Asian geology

The continent of Asia is made up of a large number of separate geological units, of varying age and origin. The Himalayan belt separates the main mass of central and northern Asia from the two separate stable blocks of peninsular India and Arabia. These southerly blocks were originally part of the ancient southern continent of Gondwanaland, and became joined to the rest of Asia only in late Phanerozoic times, after the splitting up of Gondwanaland. The northern part of Asia, part of the ancient continent of Laurasia, is made up of ancient cratons separated by mobile belts active in early Phanerozoic times. The Himalayan belt runs east–west from the Mediterranean through the Middle East to the Himalayas, then turns south through Burma, Malaysia, and Indonesia. It is a very large and long-lived mobile belt, active intermittently throughout the Phanerozoic, and still active in the present day.

Asiatic Laurasia This vast continental area to the north of the Himalayas is at present a stable massif, containing a number of ancient shield areas of Precambrian age.

The Siberian platform The largest of the shield areas, it occupies the northern part of the continent and is composed of Precambrian rocks which emerge as massifs from a thick cover of more recent sediments. In the Anabar massif, in the central part of the platform, high-grade metamorphic rocks of early to middle Precambrian age are surrounded by a flat-lying succession of late Precambrian age.

The Aldan massif, in the southeast of the Siberian platform, contains very old early Precambrian high-grade metamorphic rocks, together with middle Precambrian metasediments and granites. Gold occurs as lodes and placers in these rocks. In the southeast and southwest parts of the platform are large areas of late Precambrian rocks, representing mobile belts that were active 1,600–800 million years ago.

The Taimyr Peninsula To the north of the Siberian platform, the Taimyr Peninsula contains a massif of metamorphic Precambrian rocks that has been largely incorporated in a younger Palaeozoic fold belt. There are also a number of stable blocks of Precambrian age in eastern Asia; these have been preserved in later Phanerozoic fold belts in eastern China, Manchuria, and Korea.

The younger rocks of Asiatic Laurasia occur in a number of widely separated areas. To the west of the Siberian platform a thick sequence of Mesozoic and Tertiary sediments underlie the West Siberian lowlands; this area is bounded to the west by the Upper Palaeozoic mobile belt of the Ural Mountains. To the southwest, the Siberian platform is cut by the Lake Baikal Rift Valley System. To the south of the platform is a complex area of Palaeozoic mobile belts, while to the east in China lies a complex of Mesozoic fold belts. At the eastern margin of the continent are the island arcs of the still active circum-Pacific mobile belt.

The Baikal Rift Valley System An arcuate rift system of late Tertiary age, the Baikal Rift Valley System runs for almost 2,000 km/1,242 mi along the southern margin of the Siberian craton. A series of fault troughs contain thick accumulations of sedimentary and volcanic rocks. Lake Baikal lies in the central part of the rift some 3 km/1.8 mi below the blocks on each side of the rift valley.

Central Asia The Palaeozoic mobile belts cover the mountainous area of Mongolia and Central Asia to the south of the Siberian platform; they contain sediments of late Precambrian to Palaeozoic age and have been involved in phases of orogenic activity (see orogeny) in the late Precambrian, and in Caledonian and Hercynean times, with associated granite intrusions. Mesozoic and Tertiary sediments of continental type occur within and alongside the fold belts, and are generally undisturbed. The whole area suffered massive differential uplift, and thick conglomerates were eroded from the uplifted blocks and deposited in a number of basins.

Northeast Asia Mesozoic fold belts occur in northeast Asia, flanking the northeast margin of the Siberian platform. Thick sequences of detrital sediments of Permian to Jurassic age adjoin the platform; these are folded and intruded by Mesozoic granites containing gold, tin, and tungsten ores. Further east, towards the present continental margin, is a mobile belt active since early Phanerozoic times; this belt suffered its major orogenic activity in Alpine times. Tertiary granites and Tertiary to Quaternary andesite, dacite, and basalt lavas are widespread. To the east again this belt is fringed by the island arcs of the present active oceanic margin Kamchatka, the Kurils, Japan, and the Philippines.

The Precambrian Siberian platform itself is largely covered by shallow-water shelf-sea deposits of evaporites and non-marine sediments, ranging in age from Cambrian to Cretaceous and containing coals at a number of horizons. Thick plateau lavas, with associated basic sills and dykes, were products of Permian and Triassic times. These are known as the Siberian Traps. Diamond-bearing kimberlite pipes were intruded in and near the plateau basalt province in Triassic times.

The Indian Craton The triangular southern part of India and Sri Lanka is geologically quite distinct from the Himalayan mobile belt to the north; the southern part was originally joined to Africa as part of Gondwanaland, and has drifted north to join the continent of Asia only in geologically recent times. The Indian craton is composed of an ancient Precambrian crystalline basement, containing rocks formed during several episodes of Precambrian mobility. The oldest Precambrian rocks are of early to middle Precambrian age and are found in the Dharwar belt of southwest India, the Aravalli region between Mumbai (formerly Bombay) and Delhi, and the iron-ore belt of the Singbhum area.

The Dharwar belt is made up of an assemblage of gneisses, migmatites, and granites containing linear greenstone belts in which volcanics and sedimentary rocks, metamorphosed to a greater or lesser degree, are preserved. Some of the sediments contain rich haematite ores and manganese ores. Gold also occurs associated with basic volcanic rocks or intrusions in the greenstone belts. The whole of the Dharwar belt is intruded by swarms of basic dykes, intruded towards the end of the main metamorphic episode, 2,600–2,300 million years ago. In the Aravalli region, banded gneisses of the same general age occur underlying later cover rocks.

The iron-ore belt of the Singbhum area contains an ancient complex of metamorphic rocks, formed in an early Precambrian cycle of activity ending some 3,200 million years ago. Above these older metamorphic rocks is the iron-ore group, a folded but unmetamorphosed succession of shales, sandstones, lavas, and banded haematite-jasper, of great economic importance as one of India's main sources of iron ore. This cycle of deposition ended with the intrusion of the Singbhum granite 2,700 million years ago. A further cycle of activity resulted in the reactivation of part of this belt.

All these early massifs are truncated by mobile belts of late Precambrian age; these are the Eastern Ghats belt, lying along the east coast; the Aravalli-Delhi belt, running southwest from Delhi, and the Satpura belt, extending inland from Kolkata (formerly Calcutta). The Eastern Ghats belt is a belt of high-grade metamorphic rocks, gneisses, and charnockites, formed 2,000–1,200 million years ago. To the west of this belt late tectonic sediments and volcanics of the Eastern Ghats cycle are preserved relatively unaltered in the Cuddapah basin, northwest of Chennai (formerly Madras).

The Aravalli-Delhi belt runs north–northeast for some 800 km/500 mi through Rajasthan to Delhi. The Aravalli system at the base of the succession is a highly metamorphosed and granitised complex containing older Precambrian basement rocks. Above this are thick sedimentary accumulations, metamorphosed during further periods of orogenic activity and invaded by post-orogenic granites 1,650–1,000 million years ago.

The Satpura belt runs west–southwest from the Ganges delta for about 800 km/500 mi before disappearing under younger rocks. In the western part of the belt two formations occur: the Sakoli series, sediments metamorphosed to a relatively low grade, and the Sausar group, highly metamorphosed and migmatised rocks formed about 1,000 million years ago. The metamorphosed sediments contain rich deposits of manganese ore. In the eastern part, the Satpura belt is involved in activity of the Singbhum cycle, which also involves older rocks of the iron-ore belt. The margin of the older iron-ore belt is the thrust zone of the copper belt, containing important copper and uranium deposits. During this cycle, thick sediments and volcanics accumulated in the mobile belt to the north of the copper belt, and were later metamorphosed and folded.

Between the Aravalli-Delhi and Satpura belts and to the southwest is a huge area covered by rocks of the Vindhyan formation, shallow-water quartzites and shales laid down at various times on a stable platform after the stabilization of the main Indian craton around 900 million years ago. The Vindhyan sandstones are extensively quarried as building stones.

A mobile belt of late Precambrian to early Palaeozoic age (the Indian Ocean belt) crosses the southeast tip of India and runs through Sri Lanka and up the east coast as far as Chennai. In Sri Lanka a complex series of high-grade metamorphic rocks were formed during a series of metamorphic episodes, the final one occurring 500–600 million years ago. In late Palaeozoic times India was still part of Gondwanaland and formed a shield area, bounded to the north by the mobile belt of the Himalayas. On the platform adjoining the mobile belt sediments of Cambrian to Mesozoic age were laid down.

Glaciation Towards the end of the Palaeozoic a widespread glaciation affected Gondwanaland and thick glacial deposits accumulated. In India the Gondwana system, preserved in the northeast of the peninsula, begins with glacial tillites which pass up into coal measures, sandstones, and shales. The coal measures contain thick seams that provide most of India's coal. The upper part of the Gondwana succession consists of continental sandstones and shales of early Triassic to mid-Cretaceous age.

In late Jurassic to Cretaceous times marine sediments began to accumulate in new basins on the eastern and western margins of the craton, and in late Cretaceous to early Tertiary times thick plateau lavas, the Deccan Traps, were poured out over much of northern peninsular India. They are up to 2 km/1.25 mi thick, and were associated with the disruption of Gondwanaland, which began in Cretaceous times. From this time on, India moved north on a crustal plate, and in late Tertiary and Quaternary times this resulted in the collision of the continent with the ancient Laurasian massif, and the rise of the new Himalayan mountain ranges on the site of the mobile belt at the old continental margin. Thick Tertiary and post-Tertiary sediments were eroded from the rising mountain chains and deposited on the north India plains.

The Himalayan Mobile Belt This belt has a long history of activity, beginning in the earliest Palaeozoic and continuing up to the present day. It began as part of a long belt encircling the ancient northern continent of Laurasia, and was the later site of the collision between the fragments of Gondwanaland – Africa, Arabia, and peninsular India and Laurasia – as they moved northwards after the disruption of the supercontinent. During Palaeozoic and Mesozoic times sediments were deposited without break in the ancient Tethys Sea, along the margin of the Laurasian continent. These are preserved in the more northerly parts of the Himalayas; further south, the main part of the belt is made up of slices and fragments of the Indian subcontinent, sliced off as the continent collided with and underthrust the massif to the north. The main orogenic period began in late Mesozoic times. Fold belts generated during this cycle formed the Pamirs, the Karakoram, and the Hindu Kush ranges, linking up westwards through Afghanistan with the Alpine orogenic belt of the Middle East and Europe.

The southeast part of mainland Asia is occupied by mountain ranges running southeast from the eastern Himalayas; although the structures in these ranges are of similar Cretaceous to Quaternary age as those in the Himalayas, they do not seem actually to join the Himalayan ranges. The Malayan peninsula contains important granite intrusions from which valuable tin ores are derived.

The Pacific Island Arcs These form part of the active circum-Pacific mobile belt. They run from the Kurils in the north through Japan, the Philippines, the Mariana Islands, and on to the Tonga Isles and New Zealand. The arcs are flanked on their ocean (convex) side by deep ocean trenches, the sites where ocean floor is being thrust below the continental mass. This is therefore a region of strong seismic activity, suffering very deep earthquakes. The history of the island arcs stretches back to Palaeozoic times, and throughout their history they have been the site of frequent volcanic activity and metamorphism.