Eocene exhumation and extensional basin formation in the Copper Mountains, Nevada, USA

Abstract

Within extended orogens, records that reflect the driving processes and dynamics of early extension are often overprinted by subsequent orogenic collapse. The Copper Mountains of northeastern Nevada preserve an exceptional record of hinterland extensional deformation and high-elevation basin formation, but current geochronology and thermochronology are insufficient to relate this to broader structural trends in the region. This extension occurred concurrent with volcanism commonly attributed to Farallon slab removal. We combine thermochronology of both synextensional hanging-wall strata and footwall rocks to comprehensively evaluate the precise timing and style of this deformation. Specifically, we apply (U-Th)/(He-Pb) double dating of minerals extracted from Eocene–Oligocene Copper Basin strata with multi-mineral (U-Th)/He and 40Ar/39Ar thermochronology of rocks sampled across an ∼20 km transect of the Copper Mountains. We integrate basement and detrital thermochronology records to comprehensively evaluate the timing and rates of hinterland extension and basin sedimentation. Cooling and U-Pb crystallization ages show the Coffeepot Stock, which spans the width of the Copper Mountains, was emplaced at ca. 109–108 Ma, and then cooled through the 40Ar/39Ar muscovite and biotite closure temperatures by ca. 90 Ma, the zircon (U-Th)/He closure temperature between ca. 90 and 70 Ma, and the apatite (U-Th)/He closure temperature between 43 and 40 Ma. Detrital apatite and zircon (U-Th)/(He-Pb) double dating of late Eocene fluvial and lacustrine strata of the Dead Horse Formation and early Oligocene fluvial strata of the Meadow Fork Formation, both deposited in Copper Basin, shows that Early Cretaceous age detrital grains have a cooling history that is analogous to proximal intrusive rocks of the Coffeepot Stock. At ca. 38 Ma, cooling and depositional ages for Copper Basin strata reveal rapid exhumation of proximal source terranes (cooling rate of ∼37 °C/m.y.); in these terranes, 8–12 km of slip along the low-angle Copper Creek normal fault exhumed the Coffeepot Stock in the footwall. Late Eocene–early Oligocene slip along this fault and an upper fault splay, the Meadow Fork fault, created a half graben that accommodated ∼1.4 km of volcaniclastic strata, including ∼20 m of lacustrine strata that preserve the renowned Copper Basin flora. Single-crystal sanidine 40Ar/39Ar geochronology of interbedded tuffs in Copper Basin constrains the onset of rapid exhumation to 38.0 ± 0.9 Ma, indicating that surface-breaching extensional deformation was coincident with intense proximal volcanism. Coarse-grained syndeformational sediments of the Oligocene Meadow Fork Formation were deposited just prior to formation of an extensive regional Oligocene–Miocene unconformity and represent one of the most complete hinterland stratigraphic records of this time. We interpret this history of rapid late Eocene exhumation across the Copper Mountains, coeval volcanism, and subsequent unconformity formation to reflect dynamic and thermal effects associated with Farallon slab removal. The final phase of extension is recorded by late, high-angle normal faults that cut and rotate the early middle Miocene Jarbidge Rhyolite sequence, deposited unconformably in the hanging wall. These results provide an independent record of episodic Paleogene to Miocene exhumation documented in Cordilleran metamorphic core complexes and establish that substantial extension occurred locally in the hinterland prior to province-wide Miocene extensional break-up.

Geochemical Evolution of Eocene Lakes in the Nevada Hinterland of the North American Cordillera

“…This implies that extension began in the Eocene locally and likely influenced lake evolution. Elko Formation lithofacies and spatial accumulation patterns, however, differ in several ways from typical syn‐extensional sedimentary strata deposited in basins bound by range‐uplifting normal faults (e.g., Canada et al., 2019; Ezquerro et al., 2014; Gawthorpe & Leeder, 2000; Janecke et al., 1997). In particular, the Elko Formation lacks detectable associated coarse‐grained, footwall‐generated alluvium and has a broad, roughly symmetrical isopach pattern, with the thickest accumulations in the center (Canada et al., 2019; Smith et al., 2017), implying that an alternate basin forming mechanism(s) may have contributed to its accumulation.…”

“…Recent detrital zircon U‐Th/Pb‐He double dating and lag time analysis show that minimum nonvolcanic detrital zircon (U‐Th)/He age lag times relative to depositional age decreased to <100 m.y in 45–43 Ma strata and to <10 m.y. in 43–41 Ma strata (Canada et al., 2019). This implies that extension began in the Eocene locally and likely influenced lake evolution.…”

“…Lake Elko reached its greatest apparent depth and extent roughly coincident with the arrival of rollback magmatism in northeast Nevada (Canada et al., 2019; Henry et al., 2011; Lund Snee et al., 2016). This would locate the basin roughly above the slab hinge (Figure 9), where dynamic subsidence is predicted (Göğüş & Pysklywec, 2008; Heller & Liu, 2016).…”

Zircon provenance of Quaternary cover beds using U–Pb dating: Regional differences in the Southwestern USA

“…(Colgan et al, 2010;Lund Snee et al, 2016). The youngest reported age at this latitude is 34 Ma (John, Henry, & Colgan, 2008;Canada et al, 2019).…”

Introduction: Active Margins in Transition—Magmatism and Tectonics through Time: An Issue in Honor of Arthur W. Snoke

“…Mountains, Nevada, USA Canada et al (2019) integrate the thermochronologic record of both footwall metamorphic rocks and synextensional hanging-wall strata to document the onset and evolution of extension in part of the Sevier hinterland in northernmost Nevada in the Cordilleran orogen of the western USA. By correlating the cooling history of basement rocks using 40 Ar/ 39 Ar and (U-Th)/He thermochronology with detrital apatite and zircon (U-Th)/(He-Pb) double-dating of syntectonic strata, they document 8-12 km of latest Eocene to early Oligocene slip on the Copper Creek normal fault system.…”