Structure and Tectonics of the Indian Continental Crust and Its Adjoining Region

Deep Seismic Studies

Harish C Tewari, B.Rajendra Prasad, Prakash Kumar

• Publisher: Elsevier
• Year: 2018
• ISBN:
  9780128136867
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Geophysics

Structure and Tectonics of the Indian Continental Crust and Its Adjoining Region: Deep Seismic Studies, Second Edition, collates essential data from seismic studies of Earth's crust across India, offering an essential understanding of the tectonic development of the Indian subcontinent. Seismic studies have been carried out in various parts of India since 1972, recording crust-related seismic data for determination of velocity-depth configuration and determination of structural patterns. The book examines the details of these studies, including their synthesis and global applications. The book presents both background and applications in one cohesive volume for researchers and students of geophysics and geology.

• Presents all the information and metadata of the Indian continental crust and its neighbouring regions in a cohesive way
• Provides basic knowledge of the Indian subcontinent to support the discussion of seismic studies related to crustal structure
• Includes all new chapter covering global applications and synthesis of the findings and observations

Chapter One

Overview of Crust and Introduction to Seismic Observations on Indian Plate

Abstract

Studies of the Earth's structure over the last 100 years have proven that the Earth consists of several layers. The Earth has been subdivided into four main units: the inner core, outer core, mantle, and crust. The crust and the uppermost mantle, down to a depth of about 70–100 km under the deep ocean basins and 100–200 km under the continents, is called the lithosphere. It is rigid and forms a hard outer shell that deforms in an essentially elastic manner. The crust is the Earth's hard outer shell and is much thinner compared to other layers of the Earth. Knowledge of characteristic features of the continental crust is very important, as the present-day configuration of the continental crust is mostly an outcome of lithospheric evolution and crust-mantle interaction. The controlled source seismic study [also known as the deep seismic sounding (DSS) study] is a definitive geophysical technique for exploring the structure of the Earth's crust and uppermost mantle. These studies were carried out in various geological terrains in India for a better understanding of the geological history of the Indian plate through the velocity and structural configuration of the continental crust.

Keywords

Crust; Lithosphere; Asthenosphere; Seismic; Indian plate; Basaltic

Five billion years ago the planet Earth was formed as a large conglomerate. The immense amount of heat energy released by massive high-velocity bombardment of meteorites and comets melted the entire planet. Since then, the planet has been cooling off and the process continues even today. During the cooling process, denser materials, such as iron from meteorites, sank into the core of the Earth while lighter material, e.g., silicates, oxygen compounds and water from comets, rose near to the surface.

Studies of the Earth's structure over the last 100 years have proven that the Earth consists of several layers. Characteristic properties of each layer are different in terms of physical and chemical parameters. The chemical parameters, e.g., alkalinity, acidity, salinity, etc., and the physical parameters, e.g., pressure, temperature, density and elasticity, vary from layer to layer. The parameters of elasticity and density determine the seismic wave velocity, which normally is different for each of these layers.

From the study of the seismic wave velocity and density, the Earth has been subdivided into four main units (Fig. 1.1): the inner core, outer core, mantle and crust (Ritter Michael, 2006). The equatorial radius of the Earth is 6378 km, out of which the inner core is about 1250 km, the outer core 2200 km and the mantle 2900 km thick, respectively. The core is composed mostly of iron and is so hot that its outer part (outer core) is molten, with about 10% sulfur. The inner core is under extreme pressure and remains solid. Most of the Earth's mass is in the mantle, which is composed of iron, magnesium, aluminum, silicon and oxygen compounds. At over 1000°C, the mantle is solid but can deform slowly in a plastic manner. The crust is composed of the least dense calcium and sodium/aluminum-silicate minerals. Being relatively cold, the crust is rocky and brittle and therefore is easier to fracture.

Fig. 1.1 Generalized structure of the Earth. Depths to major boundaries are given.

1.1 Lithosphere and Asthenosphere

The crust and the uppermost mantle, down to a depth of about 70–100 km under the deep ocean basins and 100–200 km under the continents, is called the lithosphere. It is rigid and forms a hard outer shell that deforms in an essentially elastic manner. The lithosphere is composed of various plates that float on partially molten asthenosphere. Delineation of an unambiguous boundary that separates the lithosphere from the underlying asthenosphere has not yet become possible, most likely because the asthenosphere under old continental platforms is imaged as a broad zone in the seismic velocities. Here, instead of a single low-velocity zone, a series of high- and low-velocity layers are intermingled (Fuchs et al., 1987).

The upper mantle plays a crucial role in structural development of the Earth's crust. Critical levels of thermodynamic conditions prevail in individual zones of the upper mantle, under the influence of differential and thermoelastic stresses. A discontinuous increase or decrease of volume takes place due to polymorphic, phase and chemical transformation of the inhomogeneous mantle substance. Distribution of structural forms within the Earth's crust and mineral deposits on the surface have a close dependence on the processes occurring in the upper mantle.

The asthenosphere is a rheologically weak, semiviscous layer in the upper mantle. In this layer the velocities often decrease, suggesting lower rigidity. This weaker layer is thought to be partially molten; the melt may be able to flow over long periods of time like a viscous liquid or plastic solid, in a way that depends on its temperature and composition. The asthenosphere plays an important role in plate tectonics, because its viscous state allows relative motion of the overlying rigid lithospheric plates. Some of the researchers suggest that the asthenosphere should be defined not as a weak upper mantle layer but as a zone of partial melting (Pavlenkova, 1988).

The lithosphere is divided into several plates, of which the crustal component could be either continental or oceanic. Very little progress has been achieved so far in understanding the evolution of the continental lithosphere, due to the inaccessibility of its subcrustal part for direct studies. Explosion seismology studies in different tectonic settings (viz. old Precambrian shields, young continental platforms and the oceans) show that several velocity layers exist in the upper mantle (Mooney and Meissner, 1992). The most important findings are: (1) occurrence of the low velocity layers at shallow depths in the continental upper mantle, with large velocity contrasts at their boundaries; and (2) observation of unexpectedly high-compressional (P) wave velocities, up to 8.6–8.9 km s^{− 1}, and high-velocity gradients of 0.02–0.04 km s^{− 1} at depths of 10–30 km below the crust-mantle boundary (Bean and Jacob, 1990). These findings provide indirect evidence that the elastic anisotropy continues within the uppermost mantle.

1.1.1 The Crust

The crust covers the mantle and is the Earth's hard outer shell, the surface on which we are living. Compared to other layers of the Earth, the crust is much thinner, like a stamp on a football. It generally consists of solid material, but this material is not the same everywhere and is less dense and more rigid than the material of the Earth's mantle. The crust over the oceans is different in nature as compared to the rocks of the continental crust. The oceanic crust is about 6–11 km thick and the rocks in it are very young, not older than 200 million years, compared to the rocks of the continental crust. Its igneous basement consists of a thin (about 500 m thick) upper layer of superposed basaltic lava flows underlain by basaltic intrusion, the sheeted dike complex and the gabbroic layer. A greater part of the oceanic crust consists of the tholeiitic basalt (basalt without olivine), which has a dark, fine and gritty volcanic structure. It is formed out of liquid lava, which cools off quickly. The grains are so small that they are only visible under a microscope. The average density of the oceanic crust is 3000 kg m^{− 3}.

The crust under the continents and areas of shallow seabed close to their shores (continental shelves) is called the continental crust. It covers more than one-third of the Earth's surface. It is thicker than the oceanic crust, 35–40 km thick under the stable areas and 50–80 km under the young mountain ranges, and mainly consists of igneous rocks. It is divided into two layers. The upper crust mainly consists of sediment, gneiss, granite and granodiorite rocks, while the lower crust consists of basalt, gabbros, amphibolites and granulites. The average density of the upper crust is 2700 kg m^{− 3} while that of the lower crust is 2850 kg m^{− 3}. It is older than the oceanic crust; some rocks are as old as 3800 million years. When active margins of the continental crust meet the oceanic crust in subduction zones, the oceanic crust is subducted due to relatively lower density of the former. The lower density does not allow the continental crust to be subducted or recycled back into the mantle. For this reason, the oldest rocks on the Earth are within the cratons or cores of the continents, rather than in the repeatedly recycled oceanic crust. Increasingly younger units surround the older cores in the center of the continents.

Six principal types of crust are identified based on the sedimentary thickness, crustal thickness and mean seismic velocities (Beloussov and Pavlenkova, 1985). Type I, in the regions of the most recent mountains where high relief is accompanied by mountain roots, is 50–70 km thick. It has generally high-heat flow values and mean velocities are in the range of 6.4–6.5 km s^{− 1}, but in some subtypes velocities as high as 7.0 km s^{− 1} are also seen. Type II crust covers almost half the area of the continents and is common to areas with thin (< 3 km) sedimentary cover and also crystalline shields. It is about 40-km thick, mean seismic velocity in the consolidated part is around 6.5 km s^{− 1}, and has low heat flow values. Type III is an attenuated, low-velocity crust in exterior parts of the continents, e.g., West European platform. It is 25–30 km thick, has an inconsistent heat flow and a mean seismic velocity of 6.1–...