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At least 1,513 records · Page 84Linked to original sources

USGS1980

Variability of currents and sediment transport on continental shelves: Optical and current meter studies of the bottom boundary layer

The Continental Margin Sediment Dynamics program of the U.S. Geological Survey has been using a variety of optical instruments to monitor water turbidity as part of our studies of sediment transport. Although we suspect that optical devices will eventually be supplanted by more direct measures of suspended sediments, results of several field experiments indicate that for now transmissometers and nephelometers offer the best solution to the problem of long term environmental monitoring in the ocean. The Continental Margin Sediment Dynamics (CMSD) program of the U.S. Geological Survey is structured to investigate those oceanographic and geological processes which influence and control the active transport of sediments and other materials over continental margins. The focus of this program is on active sedimentary processes, the mechanisms which create them, and the effects they produce (see Figure 1 for a diagrammatic depiction). Basically, we design our experiments to investigate (1) transport mechanisms of oceanic sediments and other materials as suspended load and bed load, and (2) relationships of erosion and deposition to the dynamical characteristics of oceanic bottom boundary layers. The purpose of this report is to describe briefly those aspects of equipment and experiments that relate to our study of suspended particulate matter and water turbidity. Much of our understanding of the spatial distribution and temporal variability of suspended sediments depends upon measurements from optical instruments. In this report we describe those instruments and give examples of their use and some results of two field experiments.

Proceedings of SPIE↗

USGS1964

Composition of basalts from the Mid-Atlantic Ridge

Studies of volcanic rocks in dredge hauls from the submerged parts of the Mid-Atlantic Ridge suggest that it consists largely of tholeiitic basalt with low values of K, Ti, and P. In contrast, the volcanic islands which form the elevated caps on the Ridge are built of alkali basalt with high values of Ti, Fe 3+ , P, Na, and K. This distinct correlation between the form of the volcanic structures, elevation above the sea floor, and composition suggests that the islands of alkali basalt are derived from a parent tholeiitic magma by differentiation in shallow reservoirs. The volume of low-potassium tholeiites along the Mid-Atlantic Ridge and elsewhere in the oceans appears to be many times that of the alkali basalts exposed on oceanic islands. Tholeiitic basalts with about 0.2 K 2 O appear to be the primary and predominant magma erupted on the oceanic floor.

Science↗

USGS1986

Marine cobalt resources

Ferromanganese oxides in the open oceans are more enriched in cobalt than any other widely distributed sediments or rocks. Concentrations of cobalt exceed 1 percent in ferromanganese crusts on seamounts, ocean ridges, and other raised areas of the ocean. The cobalt-rich crusts may be the slowest growing of any earth material, accumulating one molecular layer every 1 to 3 months. Attention has been drawn to crusts as potential resources because they contain cobalt, manganese, and platinum, three of the four priority strategic metals for the United States. Moreover, unlike abyssal nodules, whose recovery is complicated by their dominant location in international waters, some of the most cobalt-rich crusts occur within the exclusive economic zone of the United States and other nations. Environmental impact statements for crust exploitation are under current development by the Department of the Interior.

Science↗

USGS1965

Investigation of initial Sr87/Sr86 ratios in the Sierra Nevada Plutonic Province

One to three whole-rock samples from each of more than a dozen discrete plutonic intrusions in the east-central Sierra Nevada batholith have been analyzed for Sr 87 /Sr 86 and Rb/Sr ratios to obtain information on initial Sr 87 abundances. The initial Sr 87 /Sr 86 ratios in the rock magmas forming this province appear to have been in the range 0.7073 ± .0010 in the majority of cases. This range is definitely higher than that found for modern alkali-type and tholeiite-type basalt magmas of oceanic regions, which commonly range between 0.703 and 0.705. However, it is much lower than the average Sr 87 /Sr 86 ratios found in Precambrian sialic regions which range from 0.71 to 0.73. It seems clear therefore that the Sierra Nevada magmas were not derived solely either from the typical source regions of oceanic basalt or from the melting of ancient crustal sial. It is possible that these magmas represent a mixture of oceanic basalt and crustal sial, as would be the case of anatexis in a geosyncline containing much volcanic material of fairly recent origin and some terrigenous sialic detritus. They may instead be of mantle derivation with admixtures of crustal material assimilated during their rise. The whole-rock Rb-Sr age results derivec from the study indicate that the Lamarck and Mount Givens Granodiorites and the alaskite of Evolution Basin and porphyritic biotite granite of Dinkey Lakes form a younger group of intrusive rocks of 90 ± 10 m.y. Although the sampling was not designed for isochron age studies, it appears that most of the remaining rock units are considerably older.

GSA Bulletin↗

USGS1971

Petrologic and geophysical nature of serpentinites

Mineralogically, serpentinites consist predominantly of lizardite, clinochrysotile, and antigorite. Recent work has shown that these minerals are not polymorphs. Chrysotile is the only mineral recognized as a synthetic product in experimental studies of the system MgO-SiO 2 -H 2 O. Antigorite seems to be stable at higher temperatures than lizardite or chrysotile. The density of individual serpentine species is dependent on their morphology; the low-density serpentinites (<2.55g/cc) consist predominantly of clino-chrysotile. Seismic velocities and magnetic susceptibilities of serpentinites are related to the degree of serpentinization. The transition of massive serpentinites from ductile to brittle behavior in laboratory experiments at high confining pressures and temperatures above 300°C has been related to dehydration which may provide a mechanism for developing deep-focus earthquakes along Benioff zones. Serpentinite is formed by direct hydration of ultramafic protolith in the crust. The most common ultramafic protoliths are harzburgite, dunite, and Iherzolite. The assemblage generally developed from these is lizardite + chrysotile + brucite + magnetite. In areas of high-grade metamorphism, antigorite is the predominant serpentine mineral. The common, large, alpine-type serpentinized ultramafic masses contain brucite and have MgO/SiO 2 ratios similar to those of their protolith, resulting in volume increase during serpentinization. Metamorphic serpentinites and some highly sheared alpine-type serpentinites have lower MgO/SiO 2 ratios than their protolith, lack brucite, and appear t o have been formed by volume-for-volume replacement with concomitant loss of magnesium or addition of silica. Many large, young masses of peridotite appear to be slabs of oceanic mantle over-thrust onto continental edges. Subsequent sedimentation, serpentinization, and tectonism have greatly modified these original slabs so that their recognition in older orogenic zones is equivocal. The concept of the tectonic evolution of ultramafic rocks from oceanic crust-mantle slabs invading continental margins and being incrementally serpentinized and moved by later tectonic events provides a working hypothesis that allows a better explanation of the many peculiar and varied occurrences of serpentinite. The evidence does not support Hess' suggestion that the third layer of the oceanic crust consists of partly serpentinized mantle peridotite.

Bulletin of the Geological Society of America↗

USGS1973

Variations in lead-isotopic compositions in Mesozoic granitic rocks of California: A preliminary investigation

Six alkali feldspar and two whole-rock samples of granitic rocks from the Sierra Nevada batholith and adjacent Klamath Mountains were analyzed for their lead-isotope compositions. The samples represented each of three 87 Sr/ 86 Sr groupings (< 0.704, 0.704 to 0.706, and > 0.706) for granitic rocks north of the Garlock fault in California. The isotopic compositions of lead in the samples from the Sierra Nevada batholith range from 18.73 to 19.37 for 206 Pb/ 204 Pb, 15.61 to 15.71 for 207 Pb/ 204 Pb, and 38.44 to 39.10 for 208 Pb/ 204 Pb. A crude parallel correspondence was found between lead and strontium isotopes, in that the specimens with the most radiogenic strontium also tend to have the most radiogenic lead similar to the previously studied Boulder batholith of Montana. A parallel correspondence is thought to imply characteristics of the source rocks for the plutons rather than consequences of partial melting or natural contamination. Lead-isotopic compositions for the Sierra Nevada batholith and the Boulder batholith differ, average values of 206 Pb/ 204 Pb being at least 18.8 for the Sierra Nevada batholith and about 18 for the Boulder batholith. In the Late Cretaceous part of the Sierra Nevada batholith, the secondary isochron “age” for the lead data in these rocks is about 2,900 m.y., far older than known Precambrian in California. Sources are proposed for these plutons from the lower continental crust and upper continental mantle or dominantly recycled continental materials, probably of intermediate composition and possibly carried down to the zone of melting by subduction. This source material may have been formed in Pre-cambrian times but did not undergo a Precambrian metamorphism greater than upper amphibolite facies which would have reduced the values of 238 U/ 204 Pb in the source rocks and resulted in Mesozoic leads like those found in the Boulder batholith and elsewhere in the Rocky Mountain region. A trondhjemite from the Klamath Mountains has a lead-isotope composition ( 206 Pb/ 204 Pb, 18.57; 207 Pb/ 204 Pb, 15.50; 208 Pb/ 204 Pb, 38.08) similar to that of oceanic volcanic rocks, particularly like those of island volcanics on oceanic ridges. Derivation of this trondhjemite from an oceanic mantle or recycled mantle material is indicated by this observation and supports the conclusion of Kistler and Peterman (1973) based on its alkali abundances and 87 Sr/ 86 Sr value.

California↗

USGS1975

The vourinos ophiolite, Greece: Cyclic units of lineated cumulates overlying harzburgite tectonite

Re-examination of the Vourinos ophiolite shows it to be composed of metamorphic tectonites, cumulates, plagiogranites, dikes, and lava. The contact between the tectonites and the cumulates is exposed and sharp. Beneath the cumulate contact, the rocks have been highly deformed and complexly folded; above that contact, they simply have been tilted vertically to expose a stratiform complex 1,500 m (4,800 ft) thick. The stratiform intrusion is characterized by cyclic units, rich in olivine at the base, and rich in feldspar at the top. Some cumulus diorites are present at the top of the section which grade into quartz diorites (plagiogranites) with hypautomorphic textures. Lineate lamination characterizes the cumulates and may indicate the direction or orientation of the Mesozoic mid-oceanic ridge crest with respect to the present position of the complex. A siliceous dike swarm cuts the upper part of the stratiform complex. The section suggests that in the case of Vourinos, a large magmatic chamber formed at a mid-oceanic ridge crest and that intrusion was a much more important process than extrusion in the formation of oceanic crust in that area. The reported presence of cumulates in many other ophiolite complexes suggests that these relations may obtain generally at most or all spreading ridges. The contact between the tectonites and the cumulates of the complex would not have corresponded with seismic M. © 1975 Geological Society of America.

Geological Society of America Bulletin↗

USGS1988

Limestone and chert in tectonic blocks from the Esk Head subterrane, South Island, New Zealand

The Esk Head subterrane is a continuous belt, generally 10-20 km wide, of tectonic mélange and broken formation on the South Island of New Zealand. This subterrane separates older and younger parts of the Torlesse terrane which is an extensive accretionary prism composed mostly of quartzo-feldspathic, submarine-fan deposits ranging from Permian to Early Cretaceous in age. The Torlesse is the most Pacific-ward of several Permian and Mesozoic accreted terranes in New Zealand that record tectonic amalgamation and ultimate accretion against the Pacific-facing Gondwana margin. The Esk Head subterrane of the Torlesse is especially informative because it includes within it conspicuous tectonic blocks of submarine basalt and a variety of basalt-associated seamount and sea-floor limestones and cherty rocks thought to be representative of the subducted plate. Limestones in tectonic blocks are of Late Triassic and probably Jurassic ages and include (1) submarine-cemented, pelagic-bivalve, geopetal packstone-grainstone; (2) brachiopod-bryozoan encrinite; and (3) radiolarian, pelagic lime mudstone. Most of the Triassic blocks have been dated using conodonts which have remarkably low color alteration index (CAI) values (<1.5). An incomplete sampling of cherts in tectonic blocks and from Holocene gravels derived from the Esk Head subterrane yields radiolarian-based ages of Late Triassic, Early Jurassic, Middle Jurassic, and Late Jurassic. Paleogeographic inferences drawn from megafossils, bioclasts, and radiolarians, as well as from carbonate cements, indicate deposition of the oceanic sedimentary rocks at paleolatitudes somewhat lower than that of the New Zealand part of the Gondwana margin, but higher than paleoequatorial latitudes. These oceanic sediments and their basaltic substrates were evidently emplaced in the Torlesse accretionary prism following off-scraping from an extensive subducting oceanic plate, probably the Phoenix plate, which was obliquely convergent with the northwest-trending Gondwana margin during Late Jurassic and/or Early Cretaceous time.

Geological Society of America Bulletin↗

USGS1994

Origin, transport, and emplacement of an exotic island-arc terrane exposed in eastern Kamchatka, Russia

The regional stratigraphy of eastern Kamchatka includes an exotic, Early-Late Cretaceous ophiolite and Late Cretaceous island-arc volcanic sequence. Integrating the existing geologic and geophysical data, we examine the origin, transport, emplacement, and postemplacement deformation of the island-arc terrane, which is named the Olyutorsky island arc. Results from several paleomagnetic studies consistently indicate that the island-arc terrane originated >1000 km to the south of where it is presently exposed. Although the formative paleolatitudes of the island-arc rocks approximately correspond to the location of the Izanagi-Farallon subduction zone, the age of the volcanic rocks postdates the cessation of Izanagi-Farallon convergence, thus indicating that an unnamed plate or back-arc basin existed in the northwest Pacific during Late Cretaceous time. We examine two possible models for northward transport of the island-arc terrane to Kamchatka: (1) infra-oceanic transport with the Pacific or Kula plates and (2) coastwise translation of the island-arc terrane after accretion to the Eurasian margin far to the south of Kamchatka. For both models, the dominant Eocene and Miocene deformation ages observed in eastern Kamchatka are used as two possible age limits for the cessation of northward transport. Although the observed paleolatitudes from paleomagnetic data correspond best with the infra-oceanic transport model, the provenance of the Paleogene "transport" stratigraphy indicates a near-shore sediment supply. Our preferred interpretation is that the island-arc terrane (1) accreted onto the Eurasian margin concurrent with cessation of island-arc volcanism (Maastrichtian-Danian) and (2) underwent northward coastwise translation along a major strike-slip fault zone ending by middle-late Eocene time (43-50 Ma). It is unclear whether the ophiolite was exposed during arc-continent collision or whether the ophiolite was obducted onto the island arc prior to collision. A consequence of either infra-oceanic transport or coastwise translation is that an open corridor between the western terminus of the Aleutian Arc and Kamchatka must have existed until middle to late Eocene time. Spreading within the Komandorsky Basin, subduction of sea-mounts, and collision of the Aleutian Arc with Kamchatka are proposed to have instigated the second Miocene phase of deformation, which uplifted and reexposed the island-arc terrane.

Kamchatka Peninsula↗

USGS1987

Arabian Shield ophiolites and Late Proterozoic microplate accretion

Fragments of Late Proterozoic ocean crust and mantle (ophiolites) occur within six major fault zones that mark sutures between crustal blocks (microplates) that were accreted between about 630 and 715 Ma to form the Arabian Shield. We report new U-Pb zircon ages for ophiolitic gabbro, diorite, and plagiogranite that range from 840 to 700 Ma and establish these complexes among the oldest proven ophiolites. By dating the ophiolitic rocks we are able to monitor the magmatic ages of sea-floor spreading events during accretion of the Arabian Shield. Comparison of the ophiolitic dates with the ages of the adjacent crustal blocks provides a more complete basis for plate-tectonics reconstruction of the shield than has previously been possible. Our new zircon ages confirm earlier Sm-Nd mineral isochrons for ophiolites of the northwestern Arabian Shield, show that the ophiolites are among the oldest oceanic rocks in each terrane, support near- or within-arc tectonic settings for the ophiolites, suggest that the Bir Umq suture extends north along the Nabitah mobile belt into the northern shield, and suggest that older (>1250 Ma) continental crustal material is locally present in the dominantly “oceanic” western shield. We suggest that Indonesia and Alaska provide Phanerozoic analogues for stages in the accretion history.

Geology↗

USGS1990

Early Cretaceous shelf-edge deltas of the Baltimore Canyon Trough: principal sources for sediment gravity deposits of the northern Hatteras Basin

We present evidence that the principal sources for Early Cretaceous (Berriasian-Valanginian) gravity-flow deposits of the northern Hatteras Basin were three large shelf-edge deltas located along the outer margin of the Baltimore Canyon Trough, &sim; 100 km southeast of Cape Charles, Virginia, Ocean City, Maryland, and Long Branch, New Jersey. Sedimentary detritus from the central Appalachian highlands and the Maryland-Virginia coastal plain was transported across the Early Cretaceous continental shelf to form the Cape Charles and Ocean City deltas, whereas deposits of the Long Branch delta came chiefly from the Adirondack and New England highlands. Each delta supplied sediment gravity flows to large slope aprons and submarine-fan complexes on the Early Cretaceous continental slope and rise. The most conspicuous distributary of sediment on the Early Cretaceous continental rise extends 500 km basinward from the Ocean City delta, where its distal deposits were cored at Deep Sea Drilling Project Site 603.

Hatteras Basin↗

USGS2015

Composite Sunrise Butte pluton: Insights into Jurassic–Cretaceous collisional tectonics and magmatism in the Blue Mountains Province, northeastern Oregon

The composite Sunrise Butte pluton , in the central part of the Blue Mountains Province, northeastern Oregon, preserves a record of subduction-related magmatism, arc-arc collision, crustal thickening, and deep-crustal anatexis. The earliest phase of the pluton (Desolation Creek unit) was generated in a subduction zone environment, as the oceanic lithosphere between the Wallowa and Olds Ferry island arcs was consumed. Zircons from this unit yielded a 206 Pb/ 238 U age of 160.2 ± 2.1 Ma. A magmatic lull ensued during arc-arc collision, after which partial melting at the base of the thickened Wallowa arc crust produced siliceous magma that was emplaced into metasedimentary rocks and serpentinite of the overthrust forearc complex. This magma crystallized to form the bulk of the Sunrise Butte composite pluton (the Sunrise Butte unit; 145.8 ± 2.2 Ma). The heat necessary for crustal anatexis was supplied by coeval mantle-derived magma (the Onion Gulch unit; 147.9 ± 1.8 Ma). The lull in magmatic activity between 160 and 148 Ma encompasses the timing of arc-arc collision (159-154 Ma), and it is similar to those lulls observed in adjacent areas of the Blue Mountains Province related to the same shortening event. Previous researchers have proposed a tectonic link between the Blue Mountains Province and the Klamath Mountains and northern Sierra Nevada Provinces farther to the south; however, timing of Late Jurassic deformation in the Blue Mountains Province predates the timing of the so-called Nevadan orogeny in the Klamath Mountains. In both the Blue Mountains Province and Klamath Mountains, the onset of deep-crustal partial melting initiated at ca. 148 Ma, suggesting a possible geodynamic link. One possibility is that the Late Jurassic shortening event recorded in the Blue Mountains Province may be a northerly extension of the Nevadan orogeny. Differences in the timing of these events in the Blue Mountains Province and the Klamath-Sierra Nevada Provinces suggest that shortening and deformation were diachronous, progressing from north to south. We envision that Late Jurassic deformation may have collapsed a Gulf of California-style oceanic extensional basin that extended from the Klamath Mountains (e.g., Josephine ophiolite) to the central Blue Mountains Province, and possibly as far north as the North Cascades (i.e., the coeval Ingalls ophiolite). The lull in magmatic activity between 160 and 148 Ma encompasses the timing of arc-arc collision (159–154 Ma), and it is similar to those lulls observed in adjacent areas of the Blue Mountains Province related to the same shortening event. Previous researchers have proposed a tectonic link between the Blue Mountains Province and the Klamath Mountains and northern Sierra Nevada Provinces farther to the south; however, timing of Late Jurassic deformation in the Blue Mountains Province predates the timing of the so-called Nevadan orogeny in the Klamath Mountains. In both the Blue Mountains Province and Klamath Mountains, the onset of deep-crustal partial melting initiated at ca. 148 Ma, suggesting a possible geodynamic link. One possibility is that the Late Jurassic shortening event recorded in the Blue Mountains Province may be a northerly extension of the Nevadan orogeny. Differences in the timing of these events in the Blue Mountains Province and the Klamath–Sierra Nevada Provinces suggest that shortening and deformation were diachronous, progressing from north to south. We envision that Late Jurassic deformation may have collapsed a Gulf of California–style oceanic extensional basin that extended from the Klamath Mountains (e.g., Josephine ophiolite) to the central Blue Mountains Province, and possibly as far north as the North Cascades (i.e., the coeval Ingalls ophiolite).

Idaho, Oregon↗

USGS2021

Late Cenozoic paleogeographic reconstruction of the San Francisco Bay Area from analysis of stratigraphy, tectonics, and tephrochronology

The Neogene stratigraphic and tectonic history of the Mount Diablo area is a consequence of the passage of the Mendocino Triple Junction (MTJ) by the San Francisco Bay area between 12 and 6 Ma, volcanism above a slab-window trailing the MTJ, and crustal transpression beginning ~8-6 Ma, when the Pacific Plate and Sierra Nevada microplate began to converge obliquely. Between ~12-6 Ma, parts of the Sierra Nevada microplate were displaced by faults splaying from the main trace of the San Andreas Fault and incorporated into the Pacific Plate. The Mount Diablo anticlinorium was formed by crustal compression within a left-stepping, restraining bend of the eastern San Andreas Fault system (SAF), with southwest-verging thrusting beneath, and with possible clockwise rotation between faults on its southeast and northwest. At ~10,5 Ma, a drainage divide formed between the northern Great Central Valley (GCV) and the ocean. Regional uplift accelerated at ~6 Ma with onset of transpression between the Pacific and North American plates. Marine deposition ceased in the eastern Coast Range basins as a consequence of the regional uplift accompanying passage of the MTJ, and trailing slab-window volcanism. From ~11 to ~5 Ma, andesitic volcanic intrusive rocks and lavas were erupted along the northwest crest of the central to northern Sierra Nevada and were deposited on its western slope, providing abundant sediment to northern Great Central Valley (GCV) and the northeastern Coast Ranges. Sediment filled the GCV, overtopped the Stockton fault and arch forming one large, south-draining system that flowed into a marine embayment at its southwestern end, the ancestral San Joaquin Sea. This marine embayment shrunk with time and by ~2.3 Ma was eventually cut off from the ocean. Fluvial drainage continued southwest in GCV until it was cut off in turn, probably by some combination of sea level fluctuations and transpression along the SAF that uplifted, lengthened and narrowed the outlet channel. As a consequence, a great lake, Lake Clyde, formed in the GCV at ~1.4 Ma, occupying all of the ancestral San Joaquin Valley and part of ancestral Sacramento Valley. The lake rose and fell with global glacial and interglacial cycles. After a long, extreme glacial period, Marine Oxygen Isotope Stage (MOIS) 16, it overtopped Carquinez sill at 0.63 Ma and drained via San Francisco valley (now Bay) and the Colma gap, into the Merced marine embayment of the Pacific Ocean. Later, a new outlet for GCV drainage formed between ~75 and ~130 ka ago., when the Colma gap closed due to transpression and right-slip on the SAF, and Duxbury Point at the south end of Pt. Reyes Peninsula moved sufficiently northwest along the SAF to unblock a bedrock notch, the feature we now call the Golden Gate.

California↗

USGS2009

Extraterrestrial demise of banded iron formations 1.85 billion years ago

In the Lake Superior region of North America, deposition of most banded iron formations (BIFs) ended abruptly 1.85 Ga ago, coincident with the oceanic impact of the giant Sudbury extraterrestrial bolide. We propose a new model in which this impact produced global mixing of shallow oxic and deep anoxic waters of the Paleoproterozoic ocean, creating a suboxic redox state for deep seawater. This suboxic state, characterized by only small concentrations of dissolved O 2 (???1 ??M), prevented transport of hydrothermally derived Fe(II) from the deep ocean to continental-margin settings, ending an ???1.1 billion-year-long period of episodic BIF mineralization. The model is supported by the nature of Precambrian deep-water exhalative chemical sediments, which changed from predominantly sulfide facies prior to ca. 1.85 Ga to mainly oxide facies thereafter. ?? 2009 Geological Society of America.

Geology↗

USGS2024

Mantle melting in regions of thick continental lithosphere: Examples from Late Cretaceous and younger volcanic rocks, Southern Rocky Mountains, Colorado (USA)

Major- and trace-element data together with Nd and Sr isotopic compositions and 40 Ar/ 39 Ar age determinations were obtained for Late Cretaceous and younger volcanic rocks from north-central Colorado, USA, in the Southern Rocky Mountains to assess the sources of mantle-derived melts in a region underlain by thick (≥150 km) continental lithosphere. Trachybasalt to trachyandesite lava flows and volcanic cobbles of the Upper Cretaceous Windy Gap Volcanic Member of the Middle Park Formation have low εNd(t) values from −3.4 to −13, 87 Sr/ 86 Sr(t) from ~0.705 to ~0.707, high large ion lithophile element/high field strength element ratios, and low Ta/Th (≤0.2) values. These characteristics are consistent with the production of mafic melts during the Late Cretaceous to early Cenozoic Laramide orogeny through flux melting of asthenosphere above shallowly subducting and dehydrating oceanic lithosphere of the Farallon plate, followed by the interaction of these melts with preexisting, low εNd(t), continental lithospheric mantle during ascent. This scenario requires that asthenospheric melting occurred beneath continental lithosphere as thick as 200 km, in accordance with mantle xenoliths entrained in localized Devonian-age kimberlites. Such depths are consistent with the abundances of heavy rare earth elements (Yb, Sc) in the Laramide volcanic rocks, which require parental melts derived from garnet-bearing mantle source rocks. New 40 Ar/ 39 Ar ages from the Rabbit Ears and Elkhead Mountains volcanic fields confirm that mafic magmatism was reestablished in this region ca. 28 Ma after a hiatus of over 30 m.y. and that the locus of volcanism migrated to the west through time. These rocks have εNd(t) and 87 Sr/ 86 Sr(t) values equivalent to their older counterparts (−3.5 to −13 and 0.7038–0.7060, respectively), but they have higher average chondrite-normalized La/Yb values (~22 vs. ~10), and, for the Rabbit Ears volcanic field, higher and more variable Ta/Th values (0.29–0.43). The latter are general characteristics of all other post– 40 Ma volcanic rocks in north-central Colorado for which literature data are available. Transitions from low to intermediate Ta/Th mafic volcanism occurred diachronously across southwest North America and are interpreted to have been a consequence of melting of continental lithospheric mantle previously metasomatized by aqueous fluids derived from the underthrusted Farallon plate. Melting occurred as remnants of the Farallon plate were removed and the continental lithospheric mantle was conductively heated by upwelling asthenosphere. A similar model can be applied to post–40 Ma magmatism in north-central Colorado, with periodic, east to west, removal of stranded remnants of the Farallon plate from the base of the continental lithospheric mantle accounting for the production, and western migration, of volcanism. The estimated depth of the lithosphere-asthenosphere boundary in north-central Colorado (~150 km) indicates that the lithosphere remains too thick to allow widespread melting of upwelling asthenosphere even after lithospheric thinning in the Cenozoic. The preservation of thick continental lithospheric mantle may account for the absence of oceanic-island basalt–like basaltic volcanism (high Ta/Th values of ~1 and εNd[t] > 0), in contrast to areas of southwest North America that experienced larger-magnitude extension and lithosphere thinning, where oceanic-island basalt–like late Cenozoic basalts are common.

Geosphere↗

USGS2015

Initiation of migration and movement rates of Atlantic salmon smolts in fresh water

Timing of ocean entry is critical for marine survival of both hatchery and wild Atlantic salmon ( Salmo salar ) smolts. Management practices and barriers to migration such as dams may constrain timing of smolt migrations resulting in suboptimal performance at saltwater entry. We modeled influences of stocking location, smolt development, and environmental conditions on ( i ) initiation of migration by hatchery-reared smolts and ( ii ) movement rate of hatchery- and wild-reared Atlantic salmon smolts in the Penobscot River, Maine, USA, from 2005 through 2014 using acoustic telemetry data. We also compared movement rates in free-flowing reaches with rates in reaches with hydropower dams and head ponds. We compared movement rates before and after (1) removal of two mainstem dams and (2) construction of new powerhouses. Initiation of movement by hatchery fish was influenced by smolt development, stocking location, and environmental conditions. Smolts with the greatest gill Na + , K + -ATPase (NKA) activity initiated migration 24 h sooner than fish with the lowest gill NKA activity. Fish with the greatest cumulative thermal experience initiated migration 5 days earlier than those with lowest cumulative thermal experience. Smolts released furthest from the ocean initiated migration earlier than those released downstream, but movement rate increased by fivefold closer to the ocean, indicating behavioral trade-offs between initiation and movement rate. Dams had a strong effect on movement rate. Movement rate increased from 2.8 to 5.4 km·h −1 in reaches where dams were removed, but decreased from 2.1 to 0.1 km·h −1 in reaches where new powerhouses were constructed. Movement rate varied throughout the migratory period and was inversely related to temperature. Fish moved slower at extreme high or low discharge. Responses in fish movement rates to dam removal indicate the potential scope of recovery for these activities.

Maine↗

USGS2021

Reduced recruitment of Chinook salmon in a leveed bar-built estuary

Estuaries are commonly touted as nurseries for salmonids, providing numerous advantages for smolts prior to ocean entry. In bar-built estuaries, sandbars form at the mouth of rivers during periods of low stream flow, closing access to the ocean and preventing outmigration. We evaluated how summer residency in a leveed bar-built estuary affects the growth, survival, and recruitment of a Chinook salmon ( Oncorhynchus tshawytscha ) population. We performed a mark–recapture study on outmigrants to determine juvenile estuary abundance, growth, and survival. We used returning adult scales and otoliths to determine the relative proportion of summer estuary residents in spawning adults. Juveniles in the estuary grew less after mouth closure, and ultimately summer estuary residents had lower smolt-to-adult survival and contributed disproportionately less to the spawning population than juveniles that reared in the ocean their first summer. Mouth closure may lower food availability and deteriorate estuary conditions by reducing marine prey influx and estuary circulation. This research demonstrates the complexity of estuary dynamics and function as salmonid nurseries, particularly when considering the extensive modification of estuaries.

California↗

USGS1983

Distribution of mineral deposits in accreted terranes and cratonal rocks of western United States

The western margin of the conterminous United States, covering roughly 300 000 mi 2 (777 000 km 2 ), is an agglomeration of tectonostratigraphic terranes accreted to the North American craton mainly during Mesozoic time. The terranes represent a number of fundamental crustal types: oceanic crust, island-arc crust, mélange, various combinations of the preceding three, batholithic, miogeoclinal, and platform.The distribution patterns of types of mineral deposits show that miogeoclinal terranes of the craton are characterized by replacement and vein-type lead–zinc–silver, skarn tungsten deposits, molybdenum, and tin, whereas accreted terranes contain all the known volcanogenic massive sulfide deposits, all chromite and chert-associated manganese, and all the large gold quartz-vein deposits, except Goldfield, Nevada. Carlin-type disseminated fine-grained gold deposits occur mostly in windows of Paleozoic miogeoclinal rocks in Nevada, but the only known fine-grained gold deposit in California is in very youthful volcanic rocks overlying oceanic-crust terrane. Large bedded-type barite deposits, although in the same general area and showing the same general trend as fine disseminated gold deposits in Nevada, are in allochthonous oceanic terrane. Mercury and antimony are dominantly in accreted terranes but antimony, in particular, also forms important deposits in cratonal rocks. Most of the large iron deposits are in the craton but a few are in accreted island-arc terranes.

California, Idaho, Nevada, Oregon, Utah, Washingto↗