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USGS2021

Zircon geochronology and geochemistry of Quaternary rhyolite domes of the Coso volcanic field, Inyo County, California

The Quaternary Coso volcanic field (CVF) is a compositionally bimodal volcanic field located within a releasing bend along the eastern range-front Sierra Nevada fault zone in California's southern Owens Valley. The erupted products of CVF silicic magmatism since ~1 Ma comprise 38 high-silica rhyolite domes, with the volumetric majority (~99%) of rhyolite emplaced within the past ~300 ka. The CVF hosts an economically important geothermal field driven by heat associated with a shallow (~5 km) igneous intrusion. The CVF is potentially an immature analog to the nearby Long Valley system, which culminated in generation and eruption of the voluminous and widespread Bishop Tuff. As such, the CVF represents a considerable volcanic hazard, making a detailed understanding of the eruptive history and pre-eruptive conditions of the system critically important. We present uranium-series isochron dates from zircon ± allanite crystal surfaces and zircon trace element geochemical data on the youngest 17 rhyolite domes at Coso, which represent ~60% (by volume) of the silicic magma erupted by the system. These data suggest: (1) a shorter emplacement duration than previously recognized for these domes, with a duration of 20 ± 5 ka; (2) 4 shorter-duration eruption pulses within this interval, all of which occur during the marine isotope stage (MIS) 5 interglacial period; (3) an uptick in the volume of CVF magma erupted between ~200 ka and ~ 78 ka relative to that emplaced over the lifetime of the system; (4) near-coeval eruption of geochemically distinct magma in close geographic proximity, either sourced from different portions of the same magma system at depth or from discrete, uncommunicating bodies; (5) ambiguity with respect to whether or not CVF magmatism is time-predictable, as previously suggested, or erupted as a series of punctuated episodes; (6) no rhyolite volcanism in the past ~78 kyr.

California↗

USGS2012

Morphology and growth history of Delgada Fan: Implications for the Neogene evolution of Point Arena Basin and the Mendocino Triple Junction

Long-range side scan (GLORIA) sonographs and seismic reflection data acquired during a survey of the western U.S. Exclusive Economic Zone in 1984, coupled with information from Deep Sea Drilling Project sites, provide new insights into the growth and evolution of the Delgada Fan. Construction of the fan commenced in the latest Miocene (∼6 Ma) following the filling of the Neogene Point Arena Basin. The fan presently covers more than 50×10 3 km 2 of the Pacific plate and contains approximately 15×10 3 km 3 of predominantly terrigenous detritus. The large size of the fan is incompatible with the small present-day supply of sediment to the canyon system. The GLORIA data show the Delgada Fan to be a hybrid-type fan, exhibiting characteristics of both elongate and radial fans. The morphology and volume of the fan, along with evidence for a decline in accumulation rates on the lower fan during the Quaternary period, suggest that the fan experienced an early growth phase (latest Miocene and Pliocene) characterized by relatively rapid progradation of elongate fan lobes followed by a period (Quaternary) of slower growth that has featured a shift of depocenters to sites closer to the canyons and a transition to distributary channels bordered by less prominent levees and overbank deposits. We examine the growth of Delgada Fan in relation to the Neogene evolution of the North American-Pacific plate boundary using a series of paleogeographic reconstructions based on recently published time displacement histories of the Mendocino triple junction (MTJ), the San Andreas fault (SAF), and the Pacific plate, upon which the fan rests. The time displacement curves for the SAF and the MTJ suggest that the MTJ and Mendocino Fracture Zone overtook and passed Point Arena Basin at about 10 Ma when the basin lay immediately southwest of the present San Francisco Bay area. We suggest that the MTJ joined the SAF at approximately that time and location, thus making the SAF the master fault in the transform system. This interpretation is compatible with evidence from seismic reflection profiles over the fan, which demonstrate that the fan and the canyon system and therefore Point Arena Basin have moved as a unit since the inception of fan growth (∼6–7 Ma). Point Arena Basin lay southwest of the San Francisco area at 10–12 Ma, and the passage of the MTJ caused the disruption of the forearc shelf and slope and the development of local uplifted and subsiding blocks. In particular, uplift of the “bay block” immediately east of the SAF may have provided the source area for the late Miocene sediments that filled Point Arena Basin and set the stage for the growth of Delgada Canyon and Fan system. The growth rate of the fan has decreased, and the style of deposition has changed as the system was tectonically transported to its present location adjacent to the small youthful drainages of the King Range.

Journal of Geophysical Research Solid Earth↗

USGS2009

Magma evolution and ascent at the craters of the moon and neighboring volcanic fields, southern Idaho, USA: Implications for the evolution of polygenetic and monogenetic volcanic fields

The evolution of polygenetic and monogenetic volcanic fields must reflect differences in magma processing during ascent. To assess their evolution we use thermobarometry and geochemistry to evaluate ascent paths for neighboring, nearly coeval volcanic fields in the Snake River Plain, in south-central Idaho, derived from (1) dominantly Holocene polygenetic evolved lavas from the Craters of the Moon lava field (COME) and (2) Quaternary non-evolved, olivine tholeiites (NEOT) from nearby monogenetic volcanic fields. These data show that NEOT have high magmatic temperatures (1205 + or - 27 degrees C) and a narrow temperature range (< 25 degrees C) at any given depth; NEOT parent magmas partially crystallize within the middle crust (14-17 km), but with little time for cooling or assimilation. In contrast, COME magmas partially crystallize at similar depths, but at any given depth exhibit lower temperatures (by ~40 degrees C), and wider temperature ranges (>50 degrees C). Prolonged storage of COME magmas allows them to evolve to higher 87 Sr/ 86 Sr and SiO 2 , and lower MgO and 143 Nd/ 144 Nd. Most importantly, ascent paths control evolution: NEOT often erupt near the axis of the plain where high-flux (Yellowstone-related), pre-Holocene magmatic activity replaces granitic middle crust with basaltic sills, resulting in a net increase in NEOT magma buoyancy. COME flows erupt off-axis, where felsic crustal lithologies sometimes remain intact, providing a barrier to ascent and a source for crustal contamination. A three-stage ascent process explains the entire range of erupted compositions. Stage 1 (40-20 km): picrites are transported to the middle crust, undergoing partial crystallization of olivine + or - clinopyroxene. COME magmas pass through unarmored conduits and assimilate 1% or less of ancient gabbroic crust having high Sr and 87 Sr/ 86 Sr and low SiO 2 . Stage 2 (20-10 km): magmas are stored within the middle crust, and evolve to moderate MgO (10%). NEOT magmas, reaching 10% MgO, are positively buoyant and migrate through the middle crust. COME magmas remain negatively buoyant and so crystallize further and assimilate middle crust. Stage 3 (15-0 km): final ascent and eruption occurs when volatile contents, increased by differentiation, are sufficient (1-2 wt % H 2 O) to provide magma buoyancy through the middle (and upper) crust.

Idaho↗

USGS1993

Petrology and isotopic composition of Quaternary basanites dredged from the Bering Sea continental margin near Navarin Basin

Quaternary basanites were recovered from shallow water depth from the continental margin of the Bering Sea (58°39.0′N, 177°12.9′W) near Navarin Basin. The basanites are highly vesicular flow rock and hyaloclastites similar to other alkalic volcanic rocks erupted repeatedly during the late Cenozoic on islands in the Bering Sea region and in mainland Alaska. K–Ar ages for the basanites indicate at least two episodes of volcanism at about 1.1 and 0.4 Ma. Similar alkalic volcanism occurred sporadically at geographically widely separated centers in the Bering Sea region for at least the past 6 Ma. Chemically, these alkalic lavas are intraplate basalts similar to those erupted from oceanic islands and in some continental settings. Trace-element data indicate these alkalic lavas have been generated by small, but variable, amounts of partial melting of a meta-somatized lherzolite source. The relatively primitive compositions (MgO > 9%), presence of mantle-derived xenoliths in some alkalic lavas, and presence of forsteritic olivine with low CaO and high NiO suggest that magma rose rapidly from greath depth without spending time in large, long-lived magma chambers. Although lavas from different volcanic centers in the Bering Sea region are similar with respect to major elements and many trace-element ratios, isotopic compositions indicate heterogeneities in the source. The Navarin basanites have higher 87 Sr/ 86 Sr and lower 143 Nd/ 144 Nd values than any other lavas so far reported from this region. The 207 Pb/ 204 Pb isotopic ratios indicate involvement of a crustal component, which may have resulted from metasomatism associated with subduction-related magmatic activity during the Early Eocene in this region. Although some volcanic episodes appear to have occurred roughly synchronously at geographically widely separated centers, no large-scale regional extension nor presence of large mantle plumes are indicated. Instead, alkalic volcanism apparently resulted from upwelling and decompressional melting of small isolated mantle diapirs in response to local lithospheric attenuation associated with jostling of blocks during adjustment to regional stresses.

Alaska↗

USGS2018

The geochemistry of loess: Asian and North American deposits compared

Loess is widely distributed over Asia and North America and constitutes one of the most important surficial deposits that serve as terrestrial records of the Quaternary. The oldest Pleistocene loess in China is likely ∼2.6 Ma, thus spanning much or all of the Pleistocene. In North America, most loess is no older than the penultimate glacial period, with the exception of Alaska, where the record may go back to ∼3.0 Ma. On both continents, loess deposits date primarily to glacial periods, and interglacial or interstadial periods are represented by paleosols. Both glacial and non-glacial sources of silts that comprise the bulk of loess deposits are found on both continents. Although loess has been considered to be representative of the average upper continental crust, there are regionally distinctive compositions of loess in both Asia and North America. Loess deposits in Asia from Yakutia, Tajikistan, and China have compositionally distinct major element compositions, due to varying abundances of silicate minerals, carbonate minerals, and clay minerals. In North America, loess in the Mississippi River valley, the Great Plains, and Alaska are also distinguishable with regard to major element composition that reflects highly diverse source sediments. Trace element geochemistry (Sc-Th-Zr and the rare earth elements) also shows regional diversity of loess bodies, in both Asia and North America. On both continents, most loess bodies show significant contributions from later-cycle, altered sedimentary rocks, as opposed to direct derivation from igneous rocks. Further, some loess bodies have detectable contributions from mafic igneous rocks as well as major contributions from average, upper-crustal, felsic rocks. Intercalated paleosols in loess sections show geochemical compositions that differ significantly from the underlying loess parent materials. Ratios of soluble-to-insoluble elements show depletions in paleosols due to chemical weathering losses of calcite, dolomite, plagioclase, mica, apatite, and smectite. In Asia and North America, the last interglacial paleosol is more weathered than equivalent modern soils, which could be due either to a climate that was warmer and more humid, a longer period of pedogenesis, or both. In Asia, early Pleistocene loess and paleosols are both more weathered than those from the middle and late Pleistocene, forming prior to a mid-Pleistocene aridification of Asia from uplift of the Tibetan Plateau. Understanding the geochemistry of loess and paleosols can tell us much about past atmospheric circulation, past temperature and moisture regimes, and even tectonic processes.

Journal of Asian Earth Sciences↗

USGS1999

Chronologic model and transgressive-regressive signatures in the late neocene siliciclastic foundation (long key formation) of the Florida keys

Recent drilling of continuous cores in southernmost Florida has documented a thick unit of upper Neogene siliciclastics subjacent to surficial shallow-water Quaternary carbonates exposed on islands of the Florida Keys. The siliciclastics comprise the Long Key Formation and were identified in two cores collected from the middle and upper Florida Keys. A chronologic model based on new planktic foraminiferal biochronology and strontium-isotope chronology suggests the timing of siliciclastic deposition and provides a basis for regional correlation. The chronologic model, supplemented by vertical trends in quartz grain size, pattern of planktic menardiiform coiling direction, and paleoenvironmental interpretations of benthic foraminiferal assemblages, shows that the Long Key Formation contains three intervals (I-III) of varying thickness, grain-size composition, and paleowater depth. Interval I is uppermost Miocene. The quartz grains in Interval I fine upward from basal very coarse sand to fine and very fine sand. Benthic foraminifera indicate an upward shift from an outer-shelf to inner-shelf depositional environment. Interval II, deposited during the late early to early late Pliocene, contains reworked upper Miocene siliciclastics and faunas. In the upper Keys, quartz grains in Interval II range from very coarse sand that fines upward to very fine sand and then coarsens to very coarse and medium sand. In situ benthic faunas indicate an upward shift from outer-shelf to inner-shelf deposition. In the middle Keys, Interval II is different, with the quartz grains ranging primarily from medium to very fine sand. In situ benthic taxa indicate deposition on an inner shelf. In both the middle and upper Keys, the upper Pliocene siliciclastics of Interval III contain quartz grains ranging from very coarse to very fine sands that were deposited on an inner shelf. A sequence boundary between Interval I and Interval II is suggested by; an abrupt shift in the strontium-isotope chemostratigraphy; coarsening in quartz grain size above the boundary; an abrupt landward shift in depositional facies in the upper Keys core; and a distinct variation in the predominant coiling direction of the menardiiform planktic foraminifera, from fluctuating dextral-sinistral to dextral in the upper Keys core. Successive siliciclastic infilling, likely associated with eustatic sea-level change and current redeposition, formed a foundation for subsequent carbonate deposition. Deep-sea biostratigraphic techniques, integrated with ages derived from strontium-isotope chemostratigraphy, can be successfully applied to coastal-margin sequences, even though a depauperate suite of faunal markers is common.

Journal of Sedimentary Research↗

USGS2016

Notes on the origin of copromacrinite based on nitrogen functionalities and δ13C and δ15N determined on samples from the Peach Orchard coal bed, southern Magoffin County, Kentucky

This paper represents the first attempt to show, by means other than just petrographic ones, that one type of macrinite, herein designated copromacrinite, may result from macrofauna feces. For that purpose a combination of coal petrography, X-ray photoelectron spectroscopy, and elemental-analysis continuous-flow isotope ratio mass spectrometry methods were used to determine nitrogen functionalities and δ 13 C and δ 15 N compositions in 1) vitrinite-rich, 2) fusinite + semifusinite-rich, and 3) macrinite-rich (with a possible coprolitic origin) samples of the high volatile A bituminous Peach Orchard coal (Bolsovian; Middle Pennsylvanian) from Magoffin County, Kentucky. There were no significant differences between pyridinic-N and quaternary-N abundance in the three samples, however, pyrrolic-N was higher (~ 54%) in the macrinite-rich sample than in the other two samples (~ 38%). The data suggest that pyridinic-N and quaternary-N are independent of maceral group composition and that pyrrolic-N is dependent on maceral composition (fusinite + semifusinite versus macrinite). δ 13 C values obtained for bulk and demineralized coal of the vitrinite- and fusinite + semifusinite-rich samples are similar with δ 13 C values of − 24.80 ± 0.01&permil; VPDB and − 24.61 ± 0.09&permil; VPDB for bulk samples and − 24.81 ± 0.07&permil; VPDB and − 24.52 ± 0.04&permil; VPDB for demineralized samples. These values are within the expected range for vitrinite-rich samples and the slightly higher δ 13 C value of the fusinite + semifusinite-rich sample is expected as δ 13 C values for inertinite are higher than for vitrinite. However, there was a significant shift to a lower δ 13 C value (− 26.80 ± 0.01&permil; VPDB for the bulk sample value) for the macrinite-rich sample. Because the samples are basically isorank, and δ 13 C (and δ 15 N) shifts do not occur during maturation until anthracite rank, the difference may be related to the presence or composition of the macrinite within the sample which lacks heat-effect indicators, such as devolatilization vacuoles and distorted pores. δ 15 N values are also similar for bulk and demineralized coal of the vitrinite- and fusinite + semifusinite-rich samples, and the bulk values were heavier in this samples (3.07 ± 0.03&permil; Air and 2.92 ± 0.10&permil; Air, respectively), and much lighter (− 2.83 ± 0.09&permil; Air) for the macrinite-rich sample. The study of Peach Orchard coal samples using reflected-light microscopy, isotopic composition, and nitrogen-forms analyses revealed that the macrinite-rich sample contains macrinite with coprolitic features (e.g. oxidation rind, mix of undigested palynomorphs, frequent and randomly located funginite, agglutination pulp of semifusinite reflectance, internal lack of bedding fabric, and suggestion of structures resulting from intestines and stomach walls), more pyrrolic-N (~ 16%), and lower δ 13 C (~ 2&permil; VPDB) and δ 15 N (~ 4&permil; Air) values than the vitrinite and semifusinite + fusinite rich samples. These findings suggest that the maceral macrinite has multiple origins based on petrography and measurable chemical differences between the macrinite, vitrinite, and semifusinite + fusinite fractions within the coal. Assuming that copromacrinite observed is an excretion then the anomalies observed may result from the symbiotic relations between the macrofauna (e.g. cockroaches) and microbiota during the digestive processes, and the nitrogen balance mechanisms inside macrofauna body.

Kentucky↗

USGS2003

Inferences on the hydrothermal system beneath the resurgent dome in Long Valley Caldera, east-central California, USA, from recent pumping tests and geochemical sampling

Quaternary volcanic unrest has provided heat for episodic hydrothermal circulation in the Long Valley caldera, including the present-day hydrothermal system, which has been active over the past 40 kyr. The most recent period of crustal unrest in this region of east-central California began around 1980 and has included periods of intense seismicity and ground deformation. Uplift totaling more than 0.7 m has been centered on the caldera's resurgent dome, and is best modeled by a near-vertical ellipsoidal source centered at depths of 6-7 km. Modeling of both deformation and microgravity data now suggests that (1) there are two inflation sources beneath the caldera, a shallower source 7-10 km beneath the resurgent dome and a deeper source ???15 km beneath the caldera's south moat and (2) the shallower source may contain components of magmatic brine and gas. The Long Valley Exploration Well (LVEW), completed in 1998 on the resurgent dome, penetrates to a depth of 3 km directly above this shallower source, but bottoms in a zone of 100??C fluid with zero vertical thermal gradient. Although these results preclude extrapolations of temperatures at depths below 3 km, other information obtained from flow tests and fluid sampling at this well indicates the presence of magmatic volatiles and fault-related permeability within the metamorphic basement rocks underlying the volcanic fill. In this paper, we present recently acquired data from LVEW and compare them with information from other drill holes and thermal springs in Long Valley to delineate the likely flow paths and fluid system properties under the resurgent dome. Additional information from mineralogical assemblages in core obtained from fracture zones in LVEW documents a previous period of more vigorous and energetic fluid circulation beneath the resurgent dome. Although this system apparently died off as a result of mineral deposition and cooling (and/or deepening) of magmatic heat sources, flow testing and tidal analyses of LVEW water level data show that relatively high permeability and strain sensitivity still exist in the steeply dipping principal fracture zone penetrated at a depth of 2.6 km. The hydraulic properties of this zone would allow a pressure change induced at distances of several kilometers below the well to be observable within a matter of days. This indicates that continuous fluid pressure monitoring in the well could provide direct evidence of future intrusions of magma or high-temperature fluids at depths of 5-7 km. ?? 2003 Elsevier B.V. All rights reserved.

California↗

USGS1991

Late Quaternary stratigraphy and depositional history of the Long Island Sound basin

The stratigraphy of Late Quaternary geologic units beneath Long Island Sound (LIS) is interpreted from 3,500 km of high-resolution, seismic-reflection profiles supplemented by vibracore data. Knowledge gained from onshore regional geologic studies and previous offshore investigations is also incorporated in these interpretations. Glacial deposits overlie and nearly fill an Inner Lowland which is floored by crystalline rocks in the north and bounded to the south by an irregular, north-facing cuesta scarp in Cretaceous Coastal-Plain strata. The thick glacial section consists largely of sediment that was deposited in glacial Lake Connecticut during the retreat of the last (late Wisconsin) ice sheet. Ice-marginal lacustrine fan deposits and submerged extensions of the southeastern Connecticut recessional moraines mark positions of the ice margin during systematic northward retreat. When the ice margin stood at positions just north of the present shoreline of Connecticut (17.6-16.5 ka), glacial Lake Connecticut was slightly larger than Long Island Sound is today. Ice-marginal deltas and near-ice marginal fluviodeltaic deposits occur along coastal Connecticut. Delta-building was diachronous; progressively younger deltas record northwesterly retreat of the ice margin and lowering lake levels. Distally from the deltas, varied lake-clays (up to 150 m thick) drape the underlying topography. The lake spillway cut across the Harbor Hill-Roanoke Point-Fishers Island-Charlestown end moraine at its lowest point just west of Fishers Island (The Race). Lake levels fell as the spillway notch was erosionally deepened, and gradually the lakebed became subaerially exposed. A largely preserved channel system cuts the lake-deposit surface; fluvial channel-fill sediments are overlain by estuarine sediments in the channel system. Perhaps as early as 15 ka, but no later than 13 ka, the sea entered the Basin through the channel at The Race and quickly transgressed westward via the lakebed channel system. As the sea rose further and spread throughout the Basin, a wave-cut surface was formed. Extensive marine deltaic deposits, up to 40 m thick, overlie the wave-cut marine unconformity in the east-central part of the Basin. These sediments were derived from erosion of the drained bed of Lake Hitchcock, in the Connecticut valley to the north, and were deposited in a -40-m relative sea at about 12.4 ka. Where quiet waters prevail, marine mud generally less than 15 m thick blankets the older deposits of the Basin. Elsewhere, especially in eastern LIS, tidal currents are actively reworking and transporting glacial and postglacial deposits.

Connecticut, New York↗

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↗

USGS2025

Postglacial eruptive history of Laguna del Maule volcanic field and constraints on its magmatic system

The Laguna del Maule (LdM) volcanic field has produced >100 explosive and extrusive events over the past 17 ka. More than two-thirds of these have been silicic, with most being postglacial rhyolites (72–78 % SiO 2 ) concentrated near the eponymous lake (LdM), an extraordinary anomaly in the Quaternary Andes and unprecedented in this 1.5–Ma-old volcanic field as a whole. The postglacial field includes >70 separate vents distributed over ∼360 km 2 that together produced the many distinct eruptive events, of which 55 are rhyolitic (73 %−77 % SiO 2 ), 18 are rhyodacitic (68 %−72 % SiO 2 ), 4 are dacitic (63 %–66 % SiO 2 ), 26 are intermediate (54–62 % SiO 2 ), and 2 are true basalts (50 %–53 % SiO 2 ). Of these, most originated from single-vent domes, cones, or craters that erupted effusive and/or explosive products, each with relatively short lifespans. Some originated from multi-vent centers, the largest one being the Barrancas complex southeast of the lake, which has as many as 18 vents that erupted over as much as 10 kyr. The LdM basin itself is ringed by 13 separate silicic centers, many of which are also multi-vent and built over time by multiple explosive and extrusive events. These surround the lake, near the middle of which is the vent for the high-silica rhyolite Plinian eruption that produced the “ Rhyolite of Laguna del Maule ”, which was the first and largest silicic event from the postglacial field. Explosive and effusive products from all these events have been put in a time-stratigraphic framework supported by radiocarbon dating and chemical analyses to reconstruct the postglacial eruptive history. Correlations of pyroclastics to eruptive vents have provided a spatial-temporal framework that helps characterize the magmatic system beneath the LdM field. Distribution of both silicic and mafic vents support the likelihood that two separate magmatic systems produced the postglacial eruptions in the volcanic field—one in the Laguna del Maule basin and the other at the Barrancas complex—with a cluster of silicic vents at each and mafic vents situated between the two. Vent distributions, compositions of eruptive products, and temporal and spatial trends of eruptive units suggest that the abundant rhyodacitic and mafic units in the LdM basin have no common magma reservoir, but instead each had its own evolutionary trend. In contrast, there is enough affinity among some of the rhyolitic units in the Basin to imply magmatic connections and/or continuity that span both time and space, although neither geographic proximity nor temporal similarity have singular control on LdM-basin rhyolite compositions. Compositional trends through time at the Barrancas center suggest the rhyolitic eruptions at West and East Barrancas were derived from separate, zoned reservoirs that were tapped in batches, not permitting development of a large high-silica reservoir such as that beneath the LdM basin.

Laguna del Maule lake basin↗

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↗

USGS2023

Stratigraphic architecture and fluvial interpretations of the Upper Cretaceous (Turonian?) Middendorf Formation, Chesterfield County, South Carolina, U.S.A.

The Upper Cretaceous (Turonian?) Middendorf Formation is a sand-rich stratigraphic unit of fluvial origin that forms a large aquifer in the U.S. Atlantic Coastal Plain. In Chesterfield County (South Carolina), which is the site of the type locality, the formation ranges in thickness from 66.5 to > 119.7 meters. The base of the formation is an unconformity above Paleozoic metasiltstone, and the upper contact is an unconformity above which lies sand of the Quaternary Pinehurst Formation. Outcrops display the following five facies assemblages: 1) sandstone to conglomeratic sandstone (fluvial bar and channel deposits), 2) beds of alternating laminae of sandstone and mudstone (fluvial overbank or floodplain deposits), 3) ≥ 1 m-thick beds of clay (swamp deposits, floodplain deposits, and/or sediment that accumulated in abandoned fluvial channels), 4) 0.2–0.5 m-thick planar to slightly undulatory beds of framework-supported sandstone with a mud matrix (traction-dominated current deposits at the top of fluvial bars, upper-flow-regime bedform deposits in subsidiary fluvial channels, or coarse-grained overbank deposits), and 5) sandstone to conglomeratic sandstone cemented by iron (interpreted as fluvial bar and channel deposits, with the iron cement being a diagenetic “groundwater ferricrete” that formed via the circulation of shallow groundwater and the oxidation of iron-bearing minerals). Kaolinite in various forms is pervasive throughout the formation and is interpreted as an early diagenetic phenomenon that formed by prolonged postdepositional weathering and flushing by meteoric water under a warm and humid paleoclimate. The fluvial system that formed the Middendorf Formation prograded into the area from the west or northwest from uplifted margins of Mesozoic rift basins and/or the Appalachian Mountains. This progradation was a response to a base-level fall and the sediment accumulated during base-level lowstand and subsequent early transgression. In Chesterfield County, the Middendorf Formation can be subdivided into three fining-upward sequences. Each sequence consists predominantly of medium to coarse sand with a greater abundance of gravel in the lower part of the sequence and a greater abundance of clay and silt beds in the upper part. Each sequence is interpreted as either a response to autogenic processes or a response to allogenic sea-level changes, specifically a higher-order (higher-frequency) progression from relative lowstand conditions to early transgression whereby coarse sand and gravel (e.g., fluvial bar and channel deposits) were preserved during initial lowstand conditions and a greater proportion of mud and finer-grained sand (floodplain deposits) were preserved during subsequent early transgression. The Middendorf Formation is correlative with several other kaolinite-rich fluvial sandstones in North America including the Raritan Formation in New Jersey, the Tuscaloosa Formation of the eastern Gulf of Mexico (Alabama, Mississippi, Louisiana), the Woodbine Formation of the central Gulf of Mexico (Texas), and the Frontier Formation of Wyoming. The accumulation and preservation of these formations occurred in response to a Turonian eustatic sea-level fall and subsequent transgression, and the early diagenetic kaolinite in these formations is attributed to similar warm and humid paleoclimate conditions.

South Carolina↗

USGS2021

Volcanic seismicity beneath Chuginadak Island, Alaska (Cleveland and Tana volcanoes): Implications for magma dynamics and eruption forecasting

Cleveland and Tana are remote volcanoes located in the central Aleutian volcanic arc on the eastern end of the Islands of Four Mountains (IFM). The persistently active Mount Cleveland volcano, on the western side of Chuginadak Island, is surrounded by several closely spaced Quaternary volcanic centers including Carlisle, Herbert, Kagamil, Tana, and Uliaga, and numerous small satellite vents on Chiginadak between Cleveland and Tana. The Alaska Volcano Observatory (AVO) installed two permanent broadband seismometers on Chuginadak Island in 2014, and we operated a temporary broadband network focused on the western side of the island in 2015–2016. Collectively, these stations provided the first seismic observations of this frequently active volcano and the surrounding Holocene-aged volcanic vents. During the study period (July 2014–January 2019), eruptive activity at Cleveland was characterized by small explosions separated by periods of lava effusion that formed small domes in the volcano's summit crater. We characterize seismicity beneath Chuginadak Island through automated analysis of event waveform frequency content, development of a one-dimensional P-wave velocity model, calculation of earthquake hypocenters, magnitudes, focal mechanisms, and identification of earthquake families. This analysis reveals the full range of seismic event types expected in a highly active volcanic environment and includes Volcano-Tectonic (VT) earthquakes, Long-Period (LP) events, and explosion signals. LP events appear to cluster at shallow depth beneath the active crater of Mount Cleveland and almost all of the explosions occur without identifiable short-term (hours to days) seismic precursors. VT earthquakes beneath Mount Cleveland occur at depths of 2 to 8 km below sea level (BSL) and range in magnitude from −0.2 to 1.8. VT focal mechanisms have horizontal P-axes that align with the regional axis of maximum stress. These observations, and a relatively slow one-dimensional seismic velocity model, are consistent with a shallow body of magma that is fed through a deeper conduit system. The time-history of VT earthquakes and shallow LP events suggest their occurrence may track the transfer of magma and fluids from the mid-crust to the shallow portions of the conduit system and may provide a means to anticipate future explosions and periods of dome growth. VT hypocenters also extend ~7 km northeast of Cleveland's summit at depths of 5 to 10 km BSL, under a group of Holocene-aged vents between Mount Cleveland and Tana. These earthquakes have vertically-oriented P-axes and a greater percentage occur in families. These observations, combined with observations of vent orientation and morphology and gas flux, suggest the area between Cleveland and Tana represents a zone of complicated volcano-tectonic interaction, similar to calderas elsewhere in the Aleutian arc. The presence of a larger volcanic system in the eastern IFM could influence magmatism and account for the multiple closely spaced volcanic centers in this region.

Alaska↗

USGS2007

Earthquake-by-earthquake fold growth above the Puente Hills blind thrust fault, Los Angeles, California: Implications for fold kinematics and seismic hazard

Boreholes and high-resolution seismic reflection data collected across the forelimb growth triangle above the central segment of the Puente Hills thrust fault (PHT) beneath Los Angeles, California, provide a detailed record of incremental fold growth during large earthquakes on this major blind thrust fault. These data document fold growth within a discrete kink band that narrows upward from ???460 m at the base of the Quaternary section (200-250 m depth) to <150 m at 2.5 m depth, with most growth during the most recent folding event occurring within a zone only ???60 m wide. These observations, coupled with evidence from petroleum industry seismic reflection data, demonstrate that most (>82% at 250 m depth) folding and uplift occur within discrete kink bands, thereby enabling us to develop a paleoseismic history of the underlying blind thrust fault. The borehole data reveal that the youngest part of the growth triangle in the uppermost 20 m comprises three stratigraphically discrete growth intervals marked by southward thickening sedimentary strata that are separated by intervals in which sediments do not change thickness across the site. We interpret the intervals of growth as occurring after the formation of now-buried paleofold scarps during three large PHT earthquakes in the past 8 kyr. The intervening intervals of no growth record periods of structural quiescence and deposition at the regional, near-horizontal stream gradient at the study site. Minimum uplift in each of the scarp-forming events, which occurred at 0.2-2.2 ka (event Y), 3.0-6.3 ka (event X), and 6.6-8.1 ka (event W), ranged from ???1.1 to ???1.6 m, indicating minimum thrust displacements of ???2.5 to 4.5 m. Such large displacements are consistent with the occurrence of large-magnitude earthquakes (Mw > 7). Cumulative, minimum uplift in the past three events was 3.3 to 4.7 m, suggesting cumulative thrust displacement of ???7 to 10.5 m. These values yield a minimum Holocene slip rate for the PHT of ???0.9 to 1.6 mm/yr. The borehole and seismic reflection data demonstrate that dip within the kink band is acquired incrementally, such that older strata that have been deformed by more earthquakes dip more steeply than younger strata. Specifically, strata dip 0.4?? at 4 m depth, 0.7?? at 20 m depth, 8?? at 90 m, 16?? at 110 m, and 17?? at 200 m. Moreover, structural restorations of the borehole data show that the locus of active folding (the anticlinal active axial surface) does not extend to the surface in exactly the same location from earthquake to earthquake. Rather, that the axial surfaces migrate from earthquake to earthquake, reflecting a component of fold growth by kink band migration. The incremental acquisition of bed dip in the growth triangle may reflect some combination of fold growth by limb rotation in addition to kink band migration, possibly through a component of trishear or shear fault bend folding. Alternatively, the component of limb rotation may result from curved hinge fault bend folding, and/or the mechanical response of loosely consolidated granular sediments in the shallow subsurface to folding at depth. Copyright 2007 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗

USGS2023

Imaging the magmatic plumbing of the Clear Lake Volcanic Field using 3-D gravity inversions

The Quaternary Clear Lake Volcanic Field (CLVF) in the Northern California Coast Range is the youngest of a string of northward-younging volcanic centers in the state. The CLVF is located within the broad San Andreas Transform Fault System and has been active intermittently for ∼2 million years. Heat beneath the CLVF supports The Geysers, one of the largest producing geothermal fields in the world. Previous geophysical studies proposed the existence of a magma reservoir beneath Mount Hannah, which is northeast of The Geysers, near the geographic center of the CLVF. The lateral extent, depth, and presence of melt within this reservoir are poorly constrained, as is the relationship between this body and the broader magmatic plumbing of the CLVF. To gain a clearer and more comprehensive picture of the CLVF magma source region, a gravity dataset was compiled and the first 3-D gravity inversions of the CLVF were completed. Field and synthetic model inversions from the current study both indicate that the gravity low roughly centered on Mount Hannah is not accurately explained by a 5–7 km thick lens of Mesozoic Great Valley Sequence ( ρ = 2.58 g/cm 3 "> �=2.58g/cm3 ) as proposed by Stanley et al. (1998) . The observed gravity low is more accurately described by one or more silicic, partial melt bodies between The Geysers and Mount Hannah. Although our inversions cannot constrain the exact depth and geometry of these bodies, the recovered models indicate the existence of a partial melt zone between 6 and 13 km depth. The prolonged eruption history of the CLVF, coupled with the compositional variation of erupted rocks over time and space, is consistent with the existence of several, potentially ephemeral, melt-bearing bodies as opposed to one large melt body. Given the density and location of the recovered anomaly, rhyolite-MELTS thermodynamic modeling suggests the existence of 10–30% rhyodacitic melt within the proposed silicic magma reservoir at about 700 °C and 8 km depth (210 MPa). Independent petrologic, geochemical, and seismic evidence indicates that this silicic partial melt zone is underlain by basaltic melt in the lower to middle crust (13 to 21 km depth), which is fed by a mantle source. Eruptions in the past ∼8.5–13.5 thousand years; high regional heat flow; 3 He enrichment of hydrothermal fluids; and our modeling, which suggests the presence of a mid-crustal, silicic partial melt zone, point to a still-active CLVF. The relatively low estimates of partial melt (10–30%) predicted by thermodynamic modeling indicates that an injection of new magma into the imaged partial melt zone is needed to generate sufficient melt to incite future eruptions. Despite the low percent melt estimates within the proposed silicic partial melt zone the potential for future volcanic eruption remains. Due to the proximity of the CLVF to cities surrounding Clear Lake and the densely populated San Francisco Bay Area, continued research and monitoring of the volcanic field are warranted. The geophysical and petrologic modeling presented here improves our understanding of the CLVF magma plumbing system and allows us to better characterize its associated volcanic hazards.

California↗