Eocene extension and meteoric fluid flow in the Wildhorse detachment, Pioneer metamorphic core complex, Idaho

Abstract

The relationship between microstructure and fluid flow traced by hydrogen isotope ratios (dD) is examined within the Wildhorse detachment system of the Pioneer metamorphic core complex in south-central Idaho. Within the detachment footwall, 100-m-thick mylonitic quartzite containing minor white mica and K-feldspar displays a NW-trending stretching lineation and consistent top-to-the-NW sense of shear criteria. Microstructures within the detachment footwall comprise two groups: quartz ribbons and relict quartz grains flattened within the foliation, with porphyroclastic white mica fish; and intensely deformed and recrystallized quartz with high-aspect-ratio white mica arranged within C′ shear bands. White mica dD values are highly negative and cluster around-145‰ in high-aspect-ratio white mica and around-120‰ in porphyroclastic white mica fish. The most negative values are interpreted to reflect interaction with meteoric fluids from a high-elevation catchment (3000-4000 m), and the less negative values are interpreted to represent incomplete hydrogen isotope exchange between the meteoric fluid and the pre-extensional metamorphic fluid dD values in the white mica porphyroclasts. A suite of tightly clustered 40 Ar/ 39 Ar ages from synkinematic white mica in the detachment footwall dates deformation, recrystallization, fluid-rock interaction, and therefore the presence of high topography at 38-37 Ma; these ages are consistent with the cooling/exhumation history of the high-grade core of the Pioneer metamorphic core complex in the late Eocene. The 38-37 Ma 40 Ar/ 39 Ar ages are substantially younger than previously published ages of high topography in British Columbia to the north (49-47 Ma), in line with the hypothesis that high topography propagated from north to south in the northern segment of the North American Cordillera through Eocene time.

Prolonged high relief in the northern Cordilleran orogenic front during middle and late Eocene extension based on stable isotope paleoaltimetry

“…This reconstruction is consistent with the high late early Eocene elevation estimated by a fossil floral physiognomy study in northern Washington (Wolfe et al, 1998). The attainment of high elevation in northern Cordillera during the early middle Eocene may represent the initiation of a renewed topographic gain in the North America Cordillera, which migrated southward and reached southern Idaho during the latest Eocene, and Nevada during the Oligocene (Mix et al, 2011;Chamberlain et al, 2012;McFadden et al, 2015).…”

Triple Oxygen Isotope Paleoaltimetry of Crystalline Rocks

“…This area has been the subject of numerous paleoaltimetry studies of both the flanking basins and crystalline core. While most of the paleoaltimetry estimates come from clay and carbonate in paleosols and lake sediments (e.g., Kukla et al, 2021) the elevation estimates for the crystalline core rely almost exclusively on hydrogen isotopes of metamorphic (McFadden et al, 2015) and igneous micas (Criss and Taylor, 1983). The Eocene plutons created large Eocene (between 45 and 37 Ma) meteoric hydrothermal systems that exchanged with the FIGURE 1 | Map of the Idaho batholith and Eocene plutons and volcanics and surrounding Eocene basins.…”

“…Shown are the sites sampled for paleoaltimetry analysis for the Eocene. The triple oxygen isotope study of the Eocene hydrothermal system (Chamberlain et al, 2020) is given as the dot (EHS), the Pioneer core complex (PC; from McFadden et al, 2015), and the Early Oligocene volcanic glass in Blue Mountains (EO; from Bershaw et al, 2019). Shown is the eastern margin of the Sevier fold and thrust belt (Sevier FTB).…”

U-Pb zircon ages of the Wildhorse gneiss, Pioneer Mountains, south-central Idaho, and tectonic implications

“…The Pioneer metamorphic core complex (Wust, 1986;O'Neill and Pavlis, 1988;Silverberg, 1990;Worl et al, 1995;Vogl et al, 2012Vogl et al, , 2014McFadden et al, 2015) is part of the belt of Cordilleran metamorphic core complexes (Fig. 1) that formed during Cenozoic postorogenic extension in the hinterland of the Sevier orogenic belt.…”