FLUID FLOW DIRECTIONS AND FLUXES IN COLLISIONAL OROGENS

We present two-dimensional (2-D) numerical modeling that reconstructs the heat and fluid flow in an orogenic overthrust setting in the absence of magmatism. Our goal is to evaluate existing hypotheses regarding the magnitudes and directions of regional fluid flow in the middle and deep crust. The models incorporate several geologic factors that have received relatively little attention in 2-D models of regional metamorphic flow: 1) retrograde hydration; 2) permeability anisotropy and spatial heterogeneity; 3) temporal evolution of permeability; and 4) heats of metamorphic reactions. Our results suggest that deep and mid-crustal flow of fluid in overthrust settings is mainly driven by metamorphic dehydration reactions; the typical fluid flow pattern is towards the surface, in a direction of decreasing temperature (down-T). The time-integrated fluid fluxes (TIFF) in the deep crust below 20 km are on the order of 10³ m³/m² over 30 Ma for a plausible background permeability of 10^-19 m². Hydrofracturing related to large fluid pressure buildups during devolatilization is predicted to be most common in the middle and lower crust where unfractured rock permeabilities are small. Transient upward flow in the up-temperature direction (up-T) is predicted for the first 1–3 million years of model evolution within the inverted geotherm region in the thickened crust, but TIFF values are small (~100 m³/m²). Limited downward fluid infiltration driven by metamorphic hydration may occur on the flanks of the model orogen and in the core of the orogen at late stages of exhumation (TIFF<100 m³/m²). Low-permeability layers within the thickened crustal section constrain flow to be sub-horizontal (but down-T) over distances corresponding to the lateral dimensions of the layers. The largest TIFF values (~10⁴ m³/m²) are predicted in regions of elevated permeability produced by fracturing, metamorphic reactions, or other processes. These regions would be likely to undergo major elemental and isotopic mass transfer as a result of the large fluid fluxes. Permeability anisotropy and/or heterogeneities do not appear to be able to produce large-scale convective, downward, or sub-horizontal up-T fluid flow patterns in amagmatic overthrust settings.