user to recognize different age populations or trends in a more graphical way. The main advantages of RadialPlotter are its straightforward display, the easy handling and the age-uncertainty plot.

The program has an automatic option which identifies age populations in a mixture, based on the mixture modelling algorithm of Galbraith and Green (1990). These authors tested their algorithm with synthetic mixtures consisting of three synthetic Durango apatite age populations of 200, 240 and

342Ma. Four different combinations using only 10 apatite grains of each population are investigated.

Even with this small amount of data, the algorithm mostly succeeds in finding the right number of populations. The proportion of each population is also estimated in this paper, but is not successful.

The problematic identification of the two different populations in the mixture of age population

200Ma and 240Ma is caused by the uncertainty on the single grain ages and the fact that the authors only used 20 grains in total. In Figure 13, a mixture of three age components is displayed and three age populations are suggested. Figure 14 shows a detrital apatite sample of Upper-Carboniferous age that is analyzed in this thesis. The Dpar values are also displayed by the yellow to red color scale. This indicates that subtle differences in age populations are difficult to detect. The more advanced

DensityPlotter software is a grain distribution tool for geochronological purposes, developed by

Vermeesch (2012). It uses Kernel Density Estimations which have a much better theoretical background than the Probability Density Plots (Vermeesch, 2012).

Chapter 1: The apatite fission track method

Figure 13: radial plot of a three components age mixture of Galbraith and Green (1990). The suggestion of three age populations is correct, but the uncertainties are very high and not corresponding to the age populations of 200, 240 and 342Ma.

Figure 14: RadialPlot of sample SK-46 on which the colours display the Dpar etch pit size (section 1.6.1). The horizontal axis is logarithmic and expresses the percentage of uncertainty divided by the single grain age. Grains with a larger uncertainty are found on the left size of the plot, while the grains with the lowest uncertainty are located at the right side.

Chapter 2: Detrital thermochronology

2.1 Introduction to detrital thermochronology Tectonic and climate processes are the main sediment-generation controlling factors in a sedimentary basin. Geo- and thermochronological techniques by single grain analysis are often used in provenance studies. The common way to identify the source and quantities of sediment is by detrital single grain analysis. Single grain analysis provides a wide spectrum of information, such as source rock lithology, geochemistry, low temperature chronology and high temperature chronology. At the moment, an extensive list of minerals is used for single grain analysis: amphibole, apatite, epidote, Fe-Ti-oxides, garnet, K-feldspar, monazite, muscovite, pyroxene, rutile, spinel, titanite, tourmaline and zircon to name just the most commonly used.

Specific low-temperature detrital thermochronologic techniques use low-temperature thermochronometers such as AFT and apatite/zircon (U-Th)/He are utulized to characterize the long- term evolution and the thermal history of a sedimentary basin. In this study, two basement samples are investigated with the zircon (U-Th)/He technique, which has a closure temperature of ~200°C

(Reiners, 2005). The AFT dating method (discussed in the previous chapter) is applied on all of the samples that are analyzed in this thesis. The AFT system has a lower closure temperature of 100±20°C

(Wagner and Van den haute, 1992) and is in that sense complementary to the zircon (U-Th)/He method. In this way, a multi-method thermochronometer approach is used to describe the evolution of a sedimentary basin and its basement on different temperature/depth scales in the continental crust.

The age of the thermochronometer of choice applied to a detrital sample is affected by the burial history of the basin. If the sample is buried by several kilometers of overlying sediment, it can be heated to temperatures higher than 100°C. This would totally reset the AFT system for example. The inherited age components (i.e. preserved signals from the source areas) disappear and the so-called central age of the AFT system will typically be much younger than the stratigraphic age of the sediment. The interpretation of the AFT age is then similar to the interpretation of a basement rock sample. A vertical transect (e.g. in an oil well) of samples for AFT analysis can be used to derive the geothermal gradient in the sedimentary basin (Figure 15). The ages of the apatite fission track system can be used to determine for example the geothermal gradient in boreholes.

When a detrital sample is buried by a thin sedimentary cover (<1km) (and not affected by fault friction heat), the central AFT age will typically be older than the stratigraphic age of the sediment. In this case, the spectrum of single grain ages can consist of multiple age populations that exceed the stratigraphic age. These age populations can provide useful information on sediment sources and are invaluable to

Chapter 2: Detrital thermochronology reconstruct the tectonic, paleo-geographical and paleo-climatic evolution/history at the time of sedimentation.

When the detrital sample is covered by ±2 km of sediment, the sample is heated to an extent that it is brought back in the apatite PAZ. Fission tracks are partially annealed, which can be recognized by a reduced mean track length. In this case, the central AFT age is typically a mixed age (i.e. meaningless age) in between the stratigraphic age and the reset age.

Figure 15: Variation of AFT central age and minimum age with depth in a borehole. The stratigraphic age is systematically younger than the AFT ages for the first 3km. Samples taken deeper in the borehole have a decreasing age because of the annealing of the fission tracks if the closure temperature of the AFT system is exceeded. A constant lag time (see further) of ±2Ma can be recognized (Gautam and Fujiwara, 2000). Although AFT dating represents a solid low-temperature thermochronometer, a few limitations have to be admitted. First of all, the single-grain AFT ages are not always very precise, due to the relatively low

U-concentration and consequential low spontaneous fission track densities compared to zircon.

Secondly, acid pore water coming from coal layers or ore bearing deposits can bring the apatite minerals in solution. Another limitation is the possibility to have a meaningless mixed age (section 1.7), when the sediments remain long enough in the partial annealing zone so that the system can be partially reset.

Datasets of AFT samples are typically smaller than detrital zircon U/Pb datasets, having usually approximately 50 single grain ages for each AFT sample (von Eynatten and Dunkl, 2012). Detrital zircon

Chapter 2: Detrital thermochronology

U/Pb datasets most of the time consist of more than 117 grains, in order to reach the 95% confidence level that a component consisting of 5% of the total age distribution is not missed (Vermeesch, 2004).

The youngest single-grain age in a non-reset AFT age system is not of any importance and does not assess the stratigraphic age in a precise way. A more meaningful age is the youngest age component that is identified after modelling the dataset with a program such as BayesMixQt (Gallagher et al.,

2009) or RadialPlotter (Vermeesch, 2009) (see section 1.8). The youngest age component of the AFT dataset indicates the maximum age of deposition, in the case that the detrital sample was not brought back into the partial annealing zone during its geological history.

The lag time of a sediment is an important concept in detrital thermochronology (Figure 16). The lag time is the difference between the youngest cooling age component and the age of sedimentation. A short lag time typically indicates rapid exhumation in the source area, whereas a long lag time suggests slow exhumation. The time from erosion to deposition is generally short and smaller than 1 million year (Brandon and Vance, 1992), meaning that the lag time records the exhumation speed of a mountain range in a foreland basin setting.

Figure 16: lag time concept. Lag time (L) stands for the age of the youngest age component minus the sedimentation age (von Eynatten and Dunkl, 2012) To conclude, some considerations on the dependence of apatite-bearing lithologies have to be made.

The dataset can suffer from a blind spot when the source rock interpretation is only based on detrital apatite analysis, because source rocks such as limestones and apatite-free metamorphic rocks are underrepresented in the apatite spectrum. Another possible blind spot is the recycling of sediments in a sedimentary basin, which is difficult to recognize with detrital single-grain thermochronology. A possible way to overcome this problem is by estimating the maturity in petrographic thin-sections.

Chapter 2: Detrital thermochronology

2.2 Erosion and sediment generation estimations In this subsection, the focus lies on exhumation controlled by climate or erosive denudation and not on tectonic denudation. Tectonic denudation and erosive denudation are the two different types of denudation. Tectonic denudation can cause very high cooling rates in basement rocks and results typically in young ages and short lag times for detrital thermochronology signals. These cooling ages caused by tectonic exhumation lead to short lag times and therefore not used in the quantification of erosion rates caused by erosive denudation. Erosive denudation is the process that causes sediment erosion and means (or is defined by) exclusive removal of the upper most layer of the crust by mechanical and chemical alteration processes followed by down-slope transport of material in the drainage system (von Eynatten and Dunkl, 2012). Consequently, it is only possible to quantify erosion rates with detrital thermochronology if it can be assured that only erosive denudation affected the source area (von Eynatten and Dunkl, 2012). Successful studies in foreland basins in the Western

European Alps indicated changing erosion rates through recent geological history and illustrated the applicability of detrital thermochronology studies on erosion rates on short time-scale (Glotzbach et al., 2011).

Chapter 2: Detrital thermochronology

Figure 17: Three evolutionary stages of a collisional mountain belt. (a) constructional phase, (b) steady-state phase, (c) destructional phase. These phases are reflected in a trend of decreasing lag time (figure below), in which phase 1,2 and 3 represent the constructional, steady-state and destructional phases respectively (Allen and Allen, 2013). Three phases of orogenic growth that generate different amounts of sediments are described in Figure

17. During the first phase, tectonic mass influx (FA) is higher than the erosion rate (FE). This inequilibrium causes crustal thickening, surface uplift and the construction of topography. The second phase (steady-state) represents a phase of equilibrium in which rivers and hillslopes transport sediment into the sedimentary basins. The third and last phase is the destructional phase in which the tectonic force (FA) decreases, topography reduces and the erosional component overcomes the tectonic component (Allen and Allen, 2013). The sketch at the bottom of Figure 17 indicates the change in lag time through an orogenic cycle.

2.2.1 Quantification of erosion rates with thermochronology

Scientific research on erosion rates quantifications with detrital thermochronology data have been done only recently (Brandon et al. 1998 and Garver et al. 1999) and during the last decades it is

Chapter 2: Detrital thermochronology optimized (Willet and Brandon, 2013). Brandon et al. (1998) and Garver et al. (1999) proposed a formula to convert lag times to erosive denudation rates (equation 2.1). The upper and lower boundaries of the rock layer are at constant temperatures Ts (surface temperature) and Ts + G0/L respectively, where G0 stands for the geothermal gradient and L is the layer thickness.

2.1

With: z = depth

E = denudation rate

K = thermal diffusivity of the crust

By coupling the equation that expresses the closure temperature Tc of a sample as a function of its cooling rate (see Dodson, 1973), the following expression for the depth (zc) at which closure occurs is obtained (Braun et al., 2006):

2.2

The final equation 2.3 relates the thermochronological age ( ) of the youngest component in the detrital FT population to the depth of closure of the system (zc):

2.3

The thermal parameters Ts, G0, L and K can be filled in as standard values/estimations, or if available, the values derived from thermal conductivity studies in the study area. The thermochronological age is ideally build up of fully reset age (and not a mixed age) from a thermochronometer of choice (Braun et al., 2006).

Figure 18: a) The depth of closure isotherm zc and exhumation rate E as functions of the lag time for four different thermochronometers. The thermal parameters used are: Ts = 10°C, G0 = 25°C/km, L = 30km and K = 25 km²/Ma (Braun et al. 2006) b) the lag time – exhumation rate relation 26

Chapter 2: Detrital thermochronology

On Figure 18 it can be seen that when the exhumation rate increases, the lag time decreases. A very short lag time will thus be typical for an area of rapid exhumation. On the contrary, a very long lag time of tens of millions of years indicates very slow exhumation in the area.

Before drawing conclusions on denudation/erosion rate, it is necessary to consider the following assumptions:

1. The geothermal gradient (°C/km) is a multiplication factor that is used in the calculation of the

denudation rate (equation 2.1 and 2.2) and also introduces some uncertainty. Also, the current

geothermal gradient is not the same as the paleo-geothermal gradient at the time of

sedimentation and exhumation. Magmatic underplating or intense volcanism can have a

dramatic effect on the geothermal gradient of the crust (von Eynatten and Dunkl, 2012).

2. The denudation process is assumed to be a continuous process. If the process occurred in

pulses, the value of denudation will be only an estimation of the long-term process (von

Eynatten and Dunkl, 2012).

3. Depth of erosion must have reached the total reset zone of the used thermochronometer. If

this is not the case, the dating will only reflect previous tectonic events (Rahl et al., 2007).

4. The mineral of the chosen thermochronometer is representative for the eroded rock unit (von

Eynatten and Dunkl, 2012). Apatite-bearing lithologies (e.g. granites and granodiorites) are

very abundant in Kyrgyzstan and are therefore assumed to be representative for the eroded

source rock and recognizable in the single grain analysis.

Chapter 2: Detrital thermochronology

2.3 Thermal maturity estimation

2.3.1 Thermal maturity estimations in sedimentary basins based on organic material

Temperature estimations in sedimentary basins are typically done in the context of hydrocarbon prospection. A frequently used technique is the study of vitrinite reflectance, which is an optical parameter and is measured on the vitrinite group of macerals. With increasing burial depth, the reflectance of the vitrinite increases. Organic-rich sediments and especially sediments containing plant material produce a big amount of vitrinite macerals. With increasing depth, the irreversible process of increasing vitrinite reflectance occurs. Another less frequently used technique for thermal maturation studies is the coal rank. The coalification process starts at the surface with the deposition of peat layers and can reach the anthracite stadium after the loss of moisture and volatiles. Especially in organic-rich parts in the often coal-rich Jurassic sediments of Kyrgyzstan, there is a difference in degree of coalification of the Jurassic coal deposits east and west of the TFF. This can indicate that the Jurassic coal sediments are buried to different degrees on both sides of the TFF. Several samples with organic- rich material were taken during the 2015 field expedition, on which vitrinite reflectance measurements and paleobotanical research can be performed. These organic-rich, vitrinite-rich sediments are the typical sediments that can give gas-rich deposits in the Fergana depression located in Uzbekistan and

Kyrgyzstan (Rozanov et al., 1960).

2.3.2 Thermal maturity estimations in sedimentary basins based on low-temperature thermochronology

Vertical sections or drill holes offer information about the geothermal gradient and illustrate the lag time concept. The central AFT age is higher than the stratigraphic age for the first kilometer because of the lag time concept (Figure 16). If the sample reaches the apatite PAZ and eventually the apatite TAZ, the age decreases and will eventually approach zero. The lower part of the PAZ and the TAZ is located in the oil and gas maturation window. A description of a local oil well in the Australian Otway basin provides detailed information on the decreasing mean track length with increasing temperature

(Figure 19, Green et al., 1989).

Chapter 2: Detrital thermochronology

Figure 19: AFT parameters observed in multiple samples in the South-Australian Otway basin wells (Green et al., 1989).

Chapter 3: From sampling to AFT analysis

3.1 Report of field work A field campaign in the Kyrgyz Tien Shan took place from the 20th of June until the 14th of July 2015 organized by the MINPET group (Department Geology, Ghent University) and the Geosciences

Department from Rennes University. An international team consisting of sedimentologists, structural geologists and thermochronologists investigated the Mesozoic deposits exposed in several areas over

Kyrgyzstan. Approximately 40 samples for AFT analysis and zircon U/Pb analysis were gathered during this expedition. Both crystalline and detrital rocks were targeted for sample collection. Additionally, samples for paleobotanical analysis and paleosol samples were collected. The detrital thermochronology samples fit in a high-resolution sedimentary log that was drawn on the field by the sedimentologists. The differences or similarities in Mesozoic deposits on the eastern and western side of the TFF was the main subject of this expedition. The team consisted of the following scientists:

- Elien De Pelsmaeker (PhD student Ghent University)

- Dr. Fedor Zhimulev (Siberian branch of the Russian Academy of Science): structural geologist

with detailed knowledge on the structural evolution of Central-Asia

- Prof. Dr. Marc Jolivet (CNRS France): experienced geochronologist in Central-Asia

- Amandine Dransart-Laborde (PhD Student University of Paris): sedimentologist

- geophysicists Dr. Vlad Batalev (Russian academy of Sciences, Bishkek, Kyrgyzstan)

- Simon Nachtergaele (Master student Ghent University)

Figure 20: field work trajectory of the field work expedition of June/July 2015

Chapter 3: From sampling to AFT analysis

3.2 AFT sample preparation

3.2.1 Heavy mineral separation

Various sample separation techniques are applied on each rock sample in the laboratory of the

MINPET research group before a heavy mineral mixture enriched in zircon and apatite fraction is obtained. The process starts with circa five kilogram of rock sample that is crushed to gravel size with the jaw breaker machine. The Fritsch disk mill thereafter reduces the grain size to the fine to very fine sand fraction, of which the (sieved) fraction from 63µm to 250µm is used. Wet and dry sieving techniques are applied to remove the coarser and finer grain sizes with a Fritsch sieving tower.

The next step is the removal of the magnetic minerals with the LB-1 Frantz magnetic separator.

Different voltages and current settings (0.1A, 0.5A, 0.8A, 1.0A and 1.2A) are used to separate the ferromagnetic from the ferrimagnetic minerals. The nonmagnetic part consists mainly of feldspars and quartz grains but also of other diamagnetic minerals such as zircon and apatite. These two extensively used minerals in geochronological studies are afterwards extracted with a heavy liquid separation. LST

(Low Sodium Toxicity heteropolytungstate) Fastfloat is a water based heavy liquid with very low viscosity, low toxicity and adjustable density, depending on the water concentration. A density of

2.82g/ml is applied on the non-magnetic fraction of the rock sample, separating the heavy minerals

(e.g. apatite and zircon) from the lighter minerals (e.g. quartz and feldspar crystals).

3.2.2 Heavy mineral selection

Handpicking of apatite and zircon crystals is executed with a needle under a ‘Leica MZ 16 FA’ stereo microscope. Apatite is recognised by its hexagonal shape, translucent to light grey colour, medium strong gloss and high relief. The selection of apatite in felsic intrusives is rather easy, because of the presence of euhedral crystal facets. On the contrary, apatites in sedimentary rocks are typically rounded and often difficult to recognize, requiring experience from the geochronologist. These identification problems can cause an apatite-poor mount in some cases. Especially the difference between well-rounded apatite and well-rounded zircons is difficult to distinguish with a stereo microscope (Figure 21). There is a subtle difference between the well-rounded apatite and well- rounded zircon grains. Apatites from sedimentary rocks are typically white and often have a yellow shine. Well-rounded zircons from sedimentary rocks are sometimes brown or red and have a stronger reflectance.

For a sedimentary sample and where possible, 150 to 200 apatite grains are aligned manually on double coated tape, while for a plutonic sample circa 100 apatites are handpicked. This large amount of grains leaves some room for loss of grains during the grinding and polishing process. Eventually, the mount is photographed for multiple purposes. The first reason is that it is not possible to

Chapter 3: From sampling to AFT analysis misidentify an apatite mount if the sample names got lost during grinding, polishing or etching.

Another reason to make photographs is that it is possible to consult the photographs when the apatite identification process has failed.

Figure 21: photograph of handpicked apatite mount on which the apatite crystals are indicated in red. The other mineral grains are well-rounded zircons that are mistaken for well-rounded apatite minerals.

3.2.3 Mounting procedure

Each apatite sample is embedded in low viscosity EpoFix resin in molds of 1.4 cm diameter. Struers

EpoFix resin and EpoFix hardener are mixed with a volumetric ratio of 15:2 respectively. A more precise way of mixing the resin with the hardener is by a weight ratio of 25/3. This way, volumetric uncertainties caused by bubbles and adhesive forces in the syringes are negligible. Maintaining a constant ambient temperature around the mount prevents the formation of unwanted shrinkage cracks in the mount. Struers silicon carbide grinding paper of different mesh sizes is used during the wet grinding step. The internal surface of the apatite minerals is carefully revealed during the #500 and

#1000 step. The last step of the grinding procedure is the #2400 step and removes the microscopic irregularities on the apatite surface. The grinding of very fine sand-sized apatite grains is often problematic and requires prudence.

The apatite surfaces are subsequently polished with a Struers DP-U4 polishing machine (150rpm). It uses a magnetic, rotating disc (MD-Disc) which supports different types of MD-Cloths (MD-Dac, MD-

Plan, MD-Dur, …). Struers DiaDuo diamond suspensions of 6µm, 3µm and 1µm are used in decreasing order. These suspensions contain both cooling lubricant and the correct amount of diamond grinding suspension. During this thesis, the polishing protocol is tested on new cloths (MD-Dac, MD-Plan, MD- 32

Chapter 3: From sampling to AFT analysis

Dur, …). Satisfactory polishing results are only obtained after 45-90 minutes with the ‘MD-Dur’ cloth.

On the contrary, by using the ‘MD-Dac’ cloth, fairly good results are already obtained after 10 to 15 minutes of polishing (Table 2). Each sample requires a different polishing treatment for a perfect polishing result and is therefore polished individually.

Table 2: polishing time of apatite mounts on three different MD-Dac cloths of 9 prepared basement samples

Sample Dac 6µm (min) Dac 3µm (min) Dur 1µm (min)

KB 121 3 5 1 KB 122 3 3.5 1.5 KB 123 3 4.5 2.5 KB 124 4 5 2.5 KB 131 5 3.5 2.5 KB 132 5 3.5 2 KB 133 5 2.5 2 KB 134 5 3 2 KB 135 5 2.5 2

The final step of the mount preparation process is the etching of the spontaneous tracks with 5.5M nitric acid (HNO3) at 21°C for 20 seconds. After the chemical etchant step is finished and the mounts are cleaned with an excess of water, an external detector (see section 1.3) can be placed on top of each sample.

3.2.4 Irradiation at nuclear reactor BR1

Two irradiation packages (M11 and M12) with samples analyzed in this thesis were compiled with apatite mounts, apatite age standards and U-doped glass dosimeters (Figure 22, Figure 23). Irradiation with thermal neutrons took place at the Belgian Reactor BR1 at the 11th–12th of January (M11) and at the 22nd–23rd of March (M12). The reactor has been operational since 1956 and is stationed at the SCK-

CEN (‘Studiecentrum voor kernenergie’ or ‘Centre d’Etude de l’énergie Nucliaire’) site in Mol (Belgium).

It is an air-cooled reactor that is moderated by graphite (±500 ton). The fuel of the reactor is still the original 25 ton of natural Uranium that is used since 1956 and barely 1% of this fuel has been used so far. Since the arrival of the BR2 reactor, the BR1 reactor has not always been operative. The BR1 is only used for research purposes, such as calibration of measurement instruments, silicon doping, training for nuclear experts and radiation tests. The reactor contains 829 channels of which only 569 are loaded. For experimental use, 70 channels of all different kind of size can be used. The reactor is used for routine irradiation for FT purposes since 2010 (De Grave et al., 2010). Both irradiation packages have been irradiated in channel X26, which is a well-thermalized channel that has an experimental determined thermal/epithermal fluence ratio of 98±3 (more information on neutron spectra in section

1.3.1) (De Grave et al., 2010). The irradiation packages M11 and M12 are filled with several apatite age standards and glass dosimeters (Figure 22 and Figure 23). Dr. Guido Vittiglio and Dr. Bart van Houdt

Chapter 3: From sampling to AFT analysis from SCK-CEN are thanked for performing the irradiation step. After a cool-down period of approximately three to four weeks, the irradiation package is ready to be dismantled. The last step in the sample preparation process is the etching of the external detectors with a 40% HF solution for

40min at a temperature of 20°C in order to reveal the induced fission tracks.

Figure 22: composition of the M11 irradiation package Figure 23: composition of the M12 irradiation package

3.3 Zeta-factor age calibration

3.3.1 AFT counting procedure

The BH-2 Olympus microscope in the MINPET research group is currently used for fission track geochronology. This microscope has a maximal magnification of 1250x and has the option to use both transmitted and incident light. This magnification is obtained with an ocular of 10x magnification, an objective of 100x magnification and a tube lens that magnifies with another 1.25x. A counting grid with a calibrated surface of 6400µm is mounted on the ocular, which is used for the conversion of the amount of fission tracks (Ns, Ni, Nd) counts to fission track densities (ρs, ρi, ρd).

Induced and spontaneous fission tracks are typically counted on a surface of the polished apatite surface and on the corresponding area of the external detector. Therefore, an exact placement (so- called repositioning method) of the etched external detector back on the apatite mount is essential

(Jonckheere et al., 2003). The number of fission tracks has to be counted on the exact same area of the apatite crystal and the external detector. Differences in U-content in both apatite populations and individual apatites cause differences in the induced track densities. If the external detector is placed in the same way as prior to irradiation, it is possible to adjust the focus plane of the microscope and

Chapter 3: From sampling to AFT analysis count the spontaneous and induced tracks on the same surface area of the grain. This overcomes the two aforementioned problems caused by heterogeneities in U-concentration (Jonckheere et al., 2003).

The counting process of fission tracks requires not only an exact positioning of the external detector, but also a well-defined counting strategy. The researcher has to make an almost instantaneously decision on the particular feature on the apatite surface. The subtle difference between an etch pit, mineral inclusions or a very shallow fission track is a rather difficult decision requiring experience by the analyst. This is one of the causes of the differences in zeta factors between the different researchers. Etch pits which are not deep enough are interpreted as mineral inclusions or point defects and therefore not counted by the author of this thesis. Neither apatite surfaces crosscut by cleavage planes, nor very small surfaces are counted. This is because a single grain age based on two or three spontaneous or induced fission tracks has great uncertainties. To conclude, each geochronologist has a different perception of an etch pit and this may vary with time. However, as long as the researcher stays consistent through time in his perception of the etch pit or shallow fission tracks, this is not a problem at all (Hurford and Green, 1983).

3.3.2 AFT length measurements

The Kontron MOP-AMO3 tablet system is used for the fission track length measurements that characterizes the thermal history of the sample (see section 1.6). The Kontron system consists of a high-resolution digitizing tablet and red laser which is projected via a mirror on the microscopic view.

A calibration of the digitizing tablet is executed prior to the track length measurements. As much length measurements as possible are executed on horizontal TINTs (Track In Tracks), because the annealing behavior of TINCLEs (Track IN CLEavage) is different (Donelick et al., 2005). Therefore, it was not always possible to obtain the required minimum of 100 track lengths for each sample.

3.3.3 AFT-analysis: calibration by glass dosimeters

The thermal neutron fluence is quantified by the track densities of the external detectors that are attached to U-doped glasses. Four U-doped glasses of which the U-concentration is well-known are irradiated in both the M11 and M12 irradiation packages. These glasses are IRMM-540 glasses which were developed in co-operation with the Institute of Reference Materials and Measurements (IRMM) of the European Commission. The IRMM-540 glass has a homogeneously distributed certified (De

Corte et al., 1998) U-content of 13.9±0.5ppm and a natural 235U/238U isotopic ratio (of 7.277±0.007*10-

3). The concentration of Thorium and Rare Earth Elements is below the detection limit of various analytical methods (De Corte et al., 1998)

In order to have a relatively low error margin on the induced track density from the glass dosimeters, at least 2000 tracks are counted in each external detector. The glass calibration curve is displayed in 35

Chapter 3: From sampling to AFT analysis

Figure 24 for irradiation M11 and for irradiation M12 in Figure 25. The glass calibration curve of the irradiation package M11 is relatively flat and indicates a relatively uniform thermal neutron fluence in the channel X26 of the BR1 reactor. A linear best-fitting curve is drawn through the individual measurements and provides an estimation of the thermal neutron fluence through the irradiation package. Eventually, each age standard and apatite mount obtains an interpolated ρd value based on the measurements in the U-doped glass dosimeters (see section 3.4). A linear interpolation determines the estimated ρd value for each co-irradiated sample and is used in the age equation (section 1.4, equation 1.6).

Irradiation package M11 y = -0.0029x + 4.6251 5.00 R² = 0.7938 4.90

4.80

4.70 4.60 4.50 4.40

d (10^5 tracks/cm²) (10^5 d 4.30 ρ 4.20 4.10 4.00 0 10 20 30 40 50 60 Relative position of the glass dosimeter in irradiation package (mm)

Figure 24: glass calibration curve of irradiation package M11. A linear interpolation is used for the quantification of the ρd value for each sample that is situated in between the U-doped glass monitors. The interpolation curve suggests a negligible axial gradient and intersects the uncertainty ranges of all four glass dosimeter.

Irradiation package M12 y = -0.0019x + 4.1048 4.50 R² = 0.1548 4.40

4.30 4.20 4.10 4.00 3.90 3.80

ρd (10^5 (10^5 ρdtracks/cm²) 3.70 3.60 3.50 0 10 20 30 40 50 60 70 Relative position of the glass dosimeter in irradiation package (mm)

Figure 25: glass calibration curve of irradiation package M12. A linear interpolation is used for the quantification of the ρd value for each interlocated apatite sample. The interpolation curve does not intersect all three the glasses, which could be caused by a non-uniform thermal neutron fluence.

Chapter 3: From sampling to AFT analysis

3.3.4 AFT-analysis: calibration by age standards

The next step is the ‘zeta’ factor age calibration step and is outlined in section 1.4.1. During this calibration step, both the spontaneous (ρs) and induced fission track densities (ρi) of very well-known age standards are determined. At least five age standard mounts have to be counted in order to obtain a well-constrained zeta-value, but it is advised to measure as much as possible. Five different

Fish Canyon Tuff (FCT) and five Durango (DUR) apatite standard mounts are used for the zeta- calibration (Table 3 and Figure 26). The Standard Weighted Mean Zeta (SWMZ) for the Durango

Apatite standard is 300.17±5.83 a*cm² and 260.06±7.99 a*cm² for the Fish Canyon Tuff. The Overall

Mean Weighted Zeta (OWMZ) is equal to 286.23±4.71 a*cm². Within the individual zeta factors for the

Durango and the Fish Canyon Tuff age standards, the individual values remain more or less stable according to the SWMZ value (Figure 26 and Table 3).

Table 3: Results of the FT-analysis of the age standards. All track densities are in 105 tracks/cm². n=amount of grains counted, Ns = number of spontaneous tracks counted, s = spontaneous track density, Ni = amount of induced tracks counted, i = induced track density s/i = the ratio of the spontaneous over the induced tracks, Nd = interpolated value of the glass dosimeter tracks, d = glass dosimeter track density,  = zeta factor in a*cm², x pos = relative position in the irradiation package, P(2) = the chi square probability test result

Standard n Ns ρs (±1σ) Ni ρi(±1σ) ρs/ρi (±1σ) Nd ρd (±1σ) ζ σ(ζ) x pos P(χ²)

DUR-E2 M11 110 938 1.332(0.044) 2183 3.101 (0.066) 0.452(0.018) 2932 4.580(0.085) 304.52 13.27 15.70 0.89 DUR-E5 M11 130 1105 1.328(0.040) 2378 2.858 (0.059) 0.483(0.018) 2900 4.529(0.084) 288.16 11.89 33.20 0.99 DUR-E8 M11 125 1013 1.266(0.040) 2326 2.908 (0.060) 0.456(0.017) 2892 4.518(0.084) 305.99 12.97 37.10 0.99 DUR-E10 M11 90 727 1.136(0.042) 1597 2.495 (0.062) 0.478(0.021) 2869 4.480(0.084) 294.33 14.37 49.95 0.94 DUR-E11 M11 125 1026 1.262(0.039) 2342 2.881 (0.060) 0.455(0.017) 2866 4.476(0.083) 309.34 13.06 51.25 0.99 FC3-E2 M11 76 785 2.041(0.073) 1740 4.393(0.105) 0.481(0.021) 2954 4.615(0.085) 248.75 12.08 3.65 0.94 FC3-E3 M11 20 138 1.634(0.139) 315 3.501(0.197) 0.476(0.049) 2924 4.567(0.084) 253.99 26.56 20.15 0.99 FC3-E4 M11 34 170 1.863(0.119) 440 4.483(0.184) 0.441(0.034) 2901 4.531(0.084) 276.46 25.74 32.35 0.86 FC3-E5 M11 41 282 1.957(0.117) 610 4.182(0.169) 0.483(0.035) 2871 4.484(0.084) 254.91 19.25 48.50 0.99 FC3-G3 M12 34 380 1.843(0.094) 772 3.775(0.136) 0.498(0.031) 2273 4.032(0.083) 277.42 18.44 36.55 0.99

Figure 26: calculated zeta-factors for Durango apatite age standards (DUR) and Fish Canyon Tuff age standards (FC3). Only the last age standard is irradiated in irradiation package M12, while the other age standards were irradiated in the M11 irradiation package. The error bars are representing the 2σ uncertainty. The personal Overall Mean Weighted Zeta value of 286.23±4.71 a*cm² is displayed in green, whereas the 1σ uncertainty is expressed with the red line. FC3-E2 M11 is the only age standard that has error bars which are not included in the error on the OMWZ value. 37

Chapter 4: Geological context

4.1 Introduction to the Central Asian Orogenic Belt The 5000km long Central Asian Orogenic Belt (CAOB) is with its mountain peaks of more than 7000 meters the largest and most active intracontinental orogenic system of the world. The CAOB is spread over a. o. the Tien Shan, Altai, Sayan and the Mongol-Okhotsk Orogenic Belt (Figure 27). The CAOB is an assembly of terranes which accreted to Eurasia during the Precambrium and the Palaeozoic.

Mesozoic accretion-collision events and the Cenozoic India-Eurasian collision represent the final steps in the complex history of the CAOB. During the Meso-Cenozoic several tectonic reactivation episodes affected the CAOB.

The Tien Shan mountain belt is a 2500km long range located mainly in Kyrgyzstan, China and

Kazakhstan. The eastern Tien Shan mountain range is located in China, while the western Tien Shan is situated in Kyrgyzstan, Tajikistan, Uzbekistan and Kazakhstan. The entire Tien Shan is crosscut with ancient Palaeozoic fault systems, which have been reactivated during the Mesozoic and late Cenozoic

(e.g. Bande et al., 2015). One of these reactivated Palaeozoic faults is the Talas Fergana Fault (TFF), which is a currently active dextral strike-slip fault that has an accumulated offset of ±200km.

Deformation of this strike-slip fault was initiated in the late Palaeozoic, progressed during the

Mesozoic and is active until now (Bande et al., 2015; Korzhenkov et al., 2014). Continent-continent collisions were the main driving force that reactivated inherited fault systems. As a consequence, it is necessary to summarize the main assemblage events of the CAOB in order to understand the tectonic events constrained thermochronology in this thesis.

4.2 Precambrian and Palaeozoic evolution of the CAOB

4.2.1 Precambrian geodynamical evolution

The history of the CAOB starts in the Late Proterozoic period (Riphean, 1.4 Ga to 0.8 Ga) when the

Palaeo-Asian Ocean (PAO) was located south of the Archean-Early Proterozoic Siberian craton. A

Neoproterozoic ophiolite belt dated with zircon U/Pb proves the existence of an early PAO at the

Latest Mesoproterozoic (1019±0.7Ma) (Khain et al., 2003). Several younger Precambrian ophiolite relicts and subduction related rocks can be found throughout Siberia, indicating the long and complex subduction-accretion history of the Siberian craton during the Latest Proterozoic and Phanerozoic

(Windley et al., 2007).

The East European craton or ‘Baltica’, the North American craton ‘Laurentia’ and Siberia collided during the world-wide Grenvillian orogenic event, which resulted in the accretion of the supercontinent

Chapter 4: Geological context

‘Rodinia’. The PAO (with its different subbasins) is located south from the Rodinia supercontinent and north from the other supercontinent called ‘Gondwana’.

Figure 27: The Tien Shan is bordered by the Tarim block in the south-east, the Junggar and Kazakh plate to the north and the Pamir block in the south-west (De Grave, 2007).

4.2.2 Palaeozoic evolution: assembly of the Kyrgyz Tien Shan

The composition of the PAO south of the Siberian craton dominates the evolution of the CAOB.

Several accretion processes of (mainly peri-Gondwanian) (micro)continents were followed by an eventual total clusure of the PAO. These continental blocks and ocean floor remnants accreted to the

Siberian craton, resulting in the complex CAOB mosaic structure at the end of the Palaeozoic (Windley et al., 2007). In this context, numerous subduction cycles and accretion events with episodes of large- scale granite intrusions are recognized in the CAOB (see geological map in appendix). Significant prove of these subduction events in the Tien Shan area are the ultra high pressure (UHP) metamorphosed rocks (blueschists and eclogites) in the Kyrgyz Tien Shan (Tagiri et al., 2010). The diamond-bearing

Precambrian basement of the Kokchetav massif in Northern Kazachstan is another important example of UHP metamorphism (Glorie et al., 2015).

Different geological domains of the Kyrgyz Tien Shan are stitched to the Kazakh terrane during the

Palaeozoic. In this sense, the Kyrgyz Tien Shan consists of three Palaeozoic E-W directed blocks: The

Chapter 4: Geological context

Northern Tien Shan (NTS), Middle Tien Shan (MTS) and Southern Tien Shan (STS) (Windley et al., 2007)

(Figure 29). These three blocks are separated from each other by E-W oriented suture zones in which ophiolite relicts occur.

Figure 28: Early Devonian (~390Ma) situation of the CAOB. The Tarim block is rifting towards the Kazakhstan continent, with in between the Turkestan Ocean. Am: Altai-Mongol, BL: Barlyk arc, BS: Beishan; ChTS: Chinese Tien Shan, CK: Central Kunlun; CTS: Central Tien Shan, EJ: East Junggar; ES: East Sayan, GA: Gorny Altai, K: Kokchetav, KHM: Khanti- Mansi, MG: Magnitogorsk, NTS: North Tien Shan; NU: North Urals, PC: Pre-Caspian basin, S: Salym, SJ: Southern Junggar, STS: South Tien Shan, TP: Timan-Pechora; WS: West Sayan, U: Ubagan (modified after Filippova et al., 2001; Windley et al., 2007) The NTS block includes the famous intramontane Issyk Kul basin (Figure 29). It accreted to the Kazakh microcontinent in the Ordovician. The NTS consists of (1) Precambrian metamorphic blocks, (2)

Cambrian to Lower-Ordovician ophiolites and marine sediments, (3) numerous I-type granites of Late-

Ordovician and Early Silurian age (Caledonides) (Biske and Seltmann, 2010) and (4) scarce Late

Palaeozoic (Hercynian) intrusives which post-date the closure of the Turkestan Ocean (Figure 28)

(Glorie et al., 2010). Emplacement of these voluminous Early Palaeozoic granites into the Precambrian basement mainly occurred during the Late Ordovician and Early Silurian (461 and 441Ma) (Kröner et al., 2013). This emplacement event was accompanied with continental arc volcanism in the Middle to early Late Ordovician (Degtyarev et al., 2012).

The NTS is divided from the MTS with an E-W directed suture zone called the ‘Nikolaev line’ (Figure

29), which is a 460Ma old suture zone (Degyarev et al., 2012). The MTS microcontinent is a 2000km oblong microcontinent that extends from north Kazakhstan to west Kyrgyzstan. The MTS is truncated by a segment of the STS that is emplaced by dextral movement of the TFF during the Mesozoic (Figure

29).

The MTS is mainly built up of Precambrian basement consisting of volcanic rocks and tillites. From the

Silurian until the Pennsylvanian (Late Carboniferous), the MTS represented the southern margin of the 40

Chapter 4: Geological context

Kazakh continent (Figure 28). This is reflected in the sedimentary record with Middle Devonian to Late

Carboniferous marine sediments consisting of shallow-marine siliciclastics and carbonates (Biske and

Seltmann, 2010). Scarce subduction-related felsic intrusives in the Middle Tien Shan west of the TFF have ages between 303 and 320Ma and are unique in the Tien Shan (Seltmann et al., 2011).

Figure 29: Schematic map of the three main domains of the Kyrgyz Tien Shan. Major lakes (Issyk-Kul and Song-Kul) are also mentioned. (Modified figure of De Grave et al., 2011)

The accretion of the Southern Tien Shan (STS) is illustrated by an ophiolite-bearing suture zone – called the Atbashi-Inylcheck suture zone (Figure 29). The STS is a Late Palaeozoic fold-and-thrust belt that nowadays is exposed from Uzbekistan to NW China. The complex STS is made up of continental slope and shelf sediments, fragments of ophiolite assemblages, volcanic and carbonate seamounts, island arcs and other marine sequences (Alexeiev et al., 2016). These various tectonic units were stacked together during the Pennsylvanian - Early Permian time period. The numerous Early Permian granite intrusives emplaced during the Early Permian (290-270Ma, Seltmann et al., 2011) indicate the final closure of the Turkestan Ocean and the final assembly of the ancestral Tien Shan with collision of the Kazakh-NTS-MTS block to the STS-Tarim block.

During the final amalgamation of the CAOB (in the Permian), older Palaeozoic fault structures were reactivated, causing large-scale counter-clockwise rotation and sinistral strike-slip fault movements in the CAOB (e.g. Buslov et al., 2003). For example the Irtysh and the Charta faults zone record sinistral displacement over more than 1000km (Buslov et al., 2004) (Figure 27). The Irtysh shear zone is an important structural boundary in the CAOB that separates Siberia, Mongolia and Altai in the north, from Kazakhstan, Tien Shan and Tarim to the south. The 2000km long TFF fault is a (currently)

Chapter 4: Geological context northwest-southeast striking dextral strike-slip fault that was already very active in the Late Palaeozoic when the Tien Shan experienced transpressional deformation (Rolland et al., 2013).

4.3 Mesozoic evolution The topography of the CAOB at the beginning of the Mesozoic is mountainous because of several accretion events in the broader framework of Pangaea assembly in the Late Palaeozoic. The ancestral

Tien Shan already constitutes the barrier between the Junggar and Kazakhstan basins to the north, and the Tarim basin to the south (Hendrix, 2000). The break-up of Pangaea is initiated by an active mantle plume which is illustrated by a flood basalt event in Siberia (Nikishin et al., 2002).

The Mesozoic intramontane basins in the ancestral Tien Shan contain (mainly) continental molasse deposits, volcanic deposits and Middle Jurassic coal bearing sequences. Three main Mesozoic orogenic pulses can be identified: Late Triassic - Early Jurassic, Late Jurassic – Early Cretaceous and the Late

Cretaceous (e.g. Hendrix et al., 1992; Allen et al., 1993; Vincent and Allen, 2001). These tectonic compressive pulses are generated by the accretion of the Cimmerian blocks during the Mesozoic.

These Cimmerian blocks (e.g. Qiangtang and Lhasa) rifted to subduction zones that consumed the oceanic lithosphere of the Tethys Ocean (Figure 30). The collisions of the Cimmerian blocks resulted in fault reactivation processes that propagated northwards and reactivated old inherited faults all over the CAOB. The complex reactivation of the Kyrgyz Tien Shan during the Mesozoic is well-documented with AFT data, but not fully understood (e.g. De Grave et al., 2007, Glorie et al., 2011, De Grave et al.

2012 and Glorie and De Grave, 2015).

Chapter 4: Geological context

Figure 30: Main continental blocks and terranes related to the CAOB and their suture zones. BH: Bayan Har, Hel: Helmand; HK: Hindu Kush; Ind: Indochina, Kh: Kohistan; Ku: Kudi, NQi: North Qiangtang; P: Pamir, Qi: Qilian Shan; SG: Songpan-Garze, Sib.: Sibumasu, SQi: South Qiangtang; WB, West Burma. Suture zones: 1: Indus-Yarlung-Zangbo suture, 2: Bangong Nujiang suture, 3: Yushu-Batang suture, 4: Jinsha Suture, 5: Longmu Co-Shangshu suture, 6: Panjao suture, 7: Solonker sutre, 8: Jilin suture, 9: Mongol-Okhotsk suture, 10: Kunlun-Anyemaqen suture (Jolivet, 2015)

4.3.1 Late Triassic – Early Jurassic event

A Late Triassic – Early Jurassic reactivation event recorded in the Tien Shan basement is widely recognised (e.g. De Grave et al., 2011). Late Triassic and Early Jurassic denudation is mainly recorded in titanite fission track (TFT), AFT and also apatite (U-Th)/He ages in the mountain ranges of the Song-Kul plateau (>3000m elevation) close to the Nikolaev line suture zone (Figure 29). The structure of the

Song-Kul plateau is preserved by the stop in erosion (i.e. denudation) since the Mesozoic and

Cenozoic. Exhumed AFT (partial) annealing zone signatures (with break-in-slope in age-elevation plots) are preserved in the adjacent mountain ranges of the Song-Kul plateau. This hence records fast cooling and exhumation of the basement during the Late Triassic (De Grave et al., 2011). Other Triassic

AFT ages in the Kyrgyz Tien Shan are preserved in Cenozoic detrital sediments of intramontane basins in the Alai basin in the STS (De Grave et al., 2012). An extra argument for a less local tectonic event is found in the adjacent Tarim basin, where a Triassic - Early Jurassic conglomerate unit is deposited

(Dimitru et al., 2001) and even further in the Kuqa Basin (Jolivet et al., 2013) where similar ages and observations were collected.

The Cimmerian Qiangtang block collided with the Kunlun terrane at the southern Eurasian margin during the Middle- to Late-Triassic (~230-190Ma) (Ratschbacher et al., 2003) (Figure 30). It is 43

Chapter 4: Geological context suggested that the Qiangtang-Eurasia collision is responsible for the reactivation of faults and basement exhumation in Central-Asia. Distant effects of the Qiangtang-Kunlun collision are assumed to be the major cause of coeval deformation in the Kyrgyz Tien Shan (De Grave et al., 2011 and De

Grave et al., 2013). The kilometers thick Early-Jurassic sequence in the East Fergana basin is believed to be a direct consequence of this Qiangtang collision, because the Qiangtang collision triggered the activity of the TFF, leading to the formation of a strike-slip basin in which kilometres of sediment accumulated (Sobel et al., 1999). Another possible triggering mechanism for basement exhumation in the Tien Shan is the Triassic collision of the Turan plate - which is another Cimmerian unit - with the west Pamir block (Alexeiev et al., 2009) or the extension and subsidence in the Tarim basin (Zhang et al., 2010).

Sedimentological evidence to verify this reactivation episode in the Kyrgyz Tien Shan is scarce because the Late-Triassic – Jurassic sediments have only few outcrops. The reason for the bad preservation is the possible reworking by later Mesozoic and Cenozoic exhumation events. The scarce Lower Jurassic outcrops will be discussed in section 4.3.4.

Initiation of tectonic activity in the Chinese Tien Shan region occurred during the Middle Jurassic, causing basin inversion and recycling of the Mesozoic sediments (Jolivet et al., 2015). Arguments for a tectonic reactivation are found in the south Junggar basin, where recycling of Jurassic sediments is visible in the detrital zircon U/Pb spectrum (Yang et al., 2013). Paleocurrent directions are towards the north in the Junggar basin and towards the south in the Tarim basin, which implies an eroding topographical relief in the Middle-Jurassic from the Tien Shan (e.g. Hendrix et al., 1992).

4.3.2 Middle-Jurassic peneplanation

The Middle-Jurassic epoch in the Kyrgyz Tien Shan is characterized by peneplanation in which the remaining topography is eroded (through efficient transportation of fine-grained sediments via rivers into the local basins). As a result of the peneplanation under humid conditions, coal-bearing Early to

Middle-Jurassic strata can be found in the Kyrgyz Tien Shan. Only on the western side of the TFF it can be assured that Middle-Jurassic sediments are deposited. The description of the Early to Middle-

Jurassic sedimentary cover of the different basins is covered in section 4.3.4.

4.3.3 Late-Jurassic – Cretaceous cooling

After the Late Triassic – Early Jurassic event, the Kyrgyz Tien Shan recorded a regional cooling basement event in Central Asia from the Late Jurassic-Cretaceous (Figure 31) (Glorie and De Grave,

2015). This is supported by a large number of AFT ages and several zircon (U-Th)/He and apatite (U-

Th)/He ages (Figure 31). Several episodes of (slowed or accelerated) exhumation can be distinguished in this wide time-span ranging from ~160-80Ma (Glorie and De Grave, 2015). 44

Chapter 4: Geological context

Figure 31: Summary of all available TFT, ZHe, AFT and AHe data in time-temperature space for the Kyrgyz Tien Shan (black circle = TFT, white diamond = ZHe, white circle = AFT and black diamond = AHe). Cooling models derived from thermal history modelling are added, just like estimates on the rate of cooling. The thermochronological ages are compared with the suggested cooling pulses. Seq. refers to additional sedimentary constraints in the area that are meaningful for the interpretation in this area (h.=hiatus, c. = alluvial conglomerates). An estimate of the topographic evolution is presented (references see Glorie and De Grave, 2015). A pooled ages density plot is generated from existing basement thermochronological data in the Kyrgyz Tien Shan (Bullen et al. 2001, 2003; Sobel et al. 2006a; Glorie et al. 2010, 2011a; De Grave et al. 2011, 2012, 2013; Macaulay et al. 2013, 2014; De Pelsmaeker et al. 2015).

A first tectonic pulse is defined by zircon (U-Th)/He and AFT ages from ~160-145Ma (i.e. Late Jurassic).

Conglomerate deposits with ages of ~150Ma in the adjacent Tarim and Junggar basins provides possible sedimentological evidence for accompanying basement denudation (Jolivet et al., 2013 and

Jolivet et al., 2015). From ~145-120Ma there is a moderate cooling event captured by the AFT and apatite (U-Th)/He data. This period of tectonic activity was thought to be coeval with the late-Jurassic –

Early Cretaceous (150-120Ma) collision of the Cimmerian Lhasa Block with Eurasia (150-120Ma) (Kapp et al., 2007). This hypothesis is highly debated, because in regions closer to Tibet and in Qaidam areas little influence by the Lhasa collision has been recorded (Jolivet et al., 2013). An alternative explanation for this exhumation event is the closure of the Mongol-Okhotsk closure, which has widely been recorded in basement of the Altai-Sayan region (e.g. Glorie et al., 2012 and De Grave et al., 2014)

Also, ophiolitic gabbros from the Bangong Nujiang suture zone (Figure 30) obtained a zircon U/Pb age of 128Ma (Chen et al., 2006). Recent studies pointed out the occurrence of a peak in volcanic activity around ~110Ma in Central Lhasa, which are supposed to be emplaced in an extensional regime of slab break-off (Sui et al, 2013; Chen et al., 2014). There are several opinions and the debate continues on when and how exactly the Lhasa block collided and caused the distant effects in the Kyrgyz Tien Shan. 45

Chapter 4: Geological context

The late Early Cretaceous cooling event is also sometimes interpreted as an isostatic response to the

Tethyan slab break-off after final collision of the Lhasa block (Glorie and De Grave, 2015). The late Early

Cretaceous event is reflected in a high concentration of AFT data around ~120-95Ma, representing the most common Mesozoic age population in the Kyrgyz Tien Shan (Figure 31). This period of accelerated basement cooling is also observed in several thermal history models (Glorie and De Grave, 2015). The

Tarim basin gives evidence for this tectonically active period with a sediment hiatus (Dumitru et al.,

2001). According to the hypothesis of Jolivet et al. (2015), this event is caused by the Lhasa block collision around ~120-110Ma. To conclude, two possible hypotheses can be made to explain this Mid-

Cretaceous cooling event: (i) isostatic response to the slab break-off of the subducting Bangong-

Nujiang Tethyan Ocean lithosphere (Sui et al., 2013, Chen et al., 2014) or (ii) the distant effect of the

Lhasa block collision (Chen et al., 2006), or in fact a combination of both.

From values of ~90-75Ma, AFT and apatite (U-Th)/He data become scarce (Figure 31), suggesting a slower cooling. A third faster cooling event in the Kyrgyz Tien Shan, from the latest Cretaceous (~75-

60Ma) to Early Paleogene event is identified by several AFT ages and increasing cooling rates depicted in thermal history models. This event could be linked to the collision of the Kohistan-Dras island arc

(Treloar et al., 1996) and the Karakoram Block to Eurasia (Schwab et al., 2004). Although also this tectonic model is a subject of polemic. Several researchers argue that the Kohistan-Dras arc first accreted to (Greater) India (e.g. Metcalfe, 2011). In that case, the Late Cretaceous-Early Paleogene ages might reflect the collision of Greater India with Eurasia (Klootwijk, 1984). In any case, these accretion events are assumed to play a role in the latest Cretaceous exhumation events in the Kyrgyz Tien Shan

(e.g. De Grave et al., 2013). Figure 31 illustrates the data from multiple thermochronometers applied to basement rocks in the Kyrgyz Tien Shan, while in Figure 32 only AFT data is retained. Based on Figure

31 and Figure 32 it can be deduced that the Late Jurassic-Cretaceous cooling phase is less strong than the Neogene cooling phase. However, this Late Jurassic-Cretaceous cooling phase is underestimated because it is systematically undersampled as more recent events can have eroded the relevant rock sections. Moreover, many samples were taken at major, active fault zones that were active during the

Neogene which relatively overestimates the latter.

Chapter 4: Geological context

Figure 32: age compilation based on basement AFT data in the Kyrgyz Tien Shan. References for the AFT ages are Bande et al., 2015; De Pelsmaeker et al., 2015; De Grave et al., 2011; De Grave et al., 2012; De Grave et al., 2013; Glorie et al., 2010; Glorie et al., 2011; Sobel et al., 2006a, Macaulay et al., 2014, Bullen et al., 2001. The AFT age dataset can be found in the appendices. 4.3.3.1 A short note on AFT ages in fault zones

In a strike-slip fault setting where compressional forces occur, it is not easy to interpret the AFT ages of the uplifted fault blocks. Before the compressional forces occur, the crust has AFT and AHe annealing zones which are typical for a normal crust. The AFT system has a higher closure temperature than the apatite (U-Th)/He (AHe) system, as illustrated by Figure 33a. The rocks that are situated below the closure temperature of the system have an AFT age of zero, while the age of the sample above the

Partial Annealing Zone is equal to the original cooling age and reflects a regional cooling trend. After the compressional fault reactivation at time t1, the original thermal structure of the crust will be exhumed. The most common AFT ages in the mountain range will thus be more influenced by the timing of the older regional cooling event than the compressional faulting induced timing. Most AFT ages will in fact represent a geologically (meaningless) cooling ages with values in between t0 and t1. Only at the base of the hanging wall, where the totally reset rocks (red in the panel Figure 33b) are possibly exhumed, AFT or AHe age data will approach the timing of the faulting t0. These tilted fault blocks are represented in the Tien Shan and record a cooling age of ~120-60Ma, which is a preserved signal of the PAZ of Cretaceous cooled basement rocks (Glorie and De Grave, 2015). Cenozoic AFT ages are typically found at lower exhumed crustal levels in the reactivated fault blocks in the Tien Shan mountains. Further away from the fault zone it is more likely that older Mesozoic ages will occur

(Glorie and De Grave, 2015).

Chapter 4: Geological context

Figure 33: Figure illustrating the evaluation of the timing of compressional forces with AFT age data (Glorie and De Grave, 2015).

The hypothesis of Glorie and De Grave (2015) explains the large amount of Cretaceous sediments in the adjacent Kyrgyz basins as a result of the reaction to the regional exhumation-denudation at 120-

95Ma. The Early Cretaceous basement exhumation and denudation invokes the removal of huge volumes of the lithostratigraphic column. The cooling signal observed in these basement blocks records the time of erosive denudation as a response on the time of exhumation. These two events are not synchronous and often have a time lag of million of years. AFT ages caused by (erosive) denudation are thus not focused in one age population, but are widespread over a wider period. This idea of denudation explains the wide range in Cretaceous cooling ages (~160-80Ma) in the Kyrgyz

Tien Shan. Moreover, a wide spread in AFT ages can indicate a part of the exhumed apatite PAZ, where a wide range in AFT ages can be expected over a relative short paleodepth interval. In the latter case

Cretaceous ages can in fact illustrate meaningless ages in the PAZ (see chapter three and Figure 33).

To overcome this problem and make an estimation of the cooling rate, several techniques can be applied:

1) Thermal history modelling based on AFT length distributions can be done in order to reveal

geological information (e.g. Gallagher, 2012). This is often the preferred method to extract

useful information from potentially meaningless AFT ages. Thermal history modelling of young

samples is often problematic, because of the low amount of confined track length

measurements.

2) The second possibility is with age-elevation profiles using AFT ages of samples taken from a

vertical profile (see section 3.7.3). This technique can suffer from low temperature isotherm

compression in (rapidly) exhumed terranes (Van der Beek et al., 2010). Another pitfall is the

estimation in which part of the annealing zone the vertical profile is located. In order to asses

this pitfall, the detection of a break-in-slope is fundamental.

Chapter 4: Geological context

3) Another possibility to extract information is by using a multi-thermochronometric approach to

estimate the thermo-tectonic history of a sample through different temperature zones. Other

commonly used thermochronometers than AFT that are appropriate for example in this sense,

are apatite (U-Th)/He, zircon FT, zircon (U-Th)/He, 40Ar/39Ar and others.

4.3.4 Mesozoic intramontane basins in the Kyrgyz Tien Shan

Before going into detail on the different Mesozoic sedimentary successions in the Kyrgyz Tien Shan, it is necessary to sketch a general picture. Triassic deposits are absent over most of Kyrgyzstan, possibly because they are reworked (or never deposited) during the Meso- and Cenozoic. Scarce outcrops of

Jurassic sediments deposited in a freshwater environment are located on both sides of the TFF. At the east side of the TFF in the NTS, small Jurassic outcrops are described and sampled during the field campaign of the MINPET research group (Ghent University): the Minkush section, Kadji-Sai section and

Jeti Oguz section (Figure 34). Jurassic deposits in the MTS are absent, while in the STS, the Jurassic is rarely exposed. At the western side of the TFF, well-exposed thick outcrops of Jurassic sediments occur in the East Fergana basin, from which two sections are studied in the field campaigns of 2013 and 2015

(Figure 34). Other studied sections west of the TFF are the Tash Kumyr and Jetim Dobo section. All of these Jurassic sediments are deposited in intramontane basins. Big differences in the thickness of the

Jurassic outcrops on both sides of the TFF can be observed. On the eastern side of the TFF, several tens of meters of Lower-Jurassic sediments are preserved, whereas on the west side of the TFF, there are both Lower- and Middle-Jurassic sediments to be found. Two different sedimentary basins are distinguished on the east side of the TFF: the Kavak basin and the Issyk Kul basin, which are located east and west respectively (Figure 34). The thickness of the sediments is more outspoken on the west side of the TFF. Especially in the intensely faulted East Fergana Basin there are several kilometres of

Jurassic sediments deposited.

Cretaceous sediments are absent on the east side of the TFF, while a thick Cretaceous sedimentary sequence is deposited on the opposite side. The Cretaceous sediments are recognized in the field by the red coloured sandstones and massive conglomerate deposits at the base. These red conglomerate deposits are widespread over Central-Asia, but the exact depositional process is in fact not fully understood. The age of the red conglomerate is estimated to be Upper-Jurassic to Lower-Cretaceous

(Jolivet et al., 2015) and will be further discussed in section 4.7.

Chapter 4: Geological context

Figure 34: studied Mesozoic sections in Kyrgyzstan. Numerous detrital samples were taken for thermochronological purposes, but often too low amounts of apatite crystals were obtained in the samples. Additional samples of the North Suyak basin sampling campaign (2013) are prepared and analyzed in this thesis (modified geological map from ‘geological maps of central Asia and adjacent areas’, published in 2008, see appendices). 4.3.4.1 East of TFF: Kavak and Issyk Kul basin

The Jurassic sections in the Min-Kush area (Kavak basin) is studied and sampled extensively during the field campaign of 2015 in which the author of this thesis participated. The Min-Kush (literally translated

“coal mine”) village is built for the extraction of the Jurassic coal layers. The Jurassic freshwater sediments consists of quartz and polymictic sandstones, siltstones and brown coals. The flora found in the coal deposits are typical for the Lower Jurassic, but some of the plants can occur in the Middle

Jurassic as well (Bachmanov et al., 2008). The thickness of the Jurassic strata ranges from 100m to

680m in the Kavak basin. A regional variation in grain size also indicates that there was a strong topography at the time of deposition (Bachmanov et al., 2008). The Jurassic deposits are covered by a sediment hiatus, implying that no Cretaceous sediments were deposited on top of the Jurassic sediments. Sedimentation starts again with a 40m thick deposit of the Paleogene Kokturpak formation, which is a sediment that is mainly built up of erosion crust material. A local facies of the Kyrgyz

Formation of the Kyrgyz formation “Minkush Conglomerate” lies on top in the Kavak basin

(Bachmanov et al., 2008). More information on the Cenozoic evolution and sedimentary sequences is given in section 4.5.

The Jurassic deposits of the Issyk Kul basin that were studied in the Kadji-Sai and Jeti Oguz region are approximately 150m and 10m thick respectively. The deposits are characterized by alluvial, swamp and lacustrine facies. It is not clear what the exact age of the Jurassic deposits is, but Early Jurassic is assumed based on palaeobotanical evidence. A first formation consisting of coarse sandstones and

Chapter 4: Geological context conglomerates represents mainly alluvial deposits. These are covered by more fine grained lacustrine sediments with interlayered weathering crusts (Figure 35).

Figure 35: Kadji-Sai Early-Jurassic sediments at the East side of the TFF. On the picture the contact between the two different Jurassic formations at the Kadji-Sai outcrop can be seen in the middle of the picture. 4.3.4.2 West of the TFF: Tash Kumyr

The Jurassic outcrop close to the Tash Kumyr city consists of 3 parts. The lower part consists of quartz microconglomerates which are deposited in a fluvial environment. The second formation is built up of mudstones with goethite crusts. The third, Middle-Jurassic part is composed of siltstones and sandstones that are assumed to be turbidites. Coal layers also occur in this formation, which could be deposited in a deltaic environment. Shark teeth dating from the Callovian period (Latest Middle-

Jurassic) are also found back in the Tash Kumyr section (Martin and Averianov, 2004).

The lowest part of the Tash Kumyr Jurassic sediments is very similar to the Jurassic sections seen east of the TFF and it is assumed that these layers have the same depositional age (i.e. Early Jurassic). This uniform sedimentation pattern at both sides of the TFF indicates that the TFF was not controlling the sedimentation regime at the time of deposition for the first part of the Tash Kumyr section. The next two parts of the Jurassic indicate the deepening of the basin: a lacustrine sedimentary facies occured and deposited fine-grained sediments. Goethite crusts in these fine-grained sediments indicate the evaporation of the lake. These lacustrine deposits are followed by a turbidite sequence with coal layers on top indicating a more turbulent environment, instead of the fresh water depositional environment

Chapter 4: Geological context of the first two formations (pers. communication with Dr. F. Zhimulev). The city of Tash Kumyr is named after the so-called ‘stone-coal’ (or anthracite) that is found in the region, which indicates stronger coalification processes. Browncoals are the type of coals that are found east of the TFF, which suggests a strong difference in coalification and burial depth between the deposits east and west of the TFF.

4.3.4.3 West of the TFF: East Fergana basin

Three big oblong sedimentary Jurassic basins west of the TFF are formed because of the dextral movement of the TFF (Burtman, 1980) (Figure 34). The smaller Leontiev strike-slip basin (Burtman,

1980) is located in Kazakhstan (close to Karatau) and is a 180 km long and 10 km wide structure

(Figure 36). Jurassic sediments up to 2km thick accumulated in the Leontiev strike-slip basin (Burtman,

1980). More to the southeast part of the TFF in South-West Kyrgyzstan, the 400km long strike-slip basin of the East Fergana basin is formed during the Jurassic (Burtman, 1980). The thickness of the

Jurassic deposits is estimated as 5km in the Fergana basin. The Chinese part of the East Fergana basin

(the Kuzigongsu basin) consists of Lower- and Middle-Jurassic sediments and the East Fergana basin is assumed to have the same age. The East Fergana basin formation was also a cause of the dextral movement of the TFF during the Jurassic (Allen et al., 2001). The South Turgay extensional basin is located in South Kazakhstan, north of the Leontiev basin. This basin accumulated ±2500m of Jurassic sediments, including bituminous shales (Allen et al., 2001). Thick Early Cretaceous sediments deposited on top of the Jurassic sediments in the East Fergana basin indicate the continuing dextral movement of the TFF and the accompanying subsidence (e.g. Burtman et al., 1980, Burtman et al., 1996, Sobel, 1999).

The transition from white Jurassic deposits to reddish Cretaceous sandstones indicates a change in climate and precipitation (see section 4.7, Figure 37).

Table 4: general overview of the overview of the thickness of the Mesozoic sediment deposits in the Kyrgyz Tien Shan. (km = kilometer scale, hm = hectometer scale, dam = decameter scale, - = no sediments preserved/deposited)

West of TFF: West of TFF: East of TFF: East of TFF: East Fergana basin North Fergana basin Kavak basin Issyk Kul basin

Studied Terek section, Tash Kumyr section, Minkush section Kadji Sai section, section(s) Jazy Valley section Jetim Dobo section Jeti Oguz section Triassic - - - - Lower km hm hm - dam hm - dam Jurassic Middle km hm - - Jurassic Upper - - - - Jurassic Cretaceous km hm - -

Chapter 4: Geological context

Figure 36: (a) cartoon of the Early-Mid Jurassic evolution and the formation of strike-slip basins. (b) Imbricate fan illustration: Each of these three splays forms a fault-propagation fold, leading to the formation of a imbricate fan. Trailing means that 1 is the first and 3 is the last (modified after Boyer and Elliot, 1982). (c) Early-Mid-Jurassic evolution of the Karatau/TFF as a dextral strike-slip system. The South Turgay basin is a trailing imbricate fan of normal faults (see b) (Allen et al., 2001). The Leontiev graben and the East Fergana basin are transtensional basins. The total offset of the TFF is illustrated by the displacement of the Palaeozoic Turkestan-Kokshaal complex (Allen et al., 2001).

Figure 37: Transition of white Jurassic sediments to red Cretaceous in the East Fergana basin (Terek section). Inset: location of photo on the geological map.

Chapter 4: Geological context

4.4 Mesozoic volcanism in the Tien Shan region

4.4.1 Early-Jurassic volcanism

Jurassic detrital zircon grains are detected in several samples in the southern Junggar basin. The zircon

U/Pb ages range from the Jurassic to the Early Cretaceous (183-127Ma) (Yang et al., 2013). This

(mainly) Jurassic peak occurs through the full Mesozoic section, i.e. in Middle Jurassic, Upper Jurassic,

Lower Cretaceous and Upper Cretaceous sediments (Yang et al., 2013). This constant signature of

Jurassic zircon ages implies that there is a source of zircons active in the Mesozoic (Yang et al., 2013).

The detrital zircon U/Pb signature of the Neogene sediments in the southern Junggar basin reveals another peak of Mesozoic ages, which is caused by sediment recycling (Yang et al., 2013).

On the contrary, in the Ulugquat section, in the southern the East Fergana (south in the inset figure of

Figure 37) basin three detrital Mesozoic samples are dated with zircon U/Pb (Yang et al., 2014). Not one Mesozoic zircon age was found in the Ulugquat section through Meso- and Cenozoic samples, possibly caused by a small sample size of only 43 zircon crystals for the Middle-Jurassic sample and 48 for the Lower-Cretaceous sample (Yang et al., 2014). Sample bias during the zircon handpicking process could also be the reason for the absence of the volcanic zircons, because volcanic zircons are typically small and big zircon crystals are typically more easy to handpick and to analyze with LA-ICP-

MS. The Early Jurassic magmatism peak of volcanism is not registered with U/Pb at this location.

Preliminary results of U/Pb dating on detrital zircons show that in a sample of the northern part of the

East Fergana basin (Jazy valley section, Figure 34) a very abundant peak (>50%) of Middle-Jurassic

(~175Ma) zircons is present (pers. com. with Elien De Pelsmaeker). Although these are just preliminary results from the zircon U/Pb dating (as a part of the PhD research of De Pelsmaeker), a peculiar difference between the two coeval samples from the same larger basin that are only located ±100km apart, is noticed. Evidence of intracontinental volcanism in the Tien Shan region is rarely found.

Furthermore, Early- and Middle-Jurassic volcanism in the Tien Shan contemporary with the large scale deposition of coal layers, which indicates a widespread peneplanation is strange. To conclude, it is necessary to further investigate the Jurassic zircon distribution in space and time in the Tien Shan region. Several samples of diverse Mesozoic age in diverse regions of Kyrgyzstan are already analyzed and will be investigated in the near future.

4.4.2 Mantle plume activity in the CAOB?

Outcrops of basalts are observed in the Kyrgyz Tien Shan, SE Kazachstan and the Junggar terrane (e.g.

Figure 38b). During the field campaign of 2015 there were a few indications of volcanic activity, interbedded in Mesozoic deposits (Figure 38a and Figure 38b).

Chapter 4: Geological context

Figure 38: a) Jeti Oguz location (Issyk-Kul basin): mafic pebbles in the lower-Jurassic. This is probably evidence of the Jurassic magmatic activity in Central-Asia and could be related to intraplate magmatic activity. b) Tash Kumyr site: Late Jurassic – Early Cretaceous wall conglomerate with on top a (black) alkali basalt that continues for 20km along the Naryn river (Simonov et al., 2008) (picture of Elien De Pelsmaeker).

The deposits of alkali basalts in the Kyrgyz Tien Shan occur over a wide area of more than 285 000km²

(Simonov et al., 2015). Geological, geochemical, petrological and geochronological evidence suggests that these alkali basalts are emplaced in four stages of intracontinental mantle plume activity in the

Tien Shan, Junggar and SE Kazakhstan (e.g. Sobel and Arnaud, 2000; Simonov et al., 2008; Simonov et al., 2015). The most recent hypothesis is discussed here and implies a four-stage model caused by a mantle plume or superplume (Simonov et al., 2015), although the existence of a plume is still a matter of debate.

The first episode occurs in the Late Carboniferous (312-305Ma), the second in the Early Permian

(282Ma), both in SE Kazachstan (Simonov et al., 2015). The third pulse of mantle plume activity occurred during the Early Jurassic (198-186Ma) in the Junggar terrane (Simonov et al., 2015). The last and most recent mantle plume activity occurred in the Late-Cretaceous-Paleocene (74-52Ma) in the

Kyrgyz Tien Shan and is observed in the Kyrgyz Tien Shan (Figure 38b).

The Early Permian alkali basalts could be related to the Early Tarim plume (Zhang et al., 2010). The

Early Jurassic plume or Junggar plume produced several alkali basalt deposits in the Junggar terrane

(Simonov et al., 2015). The Late Cretaceous-Paleocene (Tien Shan plume) emplaced multiple basalt deposits in the Kyrgyz Tien Shan over a very wide area (Figure 39). These basalts might be related to a global superplume event of the Late Cretaceous-Paleocene, which was active contemporary at that time and produced great amounts of basalt rocks in the Deccan Large Igneous Province (LIP) in India

(Simonov et al., 2015). Several Mesozoic ages in the Issyk Kul area were already found in “basement” rocks by several Russian authors and are used in a zircon U/Pb compilation (Macaulay et al., 2016)

(inset Figure 40). Further information on the source of this zircons is lacking. It is for example also a possibility that these zircons have a hydrothermal origin.

Chapter 4: Geological context

Figure 39: localities of the basalt outcrops along the Tien Shan and Junggar terrane. The triangles represent the Late Palaeozoic basalts of SE Kazachstan. The diamonds represent the Early-Jurassic basalts of the Junggar terrane. The stars represent the Late-Cretaceous – Early Cenozoic basalt outcrops in the Kyrgyz Tien Shan (figure of Simonov et al., 2015)

Figure 40: Mesozoic U/Pb ages of Macaulay et al. (2016) plotted on the topographic map. (data from Macaulay et al. (2014) and the references therein). Inset: Macaulay et al. (2016) presents a zircon U/Pb compilation dataset in the NTS block in East Kyrgyzstan. Although 11 Mesozoic ages occur in the NTS U/Pb compilation, these authors did not discuss the occurrence of this age peak in their age compilation. The reference papers are written in Russian, date back from the ‘90s. It is unclear if these ages for 11 samples are obtained on Mesozoic volcanic deposits, Mesozoic hydrothermally formed zircons or if the U/Pb ages are meaningless and not concordant.

Chapter 4: Geological context

4.5 Cenozoic evolution An AFT dataset of Cenozoic AFT ages in the Kyrgyz Tien Shan is displayed in Figure 41 and describes the Cenozoic cooling of the Kyrgyz Tien Shan. The Cenozoic starts with a small peak of AFT ages in the

Early Paleogene. The Early Paleogene is described as a period of erosion and weathering in the Tien

Shan which is characterized by the (re)deposition of weathering crusts (Kokturpak Formation) on specific locations. This Formation ranges from a few meters to 250m maximum. The presence of weathering crusts expresses the tectonic quiescence that occurred in the Early Paleogene.

Figure 41: Basement AFT age compilation showing a distinct peak in Oligocene – Miocene. References for the AFT ages are Bande et al., 2015; De Pelsmaeker et al., 2015; De Grave et al., 2011;2012;2013; Glorie et al., 2010; 2011; Sobel et al., 2006a, Macaulay et al., 2014, Bullen et al., 2001 The most abundant population of AFT age data for the Kyrgyz Tien Shan has late Cenozoic age (~33-

~3Ma) (Figure 41). This is mainly biased because the data comes from research that was mainly done on samples close to the base of the fault blocks, while at the summits of the fault blocks, older AFT ages of Cenozoic and Cretaceous age occur. An overview of the Cenozoic AFT ages close to the major fault zones of the Kyrgyz Tien Shan is given in Table 5. For example for the Central Terskey Fault it is found that the AFT ages differ from east to west, meaning that the tectonic activity of one fault zone can be diverse (Glorie and De Grave, 2015).

Chapter 4: Geological context

Table 5: Cenozoic AFT ages in the Kyrgyz Tien Shan for samples taken in close vicinity of major fault zones (Glorie and De Grave, 2015).

Fault zone AFT age (Ma) Apatite (U-Th)/He (Ma) References

South Tien Shan Suture ~60 -~7: ~60-50 at higher - (SSTS) elevations, ~30-7 at lower Glorie et al., 2011 elevations Sarydjaz Fault (SarF) ~45-23 - Glorie et al., 2011 and Macaulay et al., 2014 Central Terskey Fault (CTF) ~60 – 26, West: ~5 ~20-11 De Grave et al., 2013 and Macaulay et al., 2014 Main Terskey Fault (MTF) ~22-~7 - De Grave et al., 2013 and Macaulay et al., 2014 Issyk-Ata Fault (IAF) ~25-~4 - Bullen et al., 2001, Bullen et al. 2003, Sobel et al. 2006a Zaili Fault (ZF) ~17-~10 - De Grave et al., 2013 Karakung-Almaty (KAF) ~29-~15 - De Pelsmaeker et al., 2015 Turkestan Fault zone (TF) ~22- ~8 ~5-10 De Grave et al. 2011

The Late Paleogene – Neogene AFT age peak does not indicate a peak in renewed tectonic activity, because these ages probably reflect the exhumation of the fossil PAZ (Figure 33) (Glorie and De Grave,

2015). The timing of the true tectonic activity is Neogene. Neogene ages are typically found at the base of the hanging walls, where the deepest paleocrustal levels might be exposed (Figure 33).

Sedimentological evidence for this period of high tectonic activity lies in the deposition of the Late

Oligocene-Miocene Kyrgyz Formation (Figure 42). The Kyrgyz Formation consists of sediments that accumulated at the foot of a mountain slope in tectonically active areas. Thickness of the Kyrgyz

Formation ranges from 100 – 1500m (Burtman, 2012).

Figure 42: a) white fluvial Jurassic deposits with a few meters of orange Kokturpak Fm, covered by several tens of meters of the red Kyrgyz Fm (Minkush conglomerate variation). (NTS, Minkush area), Kavak Basin. Samples KS 126 and KS 128 are taken in this section and AFT data on this sample will be further discussed. b) red coloured Kyrgyz Formation (local facies of Jeti Oguz), Late-Oligocene – Miocene, NTS, Issyk Kul Basin. Sample SK-46 and SK-47 are sampled in this section and will be further discussed. Based on the combination of several thermochronometers, the information extracted from thermal history modelling and the sedimentary record (i.e. hiatus or thick alluvial gravels), it is possible to make an estimation of the tectonic activity in the Kyrgyz Tien Shan. The first Cenozoic phase of tectonic activity started in the Oligocene-Early Miocene (~33-22Ma) and intensified in the late Miocene (~12- 58

Chapter 4: Geological context

8Ma) and the Plio-Pleistocene (<5Ma) (Glorie and De Grave, 2015). Upper-Miocene – Pliocene sediments (Chu, Issyk-Kul and Naryn formations) are contemporary and very heterogeneous in thickness (few meters to 1800m thick). The sediments are more fine-grained and consist of clays, siltstones and sandstones. The more coarse grained (proluvium-type) Sharpyldak Formation (Upper-

Pliocene – Lower-Quaternary) is related with a final intensification of the tectonic reactivation of the mountain belt since the Late Pliocene. The thickness of the Sharpyldak Formation is also very heterogeneous, ranging from 50-1300m in thickness.

This period of high tectonic activity during the Late Paleogene – Neogene is driven by coeval

(accretion or tectonic) events in the CAOB and are assumed to be related to far-field effects of the

India-Eurasia collision. This collision represents the final closure of the Neo-Tethys, estimated at ~35-

34Ma ago with biostratigraphical arguments (Jiang et al., 2015). Intensification in basement cooling occurred during the late Miocene (~12-8Ma) and is assumed to be a result of the increased magnitude of crustal shortening and associated basement exhumation. The delamination of the mantle in the Tien

Shan (Zhiwei et al., 2009; Macaulay et al., 2014), but also beneath Tibet (Molnar and Stock, 2009) is a possible explanation for this event. The current ongoing tectonic pulse that presumably initiated in the

Pliocene (~5Ma) is assumed to be caused by a subduction of India beneath Eurasia (Capitanio et al.,

2010; Müller, 2010). The ongoing tectonic activity is responsible for crustal shortening rates of

~20mm/a in the Tien Shan (Abdrakhmatov et al., 1996).

Geophysical evidence for these active tectonics is abundant and diverse. The depth of the Moho- discontinuity is well-studied in the Kyrgyz Tien Shan. The depth of the Moho increases from 45km in the STS to 60km in the NTS (Bragin et al., 2001). The upper brittle part is 16-25 km, while the lower ductile part is 30-35 km thick (Bragin et al., 2001). Paleostress measurements revealed that the Tien

Shan area was subjected to an intensified tectonic stage since the Late Pliocene up to the Early

Pleistocene (Delvaux et al., 2013). A change in stress direction started in the Middle-Pleistocene and continued in the Holocene (Delvaux et al., 2013). Strong earthquakes are also located in the Kyrgyz

Tien Shan, e.g. the Kemin Earthquake in 1911 (M~8) (e.g. Deev and Korzhenkov, 2016). Other tectonic activity is registered in for example the TFF region (Korzhenkov et al., 2014). More than 18 strong earthquakes (M>7) occurred at the TFF region in the last 16ka (Korzhenkov et al., 2014).

4.6 Evolution of the Talas-Fergana Fault (TFF)

4.6.1 Late Palaeozoic and Mesozoic activity

Strike-slip displacement in most of the Tien Shan region occurred in the Permian as the entire area underwent a strong transpressional deformation phase. This is recorded in several 40Ar/39Ar ages in mylonite rocks sampled in the fault zones such as the TFF (Rolland et al., 2013). One Early Jurassic

Chapter 4: Geological context

40Ar/39Ar age of 195±3Ma is obtained on a muscovite mica of a pegmatitic dyke that was emplaced in the TFF fault zone. This dyke is not deformed and therefore the Ar-Ar age is most likely a crystallization age constraining the upper time limit of the Permian – Early Jurassic shear for the TFF movement. The emplacement of a the dyke in the same strike (NW-SE) of the TFF suggests a transtensional sense of the TFF (Rolland et al., 2013). This Permian to Early Jurassic ages are confirmed by Permian to Early

Jurassic 40Ar/39Ar ages obtained in syndeformed mica of mylonites samples in the Kyrgyz part of the

TFF (Konopelko et al., 2013).

The dextral motion along the TFF and Karatau Fault continued further in the Jurassic and led to the formation of pull-apart basins (Leontiev Graben, East Fergana basin) as discussed earlier. The Jurassic fault activity is accompanied by a counter-clockwise rotation of the Tarim block during the Early-

Middle Jurassic (Sobel, 1999). A Latest Jurassic to Early Cretaceous compressional regime in the East

Fergana basin during the Latest Jurassic – Early Cretaceous slowed down the sediment deposition in the East Fergana basin (Sobel, 1999).

Figure 43: Shaded relief map of the Tien Shan and Pamir with major structures. Solid circles represent AFT ages of Bande et al. (2015), diamonds represent thermochronological data from Glorie et al., 2011 and De Grave et al., 2011. Only Cenozoic AFT ages are shown.

4.6.2 Cenozoic reactivation

AFT ages of the Talas Range in the NW Tien Shan indicate that the range was rapidly exhumed during the Latest Oligocene-Early Miocene (Bande et al., 2015). During this renewed strike-slip displacement, several horsetail structures and positive flower structures were formed (e.g. eastern Chatkal range). The renewed strike slip displacement of the TFF is estimated to have commenced at c. 25Ma, which is caused by the Pamir indentation (Figure 27) (Bande et al., 2015). This Miocene reactivation along the

Tien Shan is revealed in AFT ages of samples in the upthrusted flower structures in the south East

Fergana basin, where Middle-Jurassic and Lower-Cretaceous sediments yielded an Early Miocene age

(Yang et al., 2014). AFT ages of basement samples in the STS also show this late Oligocene cooling

Chapter 4: Geological context signal (Glorie et al., 2011). Oligocene-Miocene AFT ages were also obtained on the southern part of the Kokshaal Range close to the Tarim basin (Sobel et al., 2006b).

4.7 Jurassic and Cretaceous climate Strong climate changes occurred during the Mesozoic in the Tien Shan which are intimately bound to the geodynamical evolution of the CAOB (Jolivet, 2015). The Early- and Middle-Jurassic period was a period in which a humid climate led to the development of coal layers and widespread peneplanation.

The Late Jurassic climate in the Fergana basin (Kyrgyz Tien Shan) evolved into a semi-arid climate, which intensified to an arid-hyperarid desert type climate during the Late Jurassic – Early Cretaceous transition (Jolivet et al., 2015). This peak in aridity corresponds to the alluvial fan deposits of the Late

Cretaceous – Early Jurassic Kalaza conglomerate formation that are formed before the Lhasa collision

(Jolivet et al., 2015). The widespread Kalaza Formation (and Kyrgyz equivalent, illustrated on Figure

38b) is thus mainly controlled by a climate induced change in an active tectonic setting (Jolivet et al.,

2015). The Early Cretaceous in the Junggar, Tarim and Fergana basin is characterized by more humid conditions (with a seasonal change), derived from the faunal en floral populations of the Junggar basin

(Eberth et al., 2001; Jolivet et al., 2015).

Chapter 5: Results

5.1 Sample overview Both basement and detrital samples with a diverse geographic range are analyzed in this work. Six sediment samples spread over the NTS and STS are analyzed, while 18 basement samples of the NTS,

MTS and STS are also subject of AFT analysis. The samples in Table 6 were taken during several field campaigns over diverse regions of the Kyrgyz Tien Shan. An overview of the geographic location of these samples can be found in Figure 44.

Table 6: overview of the samples analyzed in this work

Sample Latitude Longitude Alt. Location Lithology Formation or (m) depositional age

KB-121 N 41° 43.545' E 74° 46.552' 2756 Karakeche Porphyry alkaline Lower-Permian KS-126 N 41° 40' 29.9" E 074° 30' 26.1" 2200 Minkush Sandstone Lower-Jurassic KS-128 N 41° 40' 34.5" E 074° 30' 26.0" 2280 Minkush Sandstone Lower-Jurassic KB-131 N 41° 43.501' E 074° 32.353' 4143 Kavak-Tau Range Granodiorite Upper-Ordovician KB-132 N 41° 43.575' E 074° 32.065' 3802 Kavak-Tau Range Granodiorite Upper-Ordovician KB-133 N 41° 43.232' E 074° 30.996' 3387 Kavak-Tau Range Granodiorite Upper-Ordovician KB-134 N 41° 42.509' E 074° 30.545' 2980 Kavak-Tau Range Granodiorite Upper-Ordovician KB-135 N 41° 42.205' E 074° 30.337' 2822 Kavak-Tau Range Granodiorite Upper-Ordovician TF-06 N 42° 00’ 33” E 072°51’38” 1355 Talas Range, Chickhan Granodiorite Upper-Ordovician TF-15 N 42° 16’ 38” E 073° 11’ 09” 3300 Talas Range, Ötmek Granoditorite Upper-Ordovician TF-16 N 42° 13’ 19” E 073° 13’ 16” 2970 Talas Range, Ötmek Granodiorite Upper-Ordovician TF-23 N 42° 06’ 42” E 074° 06’ 06” 2270 Djumgol Range, Kozjomkul Granite Upper-Ordovician KYR-02 N 42° 19’ 04” E 073° 49’ 41” 2920 Kyrgyz range, Tuz-Asuu Granite Upper-Ordovician KYR-04 N 41° 42’ 42” E 072° 56’ 36” 1625 Fergana Range, Toktogul Mylonite Proterozoic KYR-05 N 41° 43’ 20” E 072° 58’ 05” 2110 Fergana Range, Toktogul Meta-granite Proterozoic KYR-15 N 41° 20’ 17” E 073° 38’ 49” 2030 Fergana Range, Kazarman Granodiorite Carboniferous AI-44 N 41° 18’ 13” E 073° 38’ 58” 2817 Fergana Range, Kaldama Mylonite Carboniferous AI-45 N 41° 20’ 48” E 073° 39’ 49” 2125 Fergana Range, Urumbash Granite Carboniferous F11-775 N 40° 19’ 02” E 072° 37’ 54” 1200 Kichi-Ali Range, Aral Plagiogranite Carboniferous (ophiolite) KS13-19 N 40°50'3.73" E 73°36'42.07" 1372 East Fergana basin Sandstone Early Cretaceous KS13-20 N 40°50'0.46" E 73°36'35.59" 1380 East Fergana basin Sandstone Early Cretaceous KS13-22 N 40°50'42.27" E 74° 5'59.61" 2489 East Fergana basin Sandstone Middle-Jurassic SK-46 N 42°19'38.64" E 78°14'47.70" 2153 Jeti Öguz, Issyk Kul Sandstone Upper-Carb. SK-47 N 42°20'19.74" E 78°14'7.08" 2028 Jeti Öguz, Issyk Kul Micoconglomerate Lower-Miocene

Chapter 5: results

Figure 44: Geological map with the samples discussed in this thesis. The sample numbers in blue are sediment samples. Basement samples are displayed in black. (modified geological map from ‘geological maps of central Asia and adjacent areas’, 2008, see appendices for explanation)

Chapter 5: Results

5.2 Sedimentary logs The sedimentary logs are sketched based on detailed field observations of several field campaigns through Kyrgyzstan. All the Jurassic sediments observed in these sedimentary logs are deposited in a freshwater environment. The log and interpretation of the Kadji-Sai (Figure 45) section are made by the author of this work, while the two other logs of Minkush (Figure 46) and Jeti Oguz (Figure 47) are drawn and interpreted by Prof. Dr. Marc Jolivet. The sedimentary log from Kadji Sai indicates a transition in palaeoenvironment from proximal alluvial plain to distal alluvial plain and eventually evolving in a distal lake palaeoenvironment. The Minkush section suggests that the sediments are deposited in a proximal alluvial plain environment. The small sedimentary section of a few meters thick exposed in Jeti Oguz is deposited in a distal alluvial fan setting. To confirm these hypotheses on the depositional environment in the Jurassic there is a need for thermochronological evidence (i.e. AFT), provenance analysis tools (e.g. U/Pb zircon analysis) and biostratigraphy.

Figure 45: sedimentary log and palaeo-environmental interpretation of the Kadji-Sai section of the Issyk Kul basin drawn by the author of this thesis. Scale is in meters, KS samples are for geochronological purposes and the KP sample for paleosol analysis.

Chapter 5: Results

Figure 46: Sedimentary log of the Minkush section drawn by Prof. Dr. Marc Jolivet after the field work of 2015. On the right, a rough sedimentological interpretation is suggested, based on the sediment composition and sedimentary structures. KP samples are samples for pollen or paleosol analysis, while KS samples are for geochronological analysis.

Figure 47: sedimentary log of the very thin Jurassic outcrops at the Jeti Oguz drawn by Dr. Marc Jolivet after the field campaign of 2015. On the right, a rough sedimentological interpretation is suggested based on the sediment composition and sedimentary structures. KP samples are samples for pollen or paleosol analysis, while KS samples are samples for geochronological analysis.

Chapter 5: Results

5.2.1 Paleosol occurence

As visible in the sedimentary logs above, a few paleosols and carbonated root sleeves occur in the

Mesozoic sections in Kyrgyzstan. These calcium carbonate deposits are formed in between clastic sediments in alluvial strata, typically deposited in a relatively humid climate. Stable oxygen isotopes can be used to derive climate information, such as mean annual temperature and mean annual precipitation (Kraus, 1999). It is also possible to derive the CO2 concentration in the atmosphere at the time of deposition (Kraus, 1999). Mesozoic pedogenetic paleosol calcretes and nodules are investigated in the Chinese Tien Shan for example by Heilbronn et al. (2015). This work will be done by the French team in the near future.

Chapter 5: Results

5.3 Apatite fission track (AFT) data on basement rocks Table 7: AFT data gathered on basement samples by Prof. Dr. J. De Grave and Prof. Dr. S. Glorie with a personal zeta-factor of 253.1 ± 2.4 and 259.1 ± 3.3 respectively. The samples measured by the former researcher are in indicated with °, while the samples measured by Dr. S Glorie are indicated with *. n = amount of apatite grains counted, s 5 5 = spontaneous track density expressed in 10 tracks/cm², Ns = amount of spontaneous tracks counted, i = induced track density expressed in 10 tracks/cm², Ni = amount of 5 induced tracks counted, Nd = interpolated value of the glass dosimeter tracks, d = glass dosimeter track density expressed in 10 tracks/cm², s/i = the ratio of the 2 spontaneous over the induced tracks, P( ) = the chi square probability test, t() = the zeta age in million years, lm = mean track length in µm, nl = number of track lengths measured, σ = standard deviation of the track length distribution

2 Sample n s ( 1) Ns i ( 1) Ni d ( 1) Nd s/i ( 1) P( ) t() lm nl 

TF-06* 20 10.041 (0.315) 1014 10.612 (0.325) 1064 4.284 (0.101) 1791 0.972  0.043 0.79 53.2  2.8 12.8 53 1.1 TF-15° 20 15.451 (0.430) 1292 6.170 (0.277) 495 3.658 (0.076) 2323 2.520  0.133 <0.99 115.6  6.6 12.7 100 1.9 TF-16° 7 5.261 (0.592) 79 2.427 (0.405) 36 3.654 (0.076) 2316 2.228  0.448 <0.99 102.2  20.7 ------TF-23* 20 32.741 (0.671) 2384 17.358 (0.489) 1261 3.987 (0.096) 1727 1.940  0.068 0.58 101.5  4.5 12.2 100 1.6 KYR-02* 20 18.260 (0.577) 1001 9.294 (0.415) 501 4.158 (0.099) 1775 2.081  0.114 0.54 115.5  7.1 11.7 30 1.3 KYR-04° 30 7.099 (0.407) 304 3.812 (0.302) 159 3.102 (0.072) 1855 1.882  0.184 1.00 73.5  7.4 ------KYR-05° 25 10.222 (0.352) 843 4.232 (0.229) 341 3.113 (0.072) 1861 2.495  0.160 0.91 97.5  6.7 ------KYR-15° 11 18.302 (0.732) 626 8.618 (0.510) 286 4.118 (0.080) 2635 2.131  0.152 0.60 110.1  8.2 ------AI-44° 20 20.520 (0.480) 1827 9.287 (0.325) 819 4.128 (0.080) 2642 2.281  0.096 0.98 118.1  5.6 13.5 100 1.2 AI-45* 20 20.251 (0.399) 2571 9.979 (0.281) 1265 4.030 (0.096) 1754 2.036  0.070 0.54 106.5  4.7 13.3 100 1.3 F11-775° 20 21.528 (0.600) 1286 9.461 (0.399) 560 4.104 (0.080) 2628 2.295  0.116 0.94 118.1  6.5 13.2 100 1.5

Table 8: AFT on basement samples taken during the field campaign of 2015 and analyzed by the author of this thesis. n = amount of grains counted, s = spontaneous track 5 5 density (10 tracks/cm²), Ns = amount of spontaneous tracks counted, i = induced track density (10 tracks/cm²), Ni = amount of induced tracks counted, Nd = interpolated 5 2 value of the glass dosimeter tracks, d = glass dosimeter track density (10 tracks/cm²), s/i = the ratio of the spontaneous over the induced tracks, P( ) = the chi square probability test, t() = the zeta age in million years, lTINT = mean track length, nl = number of track lengths measured, σ = standard deviation of the track length distribution

2 Sample n s ( 1) Ns i ( 1) Ni d ( 1) Nd s/i P( ) t() lTINT nl 

KB 121 20 5.296 (0.2248) 555 3.936 (0.1934) 414 4.046 (0.082) 2340 1.394 (0.091) 0.98 80.2±5.6 12.0 30 1.7 KB 131 20 15.78 (0.3511) 2020 6.758 (0.2298) 865 4.021 (0.084) 2217 2.358 (0.096) 0.95 134.6±6.5 12.6 75 1.3 KB 132 20 14.93 (0.3389) 1941 7.662 (0.2425) 998 4.018 (0.084) 2204 1.986 (0.077) 0.74 113.2±5.3 12.7 100 1.2 KB 133 20 14.00 (0.3429) 1666 8.208 (0.2625) 978 4.016 (0.084) 2190 1.730 (0.070) 0.32 98.6±4.8 12.3 100 1.2 KB 134 20 10.71 (0.2935) 1331 9.782 (0.2811) 1211 4.013 (0.084) 2177 1.121 (0.045) 0.88 64.1±3.1 11.8 69 1.8 KB 135 20 6.219 (0.2231) 777 6.698 (0.2315) 837 4.010 (0.084) 2162 0.936 (0.047) 0.78 53.5±3.0 13.0 20 1.6

Chapter 5: Results

The AFT data in Table 7 basement samples has been gathered by Prof. Dr. S. Glorie and Prof. Dr. De Grave.

Each age is calculated with the personal zeta-factor of the analyst (259.1±3.3 for Prof. Dr. Glorie and

253.1±2.4 for Prof. Dr. De Grave). Samples TF-06, TF-15, TF-16, TF-23 and KYR-02 are from Late

Ordovician, Early Silurian basement samples (granites and granodiorites) of the NTS. Samples KYR-04,

KYR-05, KYR-15, AI-44 and AI-45 are from Late Carboniferous and Early Permian granites in the MTS, very close and on the east side of the TFF. Sample F11-775 is a plagiogranite (tonalite) sample of the STS and originates from a Carboniferous ophiolite complex (pers. com. With Dr. Fedor Zhimulev). Where possible,

100 confined tracks were measured on all of these samples by the aforementioned analysts. The AFT ages of these basement samples are basically Mid Cretaceous, with a few younger ages on sample locations with a lower altitude (e.g. TF-06).

More basement AFT data on basement samples taken during the field campaign of 2015 are displayed in

Table 8. The zeta-age of the six basement samples is calculated with the personal zeta-factor

(286.23±4.71) of the author of this thesis. Sample KB 121 is an alkaline porphyritic intrusive igneous complex emplaced during the Early Permian. KB 131, KB 132, KB 133, KB 134 and KB 135 are Late

Ordovician granodiorites that are part of a vertical profile from the Kavak-Tau Range (Figure 44). The resulting AFT cooling ages are mainly Cretaceous to Early Paleogene. Only TINTs and no TINCLEs were used for length measurements in this dataset, in order to improve the thermal history model. KB 121, KB

134 and KB 135 did not yield 100 confined track (TINT) length measurements due to the low spontaneous track density. An age-elevation profile is displayed in Figure 48. A clear, normal linear younging downward trend can be observed.

Figure 48: AFT age – elevation profile of the vertical basement profile in the Kavak-Tau range (Minkush area).

Chapter 5: Results

5.3.1 Track length measurements

Track lengths are performed with a KONTRON tablet and laser system, as outlined in section 3.3.2. These track lengths will further be used for the reconstruction of the time-temperature history of the sample with the modelling program QTQt (Gallagher, 2012). These thermal history models are presented and discussed in section 5.3.2. Track length distributions are usually displayed in histograms with a bin width of 1µm, as visible on Figure 49. The number of measured tracks, mean track length and the standard deviation are values used for the interpretation of the cooling history of the sample (see section 1.6.1). TF-06 has a normal length distribution with a medium high mean track length, although only 43 confined tracks could be measured (Figure 49a). This means that the sample had a moderate fast cooling through the PAZ. TF-15 has a bimodal distribution of the track length distribution, even after 100 track length measurements

(Figure 49b). A similar, but less outspoken, pattern is found for TF-23 (Figure 49c). The mean track length of this sample (12.2µm) indicates that the sample cooled slow. The track length distribution of KYR-02 yields a low mean track length (11.7µm) on only 30 tracks, inferring that the sample could have cooled slowly (Figure 49d).

Figure 49: track length distributions for basement samples of Table 7.

Chapter 5: Results

Figure 50: Second part of the track length distribution for samples of Table 7. Three samples with a higher mean track length (>13µm) are displayed in Figure 50. AI-44 has a normal distribution of track lengths and a relative high mean track length (13.5µm), meaning that this sample experienced a rapid cooling through the PAZ (Figure 50a). Figure AI-45 - which is also sampled close to the TFF - also yielded a relatively high mean track length, implying a fast cooling (Figure 50b). Sample F11-

775 has also a high mean track length (13.2µm), but has a less smooth normal distribution of the track lengths (Figure 50c).

The following six track length distributions are obtained by the author of this thesis. Only TINTs are measured on these samples, leading to a low number of measured track lengths. For example sample KB

121 yielded only 30 TINTs (Figure 51a). The mean track length of this sample is around ~12µm, which could mean that the sample cooled slowly. Sample KB 131 has a normal track length distribution and a mean track length of 12.6µm (Figure 51b), only after measuring 75 tracks. KB 132 and KB 133 also yield a normal distribution and mean track lengths of ~12.5µm (Figure 51c and Figure 51d).

Chapter 5: Results

Figure 51: Track length distributions of the first four samples of Table 8. Samples KB 134 and KB 135 did not reach the goal of 100 TINTs and therefore have a less smooth normal distribution of the track lengths (Figure 52a and Figure 52b). This means that the conclusions on the thermal history models have to be drawn carefully.

Figure 52: Track length distribution of two samples of Table 8 that did not yield enough measurable TINTs.

Chapter 5: Results

5.3.2 Thermal history modelling results with QTQt

If sufficient confined track length measurements are available, a reconstruction of the thermal history of the sample can be made with the program QTQt (Gallagher, 2012) (see section 1.8). Even when only 30 length measurements are available, an attempt for the reconstruction of a thermal history model is made, although it has to be interpreted with prudence. The thermal history modelling provides information on the cooling history of the sampled crystalline basement through geological time. Periods of accelerated cooling, slowed cooling, stability or even reheating can be visualized. With a non-Gaussian track length distribution it is difficult to fit a curve to the data, this is especially the case for samples TF-15 and TF-23.

Therefore, thermochronological models derived from non-Gaussian track length distributions have to be interpreted carefully. Furthermore, thermal history models very often indicate a pronounced Neogene cooling. The workflow of the thermal history modelling with QTQt is described in section 1.8.

The suggested model (i.e. the expected model displayed in black) of TF-06 is complicated and visualizes the Early Paleogene cooling of this sample, followed by a strong pronounced Late Neogene cooling

(Figure 53a). The model of TF-15 is less complex and visualizes a slow and steady cooling through the crust during the Cretaceous and the Cenozoic (Figure 53b). Almost the same model Tt path is suggested for TF-23 (Figure 53c), but higher probability values are yielded for the Cretaceous evolution of TF-23. An attempt is made to model KYR-02 with limited track length information (Figure 53d). The model suggests fast cooling in the Late Jurassic – Early Cretaceous, followed by a period of stability in the Late Cretaceous and Early Paleogene. A reheating of sample KYR-02 occurs in the Latest Paleogene, followed by a steep cooling in the Late Neogene. Figure 54 displays three comparable time-temperature histories for samples

AI-44 (Figure 54a), AI-45 (Figure 54b) and F11-775 (Figure 54c). AFT ages of these samples are Early to

Late Cretaceous transition (~110Ma) and mean track lengths are >13µm, typical for fast cooled samples that did not experience reheating. This continuous cooling is illustrated in Figure 54. Although only 30

TINTs could be measured in KB 121, a thermal history model is reconstructed in Figure 55a. A Cretaceous cooling, Paleogene stabilization and Neogene reactivation is visible in the model of KB 121. The models of

KB 131, KB 132 and KB 133 generally reveal the same time-temperature history. Just like the models discussed above, there is a well-pronounced Late Jurassic to Early Cretaceous cooling, followed by a Late

Cretaceous – Paleogene stabilization (and reheating?) after which the samples follow a steep cooling path in the Neogene. A slow Cretaceous cooling is observed in the model of KB 134, followed by a steep

Neogene cooling (Figure 56a). The model of KB 135 is based on insufficient TINT lengths and is therefore not used for interpretation (Figure 56b).

Chapter 5: Results

Figure 53: thermal history models for the first four samples from Table 7. Modelling was done using QTQt (Gallagher, 2012)

Chapter 5: Results

Figure 54: thermal history models for the last three samples from Table 7.

Chapter 5: Results

Figure 55: thermal history models for the first four samples of Table 8.

Chapter 5: Results

Figure 56: thermal history models for the last two samples of Table 8.

5.4 AFT data on detrital rocks

Table 9: Apatite fission track data of the detrital samples analyzed by the author of this Master thesis. n = amount of grains counted, s = spontaneous track density 5 5 (expressed in 10 tracks/cm²), Ns = amount of spontaneous tracks counted, i = induced track density (expressed in 10 tracks/cm²), Ni = amount of induced tracks counted, Nd 5 = interpolated value of the glass dosimeter tracks, d = glass dosimeter track density (expressed in 10 tracks/cm²), s/i = the ratio of the spontaneous over the induced tracks, P(2) = the chi square probability test, t() = the zeta age in Ma, Central age = age (in Ma) calculated with more weight on the ages with small uncertainty (Vermeesch, 2008), lm = mean track length, nTINTS= number of Track In Track (TINT) lengths measured, σ = standard deviation of the track length distribution.

2 Sample n s ( 1) Ns i ( 1) Ni d ( 1) Nd s/i (( 1) P( ) t() Central age lm nTINTS 

KS-126 7 15.86 (0.6330) 628 5.325 (0.3719) 205 4.592 (0.085) 2940 2.828 (0.228) 0.76 183.2±15.4 198.0±16 13.2 14 1.3 KS-128 36 9.813 (0.2196) 1997 3.110 (0.1233) 636 4.587 (0.085) 2937 3.303 (0.150) 0.99 213.2±11.1 202.7±9.2 13.3 52 1.2 KS13-19 38 10.03 (0.3372) 885 5.638 (0.2484) 515 4.513 (0.084) 2889 1.930 (0.107) 0.98 123.4±7.5 110.0±6.1 11.9 15 1.5 KS13-20 31 10.99 (0.4222) 677 6.692 (0.3381) 424 4.508 (0.084) 2886 1.709 (0.106) 1.00 109.4±7.4 102.2±6.3 11.3 13 1.2 KS13-22 35 2.215 (0.1326) 279 12.12 (0.3036) 1595 4.504 (0.084) 2884 0.206 (0.013) 0.03 13.5±0.9 11.4±0.9 - - - SK-46 57 10.05 (0.0213) 2243 3.935 (0.313) 898 4.491 (0.084) 2875 2.618 (0.103) 1.00 166.1±7.8 158.6±6.3 12.1 80 1.0 SK-47 59 9.546 (0.2119) 2030 6.656 (0.1788) 1385 4.487 (0.084) 2873 1.552 (0.054) 0.05 98.8±4.3 93.1±4.2 11.9 39 1.2

Chapter 5: Results

The detrital AFT samples in Table 9 are analyzed by the author of this thesis. Radial plots were used for the calculation of the central age and the evaluation of multiple age populations. The central ages displayed in

Table 9 are within the 1 sigma uncertainty of the zeta age. Samples KS13-22 and SK-47 fail the chi square statistical test (<0.05), meaning that multiple populations are present in the dataset.

KS 126 is a sandstone of the Minkush sampling site which is formed in the so-called Kavak basin in the

NTS (Figure 44). This sample fits in a detailed sedimentary log of the Jurassic layers in Minkush (Figure 46).

The stratigraphic age is based on the Lower-Jurassic age of the plants in an adjacent brown coal deposit.

According to Bachmanov et al. (2008) it is possible that the coal complex adjacent to samples KS126 and

KS128 can contain plants that also occur in the Middle-Jurassic. Seven grains are counted, 14 TINTS measured with a mean length of 13.2µm and a standard deviation of 1.3µm. Single grain ages range from

~230 to ~145Ma, yielding a central age of 198±16Ma (Figure 57a).

KS 128 is another detrital sample of the Minkush sampling site and is located a few meters above KS 126 in the section (Figure 44, Figure 46). 36 grains are counted, 52 TINTS are measured with a mean length of

13.3µm and a standard deviation of 1.2µm. Single grain ages range from ~290 to ~150Ma. A central age of 202.7±9.2Ma is obtained and reflects the age of one apatite age population (Figure 57b).

KS13-19, KS13-20 and KS13-22 are all originating from the North East Fergana basin west of the TFF

(Figure 44). The stratigraphic age of KS13-19 sample is Late Jurassic to Early Cretaceous, because it is located a few meters below the Kalaza conglomerate (Jolivet et al., 2015). The stratigraphic age should be around ~150Ma. 38 grains are counted, 15 TINTS are counted with a mean length of 11.9µm and standard deviation of 1.5µm. Single grain ages range from ~220 to 70Ma, but are outliers that are within uncertainty. The central age of KS13-19 is equal to 110.0±6.1Ma (Figure 57c).

KS13-20 is sampled in the first unit of the Upper-Jurassic - Lower-Cretaceous conglomerate, close to

KS13-19 (Figure 44). The stratigraphic age should also be around ~150Ma. 31 grains are counted, 13 TINTS are measured with a mean length of 11.3µm and a standard deviation of 1.2µm. Single grain ages range from 170 to 70Ma, but are part of the main age population with a central age of 102.2±6.3Ma (Figure

57d).

KS13-22 is sampled deeper in the stratigraphic section of the East Fergana basin and has a Middle-Jurassic stratigraphic age (Figure 44). No length measurements could be executed on this sample, because of the low spontaneous track density (ρs). 35 apatite grains are counted. The chi-square statistical test failed and therefore two populations (~15Ma and ~10Ma) are distinguished with the RadialPlotter (Figure 57e).

Chapter 5: Results

Figure 57: Radial plots of six detrital samples with AFT single grain ages. a) KS 126, b) KS 128, c) KS13-19, d) KS13-20, e) KS13- 22 and f) SK-46 with Dpar values illustrated in red and yellow

Chapter 5: Results

SK-46 is an Upper-Carboniferous detrital basement sample taken at the Jeti Oguz section in the Issyk-Kul

Basin (Figure 44). 57 grains are counted, 80 TINTs with a mean length of 12.0µm and standard deviation of

1.0µm. The etch pit diameter ‘Dpar’ value (see section 1.5) is measured on sample SK-46 as a test to distinguish the different kinetic populations. Four etch pit diameter measurements are done on each apatite grain of SK-46 (Donelick et al., 2005). The Dpar values are between 2.0 and 2.7µm. Single grain ages range from 270-115Ma, but are part of the main age population of 158.6±6.3Ma (Figure 57f).

SK 47 is sampled in the Kyrgyz formation (south-east of Issyk Kul lake), which has a Neogene age according to the most recent geological map. An earliest Miocene (Aquitanian) age is estimated by magnetostratigraphic research on this section (Wack et al., 2014). 59 grains are counted, 39 TINTS are counted with a mean length of 11.9µm and a standard deviation of 1.2µm. After failing the chi square test, three different popoulations are distinguished with RadialPlotter (Figure 58). A small Late Jurassic – Early

Cretaceous age peak (~145Ma), an Early to Late Cretaceous peak (~105Ma) and a Late Cretaceous peak

(~80Ma) (Figure 58). Outliers are indicated in red (Figure 15).

Figure 58: Modified radial plot with the distinction of three different age populations for the Early Miocene sample of the Issyk Kul basin (SK-47).

Chapter 5: Results

5.4.1 Track length distributions

Track length distributions were obtained after measuring the maximal amount of confined tracks. Only

TINTs (Track IN Tracks) are measured for the detrital samples, causing a lower number of measurable track lengths but significantly improving the quality of the track length information (Donelick et al., 2005).

Only 14 tracks measurements were obtained for sample KS 126 (Figure 59a). These long track lengths are confirmed in the adjacent sample KS 128 (Figure 59b), and have a normal distribution after the measurement of 42 TINTs. The mean track length of KS 126 and KS 128 are ~13.2µm and ~13.4µm respectively. In KS13-19 and KS13-20 only 15 and 13 TINTs are could be measured respectively (Figure

59c and Figure 59d). The mean track length is 11.9µm (KS13-19) and 11.3µm (KS13-20), which is a relatively low value. More track lengths are required for thermal history modelling. SK-46 has a mean track length of 12.1µm and a narrow normal distribution, based on the measurement of 80 TINTs (Figure 59e).

SK-47 yielded a relative low mean track length of 11.9µm, based on the measurement of 39 TINTs (Figure

59f).

Figure 60 displays the geographical distribution and central AFT age of the detrital samples. Mesozoic samples west of the TFF yield a much younger AFT central age than west of the TFF. Especially the

Miocene AFT age of the Jurassic sediment (KS13-22) obtained in the East Fergana basin is different to the

Late Triassic – Early Jurassic age (~200Ma) obtained in two samples east of the TFF (KS 126 and KS 128).

Chapter 5: Results

Figure 59: track length distribution of the detrital samples of Table 9: a) KS 126, b) KS 128, c) KS13-19, d) KS13-20, e) SK-46 and f) SK-47

Chapter 5: Results

Figure 60: geological map of Kyrgyzstan on which the detrital sample locations are plotted with the central age and its uncertainty in Ma. Sample numbers can be found in Figure 44.

Chapter 5: Results

5.5 Zircon(U-Th)/He data Two basement samples (AI-45 and KYR-05) of the MTS close to the TFF are analyzed with the zircon (U-

Th)/He dating technique (see section 1.8). These analyses were carried out at the John de Laeter Center for

Isotope Research, Curtin University in Perth (Australia). The analytical procedure of the zircon (U-Th)/He analysis are described in De Grave et al. (2015). Inclusion-free zircon grains were handpicked and sealed in

Nb crucibles. These crucibles were degassed using a 1064nm Nd-YAG laser. 4He concentrations were measured with isotope dilution after spiking with 3He and calibrated with an independent 4He standard on a

Pfeiffer Prisma Quadrupole Mass Spectrometer. The fused zircon-Nb crucibles were dissolved using a 235U and 230Th spiked HF solution in Parr bombs at 240°C for 40h. Sm was not measured here due to the very low concentration. U and Th content was measured using an Agilent Quadrupole 7500 ICP-MS. The closure temperature of the (U-Th)/He zircon system is ~200°C (Reiners, 2005). In Table 10, two of the eight zircon

(U-Th)/He ages are discarded because there was a monazite inclusion (AI-45 1) and parentless excess

Helium present (KYR05B-1) (Table 5). The average age of the zircon (U-Th)/He system of AI-45 is ~160Ma for AI-45. A comparable age of ~155Ma is obtained for KYR-05. These zircon (U-Th)/He ages and the previously discussed basement AFT ages are plotted on the geological map in Figure 61.

Table 10: Zircon (U-Th)/He data for samples close to the TFF. For each sample, four single grain ages were determined. Analyses in italics were discarded for the calculation of the sample average age, because of anomalous values caused by inclusions in the zircon crystals. Ft is the alpha ejection parameter as defined by Farley et al. (1996). Uncorrected ages (Unc. age) are corrected (Cor. age) based on the Ft parameter (subdivided). TAU is the Total Analytical Uncertainty.

He Unc. Cor. U Th He TAU Th/ Sample σ σ err age σ Ft Age σ Average (Ma) (ppm) (ppm) (ncc) (%) U (%) (Ma) (Ma)

AI-45 1 979.3 19.5 1444.1 28.0 260.6 2.5 3.0 1.5 340.37 10.08 0.77 443.32 13.85 AI-45 2 1160.0 27.2 1449.5 21.1 91.1 2.5 3.1 1.2 119.48 3.73 0.76 157.69 5.84 AI-45 3 768.6 16.4 377.2 7.3 59.3 2.6 3.2 2.5 131.21 4.19 0.76 171.91 6.48 AI -45 4 1068.9 28.4 887.0 12.9 130.9 2.5 3.4 0.8 121.39 4.08 0.80 151.15 5.31 160.3±5.3 KYR05B-1 254.0 5.8 315.7 6.2 10.5 2.6 3.1 1.2 160.91 5.07 0.69 231.76 8.65 KYR05B-2 427.3 9.1 428.5 8.4 5.1 1.7 2.4 1.0 73.99 1.80 0.61 120.35 4.65 KYR05B-3 564.1 12.0 565.6 11.1 6.2 1.5 2.3 1.0 90.21 2.07 0.61 148.95 5.62 KYR05B-4 649.3 16.5 342.3 7.2 8.6 1.3 2.6 0.5 120.27 3.17 0.61 196.79 7.86 155.4±6.0

Chapter 5: Results

Figure 61: geological map of Kyrgyzstan with the different AFT ages and zircon (U-Th)/He ages (in bold and italics) on basement rocks in this work.

Chapter 6: Discussion

6.1 Basement AFT and zircon (U-Th)/He data Numerous basement AFT data is available in Kyrgyzstan and is described in chapter four. The compilation of basement AFT ages revealed exhumation related basement cooling events in the Mesozoic and

Neogene. In this work, AFT ages of basement samples are mainly Cretaceous and range from ~135 to

~95Ma with younger ages for KB 121 (80.2±5.6Ma), KYR-04 (73.5±7.4Ma), KB 134 (64.1±3.1Ma), TF-06

(53.2±2.8Ma) and KB 135 (53.5±3.0Ma) (Figure 61). Hence our data corresponds with a widely recognized

Cretaceous age peak in the Kyrgyz Tien Shan (Figure 31 and Figure 32). The majority of the basement rocks in this manuscript yields relatively short (~12µm) mean track lengths, which is typical for slow cooling of the basement through the apatite PAZ. Samples with a higher mean track length >13µm (i.e. AI-

44, AI-45 and F11-775) are indicative of faster cooling through the PAZ. Figure 62 is an age-elevation plot of the basement AFT data obtained in this work.

Figure 62: AFT age – elevation plot of the basement AFT data of this thesis. The red symbols represent samples of the NTS, green circles represent samples of the MTS and the orange circles represents one AFT age of the STS. AFT ages sampled on basically the same geographic location are connected with a dashed line, while the samples consisting of a vertical profile are connected with a full black line. The population representing the most abundant AFT cluster (130-100Ma) is indicated by green dashed lines.

Chapter 6: Discussion

The general trend inFigure 62 is a decrease in AFT age with decreasing elevation. This relation is typical for

AFT basement samples that experienced slow cooling through the apatite PAZ. The samples displayed above in the age-elevation plot are discussed in groups based on their geographical location:

- KYR-15 (110.1±8.2Ma), AI-44 (118.1±5.6Ma) and AI-45 (106.5±4.7Ma): These three basement samples

are sampled in the MTS close to the TFF and yield ages of ~110Ma. The longer mean track lengths for

AI-44 and AI-45 (>13µm) and the information derived from the thermal history models (Figure 54a and

Figure 54b) indicate that these samples experienced a fast cooling at ~120-110Ma. The zircon (U-

Th)/He age data of AI-45 reveals that this sample passed the ~200°C isotherm about 160±6Ma ago.

The 200°C isotherm is located ±3.3km below the AFT closure temperature if a normal geothermal

gradient (±30°C/km) is assumed. This indicates a cooling rate of ~60m/Ma in between the 100°C and

200°C isotherms (160-105Ma). Sample AI-44 and KYR-15 are connected by a dashed line onFigure 62.

- KYR-04 (73.5±7.5Ma) and KYR-05 (97.5±6.7Ma): These two samples of the MTS close to the TFF have a

lower elevation than the three aforementioned samples (KYR-15, AI-44 and AI-45) (Figure 44, Figure

61). The zircon Helium age of KYR-05 is in the range of ~155Ma and is comparable to the zircon (U-

Th)/He and AFT age of AI-45.

- KB 131 (134.6±6.5Ma), KB 132 (113.2±5.3Ma), KB 133 (98.6±4.8Ma), KB 134 (64.1±3.1Ma), KB 135

(53.5±3.0Ma) and KB 121 (80.2±5.6Ma): The five basement samples (KB131-135) are sampled as a

vertical profile of the NTS and follow (as expected) an almost linear trend in the age-elevation plot

(Figure 62). The ages of these five samples are dispersed over the Cretaceous until the Early Paleogene.

KB 121 is sampled more to the east of the Kavak-Tau range and follows basically the same trend. All

these samples have relative low mean track lengths of about 12.5µm. KB 135 has a mean track length

of 13.0µm, but this is only based on 20 track length measurements and should be interpreted with

caution.

- TF-06 (53.2±2.8Ma), TF-15 (115.6±6.6Ma), TF-16 (102.2±20.7Ma), TF-23 (101.5±4.5Ma) and KYR-02

(115.5±7.1Ma): These five samples are all taken on the east side of the TFF in the NTS (Figure 64). A

very rudimentary age-elevation profile could be drawn for TF-16 to TF-15, because they were sampled

on basically the same geographic location. The mean track length of these four samples is low,

although sufficient (minimum 100) track lengths could only be measured in TF-15 and KYR-02.

- F11-775 (118.1±6.5Ma): Another late Early-Cretaceous AFT age (118.1±6.5Ma) is obtained south of the

Fergana basin on sample F11-775. This sample is located in the STS west of the TFF (Figure 61). The age

Chapter 6: Discussion

is in accordance with a Latest Permian molasse sample with a ‘reset’ AFT age of 117.8±6.5Ma (and

mean track length of 12.6µm) located 50 km east of F11-775 (De Grave et al., 2012). The mean track

length of sample F11-775 is 13.2µm. There is not much Mesozoic AFT age data available in this region,

so it is difficult to draw conclusions on the regional, Mesozoic development.

6.1.1 Thermal history model compilation

An overview of the reconstructed thermal history models of this work is given in Figure 63. Three main general phases in these models can be distinguished. Fast cooling of basement rocks occurred during the

Late-Jurassic to Early Cretaceous. This is followed by a period of stabilization since the Late Cretaceous. A subtle reheating event occurs in the Paleogene and is followed by a fast cooling during the (Late)

Neogene (Figure 63). This is in agreement with other studies that analyzed basement rocks in the Kyrgyz

Tien Shan (e.g. De Grave et al., 2007; Glorie et al., 2010; De Grave et al., 2012; Glorie and De Grave, 2015).

Figure 63: thermal history model compilation of the basement rock AFT data published in this thesis. Each AFT sample is modelled individually with QTQt. The expected model of each AFT sample is eventually extracted and plotted on a absolute timescale. The general trends observed in this figure are: (1) fast cooling during the Late Jurassic - Early Cretaceous, (2) stabilization in the Late Cretaceous – Paleogene and (3) renewed cooling during the Neogene.

Chapter 6: Discussion

6.1.2 Mesozoic basement cooling in the NTS

The Mesozoic ages of the samples of the Kavak-Tau Range (KB131-135 & KB 121, Figure 62) are in the line of the observations in the age-elevation plot. Similar AFT ages were obtained on the same mountain range by Glorie et al. (2010) (Figure 64). Sedimentological arguments exist that the Kavak-Tau range has been reactivated and thrusted over the Jurassic sediments of Minkush since the Oligocene (Bachmanov et al.,

2008). This is revealed by an AFT age of 33.8±2.1 and an apatite (U-Th)/He age of 29.4±1.8Ma close to the thrust front of the North Kavak Fault (Glorie et al., 2010) (Figure 64). An intensification of this overthrusting movement along the North Kavak Fault occurred during the Quaternary (Bachmanov et al., 2008).

Figure 64: Detail of the NTS east of the TFF. The AFT and zircon (U-Th)/He obtained in this work is illustrated, while the bold font expresses AFT data and Apatite (U-Th)/He data (AHe) of Glorie et al. (2010). Data of the NTS of De Grave et al. (2012) is also used for this map. The North Kavak fault is indicated in red and the Jurassic section of Minkush in blue (KS126 & KS128).

The other group of NTS samples (TF-06, TF-15, TF-16, TF-23 and KYR-02) is located more in the north of the NTS than the samples of the Kavak-Tau Range (KB samples) (Figure 64). The thermal history models

(Figure 63) suggest a pronounced cooling phase around ~120-110Ma for these samples. The AFT age of

Chapter 6: Discussion

TF-15, TF-16, TF-23 and KYR-02 corresponds to a well-preserved age population of Early Cretaceous age in the NTS (Glorie et al., 2010). The four samples surround an elevated plateau (Figure 65). These Cretaceous

AFT ages (~150Ma and 120-95Ma) on the elevated plateau refer to Mesozoic tectonic events. The initial collision of the Cimmerian Lhasa block (~150-120Ma) and the isostatic response to the slab break-off of the subducting Bangong-Nujiang Tethyan Ocean lithosphere under the Tarim and STS (e.g. Sui et al., 2013;

Chen et al., 2014, Figure 30) are thought to have caused this phase of basement exhumation in the Kyrgyz

Tien Shan.

Figure 65: Mesozoic AFT ages of this study and of Glorie et al., 2010. The ages of the four samples above are in the same range of ~110Ma. This block/plateau is not rejuvenated by tectonic events and therefore displays an (inherited) AFT age population that is caused by a peak in exhumation around ~110Ma. Sample TF-06 (in green) is located at the border of this plateau at a much lower elevation of 1355m and has a AFT age of 53.2±2.8Ma (inset = geological map with study area).

A younger Early Paleogene age population is found in TF-06 (53.2 ±2.8Ma) and KB 135 (53.5±3.0Ma) and has already been recognized previously in the Kyrgyz Tien Shan by Glorie and De Grave (2015). A fast cooling through the apatite PAZ could be assumed based on the small number of track lengths. This cooling event is less abundant because it only affected the weaker fault zones in the Kyrgyz Tien Shan

(Figure 64), illustrated by the TF-06 and KB-135 that are both located close to fault zones (Glorie and De

Grave, 2015). The driving force of this less well-pronounced phase of basement exhumation is still debated. It is thought to be a result of the accretion of an island-arc system to the Eurasian (Tethyan) margin (Kohistan-Dras-Lordekh arc) or the leading edge of the Greater Indian Plate (Klootwijk, 1984),

Chapter 6: Discussion inducing a northwards thrusting of the Pamir nappe and reactivation of the weaker fault zones of the

Kyrgyz Tien Shan (Aitchison et al., 2000, 2007; Glorie and De Grave, 2015).

6.1.3 Mesozoic activity of the TFF based on basement AFT and zircon (U-Th)/He data

The AFT ages of the five samples of the MTS (KYR-04, KYR-05, KYR-15, AI-44 and AI-45) along the eastern part of the TFF all yield a Cretaceous age. The absence of Cenozoic ages close to the TFF indicates that there was little post-Mesozoic vertical movement in this segment of the TFF. This is in contrast to the

Miocene AFT ages obtained on basement rocks ±100km further to the NW side of the TFF (Bande et al.,

2015) and in the STS close to the TFF (De Grave et al., 2012; Figure 66). This means that the different sections of the TFF were reactivated at different timespans at least with respect to differential vertical block movements and associated denudation.

Figure 66: AFT data of basement (black font) and detrital (blue) samples taken close to the TFF or close to faults next to the TFF. White boxes display AFT data and zircon (U-Th)/He data of this work. Red boxes display data from the De Grave et al. (2012). Yellow boxes display data from Bande et al. (2015).

Chapter 6: Discussion

The Late Jurassic (~160-155Ma) zircon (U-Th)/He ages of KYR-05 and AI-45 indicate vertical movement of the TFF, which can be linked to the collision of the Lhasa block to the Eurasian margin (Kapp et al., 2007).

The AFT ages of four basement samples close to the TFF range from ~120-95Ma and indicate activity of the TFF probably as a response of the slab break-off of the subducting Bangong-Nujiang Tethyan Ocean lithosphere (e.g. Sui et al., 2013; Chen et al., 2014). The activity of the TFF at the end of the Late Cretaceous is constrained with sample KYR-04 that yields an age of 73.5±7.4Ma. This age is coeval with the collision of the Kohistan-Dras island-arc (Treloar et al., 1996) and the Karakoram Block to Eurasia (~80-70Ma; Schwab et al., 2004).

6.2 Detrital apatite fission track thermochronology

6.2.1 East Fergana basin samples (west of TFF)

6.2.1.1 KS13-22

Burial by kilometres of Cretaceous and Cenozoic sediments (and consequential heating) caused the resetting of the AFT system of this Middle-Jurassic sample. This deeper burial depth corresponds to a higher degree of coalification of the Jurassic coals in this basin. The central AFT age of the reset sample is

11.5±1.0Ma, from which no thermal history model could be reconstructed because no track lengths could be measured. This Late Miocene age is not uncommon in the STS and was already detected in basement and reset sedimentary rocks in regions close to the Pamir block (Sobel, 2006b; Glorie et al., 2011;De Grave et al., 2012; Yang et al., 2014). This Miocene reactivation is caused by the indentation of the Pamir block

(Bande et al., 2015; Sobel, 2006b; De Grave et al., 2012; Yang et al., 2014) caused by the ‘hard’ collision of

India. The Pamir indentation caused a counter-clockwise rotation of the Fergana basin, constraint by paleomagnetic data (Thomas et al., 1993). The exact timing of the onset of the exhumation is dated to

25Ma in the STS (Sobel et al., 2006b; Glorie et al., 2011), but reactivated only some areas along the TFF

(Figure 66). Here, this Miocene age represents the time when these reset sediments were exhumed to AFT registration threshold, once again.

6.2.1.2 KS 13-19 and KS 13-20

Samples KS13-19 and KS13-20 are both from the western side of the East Fergana basin. The stratigraphic age of both samples is not well-known because the sampled conglomerate wall (equivalent of Kalaza formation) lacks precise dating, but, it is assumed that these samples are of Late Jurassic to Early

Cretaceous age (Jolivet et al., 2015). The central AFT age of the sample below the conglomerate formation

(KS13-19) is 111.5±6.2Ma, while KS13-20 is taken inside the conglomerate formation and has a central age

Chapter 6: Discussion of 103.5±6.4Ma. The reduced mean track length (~11µm) and Middle-Cretaceous central AFT age indicate that the sample is partially reset by moderately deep sedimentary burial (Figure 19). A complete resetting

(e.g. KS13-22) is not the case for KS13-19 and KS13-20 here. Both central AFT ages are considered to be mixed ages, but the transition to the totally reset Jurassic samples is for the first time revealed on the

Kyrgyz side of the East Fergana basin. This transition from the total annealing zone to the partial annealing zone was already found in Lower-Cretaceous sediments on the Chinese side of the East Fergana basin

(Yang et al., 2014).

6.2.2 Lower-Jurassic samples of Minkush (east of TFF)

KS 126 and KS 128 are sampled very close to each other and are both clastic Lower-Jurassic (Bachmanov et al., 2008) sedimentary rocks deposited in the Kavak basin (NTS) at the east side of the TFF (Figure 46).

The amount of counted apatite grains is higher in sample KS 128, while only seven grains could be counted in sample KS 126. The central age of these two samples is about ~200Ma, implying that the Early

Mesozoic AFT age components are preserved. The low resolution of the single grain ages hinders a detailed interpretation and identification of the different age components. Based on the radial plot (Figure

57) and chi square statistical test (Table 9), it is assumed that only one age population is present in these samples. This age population of ~200Ma indicates that there was a relative short lag time between the time of deposition and renewed exhumation. Exhumation rates based on this lag time of 10-20Ma are approximately 200-300m/Ma in the Early Jurassic, which is a relative high value compared to the exhumation of the Cretaceous AFT samples (~60m/Ma) calculated with the multi-method approach (see section 1.8). The estimations of the exhumation rates is based on the mathematical derivations of section

2.2.1 (Figure 18). The difference between the rapid Early Jurassic and slower Early Cretaceous exhumation rate is in agreement with the observations of Glorie and De Grave (2015) (Figure 31).

The mean track length of both samples is high (±13.3µm) from which some important conclusions can be drawn. First of all, this high mean track length means that a source with fast cooled apatite minerals was already exposed in the Early Jurassic and sourced the Jurassic intramontane Kavak basin. Another important conclusion that can be drawn from the high mean track length is that the Jurassic layers have not been reheated by deep burial after deposition. This shallow burial is expected, because Jurassic sediments with on top Cretaceous sediments are nowhere exposed in the NTS. The coals exposed in the

Jurassic sediments are browncoals, which indicate that the Jurassic coals have not suffered strong coalification caused by deep burial.

Chapter 6: Discussion

6.2.3 Issyk Kul sections (east of TFF)

6.2.3.1 SK 46

Sample SK 46 is an Upper-Carboniferous detrital sedimentary sample of the east side of the Issyk-Kul basin. The central age of ~160Ma is clearly younger than the Upper-Carboniferous stratigraphic age (323-

299Ma) (Figure 57f), meaning that the sample is reset completely, and in fact can in fact be considered as a “basement” sample with respect to posterior Meso-Cenozoic events. The single grain ages are dispersed over a wide interval (~270-115Ma). The most abundant AFT age population is of Early Cretaceous age and probably represents the complete reset AFT age system in this sample. The more older ages (~250Ma) are interpreted as partially reset apatite grains with an inherited component. Dpar values (section 1.5.1) were measured as a test, leading to values from 1.95 to 2.67µm, but were not distinctive enough to distinguish age or kinetic populations. The mean track length measured on TINTs is equal to 12.1µm, indicating a slow cooling. It could also be possible that the source was a rapidly cooled basement rocks with apatite minerals with a high mean track length that have been thermally overprinted (i.e. shortened) by a

Mesozoic and/or Cenozoic event. The apatite crystals of this fine grained sample are relative large and euhedral, suggesting that the source of the apatites was proximal or that recrystallization occured.

Figure 67: satellite image of the Issyk-Kul lake in the NTS of Kyrgyzstan. An extensive dataset of AFT ages is already published by De Grave et al. (2013). Only the Mesozoic basement age data from De Grave et al. (2013) and is displayed in yellow. The detrital AFT data of this work is displayed in pink. Jurassic and Late Oligocene – Miocene outcrops are also indicated in blue and orange respectively.

Chapter 6: Discussion

The Carboniferous basement of SK-46 is located on a local anticline of the Terskey Range on the south- east side of the Issyk Kul basin (Figure 67). The Cretaceous reset age group is in line with earlier detailed

AFT research on basement rocks exposed in the mountain ranges adjacent to the Issyk Kul lake (De Grave et al., 2013). Overall Early Cretaceous AFT ages with older ages of ~150Ma occur at the Kungey Range, while younger Mesozoic ages of ~110Ma occur at the Terskey Range (De Grave et al., 2013; De Pelsmaeker et al., 2015) (Figure 67). The Early Cretaceous reset ages (~150-120Ma) of the Upper-Carboniferous sediment are in agreement with the Mesozoic AFT ages obtained in the Issyk Kul region, probably representing a distant effect of the Lhasa collision (~150-120Ma) in the Issyk Kul basement (De Grave et al., 2013).

6.2.3.2 SK 47

Sample SK-47 is taken at the south-east side of the Issyk Kul basin. It is a microconglomerate taken in the lower part of the Late Oligocene – Miocene ‘Kyrgyz’ Formation. This Formation is only exposed in some places in the Issyk Kul basin (Figure 67). The central age of SK-47 is ~93Ma and is determined with 59 single grain AFT ages. Only a few apatite grains have a Cenozoic AFT age, while the stratigraphic age of the sediment is determined with magnetostratigraphy as ±23Ma (Wack et al., 2014). The low mean track length is equal to 11.9µm. This is only based on a limited amount of length measurements, so care must be taken with respect to the interpretation. It is in any case not plausible that this low mean track length is caused by heating by deep burial, because the Kyrgyz Formation is only covered by a thin Upper-Neogene cover, and therefore an inherited shortening of the tracks within the source rock seems a more reasonable explanation. All of this seems to suggest that SK-47 is derived from basement rocks with a mean Late-

Cretaceous AFT age of about ~94Ma and short track lengths of about 12µm. The timing of the deposition is ±23Ma and is contemporary with the first Neogene phase of reactivation in the Kyrgyz Tien Shan region.

The long lag time between the youngest age population (~70Ma, Figure 68) and the age of sedimentation indicates that the exhumation process was relatively slow during the Paleocene and Eocene until the Early

Oligocene. This confirms that the Paleocene and Eocene was generally a period of tectonic quiescence.

Three age populations can be distinguished in SK-47 using the RadialPlotter (Figure 58) and DensityPlotter

(Figure 68). A broad first peak of ~70Ma, a sharp second peak of ~110Ma and a last small peak of ~145Ma can be distinguished. These populations can be linked back to AFT ages that are preserved in the surrounding exposed basement (Figure 67). In that sense, this one Early Miocene sediment preserved three

Mesozoic age signals that were already observed in the adjacent exhumed basement of the Issyk Kul basin

(De Grave et al., 2013). The study of De Grave et al. (2013) concluded that there was a Late Jurassic – Early

Chapter 6: Discussion

Cretaceous group (150-130Ma), a Middle Cretaceous (110-90Ma) and Late Cretaceous age cluster (75-

65Ma), based on a large number of AFT and apatite (U-Th)/He samples taken in the Issyk Kul region. These age peaks are linked to distant effects of Late Triassic - Cretaceous accretion of three peri-Gondwanan blocks: Qiangtang, Lhasa and Karakoram (De Grave et al., 2013). The Miocene sample thus confirms earlier analysis on basement AFT data and illustrates the strength of single-grain age detrital AFT thermochronology. An overview of the available thermochronological data is presented in Figure 69, on which all basement AFT, basement zircon (U-Th)/He and sedimentary age populations results of this study are added on the compilation figure of Glorie and De Grave (2015).

Figure 68: DensityPlot of sample SK-47 with an adjusted bin width of 10Ma. Three age peak can be distinguished in this figure: a first peak of ~70Ma, a second peak of ~110Ma and a smaller peak around 145Ma.

6.2.3.3 KS 136, KS 137, KS 138 and KS 139

KS 136, KS 137, KS 138 and KS 139 are Lower-Jurassic samples from the Issyk Kul basin (Kadji-Sai section and Jeti Oguz section, Figure 67) in which no euhedral and rounded apatite grains were found. KS 139 fits in a sedimentary log that is presented in chapter 5. The absence of apatite in the samples of course made it impossible to perform AFT analyses. The detrital zircon U/Pb spectrum of Jurassic sediments on the SE side of the Issyk Kul basin consists mainly of Ordovician-Silurian sources (Macaulay et al., 2014) (see geological map in Appendix). This means that already some Ordovician and Silurian basement rocks were exposed to the surface in the Early Jurassic Issyk Kul region, but the lack of apatite in these four Jurassic samples could be caused by bad preservation.

Chapter 6: Discussion

6.3 Geodynamic events and thermochronology in the Kyrgyz Tien Shan The Song-Kul plateau (±3000m asl) located on the border of the NTS and the MTS (Figure 29) has preserved several Late Triassic, Early Jurassic AFT ages (De Grave et al., 2011). AFT data is scarce in this region and are only available in the surrounding mountain ranges of the Song Kul plateau and MTS (Table

11). Although the Early Mesozoic signal is often removed by later Mesozoic and Cenozoic erosion, various

Late Triassic – Early Jurassic AFT ages are present all in the MTS (east of TFF) (De Grave et al., 2011; Glorie et al., 2011; Macaulay et al., 2014). Meso- and Cenozoic sediments are almost completely absent in the

MTS, with the exception of the Late Paleogene – Early Neogene sediments (Kyrgyz Formation) at the edges of the Naryn basin and the Quaternary deposits in the central basin (Figure 60). Most of the AFT ages are obtained on basement samples at the borders of the MTS. Mean track lengths (MTL) are generally high (mean MTL = 13.0µm) and indicate a rapid cooling (Table 11), as indicated on Figure 69.

Table 11: Early Mesozoic (>175Ma) AFT age data on samples of the MTS. Unc. = 1 sigma error on the AFT age. MTL = mean track length in micrometers.

Sample Latitude (°N) Longitude (°E) AFT age (Ma) Unc. (Ma) Reference MTL (µm)

KYR-16 41°45'41" 75°09'38" 187.4 12.0 De Grave et al., 2011 11.5 KYR-17 41°43'01" 75°11'19" 206.0 13.9 De Grave et al., 2011 12.6 Al-71 41°03'12.6" 75°39°10.2" 175.1 7.9 Glorie et al., 2011 14.1 Al-69 41°02'59.0" 75°39°21.4" 187.8 8.9 Glorie et al., 2011 14.0 8TS358 41.4968 77.6238 218.3 24.4 Macaulay et al., 2014 12.7 8TS361 41.5036 77.6298 201.2 9.5 Macaulay et al., 2014 n/a 8TS362 41.5108 77.5876 240.7 19.8 Macaulay et al., 2014 n/a 7TS304 42.3102 79.6467 200.1 11.5 Macaulay et al., 2014 12.9

AFT analysis on Early-Jurassic sediments (KS 126 and KS 128) close to the Song-Kul plateau confirms the existence of a rapidly exhumed basement rock that sourced the Early Jurassic sediments (section 6.2.2).

Moreover, the geographical setting of the Minkush samples (at the edge of the NTS and close to the MTS and Song-Kul plateau, Figure 64) is ideal to detect the AFT signal of the eroding sediments of the MTS and

Song-Kul plateau. The entire MTS could have been the source of sediment for the Jurassic intramontane basins in the Kyrgyz Tien Shan (i.e. Issyk Kul basin and Fergana basin) (De Grave et al., 2011) and the adjacent Junggar and Tarim basin where kilometres thick sediment deposits are present (e.g. Hendrix,

2000). Non-reset Neogene sediments in the Alai basin of the STS also yielded an age component of ~230-

200Ma and a 165-145Ma age peak (De Grave et al., 2012). The future analysis of the detrital samples taken in the Tash Kumyr region (North Fergana basin, west of the TFF) could reveal an inherited (Mesozoic) AFT age component from the adjacent MTS. The geodynamical event responsible for the Late Triassic – Early

Chapter 6: Discussion

Jurassic topography building in the Kyrgyz Tien Shan (MTS) is considered to be the collision of Qiangtang

(Cimmerian unit) (~230-200Ma) with the Eurasian margin (Ratschbacher et al., 2003; De Grave et al., 2011;

Figure 69). This fast Late Triassic – Early Jurassic cooling phase is confirmed by TFT (Titanite Fission Track), zircon (U-Th)/He, AFT and apatite (U-Th)/He data (Figure 69). The Late Triassic – Early Jurassic exhumation in the Kyrgyz Tien Shan is believed to be a distant effect of the Qiangtang collision. The rapid exhumation is not only revealed by AFT dating of basement rocks, but can also be constrained by AFT dating on detrital sediments of intramontane basins.

Figure 69: Illustrated data of all thermochronological research on multiple thermochronometers, each sensitive to different temperatures. Major topography building events in the Kyrgyz Tien Shan constrained by thermochronology data are associated with the accretion of several microcontinents to Eurasia. Hiatuses in the sedimentary environment are indicated in red. Apatite (U-Th)/He (AHe) ages of the Kyrgyz Tien Shan are indicated with black diamonds, AFT data with white dots, zircon (U-Th)/He (ZHe) with white diamonds and titanite fission tracks (TFT) data are indicated with black dots. Several models and periods of increased cooling are indicated in the figure above. Basement samples of this work are indicated in red (with uncertainty bars). Sediment AFT age populations of the detrital samples are displayed in blue (with uncertainty bars). (Glorie and De Grave (2015)).

The majority of the AFT data and two Late Jurassic zircon (U-Th)/He ages obtained in this thesis are interpreted as a result of the Late Jurassic - Early Cretaceous Lhasa collision (~150-120Ma) and/or an isostatic response to the slab break-off of the subducting Bangong-Nujiang Tethyan lithosphere (120-

95Ma) that caused an extensional regime (with volcanic activity around ~110Ma) in northern and Central

Chapter 6: Discussion

Lhasa (Chen et al., 2014). Miocene detrital sediments of the Issyk Kul basin yielded AFT populations of

~145Ma and ~110Ma, representing the different AFT age peaks found in the basement of Issyk Kul. The

Miocene sediment also yielded a third age population of ~80-60Ma, that can be explained by basement exhumation as a distant effect of the Karakoram and Kohistan-Dras island arc collision or Greater-India collision and Pamir indentation around ~80-60Ma. This exhumation event is illustrated by three AFT ages of ~80-60Ma obtained on basement rocks obtained in this work.

Two AFT ages of ~53Ma indicate that the general tectonic quiescence during the Paleocene – Eocene was disturbed by a minor event of fast fault reactivation probably caused by the accretion of an island-arc system to the Eurasian margin, but the exact mechanism and the influence of the accretion of Greater

India is still debated (Glorie and De Grave, 2015).

Zircon (U-Th)/he and AFT data on basement rocks close to the TFF revealed that vertical movement along this fault was significant and caused by several accretion events. The MTS and NTS were uplifted and eroded as a result of this vertical component faulting activity. No Cretaceous sediments were able to accumulate/preserve on the east side of the TFF, while on the opposite side of the TFF, significant amounts of Cretaceous sediments accumulated as a result of the differential vertical movement of the TFF.

Cretaceous sediments accumulated on top of the Jurassic East Fergana strike-slip associated deposits. This deep burial succeeded to reset the AFT system of the Jurassic sediments, while the Cretaceous sediments are only partially reset by burial. Miocene tectonic reactivation of the TFF affected the East Fergana basin, potentially triggered by the Late Miocene ‘hard’ collision of India with Eurasia. This collision caused dextral fault movement of the TFF and reactivated several faults of the Kyrgyz Tien Shan (Bande et al., 2015). This

Miocene compressional phase resulted in the building of the modern topography of the Kyrgyz Tien Shan

(e.g. De Grave et al., 2012; De Grave et al., 2013).

Chapter 7: Conclusion Based on the results presented in chapter 5 and the accompanied discussion in chapter 6, the following conclusions can be drawn:

(1) Late Triassic – Early Jurassic dextral strike-slip movement of the Talas-Fergana Fault (TFF) led to the

formation of the East Fergana strike-slip basin in which kilometres of Jurassic sediments accumulated.

The AFT age of a Middle Jurassic sedimentary deposit such as sample KS13-22 is reset as a result of

deep sedimentary burial. On the opposite side of the TFF, Late Triassic – Early Jurassic AFT ages with

high mean track lengths are preserved in Early Jurassic samples (KS 126 and KS 128). Fast exhumed

regions with Late Triassic – Early Jurassic AFT age were the source of the Early Jurassic intramontane

basins. Detailed sedimentary logs of the Early Jurassic intramontane basins in the North Tien Shan

prove the existence of an eroding topography. This Late Triassic – Early Jurassic exhumation in the

Kyrgyz Tien Shan might be a distant effect of the Qiangtang collision with the Eurasian margin (~230-

200Ma).

(2) The movement of the TFF changed since the Late Jurassic – Early Cretaceous as a response to the

accretion of the (Cimmerian) Lhasa block to the Eurasian margins. Thermochronological data on

basement rocks and sediments revealed three different Cretaceous phases of exhumation in the

Kyrgyz Tien Shan related to accretion events. The first phase of exhumation is constrained by zircon

(U-Th)/He data – which has a higher closure temperature than the AFT system – and AFT age data on

both basement and detrital rocks that yielded ages of ~150-120Ma. This exhumation event is caused

by the accretion of the (Cimmerian) Lhasa block to the Eurasian margin. The majority of the AFT ages

of the basement and sediments preserved late Early Cretaceous ages (120-95Ma), which are believed

to be caused as a result of the isostatic response to the slab break-off from subducting Tethyan

Ocean lithosphere. The third Cretaceous exhumation phase at the end of the Cretaceous (~80-60Ma)

might be caused by the accretion of the Kohistan-Dras and Karakoram island arc (or the collision of

Greater India) to the Eurasian margin and Pamir indentation. This age peak was also found in multiple

AFT age data on basement rocks and sediments.

(3) Significant amounts of Cretaceous sediments west of the TFF and the absence of Cretaceous

sediments on the east side of the TFF indicates that the Kyrgyz Tien Shan east of the TFF was

differentially eroded in the Cretaceous and thereafter. The supposed Cretaceous activity of the TFF is

Chapter 7: Conclusion

now confirmed by abundant Cretaceous AFT ages of basement rocks and sediments adjacent to the

TFF.

(4) Paleogene stability and possibly subtle reheating is constrained by thermal history models of the

basement samples. The long lag time of the Earliest Miocene detrital sample (SK-47) of the Issyk Kul

basin is typical for a period of almost ~50 million years of general tectonic quiescence in the Kyrgyz

Tien Shan. An Early Eocene exhumation event only reactivated weaker fault zones, in which two

basement samples yield an Early Eocene (~53Ma) AFT age.

(5) Oligocene – Early Miocene (~25-20Ma) reactivation caused by the first pulse of the India-Eurasia

collision and the Pamir indentation shaped the modern architecture of the Kyrgyz Tien Shan.

Intensification in basement cooling in the Late Miocene (~12-8Ma) is linked to the reactivation of the

TFF leading to overthrusting in the East Fergana basin, constrained by a reset Jurassic sediment that

yield an AFT age of ~11.5±1.0Ma.

Although a basic tectonic framework for the complex history of the Kyrgyz Tien Shan is already established, important questions concerning the provenance of the sediments still need to be resolved.

Therefore, I would suggest the following research topics in decreasing order of importance:

(1) Geochronological analysis on the Cretaceous samples west of the TFF in the north Fergana basin

(Tash Kumyr region) could reveal multiple Mesozoic AFT age populations, if the system is not reset

through burial. Therefore it is advised to analyze sufficient apatite crystals (>50 grains) in order to

distinguish the different populations. Additional analysis of zircon U/Pb dating on these samples can

reveal the provenance of the sediment, leading to a reconstruction of the drainage pattern based on

source rock characterization with zircon U/Pb ages.

(2) Heavy ion bombardment of the AFT samples taken along the TFF could reveal more confined tracks,

from which better constrained thermal history models can be derived. Apatite (U-Th)/He analysis on

these samples can give some extra low-temperature constraints on the Cenozoic evolution of the TFF.

(3) The exact timing of deposition of the Upper Jurassic – Late Cretaceous massive conglomerate wall

widespread over the Tien Shan, Junggar and Tarim region is based on little evidence. Biostratigraphy

on the scarce fossils of the Cretaceous deposits is necessary to find the exact timing of deposition.

The reaction of the sedimentary environment to the three Cretaceous exhumation peaks is still

unclear.

100

Chapter 7: Conclusion

(4) Biostratigraphic dating by paleobotanical analysis of the Jurassic samples taken in the field campaign

of 2015 is necessary to reveal the exact age of deposition.

(5) Zircon U/Pb analysis on prepared zircon mounts of the Jurassic samples (of areas east of the TFF)

discussed in this thesis can be performed in the very near future. These samples probably preserved

Jurassic magmatic activity in the Tien Shan. The exact cause of this Jurassic peak is still undetermined,

but some researchers believe it is caused by the activity of a mantle plume through Central Asia.

101

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xii

Appendices

Appendices:

A. Geological Map of Central Asia and Adjacent areas (version 2008):

xiii

Appendices

xiv

Appendices

B. International Chronostratigraphic chart (version 2016)

Appendices

C. Complete AFT dataset of basement samples of the Kyrgyz Tien Shan

Latitude (°N) Longitude (°E) Sample name Age (Ma) Error (Ma) Reference 42.163 72.04 10TR02 22 3.1 Bande et al., 2015 42.138 72.041 10TR03 23 2.3 Bande et al., 2015 42.109 72.043 10TR04 25.4 2.9 Bande et al., 2015 42.122 72.037 10TR05 25.1 3.1 Bande et al., 2015 42.459 71.534 10TR17 25.2 2.8 Bande et al., 2015 42.233 71.407 10UM12 51.5 2.8 Bande et al., 2015 42.24 71.41 10UM14 27.5 3.2 Bande et al., 2015 42.242 71.413 10UM15 37.6 3.8 Bande et al., 2015 41.887 70.852 10CA26 40.9 4.4 Bande et al., 2015 41.881 70.854 10CA27 43.6 3 Bande et al., 2015 41.875 70.863 10CA29 30.1 2.3 Bande et al., 2015 42.199 71.586 10TR19 29.8 2.4 Bande et al., 2015 42.186 71.574 10TR20 27.4 3.5 Bande et al., 2015 42.184 71.57 10TR21 24.5 2.6 Bande et al., 2015 42.129 71.558 10TR22 26.4 2.9 Bande et al., 2015 42.12 71.547 10TR23 11 1.6 Bande et al., 2015 41.574 70.699 10CK34 124 6.6 Bande et al., 2015 41.186 71.024 10CK37 53.6 3.8 Bande et al., 2015 41.438 71.227 10CK41 120 6.7 Bande et al., 2015 41.584 71.527 10CK43 51 5.5 Bande et al., 2015 41.566 71.529 10CK44 64.7 5 Bande et al., 2015 41.531 72.332 10SU67 19.2 5.6 Bande et al., 2015 41.613 72.364 10SU65 20.8 1.9 Bande et al., 2015 41.59 72.329 10SU61 16.4 2 Bande et al., 2015 41.609 72.356 10SU62 23.9 2.1 Bande et al., 2015 41.623 72.368 10SU64 43 2.7 Bande et al., 2015 42°54'54.0" 76°13'05.2" SK31 85.1 4.8 De Pelsmaeker et al., 2015 42°55'10.2" 76°13'00.7" SK32 120.4 7.7 De Pelsmaeker et al., 2015 43°03'21.5" 76°58'59.2" 11-27 10.1 0.6 De Pelsmaeker et al., 2015 43°02'22.3" 76°56'40.0" 11-28 16.8 0.8 De Pelsmaeker et al., 2015 42°05'35" 75°07'11" Al-91 118.6 5.4 De Grave et al., 2011 42°05'03" 75°04'41" Al-92 154.3 11.3 De Grave et al., 2011 41°53'51" 74°49'25" Al-93 152 10.8 De Grave et al., 2011 41°50'37" 74°54'04" Al-97 201.1 12.3 De Grave et al., 2011 41°53'06" 75°01'05" Al-98 138 5.7 De Grave et al., 2011 41°55'56" 75°01'44" Al-99 195.6 11 De Grave et al., 2011 41°55'41" 75°02'15" Al-100 189.3 15.4 De Grave et al., 2011 41°55'08" 75°02'21" Al-101 194.9 11.6 De Grave et al., 2011 41°54'36" 75°02'48" Al-102 182.5 8.2 De Grave et al., 2011 41°45'41" 75°09'38" KYR-16 187.4 12 De Grave et al., 2011 41°43'01" 75°11'19" KYR-17 206 13.9 De Grave et al., 2011 40°03'59" 73°32'28" KYR-10 15.9 1.5 De Grave et al., 2012 40°15'24" 73°18'21" KYR-13 117.8 6.5 De Grave et al., 2012 39°45'50" 73°34'53" Al-40 8.4 0.3 De Grave et al., 2012 39°42'51" 73°39'56" Al-42 22.3 1.1 De Grave et al., 2012 40°07'47" 73°31'25" Al-37 18.8 1.6 De Grave et al., 2012 40°08'22" 73°32'07" Al-38 14 1.4 De Grave et al., 2012 N42°41'03" E075°53'21" TS-02 128.1 13.4 De Grave et al., 2013 N42°51'19" E076°34'39" TS-04 132.3 6.4 De Grave et al., 2013 N42°43'35" E076°49'51" TS-06 137.4 6.8 De Grave et al., 2013 N42°43'23" E076°50'37" TS-07 147 8 De Grave et al., 2013 N42°43'05" E076°50'38" TS-08 147.3 6.2 De Grave et al., 2013 N42°43'01" E076°50'59" TS-09 115.9 3.9 De Grave et al., 2013 N42°43'14" E076°51'31" TS-10 134 5.3 De Grave et al., 2013 N42°42'17" E076°51'51" TS-11 138.2 7.2 De Grave et al., 2013 N42°40'40" E076°51'03" TS-12 129.7 8 De Grave et al., 2013 N42°46'09" E077°31'25" TS-13 162.8 8.7 De Grave et al., 2013 N42°48'07" E077°31'50" TS-14 153.1 7.9 De Grave et al., 2013 N42°26'57" E075°51'39" TS-15 148.6 7.7 De Grave et al., 2013

xvi

Appendices

N42°27'06" E075°51'23" TS-16 110.6 6.3 De Grave et al., 2013 N42°27'13" E075°51'25" TS-17 120.2 6.4 De Grave et al., 2013 N42°27'51" E076°00'52" TS-18 126 7.5 De Grave et al., 2013 N42°07'11" E077°08'30" TS-19 107.5 16.5 De Grave et al., 2013 N42°05'03" E077°22'04" TS-20 61.9 3.9 De Grave et al., 2013 N42°03'00" E077°09'55" TS-22 69.3 7.2 De Grave et al., 2013 N42°03'12" E077°09'31" TS-23 72 5 De Grave et al., 2013 N42°03'16" E077°09'07" TS-24 102.6 8.6 De Grave et al., 2013 N42°04'14" E077°08'19" TS-26 78.4 6.6 De Grave et al., 2013 N42°05'41" E077°04'05" TS-27 99.4 5.7 De Grave et al., 2013 N42°07'30" E077°07'00" TS-28 85.7 10.7 De Grave et al., 2013 N43°21'00" E074°57'00" KAZ-01 147 7 De Grave et al., 2013 N43°14'00" E074°45'00" KAZ-03 162.2 9.1 De Grave et al., 2013 N42°21'28" E075°50'49" IK-01 81.3 3.4 De Grave et al., 2013 N42°18'35" E075°52'37" IK-02? 96.6 3.4 De Grave et al., 2013 N41°55'54" E075°44'14" IK-07? 75.2 11 De Grave et al., 2013 N42°25'22" E079°01'15" IK-11 129.8 9.1 De Grave et al., 2013 N42°25'51" E078°57'04" IK-12 38.6 14.4 De Grave et al., 2013 N42°27'07" E078°33'06" IK-13 26.3 1.3 De Grave et al., 2013 N43°09'48" E77°02'45" ALMA3-01 22.9 4 De Grave et al., 2013 N43°08'55" E77°03'33" ALMA3-02 29.4 1.4 De Grave et al., 2013 N43°07'30" E77°05'03" ALMA3-03 70.9 4 De Grave et al., 2013 N42°05'48" E077°04'06" KYR-33 115.8 20.8 De Grave et al., 2013 N41°58'11" E077°38'10" KYR-35 38.1 2 De Grave et al., 2013 N41°52'15" E077°44'10" KYR-38 108.4 5.7 De Grave et al., 2013 N41°54'07" E077°39'27" KYR-39 110.1 6.6 De Grave et al., 2013 N42°02'22" E077°35'52" KYR-42 17.6 0.8 De Grave et al., 2013 N42°02'49" E077°09''34" AI-01 64.1 3.2 De Grave et al., 2013 N42°02'28" E077°09'29" AI-04 68.2 3.6 De Grave et al., 2013 N42°02'33" E077°09'09" AI-05 5 0.8 De Grave et al., 2013 N42°04'06" E077°08'40" AI-09 68.9 7.9 De Grave et al., 2013 41°46'48" 74°16'01" TF-17 33.8 2.1 Glorie et al., 2010 41°49'12" 74°19'44" TF-18 104.5 4.9 Glorie et al., 2010 41°49'22" 74°19'26" TF-19 97.1 4.6 Glorie et al., 2010 41°57'57" 74°09'55" TF-20 124.7 7.4 Glorie et al., 2010 42°05'36" 74°07'32" TF-21 114.8 5.5 Glorie et al., 2010 42°07'09" 74°06'22" TF-22 125 9.8 Glorie et al., 2010 42°18'36" 73°50'14" KYR-03 128.2 12.8 Glorie et al., 2010 42°18'35" 75°52'37" IK-02 96.6 3.4 Glorie et al., 2010 41°51'34" 75°43'41" IK-05 125 6.5 Glorie et al., 2010 41°52'50" 75°43'09" IK-06 157.9 8 Glorie et al., 2010 41°55'54" 75°44'14" IK-07 75.2 11 Glorie et al., 2010 42°11'59.1" 79°06'58.1" Al-20 133.5 6 Glorie et al., 2011 42°06'40.0" 79°04'06.8" Al-15 23 1.3 Glorie et al., 2011 42°03'44.5" 79°05'02.7" Al-14 12.8 0.8 Glorie et al., 2011 42°03'51.3" 79°05'13.8" Al-13 7.9 0.5 Glorie et al., 2011 42°02'31.3" 79°06'09.6" Al-11 9.8 1.1 Glorie et al., 2011 42°01'11.2" 79°08'25.3" Al-16 58.9 2.7 Glorie et al., 2011 41°44'12.1" 78°03'59.1" Al-31 126.1 8.7 Glorie et al., 2011 41°42'52.9" 78°09'48.8" Al-29 56.5 3.9 Glorie et al., 2011 41°03'26.0" 75°39°05.3" Al-73 153.9 6.7 Glorie et al., 2011 41°03'21.1" 75°39°08.8" Al-72 173.8 13 Glorie et al., 2011 41°03'12.6" 75°39°10.2" Al-71 175.1 7.9 Glorie et al., 2011 41°02'59.0" 75°39°21.4" Al-69 187.8 8.9 Glorie et al., 2011 40°58'59.8" 75°35'58.1" Al-62 62.1 4.4 Glorie et al., 2011 40°49'39.0" 75°33'23.0" Al-75 21 1.9 Glorie et al., 2011 40°49'38.2" 75°33'23.5" Al-77 19.5 1.1 Glorie et al., 2011 40°32'17.8" 75°17'37.4" Al-74 47.4 2.5 Glorie et al., 2011 40°48'31.8" 76°15'46.2" Al-79 138.2 7.9 Glorie et al., 2011 40°59'08.5" 76°36'18.8" Al-82 139.5 10 Glorie et al., 2011 42°31.108' 74°52.425' TS159 6.9 0.6 Sobel et al., 2006 42°32.364' 74°52.178' TS158 4.7 0.7 Sobel et al., 2006 42°33.391' 74°53.351' TS162 4.4 0.6 Sobel et al., 2006 42°34.854' 74°54.187' TS163 5.4 2.8 Sobel et al., 2006

xvii

Appendices

42°37.912' 74°55.100' Mav38 3.9 0.7 Sobel et al., 2006 42°32.688' 75°48.867' TS84 150.2 7.6 Sobel et al., 2006 41.9859 77.6074 CP2 27.7 2.2 Macaulay et al., 2014 41.9842 77.6113 CP3 23.3 2.6 Macaulay et al., 2014 41.9832 77.6163 CP4 21.2 1.6 Macaulay et al., 2014 41.9766 77.6229 CP5 21.8 2.7 Macaulay et al., 2014 41.9703 77.6377 NS 28.2 3.8 Macaulay et al., 2014 42.5433 78.9368 7TS330 21.9 2.2 Macaulay et al., 2014 42.5476 78.9362 7TS331 19.1 1.4 Macaulay et al., 2014 42.5517 78.938 7TS332 15.7 1.7 Macaulay et al., 2014 42.5609 78.9268 7TS333 17.9 1.8 Macaulay et al., 2014 42.5322 78.9172 SJTC-6 10.7 1.3 Macaulay et al., 2014 42.5012 78.9362 7TS328 17.2 1.9 Macaulay et al., 2014 42.4436 78.9584 9TS456 50.2 7.4 Macaulay et al., 2014 42.4345 78.9501 SJTC-5 46.7 14.9 Macaulay et al., 2014 42.4498 78.9476 9TS458 38.4 5.5 Macaulay et al., 2014 42.4173 78.9504 9TS452 37.9 6.9 Macaulay et al., 2014 42.0054 76.3093 8TS411 64.7 3.7 Macaulay et al., 2014 42.001 76.3199 8TS413 21 2.3 Macaulay et al., 2014 41.9758 76.3235 8TS414 144.8 10.5 Macaulay et al., 2014 41.9673 76.2935 8TS410 107.5 9.2 Macaulay et al., 2014 41.8735 77.7219 SP1 92.5 4.9 Macaulay et al., 2014 41.8945 77.6993 SP2 40.7 29.5 Macaulay et al., 2014 41.8956 77.6874 SP3 40.2 11.3 Macaulay et al., 2014 41.9038 77.6537 SP4 26.9 3.4 Macaulay et al., 2014 42.2718 78.8378 9TS467 72.9 6.5 Macaulay et al., 2014 42.2717 78.8452 9TS468 67.4 12.1 Macaulay et al., 2014 42.2681 78.8495 9TS469 73.3 10.4 Macaulay et al., 2014 42.2662 78.858 9TS470 70 6.8 Macaulay et al., 2014 42.2566 78.8828 9TS471 79.4 4.4 Macaulay et al., 2014 42.3888 79.0607 SJTC-4 81 3.3 Macaulay et al., 2014 42.3621 79.0455 SJTC-3 88 3.4 Macaulay et al., 2014 41.8388 76.4693 8TS408 115.3 6.6 Macaulay et al., 2014 41.7736 76.5406 8TS406 130.4 6.3 Macaulay et al., 2014 41.7349 76.751 8TS402 154.9 6.7 Macaulay et al., 2014 41.6968 76.7163 8TS401 124.4 9.6 Macaulay et al., 2014 41.6648 76.9755 8TS398 124.1 7.3 Macaulay et al., 2014 41.6722 77.0178 8TS394 125.8 6.6 Macaulay et al., 2014 41.4505 77.3417 8TS369 122 6.5 Macaulay et al., 2014 41.4493 77.3357 8TS370 53.8 76.3 Macaulay et al., 2014 41.4548 77.3285 8TS371 122.6 7.3 Macaulay et al., 2014 41.4609 77.3254 8TS372 121.1 5.8 Macaulay et al., 2014 41.4502 77.3159 8TS368 107.9 6.7 Macaulay et al., 2014 41.4968 77.6238 8TS358 218.3 24.4 Macaulay et al., 2014 41.4986 77.6227 8TS359 155.9 8.7 Macaulay et al., 2014 41.5036 77.6298 8TS361 201.2 9.5 Macaulay et al., 2014 41.5108 77.5876 8TS362 240.7 19.8 Macaulay et al., 2014 42.1212 79.0924 9TS461 88.3 9.5 Macaulay et al., 2014 42.1261 79.0931 9TS463 20.6 24.5 Macaulay et al., 2014 42.1291 79.08 9TS465 44.7 7.7 Macaulay et al., 2014 42.1743 79.0937 9TS466 75.2 7.7 Macaulay et al., 2014 42.0541 79.1014 8TS431 16.6 2.2 Macaulay et al., 2014 42.0535 79.0981 8TS432 21.5 4.8 Macaulay et al., 2014 42.0545 79.0944 8TS433 18.7 4.4 Macaulay et al., 2014 42.0548 79.0944 8TS434 15.3 2.8 Macaulay et al., 2014 42.0545 79.0913 8TS436 14.6 3.8 Macaulay et al., 2014 42.0545 79.0912 8TS435 12.4 2.1 Macaulay et al., 2014 42.0542 79.0847 8TS437 13.7 3.3 Macaulay et al., 2014 42.0569 79.0807 8TS438 23 3.7 Macaulay et al., 2014 42.0426 79.0758 SJTC-1 6.9 0.7 Macaulay et al., 2014 42.0695 79.0833 SJTC-2 11.3 1.7 Macaulay et al., 2014 42.2772 79.7506 7TS302 96.1 7.7 Macaulay et al., 2014 42.2998 79.7627 7TS305 110.9 12 Macaulay et al., 2014 42.3102 79.6467 7TS304 200.1 11.5 Macaulay et al., 2014

xviii

Appendices

42.3338 79.6848 7TS308 161.6 16.2 Macaulay et al., 2014 42.0782 79.275 8TS427 103.1 15.1 Macaulay et al., 2014 42.0796 79.2755 8TS428 68.2 62.5 Macaulay et al., 2014 42.0814 79.2463 mav65/8 105.2 8.6 Macaulay et al., 2014 42.0882 76.3906 8TS419 87.2 11.8 Macaulay et al., 2014 42.0909 76.3803 8TS420 188.8 54.8 Macaulay et al., 2014 42.0901 76.3662 8TS421 111 10.9 Macaulay et al., 2014 41.9154 77.068 9TS474 62.9 93.8 Macaulay et al., 2014 41.9141 77.0598 9TS475 87.6 4.7 Macaulay et al., 2014 42.0563 77.61 NP1 17.3 1.4 Macaulay et al., 2014 42.0624 77.6056 NP2 17.9 1.7 Macaulay et al., 2014 42.065 77.6024 NP3 6.8 9.1 Macaulay et al., 2014 42.0677 77.6026 NP4 8.3 7.6 Macaulay et al., 2014 41.9831 77.6009 CP1 38.9 2.5 Macaulay et al., 2014 41.9397 77.6564 MAV 104 58.5 6.6 Macaulay et al., 2014 212-12 20.3 5.5 Bullen et al., 2001 212-13 103.2 11.3 Bullen et al., 2001 212-8 15.9 2.2 Bullen et al., 2001 212-9 14.7 1.7 Bullen et al., 2001 98-30 10.5 2.1 Bullen et al., 2001 212-14 11.5 1.3 Bullen et al., 2001 212-16 15.6 2.3 Bullen et al., 2001 98-33 10.8 2.7 Bullen et al., 2001

xix