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At least 1,279 records · Page 71Linked to original sources
USGS2002

Interactions between onshore bedrock-channel incision and nearshore wave-base erosion forced by eustasy and tectonics

We explore the response of bedrock streams to eustatic and tectonically induced fluctuations in base level. A numerical model coupling onshore fluvial erosion with offshore wave‐base erosion is developed. The results of a series of simulations for simple transgressions with constant rate of sea‐level change ( SLR ) show that response depends on the relative rates of rock uplift ( U ) and wave‐base erosion (ɛ w ). Simple regression runs highlight the importance of nearshore bathymetry. Shoreline position during sea‐level fall is set by the relative rate of base‐level fall ( U‐SLR ) and ɛ w , and is constant horizontally when these two quantities are equal. The results of models forced by a realistic Late Quaternary sea‐level curve are presented. These runs show that a stable shoreline position cannot be obtained if offshore uplift rates exceed ɛ w . Only in the presence of a relatively stable shoreline position, fluvial profiles can begin to approximate a steady‐state condition, with U balanced by fluvial erosion rate (ɛ f ). In the presence of a rapid offshore decrease in rock‐uplift rate ( U ), short (∼5 km) fluvial channels respond to significant changes in rock‐uplift rate in just a few eustatic cycles. The results of the model are compared to real stream‐profile data from the Mendocino triple junction region of northern California. The late Holocene sea‐level stillstand response exhibited by the simulated channels is similar to the low‐gradient mouths seen in the California streams.

California↗
USGS2002

Sea level controls on the textural characteristics and depositional architecture of the Hueneme and associated submarine fan systems, Santa Monica Basin, California

Hueneme and Dume submarine fans in Santa Monica Basin consist of sandy channel and muddy levee facies on the upper fan, lenticular sand sheets on the middle fan, and thinly bedded turbidite and hemipelagic facies elsewhere. Fifteen widely correlatable key seismic reflections in high-resolution airgun and deep-towed boomer profiles subdivide the fan and basin deposits into time-slices that show different thickness and seismic-facies distributions, inferred to result from changes in Quaternary sea level and sediment supply. At times of low sea level, highly efficient turbidity currents generated by hyperpycnal flows or sediment failures at river deltas carry sand well out onto the middle-fan area. Thick, muddy flows formed rapidly prograding high levees mainly on the western (right-hand) side of three valleys that fed Hueneme fan at different times; the most recently active of the lowstand fan valleys, Hueneme fan valley, now heads in Hueneme Canyon. At times of high sea level, fans receive sand from submarine canyons that intercept littoral-drift cells and mixed sediment from earthquake-triggered slumps. Turbidity currents are confined to ‘underfit’ talweg channels in fan valleys and to steep, small, basin-margin fans like Dume fan. Mud is effectively separated from sand at high sea level and moves basinward across the shelf in plumes and in storm-generated lutite flows, contributing to a basin-floor blanket that is locally thicker than contemporary fan deposits and that onlaps older fans at the basin margin. The infilling of Santa Monica Basin has involved both fan and basin-floor aggradation accompanied by landward and basinward facies shifts. Progradation was restricted to the downslope growth of high muddy levees and the periodic basinward advance of the toe of the steeper and sandier Dume fan. Although the region is tectonically active, major sedimentation changes can be related to eustatic sea-level changes. The primary controls on facies shifts and fan growth appear to be an interplay of texture of source sediment, the efficiency with which turbidity currents transport sand, and the effects of delta distributary switching, all of which reflect sea-level changes.

California↗
USGS2008

Site characterization of Italian strong motion recording stations

A dataset of site conditions at 101 Italian ground motion stations with recorded motions has been compiled that includes geologic characteristics and seismic velocities. Geologic characterization is derived principally from local geologic investigations by ENEL that include detailed mapping and cross sections. For sites lacking such detailed geologic characterization, the geology maps of the by Servizio Geologico d'Italia are used. Seismic velocities are extracted from the literature and the files of consulting engineers, geologists and public agencies for 33 sites. Data sources utilized include post earthquake site investigations (Friuli and Irpinia events), microzonation studies, and miscellaneous investigations performed by researchers or consulting engineers/geologists. Additional seismic velocities are measured by the authors using the controlled source spectral analysis of surface waves (SASW) method for 18 sites that recorded the 1997–1998 Umbria Marche earthquake sequence. The compiled velocity measurements provide data for 51 of the 101 sites. For the remaining sites, the average seismic velocity in the upper 30 m (V s30 ) "> (V s30 ) is estimated using a hybrid approach. For young Quaternary alluvium, V s30 "> 𝑉 s30 an existing empirical relationship for California sites by Wills and Clahan (2006) is used, which we justify by validating this relationship against Italian data. For Tertiary Limestone and Italian Mesozoic rocks, empirical estimates of V s30 "> 𝑉 s30 are developed using the available data.

Conference Paper↗
USGS2012

Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis

We report the discovery in Lake Cuitzeo in central Mexico of a black, carbon-rich, lacustrine layer, containing nanodiamonds, microspherules, and other unusual materials that date to the early Younger Dryas and are interpreted to result from an extraterrestrial impact. These proxies were found in a 27-m-long core as part of an interdisciplinary effort to extract a paleoclimate record back through the previous interglacial. Our attention focused early on an anomalous, 10-cm-thick, carbon-rich layer at a depth of 2.8 m that dates to 12.9 ka and coincides with a suite of anomalous coeval environmental and biotic changes independently recognized in other regional lake sequences. Collectively, these changes have produced the most distinctive boundary layer in the late Quaternary record. This layer contains a diverse, abundant assemblage of impact-related markers, including nanodiamonds, carbon spherules, and magnetic spherules with rapid melting/quenching textures, all reaching synchronous peaks immediately beneath a layer containing the largest peak of charcoal in the core. Analyses by multiple methods demonstrate the presence of three allotropes of nanodiamond: n-diamond, i-carbon, and hexagonal nanodiamond (lonsdaleite), in order of estimated relative abundance. This nanodiamond-rich layer is consistent with the Younger Dryas boundary layer found at numerous sites across North America, Greenland, and Western Europe. We have examined multiple hypotheses to account for these observations and find the evidence cannot be explained by any known terrestrial mechanism. It is, however, consistent with the Younger Dryas boundary impact hypothesis postulating a major extraterrestrial impact involving multiple airburst(s) and and/or ground impact(s) at 12.9 ka.

Lake Cuitzeo↗
USGS2013

Space can substitute for time in predicting climate-change effects on biodiversity

“Space-for-time” substitution is widely used in biodiversity modeling to infer past or future trajectories of ecological systems from contemporary spatial patterns. However, the foundational assumption—that drivers of spatial gradients of species composition also drive temporal changes in diversity—rarely is tested. Here, we empirically test the space-for-time assumption by constructing orthogonal datasets of compositional turnover of plant taxa and climatic dissimilarity through time and across space from Late Quaternary pollen records in eastern North America, then modeling climate-driven compositional turnover. Predictions relying on space-for-time substitution were ∼72% as accurate as “time-for-time” predictions. However, space-for-time substitution performed poorly during the Holocene when temporal variation in climate was small relative to spatial variation and required subsampling to match the extent of spatial and temporal climatic gradients. Despite this caution, our results generally support the judicious use of space-for-time substitution in modeling community responses to climate change.

Proceedings of the National Academy of Sciences↗
USGS2015

Energy flow and the “grassification” of desert shrublands

In our directionally and continuously changing world, history still matters, and it does so in increasingly novel and important ways. Human adaptation to global change will rely heavily on robust baselines of historic environmental variability and detailed understanding of how both past and modern ecosystems have responded to both individual and multiple stressors. The question of global change has motivated an upsurge in paleoecological studies that span the late Quaternary and the modern era, and has inspired a growing consideration of time as a fundamental axis in ecology ( 1 ). A major challenge in developing pertinent ecological baselines remains how to fuse, into continuous time series, observations and experiments from living systems with paleoecological reconstructions from the same sites ( 2 , 3 ). Tracing and disentangling complex responses to environmental stress from paleological to present-day communities is especially daunting; for example, how climate change; accelerated land use; and biological invasions are influencing the flows of water, nutrients, and energy. The paper by Terry and Rowe in PNAS ( 4 ) is a shining example of how modern ecology and paleoecology can be spliced together to decipher how ecological processes unfold over time scales inaccessible to direct observation or experimentation, and how they can be disrupted by human impacts.

Proceedings of the National Academy of Sciences↗
USGS2015

Dynamic response of desert wetlands to abrupt climate change

Desert wetlands are keystone ecosystems in arid environments and are preserved in the geologic record as groundwater discharge (GWD) deposits. GWD deposits are inherently discontinuous and stratigraphically complex, which has limited our understanding of how desert wetlands responded to past episodes of rapid climate change. Previous studies have shown that wetlands responded to climate change on glacial to interglacial timescales, but their sensitivity to short-lived climate perturbations is largely unknown. Here, we show that GWD deposits in the Las Vegas Valley (southern Nevada, United States) provide a detailed and nearly complete record of dynamic hydrologic changes during the past 35 ka (thousands of calibrated 14 C years before present), including cycles of wetland expansion and contraction that correlate tightly with climatic oscillations recorded in the Greenland ice cores. Cessation of discharge associated with rapid warming events resulted in the collapse of entire wetland systems in the Las Vegas Valley at multiple times during the late Quaternary. On average, drought-like conditions, as recorded by widespread erosion and the formation of desert soils, lasted for a few centuries. This record illustrates the vulnerability of desert wetland flora and fauna to abrupt climate change. It also shows that GWD deposits can be used to reconstruct paleohydrologic conditions at millennial to submillennial timescales and informs conservation efforts aimed at protecting these fragile ecosystems in the face of anthropogenic warming.

Nevada↗
USGS2010

Rejuvenation of the Kuqa Foreland Basin, northern flank of the Tarim Basin, northwest China

The Kuqa depression along the northern flank of the Tarim basin is filled with a thick sequence of Neogene and Quaternary coarse elastic continental sediments. This structural depression is part of a large foreland basin that lies south of the Tianshan—an orogenic belt of intracontinental convergence resulting from the northward propagation of stress following the collision of India with the southern margin of Eurasia.

Kuqa Foreland Basin, Tarim Basin↗
USGS1989

Reversed-phase high-performance liquid chromatography of unsubstituted aminobenzoic acids

High-performance liquid chromatographic (HPLC) characteristics of three position isomers of aminobenzoic acids (potential metabolites of important anesthetic drugs), were delineated with respect to their interactions with various mobile phases and stationary phases. HPLC with five hydrocarbonaceous phase, I?-cyclodextrin silica (CDS), macrophase MP-1 polymer (MP), macroporous polystyrene/divinylbenzene (MPD), octadecylsilica (ODS), and propylphenylsilica (PPS), yielded results explicable in terms of substituent effects derived from the bifunctional amino- and carboxy groups. For cases where mobile phases contained sulfonates or quaternary ammonium salts both having longer chain alkyls, retention of analytes on all but CDS appeared to proceed predominantly via an ion-pairing mechanism. The extent of the corresponding counter-ion effects decreased in the order: MPD > ODS > PPS > MP, while the analyte retention order paralleled thier pH 2 values. On the other hand, an inverse relationship between the magnitude of capacity factors (k') and pK 1 values of the title compounds was observed in experiments that produced retention data incompatible with ion-pair interaction rationales. The unique HPLC results obtained with the CDS phase are compared with those obtained with other phases.

Journal of Liquid Chromatography↗
USGS1964

Geohydrologic analogies between the Jordan Valleys of Utah and the holy land

The biblical Jordan River Valley , which extends from Lake Tiberias (the Sea of Galilee) to the Dead Sea, is decidedly similar to the Jordan River Valley of Utah , which joins Lake Utah and Great Salt Lake. Both Jordan Rivers drain relatively large fresh-water lakes and also are major sources of discharge into large salty lakes that have no outlets to the ocean.The two Jordan River valleys and the highlands and mountains that surround them, have many physiographic, geologic, and hydrologic similarities as well as some noteworthy differences. For example, an hypothesis for the formation of the Dead Sea- Jordan Valley rift is that the east Jordan block slid northward with respect to the west Jordan block. The amount of displacement is estimated to be about 65 miles and took place partly in Miocene and possible Pliocène and partly in Pleistocene time. Tectonc activity has also been a major factor in the formation of the Jordan valley of Utah , but the movement here probably was along large normal faults in late Tertiary and Quaternary time. The sediments underlying both Jordan River valleys were deposited in ancestral lacustrine and fluvial environments. Abundant supplies of ground water are found under both valleys , but probably larger supplies of better quality water can be obtained in Utah . Both valleys contain numerous small nonthermal and a few large thermal springs.

Utah↗
USGS1997

Endictya hendeyi sp. nov., a new coastal diatom species from North America

Endictya hendeyi E. Hemphill-Haley sp. nov. is described from Recent and Quaternary, estuarine and marine deposits along the west coast of the United States. This taxon is distinguished by its 1) consistently small valves; 2) areolae of uniform size on the valve face in linear tangential rows; 3) prominent, external spatulate extensions of the rimoportulae at the valve face/mantle junction; 4) externally projecting hyaline rim or flange at the interior edge of the mantle; and 5) discontinuous pattern of pores on the basal siliceous layer. The new species is compared to the generitype, E. oceanica Ehrenberg, as well as E. minor A. Schmidt and E. margaritifera A. Mann.

Diatom Research↗
USGS1977

Submarine faults and slides on the continental shelf, northern Gulf of Alaska

Submarine faults and slides or slumps of Quaternary age are potential environmental hazards on the outer continental shelf (OCS) of the northern Gulf of Alaska. Most faults that approach or reach the seafloor cut strata that may be equivalent in age to the upper Yakataga Formation (Pliocene‐Pleistocene). Along several faults, the seafloor is vertically offset from 5 to 20 m. A few faults appear to cut Holocene sediments, but none of these shows displacement at the seafloor. Submarine slides or slumps have been found in two places in the OCS region: (1) seaward of the Malaspina Glacier and Icy Bay, an area of 1200 km 2 with a slope of less than 0.5°, and (2) across the entire span of the Copper river prodelta, an area of 1730 km 2 , having a slope of about 0.5°. Seismic profiles across these areas show disrupted reflectors and irregular topography commonly associated with submarine slides or slumps. Potential slide or slump areas have been delineated in areas of thick sediment accumulation and relatively steep slopes. These areas include (1) Kayak Trough, (2) parts of Hinchinbrook Entrance and Sea Valley, (3) parts of the outer shelf and upper slope between Kayak Island and Yakutat Bay, and (4) Bering Trough.

Alaska↗
USGS1999

Stratigraphic framework and heavy minerals of the continental shelf of Onslow and Long Bays, North Carolina

One hundred fourteen vibracores from the Atlantic continental shelf offshore of southeastern North Carolina were opened, described, and processed over several contract years (years 6-9) of the Minerals Management Service-Association of American State Geologists Continental Margins program. Reports for years 9 and 10 of the program compiled the results of the work and assembled the data for release as an interactive CD-ROM report, respectively. The continental shelf of Onslow and Long Bays consists predominantly of outcropping Cretaceous through late Tertiary geologic units. Nearshore these units are covered and incised by late Tertiary and Quaternary units. From oldest to youngest, formally recognized geologic units mapped as part of this study are the Late Cretaceous Peedee Formation-a muddy, fine- to medium-grained quartz sand with trace amounts of glauconite and phosphate; the Paleocene Beaufort Formation-a muddy, fine- to medium-grained glauconitic quartz sand with locally occurring turritelid-mold biosparrudite; the middle Eocene Castle Hayne Formation-a sandy bryozoan biomicrudite and biosparrudite; the Oligocene River Bend Formation-a sandy molluscan-mold biosparrudite; and the Miocene Pungo River Formation-a medium-grained, poorly sorted slightly shelly phosphatic sand. Informal units include a very widespread, unnamed fine- to very fine grained, well-sorted, dolomitic muddy quartz sand that is biostratigraphically equivalent to the Oligocene River Bend Formation; several large valley-fill lithosomes composed of biomicrudite, biomicrite, and biosparrudite of Plio/Pleistocene age; muddy, shelly sands and silty clays of Pliocene, Pleistocene, or mixed Plio/Pleistocene age; and loose, slightly shelly, medium- to coarse-grained sands assigned a Holocene age. Heavy minerals (SG > 2.96) comprise an average of 0.54 wt% (on a bulk-sample basis) of the sediments in 306 samples derived from the 114 vibracores. Heavy-mineral content ranges from <0.01 to 3.69 wt%. The economic heavy mineral content (EHM = ilmenite + zircon + rutile + aluminosilicates + leucoxene [altered ilmenite] + monazite) of the bulk samples averages 0.26 wt% in a range of <0.01-1.70 wt%. As a percentage of the heavy-mineral concentrate, the average EHM value is 45.78% in a range of 0.27-68.60%. The distribution of heavy minerals offshore of southeastern North Carolina is controlled by the lithostratigraphic framework. The unnamed Oligocene sand unit has the highest heavy-mineral content, averaging 0.86 wt% on a bulk-sample basis. The remaining geologic units and their heavy-mineral content (in decreasing order of abundance) are Beaufort (0.64%), Holocene sand (0.60%), Plio-Pleistocene muddy sand and silty clay (0.59%), Peedee (0.42%), River Bend (0.34%), Plio-Pleistocene carbonate (0.12%), and Castle Hayne (0.08%). The heavy-mineral assemblage is fairly consistent throughout the different units. Significantly smaller percentages of heavy minerals correlate with increased amounts of CaCO 3 in the sediments. The sediments analyzed in this study have significantly lower overall heavy-mineral content, as well as lower EHM content than sediments that are known to host commercially important heavy-mineral deposits in the southeastern United States. The potential for economic deposits of heavy minerals in the area of this study, therefore, appears to be limited.

Marine Georesources and Geotechnology↗
USGS1993

The Solubility and Stabilization of Ikaite (CaCO3·6H2O) from 0° to 25°C: Environmental and Paleoclimatic Implications for Thinolite Tufa

We determined the solubility of ikaite from 0° to 25°C to model its saturation state in natural waters and test the hypothesis that it is the precursor of the calcite pseudomorphs in thinolite tufa of Quaternary Lake Lahontan. Reversible solubility at buffered $P_{CO_{2}}$ yields the following expression for the dissolution constant of ikaite: $log K_{ikaite} = 0.15981 - 2011.1/T$. Where $T = ^{\circ}K$, and $0^{\circ}C \leq t \leq 25^{\circ}C$. Derived standard state properties are $\Delta G_{ikaite}^{\circ} = -2541.9 kJmol^{-1} \pm 0.66; \Delta H_{ikaite}^{\circ} = -2973.4 kJmol^{-1} \pm 1.02; S_{ikaite}^{\circ} = 306.6 JT^{-1}mol^{-1} \pm 1.2$. Modeling shows that ikaite is undersaturated at all temperatures in seawater and in alkaline lakes, but that it rapidly approaches saturation near 0°C. Its precipitation in near-freezing marine sediments requires large additions of $HCO_{3}$ to pore fluids from the diagenetic decomposition of organic matter. Its crystallization in tufas of alkaline lakes, however, requires only small additions of Ca from springs. Simple kinetic experiments show that ikaite is stabilized in natural environments by orthophosphate, which prevents the crystallization of the more stable anhydrous forms of $CaCO_{3}$ but does not interact with the ikaite. Therefore, the presence of ikaite or its pseudomorphs is an indicator of near-freezing conditions in environments with high concentrations of orthophosphate. If ikaite is the precursor of thinolite tufa, then the thinolite likely grew below the sediment-water interface at the site of sublacustrine springs during prolonged cold periods.

Journal of Geology↗
USGS2015

Seismic velocities within the sedimentary succession of the Canada Basin and southern Alpha-Mendeleev Ridge, Arctic Ocean: evidence for accelerated porosity reduction?

The Canada Basin and the southern Alpha-Mendeleev ridge complex underlie a significant proportion of the Arctic Ocean, but the geology of this undrilled and mostly ice-covered frontier is poorly known. New information is encoded in seismic wide-angle reflections and refractions recorded with expendable sonobuoys between 2007 and 2011. Velocity–depth samples within the sedimentary succession are extracted from published analyses for 142 of these records obtained at irregularly spaced stations across an area of 1.9E + 06 km 2 . The samples are modelled at regional, subregional and station-specific scales using an exponential function of inverse velocity versus depth with regionally representative parameters determined through numerical regression. With this approach, smooth, non-oscillatory velocity–depth profiles can be generated for any desired location in the study area, even where the measurement density is low. Practical application is demonstrated with a map of sedimentary thickness, derived from seismic reflection horizons interpreted in the time domain and depth converted using the velocity–depth profiles for each seismic trace. A thickness of 12–13 km is present beneath both the upper Mackenzie fan and the middle slope off of Alaska, but the sedimentary prism thins more gradually outboard of the latter region. Mapping of the observed-to-predicted velocities reveals coherent geospatial trends associated with five subregions: the Mackenzie fan; the continental slopes beyond the Mackenzie fan; the abyssal plain; the southwestern Canada Basin; and, the Alpha-Mendeleev magnetic domain. Comparison of the subregional velocity–depth models with published borehole data, and interpretation of the station-specific best-fitting model parameters, suggests that sandstone is not a predominant lithology in any of the five subregions. However, the bulk sand-to-shale ratio likely increases towards the Mackenzie fan, and the model for this subregion compares favourably with borehole data for Miocene turbidites in the eastern Gulf of Mexico. The station-specific results also indicate that Quaternary sediments coarsen towards the Beaufort-Mackenzie and Banks Island margins in a manner that is consistent with the variable history of Laurentide Ice Sheet advance documented for these margins. Lithological factors do not fully account for the elevated velocity–depth trends that are associated with the southwestern Canada Basin and the Alpha-Mendeleev magnetic domain. Accelerated porosity reduction due to elevated palaeo-heat flow is inferred for these regions, which may be related to the underlying crustal types or possibly volcanic intrusion of the sedimentary succession. Beyond exploring the variation of an important physical property in the Arctic Ocean basin, this study provides comparative reference for global studies of seismic velocity, burial history, sedimentary compaction, seismic inversion and overpressure prediction, particularly in mudrock-dominated successions.

Geophysical Journal International↗
USGS2021

Dynamic landscapes in northwestern North America structured populations of wolverines (Gulo gulo)

Cyclic climatic and glacial fluctuations of the Late Quaternary produced a dynamic biogeographic history for high latitudes. To refine our understanding of this history in northwestern North America, we explored geographic structure in a wide-ranging carnivore, the wolverine ( Gulo gulo ). We examined genetic variation in populations across mainland Alaska, coastal Southeast Alaska, and mainland western Canada using nuclear microsatellite genotypes and sequence data from the mitochondrial DNA (mtDNA) control region and Cytochrome b ( Cytb ) gene. Data from maternally inherited mtDNA reflect stable populations in Northwest Alaska, suggesting the region harbored wolverine populations since at least the Last Glacial Maximum (LGM; 21 Kya), consistent with their persistence in the fossil record of Beringia. Populations in Southeast Alaska are characterized by minimal divergence, with no genetic signature of long-term refugial persistence (consistent with the lack of pre-Holocene fossil records there). The Kenai Peninsula population exhibits mixed signatures depending on marker type: mtDNA data indicate stability (i.e., historical persistence) and include a private haplotype, whereas biparentally inherited microsatellites exhibit relatively low variation and a lack of private alleles consistent with a more recent Holocene colonization of the peninsula. Our genetic work is largely consistent with the early 20 th century taxonomic hypothesis that wolverines on the Kenai Peninsula belong to a distinct subspecies. Our finding of significant genetic differentiation of wolverines inhabiting the Kenai Peninsula, coupled with the peninsula’s burgeoning human population and the wolverine’s known sensitivity to anthropogenic impacts, provides valuable foundational data that can be used to inform conservation and management prescriptions for wolverines inhabiting these landscapes.

Alaska, British Columbia, Northwest Territories, N↗
USGS1990

Calc-alkaline, shoshonitic, and primitive tholeiitic lavas from monogenetic volcanoes near Crater Lake, Oregon

Quaternary monogenetic volcanism in the High Cascades of Oregon is manifested by cinder cones, lava fields, and small shields. Near Crater Lake caldera, monogenetic lava compositions include: low-K (as low as 0·09% K 2 O) high-alumina olivine tholeiite (HAOT); medium-K. calc-alkaline basalt, basaltic andesite, and andesite; and shoshonitic basaltic andesite (2·1% K 2 O, 1750 ppm Sr at 54% SiO 2 ). Tholeiites have MORB-like trace element abundances except for elevated Sr, Ba, and Th and low high field strength elements (HFSE), and they represent near-primary liquids. They are similar to HAOTs from the Cascades and adjacent Basin and Range, and to many primitive basalts from intraoceanic arcs. Calc-alkaline lavas show a well-developed arc signature of high large-ion lithophile elements (LILE) and low HFSE. Their Zr and Hf concentrations are at least partly decoupled from those of Nb and Ta; HREE are low relative to HAOT. Incompatible element abundances and ratios vary widely among basaltic andesites. Some calc-alkaline lavas vented near Mount Mazama contain abundant gabbroic microxcnoliths, and are basaltic andesitic magmas contaminated with olivine gabbro. A calc-alkaline basalt and a few basaltic andesites have MgO and compatible trace element contents that suggest only minor fractionation. There appears to be a compositional continuum between primitive tholeiitic and calc-alkaline lavas. Compositional variation within suites of comagmatic primitive lavas, both tholeiitic and calc-alkaline, mainly results from different degrees of partial melting. Sources of calc-alkaline primary magmas were enriched in LILE and LREE by a subduction component and contained residual garnet, whereas sources of HAOTs had lower LILE and LREE concentrations and contained residual clinopyroxene. High and variable LILE and LREE contents of calc-alkaline lavas reflect variations in fluid-transported subduction component added to the mantle wedge, degree of partial melting, and possibly also interaction with rocks or partial melts in the lower crust. Andesites were derived from calc-alkaline basaltic andesites by fractionation of plagioclase+augite+magnetite+apatite ± orthopyroxene or olivine, commonly accompanied by assimilation. Many andesites are mixtures of andesitic or dacitic magma and a basaltic or basaltic andesitic component, or are contaminated with gabbroic material. Mingled basalt, andesite, and dacite of Williams Crater formed by multi-component, multi-stage mixing of basaltic andesitic magma, gabbro, and dacitic magma. The wide range of compositions vented from monogenetic volcanoes near Crater Lake is a result of the thick crust coupled with mild tectonic extension superimposed on a subduction-related magmatic arc.

Oregon↗
USGS2001

Eruptive stratigraphy of the Tatara-San Pedro complex, 36°S, sourthern volcanic zone, Chilean Andes: Reconstruction method and implications for magma evolution at long-lived arc volcanic centers

The Quaternary Tatara–San Pedro volcanic complex (36°S, Chilean Andes) comprises eight or more unconformity-bound volcanic sequences, representing variably preserved erosional remnants of volcanic centers generated during ∼930 ky of activity. The internal eruptive histories of several dominantly mafic to intermediate sequences have been reconstructed, on the basis of correlations of whole-rock major and trace element chemistry of flows between multiple sampled sections, but with critical contributions from photogrammetric, geochronologic, and paleomagnetic data. Many groups of flows representing discrete eruptive events define internal variation trends that reflect extrusion of heterogeneous or rapidly evolving magma batches from conduit–reservoir systems in which open-system processes typically played a large role. Long-term progressive evolution trends are extremely rare and the magma compositions of successive eruptive events rarely lie on precisely the same differentiation trend, even where they have evolved from similar parent magmas by similar processes. These observations are not consistent with magma differentiation in large long-lived reservoirs, but they may be accommodated by diverse interactions between newly arrived magma inputs and multiple resident pockets of evolved magma and/or crystal mush residing in conduit-dominated subvolcanic reservoirs. Without constraints provided by the reconstructed stratigraphic relations, the framework for petrologic modeling would be far different. A well-established eruptive stratigraphy may provide independent constraints on the petrologic processes involved in magma evolution—simply on the basis of the specific order in which diverse, broadly cogenetic magmas have been erupted. The Tatara–San Pedro complex includes lavas ranging from primitive basalt to high-SiO 2 rhyolite, and although the dominant erupted magma type was basaltic andesite (∼52–55 wt % SiO 2 ) each sequence is characterized by unique proportions of mafic, intermediate, and silicic eruptive products. Intermediate lava compositions also record different evolution paths, both within and between sequences. No systematic long-term pattern is evident from comparisons at the level of sequences. The considerable diversity of mafic and evolved magmas of the Tatara–San Pedro complex bears on interpretations of regional geochemical trends. The variable role of open-system processes in shaping the compositions of evolved Tatara–San Pedro complex magmas, and even some basaltic magmas, leads to the conclusion that addressing problems such as arc magma genesis and elemental fluxes through subduction zones on the basis of averaged or regressed reconnaissance geochemical datasets is a tenuous exercise. Such compositional indices are highly instructive for identifying broad regional trends and first-order problems, but they should be used with extreme caution in attempts to quantify processes and magma sources, including crustal components, implicated in these trends.

Andes Mountains, Tatara-San Pedro Volcanic Complex↗