Evolution of fluid driving forces and composition within collisional orogens

Abstract

Subaerial collisional mountain belts have a predictable asymmetric two‐sided wedge geometry. The major driving forces for crustal fluid flow in this framework arc 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°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.