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1. Magmatic Surge Requires Two-Stage Model for the Laramide Orogeny

   https://doi.org/10.21203/rs.3.rs-2327151/v1  

   Schwartz, J.J.; Lackey, J.S.; Miranda, E.A.; Klepeis, K.A.; Mora-Klepeis, G.; Robles, F.; Bixler, J.D. (July 2023, Nature communications)

   The Laramide orogeny is a pivotal time in the geological development of western North America, but its driving mechanism is controversial. Most prominent models suggest this event was caused by the collision of an oceanic plateau with the Southern California Batholith (SCB) which caused the angle of subduction beneath the continent to shallow and led to shut-down of the arc. Here, we show that magmatism was surging in the SCB from 90 to 70 Ma, the lower crust was hot, and cooling occurred after 75 Ma. These data contradict plateau underthrusting and flat-slab subduction as the driving mechanism for early Laramide deformation. We propose that the Laramide orogeny is a two-stage event consisting of: 1) an arc ‘flare-up’ phase in the SCB from 90-75 Ma; and 2) a widespread mountain building phase in the Laramide foreland belt from 75-50 Ma that is linked to subduction of an oceanic plateau. 

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2. Magmatic surge requires two-stage model for the Laramide orogeny

   https://doi.org/10.1038/s41467-023-39473-7  

   Schwartz, Joshua J.; Lackey, Jade Star; Miranda, Elena A.; Klepeis, Keith A.; Mora-Klepeis, Gabriela; Robles, Francine; Bixler, Jonathan D. (June 2023, Nature Communications)

   Abstract The Laramide orogeny is a pivotal time in the geological development of western North America, but its driving mechanism is controversial. Most prominent models suggest this event was caused by the collision of an oceanic plateau with the Southern California Batholith (SCB) which caused the angle of subduction beneath the continent to shallow and led to shut-down of the arc. Here, we use over 280 zircon and titanite Pb/U ages from the SCB to establish the timing and duration of magmatism, metamorphism and deformation. We show that magmatism was surging in the SCB from 90 to 70 Ma, the lower crust was hot, and cooling occurred after 75 Ma. These data contradict plateau underthrusting and flat-slab subduction as the driving mechanism for early Laramide deformation. We propose that the Laramide orogeny is a two-stage event consisting of: 1) an arc ‘flare-up’ phase in the SCB from 90-75 Ma; and 2) a widespread mountain building phase in the Laramide foreland belt from 75-50 Ma that is linked to subduction of an oceanic plateau. 

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3. MAGMA-DEFORMATION INTERACTIONS IN THE SOUTHERN CALIFORNIA BATHOLITH DURING LATE CRETACEOUS ONSET OF THE LARAMIDE OROGENY, SAN GABRIEL MOUNTAINS, CALIFORNIA

   https://doi.org/10.1130/abs/2023AM-394141  

   Klepeis, Keith; Schwartz, Joshua; Miranda, Elena; Robles, Francine; Baskin, Jillyan; Mora-Klepeis, Gabriela (October 2023, Geological Society of America Abstracts with Programs)

   The Southern California Batholith is a ~500-km-wide segment of the Mesozoic California arc that lies between the northern Peninsular Ranges and the southern Sierra Nevada mountains. We use structural data and U-Pb zircon analyses from the eastern San Gabriel mountains to examine how the batholith responded to the onset of the Laramide orogeny during the Late Cretaceous. Zircon analyses show that the middle and lower crust of the batholith was hot and records a magmatic flareup from 90-77 Ma. From 90 to 86 Ma, tonalite of the San Sevaine Lookout intruded a thick package of metasedimentary rock that records a history of reverse displacements, crustal imbrication, and granulite metamorphism prior to tonalite intrusion. During the early stages of the magmatic flare-up, granodiorite dikes were emplaced and soon became tightly folded and disaggregated as younger sheets of comagmatic tonalite intruded. Deformation accompanied the magmatism, forming two parallel shear zones several 100 m thick. These two shear zones, which include the Black Belt Mylonite, are composed of thin (≤10 m) high-strain zones spaced several tens of meters apart. Each discrete high-strain zone contains subparallel layers of mylonite, ultramylonite, cataclasite and pseudotachylyte, all recording oblique sinistral-reverse displacements on gently and moderately dipping surfaces. This architecture, whereby individual high-strain zones are widely spaced and parallel the margins of intruding tonalite sheets, reveals the influence of magma emplacement on shear zone structure. U-Pb zircon geochronology on syn-tectonic dikes indicate that these different styles of deformation all formed within the same 89-85 Ma interval, suggesting that they reflect non-steady flow on deep seismogenic faults. Widespread (garnet) granulite-facies metamorphism and partial melting accompanied intrusion of the tonalites and sinistral- reverse displacements. The ages of undeformed dikes indicate that the deformation was over by 77-75 Ma. Together, these data show that arc magmatism and transpression within the Mesozoic California arc occurred from ~90 until ~75 Ma, implying that flat-slab subduction and the migration of the Laramide orogenic front into the North America interior occurred after ~75 Ma. 

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4. Tectonic controls on the origin and segmentation of the Cascade Arc, USA

   https://doi.org/10.1007/s00445-022-01611-2  

   Humphreys, Eugene D.; Grunder, Anita L. (December 2022, Bulletin of Volcanology)

   Abstract The magmatic response above subducting ocean lithosphere can range from weak to vigorous and from a narrow zone to widely distributed. The small and young Cascade Arc, riding on the margin of the tectonically active North American plate, has expressed nearly this entire range of volcanic activity. This allows an unusually good examination of arc initiation and early growth. We review the tectonic controls of Cascade-related magmatism from its inception to the present, with new considerations on the influences of tectonic stress and strain on volcanic activity. The Cascade Arc was created after accretion of the Siletzia oceanic plateau at ~ 50 Ma ended a period of flat-slab subduction. This (1) initiated dipping-slab subduction beneath most of the northern arc (beneath Washington and Oregon) and (2) enabled the more southerly subducting flat slab (beneath Nevada) to roll back toward California. As the abandoned flat slab fragmented and foundered beneath Oregon and Washington, vigorous extension and volcanism ensued throughout the northwest USA; in Nevada the subducting flat slab rolled back toward California. Early signs of the Cascade Arc were evident by ~ 45 Ma and the ancestral Cascade Arc was well established by ~ 35 Ma. Thus, from ~ 55–35 Ma subduction-related magmatism evolved from nearly amagmatic to regional flare-up to a clearly established volcanic arc in two different tectonic settings. The modern Cascades structure initiated ~ 7 Ma when a change in Pacific plate motion caused partial entrainment of the Sierra Nevada/Klamath block. This block pushes north and west on the Oregon Coast Ranges block, breaking the arc into three segments: a southern extensional arc, a central transitional arc, and a northern compressional arc. Extension enhances mafic volcanism in the southern arc, promoting basalt decompression melts from depleted mantle (low-K tholeiites) that are subequal in volume to subduction fluxed calcalkaline basalts. Compression restricts volcanic activity in the north; volcanism is dominantly silicic and intra-plate-like basalts cluster close to the main arc volcanoes. The transitional central arc accommodates dextral shear deformation, resulting in a wide volcanic arc with distributed basaltic vents of diverse affinities and no clear arc axis. 

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5. Regional exhumation of the Laramide Province

   https://doi.org/10.1130/B37625.1  

   Jepson, Gilby; Carrapa, Barbara; Reeher, Lauren J; DeCelles, Peter G; Afonso, Walter D; Howlett, Caden J; Caylor, Emilia A; Sherpa, Tshering ZL; Wang, Jordan W; Constenius, Kurt N (February 2025, Geological Society of America Bulletin)

   Western North America is the archetypical Cordilleran orogenic system that preserves a Mesozoic to Cenozoic record of oceanic Farallon plate subduction-related processes. After prolonged Late Jurassic through mid-Cretaceous normal-angle Farallon plate subduction that produced the western North American batholith belt and retroarc fold-thrust belt, a period of low-angle, flat-slab subduction during Late Cretaceous−Paleogene time caused upper plate deformation to migrate eastward in the form of the Laramide basement-involved uplifts, which partitioned the original regional foreland basin. Major questions persist about the mechanism and timing of flat-slab subduction, the trajectory of the flat-slab, inter-plate coupling mechanism(s), and the upper-plate deformational response to such processes. Critical for testing various flat-slab hypotheses are the timing, rate, and distribution of exhumation experienced by the Laramide uplifts as recorded by low-temperature thermochronology. In this contribution, we address the timing of regional exhumation of the Laramide uplifts by combining apatite fission-track (AFT) and (U-Th-Sm)/He (AHe) data from 29 new samples with 564 previously published AFT, AHe, and zircon (U-Th)/He ages from Laramide structures in Arizona, Utah, Wyoming, Colorado, Montana, and South Dakota, USA. We integrate our results with existing geological constraints and with new regional cross sections to reconstruct the spatial and temporal history of exhumation driven by Laramide deformation from the mid-Cretaceous to Paleogene. Our analysis suggests a two-stage exhumation of the Laramide province, with an early phase of localized exhumation occurring at ca. 100−80 Ma in Wyoming and Montana, followed by a more regional period of exhumation at ca. 70−50 Ma. Generally, the onset of enhanced exhumation occurs earlier in the northern Laramide province (ca. 90 Ma) and later in the southern Laramide province (ca. 80 Ma). Thermal history models of selected samples along regional cross sections through Utah−Arizona−New Mexico and Wyoming−South Dakota show that exhumation occurred contemporaneously with deformation, implying that Laramide basement block exhumation is coupled with regional deformation. These results have implications for testing proposed migration pathway models of Farallon flat-slab and for how upper-plate deformation is expressed in flat-slab subduction zones in general. 

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