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CHAPTER-1

Introduction

Chapter -1 Introduction

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CHAPTER-1

INTRODUCTION

1.1. Introduction

Mountain belts formed by continent-continent collision are perhaps the most

dominant geologic features of the surface of the Earth (Dewey and Burke, 1973). The

Himalaya is a classic example of an orogenic system created by continent–

continent collision ( Dewey and Bird, 1970; Dewey and Burke, 1973; Molnar and

Tapnier, 1975; Replumaz and Tapponnier, 2003; Fournier et al., 2004; Najman et al.,

2010; Hall, 2012) and the Himalaya formed by huge tectonic forces contain evidence

of the complete Wilson cycle from the Mesozoic to the Eocene, followed by post-

collisional deformation that is still active. The Himalayan-Tibetan orogeny originated

when the Tethys ocean subducted northward beneath the Asian plate, and the crust of

the Indian and Asian plates began to collide at ~ 55 Ma (Powell and Conaghan, 1973;

Coward and Butler, 1985). Himalaya has extension over 2500 km from north-west

(33o15'N, 74o36'E) to south-east (29o37'N, 95o15'E) strike with an average width

along the entire longitudinal extension ranging from 100-400 km. In the northern side,

Indus-Tsangpo Valley separates the main Himalaya from the Trans-Himalaya. Its

youthfulness and incredible exposure make the orogen best for studying various

geologic processes related to mountain building. Its potential as a guide to interpret

the feedback processes between lithospheric deformation and atmospheric circulation

has encouraged intense research in recent years on the history of the Himalayan

orogen, it has played a significant role in global climate change, and its interaction

with erosion (Harrison et al., 1998; Molnar et al., 1993; Royden et al., 1997;

Ramstein et al., 1997; Tapponnier et al., 2001; Beaumont et al., 2001; Yin et al.,

2002; Yi et al., 2011). Owing to scientific interest, the Himalayan fold-and-thrust

belts have been extensively studied since 1950 after the Himalayan territory was

opened. According to Valdiya (1988), the various postulations on evolution of the

Himalayan Mountains can be put into two categories in which one school of thought

attributes the origin to vertical movements and attendant block faulting along deep

faults and fractures which also served as channel ways for the granitic magmas

(Van Hinsbergen et al., 2011) and the other view is that the orogen came into

existence as a result of horizontal compression of marine sediments, the compression

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resulting from northward drift of the Indian subcontinent and colliding with the

Eurasian plate, the Indus-Tsangpo zone representing the junction of the two

continents (Ali and Aitchison, 2005; Gibbons et al., 2012). India and Asia continued

convergence at the rate of 5 cm/yr estimated from the magnetostratigraphy (Patriat

and Achache, 1984), and the collision was accommodated by major faults along the

Himalaya (Brunel et al., 1983; Macfarlane et al., 1993; Hodges et al., 1996, 2000;

DeCelles et al., 1998a). So south of the suture zone lies the Himalayan thrust belt

which consists of series of south vergent, southward propagating thrust faults (Fig.

1.1) that developed in response to ongoing subduction of Indian plate beneath the

Asian plate (Gansser, 1964; Coward and Butler, 1985; Searle, 1991; Srivastava and

Mitra, 1994; Yin and Harrison, 2000). Because of the ongoing convergence, uplift,

and climate interactions, the Himalayan orogenic system may be the world’s best

geological field laboratory and is the focus of integrated research involving structural

geology, sedimentology, thermobarometry, geochronology and geophysics.

Fig.1.1. Simplified Tectonic map of the Himalayan Orogen (modified after Arora et al., 2012).

The information regarding the history of the collision between India and

Eurasia (i.e. when the last oceanic lithosphere was subducted and continental

lithosphere comes into contact with other continental lithosphere) can be extracted by

examining the timing of deformation, metamorphism, erosion and sedimentation

within the collisional belt (Searle et al., 2003; Aitchison et al., 2007; Guillot et al.,

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2008; Metcalfe, 2013). In view of some authors, the evolution of the orogen involved

some distinct accretion events (Whitmarsh et al., 2001; Aitchison et al., 2007), while

others suggested a single collision event followed by a expanded history (Searle et al.,

1992, 1999; Vance and Harris.,1999; Noble et al., 2001; Walker et al., 2001;

Beaumont et al., 2004; Jamieson et al., 2006; Leech, 2008). These controversial

matters could be determined by increasing detail in terms of the analysis of what geo-

chronological and structural data within the orogen reveals in terms of the evolution

of its tectono-metamorphic stratigraphy, and of its architecture. Alternatively, the

impact of individual accretion events might be evident in plate reconstructions of the

relative motion of India to Eurasia applying ocean floor magnetic anomaly data

(White and Lister, 2012). One key piece of evidence applied to establish when the

collision of the two continents occurred is plate reconstructions of India’s motion

relative to Eurasia. Molnar and Tapponnier (1975) were the first to suggest that a

decrease in the rate of northward movement of India from 100–112 mm/year to 45–65

mm/year at ∼40 Ma represented the collision of India and Eurasia. Consequently,

plate reconstructions also observed a decline in the relative motion of India relative to

Africa, Antarctica and Eurasia (Dewey et al., 1989; Molnar et al., 1988; Patriat and

Achache, 1984; Patriat and Segoufin, 1988). Although there were differences in each

of these models, they all attribute the deceleration of the Indian plate between 55 and

36 Ma to the collision of India and Asia (Jain, 2014) and is consistent with geological

observations that suggest substantial changes occurred in the Himalayan orogen

during this time period (e.g., Rowley, 1996; Guillot et al., 2003). Van Hinsbergen et

al., (2011) suggests the deceleration of India relative to Eurasia may be related to

something other than the collision of the two continents. These researchers

highlighted that India’s motion increased at ∼90 Ma and between ∼65 and 50 Ma.

They suggested that plate acceleration and deceleration could be related to plume

head arrival and increasing continent-plume distance respectively.

Studies along the Himalayan arc that employ an understanding of the

structural architecture using the concepts of fold-thrust belt development (Dhalstrom

et al., 1969; Boyer and Elliott, 1982) have been conducted in Pakistan (Coward and

Butler, 1985), northern India (Srivastava and Mitra, 1994), eastern Nepal (Schelling

and Arita, 1991; Schelling, 1992), western Nepal (DeCelles et al., 2001; Robinson,

2006; Robinson et al., 2008), central Nepal (Pearson, 2002) and western Bhutan

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(McQuarrie et al., 2008). These orogen-scale studies provide a useful method for

understanding the deep structures of the mountain belt and calculating an amount of

upper crustal shortening after the Indo-Asia collision. The shortening values reported

on the above studies can be used to identify along-strike variability of structures and

amount of shortening. These variations in shortening might explain the response of

lithosphere to collision and location of maximum deformation in the Himalaya. These

mountain building activities involve the accumulation of stress and these accumulated

stress are released in the phased manner which leaves behind imprints, in the form of

different patterns of structural elements. The imprints shaped by different

deformational episodes are present as signatures of Himalayan and pre-Himalayan

orogens. Different models have been given by different workers concerning the

Himalayan orogeny from time to time on the basis of different criteria. The

compressional tectonics in the Himalayan region is an accepted fact of field geology

but a number of geological facts, for example limited width (

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geologic processes related to mountain building. The Himalaya forms one of the

famous and strongest features in the topography of the world. Himalayan range

outline the Indian subcontinent in a massive 2500 km arc, an icy barrier between the

tropical India and the highlands of Central Asia and lies between its eastern and

western Syntaxis by the Namche Barwa and Nanga Parbat peaks (Fig.1.2).

Fig.1.2. Digital elevation model for the Himalaya. Note the steep front of the Himalayan range towards the South and the huge Tibetan plateau in the North. The two syntaxes near the Nanga Parbat (left) and the Namche Barwa (right) are nicely visible (Yin, 2006). White mark shows the location of