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

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ID: 305389
2004
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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 ē~s~=0.32 (1.15:0.93:0.73; x, y, z, principal axes of strain) outside, to ē~s~=0.64 (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 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, 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 δ^18^O than undeformed samples which have O- and C-isotope compositions similar to those of marine carbonates. Veins are even lower in δ^18^O~V-SMOW~, with minimum values of ∼+5 permil reflecting penetration of the crust by ocean-derived precipitation with δ^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 δ^18^O of carbonates even further, based on the O-isotope compositions of veins related to younger compressional deformation and to the onset of crustal-scale extension in the Eocene. This progression toward lowered δ^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.
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openalex_W2146048240 Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors David Anastasio
Journal american journal of science
Year 2004
DOI
10.2475/ajs.304.4.333
URL
https://ajs.scholasticahq.com/article/61726.pdf
Keywords Keywords not found

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