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Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

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At least 127 records · Page 7Linked to original sources
USGS2020

Dust deposited on snow cover in the San Juan Mountains, Colorado, 2011-2016: Compositional variability bearing on snow-melt effects

Light-absorbing particles in atmospheric dust deposited on snow cover (dust-on-snow, DOS) diminish albedo and accelerate the timing and rate of snow melt. Identification of these particles and their effects are relevant to snow-radiation modeling and thus water-resource management. Laboratory-measured reflectance of DOS samples from the San Juan Mountains (USA) were compared with DOS mass loading, particle sizes, iron mineralogy, carbonaceous matter type and content, and chemical compositions. Samples were collected each spring for water years 2011-2016, when individual dust layers had merged into one (all layers merged) at the snow surface. Average reflectance values of the six samples were 0.2153 (sd, 0.0331) across the visible wavelength region (0.4-0.7 µm) and 0.3570 (sd, 0.0498) over the full-measurement range (0.4-2.50 µm). Reflectance values correlated inversely to concentrations of ferric oxide, organic carbon (1.4-10 wt. %), magnetite (0.05-0.13 wt. %), and silt (PM63-3.9; median grain sizes averaged 21.4 µm) but lacked correspondence to total iron and PM10 contents. Measurements of reflectance and Mössbauer spectra and magnetic properties indicated that microcrystalline hematite and nano-size goethite were primarily responsible for diminished visible reflectance. Positive correlations between organic carbon and metals attributed to fossil-fuel combustion, with observations from electron microscopy, indicated that some carbonaceous matter occurred as black carbon. Magnetite was a surrogate for related light-absorbing minerals, dark rock particles, and contaminants. Similar analyses of DOS from other areas would help evaluate the influences of varied dust sources, wind-storm patterns, and anthropogenic inputs on snow melt and water resources in and beyond the Colorado River basin.

Colorado↗
USGS1971

Remanent magnetization and susceptibility of late Cenozoic rocks from New Zealand

Measurements of magnetic remanence and susceptibility were made on 176 samples of volcanic rocks from 22 late Cenozoic formations from the North Island. Intensities of remanence, susceptibilities, and Q ratios are highly variable in both the ignimbrites and andesites and their ranges overlap, indicating that the magnetic anomalies produced by ignimbrite bodies would be difficult to distinguish from those produced by andesite flows. An analysis of the directions of magnetization of the ignimbrites indicates that groups of petrologically distinct ignimbrites probably formed during the same volcanic episode.

North Island↗
USGS1980

Evaporite encrustations and sulphide oxidation products from the southern Antarctic peninsula.

White evaporite encrustations collected from 66 sites scattered over 30 000km2 of S.Antarctic Peninsula are dominantly gypsum and calcite, and subordinate aragonite and thenardite. Natrojarosite occurs as locally developed yellow surface stains on c30% of the white encrustations examined. Water, which forms by the melting of snow blown into cracks in highly fractured rocks by ground blizzards, dissolves soluble ions from minerals in these rocks as it percolates into them. The white encrustations appear to be deposited when this water later rises to the surface by capillary action and evaporates.-from Author Antarctica chemical weathering evaporites sulphide oxidation.

New Zealand Journal of Geology and Geophysics↗
USGS1980

Fission-track age of the Mangaroa ash and tectonic implications at Wellington, New Zealand.

Three samples of zircon from the Mangaroa Ash, an important marker bed in the late Pleistocene deposits of the Wellington area, have been dated by fission-track analysis. The average age of the 3 zircon samples is 380 000 years. Since this tephra fell, vertical uplift at Tinakori Hill on the northwest side of the Wellington Fault has been about 266 m, at a rate of about 69 cm per 1000 years; at the Whakatiki Terraces the minimum rate of uplift on the northwest side of this fault has been about 21 cm per 1000 years. -Authors

New Zealand Journal of Geology and Geophysics↗
USGS1988

A paleomagnetic investigation of rocks from the Ohio Range and the Dry Valleys, Transantarctic Mountains, Antarctica

Two well-defined virtual geomagnetic poles (VGPs) for East Antarctica were obtained from the Middle Jurassic Ferrar Dolerite, sampled from a thick sill on Mt Schopf in the Ohio Range, and from a horizontal sheet intruding Paleozoic granitic rocks at Mt Cerberus in the Dry Valleys. The VGP from the sill at Mt Schopf lies at lat. 58°.0S, long. 129°.0W (dm=13°, dp=12°), and the VGP from Mt Cerberus lies at lat. 57°.8S, long. 135°.7W (dm=6°, dp=6°). Rocks from both localities have normal polarity magnetisations. These data, together with 15 other acceptable pole positions reported in the literature, define a Middle Jurassic paleomagnetic pole at 52°.7S, 139°.6W (α 95 =4°.4), which is not significantly different from previously reported mean Middle Jurassic paleomagnetic poles from East Antarctica. Paleomagnetic investigations in the Ohio Range of mafic inclusions in Ordovician granitic and sedimentary rocks of Devonian and Permian ages found that the rocks are magnetically unstable and are unreliable for determining a paleomagnetic pole.

New Zealand Journal of Geology and Geophysics↗
USGS1991

K‐Ar ages from the Western Dome Belt and associated rhyolitic lavas in the Maroa‐Taupo area, Taupo Volcanic Zone, New Zealand

The Western Dome Belt is a 32 km long belt of rhyolitic lava domes west of Maroa caldera. New K‐Ar age data suggest that the belt was a significant locus of extrusive volcanism between the voluminous pyroclastic Whakamaru eruption(s), and the onset of caldera formation at Maroa, during which time it marked the western boundary of rhyolitic volcanism in Taupo Volcanic Zone. A smaller, younger cluster of lava flows and domes to the south at Ben Lomond station forms part of a transitional region between the youthful Maroa and Taupo calderas.

Maroa, Taupo↗
USGS1991

Zoned garnets in metapelites in western Fiordland, southwest New Zealand: polychronic crystallisation and insight into the nature and extent of early Cretaceous regional metamorphism

Two distinct types of compositional zoning are recognised in idioblastic to subidioblastic garnets in amphibolite facies metapelites in western Fiordland. One type occurs in kyanite (?? sillimanite)-bearing rocks and is characterised by a core to rim increase in CaO content, and a sympathetic decrease in FeO and MgO, with MnO remaining essentially constant, or decreasing slightly near the rim. This zoning pattern formed during garnet growth, and records adjustment of mineral equilibria under conditions of increasing load pressure. A much less common type of zoned garnet occurs in sillimanite + K-feldspar-bearing metapelites and is characterised by a core to rim decrease in CaO and MgO contents and a sympathetic increase in FeO and MnO near the rim. This zoning pattern is interpreted to result from post-growth volume diffusion, and records metamorphism at significantly lower pressures. -from Author

New Zealand Journal of Geology and Geophysics↗
USGS1991

The Briggs Creek Amphibolite, Klamath Mountains, Oregon: Its origin and dispersal

The Briggs Creek Amphibolite (BCA) in southwestern Oregon is an allochthonous slice of metamorphosed oceanic crust emplaced during the Nevadan orogenic event. Even though the evidence is not clear, the authors prefer to have the protolith for the BCA formed in a back‐arc basin contemporaneously as part of the Josephine Ophiolite during late Callovian to early Oxfordian. Although major rearrangement of some of the chemical elements took place during metamorphism, it is possible, by use of major and rare earth elements, to suggest that the BCA formed in an oceanic spreading centre rather than an island arc setting. Meta‐morphic mineral assemblages and composition of the hornblendes indicate that metamorphism attained amphibolite facies grade. Widespread occurrence of allochthonous blocks of BCA in Western Oregon indicate that its dispersion following metamorphism can be related to the accretionary history of the continental margin. Westward thrusting of these blocks of BCA over the Chetco Arc during the Nevadan orogenic event provided blocks that slid into the outboard active Early Cretaceous trench. These blocks were then incorporated into the downgoing subduction complex and overprinted by high P/T metamorphic assemblages. Tectonic blocks within the Early Cretaceous Otter Point trench melange (Franciscan Complex equivalent) contain amphibolites correlated with the BCA and overprinted by high P/T mineral assemblages.

Oregon↗
USGS1991

Mid-Paleozoic age of granitoids in enclaves within early Cretaceous granulites, Fiordland, southwest New Zealand

Orthogneisses of granite, quartz monzonite, monzonite, and tonalite, occur locally as isolated enclaves within the Early Cretaceous granulite terrain (Western Fiordland Orthogneiss - WFO). Discordant U-Pb zircon isotopic data (seven fractions) from four granitoid samples from enclaves at George Sound, define an upper intecept age of 341??34 Ma that is interpreted as approximating the time of formation of the granitoid suite. The lower intercept age of 93??37 Ma is interpreted as approximating the time of zircon isotopic disturbance by major episodic Pb loss. The low 87Sr/ 86Sr initial ratio indicates that these mid-Paleozoic granitoids were derived from an isotopically primitive source. The granitoid enclaves within WFO show influences of several different sources. The granitoids provide evidence linking WFO to a mid-Palaeozoic country rock similar to the central Fiordland metasediments. -from Authors

New Zealand Journal of Geology and Geophysics↗
USGS2004

Timing of the most recent surface rupture event on the Ohariu Fault near Paraparaumu, New Zealand

Thirteen radiocarbon ages from three trenches across the Ohariu Fault tightly constrain the timing of the most recent surface rupture event at Muaupoko Stream valley, c. 2 km east of Paraparaumu, to between 930 and 1050 cal. yr BP. This age overlaps with previously published ages of the most recent event on the Ohariu Fault and together they further constrain the event to 1000-1050 cal. yr BP. Two trenches provide loose constraints on the maximum recurrence interval at 3-7000 yr. Tephra, most probably the Kawakawa Tephra, was found within alluvial fan deposits in two of the trenches. ?? The Royal Society of New Zealand 2004.

New Zealand Journal of Geology and Geophysics↗
USGS2005

Accurate elevation and normal moveout corrections of seismic reflection data on rugged topography

The application of the seismic reflection method is often limited in areas of complex terrain. The problem is the incorrect correction of time shifts caused by topography. To apply normal moveout (NMO) correction to reflection data correctly, static corrections are necessary to be applied in advance for the compensation of the time distortions of topography and the time delays from near-surface weathered layers. For environment and engineering investigation, weathered layers are our targets, so that the static correction mainly serves the adjustment of time shifts due to an undulating surface. In practice, seismic reflected raypaths are assumed to be almost vertical through the near-surface layers because they have much lower velocities than layers below. This assumption is acceptable in most cases since it results in little residual error for small elevation changes and small offsets in reflection events. Although static algorithms based on choosing a floating datum related to common midpoint gathers or residual surface-consistent functions are available and effective, errors caused by the assumption of vertical raypaths often generate pseudo-indications of structures. This paper presents the comparison of applying corrections based on the vertical raypaths and bias (non-vertical) raypaths. It also provides an approach of combining elevation and NMO corrections. The advantages of the approach are demonstrated by synthetic and real-world examples of multi-coverage seismic reflection surveys on rough topography. ?? The Royal Society of New Zealand 2005.

New Zealand Journal of Geology and Geophysics↗
USGS2006

Timing of late Holocene surface rupture of the Wairau Fault, Marlborough, New Zealand

Three trenches excavated across the central portion of the right-lateral strike-slip Wairau Fault in South Island, New Zealand, exposed a complex set of fault strands that have displaced a sequence of late Holocene alluvial and colluvial deposits. Abundant charcoal fragments provide age control for various stratigraphic horizons dating back to c. 5610 yr ago. Faulting relations from the Wadsworth trench show that the most recent surface rupture event occurred at least 1290 yr and at most 2740 yr ago. Drowned trees in landslide-dammed Lake Chalice, in combination with charcoal from the base of an unfaulted colluvial wedge at Wadsworth trench, suggest a narrower time bracket for this event of 1811-2301 cal. yr BP. The penultimate faulting event occurred between c. 2370 and 3380 yr, and possibly near 2680 ?? 60 cal. yr BP, when data from both the Wadsworth and Dillon trenches are combined. Two older events have been recognised from Dillon trench but remain poorly dated. A probable elapsed time of at least 1811 yr since the last surface rupture, and an average slip rate estimate for the Wairau Fault of 3-5 mm/yr, suggests that at least 5.4 m and up to 11.5 m of elastic shear strain has accumulated since the last rupture. This is near to or greater than the single-event displacement estimates of 5-7 m. The average recurrence interval for surface rupture of the fault determined from the trench data is 1150-1400 yr. Although the uncertainties in the timing of faulting events and variability in inter-event times remain high, the time elapsed since the last event is in the order of 1-2 times the average recurrence interval, implying that the Wairau Fault is near the end of its interseismic period. ?? The Royal Society of New Zealand 2006.

New Zealand Journal of Geology and Geophysics↗
USGS2016

Refined depositional history and dating of the Tongaporutuan reference section, north Taranaki, New Zealand: new volcanic ash U-Pb zircon ages, biostratigraphy and sedimentation rates

This study presents new radiometric ages from volcanic ash beds within a c. 1900 m thick, progradational, deep-water clastic slope succession of late Miocene age exposed along the north Taranaki coast of the North Island, New Zealand. The ash beds yield U–Pb zircon ages ranging from 10.63 ± 0.65 Ma to 8.97 ± 0.22 Ma. The new ages are compatible with and provide corroboration of New Zealand Tongaporutuan Stage planktic foraminiferal and bolboformid biostratigraphic events identified in the same section. The close accord between these two age datasets provides a stratigraphically consistent and coherent basis for examining margin evolution. The arrival of a prograding clastic wedge and ensuing upward shoaling is recorded by sedimentation rates c. 2000 m/Ma–1 that are an order of magnitude higher than sedimentation rates on the precursor deep basin floor. This outcrop study provides new constraints for interpreting analogous subsurface deposits in Taranaki Basin and complements the regional late Miocene biostratigraphic dating framework.

Taranaki↗
USGS2019

Ar-Ar age constraints on the timing of Havre Trough opening and magmatism

The age and style of opening of the Havre Trough back-arc system is uncertain due to a lack of geochronologic constraints for the region. 40 Ar/ 39 Ar dating of 19 volcanic rocks from across the southern Havre Trough and Kermadec Arc was conducted in three laboratories to provide age constraints on the system. The results are integrated and interpreted as suggesting that this subduction system is young (<2 Ma) and coeval with opening of the continental Taupo Volcanic Zone of New Zealand. Arc magmatism was broadly concurrent across the breadth of the Havre Trough.

Kermadec Arc–Havre Trough↗
USGS2021

Sr, Nd, Hf and Pb isotope geochemistry of Early Miocene shoshonitic lavas from the South Fiji Basin: Note

We present new Sr, Nd, Hf and Pb isotope data for Early Miocene shoshonitic and high-K lavas dredged from seamounts in the South Fiji Basin, southwest Pacific Ocean. Our analyses provide a useful reference dataset for this distinctive compositional suite. The shoshonitic lavas formed in an intra-oceanic back-arc basin setting and are broadly coeval with nearby arc and back-arc basin lavas. Further work in the region will be needed before the tectonic implications of the shoshonitic and high-K lavas are fully understood.

New Zealand Journal of Geology and Geophysics↗
USGS2025

The communication of volcano information in New Zealand - A narrative review

Communication of volcano information is critical for effective volcanic risk management. A variety of information is communicated to inform decisions and guide actions for planning, preparedness, and response. Such information needs to be reliable, and fit-for-purpose across different stages of volcanic activity (quiescence, unrest, short or long-term eruptive stages, and the post-eruptive stage). However, an understanding of communication across these different stages of volcanic activity remains limited. We undertook a narrative review of New Zealand literature to explore what information is communicated about volcanoes, across which stages of activity and by whom. Results highlight that NZ literature only documents certain aspects of volcano information and communication, specifically regarding certain locations, stages of volcanic activity (i.e. quiescence or unrest), or hazards. Literature gaps exist regarding volcano communication during unrest and post-eruptive stages, as well as how volcano information evolves between these phases, and how decision-makers use such information. Additional work would be useful to document existing examples of volcano information for different stages of activity. Further research could help in understanding the information needs of decision-makers during each of these stages to improve information and communication.

New Zealand Journal of Geology and Geophysics↗