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At least 1,063 records · Page 59Linked to original sources
USGS2019

Azimuthal seismic anisotropy of 70 Ma Pacific‐plate upper mantle

Plate formation and evolution processes are predicted to generate upper mantle seismic anisotropy and negative vertical velocity gradients in oceanic lithosphere. However, predictions for upper mantle seismic velocity structure do not fully agree with the results of seismic experiments. The strength of anisotropy observed in the upper mantle varies widely. Further, many refraction studies observe a fast direction of anisotropy rotated several degrees with respect to the paleospreading direction, suggesting that upper mantle anisotropy records processes other than 2D corner flow and plate‐driven shear near mid‐ocean ridges. We measure 6.0 ± 0.3% anisotropy at the Moho in 70 Ma lithosphere in the central Pacific with a fast direction parallel to paleospreading, consistent with mineral alignment by 2D mantle flow near a mid‐ocean ridge. We also find an increase in the strength of anisotropy with depth, with vertical velocity gradients estimated at 0.02 km/s/km in the fast direction and 0 km/s/km in the slow direction. The increase in anisotropy with depth can be explained by mechanisms for producing anisotropy other than intrinsic effects from mineral fabric, such as aligned cracks or other structures. This measurement of seismic anisotropy and gradients reflects the effects of both plate formation and evolution processes on seismic velocity structure in mature oceanic lithosphere, and can serve as a reference for future studies to investigate the processes involved in lithospheric formation and evolution.

Journal of Geophysical Research B: Solid Earth↗
USGS2019

Insights into pāhoehoe lava emplacement using visible and thermal structure-from-motion photogrammetry

We present the evolution over 3 months of a 2016–2017 pāhoehoe flow at Kīlauea as it changed from a narrow sheet flow into a compound lava field fed by a stable system of tubes. The portion of the flow located on Kīlauea's coastal plain was characterized using helicopter‐based visible and thermal structure‐from‐motion photogrammetry to construct a series of georeferenced digital surface models and thermal maps on eight different days. Results reveal key influences on the emplacement and evolution of such long‐lived pāhoehoe flows. This region of the flow grew by ~12 × 10 6 m 3 with a near‐constant time‐average discharge rate of 1.2–2.7 m 3 /s. The development of two tube systems is captured and shows an initial nascent tube enhanced by a narrow topographic confinement, which later inflated and created a topographic inversion that modulated the emplacement of a second flow lobe with its own tube system. The analysis of breakouts at various stages of the field's life suggests that the evolution of the thermal and morphological properties of the flow surface reflect its maturity. Thermal properties of breakouts were used to expand the empirical relationship of breakout cooling to longer timescales. This study contributes to the long‐term development and validation of more accurate predictive models for pāhoehoe, required during the management of long‐lasting lava flow crises in Hawai'i and elsewhere.

Hawaii↗
USGS2021

Fluid-earthquake and earthquake-earthquake interactions in southern Kansas, USA

An increase in injection activity associated with energy production in southern Kansas starting in 2013 has been linked to the occurrence of more than 130,000 earthquakes (M −1.5 to 4.9) between 2014 and 2017. Studies suggest that the dramatic increase in seismicity rate is related to wastewater injection into the highly permeable Arbuckle formation. Most of the seismicity is located in the underlying crystalline basement, for which hydrological properties and specific fault geometries are unknown. Additionally, some earthquake clusters occurred relatively far (tens of kilometers) from the main injection wells. Therefore, the effect of pore pressure diffusion may be insufficient to explain the relation between the volume of injected fluids and the spatiotemporal evolution of seismicity. Combining physical models (static stress and poroelasticity) and a statistical cluster analysis applied to a high-resolution relocated catalog, we analyze the evolution of seismicity in southern Kansas. We find that pore pressure changes (Δ p ) and Coulomb stress changes (ΔCFS) due to fluid diffusion smaller than 0.1 MPa are enough to initiate seismic sequences, which then evolve depending on their distance from the major injection wells. However, we find that earthquake sequences have different seismogenic responses to Δ p and ΔCFS in terms of triggering threshold. In regions located close to disposal wells (Harper area) our cluster analysis suggests that both earthquake interactions and fluid diffusion control the evolution of seismicity. On the other hand, at greater distances (Milan area), where clustering behavior suggests greater earthquake interactions, we find that coseismic ΔCFS are larger than Δ p .

Kansas, Oklahoma↗
USGS2021

Time-fractional flow equations (t-FFEs) to upscale transient groundwater flow characterized by temporally non-darcian flow due to medium heterogeneity

Upscaling groundwater flow is a fundamental challenge in hydrogeology. This study proposed time-fractional flow equations (t-FFEs) for upscaling long-term, transient groundwater flow and propagation of pressure heads in heterogeneous media. Monte Carlo simulations showed that, with increasing variance and correlation of the hydraulic conductivity ( K ), flow dynamics gradually deviated from Darcian flow and exhibit sub-diffusive, time-dependent evolution which can be separated into three major stages. At the early stage, the interconnected high- K zones dominated flow, while at intermediate times, the transverse flow due to mixed high- and low- K zones caused delayed rise of the piezometric head. At late times when flow in the relatively high- K domains reached stability, cells with very low- K continued to block the entry of water and generate “islands” with low piezometric head, significantly extending the temporal evolution of the piezometric head. The elongated water breakthrough curve cannot be quantified by the flow equation with an effective K , the space-fractional flow equation, or the multi-rate mass transfer (MRMT) flow model with a few rates, motivating the development of t-FFEs assuming temporally non-Darcian flow. Model applications showed that both the early and intermediate stages of flow dynamics can be captured by a single-index t-FFE (whose index is the exponent of the power-law probability density function of the random operational time for water parcels), but the overall evolution of flow dynamics, especially the enhanced retention of flow at later times, required a distributed-order t-FFE with variable indexes for different flow phases that can dominate flow dynamics at different stages. Therefore, transient groundwater flow in aquifers with spatially stationary heterogeneity can be temporally non-Darcian and non-stationary, due to the time-sensitive, combined effects of interconnected high- K channels and isolated low- K deposits on flow dynamics (which is the hydrogeological mechanism for the temporally non-Darcian flow and sub-diffusive pressure propagation), whose long-term behavior can be quantified by multi-index stochastic models.

Water Resources Research↗
USGS2022

Modeling of barrier breaching during Hurricanes Sandy and Matthew

Physical processes driving barrier island change during storms are important to understand to mitigate coastal hazards and to evaluate conceptual models for barrier evolution. Spatial variations in barrier island topography, landcover characteristics, and nearshore and back-barrier hydrodynamics can yield complex morphological change that requires models of increasing resolution and physical complexity to predict. Using the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system, we investigated two barrier island breaches that occurred on Fire Island, NY during Hurricane Sandy (2012) and at Matanzas, FL during Hurricane Matthew (2016). The model employed a recently implemented infragravity (IG) wave driver to represent the important effects of IG waves on nearshore water levels and sediment transport. The model simulated breaching and other changes with good skill at both locations, resolving differences in the processes and evolution. The breach simulated at Fire Island was 250 m west of the observed breach, whereas the breach simulated at Matanzas was within 100 m of the observed breach. Implementation of the vegetation module of COAWST to allow three-dimensional drag over dune vegetation at Fire Island improved model skill by decreasing flows across the back-barrier, as opposed to varying bottom roughness that did not positively alter model response. Analysis of breach processes at Matanzas indicated that both far-field and local hydrodynamics influenced breach creation and evolution, including remotely generated waves and surge, but also surge propagation through back-barrier waterways. This work underscores the importance of resolving the complexity of nearshore and back-barrier systems when predicting barrier island change during extreme events.

Florida, New York↗
USGS2023

Time-dependent weakening of granite at hydrothermal conditions

The evolution of a fault's frictional strength during the interseismic period is a critical component of the earthquake cycle, yet there have been relatively few studies that examine the time-dependent evolution of strength at conditions representative of seismogenic depths. Using a simulated fault in Westerly granite, we examined how frictional strength evolves under hydrothermal conditions up to 250°C during slide-hold-slide experiments. At temperatures ≤100°C, frictional strength generally increases with hold duration but, at 200 and 250°C, an initial increase in strength transitions to rapid time-dependent weakening for holds longer than 14 hr. Forward modeling of long hold periods at 250°C using the rate and state friction constitutive equations requires a second, strongly negative, state variable with a long evolution distance. This implies that significant hydrothermal alteration is occurring at 250°C, consistent with microstructural observations of dissolution and secondary mineral precipitation.

Geophysical Research Letters↗
USGS2024

Catchment coevolution and the geomorphic origins of variable source area hydrology

Features of landscape morphology—including slope, curvature, and drainage dissection—are important controls on runoff generation in upland landscapes. Over long timescales, runoff plays an essential role in shaping these same features through surface erosion. This feedback between erosion and runoff generation suggests that modeling long-term landscape evolution together with dynamic runoff generation could provide insight into hydrological function. Here we examine the emergence of variable source area runoff generation in a new coupled hydro-geomorphic model that accounts for water balance partitioning between surface flow, subsurface flow, and evapotranspiration as landscapes evolve over millions of years. We derive a minimal set of dimensionless numbers that provide insight into how hydrologic and geomorphic parameters together affect landscapes. Across the parameter space we investigated, model results collapsed to a single inverse relationship between the dimensionless relief and the ratio of catchment quickflow to discharge. Furthermore, we found an inverse relationship between the Hillslope number, which describes topographic relief relative to aquifer thickness, and the proportion of the landscape that was variably saturated. While the model generally produces fluvial topography visually similar to simpler landscape evolution models, certain parameter combinations produce wide valley bottom wetlands and non-dendritic, trellis-like drainage networks, which may reflect real conditions in some landscapes where aquifer gradients become decoupled from topography. With these results, we demonstrate the power of hydro-geomorphic models for generating new insights into hydrological processes, and also suggest that subsurface hydrology may be integral for modeling aspects of long-term landscape evolution.

Water Resources Research↗
USGS1996

Detailed observations of California foreshock sequences: Implications for the earthquake initiation process

We find that foreshocks provide clear evidence for an extended nucleation process before some earthquakes. In this study, we examine in detail the evolution of six California foreshock sequences, the 1986 Mount Lewis ( M L = 5.5), the 1986 Chalfant ( M L = 6.4), the 1986 Stone Canyon ( M L = 4.7), the 1990 Upland ( M L = 5.2), the 1992 Joshua Tree ( M W = 6.1), and the 1992 Landers ( M W = 7.3) sequence. Typically, uncertainties in hypocentral parameters are too large to establish the geometry of foreshock sequences and hence to understand their evolution. However, the similarity of location and focal mechanisms for the events in these sequences leads to similar foreshock waveforms that we cross correlate to obtain extremely accurate relative locations. We use these results to identify small-scale fault zone structures that could influence nucleation and to determine the stress evolution leading up to the mainshock. In general, these foreshock sequences are not compatible with a cascading failure nucleation model in which the foreshocks all occur on a single fault plane and trigger the mainshock by static stress transfer. Instead, the foreshocks seem to concentrate near structural discontinuities in the fault and may themselves be a product of an aseismic nucleation process. Fault zone heterogeneity may also be important in controlling the number of foreshocks, i.e., the stronger the heterogeneity, the greater the number of foreshocks. The size of the nucleation region, as measured by the extent of the foreshock sequence, appears to scale with mainshock moment in the same manner as determined independently by measurements of the seismic nucleation phase. We also find evidence for slip localization as predicted by some models of earthquake nucleation.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Links between magma-tectonic rate balances, plutonism, and volcanism

With Smith's (1979) model of ash flow magmatism as the point of departure, quantitative regimes of crustal magmatic evolution are outlined. The average extrusive regime of ash flow magmatism operates at a rate of about 0.001 km 3 /yr; progressively larger episodes of increasing repose times characterize the evolution of caldera-forming eruptions. The intrusive regime operates at an inferred average rate of about 0.01 km 3 /yr expressed in terms of the volumetric influx of mafic magma from the mantle, based on the record of silicic rates of extrusion and a 10:1 intrusive to extrusive ratio derived from evidence of zoned silicic systems. This rate is almost identical with the mean cumulative rate for the Hawaiian-Emperor system of basaltic volcanism, which is determined to be 0.015 km 3 /yr. Fluctuations about a mean rate of 10 −2 km 3 /yr lead to a threefold classification of magmatic regimes in terms of source power, depending on whether they are persistently greater than (HHER regime), about equal to (MHER regime), or lower than (LHER regime) the mean Hawaiian-Emperor rate. Ash flow magmatism correlates with the MHER regime, averaged over a sufficiently long time. Thus the conditions of Hawaiian-Emperor magmatism are identified with a characteristic power of magma transfer rates that optimize mechanisms for creation of high-level chambers of derivative and fractionated silicic magma (terminology of Smith (1979)) within sufficiently localized regions of high residence times. These conditions represent a balance point, and a bifurcation, in dynamic paths of magmatic evolution leading to widely divergent types of plutonic-volcanic associations. Categorical examples of the principal types of igneous path are discussed relative to continental basalt, silicic ash flow, batholithic granite, and intrusive porphyry systems. The Henry Mountains, Utah, are thought to represent an example of an intrusive porphyry generated in the LHER regime of a mafic source system lacking sufficient power for volcanic expression or significant mineralization. Long Valley, California (and possibly other systems along the eastern front of the Sierra Nevada), is an example operating at the mean rate with transient excursions to higher and lower rates. In regions of rapid extension and/or volcanic propagation (whether linear or areal) these regimes give rise to basaltic eruptions ranging from isolated cinder cones to flood basalts.

Journal of Geophysical Research Solid Earth↗
USGS1973

Oxidation during magmatic differentiation, Finnmarka Complex, Oslo area, Norway: Part 2, the mafic silicates

Electron-microprobe analyses are presented for pyroxene, amphibole, and biotite from monzonite, granodiorite, and granite at Finnmarka, Norway. Compositional trends measured in biotite, present in all three rock types, and in amphibole, present in the monzonite and granodiorite, are markedly atypical and are interpreted as reflecting crystallization under progressively more oxidizing conditions. The average Fe/Fe + Mg for biotites from successively more silicic rock types changes from 0.64 → 0.35 → 0.28, and for amphiboles changes from 0.58 in the monzonite to 0.29 in the granodiorite. Analyses of selected areas within amphibole grains in the granodiorite show marked chemical variations, although single-crystal X-ray photographs are sharp and do not reveal multiple phases. On the basis of 33 such analyses, four coupled substitutions are identified as operative; the most unusual finding is the relation of 1 Ti cation to 4 Al IV cations in the unit cell. Variations within individual amphibole grains of the granodiorite resemble changes noted in evolution of amphibole composition from monzonite to granodiorite and are interpreted as reflecting progressive oxidation. Consideration of these data for the mafic silicates, data for the opaque oxides, and the extensive formation of sphene in the granodiorite, has allowed development of schematic reactions and an overall picture of magmatic environment and evolution at Finnmarka. Crystallization apparently took place at PH 2 o of 1000 bars or less and a temperature of about 700 °C. The trend of oxidation during differentiation is more extreme than any heretofore reported. Amphiboles, as well as biotites, may participate in oxidation reactions and may reflect the oxidation-reduction processes that occurred during magmatic evolution.

Oslo↗
USGS2008

The chromosomes of the Didelphidae (Marsupialia) and their evolutionary significance

One hundred and seventy-seven specimens of American didelphids, representing 9 genera and 22 species have been studied for their chromosomal constitution. Didelphids are very conservative in chromosomal complements. All of the studied species can be sorted into one of three kinds of karyotypes: 2 n = 14 (three species of Didelphis, one of Lutreolina, two of Philander, and one of Chironectes) , 2 n = 14 (eight species of Marmosa, one of Metachirus, three of Caluromys, and one of Dromiciops), and 2 n = 18 (three species of Monodelphis). These karyotypes are stable, showing only minor variations within each basic pattern. It is concluded that chromosomals evolution in the Didelphidae proceededs from low numbers to higher numbers by a process of centromeric fissioning complemented by some pericentric inversions and/or translocations. The pattern of karyotypic stability is consistent with bradytely at the organismic level of evolution. This is explained by a low rate of regulatory genetic evolution promoted by epistatic selection favouring the retention of chromosomal arrangements highly advantageous for overall adaptation.

Biological Journal of the Linnean Society↗
USGS2003

Origin of the Blue Ridge escarpment along the passive margin of Eastern North America

The Blue Ridge escarpment is a rugged landform situated within the ancient Appalachian orogen. While similar in some respects to the great escarpments along other passive margins, which have evolved by erosion following rifting, its youthful topographic expression has inspired proposals of Cenozoic tectonic rejuvenation in eastern North America. To better understand the post-orogenic and post-rift geomorphic evolution of passive margins, we have examined the origin of this landform using low-temperature thermochronometry and manipulation of topographic indices. Apatite (U-Th)/He and fission-track analyses along transects across the escarpment reveal a younging trend towards the coast. This pattern is consistent with other great escarpments and fits with an interpretation of having evolved by prolonged erosion, without the requirement of tectonic rejuvenation. Measured ages are also comparable specifically to those measured along other great escarpments that are as much as 100 Myr younger. This suggests that erosional mechanisms that maintain rugged escarpments in the early post-rift stages may remain active on ancient passive margins for prolonged periods. The precise erosional evolution of the escarpment is less clear, however, and several end-member models can explain the data. Our preferred model, which fits with all data, involves a significant degree of erosional escarpment retreat in the Cenozoic. Although this suggests that early onset of topographic stability is not required of passive margin evolution, more data are required to better constrain the details of the escarpment's development.

Blue Ridge escarpment↗
USGS2004

Estimating tectonic history through basin simulation-enhanced seismic inversion: Geoinformatics for sedimentary basins

A data assimilation approach is demonstrated whereby seismic inversion is both automated and enhanced using a comprehensive numerical sedimentary basin simulator to study the physics and chemistry of sedimentary basin processes in response to geothermal gradient in much greater detail than previously attempted. The approach not only reduces costs by integrating the basin analysis and seismic inversion activities to understand the sedimentary basin evolution with respect to geodynamic parameters-but the technique also has the potential for serving as a geoinfomatics platform for understanding various physical and chemical processes operating at different scales within a sedimentary basin. Tectonic history has a first-order effect on the physical and chemical processes that govern the evolution of sedimentary basins. We demonstrate how such tectonic parameters may be estimated by minimizing the difference between observed seismic reflection data and synthetic ones constructed from the output of a reaction, transport, mechanical (RTM) basin model. We demonstrate the method by reconstructing the geothermal gradient. As thermal history strongly affects the rate of RTM processes operating in a sedimentary basin, variations in geothermal gradient history alter the present-day fluid pressure, effective stress, porosity, fracture statistics and hydrocarbon distribution. All these properties, in turn, affect the mechanical wave velocity and sediment density profiles for a sedimentary basin. The present-day state of the sedimentary basin is imaged by reflection seismology data to a high degree of resolution, but it does not give any indication of the processes that contributed to the evolution of the basin or causes for heterogeneities within the basin that are being imaged. Using texture and fluid properties predicted by our Basin RTM simulator, we generate synthetic seismograms. Linear correlation using power spectra as an error measure and an efficient quadratic optimization technique are found to be most effective in determining the optimal value of the tectonic parameters. Preliminary 1-D studies indicate that one can determine the geothermal gradient even in the presence of observation and numerical uncertainties. The algorithm succeeds even when the synthetic data has detailed information only in a limited depth interval and has a different dominant frequency in the synthetic and observed seismograms. The methodology presented here even works when the basin input data contains only 75 per cent of the stratigraphic layering information compared with the actual basin in a limited depth interval.

Geophysical Journal International↗
USGS2017

Phylogenetic distribution of a male pheromone that may exploit a nonsexual preference in lampreys

Pheromones are among the most important sexual signals used by organisms throughout the animal kingdom. However, few are identified in vertebrates, leaving the evolutionary mechanisms underlying vertebrate pheromones poorly understood. Pre-existing biases in receivers’ perceptual systems shape visual and auditory signaling systems, but studies on how receiver biases influence the evolution of pheromone communication remain sparse. The lamprey Petromyzon marinus uses a relatively well-understood suite of pheromones and offers a unique opportunity to study the evolution of vertebrate pheromone communication. Previous studies indicate that male signaling with the mating pheromone 3-keto petromyzonol sulfate (3kPZS) may exploit a nonsexual attraction to juvenile-released 3kPZS that guides migration into productive rearing habitat. Here, we infer the distribution of male signaling with 3kPZS using a phylogenetic comparison comprising six of ten genera and two of three families. Our results indicate that only P. marinus and Ichthyomyzon castaneus release 3kPZS at high rates. Olfactory and behavioral assays with P. marinus, I. castaneus and a subset of three other species that do not use 3kPZS as a sexual signal indicate that male signaling might drive the evolution of female adaptations to detect 3kPZS with specific olfactory mechanisms and respond to 3kPZS with targeted attraction relevant during mate search. We postulate that 3kPZS communication evolved independently in I. castaneus and P. marinus , but cannot eliminate the alternative that other species lost 3kPZS communication. Regardless, our results represent a rare macroevolutionary investigation of a vertebrate pheromone and insight into the evolutionary mechanisms underlying pheromone communication.

Journal of Evolutionary Biology↗
USGS2016

Local biotic adaptation of trees and shrubs to plant neighbors

Natural selection as a result of plant–plant interactions can lead to local biotic adaptation. This may occur where species frequently interact and compete intensely for resources limiting growth, survival, and reproduction. Selection is demonstrated by comparing a genotype interacting with con- or hetero-specific sympatric neighbor genotypes with a shared site-level history (derived from the same source location), to the same genotype interacting with foreign neighbor genotypes (from different sources). Better genotype performance in sympatric than allopatric neighborhoods provides evidence of local biotic adaptation. This pattern might be explained by selection to avoid competition by shifting resource niches (differentiation) or by interactions benefitting one or more members (facilitation). We tested for local biotic adaptation among two riparian trees, Populus fremontii and Salix gooddingii , and the shrub Salix exigua by transplanting replicated genotypes from multiple source locations to a 17 000 tree common garden with sympatric and allopatric treatments along the Colorado River in California. Three major patterns were observed: 1) across species, 62 of 88 genotypes grew faster with sympatric neighbors than allopatric neighbors; 2) these growth rates, on an individual tree basis, were 44, 15 and 33% higher in sympatric than allopatric treatments for P. fremontii, S. exigua and S. gooddingii , respectively, and; 3) survivorship was higher in sympatric treatments for P. fremontii and S. exigua . These results support the view that fitness of foundation species supporting diverse communities and dominating ecosystem processes is determined by adaptive interactions among multiple plant species with the outcome that performance depends on the genetic identity of plant neighbors. The occurrence of evolution in a plant-community context for trees and shrubs builds on ecological evolutionary research that has demonstrated co-evolution among herbaceous taxa, and evolution of native species during exotic plants invasion, and taken together, refutes the concept that plant communities are always random associations.

Arizona↗
USGS2016

Fluid spatial dynamics of West Nile virus in the USA: Rapid spread in a permissive host environment

The introduction of West Nile virus (WNV) into North America in 1999 is a classical example of viral emergence in a new environment, with its subsequent dispersion across the continent having a major impact on local bird populations. Despite the importance of this epizootic, the pattern, dynamics and determinants of WNV spread in its natural hosts remain uncertain. In particular, it is unclear whether the virus encountered major barriers to transmission, or spread in an unconstrained manner, and if specific viral lineages were favored over others indicative of intrinsic differences in fitness. To address these key questions in WNV evolution and ecology we sequenced the complete genomes of approximately 300 avian isolates sampled across the USA between 2001-2012. Phylogenetic analysis revealed a relatively ‘star-like' tree structure, indicative of explosive viral spread in US, although with some replacement of viral genotypes through time. These data are striking in that viral sequences exhibit relatively limited clustering according to geographic region, particularly for those viruses sampled from birds, and no strong phylogenetic association with well sampled avian species. The genome sequence data analysed here also contain relatively little evidence for adaptive evolution, particularly on structural proteins, suggesting that most viral lineages are of similar fitness, and that WNV is well adapted to the ecology of mosquito vectors and diverse avian hosts in the USA. In sum, the molecular evolution of WNV in North America depicts a largely unfettered expansion within a permissive host and geographic population with little evidence of major adaptive barriers.

Journal of Virology↗
USGS1983

Tin granites of Seward Peninsula, Alaska

Seven granite plutons, spatially and genetically related to tin metalization, are exposed in a 170-km-long belt across northwestern Seward Peninsula, Alaska. These plutons are cupolas and epizonal composite stocks that consist of several textural varieties of biotite granite, including medium- to coarse-grained seriate biotite granite, porphyritic biotite granite with an aplitic groundmass, and fine- to medium-grained equigranular biotite granite. The common accessory minerals are fluorite, allanite, apatite, and zircon. Other accessory minerals that are locally present include tourmaline, sphene, opaque oxide minerals, and late-forming (deuteric) muscovite and chlorite. The granites range in major-element contents as follows: SiO 2 , 72.5% to 76.6%; A1 2 O 3 , 12.7% to 14.3%; Na 2 O, 2.9% to 4.0%; K 2 O, 3.9% to 5.6%; and CaO, 0.6% to 1.2%. The sum of FeO + Fe 2 O 3 + MgO ranges from 0.3% to 2.4%; and the K 2 O to Na 2 O ratio from 1.1 to 1.8. The 0.1% to 0.9% F and 0.01% to 0.2% Cl reflect the over-all volatile-rich nature of the granites. The granites contain average or below-average concentrations of Co, Sc, Cr, and Zn, and generally above-average to distinctly high concentrations of Th, U, Hf, and Ta. The large cations emphasize the evolved nature of the granites; the Rb/Sr ratio is as high as 90 in some samples. Initial 87 Sr/ 86 Sr ratios range from 0.708 to as high as 0.720. The three Rb-Sr isochrons defined by the data agree with K-Ar age determinations and show that the stocks were emplaced during the Late Cretaceous, between about 70 and 80 m.y. ago. The field, petrologic, and geochemical data indicate that the plutons had a multistage origin that involved large-scale melting of sialic crust, emplacement of magmas derived from batholithic fractionation at depth, and subsequent evolution of these magmas to generate small volumes of more highly evolved residual magmas. Although evolution of the granite complexes was largely governed by crystal-melt fractionation, some minor-element variations in the highly evolved granites cannot be explained by this process. For example, the distribution of rubidium and the light rare-earths appears to have been influenced by volatile depletion at the final stages of crystallization. The field data, petrologic data, and variation trends, such as distinct shifts toward higher albite contents in the residual granites, suggest that the coexistence of a volatile phase was important in their evolution. These results require that models seeking to explain compositional gradients in high-level granite (rhyolite) systems fully consider the role of a coexisting volatile phase.

Alaska↗
USGS2021

Fast Pliocene integration of the Central Anatolian Plateau drainage: Evidence, processes, and driving forces

Continental sedimentation was widespread across the Central Anatolian Plateau in Miocene–Pliocene time, during the early stages of plateau uplift. Today, however, most sediment produced on the plateau is dispersed by a well-integrated drainage and released into surrounding marine depocenters. Residual long-term (10 6 –10 7 yr) sediment storage on the plateau is now restricted to a few closed catchments. Lacustrine sedimentation was widespread in the Miocene–Pliocene depocenters. Today, it is also restricted to the residual closed catchments. The present-day association of closed catchments, long-term sediment storage, and lacustrine sedimentation suggests that the Miocene–Pliocene sedimentation also occurred in closed catchments. The termination of sedimentation across the plateau would therefore mark the opening of these closed catchments, their integration, and the formation of the present-day drainage. By combining newly dated volcanic markers with previously dated sedimentary sequences, we show that this drainage integration occurred remarkably rapidly, within 1.5 m.y., at the turn of the Pliocene. The evolution of stream incision documented by these markers and newly obtained 10 Be erosion rates allow us to discriminate the respective contributions of three potential processes to drainage integration, namely, the capture of closed catchments by rivers draining the outer slopes of the plateau, the overflow of closed lakes, and the avulsion of closed catchments. Along the southern plateau margin, rivers draining the southern slope of the Central Anatolian Plateau expanded into the plateau interior; however, only a small amount of drainage integration was achieved by this process. Instead, avulsion and/or overflow between closed catchments achieved most of the integration, and these top-down processes left a distinctive sedimentary signal in the form of terminal lacustrine limestone sequences. In the absence of substantial regional climate wetting during the early Pliocene, we propose that two major tectonic events triggered drainage integration, separately or in tandem: the uplift of the Central Anatolian Plateau and the tectonic completion of the Anatolian microplate. Higher surface uplift of the eastern Central Anatolian Plateau relative to the western Central Anatolian Plateau promoted more positive water balances in the eastern catchments, higher water discharge, and larger sediment fluxes. Overflow/avulsion in some of the eastern catchments triggered a chain of avulsions and/or overflows, sparking sweeping integration across the plateau. Around 5 Ma, the inception of the full escape of the Anatolian microplate led to the disruption of the plateau surface by normal and strike-slip faults. Fault scarps partitioned large catchments fed by widely averaged sediment and water influxes into smaller catchments with more contrasted water balances and sediment fluxes. The evolution of the Central Anatolian Plateau shows that top-down processes of integration can outcompete erosion of outer plateau slopes to reintegrate plateau interior drainages, and this is overlooked in current models, in which drainage evolution is dominated by bottom-up integration. Top-down integration has the advantage that it can be driven by more subtle changes in climatic and tectonic boundary conditions than bottom-up integration.

Central Anatolian Plateau↗