More Like this

1. Massive Fluid Influx beneath the Colorado Plateau (USA) Related to Slab Removal and Diatreme Emplacement: Evidence from Oxygen Isotope Zoning in Eclogite Xenoliths

   https://doi.org/10.1093/petrology/egaa102  

   Hoover, William F; Page, F Zeb; Schulze, Daniel J; Kitajima, Kouki; Valley, John W (December 2020, Journal of Petrology)

   null (Ed.)

   Abstract The Colorado Plateau has undergone as much as 1·8 km of uplift over the past 80 Myr, but never underwent the pervasive deformation common in the neighboring tectonic provinces of the western USA. To understand the source, timing and distribution of mantle hydration, and its role in plateau uplift, garnets from four eclogite xenoliths of the Moses Rock diatreme (Navajo Volcanic Field, Utah, USA) were analyzed in situ for δ18O by secondary ion mass spectrometry. These garnets have the largest reported intra-crystalline oxygen isotope zoning to date in mantle-derived xenoliths with core-to-rim variations of as much as 3 ‰. All samples have core δ18O values greater than that of the pristine mantle (∼5·3 ‰, mantle garnet as derived from mantle zircon in earlier work) consistent with an altered upper oceanic crust protolith. Oxygen isotope ratios decrease from core to rim, recording interaction with a low-δ18O fluid at high temperature, probably derived from serpentinite in the foundering Farallon slab. All zoned samples converge at a δ18O value of ∼6 ‰, regardless of core composition, suggesting that fluid infiltration was widely distributed. Constraints on the timing of this fluid influx, relative to diatreme emplacement, can be gained from diffusion modeling of major element zoning in garnet. Modeling using best estimates of peak metamorphic conditions (620 °C, 3·7 GPa) yields durations of <200 kyr, suggesting that fluid influx and diatreme emplacement were temporally linked. These eclogite xenoliths from the Colorado Plateau record extensive fluid influx, pointing to complex hydration–dehydration processes related to flat-slab subduction and foundering of the Farallon plate. Extensive hydration of the lithospheric mantle during this fluid influx may have contributed to buoyancy-driven uplift of the Colorado Plateau and melt-free emplacement of Navajo Volcanic Field diatremes. 

   more » « less
2. Laramide bulldozing of lithosphere beneath the Arizona transition zone, southwestern United States

   https://doi.org/10.1130/G51194.1  

   Kapp, Paul; Jepson, Gilby; Carrapa, Barbara; Schaen, Allen J.; He, John J.Y.; Wang, Jordan W. (August 2023, Geology)

   Abstract The northwest-trending transition zone (TZ) in Arizona (southwestern United States) is an ~100-km-wide physiographic province that separates the relatively undeformed southwestern margin of the Colorado Plateau from the hyperextended Basin and Range province to the southwest. The TZ is widely depicted to have been a Late Cretaceous–Paleogene northeast-dipping erosional slope along which Proterozoic rocks were denuded but not significantly deformed. Our multi-method thermochronological study (biotite 40Ar/39Ar, zircon and apatite [U-Th-Sm]/He, and apatite fission track) of Proterozoic rocks in the Bradshaw Mountains of the west-central Arizona TZ reveals relatively rapid cooling (~10 °C/m.y.) from temperatures of >180 °C to <60 °C between ca. 70 and ca. 50 Ma. Given minimal ca. 70–50 Ma upper-crustal shortening in the TZ, we attribute cooling to exhumation driven by northeastward bulldozing of continental lower crust and mantle lithosphere beneath it by the Farallon flat slab. Bulldozing is consistent with contemporaneous (ca. 70–50 Ma) underplating and initial exhumation of Orocopia Schist to the southwest in western Arizona and Mesozoic garnet-clinopyroxenite xenoliths of possible Mojave batholith keel affinity in ca. 25 Ma TZ volcanic rocks. 

   more » « less
3. Mantle Wavespeed and Discontinuity Structure Below East Africa: Implications for Cenozoic Hotspot Tectonism and the Development of the Turkana Depression

   https://doi.org/10.1029/2022GC010775  

   Boyce, A.; Kounoudis, R.; Bastow, I. D.; Cottaar, S.; Ebinger, C. J.; Ogden, C. S. (August 2023, Geochemistry, Geophysics, Geosystems)

   Abstract Ethiopia's Cenozoic flood basalt magmatism, uplift, and rifting have been attributed to one or more mantle plumes. The Nubian plate, however, has drifted 500–1,000 km north since initial magmatism at ∼45 Ma, having developed above mantle that now underlies the northern Tanzania craton and the low‐lying Turkana Depression. Unfortunately, our knowledge of mantle wavespeed structure and mantle transition zone (MTZ) topography below these regions is poorest, due to a historical lack of seismograph stations. The same data gap means we lack constraints on lithospheric structure in and around the NW–SE trending Mesozoic Anza rift. We exploit data from new seismograph networks in the Turkana Depression and neighboring northern Uganda to develop AFRP22, a new African absolute P‐wavespeed tomographic model that resolves whole mantle structure along the entire East African rift system. We also map MTZ thickness using Ps receiver functions. East Africa's thinnest MTZ (∼25 km thinning) underlies the northwest Turkana Depression. AFRP22 reveals a co‐located, previously unrecognized, slow wavespeed plume tail, extending from the MTZ, deep into the lower mantle. This plume may thus have contributed, along with the African Superplume, to the development of the 45–30 Ma flood basalt province that preceded extension. Pervasive sub‐lithospheric slow wavespeeds imply that Turkana's present‐day low elevation is explained best by Mesozoic and Cenozoic‐age crustal thinning. At ∼100 km depth, AFRP22 illuminates a fast wavespeed SE Ethiopian plateau. In addition to governing the northernmost limit of Mesozoic Anza rifting, the refractory nature of this lithospheric block likely minimized Cenozoic flood basalt magmatism there. 

   more » « less
4. Upper-plate response to ridge subduction and oceanic plateau accretion, Washington Cascades and surrounding region: Implications for plate tectonic evolution of the Pacific Northwest (USA and southwestern Canada) in the Paleogene

   https://doi.org/10.1130/GES02629.1  

   Miller, Robert B; Umhoefer, Paul J; Eddy, Michael P; Tepper, Jeffrey H (July 2023, Geosphere)

   Hempel, Andrea (Ed.)

   Abstract The interaction between subduction zones and oceanic spreading centers is a common tectonic process, and yet our understanding of how it is manifested in the geologic record is limited to a few well-constrained modern and ancient examples. In the Paleogene, at least one oceanic spreading center interacted with the northwestern margin of North America. Several lines of evidence place this triple junction near Washington (USA) and southern British Columbia (Canada) in the early to middle Eocene, and we summarize a variety of new data sets that permit us to track the plate tectonic setting and geologic evolution of this region from 65 to 40 Ma. The North Cascades segment of the voluminous Coast Mountains continental magmatic arc experienced a magmatic lull between ca. 60 and 50 Ma interpreted to reflect low-angle subduction. During this period of time, the Swauk Basin began to subside inboard of the paleo-trench in Washington, and the Siletzia oceanic plateau began to develop along the Farallon plate–Kula plate or Farallon plate–Resurrection plate spreading center. Farther east, peraluminous magmatism occurred in the Omineca belt and Idaho batholith. Accretion of Siletzia and ridge-trench interaction occurred between ca. 53 and 49 Ma, as indicated by: (1) near-trench magmatism from central Vancouver Island to northwestern Washington, (2) disruption and inversion of the Swauk Basin during a short-lived contractional event, (3) voluminous magmatism in the Kamloops-Challis belt accompanied by major E-W extension east of the North Cascades in metamorphic core complexes and supra-detachment basins and grabens, and (4) southwestward migration of magmatism across northeastern Washington. These events suggest that flat-slab subduction from ca. 60 to 52 Ma was followed by slab rollback and breakoff during accretion of Siletzia. A dramatic magmatic flare-up was associated with rollback and breakoff between ca. 49.4 and 45 Ma and included bimodal volcanism near the eastern edge of Siletzia, intrusion of granodioritic to granitic plutons in the crystalline core of the North Cascades, and extensive dike swarms in the North Cascades. Transtension during and shortly before the flare-up led to >300 km of total offset on dextral strike-slip faults, formation of the Chumstick strike-slip basin, and subhorizontal ductile stretching and rapid exhumation of rocks metamorphosed to 8–10 kbar in the North Cascades crystalline core. By ca. 45 Ma, the Farallon–Kula (or Resurrection)–North American triple junction was likely located in Oregon (USA), subduction of the Kula or Resurrection plate was established outboard of Siletzia, and strike-slip faulting was localized on the north-striking Straight Creek–Fraser River fault. Motion of this structure terminated by 35 Ma. These events culminated in the establishment of the modern Cascadia convergent margin. 

   more » « less
5. Architectural and Tectonic Control on the Segmentation of the Central American Volcanic Arc

   https://doi.org/10.1146/annurev-earth-082420-055108  

   Gazel, Esteban; Flores, Kennet E.; Carr, Michael J. (May 2021, Annual Review of Earth and Planetary Sciences)

   null (Ed.)

   Central America has a rich mix of conditions that allow comparisons of different natural experiments in the generation of arc magmas within the relatively short length of the margin. The shape of the volcanic front and this margin's architecture derive from the assemblage of exotic continental and oceanic crustal slivers, and later modification by volcanism and tectonic activity. Active tectonics of the Cocos-Caribbean plate boundary are strongly influenced by oblique subduction, resulting in a narrow volcanic front segmented by right steps occurring at ∼150-km intervals. The largest volcanic centers are located where depths to the slab are ∼90–110 km. Volcanoes that develop above deeper sections of the subducting slab are less voluminous and better record source geochemical heterogeneity. Extreme variations in isotopic and trace element ratios are derived from different components of thesubducted oceanic lithosphere. However, the extent that volcanoes sample these signatures is also influenced by lithospheric structures that control the arc segmentation. ▪  The architecture of Central America derives from the assemblage of exotic continental and oceanic crustal slivers modified by arc magmatism and tectonic processes. ▪  Active tectonics in Central America are controlled by oblique subduction. ▪  The lithospheric architecture and tectonics define the segmentation of the volcanic front, and thus the depth to the slab below a volcanic center. ▪  The composition of the subducted material is the main control of the along arc geochemical variations observed in Central American volcanoes. ▪  Geochemical heterogeneity in each segment is highlighted by extreme compositions representing the smaller centers with variations up to 65% of the total observed range. 

   more » « less