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At least 1,261 records · Page 70Linked to original sources

USGS2017

Abundant carbon in the mantle beneath Hawai`i

Estimates of carbon concentrations in Earth ’ s mantle vary over more than an order of magnitude, hindering our ability to understand mantle structure and mineralogy, partial melting, and the carbon cycle. CO 2 concentrations in mantle-derived magmas supplying hotspot ocean island volcanoes yield our most direct constraints on mantle carbon, but are extensively modified by degassing during ascent. Here we show that undegassed magmatic and mantle carbon concentrations may be estimated in a Bayesian framework using diverse geologic information at an ocean island volcano. Our CO 2 concentration estimates do not rely upon complex degassing models, geochemical tracer elements, assumed magma supply rates, or rare undegassed rock samples. Rather, we couple volcanic CO 2 emission rates with probabilistic magma supply rates, which are obtained indirectly from magma storage and eruption rates. We estimate that the CO 2 content of mantle-derived magma supplying Hawai‘i ’ s active volcanoes is 0.97 −0.19 +0.25 wt%—roughly 40% higher than previously believed—and is supplied from a mantle source region with a carbon concentration of 263 −62 +81 ppm. Our results suggest that mantle plumes and ocean island basalts are carbon-rich. Our data also shed light on helium isotope abundances, CO 2 /Nb ratios, and may imply higher CO 2 emission rates from ocean island volcanoes.

Hawaii↗

USGS2017

Enhanced Arctic amplification began at the Mid-Brunhes Event 430,000 years ago

Arctic Ocean temperatures influence ecosystems, sea ice, species diversity, biogeochemical cycling, seafloor methane stability, deep-sea circulation, and CO 2 cycling. Today's Arctic Ocean and surrounding regions are undergoing climatic changes often attributed to "Arctic amplification" - that is, amplified warming in Arctic regions due to sea-ice loss and other processes, relative to global mean temperature. However, the long-term evolution of Arctic amplification is poorly constrained due to lack of continuous sediment proxy records of Arctic Ocean temperature, sea ice cover and circulation. Here we present reconstructions of Arctic Ocean intermediate depth water (AIW) temperatures and sea-ice cover spanning the last ~ 1.5 million years (Ma) of orbitally-paced glacial/interglacial cycles (GIC). Using Mg/Ca paleothermometry of the ostracode Krithe and sea-ice planktic and benthic indicator species, we suggest that the Mid-Brunhes Event (MBE), a major climate transition ~ 400-350 ka, involved fundamental changes in AIW temperature and sea-ice variability. Enhanced Arctic amplification at the MBE suggests a major climate threshold was reached at ~ 400 ka involving Atlantic Meridional Overturning Circulation (AMOC), inflowing warm Atlantic Layer water, ice sheet, sea-ice and ice-shelf feedbacks, and sensitivity to higher post-MBE interglacial CO 2 concentrations.

Scientific Reports↗

USGS2020

Aseismic transient slip on the Gofar transform fault, East Pacific Rise

Oceanic transform faults display a unique combination of seismic and aseismic slip behavior, including a large globally averaged seismic deficit, and the local occurrence of repeating magnitude (M) ∼ 6 "> ∼ 6 ∼6 earthquakes with abundant foreshocks and seismic swarms, as on the Gofar transform of the East Pacific Rise and the Blanco Ridge in the northeast Pacific Ocean. However, the underlying mechanisms that govern the partitioning between seismic and aseismic slip and their interaction remain unclear. Here we present a numerical modeling study of earthquake sequences and aseismic transient slip on oceanic transform faults. In the model, strong dilatancy strengthening, supported by seismic imaging that indicates enhanced fluid-filled porosity and possible hydrothermal circulation down to the brittle–ductile transition, effectively stabilizes along-strike seismic rupture propagation and results in rupture barriers where aseismic transients arise episodically. The modeled slow slip migrates along the barrier zones at speeds ∼10 to 600 m/h, spatiotemporally correlated with the observed migration of seismic swarms on the Gofar transform. Our model thus suggests the possible prevalence of episodic aseismic transients in M ∼ 6 "> ∼ 6 ∼6 rupture barrier zones that host active swarms on oceanic transform faults and provides candidates for future seafloor geodesy experiments to verify the relation between aseismic fault slip, earthquake swarms, and fault zone hydromechanical properties.

Proceedings of the National Academy of Sciences↗

USGS2010

Synthesis of crustal seismic structure and implications for the concept of a slab gap beneath Coastal California

Compilation of seismic transects across the central and northern California Coast Ranges provides evidence for the widespread tectonic emplacement beneath the margin of a slab of partially subducted oceanic lithosphere. The oceanic crust of this lithosphere can be traced landward from the former convergent margin (fossil trench), beneath the Coast Ranges, to at least as far east as the Coast Range/Great Valley boundary. Comparison of measured shear and compressional wave velocities in the middle crust beneath the Hayward fault with laboratory measurements suggests that the middle crust is a diabase (oceanic crust). Both of these observations are consistent with recent models of the high heat flow and age progression of Neogene volcanism along the Coast Ranges based on tectonic emplacement (stalling) of young, hot oceanic lithosphere beneath the margin, but appear to contradict the major predictions of the slab-gap or asthenospheric-window model. Finally, the Neogene volcanism and major strike-slip faults in the Coast Ranges occur within the thickest regions (>14 km thick) of the forearc, suggesting that the locations of Cenozoic volcanism and faulting along the margin are structurally controlled by the forearc thickness rather than being determined by the location of a broad slab gap.

International Geology Review↗

USGS2015

Marine foraging ecology influences mercury bioaccumulation in deep-diving northern elephant seals

Mercury contamination of oceans is prevalent worldwide and methylmercury concentrations in the mesopelagic zone (200–1000 m) are increasing more rapidly than in surface waters. Yet mercury bioaccumulation in mesopelagic predators has been understudied. Northern elephant seals (Mirounga angustirostris) biannually travel thousands of kilometres to forage within coastal and open-ocean regions of the northeast Pacific Ocean. We coupled satellite telemetry, diving behaviour and stable isotopes (carbon and nitrogen) from 77 adult females, and showed that variability among individuals in foraging location, diving depth and δ 13 C values were correlated with mercury concentrations in blood and muscle. We identified three clusters of foraging strategies, and these resulted in substantially different mercury concentrations: (i) deeper-diving and offshore-foraging seals had the greatest mercury concentrations, (ii) shallower-diving and offshore-foraging seals had intermediate levels, and (iii) coastal and more northerly foraging seals had the lowest mercury concentrations. Additionally, mercury concentrations were lower at the end of the seven-month-long foraging trip (n = 31) than after the two-month- long post-breeding trip (n = 46). Our results indicate that foraging behaviour influences mercury exposure and mesopelagic predators foraging in the northeast Pacific Ocean may be at high risk for mercury bioaccumulation.

California↗

USGS1987

Remote sensing of the Fram Strait marginal ice zone

Sequential remote sensing images of the Fram Strait marginal ice zone played a key role in elucidating the complex interactions of the atmosphere, ocean, and sea ice. Analysis of a subset of these images covering a 1-week period provided quantitative data on the mesoscale ice morphology, including ice edge positions, ice concentrations, floe size distribution, and ice kinematics. The analysis showed that, under light to moderate wind conditions, the morphology of the marginal ice zone reflects the underlying ocean circulation. High-resolution radar observations showed the location and size of ocean eddies near the ice edge. Ice kinematics from sequential radar images revealed an ocean eddy beneath the interior pack ice that was verified by in situ oceanographic measurements.

Science↗

USGS2003

Controls on intrusion of near-trench magmas of the Sanak-Baranof Belt, Alaska, during Paleogene ridge subduction, and consequences for forearc evolution

A belt of Paleogene near-trench plutons known as the Sanak-Baranof belt intruded the southern Alaska convergent margin. A compilation of isotopic ages of these plutons shows that they range in age from 61 Ma in the west to ca. 50 Ma in the east. This migrating pulse of magmatism along the continental margin is consistent with North Pacific plate reconstructions that suggests the plutons were generated by migration of a trench-ridge-trench triple junction along the margin. On the Kenai Peninsula the regional lower greenschist metamorphic grade of the turbiditic host rocks, texture of the plutons, contact-metamorphic assemblage, and isotopic and fluid inclusion studies suggest that the plutons were emplaced at pressures of 1.5–3.0 kbars (5.2–10.5 km) into a part of the accretionary wedge with an ambient temperature of 210–300 °C. The presence of kyanite, garnet, and cordierite megacrysts in the plutons indicates that the melts were generated at a depth greater than 20 km and minimum temperature of 650 °C. These megacrysts are probably xenocrystic remnants of a restitic or contact metamorphic phase entrained by the melt during intrusion. However, it is also possible that they are primary magmatic phases crystallized from the peraluminous melt. Plutons of the Sanak-Baranof belt serve as time and strain markers separating kinematic regimes that predate and postdate ridge subduction. Pre-ridge subduction structures are interpreted to be related to the interaction between the leading oceanic plate and the Chugach terrane. These include regional thrust faults, NE-striking map-scale folds with associated axial planar foliation, type-1 mélanges, and an arrayof faults within the contact aureole indicating shortening largely accommodated by layer-parallel extension. Syn-ridge subduction features include the plutons, dikes, and ductile shear zones within contact aureoles with syn-kinematic metamorphic mineral growth and foliation development. Many of the studied plutons have sheeted margins and appear to have intruded along extensional jogs in margin-parallel strike-slip faults, whereas others form significant angles with the main faults and may have been influenced by minor faults of other orientations. Some of the plutons of the Sanak-Baranof belt have their long axes oriented parallel to faults of an orthorhombic fault set, implying that these faults may have provided a conduit for magma emplacement. This orthorhombic set of late faults is interpreted to have initially formed during the ridge subduction event, and continued to be active for a short time after passage of the triple junction. ENE-striking dextral faults of this orthorhombic fault system exhibit mutually crosscutting relationships with Eocene dikes related to ridge subduction, and mineralized strike-slip and normal faults of this system have yielded 40Ar/39Ar ages identical to near-trench intrusives related to ridge subduction. Movement on the orthorhombic fault system accommodated exhumation of deeper levels of the southern Alaska accretionary wedge, which is interpreted as a critical taper adjustment to subduction of younger oceanic lithosphere during ridge subduction. These faults therefore accommodate both deformation of the wedge and assisted emplacement of near-trench plutons. Structures that crosscut the plutons and aureoles include the orthorhombic fault set and dextral strike-slip faults, reflecting a new kinematic regime established after ridge subduction, during underthrusting of the trailing oceanic plate with new dextral-oblique convergence vectors with the overriding plate. The observation that the orthorhombic fault set both cuts and is cut by Eocene intrusives demonstrates the importance of these faults for magma emplacement in the forearc. A younger, ca. 35 Ma suite of plutons intrudes the Chugach terrane in the Prince William Sound region, and their intrusion geometry was strongly influenced by pre-existing faults developed during ridge subduction. The generation of these plutons may be related to the sudden northward migration of the triple junction at ca. 40–33 Ma, as the ridge was being subducted nearly parallel to the trench during this interval. These younger plutons are used to provide additional constraints on the structural evolution of the wedge. Late- to post-ridge subduction fabrics include a pressure solution cleavage and additional movement on the orthorhombic fault system. After triple junction migration, subduction of the trailing oceanic plate involved a significant component of dextral transpression and northward translation of the Chugach terrane. This change in kinematics is recorded by very late gouge-filled dextral faults in the late structures of the accretionary prism.

Alaska↗

USGS1969

The structure and tectonic history of the eastern Aleutian Trench

The tectonic character of the eastern Aleutian Trench and some major events in its geologic history can be estimated from nine continuous seismic reflection records. A section of pre-trench, deep oceanic sediments rests on the down-warped crust that forms the trench. Nearly horizontal undeformed strata that unconformably overlie this deep oceanic section partially fill the trench. The trench fill is thickest near present sediment sources. A Pliocene age for development of the eastern Aleutian Trench is estimated from the thickness of deep oceanic sediment that accumulated after the trench began to fill. The eastern Aleutian Trench thus appears younger than the central Aleutian Trench—a relation which helps to explain the distribution of sediment along the two trench segments. Depression of the eastern Aleutian Trench diminished or virtually ended soon after the trench began to fill. The undeformed fill provides no evidence for a large thrust fault zone at the base of the continental slope. Nor is there any evidence that oceanic sediments have disappeared beneath the continents in late Tertiary time. These observations are difficult to reconcile with the simple model of a continental margin advanced in the hypothesis of plate tectonics.

Alaska↗

USGS1970

Quartz gabbro and anorthositic gabbro: Markers of offset along the San Andreas fault in the California Coast Ranges

Large-scale lateral movement on the San Andreas fault zone is suggested by the distribution of gabbroic rocks that may be slivered remnants of oceanic crust. Distinctive and unusual hornblende quartz gabbro and anorthositic gabbro that are virtually identical both petrographically and chemically are exposed at Logan and Gold Hill in the Coast Ranges along the San Andreas fault. The hornblende quartz gabbro is made up of labradorite to bytownite plagioclase, hornblende, and quartz, with very minor biotite and pyroxene, and accessory metallic opaques and apatite. The coarse-grained anorthositic gabbro with anorthositic layers and associated gneiss is made up mainly of labradorite to bytownite plagioclase, hornblende, lesser clinopyroxene, and, locally, orthopyroxene. The present 100 mi of separation between these two gabbro bodies is probably due to displacement along the San Andreas fault of what was originally one gabbro mass. Somewhat similar quartz gabbro and anorthositic gabbro associated with ultramafic rocks near Eagle Rest Peak in the San Emigdio Mountains may represent a source for the Logan and Gold Hill slivers. This suggests a minimum right-lateral movement of about 200 mi on the San Andreas fault zone. Anorthositic gabbro clasts from Cretaceous conglomerate in the Gualala area have strong resemblance to gabbroic rocks at Logan, Gold Hill, and Eagle Rest Peak and speculatively suggest 350 mi of right-lateral movement on the fault zone. It is tentatively suggested that the gabbro clasts of Gualala may have been shed from a large area of exposed gabbroic oceanic crust, pieces of which occur as fault slivers at Gold Hill and Logan. It is further suggested that the Eagle Rest Peak locality may be a relatively in-place exposure of this gabbroic oceanic crust that lies near a continental-oceanic interface. This interface, thought to represent a fossil subduction zone between Franciscan and Sierran basements, appears to be overlain depositionally by Eocene sedimentary rocks. If these Eocene deposits do “seal over” the possible subduction zone, it suggests that movement on such a zone ceased before the Eocene deposition, and that the new and different pattern of right-lateral movement on the San Andreas fault zone probably began sometime later.

California↗

USGS1983

Paleoceanographic implications of Miocene deep-sea hiatuses

Miocene paleoceanographic evolution exhibits major changes resulting from the opening and closing of passages, the subsequent changes in oceanic circulation, and development of major Antarctic glaciation. The consequences and timing of these events can be observed in variations in the distribution of deep-sea hiatuses, sedimentation patterns, and biogeographic distribution of planktic organisms. The opening of the Drake Passage in the latest Oligocene to early Miocene (25–20 Ma) resulted in the establishment of the deep circumpolar current, which led to thermal isolation of Antarctica and increased global cooling. This development was associated with a major turnover in planktic organisms, resulting in the evolution of Neogene assemblages and the eventual extinction of Paleogene assemblages. The erosive patterns of two widespread hiatuses (PH, 23.0–22.5 Ma; and NH 1, 20–18 Ma) indicate that a deep circumequatorial circulation existed at this time, characterized by a broad band of carbonate-ooze deposition. Siliceous sedimentation was restricted to the North Atlantic and a narrow band around Antarctica. A major reorganization in deep-sea sedimentation and hiatus distribution patterns occurred near the early/middle Miocene boundary, apparently resulting from changes in oceanic circulation. Beginning at this time, deep-sea erosion occurred throughout the Caribbean (hiatus NH 2, 16–15 Ma), suggesting disruption of the deep circumequatorial circulation and northward deflection of deep currents, and/or intensification of the Gulf Stream. Sediment distribution patterns changed dramatically with the sudden appearance of siliceous-ooze deposition in the marginal and east equatorial North Pacific by 16.0 to 15.5 Ma, coincident with the decline of siliceous sedimentation in the North Atlantic. This silica switch may have been caused by the introduction of Norwegian Overflow Water into the North Atlantic acting as a barrier to outcropping of silica-rich Antarctic Bottom Water. The main aspects of the present oceanic circulation system and sediment distribution pattern were established by 13.5 to 12.5 Ma (hiatus NH 3), coincident with the establishment of a major East Antarctic ice cap. Antarctic glaciation resulted in a broadening belt of siliceous-ooze deposition around Antarctica, increased siliceous sedimentation in the marginal and east equatorial North Pacific and Indian Oceans, and further northward restriction of siliceous sediments in the North Atlantic. Periodic cool climatic events were accompanied by lower eustatic sea levels and widespread deep-sea erosion at 12 to 11 Ma (NH 4), 10 to 9 Ma (NH 5), 7.5 to 6.2 Ma (NH 6), and 5.2 to 4.7 Ma (NH 7).

Geological Society of America Bulletin↗

USGS1995

Chemically diverse, sporadic volcanism at seamounts offshore southern and Baja California

Compositions of lavas from seven small to medium-sized seamounts, between lat 34.0°N and 30.5°N offshore southern and Baja California, include low-K 2 O tholeiitic, transitional, and mildly to moderately alkalic basalt and their differentiates. The low-K 2 O tholeiites resemble primitive (>9% MgO) mid-oceanic-ridge basalt (MORB) with low incompatible element abundances and very depleted, concave-downward, chondrite-normalized rare-earth-element (REE) patterns and lower 87 Sr/ 86 Sr and higher 143 Nd/ 144 Nd ratios than typical MORB from the East Pacific Rise. The seamounts with these MORB-like lavas are inferred to have formed at or near the spreading center. Transitional and mildly to moderately alkalic basalts have higher abundances of incompatible elements and steeper slopes for chondrite-normalized REE patterns with light REE enrichment up to 150 times chondrites. The alkalic compositions indicate more variably enriched mantle sources than those of most seamounts presently located near the East Pacific Rise, but the compositions are within the mantle array defined by other ocean-island basalts. Volcanic rocks from the upper part of Rocas Alijos, a much larger and morphologically more complex edifice than the northern seamounts, located offshore central Baja California at lat ∼25°N, are all highly differentiated trachyte and trachyandesite. Based on 40 Ar/ 39 Ar laser fusion techniques, MORB-like lava from one of the northern edifices is as old as the underlying oceanic crust (>20 Ma), indicating that it originated at a spreading center. Other seamount lava ages are much younger than the oceanic crust on which they reside, ranging from 16.8 ± 0.3 to <7 Ma for some of the northern seamounts to 270 ± 16 ka for the trachyte from Rocas Alijos. Similar highly evolved lavas cap fossil spreading centers like Guadalupe and Socorro Islands, but Rocas Alijos, based on magnetic anomalies, is not an abandoned spreading center but may instead have formed on a leaky transform fault. Some of the seamounts with transitional and alkalic lavas may have formed as part of a short, age-progressive chain formed by a short-lived mantle plume. Many others, aligned along abandoned spreading centers or faults and fracture zones which are abundant in the tectonically complex region offshore southern and peninsular California, may have resulted from upwelling mantle diapirs in response to localized extension. Some of the episodes of volcanism appear to have been contemporaneous with volcanism in the continental borderland and coastal southern California, suggesting linkage between extension along the continental margin and the seamount province farther offshore. The data available for the abundant volcanic edifices of varying sizes, shapes, and orientations in this region suggest that the seamounts formed from multiple episodes of chemically diverse volcanism, tapping variably enriched, heterogeneous mantle, which occurred sporadically from early Miocene to late Pleistocene.

Baja California↗

USGS1997

Ophiolitic basement to the Great Valley forearc basin, California, from seismic and gravity data: Implications for crustal growth at the North American continental margin

The nature of the Great Valley basement, whether oceanic or continental, has long been a source of controversy. A velocity model (derived from a 200-km-long east-west reflection-refraction profile collected south of the Mendocino triple junction, northern California, in 1993), further constrained by density and magnetic models, reveals an ophiolite underlying the Great Valley (Great Valley ophiolite), which in turn is underlain by a westward extension of lower-density continental crust (Sierran affinity material). We used an integrated modeling philosophy, first modeling the seismic-refraction data to obtain a final velocity model, and then modeling the long-wavelength features of the gravity data to obtain a final density model that is constrained in the upper crust by our velocity model. The crustal section of Great Valley ophiolite is 7-8 km thick, and the Great Valley ophiolite relict oceanic Moho is at 11-16 km depth. The Great Valley ophiolite does not extend west beneath the Coast Ranges, but only as far as the western margin of the Great Valley, where the 5-7-km-thick Great Valley ophiolite mantle section dips west into the present-day mantle. There are 16-18 km of lower-density Sierran affinity material beneath the Great Valley ophiolite mantle section, such that a second, deeper, "present-day" continental Moho is at about 34 km depth. At mid-crustal depths, the boundary between the eastern extent of the Great Valley ophiolite and the western extent of Sierran affinity material is a near-vertical velocity and density discontinuity about 80 km east of the western margin of the Great Valley. Our model has important implications for crustal growth at the North American continental margin. We suggest that a thick ophiolite sequence was obducted onto continental material, probably during the Jurassic Nevadan orogeny, so that the Great Valley basement is oceanic crust above oceanic mantle vertically stacked above continental crust and continental mantle.

California↗

USGS2014

Geochemical and Nd-Sr-Pb isotopic evolution of metabasites from rifting of continental lithosphere, Seward Peninsula, Alaska, and implications for paleogeographic reconstruction

The chemical character of mafic rocks from the Arctic Alaska–Chukotka terrane records rifting of continental crust during the early Paleozoic, possibly during the Ordovician. The mafic rocks are part of a metamorphosed Neoproterozoic to Devonian continental margin sequence preserved in a Mesozoic metamorphic terrane, the Nome Complex, of Seward Peninsula, Alaska. Protoliths of the mafic rocks include basalt and mafic clastic rocks, which were interlayered with calcareous, pelitic, and feldspathic sediments, and gabbro and diabase, likely feeder dikes and sills to the basalt. Major-element, trace-element, and rare-earth element (REE) analyses of these mafic rocks, together with analyses of Nd, Pb, and Sr isotopes, form two compositional groups. The two groups differ in Nb/Y (one plots as basalt, the other as alkali to subalkali basalt), TiO 2 , P 2 O 5 , and Nb (and other elements). The high-Ti group is characterized by enrichment of light REE; the low-Ti group lacks such enrichment. The trace-element and isotopic characteristics of the two groups resemble typical non-arc magmas derived from the mantle: the low-Ti group has compositions between normal mid-ocean ridge basalt (N-MORB) and enriched mid-ocean ridge basalt (E-MORB), while those of the high-Ti group are between E-MORB and ocean-island basalt (OIB). The two groups have overlapping positive values of ε Nd (+0.34 to +7.40). TiO 2 /Yb ratios suggest the high-Ti group formed from melts generated under normal thickness of continental crust, while the low-Ti group formed from melts generated at shallower conditions, presumably after rift-related crustal thinning had progressed. Geologic, paleontologic, and geochronologic characteristics of the Nome Complex support an origin along the NE margin of Baltica. The rift-related magmatism in the Nome Complex likely occurred during the opening of the Uralian ocean along that margin; by implication, related parts of the Arctic Alaska–Chukotka terrane may have experienced a similar origin.

Alaska↗

USGS2008

Trans-Alaska Crustal Transect and continental evolution involving subduction underplating and synchronous foreland thrusting

We investigate the crustal structure and tectonic evolution of the North American continent in Alaska, where the continent has grown through magmatism, accretion, and tectonic underplating. In the 1980s and early 1990s, we conducted a geological and geophysical investigation, known as the Trans-Alaska Crustal Transect (TACT), along a 1350-km-long corridor from the Aleutian Trench to the Arctic coast. The most distinctive crustal structures and the deepest Moho along the transect are located near the Pacific and Arctic margins. Near the Pacific margin, we infer a stack of tectonically underplated oceanic layers interpreted as remnants of the extinct Kula (or Resurrection) plate. Continental Moho just north of this underplated stack is more than 55 km deep. Near the Arctic margin, the Brooks Range is underlain by large-scale duplex structures that overlie a tectonic wedge of North Slope crust and mantle. There, the Moho has been depressed to nearly 50 km depth. In contrast, the Moho of central Alaska is on average 32 km deep. In the Paleogene, tectonic underplating of Kula (or Resurrection) plate fragments overlapped in time with duplexing in the Brooks Range. Possible tectonic models linking these two regions include flat-slab subduction and an orogenic-float model. In the Neogene, the tectonics of the accreting Yakutat terrane have differed across a newly interpreted tear in the subducting Pacific oceanic lithosphere. East of the tear, Pacific oceanic lithosphere subducts steeply and alone beneath the Wrangell volcanoes, because the overlying Yakutat terrane has been left behind as underplated rocks beneath the rising St. Elias Range, in the coastal region. West of the tear, the Yakutat terrane and Pacific oceanic lithosphere subduct together at a gentle angle, and this thickened package inhibits volcanism.

Alaska↗

USGS2011

Seawater calcium isotope ratios across the Eocene-Oligocene transition

During the Eocene-Oligocene transition (EOT, ca. 34 Ma), Earth's climate cooled significantly from a greenhouse to an icehouse climate, while the calcite (CaCO 3 ) compensation depth (CCD) in the Pacific Ocean increased rapidly. Fluctuations in the CCD could result from various processes that create an imbalance between calcium (Ca) sources to, and sinks from, the ocean (e.g., weathering and CaCO 3 deposition), with different effects on the isotopic composition of dissolved Ca in the oceans due to differences in the Ca isotopic composition of various inputs and outputs. We used Ca isotope ratios (δ 44/40 Ca) of coeval pelagic marine barite and bulk carbonate to evaluate changes in the marine Ca cycle across the EOT. We show that the permanent deepening of the CCD was not accompanied by a pronounced change in seawater δ 44/40 Ca, whereas time intervals in the Neogene with smaller carbonate depositional changes are characterized by seawater δ 44/40 Ca shifts. This suggests that the response of seawater δ 44/40 Ca to changes in weathering fluxes and to imbalances in the oceanic alkalinity budget depends on the chemical composition of seawater. A minor and transient fluctuation in the Ca isotope ratio of bulk carbonate may reflect a change in isotopic fractionation associated with CaCO 3 precipitation from seawater due to a combination of factors, including changes in temperature and/or in the assemblages of calcifying organisms.

Geology↗

USGS2019

Effects of Cenozoic subduction along the outboard margin of the Northern Cordillera: Derived from e-book on the Northern Cordillera (Alaska and Western Canada) and adjacent marine areas

This article describes the regional effects of Cenozoic subduction along the outboard margin of the Northern Cordillera (Alaska, USA, and Western Canada), and thereby acquaints the reader with several chapters of the e-book Dynamic Geology of the Northern Cordillera (Alaska, Western Canada, and Adjacent Marine Areas) . This article and the e-book are written for earth-science students and teachers. The level of writing for the article and the source e-book is that of popular science magazines, and readers are encouraged to share this article with students and laypersons. The main thrust of the article is to present and describe a suite of ten regional topographic, bathymetric, and geologic maps, and two figures portraying deep-crustal sections that illustrate the regional effects of Cenozoic subduction along the outboard margin of the North American Cordillera. The regional maps and cross sections are described in a way that a teacher might describe a map to students. Cenozoic subduction along the margin of the Northern Cordillera resulted in the formation of the following: (1) underthrusting of terranes and oceanic lithosphere beneath Southern Alaska; (2) landscapes, including narrow continental shelves along Southern and Southeastern Alaska and Western Canada (the Canadian Cordillera) and continental-margin mountain ranges, including the Alaska Peninsula, Chugach Range, Saint Elias Mountains, and Cascade Mountains; (3) sedimentary basins; (4) an array of active continental strike-slip and thrust faults (inboard of subduction zones); (5) earthquake belts related to subduction of terranes and oceanic plates; (6) active volcanoes, including continental-margin arcs (the Aleutian, Wrangell, and Cascade Arcs) linked to subduction zones, and interior volcanic belts related to strike-slip faulting or to hot spots; (7) lode and placer mineral deposits related to continental margin arcs or subduction of oceanic ridges; (8) hot springs related to continental-margin arcs; (9) plate movements as recorded from GPS measurements; and (10) underthrusting of terranes and oceanic lithosphere beneath the Northern Cordillera.

Alaska, British Columbia, Idaho, Montana, Washingt↗

USGS1982

Ancient plate boundaries in the Bering Sea region

Plate tectonic models of the Bering Sea suggest that the abyssal Bering Sea Basin is underlain by oceanic crust, a supposition supported by refraction and magnetic data. The oceanic crust is thought to be a remnant of the Kula(?) plate that was isolated within what is now the Bering Sea when the proto-Aleutian arc began to form between the Alaska Peninsula and Kamchatka in late Mesozoic or earliest Tertiary times. Prior to the formation of the Aleutian arc, the Kula(?) plate moved NW, directly underthrusting eastern Siberia; the plate’s eastern edge either obliquely underthrust or slid past the Bering Sea margin along a transform boundary. The Koryak Range in eastern Siberia is composed in part of mélange units that include Palaeozoic and Mesozoic allochthonous blocks juxtaposed within a matrix of Cretaceous sedimentary rocks. Structural trends suggest that these blocks were accreted into the Koryak area from the south along an ancient subduction zone formed by underthrusting of the Kula(?) plate. The base of the Bering Sea continental margin that extends from eastern Siberia to the Alaska Peninsula—the so-called Beringian margin—is underlain by a thick (7–10 km) sedimentary section along the base of the slope. Rocks dredged from the basement exposed farther up the slope (1500–2000 m deep) include shallow-water Upper Jurassic sandstone that is unconformably overlain by shallow-water Eocene to Miocene diatomaceous mudstone. Fauna in the dredge samples indicate that the shelf edge has subsided several kilometres since late Palaeogene time, perhaps in response to the cessation of motion relative to the adjacent oceanic plate and subsequent sediment loading of the oceanic plate. Uplift of the former plate boundary exposed in the Koryak Range occurred principally in late Cenozoic time, and collapse of the adjacent plate boundary, the Beringian margin, began in earliest Tertiary time and has continued to the present. Both tectonic events occurred after the site of active plate collision shifted south to near the present Aleutian Trench. We are uncertain as to why these two ancient, yet adjacent former plate boundaries should behave so differently, i.e. why one area was folded and uplifted while the other was extensionally deformed and subsided, both apparently in response to the cessation of convergent or strike-slip plate motion.

Geological Society, London, Special Publications↗

USGS1984

Models for the deposition of Mesozoic-Cenozoic fine-grained organic-carbon-rich sediment in the deep sea

The widespread occurrence of organic-carbon-rich strata (‘black shales’) in certain portions of Jurassic, Cretaceous and Cenozoic sequences has been well-documented from Deep Sea Drilling Project sites in the Atlantic and Pacific Oceans and from sequences, now exposed on land, originally deposited in the Tethyan ocean. These ancient black shales usually have been explained by analogy with examples of modern deep-sea sediments in which organic matter locally is preserved by (1) increasing the supply of organic matter, (2) increasing the rate of sedimentation, and/or (3) decreasing the oxygen content of the bottom water. However, detailed examination of many black shales reveals characteristics that cannot be explained by simple local models, including: their approximate coincidence in time globally; their occurrence in a variety of different environments, including open oxygenated oceans, restricted basins, deep and shallow water; their interbedding with organic-carbonpoor strata which often dominate a so-called black shale sequence; their deposition by pelagic, hemipelagic, turbiditic and other processes; and the variations in type and amount of organic matter that occur even within the same sequence. A more complex model for the origin of black shales therefore appears most appropriate, in which the cyclic preservation of organic matter depends on the interplay of the three main variables, namely supply of organic matter, sedimentation rate, and deep-water oxygenation, each of which varies independently to some extent. The variation and relative importance of these parameters in individual basins and widespread black shale deposition in general are linked globally and temporally by changes in global sea-level, climate and related changes in oceanic circulation. An important and often overlooked factor for the supply of organic matter to deep-basin sediments is the frequency and magnitude of redepositional processes. The interplay of these variables is discussed in relation to the middle Cretaceous and Cenozoic organic-carbon-rich strata, in particular, which show marked differences in the relative importance of the different variables.

Geological Society of London Special Publications↗