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

Abstract

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 deformation 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 s ‫23.0؍‬ (1.15:0.93:0.73; x, y, z, principal axes of strain) outside, to e s ‫46.0؍‬ (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 deformation and far-traveled surficial fluids. Metasomatic strain softening enhanced deformation 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, faultrelated 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 undeformed samples which have O-and C-isotope compositions similar to those of marine carbonates. Veins are even lower in ␦ 18 O V-SMOW , with minimum values of ϳ؉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 H 2 O in 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 compressional deformation and to the o...

Syntectonic infiltration by meteoric waters along the Sevier thrust front, southwest Montana

“…(1998), Bebout et al. (2001), and Anastasio et al. (2004) further west within the Montana recess showed that veins formed during Sevier deformation could vary in isotopic composition based on the amount of upstream fluid–rock interaction ( δ 18 O = +5 to +27‰).…”

“…Isotopic results suggest that meteoric fluids infiltrated into the Sevier thrust sheets to the basal décollement along the thrust front to depths of at least 4 km in southwest Montana and 7–10 km in the Lost River Range, Idaho (Bebout et al. 2001; Anastasio et al. 2004).…”

“… Cartoon of fluid flow regimes in a fold and thrust belt setting: different fluid sources, δ 18 O compositions, pathways, and driving mechanisms (modified from Koons & Craw 1991; Anastasio et al. 2004).…”

“…(1998), Bebout et al. (2001) and Anastasio et al. (2004) inferred deep penetration of meteoric fluids from the isotopic compositions of synorogenic veins and variably deformed tectonite samples.…”

“…The Montana recess of the Sevier orogeny includes the foreland thrust sheets of the Idaho-Montana thrust belt exposed from west to east in the Lost River, Lemhi, and Beaverhead Ranges and the Tendoy Mountains, which are separated by intervening Tertiary-Recent extensional basins. For the Lost River Range, which exposes Late Paleozoic carbonates deformed at depths of 5-8 km or more within the orogenic wedge, Davidson et al (1998), Bebout et al (2001) and Anastasio et al (2004) inferred deep penetration of meteoric fluids from the isotopic compositions of synorogenic veins and variably deformed tectonite samples. To the east, along the thrust front, the emergent Four Eyes Canyon and Tendoy thrust sheets emplaced Late Paleozoic carbonates and clastics into a terrestrial foreland basin (Perry & Sando 1983;Haley & Perry 1990;Harkins et al 2004a,b).…”