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At least 541 records · Page 30Linked to original sources
USGS2020

Coarse sediment dynamics in a large glaciated river system: Holocene history and storage dynamics dictate contemporary climate sensitivity

The gravel-bedded White River drains a 1279 km 2 basin in Washington State, with lowlands sculpted by continental glaciation and headwaters on an actively glaciated stratovolcano. Chronic aggradation along an alluvial fan near the river’s mouth has progressively reduced flood conveyance. In order to better understand how forecasted climate change may influence coarse sediment delivery and aggradation rates in this lowland depositional setting, we assessed the contemporary delivery and routing of coarse sediment through the watershed; this assessment was based on a rich set of topographic, sedimentologic, and hydrologic data from the past century, with a focus on repeat high-resolution topographic surveys from the past decade. We found that most of the lower river’s contemporary bed-load flux originates from persistent erosion of alluvial deposits in the lower watershed. This erosion is a response to a drop in local base level caused by a major avulsion across the fan in 1906 and then augmented by subsequent dredging. The 1906 avulsion and modern disequilibrium valley profiles reflect landscape conditioning by continental glaciation and a massive mid-Holocene lahar. In the proglacial headwaters, infrequent large sediment pulses have accomplished most of the observed coarse sediment export, with exported material blanketing downstream valley floors; during typical floods, transported bed material is largely sourced from erosion of these valley floor deposits. Throughout the watershed, we observe decadal-scale coarse sediment dynamics strongly related to the filling or emptying of valley-scale sediment storage over 10 2 −10 4 yr time scales, often in response to major disturbances that either emplace large deposits or influence their redistribution. Paraglacial responses in large watersheds are suggested to be inherently complicated and punctuated as a result of internal landform interactions and stochastic/threshold-dependent events. We argue, in combination, that Holocene disturbance, storage dynamics, and human flow modification make coarse sediment fluxes in the lower White River relatively insensitive to decadal climate variability. Results highlight the degree to which river sensitivity to contemporary disturbance, climatic or otherwise, may be contingent on local and idiosyncratic watershed histories, underscoring the need to unpack those histories while demonstrating the utility of watershed-scale high-resolution topography toward that end.

Washington↗
USGS2020

Estimating the contribution of tributary sand inputs to controlled flood deposits for sandbar restoration using elemental tracers, Colorado River, Grand Canyon National Park, Arizona

Completion of Glen Canyon Dam in 1963 resulted in complete elimination of sediment delivery from the upstream Colorado River basin to Grand Canyon and nearly complete control of spring snowmelt floods responsible for creating channel and bar morphology. Management of the river ecosystem in Grand Canyon National Park now relies on dam-release floods to redistribute tributary-derived sediment accumulated on the channel bed to higher-elevation sandbars. Here, we used multivariate mixing analysis of sediment elemental compositions to evaluate the extent to which flood deposits derive from tributary-supplied sand compared to reworked, relict predam sediment. The concentrations of seven major and trace elements (Fe, Ca, K, Ti, Rb, Sr, and Zr) were measured in very fine−, fine-, and medium-grained sand from flood deposits using X-ray fluorescence and interpreted using a Bayesian mixing model to characterize the proportion of sand originating from the Paria River, the only major tributary within the study reach. Flood deposits from the 2013 and 2014 controlled floods contained 69% ± 16% and 84% ± 20% Paria River−derived material, respectively, with substantial variation among sites. Based on a sand mass balance, we calculated that under decreasing storage conditions since 1963, ∼77%−83% of the annual Paria River sand flux needs to be retained within the mass of active sand stored in Marble Canyon each year to reach the observed concentration of Paria River sand at sample locations. This finding suggests that the use of controlled floods may continue to be effective for sandbar maintenance, provided sand inputs from the Paria River do not decline.

Arizona↗
USGS2021

Eroding Cascadia— Sediment and solute transport and landscape denudation in western Oregon and northwestern California

Riverine measurements of sediment and solute transport give empirical basin-scale estimates of bed-load, suspended-sediment, and silicate-solute fluxes for 100,000 km 2 of northwestern California and western Oregon. This spatially explicit sediment budget shows the multifaceted control of geology and physiography on the rates and processes of fluvial denudation. Bed-load transport is greatest for steep basins, particularly in areas underlain by the accreted Klamath terrane. Bed-load flux commonly decreases downstream as clasts convert to suspended load by breakage and attrition, particularly for softer rock types. Suspended load correlates strongly with lithology, basin slope, precipitation, and wildfire disturbance. It is highest in steep regions of soft rocks, and our estimates suggest that much of the suspended load is derived from bed-load comminution. Dissolution, measured by basin-scale silicate-solute yield, constitutes a third of regional landscape denudation. Solute yield correlates with precipitation and is proportionally greatest in low-gradient and wet basins and for high parts of the Cascade Range, where undissected Quaternary volcanic rocks soak in 2−3 m of annual precipitation. Combined, these estimates provide basin-scale erosion rates ranging from ∼50 t ∙ km −2 ∙ yr −1 (approximately equivalent to 0.02 mm ∙ yr −1 ) for low-gradient basins such as the Willamette River to ∼500 t ∙ km −2 ∙ yr −1 (∼0.2 mm ∙ yr −1 ) for steep coastal drainages. The denudation rates determined here from modern measurements are less than those estimated by longer-term geologic assessments, suggesting episodic disturbances such as fire, flood, seismic shaking, and climate change significantly add to long-term landscape denudation.

California, Oregon, Washington↗
USGS2021

No ring fracture in Mono Basin, California

In Mono Basin, California, USA, a near-circular ring fracture 12 km in diameter was proposed by R.W. Kistler in 1966 to have originated as the protoclastic margin of the Cretaceous Aeolian Buttes pluton, to have been reactivated in the middle Pleistocene, and to have influenced the arcuate trend of the chain of 30 young (62−0.7 ka) rhyolite domes called the Mono Craters. In view of the frequency and recency of explosive eruptions along the Mono chain, and because many geophysicists accepted the ring fracture model, we assembled evidence to test its plausibility. The shear zone interpreted as the margin of the Aeolian Buttes pluton by Kistler is 50−400 m wide but is exposed only along a 7-km-long set of four southwesterly outcrops that subtend only a 70° sector of the proposed ring. The southeast end of the exposed shear zone is largely within the older June Lake pluton, and at its northwest end, the contact of the Aeolian Buttes pluton with a much older one crosses the shear zone obliquely. Conflicting attitudes of shear structures are hard to reconcile with intrusive protoclasis. Also inconsistent with the margin of the ovoid intrusion proposed by Kistler, unsheared salients of the pluton extend ∼1 km north of its postulated circular outline at Williams Butte, where there is no fault or other structure to define the northern half of the hypothetical ring. The shear zone may represent regional Cretaceous transpression rather than the margin of a single intrusion. There is no evidence for the Aeolian Buttes pluton along the aqueduct tunnel beneath the Mono chain, nor is there evidence for a fault that could have influenced its vent pattern. The apparently arcuate chain actually consists of three linear segments that reflect Quaternary tectonic influence and not Cretaceous inheritance. A rhyolitic magma reservoir under the central segment of the Mono chain has erupted many times in the late Holocene and as recently as 700 years ago. The ring fracture idea, however, prompted several geophysical investigations that sought a much broader magma body, but none identified a low-density or low-velocity anomaly beneath the purported 12-km-wide ring, which we conclude does not exist.

California↗
USGS2021

New insights into organic matter accumulation from high-resolution geochemical analysis of a black shale: Middle and Upper Devonian Horn River Group, Canada

Organic matter (OM) accumulation in organic matter-rich mudstones, or black shales, is generally recognized to be controlled by combinations of bioproductivity, preservation, and dilution. However, specific triggers of OM deposition in these formations are commonly difficult to identify with geochemical proxies, in part because of feedbacks that cause geochemical proxies for these controls to vary synchronously. This apparent synchronicity is partly a function of sample spacing, commonly at decimeter to meter intervals, which may represent longer periods of time than is required for the development of feedbacks. Higher resolution data sets may be required to fully interpret OM accumulation. This study applies a novel combination of technologies to develop a high-resolution geochemical data set, integrating energy-dispersive X-ray fluorescence (EDXRF) and infrared imagery analyses, to record proxies for redox conditions, bioproductivity, and clastic and carbonate dilution in millimeter-resolution profiles of 133 core slabs from the Middle and Upper Devonian Horn River shale in the Western Canada Sedimentary Basin, which provides decadal-scale temporal resolution. A comparison to a more coarsely sampled data set from the same core results in substantially different interpretations of variations in bioproductivity, redox, and dilution proxies. Stratigraphic distributions of organic matter accumulation patterns (bioproductivity-control, siliciclastic/carbonate-dilution, and redox conditions-control) show that organic enrichment events were highly varied during deposition of the shale and were closely related to second- and third-order sea-level changes. High-resolution profiles indicate that bioproductivity was the predominant trigger for organic matter accumulation in a second-order highstand, particularly during deposition of third-order transgressive systems tracts. Organic matter accumulation was largely controlled by dilution from either carbonate or clastic sediments in a second-order lowstand. Bioproductivity-redox feedbacks developed on timescales of decades to centuries.

British Columbia, Northwest Territories↗
USGS2022

How similar was the 1983 Mw 6.9 Borah Peak earthquake rupture to its surface-faulting predecessors along the northern Lost River fault zone (Idaho, USA)?

We excavated trenches at two paleoseismic sites bounding a trans-basin bedrock ridge (the Willow Creek Hills) along the northern Lost River fault zone to explore the uniqueness of the 1983 M w 6.9 Borah Peak earthquake compared to its prehistoric predecessors. At the Sheep Creek site on the southernmost Warm Springs section, two earthquakes occurred at 9.8−14.0 ka (95% confidence) and 6.5−7.1 ka; each had ∼1.9 m of vertical displacement. About 4 km to the southeast, across the Willow Creek Hills, two ruptures at the Arentson Gulch site on the northernmost Thousand Springs section occurred at 9.0−14.7 ka and 6.1−7.5 ka with ∼1.9 m of vertical displacement each. We synthesize these and previous paleoseismic results into a model of five postglacial (<15 ka) ruptures along a ∼65 km reach of the northern Lost River fault zone. Our results show that the Borah Peak earthquake (34 km; 0.9 m mean displacement) was unique compared to previous ruptures that had both longer and shorter rupture lengths (∼25−38 km), more displacement (mean of ∼1.3−1.4 m), and equal or greater magnitude ( M w 6.9−7.1) than that in the 1983 earthquake. These ruptures support a hypothesis of variable rupture length and displacement on the northern Lost River fault zone and show that predecessors to the 1983 rupture have passed unimpeded through the Willow Creek Hills. Our work demonstrates that normal faults are capable of producing variable spatial-temporal patterns of rupture that, together with comparisons of fault geometry and historical rupture length, improve our understanding of fault segmentation and help inform models of earthquake rupture probability.

Idaho↗
USGS2022

The dynamic floor of Yellowstone Lake, Wyoming, USA: The last 14 k.y. of hydrothermal explosions, venting, doming, and faulting

Hydrothermal explosions are significant potential hazards in Yellowstone National Park, Wyoming, USA. The northern Yellowstone Lake area hosts the three largest hydrothermal explosion craters known on Earth empowered by the highest heat flow values in Yellowstone and active seismicity and deformation. Geological and geochemical studies of eighteen sublacustrine cores provide the first detailed synthesis of the age, sedimentary facies, and origin of multiple hydrothermal explosion deposits. New tephrochronology and radiocarbon results provide a four-dimensional view of recent geologic activity since recession at ca. 15–14.5 ka of the >1-km-thick Pinedale ice sheet. The sedimentary record in Yellowstone Lake contains multiple hydrothermal explosion deposits ranging in age from ca. 13 ka to ~1860 CE. Hydrothermal explosions require a sudden drop in pressure resulting in rapid expansion of high-temperature fluids causing fragmentation, ejection, and crater formation; explosions may be initiated by seismicity, faulting, deformation, or rapid lake-level changes. Fallout and transport of ejecta produces distinct facies of subaqueous hydrothermal explosion deposits. Yellowstone hydrothermal systems are characterized by alkaline-Cl and/or vapor-dominated fluids that, respectively, produce alteration dominated by silica-smectite-chlorite or by kaolinite. Alkaline-Cl liquids flash to steam during hydrothermal explosions, producing much more energetic events than simple vapor expansion in vapor-dominated systems. Two enormous explosion events in Yellowstone Lake were triggered quite differently: Elliott’s Crater explosion resulted from a major seismic event (8 ka) that ruptured an impervious hydrothermal dome, whereas the Mary Bay explosion (13 ka) was triggered by a sudden drop in lake level stimulated by a seismic event, tsunami, and outlet channel erosion.

Wyoming↗
USGS2022

The 180-km-long Meers-Willow Fault System in the Southern Oklahoma Aulacogen: A potential U.S. mid-continent seismic hazard

We integrate new high-resolution aeromagnetic data with seismic reflection data, well logs, satellite remote sensing, and field observations to provide a regional view of buried and exposed structures in the Southern Oklahoma Aulacogen and to assess their potential for future seismicity. Trends ranging from NW−SE to ∼E−W, peaking at 330° ± 4.5° and 280° ± 3°, dominate the magnetic lineaments of the Southern Oklahoma Aulacogen, reflecting basement contacts, dikes, and faults, including a previously unmapped ∼100-km-long basement fault, which is herein referred to as the Willow fault. The fault disrupts, truncates, and vertically offsets basement-related seismic reflectors and overlying Paleozoic strata up through the Permian reflectors. Surface deformation along the trend includes fault-parallel monoclinal folds, pervasive fractures, and fracture-hosted mud dikes in Permian evaporite units. These structures indicate a Permian or post-Permian reactivation of the fault. Along-strike, the Willow fault connects to the NW-trending, seismically active Meers Fault to comprise the ∼180-km-long Meers-Willow fault system, which potentially represents a major seismic hazard along the Southern Oklahoma Aulacogen. Fault slip potential analyses of the mapped potential fault traces show that seismic hazards are elevated where faults have steeper dips. Given some uncertainty in the regional stress state, we also show that hazards along the NW−SE to E−W trending faults vary considerably within the uncertainty range. We propose that the Meers-Willow fault system originated as a Cambrian aulacogen-scale, basement-rooted fault that was later reactivated as a left-lateral strike-slip fault (with ∼40 km displacement) during the late Paleozoic Ancestral Rocky Mountain orogeny, highlighting that lateral offset accommodated a major component of deformation during the orogen.

Oklahoma↗
USGS2022

Geophysical extent of the Wyoming Province, western USA: Insights into ancient subduction and craton stability

A new 3-D resistivity model, estimated from inversion of magnetotelluric data, images crustal and upper-mantle structure of the Wyoming Province and adjacent areas. The Archean province is imaged as a coherent resistive domain, in sharp contrast to active tectonic domains of the western U.S. Prominent high-conductivity belts define the northern, eastern, and southern margins of the Wyoming Province and are interpreted as sutures marking the remnants of Paleoproterozoic orogens. The model results suggest the northern boundary of the Wyoming Province is located 150 km south of its traditional placement and adjacent to a composite orogen separating the Wyoming Province and Medicine Hat block. The eastern province boundary is clearly imaged along the Black Hills, whereas the western margin is obscured by Cenozoic extension and magmatism. An internal boundary within the Wyoming Province is interpreted to represent a Neoarchean suture; in stark contrast to Proterozoic sutures, though, it is not marked by a high-conductivity belt. This difference in conductivity is speculated to reflect changes in the subduction process through time. The absence of high-conductivity along Archean sutures appears to be global in nature and related to reduced continental freeboard in the Archean which limited continental weathering and the delivery of carbon-rich sediments to the seafloor. Although the entire Wyoming Province has been proposed to have undergone lithospheric modification that lessened its stability, the resistivity model suggests a thick lithospheric root remains in place except along its western margin. These results suggest that Archean cratons may be more resistant to lithospheric modification by influx of heat and fluids associated with extension and plumes than previously thought, and that metasomatism does not necessarily weaken the lithosphere and set a craton on the path to destruction.

Colorado, Montana, Utah, Wyoming↗
USGS2023

Crustal block-controlled contrasts in deformation, uplift, and exhumation in the Santa Cruz Mountains, California, USA, imaged through apatite (U-Th)/He thermochronology and 3-D geological modeling

Deformation along strike-slip plate margins often accumulates within structurally partitioned and rheologically heterogeneous crustal blocks within the plate boundary. In these cases, contrasts in the physical properties and state of juxtaposed crustal blocks may play an important role in accommodation of deformation. Near the San Francisco Bay Area, California, USA, the Pacific−North American plate-bounding San Andreas fault bisects the Santa Cruz Mountains (SCM), which host numerous distinct, fault-bounded lithotectonic blocks that surround the San Andreas fault zone. In the SCM, a restraining bend in the San Andreas fault (the SCM bend) caused recent uplift of the mountain range since ca. 4 Ma. To understand how rheologic heterogeneity within a complex fault zone might influence deformation, we quantified plausible bounds on deformation and uplift across two adjacent SCM lithotectonic blocks on the Pacific Plate whose stratigraphic and tectonic histories differ. This was accomplished by combining 31 new apatite (U-Th)/He ages with existing thermochronological datasets to constrain exhumation of these two blocks. Additionally, surface exposures of the latest Miocene to late Pliocene Purisima Formation interpreted in 18 structural cross sections spanning the SCM allowed construction and restoration of Pliocene deformation in a three-dimensional geologic model. We found that rock uplift and deformation concentrated within individual Pacific Plate lithotectonic blocks in the SCM. Since 4 Ma, maximum principal strain computed for the more deformed block adjacent to the fault exceeded that computed for the less deformed block by at least 375%, and cumulative uplift has been more spatially extensive and higher in magnitude. We attribute the difference in uplift and deformation between the two blocks primarily to contrasts in lithotectonic structure, which resulted from diverging geologic histories along the evolving plate boundary.

California↗
USGS2023

Magnetotelluric insights into the formation and reactivation of trans-crustal shear zones in Precambrian basement of the eastern U.S. Midcontinent

Three-dimensional inversion of regional long-period magnetotelluric (MT) data reveals the presence of two distinct sets of high-conductivity belts in the Precambrian basement of the eastern U.S. Midcontinent. One set, beneath Missouri, Illinois, Indiana, and western Ohio, is defined by northwest–southeast-oriented conductivity structures; the other set, beneath Kentucky, West Virginia, western Virginia, and eastern Ohio, includes structures that are generally oriented northeast–southwest. The northwest-trending belts occur mainly in Paleoproterozoic crust, and we suggest that their high conductivity values are due to graphite precipitated within trans-crustal shear zones from intrusion-related CO 2 -rich fluids. Our MT inversion results indicate that some of these structures dip steeply through the crust and intersect the Moho, which supports an interpretation that the shear zones originated as “leaky” transcurrent faults or transforms during the late Paleoproterozoic or the early Mesoproterozoic. The northeast-trending belts are associated with Grenvillian orogenesis and also potentially with Iapetan rifting, although further work is needed to verify the latter possibility. We interpret the different geographic positions of these two sets of conductivity belts as reflecting differences in origin and/or crustal rheology, with the northwest-trending belts largely confined to older, stable, pre-Grenville cratonic Laurentia, and the northeast-trending belts largely having formed in younger, weaker marginal crust. Notably, these high-conductivity zones spatially correlate with Midcontinent fault-and-fold zones that affect Phanerozoic strata. Stratigraphic evidence indicates that Midcontinent fault-and-fold zones were particularly active during Phanerozoic orogenic events, and some remain seismically active today, so the associated high-conductivity belts likely represent long-lived weaknesses that transect the crust.

Illinois, Indiana, Iowa, Kentucky, Michigan, Misso↗
USGS2024

A far-traveled basalt lava flow in north-central Oregon, USA

Widely separated basalt lava-flow outcrops in north-central Oregon, USA, expose products of a single eruptive episode. A Pliocene lava flow, here informally termed the Tetherow basalt, issued from vents near Redmond, in the Deschutes basin of Oregon, as a plains-forming basalt now exposed in continuous outcrops northward for 60 km. A similar basalt crops out 47 km farther north, near Maupin, within what was then a slightly incised ancestral Deschutes River canyon. The northernmost outcrops of this lava flow lie on Fulton Ridge, in the Dalles basin, near the confluence of the Deschutes and Columbia Rivers. Complementary lines of evidence confirm these rocks are all from the same volcanic eruption. Outcrops in the Deschutes and Dalles basins are chemically similar high-titanium basalts, petrographically similar to each other and distinct from other lava flows in the area. Paleomagnetic directions from 11 scattered sites are similar and indistinguishable by various tests for a common mean. Three new 40 Ar/ 39 Ar ages indicate the Tetherow basalt eruption occurred between 5.5 Ma and 5.0 Ma, likely at ca. 5.2 Ma. The widely separated outcrops of this lava flow span 160–180 km along the ancestral Deschutes River and downstream Columbia River. The lava flow’s length and erupted volume of 15–20 km 3 are extraordinarily large in a non-flood-basalt setting. This lava flow provides a datum with which to describe regional physiographic history, assess incision rates, and infer tectonic history. Spanning different depositional basins, the Tetherow basalt is a useful chronologic and stratigraphic marker bed.

Oregon↗
USGS2024

Fault activity in the San Gabriel Mountains, southern California, USA: Insights from landscape morphometrics, erosion rates, and fault-slip rates

Many studies use landscape form to determine spatial patterns of tectonic deformation, and these are particularly effective when paired with independent measures of rock uplift and erosion. Here, we use morphometric analyses and 10 Be catchment-averaged erosion rates, together with reverse slip rates from the Sierra Madre−Cucamonga fault zone, to reveal patterns in uplift, erosion, and fault activity in the range front of the San Gabriel Mountains in southern California, USA. Our analysis tests two prevailing hypotheses: (1) the range front of the San Gabriel Mountains is at steady state, in which rock uplift balances erosion and topographic elevations are stable throughout time, and (2) that west-to-east increases in elevation, relief, erosion rate, and stream-channel steepness across the interior of the massif reflect a parallel reverse-slip rate gradient on the range-bounding Sierra Madre−Cucamonga fault zone. We show that although deviations from steady state occur, the range-front hillslopes and stream channels are typically both well-connected and adjusted to patterns in Quaternary uplift driven by motion on the range-front fault network. Accordingly, landscape morphometrics, 10 Be erosion rates, and model erosion rates effectively image spatial and temporal patterns in uplift. Interpreted jointly, these data reveal comparable peak slip rates on the Sierra Madre−Cucamonga fault zone and show that they do not monotonically increase from west to east. Thus, the eastward-increasing gradients developed within the interior of the massif are not solely related to reverse slip on the range-front faults. Evaluated on shorter length scales (<10 km), morphometric data corroborate earlier descriptions of the Sierra Madre−Cucamonga fault zone as multiple individual faults or fault sections, with slip rates tapering toward fault tips. We infer that these patterns imply the predominance of independent fault or fault section ruptures throughout the Quaternary, though data cannot rule out the possibility of large, connected Sierra Madre−Cucamonga fault zone ruptures. Deeper in the hanging wall of the Sierra Madre−Cucamonga fault zone, secondary faults accommodate range-front uplift. Motion on these faults may contribute to active uplift of the highest topography within the massif, in addition to partly reconciling differences between geologic and geodetic Sierra Madre−Cucamonga fault zone reverse-slip rates. This study provides a new, unified perspective on tectonics and landscape evolution in the San Gabriel Mountains.

California↗
USGS2024

Turbidite correlation for paleoseismology

Marine turbidite paleoseismology relies on the assumption of synchronous triggering of turbidity currents by earthquake shaking to infer rupture extent and recurrence. Such inference commonly depends on age dating and correlation of the physical stratigraphy of deposits carried by turbidity currents (i.e., turbidites) across great distances. Along the Cascadia subduction zone, which lies offshore the Pacific Northwest, USA, turbidite facies in core photographs, X-ray computed tomography images, and magnetic susceptibility (MS) data exhibit differences in character over relatively short distances, which implies that not all deposits can be correlated with confidence. Thus, subjective correlation based on expected similarity over great distances and weak age constraints does not independently support paleoseismic models. We present a new method for correlating turbidites along the Cascadia margin that can yield a more objective and repeatable stratigraphic framework to underpin earthquake recurrence. We use dynamic time warping to correlate MS logs and measure correlation coefficients of core pairs to evaluate correlation strength. We then compare these measures to a distribution of correlation coefficients of randomly generated turbidite sequences and find that only a small number of core pairs can be correlated more confidently than randomly stacked turbidites. This methodology promises a more robust correlation strategy for future stratigraphic studies.

Oregon, Washington↗
USGS2025

Re-evaluating the tectonic affinity of Proterozoic crustal provinces in the Southwest USA: Detrital zircon evidence for a Laurentian source for the Yavapai and Mojave Provinces

Models for crustal growth commonly involve the accretion of dominantly juvenile crust to continental margins. However, tracking the provenance and tectonic affinity of dominantly juvenile crustal provinces is challenging. This difficulty is highlighted by uncertainty over whether the Yavapai and Mojave Provinces, part of the >1300-km-wide system of Proterozoic orogens in southwestern Laurentia, (1) have similar crustal and tectonic histories and (2) if they formed on or near Laurentian, Australian, or Antarctic cratons. Here, we contribute new large-n detrital zircon U-Pb geochronology and Sm-Nd whole-rock isotope geochemistry to help constrain the provenance of the Yavapai Province and address these broader questions. Yavapai Province metasedimentary rocks from central Colorado in the southwestern USA have abundant pre-1.80 Ga detrital zircon grains, with ca. 1.85 Ga, 2.30 Ga, and 2.50−2.70 Ga peaks, and variable amounts of 1.79−1.78 Ga grains. Evolved whole-rock Sm-Nd isotopic compositions from these rocks, including 2.36−2.08 Ga model ages, also suggest mixing between 1.79 Ga and 1.78 Ga Yavapai Province arcs and early Proterozoic to Archean sources. Nearly identical pre-1.8 Ga detrital and inherited zircon age distributions suggest that the Yavapai and Mojave Provinces formed on and/or incorporated similar material. The Trans-Hudson orogen, and to a slightly lesser extent the Penokean orogen, provide the closest matches to the pre-1.80 Ga material in the Yavapai and Mojave Provinces. This similarity, coupled with a weaker resemblance to Australian and Antarctic sources, support a Laurentian affinity for the Yavapai and Mojave Provinces. We envision Paleoproterozoic arc formation on both oceanic crust and material of Laurentian affinity and multiple phases of arc-back-arc genesis, closure, and accretionary tectonism along the long-lived margin of the supercontinent Columbia (Nuna).

Arizona, Colorado, Idaho, New Mexico, Utah, Wyomin↗
USGS2025

Origin and evolution of mafic volcanism associated with 3 m.y. of andesite production at the Goat Rocks volcanic cluster, southern Washington Cascade Range

More than 3 m.y. of mafic volcanism near the Goat Rocks volcanic cluster in the southern Washington Cascade Range, USA, lends insight into the evolution of basalts and the subarc mantle at a long-lived, major arc volcanic locus. We contribute field observations, 40 Ar/ 39 Ar dates, paleomagnetic directions, and bulk rock and mineral compositions to characterize nine mafic units that erupted in association with the Goat Rocks volcanic cluster. The time frame of mafic volcanism, ca. 3.6 Ma to 60 ka, encompasses the lifespan of the central volcanic cluster (3.1 Ma to 115 ka), with a lull from ca. 2.7 Ma to 1.4 Ma. A climactic period of voluminous mafic activity and far-traveled lava flows, including construction of the Hogback Mountain shield volcano, coincided with voluminous andesite eruptions from the central volcanic cluster. The basaltic rocks in the Goat Rocks area are calc-alkaline to barely tholeiitic and have high field strength element depletion relative to large-ion lithophile elements characteristic of calc-alkaline basalts (CAB) of the Cascade volcanic arc. Unlike at neighboring andesitic volcanic centers (Mounts Adams, St. Helens, and Rainier), no other mafic end members such as high-aluminum olivine tholeiite (HAOT) or intraplate-type basalt (IPB) are present at or near the Goat Rocks volcanic cluster, although some of the calc-alkaline basalts in this study have IPB-like affinities. The Goat Rocks mafic units exhibit two main temporal trends in composition: (1) the most primitive basalts erupted earlier, compared to less primitive and more evolved compositions later, and (2) high field strength element concentrations are higher in the younger basalt units relative to the oldest two. In contrast to these temporal trends, the mafic units define two compositional groups that recur through time, a low-Sr and a high-Sr group, each with distinct trace element and Sr and Nd isotope ratios. Although radiogenic isotope ratios are generally aligned with High Cascades CAB and HAOT, some extend toward IPB of Mount Adams and Simcoe Mountains volcanic field. Olivine-dominated crystal fractionation at shallow pressure from a small range of parent magma compositions accounts for much of the variation among the basalts and basaltic andesites. A high-pressure fractionation model is plausible for only one of the youngest basalt units (basalt of Walupt Lake volcano). Mafic recharge and crustal assimilation accounts for the incompatible-element enriched composition of basaltic andesites erupted during construction of the largest andesitic centers, further supporting sustained basalt mass flux and thermal energy driving andesite genesis. We model the most primitive members of the Goat Rocks mafic units as partial melts of successively less depleted mantle in time. Variable degrees of fluxing with fluids and melts from subduction explain the distinction between high-Sr and low-Sr groups. We propose that mantle metasomatism by ancestral subduction and fluid-flux melting is heterogeneously distributed through the local subarc mantle and played a greater role in the genesis of the high-Sr basalt group. The limited range of primitive basalt types around the Goat Rocks volcanic cluster contrasts with the much greater diversity of basalts throughout the southern Washington to northern Oregon Cascade arc. On the other hand, the central volcanic cluster encompasses nearly the entire diversity observed at neighboring composite volcanoes. In the case of the Goat Rocks area at least, and perhaps attributable to the entire region, this means that the genesis of diverse intermediate magmas is independent from and does not require vastly different parental basalt compositions.

California, Oregon, Washington↗
USGS2025

An empirical test of maximum depositional age through paired LA-ICP-MS and CA-ID-TIMS detrital zircon analysis with implications for the “Nevadan orogeny” in the Sierra Nevada foothills, California, U.S.A.

The interpretation of maximum depositional age (MDA) from U-Pb detrital zircon data acquired via laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) or secondary ion mass spectrometry (SIMS) is now routine; however, to date only a few studies have presented tests of such MDAs by subsequently analyzing a subset of the same detrital zircon grains with the more accurate and precise chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) method. We first generated LA-ICP-MS dates and MDAs from three turbidite sandstone samples of the Late Jurassic Mariposa Formation (Sierra Nevada foothills, California) containing large proportions of young detrital zircon grains (at the time of sediment deposition). We then removed 4–5 of the apparent youngest grains from the epoxy mount and analyzed them with CA-ID-TIMS. Despite a lack of low Th/U or high U (ppm) characteristics that might indicate U-Pb system disturbance, all LA-ICP-MS dates are younger than corresponding CA-ID-TIMS dates, on the same grain, by up to 11.8 m.y. when considered as point estimates. Only two out of 13 paired analyses overlap within error at 95% confidence. We interpret MDAs from our CA-ID-TIMS dates to be the youngest CA-ID-TIMS dates (YTDs). LA-ICP-MS MDAs based on fewer dates from the young tail of the youngest date distribution perform the worst (i.e., furthest from CA-ID-TIMS MDAs), whereas LA-ICP-MS MDA methods that incorporate more dates from the youngest date distribution perform better; the best performing method in all cases is the maximum likelihood algorithm–minimum. The performance of tested LA-ICP-MS MDA methods is improved by removing statistical outlier dates and by removing visually young “outlier” dates that drift away from the primary distribution of young dates. Our paired LA-ICP-MS/CA-ID-TIMS MDA workflow shows that the accuracy of MDA can be significantly improved by conducting CA-ID-TIMS on as few as four grains from a sample. The combination of our new CA-ID-TIMS MDAs with published CA-ID-TIMS analyses and petrochronology on the nearby Guadalupe igneous complex provides sub-million-year resolution of contemporaneous igneous and sedimentary systems during deformation and clarifies the timing of regional deformation that defines the local “Nevadan orogeny.” Rocks in the upper plate of the Bear Mountains fault zone have a CA-ID-TIMS YTD MDA of 151.71 ± 0.23 Ma, and rocks of the lower plate have a CA-ID-TIMS YTD MDA of 149.92 ± 0.11 Ma. These new MDAs, in conjunction with the observation of fabric-bearing Mariposa Formation xenoliths in the ca. 149.65 ± 0.10 Ma Guadalupe igneous complex, suggest that rocks in the upper plate of the Bear Mountains fault zone represent a slightly older (~2 m.y.) section of Mariposa Formation that was deformed and intruded prior to being juxtaposed against, and further deformed with, a slightly younger lower-plate section of Mariposa Formation in actively deforming, fault-bounded basins. Our observations are not consistent with traditional models that require that sedimentation of the Mariposa Formation ended by ca. 155 Ma. Instead, we interpret our data to be consistent with other evidence for a continuum of deformation in Late Jurassic to Early Cretaceous time and document that the regional “slatey cleavage” observed in the greater Mariposa Formation and used to define the “Nevadan orogeny” in our study area is largely younger than 149.92 ± 0.11 Ma.

California↗
USGS2025

A 400-k.y. perspective on arc volcanism: An exceptional explosive eruption record from Central Mexico

Volcanic eruption records provide key information for hazard planning but suffer from a time-dependent loss of resolution, hindering long-term evaluations of volcano behavior. Here, we investigate an exceptional sedimentary sequence from Lake Chalco, on the SE edge of Mexico City, which provides a perspective on volcanic activity over hundred-thousand-year time scales, and we develop a methodological and analytical protocol applicable to core datasets globally. The lava-dominated base of the sequence (~105 m) is followed by ~155 m of lateral-collapse derived deposits and overlain by ~295 m of lacustrine sediments containing at least 450 visible tephra fall deposits (TFDs), spanning 400 k.y. These TFDs include 205 events sourced from large-magnitude (predominantly volcanic explosivity index [VEI] ≥5) silicic explosive eruptions, principally from regional polygenetic sources, and 205 deposits from the nearby Sierra Chichinautzin volcanic field (SCVF). A gradual decline in both frequency and apparent magnitude in the silicic eruption record is consistent with the gradual migration of volcanism south along the adjacent Sierra Nevada volcanic range, toward Popocatépetl. In contrast, the SCVF-derived deposits imply persistent but episodic activity, on time scales of 40–70 k.y., suggesting that the SCVF has been more continuously active than previously recognized. Decoupled trends between the SCVF and regional silicic sources implies that the total magmatic flux is not dictated at the arc scale, or by external (e.g., climatic) drivers, but instead reflects the independent development of individual volcanic systems. The records indicate a minimum long-term frequency of impactful eruptions on Mexico City to be one per 900 years (>1 cm tephra deposited) or 9000 years (≥10 cm tephra).

Central Mexico↗