The thermodynamics of Himalayan orogenesis

This paper is included in the Special Publication entitled 'What drives metamorphism and metamorphic reactions?' edited by P.J. Treloar and P.J. O'Brien. Geological and geochemical research in the internal zone of the Himalayan orogen reveal evidence for complex relationships between regional metamorphism, anatexis, thrust faulting and normal faulting in Miocene time. Such interactions share many characteristics with those that define the behaviour of non-equilibrium thermodynamic systems, like chemical oscillators. The internal ordering of such systems arises spontaneously and is maintained by continual exchange of energy with the outside world. Mountain ranges are open thermodynamic systems, equally capable of displaying self-organized behaviour. They accumulate energy through a variety of mechanisms, particularly crustal thickening, but their non-equilibrium structure is maintained through compensating mechanisms of energy dissipation. The simultaneous operation of normal faults and thrust faults in the Himalayan system, ejecting a wedge of middle crust southward from the orogen towards the Indian foreland, was a remarkably efficient method of mass (and therefore energy) dissipation in Miocene time. Research in many branches of science suggests that thermodynamic systems forced far from equilibrium by their boundary conditions may evolve towards establishing the most efficient possible mechanisms for counteracting those conditions. For the Himalaya, one possible explanation for development of a process set characterized by simultaneous normal faulting and thrust faulting, linked through metamorphism and anatexis, is that simple physical erosion alone was inadequate to moderate the extraordinary crustal thickness and topographic gradients that characterized the southern flank of the range in Miocene time. The orogenic system may have adapted to this condition through a new and more efficient mechanism of energy dissipation that required the coordination of a variety of thermal and deformational processes.