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259 records · Page 15Linked to original sources
USGS2025

Timing of and pressure-temperature constraints on deformation in the Toxaway dome, eastern Blue Ridge: Evidence for continuous deformation from the Neoacadian orogeny to the Alleghanian orogeny

Many mountain belts are built through repeated collision, and in the case of orogenies closely spaced in time, determining when one orogeny ends and another begins can be challenging. The southern Appalachian mountains were formed by three mountain-building events closely spaced in time, including the Taconic (ca. 480–440 Ma), Neoacadian (ca. 375–340 Ma), and Alleghanian (ca. 330–265 Ma) orogenies. Notably, the end of the Neoacadian and the beginning of the Alleghanian are only separated by ~10 m.y., and some published dates record deformation and metamorphism in the eastern Blue Ridge during this interval, blurring the boundary between these two discrete events. The Toxaway dome, located along the North Carolina–South Carolina, USA, border at the eastern edge of the eastern Blue Ridge, is a structural dome cored by Mesoproterozoic Toxaway Gneiss and surrounded by the younger Tallulah Falls Formation. Previous ages constraints from the Toxaway dome (343 and 338 Ma U-Pb zircon ages) make it an ideal location to explore whether there was continuous deformation during this period of supposed quiescence between the Neoacadian and Alleghanian orogenies. We used optical microscopy and electron backscatter diffraction in quartz to determine deformation temperatures, thermobarometry to determine metamorphic pressure-temperature conditions, and monazite petrochronology to determine the timing of deformation. Quartz and feldspar recrystallization fabrics parallel to dome-defining fabrics indicate deformation occurred at amphibolite-facies conditions, which is corroborated by our pressure-temperature estimates of 0.67–0.8 ± 0.12 GPa and 661–689 ± 25 °C. Monazite grains that record the timing of reactions of garnet growth and breakdown range from 342 ± 4.8 Ma to 296 ± 10.8 Ma, bridging the interval between the Neoacadian and Alleghanian orogenies. Three samples from the nearby Tallulah Falls dome, which occupies a similar structural position along the edge of the eastern Blue Ridge in Georgia, record monazite dates of 334 ± 4.2 Ma to 304 ± 4.8 Ma, indicating there was tectonic activity in this region before the commonly defined beginning of the Alleghanian orogeny. We propose (1) there was no period of quiescence between the Neoacadian and Alleghanian orogenies in the eastern Blue Ridge, and (2) deformation during this time was at higher temperatures and pressures than previously reported.

Georgia, North Carolina, South Carolina↗
USGS2026

Tectonic implications of transitional melting regimes from petrological, geochronological, and compositional characterization of the ophiolitic Seventymile terrane, Alaska, USA

New geochemical, U-Pb geochronology, and Sr-Nd-Hf isotope data provide evidence for the tectonic evolution of the Seventymile terrane in interior Alaska, USA. Ultramafic and mafic rocks of the Seventymile terrane are thought to represent components of a dismembered ophiolite and provide unique constraints on regional terrane evolution and accretion. The Seventymile ophiolite represents fragments of the Devonian to Permian Slide Mountain Ocean (SMO) that separated allochthonous and parautochthonous continental fragments of western North America. It now occurs as multiple thrust sheets containing Permian mafic and ultramafic rocks overlying and/or possibly imbricated with amphibolite-facies supracrustal rocks of allochthonous Yukon-Tanana terrane and parautochthonous North America. Seventymile klippen contain variably serpentinized peridotite, primarily harzburgite, low-grade meta-mafic rocks, and minor oceanic sedimentary rocks (argillite, chert, limestone, and metasandstone). Mafic rocks include gabbro to diabase, typically as dikes, veinlets, or rare massive stocks intruding peridotite. Mafic rocks also include greenstones of the Seventymile assemblage in klippen structurally underlying, and in shear zone contact with, ultramafic klippen. New trace element and radiogenic isotope data from mafic magmatic rocks associated with the Seventymile ultramafic bodies show evidence for a weakly subduction-modified mantle source, like the mantle source of normal mid-ocean-ridge basalt (N-MORB) or back-arc basin basalt (BABB). Seventymile assemblage greenstones are more heterogeneous. They range from N-MORB to enriched mid-ocean-ridge basalt (E-MORB) and ocean-island basalt (OIB), with a subset of samples indicative of continental arc affinity. These geochemistry results indicate that distinct tectonic environments are represented by at least two, and possibly three, lithological and structural units comprising the Seventymile terrane. Hf-Nd isotope systematics are consistent with a depleted MORB mantle (DMM)−like component that overlaps with Pacific MORB. Primary zircon is rare, but new in situ U-Pb data for gabbro and greenstone indicate ca. 274−272 Ma peak zircon and titanite crystallization. Scattered younger zircons define a ca. 255 Ma zircon peak and correspond to secondary crystallization associated with baddeleyite reaction of high-Si fluids during low-grade metamorphism. If Seventymile suites are contemporaneous, obduction associated with the closure of the SMO resulted in the stacking of ophiolitic packages representing distinct tectonomagmatic settings across the transition from pericontinental, to epicontinental, to distal ocean back-arc. Intrusions hosted in klippe of ultramafic rocks, plus the least subduction-modified greenstones underlying them, geologically and compositionally resemble Slide Mountain rocks of the Campbell Range formation in eastern Yukon and may provide a new piercing point across the Tintina fault.

Alaska, British Columbia↗
USGS2025

Landsliding follows signatures of wildfire history and vegetative regrowth in a steep coastal shrubland

Five years after the deadly and destructive 9 January 2018 Montecito debris flows (Santa Barbara County, California, USA), an atmospheric river storm on 9 January 2023 triggered widespread landsliding that affected many of the same drainages in the Santa Ynez Mountains. Using high-resolution aerial imagery, we identified >10,000 landslides over an ∼160 km 2 area. Most of the landslides were shallow (<1 m in depth) translational debris slides that initiated on steep (∼40°), south-facing hillslopes, with the highest incidence of landsliding in a sandstone-dominated bedrock unit. The landslides mobilized into debris flows and delivered substantial quantities of sediment downstream, which contributed to costly infrastructure impairments. We detected order-of-magnitude differences in landslide density across the study area that could not be attributed to variations in geomorphology (topographic aspect and slope), geology (bedrock type), or hydrology (seasonal antecedent rainfall, peak hourly storm rainfall intensity, total storm rainfall), which are usually considered relevant factors for shallow landsliding. Rather, we found that vegetation regrowth following wildfire was likely a relevant factor associated with the highly variable landslide densities. Hillslopes with less and different types of vegetation regrowth after fire appear to have been more susceptible to shallow landslides. We identify a possible vegetation control on postfire landsliding, which highlights an opportunity for hypothesis testing using more advanced techniques to track the evolution of vegetation cover and vegetation type in steep shrubland environments following wildfire.

California↗
USGS2026

Syn-magmatic subsidence during the early stages of continental rifting in the Mesoproterozoic—A reanalysis of legacy data for the Midcontinent Rift, western Lake Superior

The Midcontinent Rift system (ca. 1.1 Ga) is a 2000-km-long series of elongated volcanic and sedimentary troughs and associated intrusive centers exposed chiefly in the Lake Superior region of North America. The rift system represents a long history of intense magmatism and subsequent sedimentation that was arrested by far-field tectonic events before sea-floor spreading was established. The premature cessation preserved a record of processes related to the beginning of continental rifting. The rift system under Lake Superior has been long studied using seismic-reflection data collected as part of the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE). We reexamine GLIMPCE Line C by developing a detailed velocity model for time to depth conversion constrained by other legacy data. We corroborate the model and develop a geologic interpretation using gravity and magnetic modeling and ties to geology mapped onshore. We recognize superposed subsiding sedimentary and volcanic basins for the southern half of the Line C depth section. This interpretation differs from previous paradigms that show major crustal faults that bound half-grabens or full grabens. We conclude that high-velocity (6.9 km/s) intrusive zones rather than major crustal faults border the sides of the basins. We speculate that the volcanic basin represents the initiation of seaward dipping reflectors. The syn-magmatic subsidence can be explained by dike injection and volcanic loading. Discrete lava basins throughout the region likely subsided at different times in a disorganized manner along the rift trend, raising questions about the long-term role of lithospheric thinning and melt generation.

Michigan, Minnesota, Ontario, Wisconsin↗
USGS2026

Characterizing changes in postfire debris-flow hazard as burned areas recover

Emergency assessments of postfire debris-flow hazards that are performed by the U.S. Geological Survey (USGS) provide estimates of debris-flow likelihood and rainfall triggering conditions that are used for evaluating and managing runoff-generated debris-flow hazards in recently burned areas throughout the western United States. Although the immediate postfire period, within roughly one year after fire, is typically the most susceptible to runoff-generated debris flows, the hazard evolves in time and space as the burned area recovers. The recovery trajectory a given burned area will take depends on local climate and weather and can be difficult to predict. Some burned areas recover quickly, whereas others experience debris flows for multiple years after fire. As a result, extending our ability to update debris-flow likelihood estimates and rainfall thresholds based on observed recovery of the burned area would be beneficial. We present a method for multi-year runoff-generated debris-flow hazard assessment that leverages the USGS “M1” debris-flow likelihood model and integrates updated, satellite-derived, normalized burn ratio data to estimate vegetation recovery. We predict recovery-aware rainfall thresholds and validate them against a multi-year debris-flow hazard prediction and could be adapted for use with other debris-flow models that incorporate burn severity data.

Arizona, California, Colorado, New Mexico, Washing↗
USGS2026

Ignimbrites of the Sour Creek dome, Yellowstone volcano, USA: Implications for the Lava Creek Tuff eruption and Yellowstone Caldera

Field observations plus sanidine compositional and 40 Ar/ 39 Ar age data collected on rocks from the Sour Creek dome (SCD) area of Yellowstone volcano (United States) serve to identify five new ignimbrite packages with ages (average 631.5 ± 0.9 ka) that are analytically indistinguishable from those of the Lava Creek Tuff (LCT), challenging the current framework of two LCT eruptive members. Newly recognized package 1 occurs as LCT-aged recycled clasts within a proximal lag breccia, reflecting a major time break in the LCT eruption sequence. Conformably above, the scoria-bearing ignimbrite of package 2 is extensively found in the dome, having been previously mis-mapped as Huck-leberry Ridge Tuff and LCT member A. To the south, package 3 is a crystal-rich, densely welded ignimbrite (inferred to be younger than package 2) that is conformably overlain by package 4, a fine-grained ignimbrite that contains abundant clasts of recycled tuff. Lastly, the topographically highest parts of the dome are made up of package 5, a multicomponent ignimbrite containing scoria, recycled clasts, and pumices, potentially correlative with package 2 and/or package 4. Cathodoluminescence imaging and zoning assessment was integrated with sanidine geochemistry from all five packages, including their recycled clasts. Four of the five packages were sourced from separate magmatic bodies within a larger plumbing system, each defined by their sanidine Ba distributions and componentry. Although the SCD was originally mapped as a resurgent dome and is actively deforming today, faults cutting the region show comparable offsets for NW-SE “resurgent faulting” as for orthogonal NE-SW faulting, the latter of which also cuts the 162 ka Elephant Back lava flow. We infer that faulting in the SCD well postdates eruption and that the SCD instead represents a domical accumulation of ignimbrite, not a resurgent structure. The abundance of recycled materials (and absence of Absaroka country rocks) in the SCD ignimbrites implies that vent locations were well inboard of the mapped caldera boundary, with some potentially along the line of the Yellowstone River, requiring a revision of the currently accepted LCT caldera outline. The LCT eruptive events were thus far more complex than currently recognized, with multiple deposits fed from multiple magma bodies over a time scale that cannot be resolved by radiometric dating.

Wyoming↗
USGS2020

Pulsed Mesozoic deformation in the Cordilleran hinterland and evolution of the Nevadaplano: Insights from the Pequop Mountains, NE Nevada

Mesozoic crustal shortening in the North American Cordillera’s hinterland was related to the construction of the Nevadaplano orogenic plateau. Petrologic and geochemical proxies in Cordilleran core complexes suggest substantial Late Cretaceous crustal thickening during plateau construction. In eastern Nevada, geobarometry from the Snake Range and Ruby Mountains-East Humboldt Range-Wood Hills-Pequop Mountains (REWP) core complexes suggests that the ~10–12 km thick Neoproterozoic-Triassic passive-margin sequence was buried to great depths (>30 km) during Mesozoic shortening and was later exhumed to the surface via high-magnitude Cenozoic extension. Deep regional burial is commonly reconciled with structural models involving cryptic thrust sheets, such as the hypothesized Windermere thrust in the REWP. We test the viability of deep thrust burial by examining the least-deformed part of the REWP in the Pequop Mountains. Observations include a compilation of new and published peak temperature estimates ( ⁠ n = 60 "> n = 60 n=60 ⁠ ) spanning the Neoproterozoic-Triassic strata, documentation of critical field relationships that constrain deformation style and timing, and new 40 Ar/ 39 Ar ages. This evidence refutes models of deep regional thrust burial, including (1) recognition that most contractional structures in the Pequop Mountains formed in the Jurassic, not Cretaceous, and (2) peak temperature constraints and field relationships are inconsistent with deep burial. Jurassic deformation recorded here correlates with coeval structures spanning western Nevada to central Utah, which highlights that Middle-Late Jurassic shortening was significant in the Cordilleran hinterland. These observations challenge commonly held views for the Mesozoic-early Cenozoic evolution of the REWP and Cordilleran hinterland, including the timing of contractional strain, temporal evolution of plateau growth, and initial conditions for high-magnitude Cenozoic extension. The long-standing differences between peak-pressure estimates and field relationships in Nevadan core complexes may reflect tectonic overpressure.

Nevada↗