Modeling the Magnitudes and Directions of Regional Metamorphic Fluid Flow in Collisional Orogens

We present two-dimensional numerical models that simulate fluid flow in an orogenic overthrust setting in the absence of magmatism. The modeling is intended to test hypotheses regarding the direction of regional fluid flow in the middle and deep crust, including upward, convective, sub-horizontal, up-temperature, and down-temperature flow regimes. Several geological factors that have received relatively little attention in previous numerical models of regional metamorphic fluid flow are considered: (1) retrograde hydration metamorphic reactions; (2) anisotropy and spatial heterogeneity in crustal permeability structure; (3) temporal evolution of permeability as a result of porosity production and consumption during metamorphic reactions; (4) heat of metamorphic reactions. Our results suggest that the typical fluid flow pattern in overthrust settings is towards the surface, in the direction of decreasing temperature. The flow is mainly driven by metamorphic dehydration reactions; the integrated fluxes in the deep crust below 20 km are of the order of 10(3) m(3)/m(2) over 30 Myr for a background permeability of 10(-19) m(2). At higher crustal permeability, the buoyancy flux generally exceeds the metamorphic flux and large-scale fluid circulation becomes possible. Transient upward flow in the up-temperature direction is observed during the first 1-3 Myr of the model evolution within the thrust core that undergoes retrograde hydration; this flow is the result of the initially inverted geotherm in the thickened crustal section. Downward fluid infiltration caused by metamorphic hydration may occur in the flanks of the model orogen, but the integrated fluxes in these areas are only similar to 10 m(3)/m(2). Hydration-driven downward flow may also develop in the core of the orogen at late stages of exhumation. Low-permeability layers lead to sub-horizontal (but down-temperature) flow over distances corresponding to the lateral dimensions of such layers. Heterogeneities in permeability structure, as well as porosity-permeability variations produced by metamorphic reactions, result in focusing of fluid flow and locally elevated fluid fluxes (up to 10(4) m(3)/m(2)). These fluxes are large enough to produce significant chemical and isotopic metasomatism, but are not sufficient to significantly affect the temperature distribution in the model orogen. However, metamorphic hydration and dehydration reactions are found to exert considerable control on the regional thermal structure. We conclude that convective, downward, or up-temperature fluid flow is probably very limited during amagmatic prograde metamorphism of the middle and lower crust, even in the presence of permeability anisotropy, high-permeability channels, or low-permeability barriers.