Fluid Flow During Orogenesis

Fluids play a key role in most orogenic processes where fluid flow is usually structurally controlled in faults, fractures, and ductile deformation zones. My recent research with G. Bebout and students Holl and Johnson into the regional paleohydrogeology of the Idaho-Montana thrust belt challenges the current paradigm that thrust belts are closed systems, flushed by basinal brines from the orogenic interior, while foreland basins are open to infiltration by meteroric waters. Data from the Montana recess of the Sevier orogen (Bebout et al. 2001, Anastasio et al. in press), Johnson et al. in prep.) shows a quantitative relationship between deformation from cm-km scales and fluid infiltration and metasomatism resulting from positive feedback between deformation and far-traveled meteroric fluids. These fluid-rock interactions exhibit both small-scale heterogeneity and regional-scale systematics. Geometric and chemical strain softening from authigenic clay growth in deformation zones focused strain and enhanced permeability. Variably deformed rocks are lower in d18O than undeformed rocks, which have O- and C-isotopic compositions similar to those of marine carbonates. Multiple vein generations have d18O values, which reflect a variety of up-stream fluid-rock interactions with meteroric fluids with a strong seawater influence, consistent with the paleogeographic reconstructions of the Great Western Interior Seaway. Surface winds and currents modeled for Cretaceous storms within the seaway predict interior ocean derived precipitation at the latitude of the Montana recess.

Our geochemical and structural studies have led to a conceptual model for the closed- and open-system processes leading to cleavage formation and to a model for the temporal evolution of permeability in thrust structures. Our studies suggest (1) that the system evolved from closed to open as a result of deformation, (2) that fluid flow evolved from porosity-based to discontinuity-based (deformation zones, cleavage, faults, and fracture networks) as deformation progresses (i.e. fluid flow became enhanced and focused by deformation), (3) that mass transfer was accommodated by diffusion early and advection later in the deformation history, and (4) that zones of focused fluid flow were kinematically related to larger-scale structures (faults and fault-related folds). The inferred addition of surficial fluids to these rocks during their later-stage deformation implies a fluid regime involving significant topographically driven recharge.