Understanding an Orogenic Belt

The chapter provides basic information about stress and different types of strain. Significance of orientation of layering with respect to stress orientation is described. Homogeneous and inhomogeneous strains are illustrated. Flinn diagram is described to represent shapes of three-dimensional strain ellipsoids on a two dimensional diagram. Different methods for determination of finite strain are based on certain assumptions that should be kept in mind while interpreting the results. The relevant precautions are briefly described such as: (i) Strain data obtained from one rock type is generally not representative of the regional or bulk strain ellipsoid. Strain values can also vary along a fold profile depending on the mechanism of folding. (ii) When a method is based on measurement of grain shapes, competence contrast between the grain and matrix should be minimum otherwise only the matrix will undergo deformation (if least competent) and the grain will not reveal the true strain ellipsoid. Similarly a deformed fossil will indicate strain suffered by the fossil, not by the entire rock. (iii) Reduction spots can be used as strain markers provided the reduction pre-dates the strain. (iv) Methods based on measurement of buckle shortening ignore initial layer parallel strain (homogeneous shortening) that has occurred prior to development of the fold. (v) Magnetic strain, obtained by anisotropy of magnetic susceptibility method, provides bulk strain, i.e. of the entire rock sample including the matrix. Hence petrofabric and magnetic strains can differ noticeably.

Anisotropy of magnetic susceptibility is an important technique which depicts preferred orientation of magnetic minerals in a rock or unconsolidated sediments. Hence the property is used for study of primary structures and rock fabric. The technique is non-destructive and can be used in nearly all types of rocks because it does not need a rock to contain specific strain markers like deformed fossils, reduction spots, ooids, etc. The method has an advantage as it can determine weak deformation even where lineation and foliation have not developed. In rocks with well developed tectonic fabrics, the principal magnetic susceptibility directions are closely related to orientation of structural features (e.g. fold, fault, foliation, lineation). Different types of AMS fabrics are described. Differences between magnetic and petrofabric strains are highlighted. Importance of sampling in a region of superimposed deformation is described. It is emphasized that objectives of the study should be formulated prior to selection of sample sites. Hrouda diagram is described for understanding the roles of simple and pure shear deformations in a region of simultaneous development of folding and thrusting. The technique has been successfully employed to ascertain the displacement patterns along some of the prominent Lower Himalayan thrusts.

Some of the basic information related to development of folds are provided, e.g. why, where, and how do they develop. Fold propagation and fold geometries resulting from interference of simultaneously developing folds are described in two and three dimensions along with causes of noncylindricity. Variations of fold geometry along the hinge line, and with depth along the axial surface are shown by natural examples. Attention is drawn to importance of culmination point along a fold hinge line. Sheath and eyed folds are described as separate entities formed by different mechanisms. Late stages in the development history of folds are explained by using deformed physical models. Significance of buckle shortening and layer parallel strain across fold profiles is highlighted. Development of boudins, normal, and thrust faults on fold limbs are discussed. The sequence of deformation suggests that the second order folds should not be described as parasitic folds because the term does not have a correct genetic implication. The chapter highlights the importance of folding in mountain building and how a clear understanding of early and superposed folding can help in resolving the complete deformation history of a region.

Thrust fault and its classification is described. Formation of normal and reverse drags is explained in terms of frictional effects along the thrust surface. Flat and ramp, and smooth trajectory thrusts are illustrated along with the associated cleavage patterns. Evolution of planar and listric faults is depicted in an inverted basin. A new model is proposed for development of duplex structures. Decollement upwarps have been identified in the foreland basin of the Himalaya and a model is proposed for their evolution. The model implies that formation of upwarp structures inhibits displacement along basal decollements. Formation of oblique thrust ramps and associated structures are highlighted. A new model, called as basement wedge klippe model, is proposed for formation of klippe structure. Problems related to restoration of deformed sections are described. An unrestorable section does not mean a wrong construction. The thin-skinned model for evolution of fold and thrust belts may not be universally applicable to all the orogenic belts.

Initiation, propagation, and modification of normal fault geometry from planar to sigmoidal shape are described. How normal faults of one generation can have a variety of dips, geometry and displacements are discussed. Relay ramps and bookshelf gliding are illustrated. Co-existence of normal and (pseudo) thrust faults are shown. Formation of half grabens, angular unconformities and associated structures are shown diagrammatically. Displacement patterns of planar and listric (both positive and negative) faults are described. Models of lithospheric extension are highlighted. Finally, identification of a rift phase by geochemical methods is discussed.

Stress orientation responsible for formation of strike slip fault is described along with rheological controls on development of the fault. Conjugate strike slip faults are discussed with special reference to necessity of rotation of faults with progressive deformation. Variation of displacement along the fault length and how this results in different types of fault terminations under different rheological conditions are discussed. Transpression and transtension zones occurring at curvature of strike slip faults are described. Positive and negative flower structures are illustrated. Relationship between folds and strike slip faults is discussed using natural example of the Jura mountains. Development of oblique fault ramps and significant criteria to distinguish between strike slip faults and oblique fault ramps are described.

A large amount of information is now available on the development of folds and faults but these structures have been studied in isolation and their simultaneous development has not been studied in detail. The simultaneous development of folds and faults has been described under the following headings.

Global positioning system is now generally used during field investigations for obtaining precise locations but more importantly it has been employed to determine the plate movements. The results are used for determining the strain build-up in an area and seismic predictions. However, the inferences are based upon surface observations whereas geological structures require study in three-dimensions. Velocity vectors are determined using a few fixed points. However, it is not possible to find fix points on the surface of the moving plates. Hence the velocity vectors can change their magnitude and direction with change in location of the fix points, leading to incorrect conclusions. Hence extra precautions are required when fix points are selected on moving surfaces. Some of these precautions are discussed in the light of an experiment where a physical model was deformed under controlled boundary conditions. Importance of location of GPS stations in an active orogenic belt, like the Himalaya, is emphasized where all the three types of faults (i.e. thrust, normal, strike slip) are developing simultaneously.

A brief introduction of the Himalaya is given. Evolution of the Himalaya began with break-up of the supercontinent Gondwanaland into Antarctica, Africa, Australia, and India around 140 Ma. The Indian plate moved towards north and the Indo-Eurasian collision took place somewhere between ~65 and ~43 Ma. The Himalaya started rising and led to monsoonal rains in the Indian region. Now the mountain has a vast reserve of fresh water in form of ice. It supports thick forests and is a source of many perennial rivers of north India. However, despite of several blessings, the active mountain has few inherent dangers as well, e.g. seismicity, landslides, floods, glacial lake outburst, etc. Hence the study of Himalaya is important not only from the scientific point of view but also from the societal point of view. Tectonic sub divisions of the Himalaya are illustrated.

The foreland basin is the southernmost part of the Himalaya. It is separated from the Indo-Gangetic Alluvial Plain by the Himalayan Frontal Thrust (HFT). The HFT is an active thrust evidenced by scarps, uplifts, folding of Late Quaternary and Holocene deposits, and paleo-seismic studies. Available data on slip rates suggest a general increase from North-west to Central (Nepal) Himalaya. Thrusting in the region has resulted in fault propagation folds. The basin has formed during uplift of the Himalaya and is characterized by sedimentary succession of rocks. Stratigraphic sequence and sedimentological characters are described along with a brief remark on paleoclimate. Two generations of strike slip faults formed during the early and superposed deformations are present. A pull-apart basin is described from the Kangra region. Normal faults are the youngest structures. Results of cross-section balancing and various problems related to the balancing are discussed in the light of development of foreland basins, along with possible solutions.

Litho-tectonic set-up and inverted metamorphism are described. A large number of curvatures in the trend of the Main Boundary Thrust are attributed to oblique fault ramps formed during the pre-Himalayan tensional regime. The pre-Himalayan origin of the curvatures differentiates it from arcuate structures observed in other fold and thrust belts of the world. A model is proposed for arcuate geometry of the Mandi-Karsog Pluton. The Kangra recess is the longest oblique ramp of the Himalaya. The structural evolution is proposed using a three-dimensional model. A comparison is made between petrofabric and magnetic strains measured in the region. Reasons for low magnitudes of magnetic strain are mentioned. Geometrical relationships are established between displacement along oblique fault ramps and displacement out of the tectonic transport plane. Models are proposed for allochthonous (Simla) and pop-up klippen (Satengal, Banali, and Garhwal) models along with field examples. The new evidence suggests that the actual thrust displacement is much less than the previously conceived large amounts. Using the structural data from the Uttarkashi area (Garhwal Himalaya) a model is proposed for simultaneous development of thrust and normal faults at different structural levels.

Age of the Main Central Thrust, at different places, obtained by different methods is described. Litho-tectonic subdivisions are mentioned along with magmatic events. Both Vaikrita and Munsiari rocks represent two distinct rock assemblages and both have undergone pre-Himalayan metamorphism. A model is proposed to explain the occurrence of younger Vaikrita rocks on the thrust hanging wall and older Munsiari rocks in footwall of the Vaikrita thrust. The model is based on reactivation of early rift related normal fault as thrust fault. Details of structural features along the Satluj valley are described primarily to explain the interference between Karcham oblique fault ramp and two generations of folds. Change in orientation of early and superposed folds in vicinity of Vaikrita thrust is illustrated. Field evidence suggests that after locking of the Vaikrita thrust, the maximum extension has taken place along the strike of the thrust. Different models proposed for structural evolution of the High Himalaya are reviewed.

Lithostratigraphy (including leucogranites) is described along with structural features. The nature and age of the intrusive granite are discussed. The South Tibetan Detachment System (STDS) has a complicated history of fault reactivations. It initiated as a normal fault during pre-Himalayan rifting and reactivated as thrust during early phase of Himalayan orogeny. Sheath folds in the hanging wall have formed during thrusting. During the late stages of deformation, it showed normal faulting. Two trends of normal faults, parallel and transverse to the Himalayan trend, and their origin are discussed. Some of the normal faults are a result of gravity gliding. Formation of transverse extensional faults (e.g. Leopargial Horst) in a predominant compressional regime is explained. The latest displacement along the STDS is that of right lateral slip. Evidence for the reactivations are discussed.

The timing of collision between the Indian and Tibetan plates along with significant evidence is described. The tectono-stratigraphy of the region is described under four major zones, Zanskar, Indus-Suture, Shyok Suture, and Karakoram. The plate tectonics model of subduction of the Indian plate beneath the Tibetan plate is illustrated. Tectonics of the dextral-slip Karakoram strike-slip fault is discussed with special reference to age, temperature of formation, and displacement. Deformation features characteristic of a transpressional zone are described from Tangtse area. Sequential formation of asymmetric folds, minor thrust faults, and subsequent extension resulting in irregular orientations of aplite veins is illustrated. Zanskar shear zone and simultaneous development of normal and strike-slip faults in the Tibetan region are described. The model proposed for simultaneous development of different types of faults in the Himalaya appears to be applicable to the Tibetan region.

Geochronological data from some of the important tectonic events of the Himalaya are summarized. A brief introduction of pre-Himalayan structures, including the basic rocks and four phases of acid magmatism, is given along with distribution of volcanic rocks in the Himalaya. Distribution of significant structural features, which form the basis for the proposed model are described. These include evidence of seismicity below the plane of basal detachment, inconsistency between surface and subsurface fold geometries, arcuate shape of the Himalaya as a primary structure, locking of the prominent Himalayan thrusts followed by formation of strike-slip faults, occurrence of younger rocks on the thrust hanging wall, and formation of superposed folds, which indicate maximum compression in E–W direction. The model is based on inversion tectonics and explains many of the previously unexplained features. Finally, the recent concept of tectonics versus climate is briefly discussed. It is concluded that the Himalayan structures are a result of gigantic tectonic forces that drive the plates and make the Indian plate to subduct beneath the Tibetan plate.