The Potential for Metamorphic Thermal Pulses to Develop During Compaction‐Driven Fluid Flow

Abstract

AbstractCompaction‐driven fluid flow below the brittle‐ductile transition may be a means of transporting fluids during metamorphism. In particular, when a decompaction weakening mechanism is introduced to account for the rock viscosity reduction due to fluid overpressures, channeling instabilities evolve into high‐porosity/permeability fluid conduits that focus mass and energy transfer. In this study, we consider a crustal rheology that accounts simultaneously for upward‐increasing viscosity and decompaction weakening to examine the nucleation and evolution of fluid channelization in two dimensions (2‐D). The model shows that plume‐shaped flow patterns can develop on time scales as short as 104 years, during which the plume tails act as fluid conduits and the plume heads act as fluid dispersion zones near the brittle‐ductile transition. Collection of fluids into conduits is accomplished by a basal fluid catchment zone characterized by strong lateral fluid pressure gradients but low porosity/permeability. Relatively narrow ranges of viscous activation energy (∼100 kJ mol−1) and decompaction weakening factor (∼10−4) are constrained if the fluid conduits are of kilometer scale in width. Significant thermal excursions (∼65 °C) can be induced if a high flow rate, potentially from rapid intermittent dehydration, is realized within channels. Moreover, if the focused fluids emanate from external anomalously hot sources (e.g., magma intrusion), thermal pulses (>100°C), and steep lateral temperature gradients (>50°C km−1) can be generated. Given the focusing efficiency estimated from our 2‐D compaction model, simple 3‐D modeling further shows that tubular conduits have the potential to cause thermal pulses >200°C within 104 years.