A familiar feature of orogens is the presence of a sedimentary basin, termed a foredeep basin, situated ahead of the orogen. The foredeep basins are filled in the main by the erosional detritus from the adjacent orogen, and good examples are the Siwalik basin of the Himalaya and the molasse basins of the Alps and Andes. The driving force for the erosion is mountain uplift. Before considering examples of foredeep basins we must first look at the controls on basin formation.

Isostasy and Bouguer anomalies

A full understanding of the physical relations between a mountain chain and the adjacent foreland basins calls for some knowledge of the gravitational factors involved in mountain building. The famous experiments in the Peruvian Andes by Bouguer in 1735 and 1745 established that there is a mass deficiency in mountain belts. Bouguer demonstrated this by measuring the deflection of a plumb line towards the Andes and showing that it was much less than expected from the huge bulk of the mountains. The gravity anomalies thus demonstrated are now named after him. Thus under mountains there is a negative Bouguer anomaly (Figs. 9.1, 9.2).

In 1928 Arthur Holmes suggested that the mechanism for continental drift is cells of convection in the mantle. This was a remarkable insight, although many would now question the one-to-one connection between plate motion and mantle convection. So what is the modern view on the driving force for plate movements? There are two models in which the plates drive themselves. The first is called ‘slab pull’, which means that the dense ocean crust exerts a pull on the ocean floor during subduction as it plunges into hot asthenosphere. In contrast, the less dense continental crust is relatively buoyant. Sometimes the subducted slab of ocean crust breaks off and sinks into the hot asthenosphere, but if it survives it will exert a traction and in effect pull the ocean crust away from the Mid Ocean Rise. The opposite view is ‘slab push’, which means that the driving force for the moving ocean floor is situated at the Mid Ocean Rise which is opening under extension to allow in the new ocean crust.

Perhaps it should not be either/or here. Phillip England (1982) calculated the required stresses at the Mid Ocean Rise in the Indian Ocean if slab push were to be responsible for the northward movement of the Indian plate carrying the Indian continent. The forces acting on a plate boundary must do work against gravity during the raising of high mountains and plateaux. The force balance must take into account the Argand number, which expresses the relative magnitudes of the buoyancy forces arising from contrasts in crustal thickness and the forces required to deform the medium. England's results show that the horizontal stress arising from slab push is enough to explain not only the motion of the Indian plate before collision but also the continuation of motion after the India–Asia collision, with the result that India indents Asia, and a wave of deformation has spread across the Asian continent for over 2000 km north of the Himalaya.

This chapter addresses the question of mountain support, isostasy and the tectonometamorphic and rheological processes operating in the deep structure of mountains that greatly influence their evolution. Referring to the outer part of the Earth, Barrell (1914) coined the terms lithosphere, the rocky part, and asthenosphere, the weaker lower part. This perceptive observation together with the concept of isostasy forms the basis for most of our thinking about mountains and their support. Clearly the mass of a mountain range or high plateau standing high above sea level requires a mechanism for its support. The creation of an elevated terrane means that work is done against gravity. The Tibetan Plateau could not stand at its present height without the support of a horizontal deviatoric stress, in this case the continuing plate convergence between India and Eurasia. Removal of this support would mean that the plateau would flow away – this is called orogenic collapse. It has been pointed out that the paradox in the Himalaya–Tibet example is that although the plate convergence continues steadily and the horizontal stress is maintained, the plateau is undergoing normal faulting which for many scientists implies that it is collapsing. This observation led to the idea that the lower part of the mantle lithosphere below Tibet has been delaminated and hence the Tibetan lithosphere acquired buoyancy.

Before going into the details of the geophysics of orogens it is necessary to consider models which show strength profiles through the lithosphere. There is ongoing debate between the ‘jelly sandwich’ model for the lithospheric rheology and the newer ‘crème brûlée’ model (Fig. 10.1). The jelly sandwich model is well established and might be regarded as the standard model for the rheology of the lithosphere; it is so called because it postulates a strong upper crust and upper mantle and a weak lower crust, whereas the more recent crème brûlée model, as proposed by Jackson (2002) and others, invokes a strong upper layer made up of the entire crust above a weak mantle.

The effects that mountain building may have on global climates and climate change have received considerable attention in recent years. However, the jury is still out on whether a direct causal link has been established between mountain building and climate or climate change, or at least the degree of influence of the one on the other. Many other factors are involved, not least the amount of CO2 in the Earth's atmosphere. There is agreement that the proposition calls for not only high mountains but also ones covering a large area of the Earth's surface. Climatic modelling has emphasised that in order to influence climate a huge area of high ground is needed, and the modellers consider that the Alps or Himalaya are not big enough, although they may well disrupt a north–south air flow and thereby cause local climatic effects.

Mountains influence climate because they are obstacles to air circulation. Additionally, they are sources of elevated latent heat and they change the water exchanges between continental surfaces and the atmosphere. In this chapter there is no room for an expansive account of this topic, but we will highlight some features and in particular enter the discussion of the role of orogenesis in changing the climate in southern Asia. Most of this chapter is devoted to the ongoing controversies surrounding the hypothesis that the rise of the Tibetan Plateau influenced and strengthened the monsoon in the late Miocene.

In this chapter we will be looking at the evolution of several types of Phanerozoic orogenic belts. Precambrian orogenesis will be dealt with in Chapter 12. The Himalaya and the Alps are part of a huge belt of Cenozoic age which runs from the Pyrenees through the Balkans into Turkey and on to the Middle East, Pakistan and India into Burma. There is also a leg from the Betic Cordillera to the Rif in North Africa and via Corsica to the Ligurian and Internal Western Alps. These parts were the result of the collision of Gondwanaland (the Late Palaeozoic assemblage of South America, Africa, India and Antarctic) and Eurasia (Europe and Asia). We also consider the Andes and the Caledonides in order to illustrate different types of orogens. For the present, examples are confined to the Cenozoic orogens because, as mentioned above, the younger mountain belts offer a better chance of understanding evolutionary processes in orogenesis than the older deeply eroded belts in which much of the evidence is missing (see Chapter 12).

In the now discarded geosynclinal theory of orogenesis as set out for example in Holmes's Principles of Physical Geology (Holmes, 1944), the pre-orogenic phase was a precursor of the orogenesis because the sedimentary and igneous rocks deposited in the geosyncline were already undergoing compression and so were predestined to become involved in orogeny, the point being reinforced by the postulated downward flow of mantle convection cells which led the whole process. Plate tectonics introduced a paradigm shift which included a denial of any link between the events occurring before orogenesis and the orogeny itself; this is well demonstrated by the Swiss Alps which were undergoing extension not compression before orogeny. The attempt to separate temporally extensional and compressional strain events is much too simple. For example, compressional strain in forearc wedges may be synchronous with extensional strain in the back arc, as for example in South America where overall convergence during the Jurassic–Early Cretaceous between the oceanic and continental plates involved synchronous extensional and compressional strains. This is a common feature around the Pacific where back-arc basins are opening during subduction. In addition, as Royden (1993 a,b) has shown, the roll-back and advance of the subduction zone produces alternations of extension and compression of continental margins.

Preface

Mountains have attracted the attention of mankind at least since Rousseau (or did Petrarch precede him?*) who devoted much thought to nature, perhaps because the height and scale of mountains induced a sense of awe. A love of nature showed itself in the fairly recent desire to get to the top of mountains. George Mallory gave his reason for wanting to climb Everest as “because it is there”, but long before that mountains were important for humankind, because they formed natural barriers for trade and the movement of armies. Perhaps the ancient Egyptians tried to simulate mountains in the pyramids of Giza. The same is true of builders of Gothic cathedrals, which were built ever higher so as to imitate mountains which reach up to heaven. The Greeks worshipped the gods on Mount Olympus, and mountains appeared often in Greek mythology; Prometheus, for example, was chained to a mountain side. The Greeks saw mountains as mysterious and frightening places, and even today for Hindus and Buddhists there are sacred mountains in the Himalaya such as Nanda Devi, Kailas and Everest – Qomolungma, the goddess mother of the Earth. Badrinath near the source of the Ganges in the High Himalaya is the home of the gods and a place of pilgrimage. Moses came down from a mountain bearing his famous tablets. Noah is supposed to have docked his ship on Mount Ararat. The Bible states “the mountains shall melt before the Lord” (Judges 5:5), but perhaps the reference was to volcanoes rather than orogenic mountains.

Many artists, too, have been fascinated by mountains. Leonardo Da Vinci realised that the fossils in the rocks of the Apennines showed that the rocks were once below sea level, and he and other painters used mountain scenes as backgrounds. Cezanne painted many pictures of Mont St. Victoire in Provence.