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USGS2024

GRACE and GRACE Follow-On gravity observations of intermediate-depth earthquakes contrasted with those of shallow events

Earthquakes involve mass redistribution within the solid Earth and the ocean, and as a result, perturb the Earth's gravitational field. For most of the shallow (<60 km) earthquakes with M w > 8.0, the GRACE satellite gravity measurements suggest considerable volumetric disturbance of rocks. At a spatial scale of hundreds of km, the effect of volumetric change exceeds gravity change by vertical deformation; for example, negative gravity anomalies associated with volumetric expansion are characteristic patterns after shallow thrust events. In this study, however, we report contrasting observations of gravity change from two intermediate-depth (100–150 km) earthquakes of 2016 & 2017 Mw 8.0 (two combined) Papua New Guinea thrust faulting events and 2019 Mw 8.0 Peru normal faulting and highlight the importance of compressibility in earthquake deformation. The combined 2016/17 thrust events resulted in a positive gravity anomaly of 5–6 microGal around the epicenter, while the 2019 normal faulting produced a negative gravity anomaly of 3–4 microGal. Our modeling found that these gravity changes are manifestation of vertical deformation with limited volumetric change, distinct from gravity changes after the shallow earthquakes. The stronger resistance of rocks to volume change at intermediate-depth results in largely incompressible deformation and thus in a gravity change dominated by vertical deformation. In addition, malleable rocks under high pressure and temperature at depth facilitated substantial afterslip and/or fast viscoelastic relaxation causing additional vertical deformation and gravity change equivalent to the coseismic change. For the Papua New Guinea events, this means that postseismic relaxation enhanced coseismic uplift and relative sea level decrease.

Journal of Geophysical Research: Solid Earth↗
USGS2012

Strangways Crater, Northern Territory, Australia: Siderophile element enrichment and lithophile element fractionation

The Strangways Crater, Northern Territory, Australia (15°12′S, 133°35′ E ), has a central core, about 10 km in diameter, of shocked granitic gneiss and amphibolite, and some remnants of a melt rock sheet, surrounded by outer rings of quartzite and siltstone to a diameter of 20–25 km. Seven samples of melt rock (six granitic melts, one shale melt clast) and four samples of country rock (granitic gneiss, amphibolite, shale, quartzite) were analyzed by neutron-activation analysis: for Sc, Cr, Fe, Co, Zn, Rb, Zr, Sb, Cs, Ba, rare earth elements, Hf, Ta, Th, and U, the samples were analyzed instrumentally; and for Ni, Se, Pd, Ag, Cd, Re, Os, Ir, and Au, they were analyzed radiochemically. Siderophile elements are significantly enriched in the granitic melt rocks relative to country rocks; for example, the Ir enrichments range from 0.6 to 2.8 ppb. The low Ir/Ni ratio (∼0.16 relative to C1 chondrites) excludes a chondritic impacting body, and Cr enrichment argues against impact by an iron meteorite. The Strangways Crater may have been formed by the impact of an olivine-rich achondrite and melt rocks appear to contain about 3 wt.% of projectile material. The composition of the granitic melt rocks cannot be reproduced by any simple mixture of analyzed country rock types and chemical fractionation by selective shock melting appears to have taken place.

Journal of Geophysical Research Solid Earth↗
USGS2012

Secular, annual, and semiannual changes in the baseline level of the earth's magnetic field at North American locations

Secular, annual, and semiannual changes were obtained from the midnight values of field on specially determined days of quiet magnetospheric conditions. From a selection of North American observatories the latitude variation of these changes was evaluated from 20° to 65°. Results for the active and quiet sun years (1958 and 1965) were compared, and changes at Tucson were traced from 1958 through 1974. The year-to-year variance in the determination of a secular change seemed to be decreased by the present analysis method in comparison to a method which uses the annual mean of hourly values. The annual horizontal field variations were largely northward in early summer and southward in early winter; quiet year amplitudes of 2 to 3 γ about doubled in the solar active year. The amplitudes of the semiannual field vectors were about one half that of the annual vectors; the directions were apparently inconsistent from year to year.

Journal of Geophysical Research Solid Earth↗
USGS2012

Root zone of the late Proterozoic Salma caldera, northeastern Arabian Shield, Kingdom of Saudi Arabia

The eroded root of the late Proterozoic Salma caldera crops out in a striking, roughly elliptical feature, about 27 km long and 22 km wide, near the northeastern edge of the Arabian Shield. The caldera is genetically part of an elongate alkalic granitic massif (Jabal Salma) that extends 35 km from the caldera to the southwest. Comenditic ash flow tuff and lava(?) of the caldera fill, probably more than 1 km thick, are the oldest recognized rocks of the caldera complex. These rocks were erupted during caldera collapse associated with the rapid evacuation of the upper, mildly peralkalic part of a zoned magma reservoir. Within the caldera fill, a massive, lithic-rich intracaldera rhyolite, probably a lava in excess of 1 km thick, is overlain by a layered ash flow sequence. Numerous megabreccia blocks, probably derived from the caldera wall, occur in the massive rhyolite. Open folds in the layered volcanic rocks may be due to high-temperature slumping of the rocks toward the center of the caldera following collapse. Later peralkalic granite that intruded the caldera ring fracture zone occurs in an arcuate pattern outside the area of exposed caldera fill. After caldera collapse, metaluminous to peraluminous magma rose beneath the caldera at approximately 580 Ma and solidified as biotite alkali granite, rim syenogranite, and late, high-level granophyre. Rare earth element abundances indicate that the layered rhyolite tuff, peralkalic granite, and granophyre are chemically more evolved than the biotite alkali granite and rim syenogranite. The granophyre intruded the caldera fill as a dome-shaped body composed of numerous sheetlike masses. Granophyric texture resulted from rapid pressure release and quenching accompanying the intrusion of each sheet. Maximum penetration of the granophyre into overlying rocks occurred in the central region and along the west side of the caldera, where the caldera fill volcanic rocks have been removed by erosion. No apparent structural doming of the exposed volcanic rocks along the east side of the caldera took place; the layered ash flows commonly dip steeply toward the center of the caldera. Postemplacement deformation and metamorphism of the caldera are minimal. Small-displacement strike-slip faults cut the complex, which is tilted to the northeast by no more than about 2°.

Journal of Geophysical Research Solid Earth↗
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↗
USGS2012

The McKinley Sequence of granitic rocks: A key element in the accretionary history of southern Alaska

The McKinley sequence of granitic rocks consists of several discrete plutons in the central Alaska Range. Most of these plutons crop out south of the Denali fault system (DFS) in the Talkeetna quadrangle. Plutons of the McKinley sequence largely intrude deformed upper Meszoic flysch between the DFS and the northern edges of Wrangellia and the Peninsular terrane, which jointly make up the Talkeetna superterrane. The average K-Ar age of biotite from nine granites of the McKinley sequence is 57.3 Ma; Rb-Sr data for whole rock samples indicate that the McKinley sequence cannot be older than 60 Ma. A selected suite of 20 samples of granite and granodiorite range in SiO 2 from 65.9 to 77.6%. All 20 samples are corundum normative, and 18 are moderately peraluminous. Initial 87 Sr/ 86 Sr ratios range from 0.7054 to 0.7085. The σ 18 O values range from +11.2 to +14.6‰. These high and variable Sr isotopic ratios, peraluminous nature, rare earth element patterns, and high σ 18 O values suggest that granitic rocks of the McKinley sequence crystallized from hybrid magmas produced by assimilation of sedimentary rocks by a mantle-derived melt. Mesozoic flysch is the likely source of the crustal component of the hybrid magmas. Geologic evidence suggests that the Talkeetna superterrane collided with stable Alaska after Early Cretaceous time. The flysch basin, lying south of stable Alaska, was closed by northward movement of the Talkeetna superterrane; maximum age for basin closure and terrane accretion is middle Cretaceous (Cenomanian). Paleomagnetic evidence indicates that all terranes north of the DFS have been part of stable Alaska since the Paleocene and that northward movement of Wrangellia was completed by 50 Ma. Granitic rocks of the McKinley sequence may be products of terrane accretion; the granitic rocks crystallized from hybrid magmas produced during terrane collision and deformation of the flysch basin. Isotopic ages of the McKinley sequence establish the time of final accretion of the Talkeetna superterrane as Paleocene.

Journal of Geophysical Research Solid Earth↗
USGS2024

Physics-based forecasts of eruptive vent locations at calderas

Constraining stresses in the Earth's crust in volcanic regions is critical for understanding many mechanical processes related to eruptive activity. Dike pathways, in particular, are shaped by the orientation of principal stress axes. Therefore, accurate models of dike trajectories and future vent locations rely on accurate estimates of stresses in the subsurface. This work presents a framework for probabilistic constraint of the stress state of calderas by combining three-dimensional physics-based dike pathway models with observed past vent locations using a Monte Carlo approach. The retrieved stress state is then used to produce probability maps of future vent opening across a caldera. We test our stress inversion and vent forecast approach on synthetic scenarios, and find it successful depending on the distribution of the available vents and the complexity of the volcano's structural history. We explore the potential and limitations of the approach, show how its performance is sensitive to the assumptions in the models and available prior information, and discuss how it may be applied to real calderas.

Journal of Geophysical Research: Solid Earth↗
USGS2012

Nonlinear strain buildup and the earthquake cycle on the San Andreas fault

Two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault, accompanied by a progressive broadening of the zone of deformation adjacent to it. In the thin lithosphere model, transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere. For an earth model with a thick elastic lithosphere (plate thickness » depth of seismic slip), transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane. Data from the San Andreas fault indicate a long-term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle. Observations support a cycle-long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar. If so, the usually lower strain rates and broader deformation zone currently observed on the southern San Andreas represent a later evolutionary stage of the northern locked section, where a great earthquake is a more recent occurrence. Although the data allow some extreme models to be discarded, no sufficiently strong constraints exist to decide between the thin and thick lithosphere models. Regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time-predictable model.

Journal of Geophysical Research Solid Earth↗
USGS2012

Implications of regional gravity for state of stress in the earth's crust and upper mantle

Topography is maintained by stress differences within the earth. Depending on the distribution of the stress we classify the support as either local or regional compensation. In general, the stresses implied in a regional compensation scheme are an order of magnitude larger than those corresponding to local isostasy. Gravity anomalies, a measure of the earth's departure from hydrostatic equilibrium, can be used to distinguish between the two compensation mechanisms and thus to estimate the magnitude of deviatoric stress in the crust and upper mantle. Topography created at an oceanic ridge crest or in a major continental orogenic zone appears to be locally compensated. Such features were formed on weak crust incapable of maintaining stress differences much greater than the stress from the applied load. Oceanic volcanoes formed on an already cooled, thickened lithosphere are regionally supported with elastic stresses. Similarly, the broad topographic rise seaward of subduction zones is elastically supported as the lithosphere is bent near the plate margin. Although the implied stress is to some degree dependent on the rheological model assumed, the gravity anomalies and surface deformation produced by these features demonstrate that the upper 30–40 km of the oceanic lithosphere is capable of regionally supporting stress differences in the 100-MPa range. Given certain conditions of load emplacement, continental crust can also support loads regionally over 100-m.y. time scales, but the effects of erosion only allow an estimate of a lower bound on stress. Data from space probes indicate that the upper layers of other terrestrial planets also support topographic-induced stress differences in excess of 100 MPa.

Journal of Geophysical Research Solid Earth↗
USGS2012

Petrogenesis of garnet two-mica granites in the Ruby Mountains, Nevada

Garnet-two-mica granites of Jurassic (160 ± 3 m.y.) and Cretaceous (83 ± 1.3 m.y.) ages intrude amphibolite facies metasedimentary rocks of Precambrian age and lower Paleozoic sedimentary rocks in the northern part of the Ruby Mountains, Nevada. High initial 87 Sr/ 86 Sr and high values of δ 18 O for minerals suggest a dominantly sedimentary source for the magmas. Total rare-earth contents are low and less fractionated (Ce N /Yb N = 10) than granitic rocks of similar composition in the Sierra Nevada and show a negative Eu anomaly (Eu/Eu* = 0.57). These data are consistent with melting in the stability field of plagioclase with little contribution to the melt from refractory minerals such as zircon, sphene, and apatite. Minor euhedral garnets are manganese-rich and magnesium-poor (al 60 sp 34 py 3.6 gr 2.8 ) compared to garnet (al 79 sp 6 py 13 gr 2 ) in the intruded metamorphic rocks and show an increase in manganese and a decrease in calcium in the outer few microns. Fractionation of magnesium-iron between garnet and biotite, and of sodium-potassium between K-feldspar and plagioclase in the granites suggest submagmatic equilibration temperatures, in the range of 365° to 505°C, whereas oxygen isotope equilibration temperatures for two granites are higher at 480° and 570°C. The compositions of the granites plot near minimum melting compositions in the water-saturated quartz-albite-orthoclase system. Comparison of muscovite plus quartz stability, the water-saturated granite solidus, and experimental garnet-melt equilibria suggests pressures of crystallization no lower than about 3.5 kbar. This pressure is compatible with that estimated from garnet-plagioclase-sillimanite-quartz equilibria for the intruded metasedimentary rocks.

Nevada↗
USGS2012

Pillow basalts of the Angayucham terrane: Oceanic plateau and island crust accreted to the Brooks Range

The Angayucham Mountains (north margin of the Yukon-Koyukuk province) are made up of an imbricate stack of four to eight east-west trending, steeply dipping, fault slabs composed of Paleozoic (Devonian to Mississippean), Middle to Late Triassic, and Early Jurassic oceanic upper crustal rocks (pillow basalt, subordinate diabase, basaltic tuff, and radiolarian chert). Field relations and geochemical characteristics of the basaltic rocks suggest that the fault slabs were derived from an oceanic plateau or island setting and were emplaced onto the Brooks Range continental margin. The basalts are variably metamorphosed to prehnite-pumpellyite and low-greenschist facies. Major element analyses suggest that many are hypersthene-normative olivine tholeiites. Classification based on immobile trace elements confirms the tholeiitic character of most of the basalts but suggests that some had primary compositions transitional to alkali basalt. Although field and petrographic features of the basalts are similar, trace element characteristics allow definition of geographically distinct suites. A central outcrop belt along the crest of the mountains is made up of basalt with relatively flat rare earth element (REE) patterns. This belt is flanked to the north and south by LREE (light rare earth element)-enriched basalts. Radiolarian and conodont ages from interpillow and interlayered chert and limestone indicate that the central belt of basalts is Triassic in age, the southern belt is Jurassic in age, and the northern belt contains a mixture of Paleozoic and Mesozoic ages. Data for most of the basalts cluster in the “within-plate basalt” fields of trace element discriminant diagrams; none have trace-element characteristics of island arc basalt. The Triassic and Jurassic basalts are geochemically most akin to modern oceanic plateau and island basalts. Field evidence also favors an oceanic plateau or island setting. The great composite thickness of pillow basalt probably resulted from obduction faulting, but the lack of fault slabs of gabbro or peridotite suggests that obduction faults did not penetrate below oceanic layer 2, a likely occurrence if layer 2 were anomalously thick, as in the vicinity of an oceanic island. The presence of basaltic tuff interbeds indicates proximity to an explosive basaltic eruptive center. The juxtaposition of submarine basalts of differing chemical affinity and age, adjacent to higher-grade Paleozoic metamorphic rocks of the Brooks Range to the north, may be explained by obduction of internally complex (thickened) oceanic crust formed in an ocean plateau setting. Emplacement and rotation of thrust plates to steep attitudes occurred during accretion of the Brooks Range passive margin, probably beginning in the Late to Middle Jurassic.

Journal of Geophysical Research Solid Earth↗
USGS2012

Deuterium in interstitial water from deep-sea cores

As part of the Joint Oceanographic Institutions Deep Earth Sampling project, the interstitial waters of cores from 69 holes were sampled for deuterium analysis. Sixteen of the cores penetrated sediments as old as Eocene, and several sampled Cretaceous sediments, which allowed us to examine changes in the deuterium content of the oceans with time. Deuterium is shown to be a conservative constituent of the pore water. Its abundance in the pore fluids can be changed by diffusion, but the rate is slow, and corrections for this effect have been made. Changes in the abundance of deuterium can be related to changes in the amount of ice stored in continental glaciers, inasmuch as precipitation in the form of snow is highly depleted in deuterium compared with the oceans. Many of the cores show a change in isotopic composition of samples from early to late Miocene that can be ascribed to the buildup of the Antarctic ice sheets. After correcting for the role of diffusion in reducing the isotopic contrast between samples from a single core, we estimate an increase of 10 per mil (‰) δD (corresponding to a° δ 18 O change of about 1.2‰) between the early and late Miocene. A similiar analysis of Pleistocene to Holocene changes indicates a δD rise of 8‰ during the time of maximum continental ice, which corresponds to a δ 18 O increase of about 1.0‰. On the basis of limited data, we find no δD change in the oceans from Cretaceous to Miocene.

Journal of Geophysical Research Solid Earth↗
USGS2022

Insights on multistage rock avalanche behavior from runout modeling constrained by seismic inversions

Inversion of low-frequency regional seismic records to solve for a time series of bulk forces exerted on the earth by a landslide (a force-time function) is increasingly being used to infer volumes and dynamics of large, highly energetic landslides, such as rock avalanches and flowslides, and to provide calibration information on event dynamics and volumes for numerical landslide runout models. Much of the work to date using landslide runout modeling constrained by seismic data has focused on using single-phase models with frictional or velocity-weakening rheologies. Awareness of multistage landslide initiations is increasing, with discrete failures separated in time contributing to the final impact of an event. Our work utilizes a method for incorporating seismic data as a calibration constraint for landslide runout models, considering variable rheologies and different initiation conditions. This study presents a systematic examination of multiple rheologies and initiation conditions, and shows how these factors affect the force-time function derived from the landslide runout model. Our work confirms that, while rheology and fragmenting or initially coherent initiations affect the force-time function, multiple collapses separated by tens of seconds have the greatest impact on the shape and amplitude. We apply this method to the analysis of three real rock avalanches to better constrain plausible initiation conditions and rheology parameters using both seismic and field data. This study provides insights on how assumptions about the initiation dynamics of the source zone and the runout model definition can aid in the interpretation of seismic inversions for multistage rock avalanches.

Journal of Geophysical Research: Solid Earth↗
USGS2026

Constraining the permeability and outer-rise hydration at the Central America margin

Subduction zones are important drivers of Earth's geochemical evolution; the presence of water within the subduction system has substantial implications for geohazards, including the style of seismicity and the volume of magma generated at arc volcanoes. We utilize estimates of pore fluid distributions in the outer-rise region of the Central American margin imaged using controlled-source electro-magnetic surveys and novel two-dimensional forward models simulating poro-elastic deformation of slab bending to constrain the background permeability structure of the Cocos Plate and within outer-rise fault damage zones. The model results suggest that a relatively low background permeability structure is required to match the observed porosity at depths greater than 3 km (less than ~10 −21 m 2 . Similarly, increasing the permeability within the damage zones of outer-rise faults by more than a factor of 10 also over-predicts observed porosity distributions at depths greater than 3 km. However, the simulations consistently underpredict pore fluid distributions between depths of 1–3 km, which may reflect nonlinear elastic or anelastic deformation processes not included in the simulations or inherent uncertainty in the conversion of resistivity measurements to porosity distributions. The best-fitting depth-permeability relationship is relatively low, which has important implications for the timing of pore fluid release down-dip of the trench, as it restricts shallow fluid release, likely leading to higher pore fluid pressures, more widespread hydraulic fracturing, and deeper release of pore fluid along the plate interface.

Journal of Geophysical Research, Solid Earth↗
USGS2012

Minor and trace element geochemistry of volcanic rocks dredged from the Galapagos spreading center: Role of crystal fractionation and mantle heterogeneity

A wide range of rock types (abyssal tholeiite, Fe-Ti-rich basalt, andesite, and rhyodacite) were dredged from near 95°W and 85°W on the Galapagos spreading center. Computer modeling of major element compositions has shown that these rocks could be derived from common parental magmas by successive degrees of fractional crystallization. However, the P 2 O 5 /K 2 O ratio averages 0.83 at 95°W and 1.66 at 85°W and implies distinct mantle source compositions for the two areas. These source regions also have different rare earth element (REE) abundance patterns, with [La/Sm] EF = 0.67 at 95°W and 0.46 at 85°W. The sequence of fractionated lavas differs for the two areas and indicates earlier fractionation of apatite and titanomagnetite in the lavas from 95°W. The mantle source regions for these two areas are interpreted to be depleted in incompatible (and volatile?) elements, although the source region beneath 95°W is less severely depleted in La and K. Incompatible trace element abundances in 26 samples are used to infer that the range of Fe-Ti-rich basalt from 85°W represents 19 to 35% residual liquid following crystal fractionation of a mineral assemblage of plagioclase, clinopyroxene, and lesser olivine. The most highly differentiated samples have also had less than 1% titanomagnetite removed. Most samples from 85°W can be related to a common parental magma that contained approximately 9 wt % FeO*, 1 wt % TiO 2 , and had an Mg number (Mg# = 100 Mg/(Mg + Fe 2+ )) of about 65. Although the samples from 95°W cannot all be derived from a common parental magma, the inferred parental magmas may have been derived by varying degrees of partial melting of a common source. The fractionation sequence consists of two parts: an initial iron enrichment trend followed by a silica enrichment trend. We interpret the trace element data to indicate that the most iron rich lavas represent about 32% residual liquid derived by crystal fractionation of plagioclase, clinopyroxene, and lesser olivine from a parental magma with an Mg number of about 66. The silica enrichment trend results from crystallization of titanomagnetite and some apatite. Fractionation of pigeonite, which is a minor phase in the major element models, cannot be distinguished from clinopyroxene fractionation by using trace elements.

Journal of Geophysical Research Solid Earth↗
USGS2012

The nature of the crust in the Yukon-Koyukuk province as inferred from the chemical and isotopic composition of five Late Cretaceous to Early Tertiary volcanic fields in western Alaska

Late Cretaceous and early Tertiary volcanic and plutonic rocks in western Alaska comprise a vast magmatic province extending from the Alaska Range north to the Arctic Circle, south to Bristol Bay, and west to the Bering Sea Shelf. The chemical and isotopic composition of five of these Late Cretaceous to early Tertiary volcanic fields in the north central part of this province were studied to determine if Paleozoic or older continental crust underlies the Yukon-Koyukuk province. Three of the fields, the Blackburn Hills, Yukon River, and Kanuti, occur within the Yukon-Koyukuk province and two, the Sischu and Nowitna, overlie bordering Precambrian and Paleozoic metamorphic terranes to the southeast. High initial 87 Sr/ 86 Sr of 0.7075–0.7079 and moderate initial 143 Nd/ 144 Nd of 0.51244–0.51247 of rhyolite, dacite, and high-silica andesite of the Sischu volcanic field indicate that the magmas have interacted with the underlying Paleozoic or older continental crust. The relatively limited variation of isotopic (initial 87 Sr/ 86 Sr = 0.7044–0.7051; initial 143 Nd/ 144 Nd = 0.51256–0.51257) and elemental compositions of andesites from the Nowitna field can be accounted for by assimilation of small amounts of Paleozoic or older continental crust during crystal fractionation of andesite parent magmas at crustal levels. The Blackburn Hills field, which consists of medium-K basalt, andesite, and rhyolite intruded by a small granitic pluton, has a large range in initial 87 Sr/ 86 Sr and initial 143 Nd/ 144 Nd that plot in the field for 60 Ma mantle, from near mid-ocean ridge basalts to near “bulk-earth” compositions (initial 87 Sr/ 86 Sr = 0.7033–0.7052; initial 143 Nd/ 144 Nd = 0.51253–0.51290). Andesites and basalts from the Blackburn Hills are divided into two group on the basis of rare earth element (REE) and isotopic composition. Isotopic variation in the more primitive group 1 is best explained by assimilation of the lower crust of the Jurassic to Early Cretaceous Koyukuk terrane by mantle-derived basalts during crystal fractionation, though part of the isotopic variation may be due to metasomatism of an oceanic island basalt type mantle source by fluids derived from subducted sediments. Group 2 andesites from the Blackburn Hills have lower heavy REE abundances and more enriched isotopic compositions. These group 2 andesites and dacites from the Kanuti field, which have ( 87 Sr/ 86 Sr) i = 0.7043–0.7048 and ( 143 Nd/ 144 Nd) i = 0.51248–0.51267, appear to have formed by partial melting of the lower crust of the Koyukuk terrane. The Yukon River field consists of basalt, andesite, dacite, and rhyolite having ( 87 Sr/ 86 Sr) i = 0.7037–0.7051 and ( 143 Nd/ 144 Nd) i = 0.51266–0.51280; its isotopic composition does not require the presence of Paleozoic or older continental crust under the volcanic field and may have formed by interaction between mantle-derived melts and the oceanic Angayucham/Tozitna or island arc Koyukuk terrane. Most of the intrusive rocks and rhyolite domes from the Blackburn Hills volcanic field have ( 87 Sr/ 86 Sr) i = 0.7038–0.7041 and dacites from the Kanuti volcanic field have ( 87 Sr/ 86 Sr) i = 0.7043–0.7048. Thus little or no old continental crust was involved in the genesis of the Late Cretaceous and early Tertiary rocks and therefore probably does not extend beneath this part of the Yukon-Koyukuk province. However, the ultimate source of the small volumes of enriched shoshonitic andesite ( 87 Sr/ 86 Sr = 0.7075, 143 Nd/ 144 Nd = 0.5125) erupted at 118 Ma in the Yukon-Koyukuk province may be continental lithosphere, which may have been thrust under this part of the Yukon-Koyukuk province during arc-continent collision in the Early Cretaceous.

Journal of Geophysical Research Solid Earth↗
USGS2014

Source and progression of a submarine landslide and tsunami: The 1964 Great Alaska earthquake at Valdez

Like many subduction zone earthquakes, the deadliest aspects of the 1964 M = 9.2 Alaska earthquake were the tsunamis it caused. The worst of these were generated by local submarine landslides induced by the earthquake. These caused high runups, engulfing several coastal towns in Prince William Sound. In this paper, we study one of these cases in detail, the Port Valdez submarine landslide and tsunami. We combine eyewitness reports, preserved film, and careful posttsunami surveys with new geophysical data to inform numerical models for landslide tsunami generation. We review the series of events as recorded at Valdez old town and then determine the corresponding subsurface events that led to the tsunami. We build digital elevation models of part of the pretsunami and posttsunami fjord‐head delta. Comparing them reveals a ~1500 m long region that receded 150 m to the east, which we interpret as the primary delta landslide source. Multibeam imagery and high‐resolution seismic reflection data identify a ~400 m wide chute with hummocky deposits at its terminus, which may define the primary slide path. Using these elements we run hydrodynamic models of the landslide‐driven tsunamis that match observations of current direction, maximum inundation, and wave height at Valdez old town. We speculate that failure conditions at the delta front may have been influenced by manmade changes in drainage patterns as well as the fast retreat of Valdez and other glaciers during the past century.

Alaska↗
USGS2017

Partitioning of pyroclasts between ballistic transport and a convective plume: Kīlauea volcano, 19 March 2008

We describe the discrete ballistic and wind-advected products of a small, but exceptionally well-characterized, explosive eruption of wall-rock-derived pyroclasts from Kīlauea volcano on 19 March 2008 and, for the first time, integrate the size distribution of the two subpopulations to reconstruct the true size distribution of a population of pyroclasts as it exited from the vent. Based on thinning and fining relationships, the wind-advected fraction had a mass of 6.1 × 10 5 kg and a thickness half distance of 110 m, placing it at the bottom end of the magnitude and intensity spectra of pyroclastic falls. The ballistic population was mapped, in the field and by using structure-from-motion techniques, to a diameter of > 10–20 cm over an area of ~0.1 km 2 , with an estimated mass of 1 × 10 5 kg. Initial ejection velocities of 50–80 m/s were estimated from inversion of isopleths. The total grain size distribution was estimated by using a mass partitioning of 98% of wind-advected material and 2% of ballistics, resulting in median and sorting values of −1.7 ϕ and 3.1 ϕ . It is markedly broader than those calculated for the products of magmatic explosive eruptions, because the grain size of 19 March 2008 clast population is unrelated to a volcanic fragmentation event and instead was “inherited” from a population of talus clasts that temporary blocked the vent prior to the eruption. Despite a conspicuous near-field presence, the ballistic subpopulation has only a minor influence on the grain size distribution because of its rapid thinning and fining away from source.

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