EVOLUTION OF FLUID DRIVING FORCES AND COMPOSITION WITHIN COLLISIONAL OROGENS

Subaerial collisional mountain belts have a predictable asymmetric two-sided wedge geometry. The major driving forces for crustal fluid flow in this framework are thermal and topographic gradients. It is possible to predict both a time- averaged fluid state and the evolution of the fluid regime during orogenesis, with three distinct fluid flow regimes: the outboard toe where material is incorporated into the orogen, a transitional zone of concentrated deformation, and a zone of rapid exhumation along the inboard slope adjacent to the indentor. As rocks move into the outboard wedge, relatively low temperature, compaction and head driven fluid flow dominates. Mid-crustal material undergoes disequilibrium dehydration as rock-fluid packets move into the transition zone, producing a dominantly aqueous 'metamorphic fluid'. In the inboard region, rapid uplift produces geothermal gradients in excess of 80-degrees-C/km which encourage vigorous free convection. Advecting fluids evolve through immiscibility towards increasing CO2 content during uplift of the rock-fluid packets. Mixing of thermally driven and head driven fluids occurs at depths of 5-10 km. The Southern Alps of New Zealand, the European Alps and the Himalaya represent variants of this orogenic-hydrothermal system.