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At least 271 records · Page 15Linked to original sources
USGS2012

Paleomagnetic rotations and the Cenozoic tectonics of the Cascade Arc, Washington, Oregon, and California

Paleomagnetic results from Cenozoic (62–12 Ma) volcanic rocks of the Cascade arc and adjacent areas indicate that moderate to large clockwise rotations are an important component of the tectonic history of the arc. Two mechanisms of rotation are suggested by the regional pattern of paleomagnetic rotations. The progressive increase in rotation toward the coast in arc and forearc rocks results from distributed dextral shear, which is likely driven by oblique subduction of oceanic plates to the west. Simple shear rotation is accommodated in the upper crust by strike-slip faulting. The right-lateral Mount St. Helens seismic zone may be an active manifestation of this process. Dextral shear probably obscures a subequal contribution to arc and forearc rotation that is driven by intraarc or backarc extension. This rotation is suggested by the average southward increase in continental margin rotations into the region outboard of the Basin and Range. The southward increase in rotation parallels a change in the arc tectonic regime from largely compressional in northern Washington to extensional in Oregon. Concomitant with this change is a southward increase in the volume of eruptive rocks and the number of basaltic vents in the arc. A progressive eastward shift of the arc volcanic front with time in the rotated arc terrane is the result of the westward pivoting of the arc block in front of a zone of extension since Eocene time. Westward migration of bimodal Basin and Range volcanism since at least 16 Ma is tracking westward rotation of the frontal arc block and growth of the Basin and Range in its wake.

Journal of Geophysical Research Solid Earth↗
USGS2012

Edge-driven microplate kinematics

It is known from plate tectonic reconstructions that oceanic microplates undergo rapid rotation about a vertical axis and that the instantaneous rotation axes describing the microplate's motion relative to the bounding major plates are frequently located close to its margins with those plates, close to the tips of propagating rifts. We propose a class of edge-driven block models to illustrate how slip across the microplate margins, block rotation, and propagation of rifting may be related to the relative motion of the plates on either side. An important feature of these edge-driven models is that the instantaneous rotation axes are always located on the margins between block and two bounding plates. According to those models the pseudofaults or traces of disrupted seafloor resulting from the propagation of rifting between microplate and major plates may be used independently to approximately trace the continuous kinematic evolution of the microplate back in time. Pseudofault geometries and matching rotations of the Easter microplate show that for most of its 5 m.y. history, block rotation could be driven by the drag of the Nazca and Pacific plates on the microplate's edges rather than by a shear flow of mantle underneath.

Journal of Geophysical Research B: Solid Earth↗
USGS2021

Estimating the impact of seep methane oxidation on ocean pH and dissolved inorganic radiocarbon along the U.S. mid‐Atlantic Bight

Ongoing ocean warming can release methane (CH 4 ) currently stored in ocean sediments as free gas and gas hydrates. Once dissolved in ocean waters, this CH 4 can be oxidized to carbon dioxide (CO 2 ). While it has been hypothesized that the CO 2 produced from aerobic CH 4 oxidation could enhance ocean acidification, a previous study conducted in Hudson Canyon shows that CH 4 oxidation has a small short‐term influence on ocean pH and dissolved inorganic radiocarbon. Here we expand upon that investigation to assess the impact of widespread CH 4 seepage on CO 2 chemistry and possible accumulation of this carbon injection along 234 km of the U.S. Mid‐Atlantic Bight. Consistent with the estimates from Hudson Canyon, we demonstrate that a small fraction of ancient CH 4 ‐derived carbon is being assimilated into the dissolved inorganic radiocarbon (mean fraction of 0.5 ± 0.4 %). The areas with the highest fractions of ancient carbon coincide with elevated CH 4 concentration and active gas seepage. This suggests that aerobic CH 4 oxidation has a greater influence on the dissolved inorganic pool in areas where CH 4 concentrations are locally elevated, instead of displaying a cumulative effect downcurrent from widespread groupings of CH 4 seeps. An upper limit approximation of the input rate of ancient‐derived DIC into the waters overlying the northern U. S Mid‐Atlantic Bight further suggests that oxidation of ancient CH 4 ‐derived carbon is not negligible on the global scale and could contribute to deep‐water acidification over longer time scales.

Journal of Geophysical Research- Biogeosciences↗
USGS2012

Terranes and suture zones in east central Alaska

East central Alaska, with its 17 terranes, forms a part of the mosaic of allochthonous terranes that join the North American and Siberian plates. These terranes range from continental and continental margin, such as the Tatonduk with its thick well-bedded succession of marine shelf rocks, to seamount, arc, and ocean floor terranes. The Yukon crystalline terrane, the largest described here, is a composite of at least four subterranes juxtaposed across the Tintina fault with the Tatonduk terrane, a northwestern extension of the North American plate in Alaska. Inboard of the Yukon crystalline terrane are packets of closely appressed microterranes separated from the Tatonduk and other terranes belonging to North America by major suture zones. These microterranes lie between North America and the mosaic of accretionary terranes that form the more southerly part of Alaska. The most obviously allochthonous microterranes within the suture zones are the Woodchopper Canyon, an Early Devonian basaltic seamount, and the White Mountains, an Ordovician volcanic arc terrane capped by Silurian and Devonian carbonate bank deposits. The nearest counterpart of these terranes is the Alexander terrane in southeastern Alaska. The Tintina fault of Mesozoic and Cenozoic age, like the Denali fault, primarily follows old suture zones that separate terranes. Strike slip faulting developed after collision in places where further convergence was oblique to the terrane margins. Where terranes met head-on, their leading edges lie along a multiple set of high-angle faults that outline microterranes in accretion zones.

Alaska↗
USGS2011

Process-based modeling of tsunami inundation and sediment transport

The infrequent and unpredictable nature of tsunamis precludes the use of field experiments to measure the hydrodynamic and sediment transport processes that occur. Instead, these processes are often approximated from laboratory, numerical, and theoretical studies or inferred from observations of the resultant sediment deposits. Here Delft3D, a three-dimensional numerical model, is used to simulate the inundation and sediment transport of a tsunami similar in magnitude to the 26 December 2004 Indian Ocean tsunami over one measured and three idealized morphologies. The model is first shown to match well the observations taken at Kuala Meurisi, Sumatra, and then used to examine in detail the processes that occur during the tsunami. The model predicts that at a given cross-shore location the onshore flow accelerates rapidly to a maximum as the wavefront passes, and then gradually decelerates before reversing direction and flowing offshore. The onshore flow does not tend to zero everywhere at maximum inundation, but instead flow reversal occurs near the shoreline even as the wavefront continues to inundate landward. While some sediment is eroded by the passing wavefront, the suspension of sandy sediment is dominated by the long-duration, high-velocity backwash that occurs along the beach face and offshore of the shoreline. Some of the sediment suspended during backwash is advected shoreward by the subsequent wave, creating large spatial gradients in the suspended sediment concentrations, which may not be in equilibrium with the local hydrodynamics. The inundation and transport of sediment during a tsunami can be affected by complexities in the morphological profile and interactions between multiple waves, and many of the hydrodynamic and sediment transport processes predicted here are similar to analogous processes previously observed in the swash zone. Copyright 2011 by the American Geophysical Union.

Journal of Geophysical Research F: Earth Surface↗
USGS2012

Petrology and age of alkalic lava from the Ratak Chain of the Marshall Islands

Volcanic rock dredged from the flanks of four volcanic edifices in the Ratak chain of the Marshall Islands consist of alkalic lava that erupted above sea level or in shallow water. Compositions of recovered samples are predominantly differentiated alkalic basalt and hawaiite but include strongly alkalic melilitite. Whole rock 40 Ar/ 39 Ar total fusion and incremental heating ages of 87.3±0.6 Ma and 82.2±1.6 Ma determined for samples from Erikub Seamount and Ratak Guyot, respectively, are within the range predicted by plate rotation models but show no age progression consistent with a simple hot spot model. Whole rock major element, and mineral chemistry are similar to those of alkalic lavas from other oceanic islands, but isotopic and certain trace element ratios reveal distinctive mantle source characteristics. Zr/Nb ratios of ∼3.0±0.2 for Bikar and Ratak Guyot, compared to 5.2±0.3 for Majuro Atoll and Erikub Seamount, at comparable degree of differentiation, suggest a more alkalic trend for the northern sites. The 143 Nd/ 144 Nd isotopic ratios range from 0.51283 to 0.51289 and 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb ratios range from 20.55 to 21.11, 15.71 to 15.77, and 39.93 to 40.75, for samples from Erikub and Ratak. Variations in isotopic and some incompatible element ratios suggest interisland heterogeneity. Similar highly radiogenic lead isotopes, coupled with distinct incompatible element ratios, especially with low Ba/Nb ratios (∼6), are observed for St. Helena in the South Atlantic and for Tubuai and Mangaia in the South Pacific. The similar mantle signature shown by lavas from Tubuai and Mangaia in the Austral-Cook chain, which are located at approximately the position where the Ratak edifices formed during the Late Cretaceous, indicates that this mantle anomaly is not of recent origin but has erupted distinctive lava compositions for at least 87 m.y.

Journal of Geophysical Research Solid Earth↗
USGS2012

A seismic refraction survey of the Imperial Valley Region, California

The U.S. Geological Survey conducted an extensive seismic refraction survey in the Imperial Valley region of California in 1979. The Imperial Valley is located in the Salton Trough, an active rift between the Pacific and North American plates. Forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0.5–1 km. More than 1300 recording locations were occupied, and more than 3000 usable seismograms were obtained. We analyzed five profiles using a standard ray‐tracing program, constructed a contour map of reduced travel times from our most widely recorded shot point, and modeled an existing gravity profile across the Salton Trough. Results are itemized: (1) All models have in common a sedimentary layer ( V p = 1.8–5.0 km/s), a “transition zone” ( V p = 5.0–5.65 km/s), a basement ( V p = 5.65 km/s in the Imperial Valley, 5.9 km/s on the bordering mesas), and subbasement ( V p = 7.2 km/s). (2) The sedimentary layer ranges in thickness along the axis of the Salton Trough from 3.7 km (Salton Sea) to 4.8 km (U.S.‐Mexican border). On the bordering mesas it is quite variable in thickness. (3) The “transition” zone is about 1 km thick in most places. In the Imperial Valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement. On the bordering mesas, however, there is a discontinuity at the top of this zone. (4) There are apparently two types of basement. On the bordering mesas, basement is crystalline igneous and metamorphic rocks. In the Imperial Valley, basement is mostly lower‐greenshist‐facies sedimentary rocks, based primarily on the smooth transition in character from sediment to basement arrivals, the low value of basement velocity, and the fact that deep (4 km) wells in the valley penetrate only the upper part of the known Cenozoic stratigraphic column for the Salton Trough. (5) The subbasement, or intermediate crustal layer, ranges in depth along the axis of the Salton Trough from 16 km (Salton Sea) to 10 km (U.S.‐Mexican border). Gravity modeling requires that this layer deepen and/or pinch out beneath the bordering mesas and mountain ranges. Based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the Imperial Valley, the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust. (6) Several structures are seen that affect basement, transition zone, and deeper parts of the sedimentary layer. They include a scarp along the Imperial fault, as much as 1 km down to the northeast, and a scarp passing roughly along the topographic boundary between the Imperial Valley and the bordering mesa to the west, as much as 3½ km down to the east. We interpret the latter scarp to be the suture, or rift boundary, between the older crystalline basement on the mesa and the younger metasedimentary basement in the Imperial Valley. (7) On a contour map of reduced travel time from our most widely recorded shot point, subtle patches of early arrivals among otherwise late arrivals in the central Imperial Valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150°C. Apparently the Salton Trough is a location where new crust is being generated. As the rift opens, mafic intrusive rocks fill it from below as sedimentary rocks fill it from above. Rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth (metasedimentary basement in the Imperial Valley) and thus consolidates the new crust.

California↗
USGS1996

Origin of Bermuda's clay-rich Quaternary paleosols and their paleoclimatic significance

Red clayey paleosols that are chiefly the product of aerosolic dust deposition are interbedded in the Quaternary carbonate formations of the Bermuda oceanic island system. These paleosols provide a basis for reconstructing Quaternary atmospheric circulation patterns in the northwestern Atlantic. Geochemical analyses were performed on representative paleosol samples to identify their parent dust source. Fine-grained fractions were analyzed by energy-dispersive X ray fluorescence to determine trace element (Zr, Y, La, Ti, and Nb) concentrations and to derive geochemical signatures based on immobile element ratios. These ratios were compared with geochemical signatures determined for three possible sources of airborne dust: (1) Great Plains loess, (2) Mississippi River Valley loess, and (3) Saharan dust. The Zr/Y and Zr/La ratios provided the clearest distinction between the hypothesized dust sources. The low ratios in the paleosol B horizons most closely resemble Saharan dust in the <2-μm size class fraction. Contributions from the two North American loessial source areas could not be clearly detected. Thus Bermuda paleosols have a predominantly Saharan aerosolic dust signature. Saharan dust deposition on Bermuda during successive Quaternary glacial periods is consistent with patterns of general circulation models, which indicate that during glacial maxima the northeast summer trade winds were stronger than at present and reached latitudes higher than 30°N despite lower-than-present sea surface temperatures in the North Atlantic.

Journal of Geophysical Research D: Atmospheres↗
USGS1998

Observations on basaltic lava streams in tubes from Kilauea Volcano, island of Hawai'i

From 1986 to 1997, the Pu'u 'O'o-Kupaianaha eruption of Kilauea produced a vast pahoehoe flow field fed by lava tubes that extended 10–12 km from vents on the volcano's east rift zone to the ocean. Within a kilometer of the vent, tubes were as much as 20 m high and 10–25 m wide. On steep slopes (4–10°) a little farther away from the vent, some tubes formed by roofing over of lava channels. Lava streams were typically 1–2 m deep flowing within a tube that here was typically 5 m high and 3 m wide. On the coastal plain (<1°), tubes within inflated sheet flows were completely filled, typically 1–2 m high, and several tens of meters wide. Tubes develop as a flow's crust grows on the top, bottom, and sides of the tubes, restricting the size of the fluid core. The tubes start out with nearly elliptical cross-sectional shapes, many times wider than high. Broad, flat sheet flows evolve into elongate tumuli with an axial crack as the flanks of the original flow were progressively buried by breakouts. Temperature measurements and the presence of stalactites in active tubes confirmed that the tube walls were above the solidus and subject to melting. Sometimes, the tubes began downcutting. Progressive downcutting was frequently observed through skylights; a rate of 10 cm/d was measured at one skylight for nearly 2 months.

Journal of Geophysical Research B: Solid Earth↗
USGS2003

Fault slip and seismic moment of the 1700 Cascadia earthquake inferred from Japanese tsunami descriptions

The 1700 Cascadia earthquake attained moment magnitude 9 according to new estimates based on effects of its tsunami in Japan, computed coseismic seafloor deformation for hypothetical ruptures in Cascadia, and tsunami modeling in the Pacific Ocean. Reports of damage and flooding show that the 1700 Casscadia tsunami reached 1-5 m heights at seven shoreline sites in Japan. Three sets of estimated heights express uncertainty about location and depth of reported flooding, landward decline in tsunami heights from shorelines, and post-1700 land-level changes. We compare each set with tsunami heights computed from six Cascadia sources. Each source is vertical seafloor displacement calculated with a three-dimensional elastic dislocation model, for three sources the rupture extends the 1100 km length of the subduction zone and differs in width and shallow dip; for the other sources, ruptures of ordinary width extend 360-670 km. To compute tsunami waveforms, we use a linear long-wave approximation with a finite difference method, and we employ modern bathymetry with nearshore grid spacing as small as 0.4 km. The various combinations of Japanese tsunami heights and Cascadia sources give seismic moment of 1-9 ?? 1022 N m, equivalent to moment magnitude 8.7-9.2. This range excludes several unquantified uncertainties. The most likely earthquake, of moment magnitude 9.0, has 19 m of coseismic slip on an offshore, full-slip zone 1100 km long with linearly decreasing slip on a downdip partial-slip zone. The shorter rupture models require up to 40 m offshore slip and predict land-level changes inconsistent with coastal paleoseismological evidence. Copyright 2003 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗
USGS2005

Nearshore wave-induced cyclical flexing of sea cliffs

[1] Evolution of a tectonically active coast is driven by geomorphically destructive energy supplied by ocean waves. Wave energy is episodic and concentrated; sea cliffs are battered by the geomorphic wrecking ball every 4-25 s. We measure the response of sea cliffs to wave assault by sensing the ground motion using near-coastal seismometers. Sea cliffs respond to waves in two distinct styles. High-frequency motion (20 Hz) reflects the natural frequency of the sea cliff as it rings in response to direct wave impact. Low-frequency motion in the 0.1-0.05 Hz (10-20 s) band consistently agrees with the dominant nearshore wave period. Integrating microseismic velocities suggests 50 ??m and 10 ??m displacements in horizontal and vertical directions, respectively. Displacement ellipsoids exhibit simultaneous downward and seaward sea cliff motion with each wave. Video footage corroborates the downward sea cliff flex in response to the imposed water load on the wave cut platform. Gradients in displacement amplitudes documented using multiple seismometers suggest longitudinal and shear strain of the flexing sea cliff on the order of 0.5-4 ?? strains during each wave loading cycle. As this sea cliff flexure occurs approximately 3 million times annually, it has the potential to fatigue the rock through cyclical loading. Local sea cliff retreat rates of 10 cm/yr imply that a given parcel of rock is flexed through roughly 109 cycles of increasing amplitude before exposure to direct wave attack at the cliff face. Copyright 2005 by the American Geophysical Union.

Journal of Geophysical Research F: Earth Surface↗
USGS2012

Igneous history of the Koyukuk terrane, western Alaska: Constraints on the origin, evolution, and ultimate collision of an accreted island arc terrane

The Koyukuk terrane of western Alaska consists of volcanic, volcaniclastic, and plutonic rocks which range from Late Paleozoic to Early Cretaceous in age. The terrane crops out in a U-shaped belt which is roughly paralleled by outer belts of ultramafic rocks, oceanic plate basalts and cherts, and retrograded blueschist facies rocks of continental protolith. These rocks have been interpreted as components of a volcanic arc terrane that collided with the North American continental margin in Early Cretaceous time. The Koyukuk terrane consists of four time-stratigraphic units: (1) pre-Middle Jurassic basalts, (2) Middle and Late Jurassic granitic rocks, (3) lower Lower Cretaceous volcanic rocks, and (4) upper Lower Cretaceous volcanic rocks. Limited chemical data from the basalts of unit 1 indicate that they were erupted in a nonarc tectonic environment, possibly in an oceanic island or back arc setting. Units 2, 3, and 4 have the characteristics of subduction-related volcanic rocks (i.e., depleted Nb and Ta and enriched alkaline elements, relative to the light rare earth elements). Unit 3 contains tholeiitic, calc-alkaline, and alkaline rocks with chondrite-normalized rare earth element patterns that range from flat (La N /Yb N = 1) to highly light rare earth element enriched (La N /Yb N > 15). The highly alkaline or shoshonitic lavas were erupted toward the end of unit 3 time (Valanginian) during the final stages of arc-continent collision. These alkaline lavas could have been derived by very small degrees of partial melting of a similar source to that of the earlier arc lavas. Unit 4 lavas are also alkaline or shoshonitic, but their incompatible element composition indicates that they were derived from a different source than that of the earlier arc lavas. These late alkaline lavas are chemically similar to crosscutting mid-Cretaceous plutons whose isotopic compositions (Arth et al., this issue ( a )) suggest derivation by partial melting of distinctly older subcontinental lithosphere. We speculate that the parental magmas of unit 4 lavas may also have been derived by partial melting of this subcontinental mantle which was underthrust beneath the Koyukuk arc terrane during the final stage of arc-continent collision.

Journal of Geophysical Research Solid Earth↗
USGS2018

Evolving environmental and geometric controls on Columbia Glacier’s continued retreat

Geometry strongly controls the dynamic behavior of marine‐terminating (tidewater) glaciers, significantly influencing advance and retreat cycles independent of climate. Yet the recent, nearly ubiquitous retreat of tidewater glaciers suggests that changes in atmospheric and oceanic forcing may also drive dynamic change. To isolate the influence of geometry on tidewater glacier dynamics, we analyzed detailed observational time series from 2012 to 2016 for two tidewater glaciers with shared dynamic histories and environmental forcing: Columbia Glacier and its former tributary (Post Glacier) in southcentral Alaska. We find that although terminus retreat has driven decadal‐scale changes in dynamics of the Columbia‐Post system, environmental factors contribute to short‐term (i.e., seasonal) dynamic variability. In particular, analysis of force balance time series indicates that observed variations in speed result from seasonal changes to the subglacial hydrologic system and associated changes in basal drag. Variations in terminus position only drive noticeable speed change when the terminus retreats from regions of relatively high basal drag. In agreement with long‐term analyses of Columbia Glacier, we find that terminus geometry can perturb the timing of seasonal ice flow patterns. Specifically, our data support the idea that retreat of a glacier terminus into deeper water is accompanied by a shift in the primary control on frontal ablation. Although our analysis focuses on two Alaskan glaciers, our data suggest that changes in the relative importance of surface meltwater and buoyancy effects on submarine melting and/or calving may manifest as a shift in terminus change seasonality and offer a mechanism to identify frontal ablation controls.

Alaska↗
USGS2010

Rheologic and structural controls on the deformation of Okmok volcano, Alaska: FEMs, InSAR, and ambient noise tomography

Interferometric synthetic aperture radar (InSAR) data indicate that the caldera of Okmok volcano, Alaska, subsided more than a meter during its eruption in 1997. The large deformation suggests a relatively shallow magma reservoir beneath Okmok. Seismic tomography using ambient ocean noise reveals two low‐velocity zones (LVZs). The shallow LVZ corresponds to a region of weak, fluid‐saturated materials within the caldera and extends from the caldera surface to a depth of 2 km. The deep LVZ clearly indicates the presence of the magma reservoir beneath Okmok that is significantly deeper (>4 km depth) compared to previous geodetic‐based estimates (3 km depth). The deep LVZ associated with the magma reservoir suggests magma remains in a molten state between eruptions. We construct finite element models (FEMs) to simulate deformation caused by mass extraction from a magma reservoir that is surrounded by a viscoelastic rind of country rock embedded in an elastic domain that is partitioned to account for the weak caldera materials observed with tomography. This configuration allows us to reduce the estimated magma reservoir depressurization to within lithostatic constraints, while simultaneously maintaining the magnitude of deformation required to predict the InSAR data. More precisely, the InSAR data are best predicted by an FEM simulating a rind viscosity of 7.5 × 10 16 Pa s and a mass flux of −4.2 × 10 9 kg/d from the magma reservoir. The shallow weak layer within the caldera provides a coeruption stress regime and neutral buoyancy horizon that support lateral magma propagation from the central magma reservoir to extrusion near the rim of the caldera.

Alaska↗
USGS2004

Geodetic and seismic constraints on some seismogenic zone processes in Costa Rica

New seismic and geodetic data from Costa Rica provide insight into seismogenic zone processes in Central America, where the Cocos and Caribbean plates converge. Seismic data are from combined land and ocean bottom deployments in the Nicoya peninsula in northern Costa Rica and near the Osa peninsula in southern Costa Rica. In Nicoya, inversion of GPS data suggests two locked patches centered at 14 ± 2 and 39 ± 6 km depth. Interplate microseismicity is concentrated in the more freely slipping intermediate zone, suggesting that small interseismic earthquakes may not accurately outline the updip limit of the seismogenic zone, the rupture zone for future large earthquakes, at least over the short (∼1 year) observation period. We also estimate northwest motion of a coastal “sliver block” at 8 ± 3 mm/yr, probably related to oblique convergence. In the Osa region to the south, convergence is orthogonal to the trench. Cocos-Caribbean relative motion is partitioned here, with ∼8 cm/yr on the Cocos-Panama block boundary (including a component of permanent shortening across the Fila Costeña fold and thrust belt) and ∼1 cm/yr on the Panama block–Caribbean boundary. The GPS data suggest that the Cocos plate–Panama block boundary is completely locked from ∼10–50 km depth. This large locked zone, as well as associated forearc and back-arc deformation, may be related to subduction of the shallow Cocos Ridge and/or younger lithosphere compared to Nicoya, with consequent higher coupling and compressive stress in the direction of plate convergence.

Nicoya Peninsula, Osa Peninsula↗
USGS2010

Aviation response to a widely dispersed volcanic ash and gas cloud from the August 2008 eruption of Kasatochi, Alaska, USA

The extensive volcanic cloud from Kasatochi's 2008 eruption caused widespread disruptions to aviation operations along Pacific oceanic, Canadian, and U.S. air routes. Based on aviation hazard warnings issued by the National Oceanic and Atmospheric Administration, U.S. Geological Survey, the Federal Aviation Administration, and Meteorological Service of Canada, air carriers largely avoided the volcanic cloud over a 5 day period by route modifications and flight cancellations. Comparison of time coincident GOES thermal infrared (TIR) data for ash detection with Ozone Monitoring Instrument (OMI) ultraviolet data for SO 2 detection shows congruent areas of ash and gas in the volcanic cloud in the 2 days following onset of ash production. After about 2.5 days, the area of SO 2 detected by OMI was more extensive than the area of ash indicated by TIR data, indicating significant ash depletion by fall out had occurred. Pilot reports of visible haze at cruise altitudes over Canada and the northern United States suggested that SO 2 gas had converted to sulfate aerosols. Uncertain about the hazard potential of the aging cloud, airlines coped by flying over, under, or around the observed haze layer. Samples from a nondamaging aircraft encounter with Kasatochi's nearly 3 day old cloud contained volcanic silicate particles, confirming that some fine ash is present in predominantly gas clouds. The aircraft's exposure to ash was insufficient to cause engine damage; however, slightly damaging encounters with volcanic clouds from eruptions of Reventador in 2002 and Hekla in 2000 indicate the possibility of lingering hazards associated with old and/or diffuse volcanic clouds.

Alaska↗
USGS2013

Seismic evidence for a slab tear at the Puerto Rico Trench

The fore-arc region of the northeast Caribbean plate north of Puerto Rico and the Virgin Islands has been the site of numerous seismic swarms since at least 1976. A 6 month deployment of five ocean bottom seismographs recorded two such tightly clustered swarms, along with additional events. Joint analyses of the ocean bottom seismographs and land-based seismic data reveal that the swarms are located at depths of 50–150 km. Focal mechanism solutions, found by jointly fitting P wave first-motion polarities and S/P amplitude ratios, indicate that the broadly distributed events outside the swarm generally have strike- and dip-slip mechanisms at depths of 50–100 km, while events at depths of 100–150 km have oblique mechanisms. A stress inversion reveals two distinct stress regimes: The slab segment east of 65°W longitude is dominated by trench-normal tensile stresses at shallower depths (50–100 km) and by trench-parallel tensile stresses at deeper depths (100–150 km), whereas the slab segment west of 65°W longitude has tensile stresses that are consistently trench normal throughout the depth range at which events were observed (50–100 km). The simple stress pattern in the western segment implies relatively straightforward subduction of an unimpeded slab, while the stress pattern observed in the eastern segment, shallow trench-normal tension and deeper trench-normal compression, is consistent with flexure of the slab due to rollback. These results support the hypothesis that the subducting North American plate is tearing at or near these swarms. The 35 year record of seismic swarms at this location and the recent increase in seismicity suggest that the tear is still propagating.

Puerto Rico↗
USGS2012

Mineralogy and geochemistry of a sediment‐hosted hydrothermal sulfide deposit from the Southern Trough of Guaymas Basin, Gulf of California

Samples dredged from a 15‐m‐high hydrothermal mound atop the flat turbidite pond in the Southern Trough of Guaymas Basin consist of pyrrhotite‐rich massive sulfide, barite, barite + calcite, talc, and opaline silica as well as substrate material composed of fossiliferous, clay‐rich ooze. An 11‐m‐long sediment core taken near the dredge site shows increasing hydrothermal alteration with depth; anhydrite‐filled fractures near the base of the core appear to be channels for hydrothermal discharge. Oxidation of the sulfide‐rich samples to an assemblage of geothite, lepidocrocite, and amorphous Fe oxyhydroxide is ubiquitous. Compared to other massive sulfide deposits on sediment‐starved oceanic ridges, the hydrothermal deposit dredged in Guaymas Basin has a high pyrrhotite/pyrite ratio, a low Zn sulfide and combined ore metal (Cu + Zn + Pb + Ag + Cd) content, and a greater abundance of sulfate, carbonate, and silicate phases. Venting hydrothermal solutions are alkaline with moderately high p H; high Ca, Ba, and SiO 2 content; low ƒS 2 and ƒo 2 ; and very low transition metal content. Disequilibrium assemblages of pyrrhotite and sulfate minerals form during rapid mixing of this evolved vent fluid with ambient bottom waters at the discharge site. Talc is formed at a temperature near 270°C by mixing or entrainment of Mg‐rich bottom water or pore fluid with upwelling hydrothermal fluid that is saturated with silica. Calcite may precipitate from the alkaline, Ca‐rich fluid during degassing of CO 2 . The minimum temperature range for sulfide and nonsulfide deposition is approximately 190°–326°C. The composition of hydrothermal deposits, vent solutions, and altered sediment requires that circulating fluids evolve during deep penetration into the basaltic basement complex, further interaction with the organic‐and carbonate‐rich sediment pile, and near‐surface mixing with ambient seawater. Although the stable assemblage albite‐epidote‐clinochlore present at depth in the sediment pile requires very low dissolved Mg and Fe in the altering fluid, the addition of Mg to deeply buried sediment indicates significant recharge of the system by Guaymas Basin bottom water.

Southern Trough of Guaymas Basin↗