FLUID FLOW AND HEAT TRANSFER DURING EXHUMATION OF METAMORPHIC CORE COMPLEXES

Metamorphic core complexes are defined by the juxtaposition of high-grade and low-grade rocks across a low-angle detachment zone. Large metamorphic gradients across thin detachments ask the question of the mechanisms of heat transfer. In this dynamic system, three main mechanisms apply: conduction, rock + heat advection, and fluids + heat advection. The latter mechanism is investigated by combining field-based analysis in the Columbia River detachment (eastern edge of Shuswap metamorphic core complex, British Columbia) and numerical modeling. Structural, metamorphic, stable isotopic, and geo/thermochronological studies of this core complex suggest that the thick orogenic crust, prior to collapse, consisted of a cold and rigid upper crust separated from a flowing partially molten layer by a detachment. During incipient collapse at ~50 Ma, meteoric fluids precipitated at high elevation (>4000 m) may permeate the upper crust and flow into the detachment. The meteoric isotopic composition is preserved in the hydrogen isotopes of white mica fish found in quartzite mylonites throughout the 800 m thick Columbia River detachment. Numerical modeling of fluid flow in a detachment/upper crustal normal faults system shows that free convection, driven by high thermal gradients across the detachment, can be an effective mechanism for local heat transfer. Following rapid orogenic collapse and exhumation of the metamorphic core complex, fluid flow becomes dominated by forced convection driven by Basin-and-Range style topography. Topographically high regions that develop as recharge areas are cooled by downward fluid flow and preserve Eocene apatite fission-track cooling ages (45-40 Ma). Away from these recharge areas, upward flow of hot fluids maintains a high heat flow in the upper crust, and apatite fission-track ages are reset to Oligo-Miocene ages. In these regions, ages range from 23.6 to 19.4 Ma, with mean confined track lengths of 12.4 to 11.8 microns, respectively, suggesting prolonged heating (1-10 Myr) at T~100°C. The interpretation of cooling ages, and therefore late-stage exhumation mechanisms of metamorphic core complexes, requires a more complete understanding of fluid flow in these systems.