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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

  • Chapter -1 Introduction


    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.,

  • Chapter -1 Introduction


    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

  • Chapter -1 Introduction


    (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 (

  • Chapter -1 Introduction


    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

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