Timing of mid-crustal ductile extension in the northern Snake Range metamorphic core complex, Nevada: Evidence from U/Pb zircon ages

Abstract

Lee et al. | Timing of ductile extension, northern Snake Range metamorphic core complex GEOSPHERE | Volume 13 | Number 2Timing of mid-crustal ductile extension in the northern Snake Range metamorphic core complex, Nevada: Evidence from U/Pb zircon ages ABSTRACT Metamorphic core complexes within the western U.S. record a history of Cenozoic ductile and brittle extensional deformation, metamorphism, magmatism, and exhumation within the footwalls of high-angle Basin and Range normal faults. In models proposed for the formation of metamorphic core complexes there is a close temporal and spatial link between upper crustal normal faulting, lower crustal ductile extension and flow, and detachment faulting. To provide constraints on the timing of ductile extension in the northern Snake Range metamorphic core complex (Nevada) and thereby test these models, we present new 238 U-206 Pb dates on zircons from both deformed and undeformed rhyolite dikes intruded into this core complex. The older age bracket is from the northern dike swarm, which was emplaced in the northwestern part of the range pretectonic to syntectonic with ductile extension. The younger age bracket is from the Silver Creek dike swarm, which was emplaced in the southern part of the range after ductile extensional defor mation. The 238 U-206 Pb zircon ages from these dikes provide tight bounds on the timing of ductile extension, between 37.806 ± 0.051 Ma and 22.49 ± 0.36 Ma. Our field observations, petrography, and 238 U-206 Pb zircon ages on these dikes combined with published data on the geology and kinematics of extension, moderate-and low-temperature thermochronology on lower plate rocks, and age and faulting histories of Cenozoic sedimentary basins, are interpreted as recording an episode of localized upper crustal brittle extension during the late Eocene that drove upward ductile extensional flow of hot middle crustal rocks from beneath the northern Snake Range detachment soon after, or simultaneously with, emplacement of the older dike swarm. Exhumation of the lower plate continued in a rolling hinge-isostatic rebound style; the western part of the lower plate was exhumed first and the eastern part extended ductilely either episodically or continuously until the latest Oligocene-earliest Miocene, when the post-tectonic younger dike swarm was emplaced. Major brittle slip along the eastern part of the northern Snake Range detachment and along high-angle normal faults exhumed the lower plate during middle Miocene. Age progressionCenozoic volcanism C o lv il le Ig n e o u s C o m p le x Challis Volcanics Absaroka Volcanics Columbia River Basalts C a s c a d e m a g m a t is m Sevier thrust belt ARG (~41-20 Ma) Snake Range (37.8-22.5 Ma) REH (~50-21 Ma) Funeral Mtns Shuswap Okanogan Priest River Pioneer N V ID O R Bitterroot Basin and Range S ie rr a N ev ad a Idaho Batholith S n a k e R iv e r P la in Colorado Plateau Laramide

Geometry and magnitude of extension in the Basin and Range Province (39°N), Utah, Nevada, and California, USA: Constraints from a province-scale cross section

“…Volcanic rocks of the ignimbrite flare-up overlie Paleozoic-Mesozoic rocks across a regionally distributed Paleogene unconformity, which represents a postorogenic erosion surface that predates extension in most places (e.g., Armstrong, 1972;Gans and Miller, 1983;Long, 2012Long, , 2015. In eastern Nevada and western Utah, some areas experienced Eocene-Oligocene extension (e.g., Gans et al, 1989Gans et al, , 2001Potter et al, 1995;Constenius, 1996;Evans et al, 2015;Long and Walker, 2015;Lee et al, 2017). However, extension was localized, and paleoaltimetry data indicate that surface elevations were still high during this time (Wolfe et al, 1997;Horton et al, 2004;Cassel et al, 2014).…”

“…The Snake Range core complex has been extensively studied over the past 40 yr (e.g., Coney, 1974;Gans and Miller, 1983;Miller et al, 1983Miller et al, , 1999bBartley and Wernicke, 1984;Gans et al, 1985;Lee et al, 1987Lee et al, , 2017Lee, 1995;Lewis et al, 1999;Cooper et al, 2010;Evans et al, 2015). However, many aspects of its development remain debated, in particular the tectonic significance of the E-vergent Northern Snake Range décollement, the primary extensional structure in the range.…”

Syncontractional deposition of the Cretaceous Newark Canyon Formation, Diamond Mountains, Nevada: Implications for strain partitioning within the U.S. Cordillera

“…During the Late Eocene and Oligocene, a northeast to southwest migration of magmatism known as the Great Basin ignimbrite flare-up swept across Nevada and is interpreted to be related to post-Laramide rollback of the Farallon slab (e.g., Humphreys, 1995;Best et al, 2009;Smith et al, 2014). During the ignimbrite flare-up, several areas in eastern Nevada experienced localized extension (e.g., Gans and Miller, 1983;Druschke et al, 2011;Lee et al, 2017;Long et al, 2019;Long, 2019). However, paleoaltimetry data indicate that elevations were still high (~2.5-3.5 km) during, and possibly in response to, the ignimbrite flare-up, and therefore the Nevadaplano still existed during the mid-Cenozoic .…”

Rapid Oligocene to Early Miocene Extension Along the Grant Range Detachment System, Nevada, USA: Insights From Multipart Cooling Histories of Footwall Rocks

“…Spatially isolated Late Cretaceous to Paleocene (~80-60 Ma) extension, which was contemporaneous with the final stages of shortening in the Sevier thrust belt, has been documented (e.g., Camilleri & Chamberlain, 1997;Druschke, Hanson, Wells, Rasbury, et al, 2009;Hodges & Walker, 1992;, and has been interpreted to have been initiated by lithospheric delamination (Wells & Hoisch, 2008). Eocene-Oligocene extension has also been documented (e.g., Gans et al, 1989Gans et al, , 2001Evans et al, 2015;Lee et al, 2017;Long & Walker, 2015), and was often associated spatially and temporally with the Great Basin ignimbrite flare-up, a NE to SW sweep of silicic volcanism interpreted to have accompanied post-Laramide slab rollback (Figure 1a; e.g., Dickinson, 2002;Humphreys, 1995). The initiation of widespread extension that formed the Basin and Range Province, which is attributed to establishment of the San Andreas transform system (e.g., Atwater, 1970), was not until the middle Miocene (e.g., Cassel et al, 2014;Colgan & Henry, 2009;Dickinson, 2002).…”

“…The Grant Range detachment system, along with the Snake Range core complex and Stampede detachment system, represent three regional-scale fault systems on the area of Figure 11 that accommodated much or all of their overall motion during the late Eocene-Oligocene. In the Snake Range, denudation-related cooling of footwall rocks and dated field relations with volcanic, intrusive and sedimentary rocks defines a protracted history of extension that spans from the early Eocene to the middle Miocene (Evans et al, 2015;Gans et al, 1989;Gébelin et al, 2014;Lee & Sutter, 1991;Lee, 1995;Lee et al, 2017;Miller et al, 1999). The Stampede detachment system was active during deposition of~34-31 Ma sedimentary rocks and prior to~29-31 Ma volcanism (Axen et al, , 1993Bartley et al, 1988;Taylor, 1990;Taylor & Bartley, 1992).…”