Cooling and exhumation of the southern Idaho batholith

Abstract

We conducted a (U-Th)/He zircon thermochronology study of the southern part of the Idaho batholith (central Idaho, USA) to constrain cooling through ~200 °C and exhumation of the batholith. Samples were collected adjacent to the Idaho-Oregon (IDOR) seismic transect and at localities where U-Pb zircon, geochemical, and fabric analyses were conducted. The rocks affected by the western Idaho shear zone and associated border zone suite of the batholith cooled through the closure temperature for He in zircon prior to ca. 60 Ma, before or during emplacement of the voluminous Atlanta lobe. In contrast, the Atlanta lobe (Atlanta peraluminous suite) records a relatively constant cooling rate, in which the (U-Th)/He zircon ages are systematically ~30 m.y. younger than the U-Pb zircon ages. We interpret this data to reflect post-magmatic isobaric cooling with little or no unroofing. The only deviation from a smooth regional cooling pattern occurs near Sawtooth Valley, where samples from the Sawtooth Range on the west side of the valley show distinctly younger ages than those from the White Cloud Peaks to the east. We interpret this difference to reflect recent cooling and exhumation associated with extensional deformation. The regionally consistent pattern of cooling and hence exhumation indicates that the current exposure level of the Idaho batholith was <5 km deep (assuming a geothermal gradient of 40 °C/km) at 50 Ma during the initiation of Challis magmatism. Our data are consistent with the existence of a crustal plateau during formation of the Atlanta lobe of the Idaho batholith.

Introduction: EarthScope IDOR project (deformation and magmatic modification of a steep continental margin, western Idaho–eastern Oregon) themed issue

“…Second, rocks from the Atlanta lobe exhibit a pattern of cooling ages as a function of elevation, which are interpreted to indicate isobaric cooling in the absence of tectonic activity. Fayon et al (2017) indicate that the cooling age pattern is consistent with the Atlanta lobe forming in a crustal plateau. Third, younger (U-Th)/He zircon ages in the Sawtooth Range indicate recent exhumation associated with extensional deformation.…”

“…Byerly et al (2016) suggest that the Atlanta lobe may have been intruded during a time of crustal plateau formation, and the lack of consistently oriented fabric resulted from emplacement in a neutral tectonic stress environment (due to topographic effects), in an overall contractional setting, and/or sill-like emplacement. Fayon et al (2017) use (U-Th)/He zircon thermochronometry to constrain the exhumation of the southern part of the Idaho batholith. Samples were taken throughout the border zone, early metaluminous, Atlanta lobe, and Challis magmatic suites.…”

“…The other aspect illuminated by IDOR research is the nature of the Idaho batholith, based on the geochronological and geochemical data (Gaschnig et al, 2010(Gaschnig et al, , 2011(Gaschnig et al, , 2013(Gaschnig et al, , 2016. Byerly et al (2016) and Fayon et al (2017) both indicate that the Idaho batholith likely intruded into a crustal plateau, continuous with the proposed crustal plateau in central Nevada (Nevadaplano; e.g., Wolfe et al, 1997;DeCelles, 2004). Notably, the WISZ is not intruded by younger (ca.…”

Crustal Shear Wave Velocity Structure of Central Idaho and Eastern Oregon From Ambient Seismic Noise: Results From the IDOR Project

“…Both IB lobes are part of a much larger suite of Mesozoic batholiths outcropping virtually ubiquitously along strike of the North American Cordillera but whose longitudinal position varies significantly: North of 50° N and south of 32° N, the batholiths currently occupy near‐coastal locations; however, the IB lies anomalously some 500 km to the east of the west coast of North America. The anomalous location of the IB and its development largely from melting of continental material are consistent with its formation as part of a large Cordilleran plateau similar to the Andean Altiplano (Coney & Harms, ; Fayon et al, ; Foster et al, ; Gaschnig et al, ; Jordan & Allmendinger, ; Miller & Bradfish, ; Russo & Silver, ; Sewall & Fricke, ).…”

“…Immediately east of the WISZ, the granitic rocks of the IB intruded Selway and Grouse Creek basement and the Proterozoic‐Paleozoic cover rocks. Current exposures of the southern parts of the batholith were emplaced at shallow to midcrustal depths (Fayon et al, ). Figure shows the two major lobes of the batholith: the Bitterroot lobe to the north and the larger Atlanta lobe to the south.…”