Extra-basinal fluid infiltration, mass transfer, and volume strain during folding: Insights from the Idaho-Montana thrust belt

Finite strain and geochemical variations along strain gradients were used to study cleavage development in carbonates in the Lost River Range, Idaho. Deformation accommodating layer-parallel shear was partitioned into thin deforma- tion zones during folding of Willow Creek anticline. Cleavage intensity is strong to very strong in deformation zones and weak in surrounding rocks. Strain magnitudes range from e s0.32 (1.15:0.93:0.73; x, y, z, principal axes of strain) outside, to e s0.64 (1.29:0.80:0.52) inside, deformation zones. We found a positive linear correlation between strain, cleavage development, and negative dilation. Volume loss, at a cubic centimeter scale, ranges from 12 percent to 49 percent. Cleavage selvages are depleted in Ca and 18 O and enriched in other elements relative to microlithons and less deformed protolith carbonates. Mass-balance considerations indicate that cleavage was formed by incongruent pressure solution leading to a passive concentration of less soluble components during Ca loss and metasomatic additions of Si, Al, and K to produce authigenic clay minerals in selvages. Data for the Willow Creek locality and elsewhere in the Lost River Range (Davidson and others, 1998), Pioneer Mountains, and the Tendoy Range (Bebout and others, 2001), show that across the Sevier orogen, fluid infiltration was heterogeneous at centimeter-kilometer scales and resulted from positive feedback between deforma- tion and far-traveled surficial fluids. Metasomatic strain softening enhanced deforma- tion zone development, which generated increased permeability as the thrust belt evolved from a porosity-based closed system to a discontinuity-based open system. Increased fluid infiltration and isotopic exchange was associated with volume loss, increasingly prolate strains, and crystallization of clays, to produce the cleavage selvages. Mass transfer was accommodated by diffusion early and advection later in the deformation history. Mesoscopic structures (deformation zones, faults, veins) focused fluid flow and were kinematically related to larger-scale structures (faults, fault- related folds). The inferred addition of surficial fluids to depths of >7 kilometers in the thrust belt implies a fluid regime involving significant topographically driven recharge. Deformed whole-rocks and microsamples are lower in 18 O than unde- formed samples which have O- and C-isotope compositions similar to those of marine carbonates. Veins are even lower in 18 OV-SMOW, with minimum values of 5 permil reflecting penetration of the crust by ocean-derived precipitation with 18 O near to somewhat lower than 0 permil (range of 7.5 to 2.5‰ calculated for H2 Oi n equilibrium with these veins). The inferred penetration, into the thrust belt, of nearshore meteoric waters is consistent with proximity to the reconstructed Western Interior Seaway. Later fluid infiltration locally lowered the 18 O of carbonates even further, based on the O-isotope compositions of veins related to younger compres- sional deformation and to the onset of crustal-scale extension in the Eocene. This progression toward lowered 18 O of the surficial waters is compatible with the retreat of the seaway during emergence of the thrust wedge and Paleogene extension, uplift, and subaerial volcanism.