Jurassic Barrovian metamorphism in a western U.S. Cordilleran metamorphic core complex, Funeral Mountains, California

Abstract

Large-magnitude retroarc shortening of Cretaceous age is well documented in the Sevier orogenic belt of the western United States, and has been associated with eastward Franciscan subduction that began in the Middle-Late Jurassic, but evidence for major Late Jurassic retroarc shortening has been lacking. Here we report new Lu-Hf garnet geochronology, pressuretemperature (P-T) paths, and 40 Ar/ 39 Ar thermochronology data that document tectonic burial of Late Jurassic age in a Barrovian metamorphic terrain, the Funeral Mountains metamorphic core complex, California, located in the hinterland of the Sevier orogenic belt. The P-T paths determined from growth-zoned garnets in upper greenschist facies pelitic schist show steep P-T trajectories consistent with metamorphism during thrust loading. The age of thrust loading is constrained by a fi ve-point Lu-Hf garnet isochron to be 158.2 ± 2.6 Ma (2σ). Partial exhumation, recorded in 146-153 Ma 40 Ar/ 39 Ar muscovite cooling ages, closely followed garnet growth. Late Jurassic Barrovian metamorphism has not been previously recognized in Cordilleran metamorphic core complexes, possibly due to being obscured by Late Cretaceous to Tertiary deformation, magmatism, and metamorphism. This study fi nds that the period of large-magnitude crustal shortening in the retroarc extended into the Late Jurassic, and may have closely followed the formation of the coherent orogenic system associated with east-dipping Franciscan subduction.

Fault Slip and Exhumation History of the Willard Thrust Sheet, Sevier Fold‐Thrust Belt, Utah: Relations to Wedge Propagation, Hinterland Uplift, and Foreland Basin Sedimentation

“…Farther south in southern Nevada to southeast California, 160-145-Ma ZHe ages record Late Jurassic onset of major slip along the Wheeler Pass thrust in the western Sevier belt, which was located closer to the magmatic arc in this area (Giallorenzo et al, 2018). Early shortening in the Sevier belt here overlapped with crustal thickening and regional metamorphism in hinterland rocks of the Funeral Mountains (Hoisch et al, 2014). The Wheeler Pass sheet and foreland experienced Late Jurassic erosion, likely related to broad uplift near the magmatic arc, with sediment transported northeastward and likely deposited in the Morrison Formation.…”

Late Jurassic flare‐up of the Coast Mountains arc system, NW Canada, and dynamic linkages across the northern Cordilleran orogen

“…See text for interpretation. Arc and arc proximal: 1—Saint Elias Mountains, Yukon (this study); 2—Eastern Coast Mountains, British Columbia [ Gehrels et al ., ]; 3—Western Coast Mountains [ Gehrels et al ., ]; 4—Blue Mountains, Oregon [ LaMaskin et al ., , and references therein]; 5—Sierra Nevada batholith, California [ Ducea , ]; 6—Alexander‐Wrangellia‐Peninsular composite terrane forearc and arc, Alaska [ MacKevett , ; Trop and Ridgway , ]; 7—Coast belt, British Columbia [ Greig , ]; 8—Franciscan Complex, California [ Wakabayashi and Dumitru , ]; 9—Sierra Nevada arc and host rocks, California [ Cao et al ., ]; 10—Chugach terrane (McHugh complex) strata, Alaska [ Amato and Pavlis , ]; 11—Gravina belt strata, southeastern Alaska and British Columbia [ Yokelson et al ., ]; Retroarc: 12—Omineca belt, Yukon [ Allan et al ., ; Bailey , ; Staples et al ., ]; 13—Intermontane belt, Yukon and British Columbia [ Evenchick et al ., ; White et al ., ]; 14—Omineca and Foreland belts, Alberta and British Columbia [ Colpron et al ., , Evenchick et al ., ; Pană and van der Pluijm , , and references therein]; 15—Luning‐Fencemaker belt, Nevada [ Wyld , ; DeCelles , ]; 16—Sevier hinterland, Idaho, Utah, and California [ Hoisch et al ., ; Cruz‐Uribe et al ., ]; 17—Whitehorse trough, Yukon [ Colpron et al ., ]; 18—Bowser basin, British Columbia [ Evenchick et al ., ]; 19—Alberta foreland basin [ Poulton , , Raines et al ., , and references therein]; and 20—Montana foreland basin [ Fuentes et al ., , and references therein]. Geological timescale of Cohen et al .…”

“…In the western United States, there is compelling evidence that Late Jurassic high‐volume magmatism was linked to retroarc hinterland and thrust belt development [ DeCelles and Graham , ]. The 160–150 Ma flare‐up of the Sierra Nevada batholith occurred during and immediately after a late Middle to early Late Jurassic episode of craton‐directed thrusting and Barrovian metamorphism in the adjacent Luning‐Fencemaker thrust belt in Nevada [e.g., Wyld , ; Wyld et al ., ] (LFTB in Figure ) and proto‐Sevier thrust belts in California [ Hoisch et al ., ] and northwestern Utah and southern Idaho (Figure ) [ Cruz‐Uribe et al ., ].…”

“…A west‐to‐east transect across the western United States (~38°N) spans the Franciscan Complex (accretionary wedge), Great Valley Group (forearc), Sierra Nevada batholith (continental arc), “Nevadaplano” Plateau (hinterland metamorphic belt), and Sevier thrust belt and adjacent basins (retroarc thrust belt and foreland basin system). A dynamic linkage between these Cordilleran elements is recorded by Late Jurassic deformation, metamorphism, and high‐volume magmatism (apparent intrusive rates of ~300 km 2 /Myr) in the Sierra Nevada arc system [ Wolf and Saleeby , ; Dunne and Walker , ; Wakabayashi and Dumitru , ; DeCelles et al ., ; Cao et al ., ; Saleeby and Dunne , ] that occurred during or after retroarc shortening and Barrovian metamorphism in the Luning‐Fencemaker and proto‐Sevier thrust belts to the east [ Schweickert and Lahren , ; Wyld et al ., ; DeCelles , ; Hoisch et al ., ; Cruz‐Uribe et al ., ].…”

Linking thermodynamic modelling, Lu–Hf geochronology and trace elements in garnet: new P–T–t paths from the Sevier hinterland

“…Heller et al ., ; DeCelles, ). Amphibolite facies pelitic schists from the Raft River, Albion and Grouse Creek mountains of northwest Utah and southern Idaho provide a unique opportunity to study the tectonic evolution of the Sevier hinterland through garnet geochemistry and geochronology (Harris et al ., ; Wells et al ., ; Hoisch et al ., ). Garnet has been identified as a critical mineral for linking small‐scale petrological processes to large‐scale orogenic processes because of its usefulness both as a geochronometer and for recording changes in P–T conditions during its growth.…”