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USGS2012

Tectonics and conductivity structures in the Southern Washington Cascades

The tectonic setting of the southern Washington Cascades has been studied with the aid of magnetotelluric (MT) and other geophysical data. The main feature of interest in the geophysical data is a broad high-conductivity anomaly mapped with MT and geomagnetic variation (GMV) data. This anomaly is located roughly within the triangle formed by the volcanoes Mount Rainier, Mount St. Helens, and Mount Adams but exceeds beyond Mount Rainier to the northwest. We interpret the cause of the anomaly to be conductive rocks with resistivities of 1–4 ohm m and thicknesses possibly greater than 15 km. These conductive rocks are found 2–8 km beneath the overlying less conductive volcanic and sedimentary rocks at the surface. Two aeromagnetic lows follow the trend of the conductivity anomaly, and linear belts of strike-slip seismicity are coincident with both these magnetic lows. One of the aeromagnetic lows is coincident with the western margin of the conductivity anomaly. The geophysical data appear to outline a suture zone of probable Eocene age caused by accretion of a large seamount complex (Siletzia) and that may contain large thicknesses of compressed forearc basin and accretionary prism sedimentary rocks of Cretaceous to Eocene age. Part of the shallower conductive rocks may be associated with carbonaceous continental and transitional marine sedimentary rocks of the Puget Group. The contact between the hypothesized compressed basin and the accreted terranes to the west may localize the release of shear stresses in this region of oblique subduction. Several possible explanations for the conductivity anomaly are considered in addition to the compressed basin hypothesis.

Washington↗

USGS1985

Ring distributions in alkali- and alkaline-earth aluminosilicate framework glasses- A raman spectroscopic study

Raman spectra of crystalline polymorphs of a number of tectosilicate minerals having various sizes of smallest rings of TO 4 tetrahedra (T = Si, Al) have been investigated to identify the bands that are sensitive indicators of the smallest rings in the network. The information obtained from the Raman spectra of tectosilicate minerals (e.g., SiO 2 polymorphs, NaAlSi 3 O 8 (Ab), NaAlSiO 4 (Ne), KAlSi 3 O 8 (Or), and KAlSi 2 O 6 (Lc)) is used to interpret the Raman spectra of the isochemical glasses. It is shown that the frequency of the dominant ν s (TOT) band in the spectra of both crystals and glasses is related to the dominant size of TO 4 rings in the structure. In agreement with previous X-ray RDF work, it is found that in the glasses of Ab and Jd (NaAlSi 2 O 6 ) compositions, six-membered rings of TO 4 tetrahedra predominate. The Raman spectrum of Or glass, however, indicates that clusters of intermixed four- and six-membered rings of TO 4 tetrahedra, similar to those existing in crystalline leucite, are also present in the glass. Raman evidence indicates that four-membered rings of TO 4 tetrahedra predominate in the glass of An composition. Similarly, the higher frequency of the ν s (TOT) band in the spectrum of Ne glass as compared with the frequency the ν s (TOT) band in the spectra of crystalline cargenieite and nephelite indicates either an admixture of the four- and six-membered rings or the puckering of six-membered rings in the glass structure.

Journal of Non-Crystalline Solids↗

USGS2005

Petrologic constraints on the thermal structure of the Cascades arc

Primitive late Cenozoic basaltic lavas from the Cascades volcanic arc near latitude 46°N comprise two distinct compositional groups. Group I includes samples with low Ba/Nb (<20) and other compositional similarities to oceanic island and MORB lavas from within-plate settings. In contrast, Group II exhibits enrichment of Ba and large-ion lithophile elements (LILE) and depletion of Nb and high-field strength elements (HFSE) as seen commonly in calcalkalic lavas from other volcanic arcs. Lavas of both groups are widely distributed across the transect, and Group I lavas are found as much as 30–40 km trenchward of stratovolcanoes that define the High Cascades ‘volcanic front (VF)’. The most primitive lavas are sparsely porphyritic, have elevated Ni, Cr, and Mg#, high calculated magmatic temperatures (1200–1300 °C), and lack evidence of shallow (crustal level) storage and crystallization. Compositions of parental liquids were calculated for each primitive sample on the premise of Fe–Mg equilibrium with mantle peridotite. Assuming that such magmas ascended rapidly from accumulation zones in the mantle, we estimate P and T of segregation. We infer that (a) Group I magmas ascended from systematically greater depths (∼50–70 km) than Group II (∼30–50 km), implying the possible existence of compositional stratification in the mantle wedge; (b) Group I basalts show the least evidence for slab-derived contributions in their sources despite their apparently greater segregation depths (approaching the locus of the Cascadia slab beneath the frontal arc region); (c) Group II lavas with the strongest slab compositional signature have temperatures far exceeding the wet peridotite solidus at high pressure; and (d) the inferred thermal structure of the mantle wedge is very warm, implying a significant component of mantle upwelling and convection. Group I lavas are interpreted as decompression melts from this mantle, and their compositions suggest that their source was little modified by slab-derived contributions. We speculate that melting to produce Group II magmas occurs in the shallow mantle, possibly in response to heating by hot ascending Group I magmas. If true, it seems unlikely that the slab-like signal in Group II lavas can be attributed to modern slab inputs; rather, we postulate that this signature may reflect melting of lithospheric mantle domains containing a ‘stored’ slab-derived component inherited from earlier stages of Cascadia subduction. This scenario differs from the standard paradigm for subduction zones (SZs), and stresses the importance of convecting asthenospheric mantle in driving arc magmatism, particularly in warm subduction zones where slab fluid contributions likely are minimal. In contrast, because tectonic conditions in more typical volcanic arcs favor subduction of cooler, less dehydrated oceanic lithosphere, slab-derived fluids may promote extensive flux-melting in the wedge. Such melts may dominate the magmatic output and mask wedge contributions. The Cascade arc thus provides rarely afforded insights into arc magma genesis.

British Columbia, California, Oregon, Washington↗

USGS2007

Generation of fine hydromagmatic ash by growth and disintegration of glassy rinds

The deposits of mafic hydromagmatic eruptions are more fine grained and variable in vesicularity than dry magmatic deposits. Blocky, equant shapes of many hydromagmatic clasts also contrast with droplet, thread, and bubble wall morphology of dry magmatic fragments. Small (<∼180 μ m), blocky hydromagmatic pyroclasts have traditionally been interpreted to result from discrete vapor explosions, although such explosions tend to occur only under certain conditions. This paper considers a process of hydromagmatic ash formation that involves repeated growth and disintegration of glassy rinds on pyroclast surfaces as they deform within turbulent flows. This process, termed “turbulent shedding”, may occur during the expansion phase of vapor explosions or during turbulent but nonexplosive mixing of magma with water, steam, or water sprays. The occurrence of turbulent shedding and the resulting fragment sizes depend on the timescale for rind growth and the timescale between disturbances that remove or disintegrate glassy rinds. Turbulent shedding is directly observable in some small littoral jets at Kilauea. Calculations suggest that, in the presence of liquid water or water sprays, glassy rinds having a thickness of microns to millimeters should form in milliseconds to seconds. This is similar to the timescale between turbulent velocity fluctuations that can shred lava globules and remove such rinds. The fraction of a deposit consisting of fine ash should increase with the duration of this process: Large‐scale Surtseyan jets generate hundreds or thousands of shedding events; bubble bursts or tephra jets at Kilauea's coast may produce only a few.

Journal of Geophysical Research B: Solid Earth↗

USGS2007

Hydrothermal fluid flow and deformation in large calderas: Inferences from numerical simulations

[1] Inflation and deflation of large calderas is traditionally interpreted as being induced by volume change of a discrete source embedded in an elastic or viscoelastic half-space, though it has also been suggested that hydrothermal fluids may play a role. To test the latter hypothesis, we carry out numerical simulations of hydrothermal fluid flow and poroelastic deformation in calderas by coupling two numerical codes: (1) TOUGH2 [Pruess et al., 1999], which simulates flow in porous or fractured media, and (2) BIOT2 [Hsieh, 1996], which simulates fluid flow and deformation in a linearly elastic porous medium. In the simulations, high-temperature water (350°C) is injected at variable rates into a cylinder (radius 50 km, height 3–5 km). A sensitivity analysis indicates that small differences in the values of permeability and its anisotropy, the depth and rate of hydrothermal injection, and the values of the shear modulus may lead to significant variations in the magnitude, rate, and geometry of ground surface displacement, or uplift. Some of the simulated uplift rates are similar to observed uplift rates in large calderas, suggesting that the injection of aqueous fluids into the shallow crust may explain some of the deformation observed in calderas.

Journal of Geophysical Research B: Solid Earth↗

USGS2012

Crustal structure of the northwestern Basin and Range Province from the 1986 Program for Array Seismic Studies of the Continental Lithosphere Seismic Experiment

A portion of northwestern Nevada was imaged to determine the crustal structure and to assess reported differences between refraction versus reflection determinations of Moho depth and how the crustal composition and structure has been influenced by volcanic and extension mechanisms. Interpretation of the refraction/wide-angle reflection data suggests that the crust is fairly uniform in thickness and varies by less than 5 km over the 280 km east-west profile and 3 km over its 220 km north-south length. The velocity structure is characterized by five layers: 1) an uppermost crust, composed of sedimentary rocks and basement that has an average velocity of 5.7 km s-1; 2) a middle crust that extends to a depth of 18-22 with an average velocity of 6.1 km s-1; 3) a 10-12 km thick lower crust with an average velocity of 6.6 km s-1; 4) a 2-5 km thick transitional crust-mantle boundary defined by a 7.6 km s-1 velocity; and 5) an upper mantle with an average Pn velocity of 7.9-8.0 km s-1. A uniform upper mantle composition across the Basin and Range is suggested and the homogeneity of the velocity structure beneath the western Basin and Range argues for a youthful Moho and crust that has been reworked by province-wide late Cenozoic extension, episodic magmatism, and underplating. -from Authors

Journal of Geophysical Research↗

USGS1994

Evidence from xenoliths for a dynamic lower crust, eastern Mojave Desert, California

Garnet-rich xenoliths in a Tertiary dike in the eastern Mojave Desert, California, preserve information about the nature and history of the lower crust. These xenoliths record pressures of ∼ 10–12 kbar and temperatures of ∼ 750–800°C. Approximately 25% have mafic compositions and bear hornblende + plagioclase + clinopyroxene + quartz in addition to garnet. The remainder, all of which contain quartz, include quartzose, quartzofeldspathic, and aluminous (kyanite±sillimanite-bearing) varieties. Most xenoliths have identifiable protoliths—mafic from intermediate or mafic igneous rocks, quartzose from quartz-rich sedimentary rocks, aluminous from Al-rich graywackes or pelites, and quartzofeldspathic from feldspathic sediments and/or intermediate to felsic igneous rocks. However, many have unusual chemical compositions characterized by high FeO(t), FeO(t)/MgO, Al 2 O 3 , and Al 2 O 3 /CaO, which correspond to high garnet abundance. The mineralogy and major-and trace-element compositions are consistent with the interpretation that the xenoliths are the garnet-rich residues of high-pressure crustal melting, from which granitic melt was extracted. High 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd, together with highly discordant zircons from a single sample with Pb/Pb ages of ∼ 1.7 Ga, demonstrate that the crustal material represented by the xenoliths is at least as old as Early Proterozoic. This supracrustal-bearing lithologic assemblage may have been emplaced in the lower crust during either Proterozoic or Mesozoic orogenesis, but Sr and Nd model ages> 4 Ga require late Phanerozoic modification of parent/daughter ratios, presumably during the anatectic event. Pressures of equilibration indicate that peak metamorphism and melting occurred before the Mojave crust had thinned to its current thickness of <30 km. The compositions of the xenoliths suggest that the lower crust here is grossly similar to estimated world-wide lower-crustal compositions in terms of silica and mafic content; however, it is considerably more peraluminous, has a lower mg -number, and is distinctive in some trace element concentrations, reflecting its strong metasedimentary and restitic heritage.

Journal of Petrology↗

USGS2021

Insights on the characteristics and sources of gas from an underground coal mine using compositional data analysis

Coal mine gas originates from the gas emission zone (GEZ) of the mine, as well as the longwall face and pillars. Gas emissions are controlled directly at the sources using horizontal or vertical boreholes drilled from surface or from the entries in advance of mining, or it is captured from the fractured and caved zones (gob) using ventholes during mining. The rest of the gas, especially that gas that originates from the longwall face and caved zone, mixes with the ventilation air and travels through bleeder and return entries before being exhausted to atmosphere from air shafts. Although the gas associated with mining mostly focuses on methane, the gas is not pure methane but is a mixture (where both the components and their quantities vary depending on the sampling location). Understanding the evolution of the composition of the gas from source to different sampling and evaluation points in mines using proper statistical analysis and interpretation methods can lead to better designed degasification and ventilation systems and the selection of the most adequate utilization method for generating energy from the gas. In this work, we present the results of compositional data analysis (CoDa) of gases sampled at different locations in a longwall mine operating in Pennsylvanian coal-bearing strata in the Northern Appalachian coal basin. Sampling locations were from accessible parts of the bleeder entries, returns, bleeder evaluation points and shafts within the mine, and also from gob gas ventholes (GGV) and coal degasification boreholes drilled in the panel areas. In addition, desorbed gas samples from seams that are important for the mine were included in the analyses for comparison. The compositional data analysis showed that the gas composition shifts based on the sampling location, putting in-mine and pure coalbed gases on the opposite ends. Removal of air from the samples did not change this observation suggesting that oxygen is depleted especially for in-mine samples, which is due to oxidation. Results also suggested that desorbed gas samples may not represent the composition of the coal seam gas.

International Journal of Coal Geology↗

USGS2012

Deformation in the White Mountain seismic gap, California-Nevada, 1972-1982

A 100×40 km trilateration network extending from Bishop, California, to near Hawthorne, Nevada, crosses the east end of the Long Valley caldera, site of renewed magma inflation in the 1979–1980 interval, and spans most of the White Mountain seismic gap. The network was surveyed in 1972, 1973, 1976, 1979, 1980, and 1982. The 1980 survey may be contaminated by a scale error. In addition, leveling surveys across the caldera have been run in 1932, 1957, 1975, 1980, 1982, and 1983. Interpretation of the deformation is complicated by the occurrence of the May 1980 Mammoth Lakes earthquake sequence (four earthquakes M L ≥6) at the south edge of the caldera as well as other moderate earthquakes within the White Mountain seismic gap. The vertical deformation is largely accounted for by 0.10‐ to 0.15‐km 3 expansion of a spherical magma chamber 8–10 km beneath the resurgent dome within the Long Valley caldera sometime between July 1979 and September 1980 with an additional expansion of perhaps 0.05 km 3 between September 1980 and July 1982. Some additional sources of deformation within the aftershock zone of the Mammoth Lakes earthquakes seem to be required to explain the horizontal deformation. We show that right‐lateral slip on vertical faults extending WNW from each of the three largest earthquakes in the Mammoth Lakes sequence provides the required additional deformation, but this solution is by no means unique. There are simply too few data to define the rather complex deformation that apparently occurred within the aftershock zone. There is little doubt, however, that inflation of a magma chamber beneath the resurgent dome within the Long Valley caldera was involved in the deformation.

California↗

USGS2000

Isotopic evidence for the sources of Cretaceous and tertiary granitic rocks, east-central Alaska: Implications for the tectonic evolution of the Yukon-Tanana terrane

Magnetotelluric traverses across the southern Yukon-Tanana terrane (YTT) reveal the presence of a thick conductive layer (or layers) beneath Paleozoic crystalline rocks. These rocks have been interpreted to be flysch of probable Mesozoic age, on the basis of the occurrence of Jurassic-Cretaceous flysch in the Kahiltna assemblage and Gravina-Nutzotin belt flanking the YTT to the southwest and southeast, respectively. The Pb, Nd, Sr, and O isotopes in Cretaceous and Tertiary granitic rocks that crop out throughout the YTT were measured to determine if these rocks do in fact contain a component of flysch. Previous limited analyses indicated that the Pb isotopes of the granitic rocks could be a mixture of radiogenic Pb derived from Paleozoic crystalline rocks of the YTT with an increasing component of relatively nonradiogenic Pb with decreasing age. Our Nd, Sr, and O data, along with additional Pb isotope data, eliminate flysch as a likely source and strongly suggest that the nonradiogenic end-member was derived from mafic rocks, either directly from mantle magma or by melting of mafic crust. The lack of a sedimentary component in the granitic plutons suggests either that the plutons did not incorporate significant amounts of flysch during intrusion or that the conductive layer beneath the YTT crystalline rocks is not flysch.

Alaska↗

USGS2012

The high-pressure electronic structure of magnesiowustite (Mg, Fe)O: applications to the physics and chemistry of the lower mantle

The electronic structure of magnesiowustite is investigated using self-consistent field X α scattered wave (SCF- X α-SW) molecular orbital calculations on (FeO 6 ) 10− and (FeMg 12 O 14 ) 2− clusters. Calculated one-electron transition energies are used to interpret the optical spectrum of (Mg, Fe)O. The results are applied to the electrical and thermal conductivity of the lower mantle. The spin pairing of Fe 2+ and the effect of pressure on bonding in magnesiowustite, with some inferences regarding the incorporation of oxygen in the outer core, is also addressed. The approach used here appears to give a reliable description of the energy and pressure dependence of the spin-allowed 5 T 2 g → 5 E g ligand field transition and the spin-pairing transition of Fe 2+ in (Mg, Fe)O. However, the oxygen to metal charge transfer transitions in (Mg, Fe)O are not as reliably determined insofar as the p - d band gap varies with cluster size and the energies of the charge transfer states cannot be found without including configurational interaction. Nevertheless, it is argued that the charge transfer transitions that are intrinsic to (Fe, Mg)O are of a sufficiently high energy to be irrelevant to the electrical and thermal conductivity of the lower mantle. This is especially true if Fe 2+ adopts the low-spin configuration. The geophysically significant properties of (Fe, Mg)O probably result from defect Fe 3+ .

Journal of Geophysical Research↗

USGS2013

Radio-transmitters do not affect seasonal productivity of female Golden-winged Warblers

Investigating the potential effects of handling and marking techniques on study animals is important for correct interpretation of research results and to effect progress in data-collection methods. Few investigators have compared the reproductive output of radio-tagged and non-radio-tagged songbirds, and no one to date has examined the possible effect of radio-tagging adult songbirds on the survival of their fledglings. In 2011 and 2012, we compared several parameters of reproductive output of two groups of female Golden-winged Warblers ( Vermivora chrysoptera ) breeding in Minnesota, including 45 females with radio-transmitters and 73 females we did not capture, handle, or mark. We found no difference between groups in clutch sizes, hatching success, brood sizes, length of incubation and nestling stages, fledging success, number of fledglings, or survival of fledglings to independence. Thus, radio-tags had no measurable impact on the productivity of female Golden-winged Warblers. Our results build upon previous studies where investigators have reported no effects of radio-tagging on the breeding parameters of songbirds by also demonstrating no effect of radio-tagging through the post-fledging period and, therefore, the entire breeding season.

Journal of Field Ornithology↗

USGS1988

Pyrite formation in the Lower Cretaceous Mowry Shale: Effect of organic matter type and reactive iron content

Geochemical analysis of the Lower Cretaceous Mowry Shale in Wyoming and Montana shows that organic matter type and reactive iron content limited pyrite formation during early diagenesis. Consequently, direct interpretation of paleosalinity cannot be made from a carbon/sulfur plot. Mowry Shale samples with hydrogen index (HI) less than 150 (mg hydrocarbon/ organic carbon) contain low amounts of sulfur. In these samples, bacterial reduction of sulfate may have been inhabited by low nutritive content of the organic matter. For Mowry samples with HI greater than 150 there is a range of sulfur values and a positive correlation between iron and sulfur. The iron sulfur regression is approximately parallel to the stoichiometric pyrite line and intersects the iron axis at 0.5 percent. On average, 60 percent of the iron present was nonreactive; the remaining iron reacted with sulfide to form pyrite. For samples with HI greater than 150, pyrite formation was limited by sulfide or organic carbon content but by reactive iron content. In one modern setting, where salinity is greater than one-half normal seawater, sulfate content does not limit pyrite formation (Berner and Raiswell, 1984). From this analysis then, it can be estimated that the Mowry Sea was not a fresh water body but was either of normal marine salinity or contained brackish water with greater than one-half normal salinity.

Wyoming↗

USGS2007

Organic petrology of Paleocene Marcelina Formation coals, Paso Diablo mine, western Venezuela: Tectonic controls on coal type

About 7??Mt of high volatile bituminous coal are produced annually from the four coal zones of the Upper Paleocene Marcelina Formation at the Paso Diablo open-pit mine of western Venezuela. As part of an ongoing coal quality study, we have characterized twenty-two coal channel samples from the mine using organic petrology techniques. Samples also were analyzed for proximate-ultimate parameters, forms of sulfur, free swelling index, ash fusion temperatures, and calorific value. Six of the samples represent incremental benches across the 12-13??m thick No. 4 bed, the stratigraphically lowest mined coal, which is also mined at the 10??km distant Mina Norte open-pit. Organic content of the No. 4 bed indicates an upward increase of woody vegetation and/or greater preservation of organic material throughout the life of the original mire(s). An upward increase in telovitrinite and corresponding decrease in detrovitrinite and inertinite illustrate this trend. In contrast, stratigraphically higher coal groups generally exhibit a 'dulling upward' trend. The generally high inertinite content, and low ash yield and sulfur content, suggest that the Paso Diablo coals were deposited in rain-fed raised mires, protected from clastic input and subjected to frequent oxidation and/or moisture stress. However, the two thinnest coal beds (both 0.7??m thick) are each characterized by lower inertinite and higher telovitrinite content relative to the rest of Paso Diablo coal beds, indicative of less well-established raised mire environments prior to drowning. Foreland basin Paleocene coals of western Venezuela, including the Paso Diablo deposit and time-correlative coal deposits of the Ta??chira and Me??rida Andes, are characterized by high inertinite and consistently lower ash and sulfur relative to Eocene and younger coals of the area. We interpret these age-delimited coal quality characteristics to be due to water availability as a function of the tectonic control of subsidence rate. It is postulated that slower subsidence rates dominated during the Paleocene while greater foreland basin subsidence rates during the Eocene-Miocene resulted from the loading of nappe thrust sheets as part of the main construction phases of the Andean orogen. South-southeastward advance and emplacement of the Lara nappes during the oblique transpressive collision of the Caribbean and South American tectonic plates in the Paleocene was further removed from the sites of peat deposition, resulting in slower subsidence rates. Slower subsidence in the Paleocene may have favored the growth of raised mires, generating higher inertinite concentrations through more frequent moisture stress. Consistently low ash yield and sulfur content would be due to the protection from clastic input in raised mires, in addition to the leaching of mineral matter by rainfall and the development of acidic conditions preventing fixation of sulfur. In contrast, peat mires of Eocene-Miocene age encountered rapid subsidence due to the proximity of nappe emplacement, resulting in lower inertinite content, higher and more variable sulfur content, and higher ash yield.

International Journal of Coal Geology↗

USGS2023

Elucidating the magma plumbing system of Ol Doinyo Lengai (Natron Rift, Tanzania) Using satellite geodesy and numerical modeling

Ol Doinyo Lengai, located in the southern Eastern Branch of the East African Rift had several eruptive episodes with ash falls and lava flows (VEI 3) that caused damage to the nearby communities between 2007 and 2010. The volcano is remote and access is difficult. Although this volcano has been studied for decades, its plumbing system is still poorly understood, in part, because of the lack of precise observations of surface deformation during periods of quiet and unrest. This study investigates the volcanic plumbing system of Ol Doinyo Lengai and its surroundings using data from the network of permanent Global Navigation Satellite System (GNSS) sites monitoring the volcano (the TZVOLCANO network) around the flanks of the volcano and Interferometric Synthetic Aperture Radar (InSAR) observations. We constrain surface motions using 6 GNSS sites distributed around Ol Doinyo Lengai, operating between 2016 and 2021, and InSAR data covering nearly the same time period. Because of the complex local tectonics, the interpretation of the deformation pattern is not straightforward. We first invert the GNSS deformation and InSAR observations independently to infer potential deformation sources. Then we perform a joint inversion of both GNSS and InSAR datasets to verify our findings. We compare the results from the joint inversion with the results from inverting each dataset independently. The GNSS, InSAR, and joint inversion results point to a deflating source, located east of Ol Doinyo Lengai and southwest of the dormant volcano Gelai at a depth of 3.49 ± 0.03 km (GNSS inversion), 5.2 ± 1.2 km (InSAR inversion) and 3.49 ± 0.06 km (joint inversion) relative to the summit (vent) and with a volume change ∆V of −0.04 ± 0.05 × 10 6 m 3 (GNSS inversion), −0.39 ± 0.29 × 10 6 m 3 (InSAR inversion), and − 0.04 ± 0.01 × 10 6 m 3 (joint inversion). Although this is non-unique modeling of geodetic datasets with small signals, the inversion results suggest that Ol Doinyo Lengai could be fed by an offset multi-reservoir system that includes a shallow magma reservoir (<5 km) east of Ol Doinyo Lengai, possibly connected to a deeper magma reservoir.

Natron Rift, Ol Doinyo Lengai↗

USGS2012

The geology and distribution of impact craters on Venus: What are they telling us?

Magellan has revealed an ensemble of impact craters on Venus that is unique in many important ways. We have compiled a data base describing the 842 craters on 89% of Venus' surface mapped through orbit 2578. (The craters range in diameter from 1.5 to 280 km.) We have studied the distribution, size‐density, morphology, geology, and associated surface properties of these craters both in the aggregate and, for some craters, in greater detail. We find that (1) the spatial distribution of craters is highly uniform; (2) the size‐density distribution of large craters (diameters ≥35 km) is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0.5 Ga (based on the estimated population of Venus‐crossing asteroids); (3) unlike the case on other planets, the density of small craters (diameters ≤35 km) declines rapidly with decreasing diameters because of atmospheric filtering; (4) the spectrum of crater modification differs greatly from that on other planets: 62% of all craters are pristine, only 4% are embayed by lavas, and the remainder are affected by tectonism, but none are severely and progressively depleted (as extrapolated from the size‐density distribution of larger craters); (5) large craters have a progression of morphologies generally similar to those on other planets, but small craters are typically irregular or multiple rather than bowl shaped; (6) diffuse radar‐bright or ‐dark features surround some craters, and 367 similar diffuse “splotches” with no central crater are observed; and (7) other crater features unique to Venus include radar‐bright or ‐dark parabolic arcs opening westward and extensive outflows originating in crater ejecta. The first three of these observations are entirely unexpected. We interpret them as indicating that the planet's cratering record was erased by a global resurfacing event or events, the latest ending about 0.5 Ga, after which volcanic activity declined (but did not cease entirely). Since the last resurfacing event, a maximum of 10% of the planet has been resurfaced and only about 4% of the craters have been obliterated. Convective thermal evolution models support this interpretation (Arkani‐Hamed and Toksoz, 1984). Observations 3–7 confirm quantitatively the expectation that the dense atmosphere of Venus has strongly affected the production of craters. Large impactors have been relatively unaffected, intermediate‐sized ones have been fragmented and have produced overlapping or multiple craters, a narrow size range has produced shock‐induced “splotches” but no craters, and the smallest bodies have had no observable effect on the surface. The number of craters eliminated by the “atmospheric filter” is enormous, about 98% of the craters between 2 and 35 km in diameter that Magellan might have observed on a hypothetical airless Venus. Unique crater‐related features such as parabolas and outflow deposits demonstrate the roles of Venus' high atmospheric density and temperature in modifying the crater formation process. Finally, heavily fractured craters and lava‐embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting Aphrodite Terra and Atla, Beta, Themis, and Phoebe regiones. These craters thus provide physical evidence for recent volcanic and tectonic activity at a low level.

Journal of Geophysical Research E: Planets↗

USGS2015

What lies beneath: geophysical mapping of a concealed Precambrian intrusive complex along the Iowa–Minnesota border

Large-amplitude gravity and magnetic highs over northeast Iowa are interpreted to reflect a buried intrusive complex composed of mafic–ultramafic rocks, the northeast Iowa intrusive complex (NEIIC), intruding Yavapai province (1.8–1.72 Ga) rocks. The age of the complex is unproven, although it has been considered to be Keweenawan (∼1.1 Ga). Because only four boreholes reach the complex, which is covered by 200–700 m of Paleozoic sedimentary rocks, geophysical methods are critical to developing a better understanding of the nature and mineral resource potential of the NEIIC. Lithologic and cross-cutting relations interpreted from high-resolution aeromagnetic and airborne gravity gradient data are presented in the form of a preliminary geologic map of the basement Precambrian rocks. Numerous magnetic anomalies are coincident with airborne gravity gradient (AGG) highs, indicating widespread strongly magnetized and dense rocks of likely mafic–ultramafic composition. A Yavapai-age metagabbro unit is interpreted to be part of a layered intrusion with subvertical dip. Another presumed Yavapai unit has low density and weak magnetization, observations consistent with felsic plutons. Northeast-trending, linear magnetic lows are interpreted to reflect reversely magnetized diabase dikes and have properties consistent with Keweenawan rocks. The interpreted dikes are cut in places by normally magnetized mafic–ultramafic rocks, suggesting that the latter represent younger Keweenawan rocks. Distinctive horseshoe-shaped magnetic and AGG highs correspond with a known gabbro, and surround rocks with weaker magnetization and lower density. Here, informally called the Decorah complex, the source body has notable geophysical similarities to Keweenawan alkaline ring complexes, such as the Coldwell and Killala Lake complexes, and Mesoproterozoic anorogenic complexes, such as the Kiglapait, Hettasch, and Voisey’s Bay intrusions in Labrador. Results presented here suggest that much of the NEIIC is composed of such complexes, and broadly speaking, may be a discontinuous group of several intrusive bodies. Most units are cut by suspected northwest-trending faults imaged as magnetic lineaments, and one produces apparent sinistral fault separation of a dike in the eastern part of the survey area. The location, trend, and apparent sinistral sense of motion are consistent with the suspected faults being part of the Belle Plaine fault zone, a complex transform fault zone within the Midcontinent rift system that is here proposed to correspond with a major structural discontinuity.

Iowa, Minnesota↗

USGS2006

Geographical patterns of human diet derived from stable-isotope analysis of fingernails

Carbon and nitrogen isotope ratios of human fingernails were measured in 490 individuals in the western US and 273 individuals in southeastern Brazil living in urban areas, and 53 individuals living in a moderately isolated area in the central Amazon region of Brazil and consuming mostly locally grown foods. In addition, we measured the carbon and nitrogen isotope ratios of common food items to assess the extent to which these isotopic signatures remain distinct for people eating both omnivorous and vegetarian diets and living in different parts of the world, and the extent to which dietary information can be interpreted from these analyses. Fingernail ??13C values (mean ?? standard deviation) were -15.4 ?? 1.0 and -18.8 ?? 0.8??? and ??15N values were 10.4 ?? 0.7 and 9.4 ?? 0.6??? for southeastern Brazil and western US populations, respectively. Despite opportunities for a "global supermarket" effect to swamp out carbon and nitrogen isotope ratios in these two urbanized regions of the world, differences in the fingernail isotope ratios between southeastern Brazil and western US populations persisted, and appeared to be more associated with regional agricultural and animal production practices. Omnivores and vegetarians from Brazil and the US were isotopically distinct, both within and between regions. In a comparison of fingernails of individuals from an urban city and isolated communities in the Amazonian region, the urban region was similar to southeastern Brazil, whereas individuals from isolated nonurban communities showed distinctive isotopic values consistent with their diets and with the isotopic values of local foods. Although there is a tendency for a "global supermarket" diet, carbon and nitrogen isotopes of human fingernails hold dietary information directly related to both food sources and dietary practices in a region. ?? 2006 Wiley-Liss, Inc.

American Journal of Physical Anthropology↗