Abstract
Hydrogen isotopic composition (δD) and 40Ar/39Ar geochronological data of neomineralized clay minerals from fault rocks and cleavage seams in the Sevier belt record primary involvement of surface-sourced fluids during Cretaceous–Paleogene mountain building events. δD values of authigenic clays range from −78.8±1.7‰ to −97.2±3.2‰. Given temperature constraints of mineral formation, we calculate δD values of mineralizing fluids as ∼−63 to −96‰, which fall within the range of δD values of Cretaceous–Paleogene surface waters in the region. The potential infiltration and presence of surface fluids at depths of several kilometers during deformation implies that pore spaces and fractures acted as conduits for surface water. Uplift of thrust sheets generated topography that favored meteoric recharge and flow of fluids into margins of the foreland basin, which were subsequently incorporated into the propagating thrust wedge. This is in contrast to the hypothesis that the release of overpressured metamorphic fluids dominates during fold–thrust belt evolution. As a result of shortening and tectonic uplift, fluid flows through basin sediments toward the foreland basin as meteoric waters recharge sediments at the basin margins. This study shows that stable isotopic compositions of shallow, deformation-related clay minerals are robust and may be used alongside other surface fluid proxies to probe the origin and composition of ancient fluids in orogenic systems.
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