The thermodynamics of Himalayan orogenesis

Abstract

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 selforganized 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 lrmderate 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.One of the great success stories in the Earth sciences over the past quarter-century has been the integration of petrology, geochemistry, structural geology and geophysics to improve our understanding of the relationships between thermal and deformational processes in mountain belts. When faced with a question like 'What drives metamorphism?', physical chemistry tell us that the answer is largely 'Heat', but the metamorphic evolution of an orogen is dictated by complex interactions between heat and mass transfer processes that can only be fully appreciated from a multidisciplinary perspective. Though the more optimistic among us might argue that sophisticated numerical simulations, systematic structural studies, and a burgeoning petrologic and thermochronologic database have provided us with a good, basic understanding of how thermal, erosional and deformational processes are related in mountainous regions, a more difficult question is 'Why?'.

Timing of metamorphism, melting and exhumation of the Leo Pargil dome, northwest India

“…In contrast, melting could develop in response to crustal extension by decompression driven melting (e.g. Hodges, 1998; Whitney et al. , 2003).…”

“…In contrast, melting could develop in response to crustal extension by decompression driven melting (e.g. Hodges, 1998;Whitney et al, 2003). Aoya et al (2005) suggest that melting can weaken the crust which can promote shear zone development, and initiation of extension can trigger decompression-melting which enhances exhumation.…”

Exhumation of orogenic crust: Diapiric ascent versus low-angle normal faulting

“…We have focused on diapirism as a possible agent for the isothermal decompression segment of the P-T path, and clearly normal faulting occurs in the upper crust of these systems. Possible relationships between normal faulting and gneiss dome formation by diapirism include (1) diapirism drives normal faulting by causing the upper crust to extend as the diapir pierces the upper crust; (2) normal faulting drives diapirism by allowing an instability at depth to ascend into the space created by extension; and (3) the two processes are coupled, resulting in positive feedback between upper crustal extension and buoyant ascent of deep crust (England and Molnar, 1993;Hodges, 1998).…”

The Grenville Province as a large hot long-duration collisional orogen – insights from the spatial and thermal evolution of its orogenic fronts

“…The constructive phase is characterized by increasing crustal thickness and potential energy (Hodges 1998), whereas during the destructive phase the stored potential energy gradually decays as the orogenic crust thins by gravitational collapse and erosion (Dewey 1988;Rey et al 2001).…”