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

Abstract

In Nevada and Utah, the Cordilleran orogen underwent a protracted Cenozoic transition to an extensional setting. However, the geodynamic processes that controlled this transition are poorly understood, in part because the space‐time patterns of extension are not known in many areas. Localities of pre‐Neogene extension have the potential to elucidate the dynamics of the Cordilleran crust during the final stages of subduction. Here we present data that constrain the timing of extension in the Grant Range in eastern Nevada, which was deformed by a low‐angle normal fault system. We present temperature‐time histories of eight granite samples exhumed by this fault system, constrained by muscovite 40Ar/39Ar multi‐diffusion domain modeling and fission track and (U‐Th)/He ages from zircon and apatite. These data demonstrate rapid cooling (20–35 °C/Myr) from 350–425 to 25–50 °C between 28–31 and 15–19 Ma. The fault system accommodated ~24 km of extension (~115%), and exhumed the granite samples from 7–9 km depths to the near‐surface. Rapid Oligocene‐early Miocene cooling is interpreted to date the duration of motion on the fault system, and defines an extension rate of 1.5–2.6 km/Myr. This was one of the most significant fault systems active during an episode of spatially distributed late Eocene‐Oligocene extension, which overlaps temporally with volcanism generated by slab rollback. Reduced interplate coupling that accompanied slab rollback is interpreted as the primary driver of extension of the Cordilleran plateau during the final stages of subduction. This supports a scenario of orogenic collapse that proceeded in distinct episodes that were initiated by external geodynamic events.

Interpreting and reporting 40Ar/39Ar geochronologic data

“…However, it is clear from numerous applications of MDD K-feldspar thermochronology to structural problems (Batt et al, 2004;Benowitz et al, 2011), including comparisons to other thermochronometers from the same rock sample (McDannell et al, 2019), that MDD results can provide very robust regional thermal constraints. A relatively new finding that appears to show great promise for 40 Ar/ 39 Ar thermochronometry is to extend MDD-style analyses to muscovite (Harrison and Lovera, 2014;Long et al, 2018). The challenge towards taking advantage of this potential opportunity is to demonstrate that the metastability of muscovite in the temperature range of the laboratory degassing experiment does not impact negatively on determining geologically relevant diffusion parameters.…”

The Role of Shortening in the Sevier Hinterland Within the U.S. Cordilleran Retroarc Thrust System: Insights From the Cretaceous Newark Canyon Formation in Central Nevada

“…Further to the south in the CNTB, in the Grant Range, the recumbent Timber Mountain anticline (Fryxell, 1988), which is interpreted as a fault‐propagation fold that formed above the east‐vergent Schofield Canyon thrust (Long et al., 2018) is intruded by the ∼86 Ma (Taylor et al., 2000) Troy granite stock, which indicates that folding was completed prior to this time. In the southern Grant Range, the Rimrock thrust, which is the northernmost segment of the east‐vergent Rimrock‐Lincoln‐Freiberg thrust system, is cut by the ∼90–98 Ma Lincoln stock, which brackets the thrust system as Late Cretaceous or older (Bartley & Gleason, 1990; Taylor et al., 2000).…”

“…Following the termination of Cordilleran shortening, a northeast to southwest sweep of silicic volcanism across Nevada between the late Eocene and early Miocene, known as the “Great Basin ignimbrite flareup” (e.g., Best et al., 2009; Henry & John, 2013), has been attributed to rollback of the subducting Farallon plate (e.g., Humphreys, 1995; Smith et al., 2014). The ignimbrite flareup was broadly synchronous with early extension in several areas of eastern Nevada and western Utah (e.g., Druschke et al., 2011; Gans & Miller, 1983; Lee et al., 2017; Long, 2019; Long et al., 2018). During the Miocene, the western North American plate boundary underwent a significant reorganization, which resulted in the progressive termination of subduction of the Farallon plate and the concurrent growth of the San Andreas continental transform system (e.g., Atwater, 1970; Dickinson, 2002, 2006).…”

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 .…”