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USGS2021

Detrital zircon record of magmatism and sediment dispersal across the North American Cordilleran arc system (28-48°N)

As zircon U-Pb geochronology has become a leading method in sediment provenance studies and basin analysis over the past 20 years, the volume of detrital zircon data made available in published literature has enabled researchers to go beyond source-to-sink provenance studies to explore increasingly complex geologic problems. In this review, we utilize the growing body of detrital zircon data acquired from Jurassic-Paleocene forearc and foreland basin strata of the North American Cordillera to investigate the Mesozoic to earliest Cenozoic evolution of the arc and its associated basins between 28°N and 48°N. Our compilation includes 830 detrital zircon samples (101,898 individual ages) from 70 studies published between 2000 and 2020. For comparative purposes, we also compile 1307 igneous zircon U-Pb ages that characterize the magmatic history of the arc. We place primary emphasis on detrital zircon ages between 251 and 56 Ma that we infer to be uniquely derived from magmatic sources in the arc. Informed by existing knowledge of magmatic, structural, and sedimentological processes that acted on the orogen, we investigate spatial and temporal trends in these “arc-derived zircon” to establish a detrital record of arc magmatism, investigate source-to-sink relationships between the arc and adjacent basins, and discuss controls on sediment dispersal across the orogen. Our review shows that compilations of detrital zircon data from the Cordilleran forearc and foreland basin systems are excellent proxies for arc magmatism because the basins are enriched in arc-derived zircon and compilations provide space- and time-integrated records of crystallization ages. The compiled detrital zircon data support a history of continuous arc magmatism throughout Mesozoic and earliest Cenozoic time, characterized by low-volume magmatism from Triassic-Early Jurassic time (~251–174 Ma) and episodic higher-volume magmatism from Middle Jurassic-Late Cretaceous time (~174–66 Ma). These trends elucidate the initiation and timing of magmatic events at the orogen-scale and corroborate our understanding of cyclic arc behavior. Detrital zircon distributions are spatially and temporally variable both within and across basins, which we discuss relative to topographic development of the orogen and attendant responses of sediment dispersal systems. Detrital zircon distributions in the forearc signal rapid transfer of sediment from the arc to basins dominantly via fluvial processes. In contrast, detrital zircon distributions across the foreland reflect the presence of topographic barriers in the hinterland region of the arc that effectively isolated parts of the foreland. The presence of hinterland topography in turn highlights the important role of ash-fall events in delivering arc-derived zircon to the foreland, underscoring the need to consider ash-fall processes in paleodrainage reconstructions. These broad regional trends, and in general the close linkage between orogenic process and sediment dispersal, emerge from our compilation because it averages out much of the local variability observed in studies of more limited geographic or temporal extent.

North American Cordilleran arc system↗

USGS2016

Tree island pattern formation in the Florida Everglades

The Florida Everglades freshwater landscape exhibits a distribution of islands covered by woody vegetation and bordered by marshes and wet prairies. Known as “tree islands”, these ecogeomorphic features can be found in few other low gradient, nutrient limited freshwater wetlands. In the last few decades, however, a large percentage of tree islands have either shrank or disappeared in apparent response to altered water depths and other stressors associated with human impacts on the Everglades. Because the processes determining the formation and spatial organization of tree islands remain poorly understood, it is still unclear what controls the sensitivity of these landscapes to altered conditions. We hypothesize that positive feedbacks between woody plants and soil accretion are crucial to emergence and decline of tree islands. Likewise, positive feedbacks between phosphorus (P) accumulation and trees explain the P enrichment commonly observed in tree island soils. Here, we develop a spatially-explicit model of tree island formation and evolution, which accounts for these positive feedbacks (facilitation) as well as for long range competition and fire dynamics. It is found that tree island patterns form within a range of parameter values consistent with field data. Simulated impacts of reduced water levels, increased intensity of drought, and increased frequency of dry season/soil consuming fires on these feedback mechanisms result in the decline and disappearance of tree islands on the landscape.

Florida↗

USGS2014

Development of a generic auto-calibration package for regional ecological modeling and application in the Central Plains of the United States

Process-oriented ecological models are frequently used for predicting potential impacts of global changes such as climate and land-cover changes, which can be useful for policy making. It is critical but challenging to automatically derive optimal parameter values at different scales, especially at regional scale, and validate the model performance. In this study, we developed an automatic calibration (auto-calibration) function for a well-established biogeochemical model—the General Ensemble Biogeochemical Modeling System (GEMS)-Erosion Deposition Carbon Model (EDCM)—using data assimilation technique: the Shuffled Complex Evolution algorithm and a model-inversion R package—Flexible Modeling Environment (FME). The new functionality can support multi-parameter and multi-objective auto-calibration of EDCM at the both pixel and regional levels. We also developed a post-processing procedure for GEMS to provide options to save the pixel-based or aggregated county-land cover specific parameter values for subsequent simulations. In our case study, we successfully applied the updated model (EDCM-Auto) for a single crop pixel with a corn–wheat rotation and a large ecological region (Level II)—Central USA Plains. The evaluation results indicate that EDCM-Auto is applicable at multiple scales and is capable to handle land cover changes (e.g., crop rotations). The model also performs well in capturing the spatial pattern of grain yield production for crops and net primary production (NPP) for other ecosystems across the region, which is a good example for implementing calibration and validation of ecological models with readily available survey data (grain yield) and remote sensing data (NPP) at regional and national levels. The developed platform for auto-calibration can be readily expanded to incorporate other model inversion algorithms and potential R packages, and also be applied to other ecological models.

Ecological Informatics↗

USGS2009

Predicting bed shear stress and its role in sediment dynamics and restoration potential of the Everglades and other vegetated flow systems

Entrainment of sediment by flowing water affects topography, habitat suitability, and nutrient cycling in vegetated floodplains and wetlands, impacting ecosystem evolution and the success of restoration projects. Nonetheless, restoration managers lack simple decision-support tools for predicting shear stresses and sediment redistribution potential in different vegetation communities. Using a field-validated numerical model, we developed state-space diagrams that provide these predictions over a range of water-surface slopes, depths, and associated velocities in Everglades ridge and slough vegetation communities. Diminished bed shear stresses and a consequent decrease in bed sediment redistribution are hypothesized causes of a recent reduction in the topographic and vegetation heterogeneity of this ecosystem. Results confirmed the inability of present-day flows to entrain bed sediment. Further, our diagrams showed bed shear stresses to be highly sensitive to emergent vegetation density and water-surface slope but less sensitive to water depth and periphyton or floating vegetation abundance. These findings suggested that instituting a pulsing flow regime could be the most effective means to restore sediment redistribution to the Everglades. However, pulsing flows will not be sufficient to erode sediment from sloughs with abundant spikerush, unless spikerush density first decreases by natural or managed processes. Our methods provide a novel tool for identifying restoration parameters and performance measures in many types of vegetated aquatic environments where sediment erosion and deposition are involved.

Ecological Engineering↗

USGS2012

An economic approach to assessing import policies designed to prevent the arrival of invasive species: the case of Puccinia psidii in Hawai'i

Since its first documented introduction to Hawai‘i in 2005, the rust fungus Puccinia psidii has already severely damaged Syzygium jambos (Indian rose apple) trees and the federally endangered Eugenia koolauensis (nioi). Fortunately, the particular strain has yet to cause serious damage to Metrosideros polymorpha (‘ōhi‘a), which comprises roughly 80% of the state's native forests and covers 400,000 ha. Although the rust has affected less than 5% of Hawaii's ‘ōhi‘a trees thus far, the introduction of more virulent strains and the genetic evolution of the current strain are still possible. Since the primary pathway of introduction is Myrtaceae plant material imported from outside the state, potential damage to ‘ōhi‘a can be minimized by regulating those high-risk imports. We discuss the economic impact on the state's florist, nursery, landscaping, and forest plantation industries of a proposed rule that would ban the import of non-seed Myrtaceae plant material and require a 1-year quarantine of seeds. Our analysis suggests that the benefits to the forest plantation industry of a complete ban on non-seed material would likely outweigh the costs to other affected sectors, even without considering the reduction in risk to ‘ōhi‘a. Incorporating the value of ‘ōhi‘a protection would further increase the benefit–cost ratio in favor of an import ban.

Hawai'i↗

USGS2011

A data-driven approach for modeling post-fire debris-flow volumes and their uncertainty

This study demonstrates the novel application of genetic programming to evolve nonlinear post-fire debris-flow volume equations from variables associated with a data-driven conceptual model of the western United States. The search space is constrained using a multi-component objective function that simultaneously minimizes root-mean squared and unit errors for the evolution of fittest equations. An optimization technique is then used to estimate the limits of nonlinear prediction uncertainty associated with the debris-flow equations. In contrast to a published multiple linear regression three-variable equation, linking basin area with slopes greater or equal to 30 percent, burn severity characterized as area burned moderate plus high, and total storm rainfall, the data-driven approach discovers many nonlinear and several dimensionally consistent equations that are unbiased and have less prediction uncertainty. Of the nonlinear equations, the best performance (lowest prediction uncertainty) is achieved when using three variables: average basin slope, total burned area, and total storm rainfall. Further reduction in uncertainty is possible for the nonlinear equations when dimensional consistency is not a priority and by subsequently applying a gradient solver to the fittest solutions. The data-driven modeling approach can be applied to nonlinear multivariate problems in all fields of study.

Environmental Modelling and Software↗

USGS2014

A computer program for uncertainty analysis integrating regression and Bayesian methods

This work develops a new functionality in UCODE_2014 to evaluate Bayesian credible intervals using the Markov Chain Monte Carlo (MCMC) method. The MCMC capability in UCODE_2014 is based on the FORTRAN version of the differential evolution adaptive Metropolis (DREAM) algorithm of Vrugt et al. (2009), which estimates the posterior probability density function of model parameters in high-dimensional and multimodal sampling problems. The UCODE MCMC capability provides eleven prior probability distributions and three ways to initialize the sampling process. It evaluates parametric and predictive uncertainties and it has parallel computing capability based on multiple chains to accelerate the sampling process. This paper tests and demonstrates the MCMC capability using a 10-dimensional multimodal mathematical function, a 100-dimensional Gaussian function, and a groundwater reactive transport model. The use of the MCMC capability is made straightforward and flexible by adopting the JUPITER API protocol. With the new MCMC capability, UCODE_2014 can be used to calculate three types of uncertainty intervals, which all can account for prior information: (1) linear confidence intervals which require linearity and Gaussian error assumptions and typically 10s–100s of highly parallelizable model runs after optimization, (2) nonlinear confidence intervals which require a smooth objective function surface and Gaussian observation error assumptions and typically 100s–1,000s of partially parallelizable model runs after optimization, and (3) MCMC Bayesian credible intervals which require few assumptions and commonly 10,000s–100,000s or more partially parallelizable model runs. Ready access allows users to select methods best suited to their work, and to compare methods in many circumstances.

Environmental Modelling and Software↗

USGS2021

Online-coupling of widely-ranged timescales to model coral reef development

The increasing pressure on Earth's ecosystems due to climate change is becoming more and more evident and the impacts of climate change are especially visible on coral reefs. Understanding how climate change interacts with the physical environment of reefs to impact coral growth and reef development is critically important to predicting the persistence of reefs into the future. In this study, a biophysical model was developed including four environmental factors in a feedback loop with the coral's biology: (1) light; (2) hydrodynamics; (3) temperature; and (4) pH. The submodels are online coupled, i.e. regularly exchanging information and feedbacks while the model runs. This ensures computational efficiency despite the widely-ranged timescales. The composed biophysical model provides a significant step forward in understanding the processes that modulate the evolution of coral reefs, as it is the first construction of a model in which the hydrodynamics are included in the feedback loop.

Environmental Modeling and Software↗

USGS2005

The South China sea margins: Implications for rifting contrasts

Implications regarding spatially complex continental rifting, crustal extension, and the subsequent evolution to seafloor spreading are re-examined for the northern and southern-rifted margins of the South China Sea. Previous seismic studies have shown dramatic differences in the present-day crustal thicknesses as the manifestations of the strain experienced during the rifting of the margin of south China. Although the total crustal extension is presumed to be the same along the margin and adjacent ocean basin, the amount of continental crustal extension that occurred is much less along the east and central segments of the margin than along the western segment. This difference was accommodated by the early formation of oceanic crust (creating the present-day South China Sea basin) adjacent to the eastern margin segment while continued extension of continental crust was sustained to the west. Using the observed cross-sectional areas of extended continental crust derived from deep penetration seismics, two end-member models of varying rift zone widths and varying initial crustal thicknesses are qualitatively examined for three transects. Each model implies a time difference in the initiation of seafloor spreading inferred for different segments along the margin. The two models examined predict that the oceanic crust of the South China Sea basin toward the west did not begin forming until sometime between 6-12 my after its initial formation (???32 Ma) toward the east. These results are compatible with crustal age interpretations of marine magnetic anomalies. Assuming rifting symmetry with conjugate margin segments now residing along the southern portions of the South China Sea basin implies that the total width of the zone of rifting in the west was greater than in the east by about a factor of two. We suggest the most likely causes of the rifting differences were east-west variations in the rheology of the pre-rift crust and associated east-west variations in the thermal structure of the pre-rift lithosphere. The calculated widths of rifted continental crust for the northern and southern margins, when combined with the differential widths of seafloor generated during the seafloor spreading phase, indicate the total crustal extension that occurred is about 1100 km and is remarkably consistent to within ???10% for all three (eastern, central, western) segments examined. ?? 2005 Elsevier B.V. All rights reserved.

Earth and Planetary Science Letters↗

USGS2006

Temporal variations in slip rate of the White Mountain Fault Zone, Eastern California

The evolution of fault slip through time may yield insight into the geodynamics of deforming lithosphere. Precise determination of temporal variations in fault slip is often hindered, however, by a dearth of markers of varying age from which to reconstruct fault slip. Here we determine slip rates across the White Mountain Fault Zone over the past ca. 0.8??Ma from displaced alluvial deposits preserved along the flank of the White Mountains. Displacement histories inferred from deposits containing the ??? 760??ka Bishop Tuff contrast strongly with those inferred from Late Pleistocene alluvial fans dated by cosmogenic 36Cl, indicating that the fault has experienced significant temporal variations in slip rate. Oblique-slip rates over the past ??? 760??ky are determined to have been > 0.9??m/ky parallel to a net slip vector plunging shallowly (< 20??) toward 340??-350??. Displacement of markers in Late Pleistocene alluvial deposits, in contrast, yield slip rates between 0.3 and 0.4??m/ky parallel to a vector plunging somewhat more steeply (20??-40??) toward 320??-340??. These variations in slip rate appear to have operated over timescales > ca. 70??ky, yet occurred on a well-established fault system. Moreover, the timing and magnitude of slip rate variation mimics behavior documented for the northern Fish Lake Valley fault zone. Together, our data provide evidence for coordinated slip rate variations across the Eastern California Shear Zone during the Pleistocene. ?? 2006 Elsevier B.V. All rights reserved.

Earth and Planetary Science Letters↗

USGS2009

Methane sources and production in the northern Cascadia margin gas hydrate system

The oceanographic and tectonic conditions of accretionary margins are well-suited for several potential processes governing methane generation, storage and release. To identify the relevant methane evolution pathways in the northern Cascadia accretionary margin, a four-site transect was drilled during Integrated Ocean Drilling Program Expedition 311. The δ13C values of methane range from a minimum value of − 82.2‰ on an uplifted ridge of accreted sediment near the deformation front (Site U1326, 1829 mbsl, meters below sea level) to a maximum value of − 39.5‰ at the most landward location within an area of steep canyons near the shelf edge (Site U1329, 946 mbsl). An interpretation based solely on methane isotope values might conclude the 13C-enrichment of methane indicates a transition from microbially- to thermogenically-sourced methane. However, the co-existing CO2 exhibits a similar trend of 13C-enrichment along the transect with values ranging from − 22.5‰ to +25.7‰. The magnitude of the carbon isotope separation between methane and CO2 (εc = 63.8 ± 5.8) is consistent with isotope fractionation during microbially mediated carbonate reduction. These results, in conjunction with a transect-wide gaseous hydrocarbon content composed of > 99.8% (by volume) methane and uniform δDCH4 values (− 172‰ ± 8) that are distinct from thermogenic methane at a seep located 60 km from the Expedition 311 transect, suggest microbial CO2 reduction is the predominant methane source at all investigated sites. The magnitude of the intra-site downhole 13C-enrichment of CO2 within the accreted ridge (Site U1326) and a slope basin nearest the deformation front (Site U1325, 2195 mbsl) is ~ 5‰. At the mid-slope site (Site U1327, 1304 mbsl) the downhole 13C-enrichment of the CO2 is ~ 25‰ and increases to ~ 40‰ at the near-shelf edge Site U1329. This isotope fractionation pattern is indicative of more extensive diagenetic alteration at sites with greater 13C-enrichment. The magnitude of the 13C-enrichment of CO2 correlates with decreasing sedimentation rates and a diminishing occurrence of stratigraphic gas hydrate. We suggest the decreasing sedimentation rates increase the exposure time of sedimentary organic matter to aerobic and anaerobic degradation, during burial, thereby reducing the availability of metabolizable organic matter available for methane production. This process is reflected in the occurrence and distribution of gas hydrate within the northern Cascadia margin accretionary prism. Our observations are relevant for evaluating methane production and the occurrence of stratigraphic gas hydrate within other convergent margins.

Earth and Planetary Science Letters↗

USGS2011

Strong climate and tectonic control on plagioclase weathering in granitic terrain

Investigations to understand linkages among climate, erosion and weathering are central to quantifying landscape evolution. We approach these linkages through synthesis of regolith data for granitic terrain compiled with respect to climate, geochemistry, and denudation rates for low sloping upland profiles. Focusing on Na as a proxy for plagioclase weathering, we quantified regolith Na depletion, Na mass loss, and the relative partitioning of denudation to physical and chemical contributions. The depth and magnitude of regolith Na depletion increased continuously with increasing water availability, except for locations with mean annual temperature < 5 °C that exhibited little Na depletion, and locations with physical erosion rates < 20 g m − 2 yr − 1 that exhibited deep and complete regolith Na depletion. Surface Na depletion also tended to decrease with increasing physical erosion. Depth-integrated Na mass loss and regolith depth were both three orders of magnitude greater in the fully depleted, low erosion rate sites relative to other locations. These locations exhibited strong erosion-limitation of Na chemical weathering rates based on correlation of Na chemical weathering rate to total Na denudation. Sodium weathering rates in cool locations with positive annual water balance were strongly correlated to total Na denudation and precipitation, and exhibited an average apparent activation energy ( Ea ) of 69 kJ mol − 1 Na. The remaining water-limited locations exhibited kinetic limitation of Na weathering rates with an Ea of 136 kJ mol − 1 Na, roughly equivalent to the sum of laboratory measures of Ea and dissolution reaction enthalpy for albite. Water availability is suggested as the dominant factor limiting rate kinetics in the water-limited systems. Together, these data demonstrate marked transitions and nonlinearity in how climate and tectonics correlate to plagioclase chemical weathering and Na mass loss.

Earth and Planetary Science Letters↗

USGS2013

A new model for the growth of basaltic shields based on deformation of Fernandina volcano, Galápagos Islands

Space-geodetic measurements of surface deformation produced by the most recent eruptions at Fernandina – the most frequently erupting volcano in the Galápagos Archipelago – reveal that all have initiated with the intrusion of subhorizontal sills from a shallow magma reservoir. This includes eruptions from fissures that are oriented both radially and circumferentially with respect to the summit caldera. A Synthetic Aperture Radar (SAR) image acquired 1–2 h before the start of a radial fissure eruption in 2009 captures one of these sills in the midst of its propagation toward the surface. Galápagos eruptive fissures of all orientations have previously been presumed to be fed by vertical dikes, and this assumption has guided models of the origin of the eruptive fissure geometry and overall development of the volcanoes. Our findings allow us to reinterpret the internal structure and evolution of Galápagos volcanoes and of similar basaltic shields. Furthermore, we note that stress changes generated by the emplacement of subhorizontal sills feeding one type of eruption may control the geometry of subsequent eruptive fissures. Specifically, circumferential fissures tend to open within areas uplifted by sill intrusions that initiated previous radial fissure eruptions. This mechanism provides a possible explanation for the pattern of eruptive fissures that characterizes all the western Galápagos volcanoes, as well as the alternation between radial and circumferential fissure eruptions at Fernandina. The same model suggests that the next eruption of Fernandina will be from a circumferential fissure in the area uplifted by the 2009 sill intrusion, just southwest of the caldera rim.

GalÃ¡pagos Islands↗

USGS2013

Crustal-scale recycling in caldera complexes and rift zones along the Yellowstone hotspot track: O and Hf isotopic evidence in diverse zircons from voluminous rhyolites of the Picabo volcanic field, Idaho

Rhyolites of the Picabo volcanic field (10.4–6.6 Ma) in eastern Idaho are preserved as thick ignimbrites and lavas along the margins of the Snake River Plain (SRP), and within a deep (>3 km) borehole near the central axis of the Yellowstone hotspot track. In this study we present new O and Hf isotope data and U–Pb geochronology for individual zircons, O isotope data for major phenocrysts (quartz, plagioclase, and pyroxene), whole rock Sr and Nd isotope ratios, and whole rock geochemistry for a suite of Picabo rhyolites. We synthesize our new datasets with published Ar–Ar geochronology to establish the eruptive framework of the Picabo volcanic field, and interpret its petrogenetic history in the context of other well-studied caldera complexes in the SRP. Caldera complex evolution at Picabo began with eruption of the 10.44±0.27 Ma (U–Pb) Tuff of Arbon Valley (TAV), a chemically zoned and normal-δ 18 O (δ 18 O magma=7.9‰) unit with high, zoned 87 Sr/ 86 Sr i (0.71488–0.72520), and low-ε Nd (0) (−18) and ε Hf (0) (−28). The TAV and an associated post caldera lava flow possess the lowest ε Nd (0) (−23), indicating ∼40–60% derivation from the Archean upper crust. Normal-δ 18 O rhyolites were followed by a series of lower-δ 18 O eruptions with more typical (lower crustal) Sr–Nd–Hf isotope ratios and whole rock chemistry. The voluminous 8.25±0.26 Ma West Pocatello rhyolite has the lowest δ 18 O value (δ 18 O melt =3.3‰), and we correlate it to a 1,000 m thick intracaldera tuff present in the INEL-1 borehole (with published zircon ages 8.04–8.35 Ma, and similarly low-δ 18 O zircon values). The significant (4–5‰) decrease in magmatic-δ 18 O values in Picabo rhyolites is accompanied by an increase in zircon δ 18 O heterogeneity from ∼1‰ variation in the TAV to >5‰ variation in the late-stage low-δ 18 O rhyolites, a trend similar to what is characteristic of Heise and Yellowstone, and which indicates remelting of variably hydrothermally altered tuffs followed by rapid batch assembly prior to eruption. However, due to the greater abundance of low-δ 18 O rhyolites at Picabo, the eruptive framework may reflect an intertwined history of caldera collapse and coeval Basin and Range rifting and hydrothermal alteration. We speculate that the source rocks with pre-existing low-δ 18 O alteration may be related to: (1) deeply buried and unexposed older deposits of Picabo-age or Twin Falls-age low-δ 18 O volcanics; and/or (2) regionally-abundant late Eocene Challis volcanics, which were hydrothermally altered near the surface prior to or during peak Picabo magmatism. Basin and Range extension, specifically the formation of metamorphic core complexes exposed in the region, could have facilitated the generation of low-δ 18 O magmas by exhuming heated rocks and creating the large water-rock ratios necessary for shallow hydrothermal alteration of tectonically (rift zones) and volcanically (calderas) buried volcanic rocks. These interpretations highlight the major processes by which supereruptive volumes of magma are generated in the SRP, mechanisms applicable to producing rhyolites worldwide that are facilitated by plume driven volcanism and extensional tectonics.

Idaho↗

USGS2015

The effect of pressurized magma chamber growth on melt migration and pre-caldera vent locations through time at Mount Mazama, Crater Lake, Oregon

The pattern of eruptions at long-lived volcanic centers provides a window into the co-evolution of crustal magma transport, tectonic stresses, and unsteady magma generation at depth. Mount Mazama in the Oregon Cascades has seen variable activity over the last 400 ky, including the 50 km 3 climactic eruption at ca. 7.7 ka that produced Crater Lake caldera. The physical mechanisms responsible for the assembly of silicic magma reservoirs that are the precursors to caldera-forming eruptions are poorly understood. Here we argue that the spatial and temporal distribution of geographically clustered volcanic vents near Mazama reflects the development of a centralized magma chamber that fed the climactic eruption. Time-averaged eruption rates at Mount Mazama imply an order of magnitude increase in deep magma influx prior to the caldera-forming event, suggesting that unsteady mantle melting triggered a chamber growth episode that culminated in caldera formation. We model magma chamber–dike interactions over &sim;50 ky preceding the climactic eruption to fit the observed distribution of surface eruptive vents in space and time, as well as petrologically estimated deep influx rates. Best fitting models predict an expanding zone of dike capture caused by a growing, oblate spheroidal magma chamber with 10–30 MPa of overpressure. This growing zone of chamber influence causes closest approaching regional mafic vent locations as well as more compositionally evolved Mazama eruptions to migrate away from the climactic eruptive center, returning as observed to the center after the chamber drains during the caldera-forming eruption.

Oregon↗

USGS2016

Radiometric dating of marine-influenced coal using Re–Os geochronology

Coal deposits are integral to understanding the structural evolution and thermal history of sedimentary basins and correlating contemporeous estuarine and fluvial delatic strata with marine sections. While marine shales may readily lend themselves to Re–Os dating due to the dominance of hydrogenous Re and Os, the lack of a chronometer for near-shore sedimentary environments hampers basinwide correlations in absolute time. Here, we employ the Re–Os geochronometer, along with total organic carbon (TOC) and Rock–Eval data, to determine the timing and conditions of a marine incursion at the top of the Matewan coal bed, Kanawha Formation, Pottsville Group, West Virginia, USA. The observed range for hydrogen index (HI: 267–290 mg hydrocarbon/gram total organic carbon) for these coal samples suggests dominance of aliphatic hydrocarbons with low carbon (<C 19 ) chain length. Average Re ( 107.6±16.4 ng/g ) and Os ( 0.52±0.09 ng/g ) concentrations of the marine-influenced Matewan coal are higher by few orders of magnitude than published data for terrestrial coal. A Re–Os isochron for the Matewan coal provides an age of 325±14 Ma (Model 3; MSWD = 12; n=19 ; 2 σ ). This is the first Re–Os age derived from coal samples; the age overlaps a new composite Re–Os age of 317±2 Ma for the immediately overlying Betsie Shale Member. External precision for replicate Os analyses carried out for several Matewan coal samples shows a positive correlation with their HI. The HI, which is low in terrestrial organic matter, reflects the degree of marine influence. Thus, samples with the most profound marine influence also have the best analytical reproducibility. Equilibration of Os isotopes with seawater under marine conditions overwhelms variability inherited from terrestrial plant debris, decreasing scatter on the isochron. The 187 Re/ 188 Os ratios of the Matewan coal (&sim;3300–5135) are higher than most of those previously published for Phanerozoic black shale (mostly <2000). Mass balance calculations based on Re/TOC and Os/TOC ratios for the Matewan coal indicate that both Re and Os are primarily marine in origin, and their high 187 Re/ 188 Os ratios confirm efficient removal of both elements from a sulfidic water column into the coal. We show that Re–Os geochronology of marine-influenced coal can be a viable tool for constraining depositional ages.

West Virginia↗

USGS2016

Large along-strike variations in the onset of Subandean exhumation: Implications for Central Andean orogenic growth

Plate tectonics drives mountain building in general, but the space-time pattern and style of deformation is influenced by how climate, geodynamics, and basement structure modify the orogenic wedge. Growth of the Subandean thrust belt, which lies at the boundary between the arid, high-elevation Central Andean Plateau and its humid, low-elevation eastern foreland, figures prominently into debates of orogenic wedge evolution. We integrate new apatite and zircon (U-Th)/He thermochronometer data with previously published apatite fission-track data from samples collected along four Subandean structural cross-sections in Bolivia between 15° and 20°S. We interpret cooling ages vs. structural depth to indicate the onset of Subandean exhumation and signify the forward propagation of deformation. We find that Subandean growth is diachronous south (11 ± 3 Ma) vs. north (6 ± 2 Ma) of the Bolivian orocline and that Subandean exhumation magnitudes vary by more than a factor of two. Similar north-south contrasts are present in foreland deposition, hinterland erosion, and paleoclimate; these observations both corroborate diachronous orogenic growth and illuminate potential propagation mechanisms. Of particular interest is an abrupt shift to cooler, more arid conditions in the Altiplano hinterland that is diachronous in southern Bolivia (16-13 Ma) vs. northern Bolivia (10-7 Ma) and precedes the timing of Subandean propagation in each region. Others have interpreted the paleoclimate shift to reflect either rapid surface uplift due to lithosphere removal or an abrupt change in climate dynamics once orographic threshold elevations were exceeded. These mechanisms are not mutually exclusive and both would drive forward propagation of the orogenic wedge by augmenting the hinterland backstop, either through surface uplift or spatially variable erosion. In summary, we suggest that diachronous Subandean exhumation was driven by piecemeal hinterland uplift, orography, and the outward propagation of deformation.

Andes↗

USGS2017

Thermal and petrologic constraints on lower crustal melt accumulation under the Salton Sea Geothermal Field

In the Salton Sea region of southern California (USA), concurrent magmatism, extension, subsidence, and sedimentation over the past 0.5 to 1.0 Ma have led to the creation of the Salton Sea Geothermal Field (SSGF)—the second largest and hottest geothermal system in the continental United States—and the small-volume rhyolite eruptions that created the Salton Buttes. In this study, we determine the flux of mantle-derived basaltic magma that would be required to produce the elevated average heat flow and sustain the magmatic roots of rhyolite volcanism observed at the surface of the Salton Sea region. We use a 2D thermal model to show that a lower-crustal, partially molten mush containing < 20 – 40 % "> <20–40% interstitial melt develops over a ∼ 10 5 "> ∼105 -yr timescale for basalt fluxes of 0.008 to 0.010 m 3 / m 2 / yr "> 0.010m3/m2/yr (∼0.0008 to ∼0.001 km / 3 yr "> /3yr injection rate) given extension rates at or below the current value of ∼0.01 m/yr ( Brothers et al., 2009 ). These regions of partial melt are a natural consequence of a thermal regime that scales with average surface heat flow in the Salton Trough, and are consistent with seismic observations. Our results indicate limited melting and assimilation of pre-existing rocks in the lower crust. Instead, we find that basalt fractionation in the lower crust produces derivative melts of andesitic to dacitic composition. Such melts are then expected to ascend and accumulate in the upper crust, where they further evolve to give rise to small-volume rhyolite eruptions (Salton Buttes) and fuel local spikes in surface heat flux as currently seen in the SSGF. Such upper crustal magma evolution, with limited assimilation of hydrothermally altered material, is required to explain the slight decrease in δ 18 O "> δ18O values of zircons (and melts) that have been measured in these rhyolites.

California↗