Imaging Tectonic Plates - Structure and Deformation of North America

The outermost layer of the Earth is broken into tectonic plates whose motions cause earthquakes and volcanism. Much of what we know about the structure of these plates comes from studying the Earth's surface. Seismic waves recorded by the EarthScope Transportable Array, however, show us the internal structure of plates down to 200 km deep and help us understand the relationship between deep structure and the Earth's surface. EarthScope images for the western United States hint at the presence of the mid-depth discontinuities. To better understand these features, Dr. Lekic relates them to the geologic history of the west.

Atmospheric CO₂ response to volcanic eruptions: The role of ENSO, season, and variability

Tropical explosive volcanism is one of the most important natural factors that significantly impact the climate system and the carbon cycle on annual to multi-decadal time scales. The three largest explosive eruptions in the last 50�years�Agung, El Chichón, and Pinatubo�occurred in spring/summer in conjunction with El Niño events and left distinct negative signals in the observational temperature and CO2 records. However, confounding factors such as seasonal variability and El Niño-Southern Oscillation (ENSO) may obscure the forcing-response relationship. We determine for the first time the extent to which initial conditions, i.e., season and phase of the ENSO, and internal variability influence the coupled climate and carbon cycle response to volcanic forcing and how this affects estimates of the terrestrial and oceanic carbon sinks. Ensemble simulations with the Earth System Model (Climate System Model 1.4-carbon) predict that the atmospheric CO2 response is �60 larger when a volcanic eruption occurs during El Niño and in winter than during La Niña conditions. Our simulations suggest that the Pinatubo eruption contributed 11�±�6 to the 25�Pg terrestrial carbon sink inferred over the decade 1990�1999 and �2�±�1 to the 22�Pg oceanic carbon sink. In contrast to recent claims, trends in the airborne fraction of anthropogenic carbon cannot be detected when accounting for the decadal-scale influence of explosive volcanism and related uncertainties. Our results highlight the importance of considering the role of natural variability in the carbon cycle for interpretation of observations and for data-model intercomparison.

Timing of Tertiary Extension in the Railroad Valley Pioche Transect, Nevada: Constraints from Ar-40/Ar-39 Ages of Volcanic Rocks

Time-space relations of extension and volcanism place critical constraints on models of Basin and Range extensional processes. This paper addresses such relations in a 130-km-wide transect in the eastern Great Basin, bounded on the east by the Ely Springs Range and on the west by the Grant and Quinn Canyon ranges. Stratigraphic and structural data, combined with 40Ar/39Ar isotopic ages of volcanic rocks, document a protracted but distinctly episodic extensional history. Field relations indicate four periods of faulting. Only one of these periods was synchronous with nearby volcanic activity, which implies that volcanism and faulting need not be associated closely in space and time. Based on published dates and the analyses reported here, the periods of extension were (1) prevolcanic (pre-32 Ma), (2) early synvolcanic (30 to 27 Ma), (3) immediately postvolcanic (about 16 to 14 Ma), and (4) Pliocene to Quaternary. The break between the second and third periods is distinct. The minimum gap between the first two periods is 2 Ma, but the separation may be much larger. Temporal separation of the last two periods is only suggested by the stratigraphic record and cannot be rigorously demonstrated with present data. The three younger periods of faulting apparently occurred across the entire transect. The oldest period is recognized only at the eastern end of the transect, but appears to correlate about 150 km northward along strike with extension in the Northern Snake Range-Kern Mountains area. Therefore the oldest period also is regional in extent, but affected a different area than that affected by younger periods. This relation suggests that distinct extensional structures and master detachment faults were active at different times. The correlation of deformation periods of a few million years duration across the Railroad Valley-Pioche transect suggests that the scale of active extensional domains in the Great Basin may be greater than 100 km across strike.

Ice Core Paleovolcanic Records from the St. Elias Mountains, Yukon, Canada

We previously reported a record of regionally significant volcanic eruptions in the North Pacific using an ice core from Eclipse Icefield (St. Elias Mountains, Yukon, Canada). The acquisition of two new ice cores from Eclipse Icefield, along with the previously available Eclipse Icefield and Mount Logan Northwest Col ice cores, allows us to extend our record of North Pacific volcanism to 550 years before present using a suite of four ice cores spanning an elevation range of 3 - 5 km. Comparison of volcanic sulfate flux records demonstrates that the results are highly reproducible, especially for the largest eruptions such as Katmai ( A. D. 1912). Correlation of volcanic sulfate signals with historically documented eruptions indicates that at least one-third of the eruptions recorded in St. Elias ice cores are from Alaskan and Kamchatkan volcanoes. Although there are several moderately large ( volcanic explosivity index (VEI) >= 4) eruptions recorded in only one core from Eclipse Icefield, the use of multiple cores provides signals in at least one core from all known VEI >= 4 eruptions in Alaska and Kamchatka since A. D. 1829. Tephrochronological evidence from the Eclipse ice cores documents eruptions in Alaska (Westdahl, Redoubt, Trident, and Katmai), Kamchatka (Avachinsky, Kliuchevoskoi, and Ksudach), and Iceland (Hekla). Several unidentified tephra-bearing horizons, with available geochemical evidence suggesting Alaskan and Kamchatkan sources, were also found. We present a reconstruction of annual volcanic sulfate loading for the North Pacific troposphere based on our ice core data, and we provide a detailed assessment of the atmospheric and climatic effects of the Katmai eruption.

Volcanostratigraphic Controls on the Occurrence of Massive Sulfide (VMS) Deposits in the Semail Ophiolite, Oman

The Semail ophiolite in Oman is capped by up to 2 km of basaltic-andesitic lavas that host copper-dominant, Cyprus-type, volcanogenic massive sulfide (VMS) deposits. This study identifies multiple volcanostratigraphic horizons on which the deposits are situated, based on characterization of footwall and hanging-wall lavas from 16 deposits or deposit clusters. Comparison of field and petrographic features, compositions of igneous clinopyroxenes, and whole-rock geochemical signatures permits classification of the lavas within a modified version of the established regional volcanostratigraphy. Four extrusive units host deposits: Geotimes (earliest), Lasail, Alley, and Boninitic Alley (latest). The latter was previously known only at few localities, but this study reveals its regional extent and significance as a host for VMS deposits. The Geotimes and Lasail units represent Late Cretaceous, ocean spreading ridge and related off-axis volcanic environments, respectively. The Alley and Boninitic Alley units represent younger, subduction-related volcanism prior to Coniacian-Santonian obduction of the ophiolite. Our results show that VMS deposits occur on or near the Geotimes/Lasail and Geotimes/Alley contacts as well as entirely within the Geotimes, Lasail, Alley, and Boninitic Alley units. Highest Cu grades tend to occur in deposits lying on or within the Geotimes, whereas highest Au grades occur in deposits within the Boninitic Alley. In contrast to earlier studies, we conclude that essentially every horizon marking a hiatus in lava deposition in the Semail ophiolite, i.e., contacts between the four major eruptive units, and umbers and sedimentary chert layers within the units, has exploration potential for Cu-Au VMS deposits.

Holocene palaeoclimate and palaeovegetation on the islands of Flores and Pico

Palaeoclimatic stability is regarded as an important factor in explaining patterns of endemism in the Azorean flora. However, modelling simulations and quantitative reconstructions for the last 6000 years suggest considerable palaeoclimatic variability. Here we explore the link between Holocene palaeoclimate and palaeovegetation on the islands of Flores and Pico. Modern pollen assemblages indicate that most major plant communities can be detected using pollen analysis and that, in some cases, the pre-colonisation vegetation was quite similar to present-day relict vegetation. A 200–500-year pollen record from Alagoinha, a low-elevation mire in western Flores, shows that Juniperus brevifolia-dominated communities were widespread at lower elevations prior to large-scale deforestation. Today these communities are generally restricted to higher elevations. While our results are preliminary, there appears to be a weak link between palaeovegetation (which was primarily influenced by volcanism, soil formation and human impact) and palaeoclimatic changes detected through geochemical proxies. Even if the Azorean palaeoclimate varied substantially, its impact on the pristine vegetation, at least in terms of pollen production, was relatively small.

Volcanic terrain and the possible periglacial formation of "excess ice" at the mid-latitudes of Utopia Planitia, Mars

At the mid-latitudes of Utopia Planitia (UP), Mars, a suite of spatially-associated landforms exhibit geomorphological traits that, on Earth, would be consistent with periglacial processes and the possible freeze-thaw cycling of water. The suite comprises small-sized polygonally-patterned ground, polygon-junction and -margin pits, and scalloped, rimless depressions. Typically, the landforms incise a dark-toned terrain that is thought to be ice-rich. Here, we investigate the dark-toned terrain by using high resolution images from the HiRISE as well as near-infrared spectral-data from the OMEGA and CRISM. The terrain displays erosional characteristics consistent with a sedimentary nature and near-infrared spectra characterised by a blue slope similar to that of weathered basaltic-tephra. We also describe volcanic terrain that is dark-toned and periglacially-modified in the Kamchatka mountain-range of eastern Russia. The terrain is characterised by weathered tephra inter-bedded with snow, ice-wedge polygons and near-surface excess ice. The excess ice forms in the pore space of the tephra as the result of snow-melt infiltration and, subsequently, in-situ freezing. Based on this possible analogue, we construct a three-stage mechanism that explains the possible ice-enrichment of a broad expanse of dark-toned terrain at the mid-latitudes of UP: (1) the dark-toned terrain accumulates and forms via the regional deposition of sediments sourced from explosive volcanism; (2) the volcanic sediments are blanketed by atmospherically-precipitated (H2O) snow, ice or an admixture of the two, either concurrent with the volcanic-events or between discrete events; and, (3) under the influence of high obliquity or explosive volcanism, boundary conditions tolerant of thaw evolve and this, in turn, permits the migration, cycling and eventual formation of excess ice in the volcanic sediments. Over time, and through episodic iterations of this scenario, excess ice forms to decametres of depth. (C) 2015 Elsevier B.V. All rights reserved.

Isotope ratios of basalts from ODP Leg 115 holes (Table 1)

Strontium, neodymium, and lead isotope ratios are reported for 13 Leg 115 basalts as well as 3 basalts from Texaco drill hole SM-1 on the Mascarene Plateau. The 87Sr/86Sr ratios and eNd range from 0.70330 to 0.70439 and 5.5 to 7.4, respectively, although 87Sr/86Sr ratios higher than 0.70383 are found only in SM-1 basalts. The high 87Sr/86Sr values are thought to reflect seawater Sr in secondary phases, although all samples were strongly leached in HC1 before analysis. 206Pb/204Pb ratios range from 18.53 to 18.80, and sho high 207Pb/204Pb and 208Pb/204Pb ratios relative to 206Pb/204Pb ratios, typical of Indian Ocean mid-ocean ridge (MORB) and oceanic-island basalts (OIB). Isotopic compositions of Leg 115 basalts generally fall between fields for MORB and Reunion Island basalts, consistent with the conclusion drawn from geochronological studies that Deccan flood basalt volcanism, the Chagos-Laccadive Ridge, and the Mascarene Plateau are all products of the Reunion mantle plume. Isotopic compositions of magmas produced by this plume have varied systematically with time in the direction of less "depleted," less MORB-like isotopic signatures. This compositional change has been accompanied by a decrease in eruption rate. We interpret Deccan volcanism as the voluminous beginning of the plume. Reduced entrainment of asthenosphere following melting of the plume head resulted in less MORB-like isotope ratios in magmas and a decrease in eruptive activity with time.

Accumulation rates of volcanic glass and ice-rafted debris in sediments of the North Pacific Ocean

Mass accumulation rates (MAR) of different components of North Pacific deep-sea sediment provide detailed information about the timing of the onset of major Northern Hemisphere glaciation that occurred at 2.65 Ma. An increase in explosive volcanism in the Kamchatka-Kurile and Aleutian arcs occured at this same time, suggesting a link between volcanism and glaciation. Sediments recovered by piston-coring techniques during ODP Leg 145 provide a unique opportunity to undertake a detailed test of this possibility. Here we use volcanic glass as a proxy for explosive volcanism and ice-rafted debris (IRD) as a proxy for glaciation. The MAR of both glass and IRD increase markedly at 2.65 Ma. Further, the flux of the volcanic glass increased just prior the flix of ice-radted material, suggesting that the cooling resulting from explosive volcanic eruptions may have been the ultimate trigger for the mid-Pliocene glacial intensification.

(Table 1) Occurrences of volcanic ash at DSDP Leg 78A Holes

Air-fall volcanic ash recovered at Deep Sea Drilling Project Sites 541, 542, and 543 on and east of the toe of the Barbados Ridge delineate middle and late Miocene, early Pliocene, and Pleistocene-Quaternary pulses of explosive volcanism in the Lesser Antilles arc. The ash beds at Site 541 allow precise correlation of intervals repeated by a probable reverse fault at this convergent margin.

Carbon and oxygen isotopic compositions of Cibicidoides spp. and fine fraction separates (<10 µm) from ODP Holes 113-689B, 119-738B, and 120-748B

A prominent middle Eocene warming event is identified in Southern Ocean deep-sea cores, indicating that long-term cooling through the middle and late Eocene was not monotonic. At sites on Maud Rise and the Kerguelen Plateau, a distinct negative shift in d18O values (~1.0 per mil) is observed ca. 41.5 Ma. This excursion is interpreted as primarily a temperature signal, with a transient warming of 4°C over 600 k.y. affecting both surface and middle-bathyal deep waters in the Indian-Atlantic region of the Southern Ocean. This isotopic event is designated as the middle Eocene climatic optimum, and is interpreted to represent a significant climatic reversal in the midst of middle to late Eocene deep-sea cooling. The lack of a significant negative carbon isotope excursion, as observed during the Paleocene-Eocene thermal maximum, and the gradual rate of high-latitude warming suggest that this event was not triggered by methane hydrate dissociation. Rather, a transient rise in pCO2 levels is suspected, possibly as a result of metamorphic decarbonation in the Himalayan orogen or increased ridge/arc volcanism during the late middle Eocene.

Appendix A. Whole-rock major, trace and rare earth element analyses of the Ulubey volcanics, Appendix B. Analytical standards and detection limits for analyses

Collisional and post-collisional volcanic rocks in the Ulubey (Ordu) area at the western edge of the Eastern Pontide Tertiary Volcanic Province (EPTVP) in NE Turkey are divided into four suites; Middle Eocene (49.4-44.6 Ma) aged Andesite-Trachyandesite (AT), Trachyandesite-Trachydacite-Rhyolite (TTR), Trachydacite-Dacite (TD) suites, and Middle Miocene (15.1 Ma) aged Trachybasalt (TB) suite. Local stratigraphy in the Ulubey area starts with shallow marine environment sediments of the Paleocene-Eocene time and then continues extensively with sub-aerial andesitic to rhyolitic and rare basaltic volcanism during Eocene and Miocene time, respectively. Petrographically, the volcanic rocks are composed primarily of andesites/trachyandesites, with minor trachydacites/rhyolites, basalts/trachybasalts and pyroclastics, and show porphyric, hyalo-microlitic porphyric and rarely glomeroporphyric, intersertal, intergranular, fluidal and sieve textures. The Ulubey (Ordu) volcanic rocks indicate magma evolution from tholeiitic-alkaline to calc-alkaline with medium-K contents. Primitive mantle normalized trace element and chondrite normalized rare earth element (REE) patterns show that the volcanic rocks have moderate light rare earth element (LREE)/heavy rare earth element (HREE) ratios relative to E-Type MORB and depletion in Nb, Ta and Ti. High Th/Yb ratios indicate parental magma(s) derived from an enriched source formed by mixing of slab and asthenospheric melts previously modified by fluids and sediments from a subduction zone. All of the volcanic rocks share similar incompatible element ratios (e.g., La/Sm, Zr/Nb, La/Nb) and chondrite-normalized REE patterns, indicating that the basic to acidic rocks originated from the same source. The volcanic rocks were produced by the slab dehydration-induced melting of an existing metasomatized mantle source, and the fluids from the slab dehydration introduced significant large ion lithophile element (LILE) and LREE to the source, masking its inherent HFSE-enriched characteristics. The initial 87Sr/86Sr (0.7044-0.7050) and eNd (-0.3 to +3.4) ratios of the volcanics suggest that they originated from an enriched lithospheric mantle source with low Sm/Nd ratios. Integration of the geochemical, petrological and isotopical with regional and local geological data suggest that the Tertiary volcanic rocks from the Ulubey (Ordu) area were derived from an enriched mantle, which had been previously metasomatized by fluids derived from subducted slab during Eocene to Miocene in collisional and post-collisional extension-related geodynamic setting following Late Mesozoic continental collision between the Eurasian plate and the Tauride-Anatolide platform.