Lateral climatic variation in the Himalaya since the Miocene

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Major tectonic units

The Himalaya south of the Indus-Tsangpo suture zone is classically divided into four major lithotectonic units, the Tethyan Sedimentary Series (TSS), the High Himalayan Crystalline Series (HHCS), the Lesser Himalayan Series (LHS) and the Sub-Himalayas (SH). These lithotectonic units are bound by a late-Cenozoic north dipping fault system, which constists from north to south, the South Tibetan Detachment (STD), the Main Central Thrust (MCT), the Main Boundary Thrust (MBT) and the Main Frontal Thrust (MFT) (DeCelles et al., 2001; Le Fort, 1986; Yin and Harrison, 2000). The Himalayan units are bound by the Indus-Tsangpo suture zone (ITSZ) north of the South Tibetan Detachment system and the Himalayan foreland basin (Indus, Ganges, Brahmaputra) in the south (Figure 1.2)
The Indus-Tsangpo suture zone (ITSZ), which separates the Indian and Asian crusts, consists of sedimentary rocks, melange, ophiolites of the Neotethys ocean and the Cretaceous-Tertiary Gangdese Batholith. Two Cenozoic thrusts form the boundaries of the suture zone (Yin and Harrison, 2000).
The Tethyan Sedimentary Series (TSS) is a Paleozoic-Eocene sedimentary succession deposited on the northern passive margin of India (e.g. Gaetani and Garzanti, 1991). The TSS consists of sedimentary and low grade metasedimentary rocks: mainly phyllites, limestones and quartzose sandstones (DeCelles et al., 2001). They are interbedded with Paleozoic and Mesozoic volcanic rocks (Yin, 2006). The TSS are located between the ITSZ and the STD.
The Higher Himalayan Crystalline Series (HHCS) are bound by the STD in the north and the MCT in the south. They consist of high grade metamorphic rocks including paragneiss, schist, migmatite, marble and orthogneiss (DeCelles et al., 2001). Miocene leucogranites are widespread in the HHCS (e.g. Gansser, 1964).
The MCT and the MBT are the boundaries of the Lesser Himalayan unit in the north and the south, respectively, with an internal duplex structure (DeCelles et al., 1998a). Rocks of the LHS mainly consist of Proterozoic-Cambrian clastic sediments, metasedimentary and metavolcanic rocks and some augengneiss (Yin, 2006)). The LHS has been sub-divided into the Inner (iLH) and Outer (oLH) Lesser Himalaya, based on depositional age and different Nd isotopic signatures. The Lower-Proterozoic iLH has a more negative Nd, whereas the Middle-Upper Proterozoic oLH has Nd similar to the HHCS (Ahmad et al., 2000). Locally the LHS is crosscut by mafic and felsic intrusions (Guillot et al., 2008).
The rocks of the southernmost Sub-Himalayan unit are non-metamorphosed syntectonic clastic sediments of the pre-Siwalik and Siwalik Groups, which formed in the Himalayan foreland basin and are thrusted over the Indus-, Ganges- and Brahmaputra- alluvial plains along the MFT. They crop out in the foothills of the Himalaya and span the entire mountain front.

Foreland basin sediments – Pre-Siwaliks/ Siwaliks

The oldest pre-Siwalik sedimentary rocks in the Himalayan foreland basin are the late Oligocene/ early Miocene Dharamsala Group in north western India and the Dumri formation in Nepal (DeCelles et al., 1998a) They consist of continental fluvial, lacustrine or deltaic sediments (White et al., 2001). Their sections are indicated in red. JN: Joginder Nagar; JW: Jawalamukhi; HK: Haripur Kolar; KM: Kameng.
deposition is followed by the deposition of the Siwaliks, which form the foothills of the Himalaya and reach along the entire mountain front from Pakistan to the northeastern states of India. They represent the Miocene to Pliocene fill of the Himalayan foreland basin, with some longitudinal age variation along strike (Chirouze et al., 2012). An overall thickening of beds and coarsening upward is observed over the entire Siwalik Group and is interpreted to reflect a gradual transition from distal to proximal facies, as the Himalayan deformation propagated. They are divided into the Lower, Middle and Upper Siwaliks, with gradual boundaries. These informal subgroups locally correspond to formations with varying names. The Lower Siwaliks (LS) consist of an alteration between mudstone with some paleosols and fine- to medium-grained sandstone. The layers are a few meters thick. The LS is associated with a depositional environment of high-sinuosity streams and overbank deposits. The Middle Siwaliks (MS) are a stack of tens of meters thick medium- to coarse-grained sandstone beds often rich in mica deposited by a large braided river system. The Upper Siwaliks (US) contain beds of tens of meter thick conglomerates with interlayers of sand- and less frequently mudstone. They were deposited closer to the mountain front by a gravelly braided river system (Chirouze et al., 2013; Nakayama and Ulak, 1999). Paleosols within the pre-Siwalik and Siwalik sediments are associated with soil carbonate nodule. The abundance of soil carbonate nodules varies laterally, and such nodules have never been reported further east than Nepal. For this study three pre-Siwalik and Siwalik were sampled in north western India and one Siwalik section in north eastern India (Figure 1.2) and are described in following sub-chapters.

Foreland basin sediments – Pre-Siwaliks/ Siwaliks

Joginder Nagar, Jawalamukhi and Haripur Kolar sections, Himachal Pradesh, NW India

The three sedimentary sections of Joginder Nagar (JN) (pre-Siwalik), Jawalamukhi (JW) and Haripur Kolar (HK) (both Siwalik) indicated on Figure 1.2 have been dated by magnetostratigraphy and span from 20-1 Ma (Meigs et al., 1995; Sangode et al., 1996; White et al., 2001). The Joginder Nagar section (Figure 1.3) is mostly pre-Siwalik and contains rocks of the Lower and Upper Dharamsala Formations. The uppermost unit is composed of Lower Siwalik rocks/sediments. Boundaries for the Lower/Upper Dharamsala are set at 16.5 Ma and at 12.5 Ma for the Upper Dharamsala/LS (White et al., 2001). The Dharamsala Formation consist of fine- to medium- grained sandstone, siltstones and mudstones, associated with development of paleosols, with abundant soil carbonate nodules (White et al., 2001).
In the Jawalamukhi section, LS, MS and US rocks are present (Figure 1.4), whereas the Haripur Kolar section contains only MS and US sediments (Thomas et al., 2002). The boundary between the LS and MS in the JW section is set at 10.9 Ma and between MS and US at 6.8 Ma, respectively (Meigs et al., 1995). The Jawalamukhi section contains a coarsening upward sequence with beds of mudstone, siltstone and sandstone at the base of the section and conglomerate becoming more abundant towards the top (Meigs et al., 1995; Najman et al., 2009). In the Jawalamukhi section the MS are characterized by the abundance of conglomerate, which is not abundant in other Siwalik sections. Paleosols are developed and carbonate nodules are present.
In the HK section (Figure 1.5) the boundary between the MS and US is set at 5.23 Ma (Sangode et al., 1996). Sediments of the HK section are characterized by alternating mudstone and sandstone. Mudstones are associated with well-developed paleosols with presence, of carbonate nodules Thomas et al. (2002). Figure 1.6 shows characteristic sedimentary features of the sections.
Figure 1.5: Haripur Kolar section location map (UTM Zone 43N); magnetostratigraphy after Sangode et al. (1996); the stratigraphy was described by Thomas et al. (2002).
Figure 1.6: Paleosols of the Haripur Kolar section near HK14-2; B: Upper Siwalik conglomerates of the Jawalamukhi section near JW14-7; C: Paleosol of the Joginder Nagar section near JN14-28; D: Soil carbonate nodule in a paleosol of the Joginder Nagar section near JN14-10.

Kameng section, Arunachal Pradesh, NE India

The Kameng River Section is located near the town of Bhalukpong, Arunachal Pradesh in North Eastern India (Fig 5.1). The modern Kameng River drains the HHCS and the LHS before it flows through the Siwaliks and finally enters the floodplain where it flows into the Brahmaputra River. In Arunachal, the Siwaliks are divided into the Dafla, Subansiri and Kimin Formations (Karunakaran and Rao, 1976; Kumar, 1997) which have been correlated with the LS, MS and US, respectively (Yin, 2006). The Kameng River Section has been magnetostratigraphically dated by Chirouze et al, (2012) and spans from 13 to 1 Ma. The exposed LS range from ca. 13 Ma to 10.5 Ma. The MS/US boundary was set at 2.6 Ma (Chirouze et al., 2012). The Kameng River Section is approximately 6 km thick and bound by the MBT in the north, and the MFT in the south, placing Siwalik rocks onto Quaternary sediments of the Brahmaputra plain (Burgess et al., 2012). Within the Kameng section the Tippi Thrust places the LS over the US, therefore the younger part with MS and US is found in the footwall of the Tippi thrust to the south, whereas the older part of the section is in the hanging wall to the north (Chirouze et al., 2013) (Figure 1.7). The LS contain layers of mudstones alternating with sandstones. The MS are characterized by tens of meters thick layers of medium- to coarse-grained “salt and pepper” sandstones. The US consist of sandstone with more frequent layers of conglomerate. Mudstones become less abundant. Generally, paleosols are poorly developed and soil carbonate nodules are lacking in the Kameng section. Figure 1.8 shows characteristic sedimentary features of the Kameng section.

Table of contents :

Introduction 
1 Geological setting 
1.1 Evolution of the Himalayan orogen – India-Asia collision
1.2 Major tectonic units
1.3 Foreland basin sediments – Pre-Siwaliks/ Siwaliks
1.3.1 Joginder Nagar, Jawalamukhi and Haripur Kolar sections, Himachal Pradesh, NW India
1.3.2 Kameng section, Arunachal Pradesh, NE India
2 Lateral climatic variation in the Himalaya since the Miocene: aWest – East comparison 
2.1 Introduction
2.2 Geological Setting and methods
2.3 Results
2.4 Discussion and conclusions
3 Weathering regime in the Eastern Himalaya since the mid-Miocene: Implication from detrital geochemistry and clay mineralogy of the Kameng River Section, Arunachal Pradesh, India 
3.1 Introduction
3.2 Geological Setting
3.2.1 Himalayan Geology
3.2.2 Foreland basin sediments – The Siwalik Group
3.3 Sampling and Methods
3.3.1 Sampling strategy
3.3.2 Methods
3.3.2.1 Heavy Minerals and petrography
3.3.2.2 Clay minerals
3.3.2.3 Major elements
3.4 Results
3.4.1 Heavy minerals and petrography
3.4.2 Clay minerals
3.4.3 Major elements and H2O+
3.5 Discussion
3.5.1 Provenance evolution of the Kameng sediments
3.5.2 Diagenesis
3.5.3 Weathering in the Eastern Himalaya since 13 Ma
3.6 Conclusion
3.7 Supplementary Information
3.7.1 Heavy mineral and petrography: report
3.7.1.1 Method: Sand/stone petrography
3.7.1.2 Method: Heavy minerals
3.7.1.3 Results
3.7.2 n-alkane analysis on the Kameng river section
3.7.2.1 Introduction
3.7.2.2 Method
3.7.2.3 Results
3.7.2.4 Discussion
3.8 Appendices
4 Weathering in the Himalayas, an East-West comparison: Indications from major elements and clay mineralogy 
4.1 Introduction
4.2 Geological Setting
4.3 Sampling and Methods
4.3.1 Sampling strategy
4.3.2 Methods
4.3.2.1 Clay mineralogy
4.4 Results
4.4.1 Clay mineralogy
4.4.2 Major elements
4.5 Discussion
4.5.1 Provenance and diagenesis in the western sections
4.5.2 West-east comparison of weathering regimes
4.6 Conclusions
5 Lithium isotope record of the Siwalik Group- a new approach to studying paleo-weathering regimes 
5.1 Introduction
5.2 Sampling and methods
5.2.1 Sampling
5.2.2 Method
5.3 Results
5.4 Discussion
5.5 Preliminary conclusions and perspectives
Conclusions and perspectives 
Bibliographie 
A Sampling points
A.1 Kameng
A.2 Joginder Nagar
A.3 Jawalamukhi
A.4 Haripur Kolar
A.5 Modern Rivers
B Water Analysis of Eastern Himalayan Rivers

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