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At least 199 records · Page 11Linked to original sources
USGS2012

Active tectonic and magmatic processes beneath Long Valley Caldera, eastern California: An overview

Geological, chronological, and structural studies of the Long Valley-Mono/Inyo Craters area document a long history of related volcanic eruptions and earthquakes controlled by regional extensional tectonics of the Basin and Range province. This activity has persisted for hundreds of thousands of years and is likely to continue. The Long Valley magma chamber had a volume approaching 3000 km 3 prior to its climatic caldera-forming eruption 0.7 m.y. ago but has been reduced to less than a third of this volume by cooling, eruption, and crystallization. Seismic evidence indicates that the main mass of the present Long Valley magma chamber is about 10 km in diameter and that its roof is 8–10 km deep with smaller cupolas as shallow as 4–5 km. Although a chamber of this size is probably capable of producing an eruption approaching 30 km 3 of lava, the record over the past 0.5 m.y. suggests that eruptions of 1 km 3 or less are far more likely. Models proposed for the current ground uplift and seismicity within the caldera require inflation of 0.1–0.2 km 3 by additional magma since mid-1979, and some models suggest that inflation was accompanied by injection of a thin dike or dikes (probably of silicic magma) into the ring fracture zone beneath the south moat. Several of the M 5.8–6.2 earthquakes that occurred in the region beginning in 1978 had non-double-couple focal mechanisms. Whether these unusual mechanisms indicate injection of mafic (low-viscosity) magma at midcrustal depths in the Sierra Nevada block south of the caldera remains debatable. Studies of calderas of various ages throughout the world indicate that episodes of unrest are relatively common and do not invariably culminate in eruptions. Although current unrest is concentrated in the south moat of Long Valley caldera, the Inyo/Mono Craters probably hold a greater potential for producing an eruption in the foreseeable future. The Inyo/Mono Craters have erupted at 500-year intervals over the past 2000–3000 years, whereas the Long Valley magma chamber has erupted at about 200,000-year intervals over the past 700,000 years. In either case, a major earthquake near the caldera could strongly influence the course of volcanic activity.

Journal of Geophysical Research Solid Earth↗
USGS2012

Evolution and present state of the hydrothermal system in Long Valley caldera

Evidence for previous periods of hydrothermal activity in Long Valley caldera exists in the form of extensive deposits of hydrothermal alteration products at several locations within the caldera and saline deposits in Searles Lake which contain mineral assemblages contributed by hot spring discharge from Long Valley. Hydrothermal activity was more intense in the past and probably involved fluid circulation to depths of several kilometers or more with heat supplied by the Long Valley magma chamber. During the past 40,000 years the heat source may have shifted to the Inyo-Mono magmatic system beneath the west moat, where deep fluid circulation supplied hot water to shallower zones of lateral flow within the Bishop Tuff beneath the resurgent dome. The present-day hydrothermal system in Long Valley appears to consist of two principal zones in which hot water flows laterally from west to east at depths of less than 1 km within and around the resurgent dome. Maximum measured temperatures within these zones are near 170°C, but estimates from chemical geothermometers and extrapolation of a high-temperature gradient measured in a recent drill hole indicate that a source reservoir at temperatures near 240° may exist at greater depths within the Bishop Tuff beneath the west moat. Regions possibly containing silicic melt detected by shear wave attenuation at depths of 4–5 km beneath the resurgent dome have probably not been in place long enough to influence sensibly the overlying thermal regime within the upper 2 km of caldera fill.

Journal of Geophysical Research Solid Earth↗
USGS2012

Links between magma-tectonic rate balances, plutonism, and volcanism

With Smith's (1979) model of ash flow magmatism as the point of departure, quantitative regimes of crustal magmatic evolution are outlined. The average extrusive regime of ash flow magmatism operates at a rate of about 0.001 km 3 /yr; progressively larger episodes of increasing repose times characterize the evolution of caldera-forming eruptions. The intrusive regime operates at an inferred average rate of about 0.01 km 3 /yr expressed in terms of the volumetric influx of mafic magma from the mantle, based on the record of silicic rates of extrusion and a 10:1 intrusive to extrusive ratio derived from evidence of zoned silicic systems. This rate is almost identical with the mean cumulative rate for the Hawaiian-Emperor system of basaltic volcanism, which is determined to be 0.015 km 3 /yr. Fluctuations about a mean rate of 10 −2 km 3 /yr lead to a threefold classification of magmatic regimes in terms of source power, depending on whether they are persistently greater than (HHER regime), about equal to (MHER regime), or lower than (LHER regime) the mean Hawaiian-Emperor rate. Ash flow magmatism correlates with the MHER regime, averaged over a sufficiently long time. Thus the conditions of Hawaiian-Emperor magmatism are identified with a characteristic power of magma transfer rates that optimize mechanisms for creation of high-level chambers of derivative and fractionated silicic magma (terminology of Smith (1979)) within sufficiently localized regions of high residence times. These conditions represent a balance point, and a bifurcation, in dynamic paths of magmatic evolution leading to widely divergent types of plutonic-volcanic associations. Categorical examples of the principal types of igneous path are discussed relative to continental basalt, silicic ash flow, batholithic granite, and intrusive porphyry systems. The Henry Mountains, Utah, are thought to represent an example of an intrusive porphyry generated in the LHER regime of a mafic source system lacking sufficient power for volcanic expression or significant mineralization. Long Valley, California (and possibly other systems along the eastern front of the Sierra Nevada), is an example operating at the mean rate with transient excursions to higher and lower rates. In regions of rapid extension and/or volcanic propagation (whether linear or areal) these regimes give rise to basaltic eruptions ranging from isolated cinder cones to flood basalts.

Journal of Geophysical Research Solid Earth↗
USGS2012

An ocean bottom seismometer study of shallow seismicity near the Mid- America Trench offshore Guatemala

Five ocean bottom seismometers recorded seismicity near the Mid-America Trench offshore Guatemala for 27 days in 1979. The array was emplaced in the lower slope region, just above the topographic trench, in the area investigated during Deep Sea Drilling Project legs 67 and 84. Approximately 170 events were recorded by three or more seismometers, and almost half were located with statistical hypocentral errors of less than 10 km. Most epicenters were located immediately landward of the trench axis, and many were further confined to a zone northwest of the array. In terms of depth, most events were located within the subducting Cocos plate rather than in the overlying plate or at the plate-plate boundary. Their apparent concentration in the lower crust and upper mantle may suggest that the upper crust does not have the strength to support earthquake-generating stresses. The data permit construction of a magnitude-duration scale, calibrated with m b magnitudes for events located by the World-Wide Standard Seismograph Network (WWSSN) and recorded by our array and by the network recording foreshocks and aftershocks of the 1979 Petatlan earthquake. Most magnitudes ranged between 3.0 and 4.0 m b , and the threshold magnitude of locatable events was about 2.8 m b . Two distinct composite focal mechanisms were determined. One appears to indicate high-angle reverse faulting in the subducting plate, in a plane parallel to trench axis strike. The other, constructed for some earthquakes in the zone northwest of the array, seems to show normal faulting along possible fault planes oriented quasi-perpendicular to the trench axis. The normal faulting is consistent with the segmentation of the Cocos plate that has been proposed from land evidence. Such segmentation might be evidenced offshore by normal faulting along planes subperpendicular to trench strike. Alternatively, the seismicity zone and associated normal faulting mechanism may be the subsurface expression of the tectonics responsible for the San Jose Canyon, a prominent submarine canyon located farther upslope. Finally, projection of our seismicity sample and of well-located WWSSN events from 1954 to 1980 onto a plane perpendicular to the trench axis shows a distinct gap between the shallow seismicity located by our array, and the deeper Wadati-Benioff zone seismicity located by the WWSSN. We tentatively ascribe this gap to inadequate sampling, but we suggest that it requires further investigation.

Journal of Geophysical Research Solid Earth↗
USGS2012

Reservoir processes and fluid origins in the Baca geothermal system, Valles Caldera, New Mexico

At the Baca geothermal field in the Valles caldera, New Mexico, 19 deep wells were drilled in an attempt to develop a 50-MW e (megawatts electric) power plant. The chemical and isotopic compositions of steam and water samples have been used to indicate uniquely the origin of reservoir fluids and natural reservoir processes. Two distinct reservoir fluids exist at Baca. These fluids originate from the same deep, high-temperature (335°C), saline (2500 mg/kg Cl) parent water but have had different histories during upflow. One fluid (from wells 4 and 13) is isotopically light, high in radiogenic noble gases, CO 2 and HCO 3 , and low in Ca. It has a temperature of 290°–295°C and a reservoir chloride near 1900 mg/kg. This fluid resulted from rapid upward flow through 1.1- to 1.4-m.y.-old Bandelier Tuff reservoir rocks after long residence in pre-Bandelier (>7 m.y.) sediments and Precambrian basement rocks and 25% dilution with high-altitude cold groundwater from Redondo Peak. The other water (from wells 15, 19, and 24) moved slowly through the Bandelier Tuff and cooled conductively (with minor steam loss for well 19) from 335°C to 280°–260°C. Apparently, short residence in old basement rocks has left this water with low radiogenic gases. Conductive cooling without mixing has kept the original chloride and relatively heavy isotope composition of the deep water. The recharge to the deep parent water is not well understood but may be from lower elevation precipitation outside the Valles caldera area. Gases are in equilibrium in all-liquid reservoir fluids at near reservoir temperatures, and the concentrations of atmospheric gases are similar to those of air-saturated water, indicating little boiling and steam loss. All water, solutes, and gases in the reservoir fluids originate from air-saturated meteoric recharge water, watermineral reactions, and rock leaching, with the possible exception of excess 3He that must have an ultimate mantle source. This gas could originate directly from magma or from leaching of intrusive volcanic rocks beneath the Bandelier.

Journal of Geophysical Research Solid Earth↗
USGS2012

Upper mantle structure from teleseismic P wave arrivals in Washington and northern Oregon

Teleseismic P wave travel time residuals are used to detect lateral velocity heterogeneities in the upper mantle beneath Washington and northern Oregon. The results of an inversion for three-dimensional velocity variations resolves an east dipping high-velocity zone that we interpret as the subducting Juan de Fuca plate. The plate is characterized by 3–8% higher velocities than those in the surrounding upper mantle. Inversion of the travel time data and ray trace modeling indicate that the plate extends to a depth of 200–300 km. The plate dips at a moderate angle of 45° to the east-northeast beneath the central Washington Cascade Range north of Mount Rainier, with 5% faster velocities than the surrounding upper mantle. Beneath the North Cascade Range of Washington, the plate strikes to the northwest and has 6–8% faster velocities than the upper mantle to the west. South of 47°N, beneath the Cascade Range in southern Washington and northern Oregon, the plate dips steeply to the east and has 3–4% faster velocities than the surrounding upper mantle. Based on changes in the geometry and velocity structure of the subducted Juan de Fuca plate east of about 123°W, we propose that the subducted slab is segmented into three sections beneath Washington and northern Oregon.

Journal of Geophysical Research Solid Earth↗
USGS2012

Use of focal mechanisms to determine stress: A control study.

To allow focal mechanisms to be inverted for the stress field requires a different inversion algorithm than for slickenside data because focal mechanisms do not represent fault slip data unless one can decide which nodal plane is the fault plane. If one can decide which nodal plane is the fault plane, then the focal mechanisms can be inverted with the slickenside inversion algorithm. This decision cannot always be made, so algorithms for inverting focal mechanisms for the stress field are studied. These algorithms either use both of the possible fault planes or attempt to choose the correct fault plane while determining the stress tensor. Simulated focal mechanisms are made from slickenside data and used to provide a control study for the focal mechanism inversion algorithms. The results of this control study show that focal mechanisms can be inverted to find the best stress tensor, but the resolution is decreased unless the fault planes can be picked a priori. The resolution can also be increased by including constraints on the magnitude of the tangential traction on the fault plane. Therefore, using focal mechanisms to study small variations in the stress field requires that other data (e.g., studies of the hypocenters, surface faulting, or structural information concerning the region) be introduced to pick which of the nodal planes is the fault plane. This study also introduces the method of bootstrap resampling to the statistics of this problem. The non-Gaussian nature of the data makes the nonparametric formulation of the bootstrap approach ideal for this problem.

Journal of Geophysical Research Solid Earth↗
USGS2012

Usefulness of weak bands in midinfrared remote sensing of particulate planetary surfaces

Midinfrared (2.5–25 μm) reflectance spectra of minerals are often used to predict emittance qualitatively. These spectra display weak overtone and combination tone bands, which may be as diagnostic of composition as the strong fundamental molecular vibration bands usually considered for remote sensing applications, but which have been widely ignored. However, unlike the strong bands, the contrast of weak bands relative to the continuum in the midinfrared usually does not decrease with decreasing particle size, but typically increases. To illustrate this behavior, transmittance and reflectance spectra of calcite and quartz are presented for the wavelength range from 4000 to 400 cm −1 (2.5–25 μm). It is the purpose of this paper to point out that these weak bands are potentially useful for compositional remote sensing of particulate planetary surfaces; this will require completion of supporting laboratory studies to document the occurrence of weak bands.

Journal of Geophysical Research Solid Earth↗
USGS2012

Age of tilted reefs, Hawaii

Submerged carbonate reefs are preserved as a series of submarine terraces between Molokai and Hawaii along a 200-km span of the southeastern Hawaiian Ridge. Limestones from two of the terraces have been sampled from submersibles and dated radiometrically at 13 and 120 ka. Recognition that the terraces are tilted permits assignment of about a dozen terraces from 150 to 1300 m depth to eight general reef platforms. These reefs were drowned by the combined effects of island subsidence and sea level rise at the end of successive glacial stages from 13 to 647 ka. The platforms are tilted 5 m/km southeast toward the locus of volcanic loading centered on the island of Hawaii. The end of the active period of volcanic shield building is represented on each nonactive volcano by a submerged change in slope. Dating of this slope change by reference to the dated reefs provides independent evidence that shield building ended at about the same time that the volcano changed from eruption of tholeiitic to alkalic basalt. The variation in depth of the dated tilted reefs along the ridge indicates that the end of subsidence follows the end of shield building by about one half million years. The present northwest margin of major subsidence probably crosses southeast Maui. During the last half million years the rate of southeast migration of the beginning of both subsidence and of tholeiitic volcanism does not appear to have increased. However, the rate of southeast migration of the end of both subsidence and of tholeiitic volcanism has apparently increased. This shortening of the life history of the younger volcanoes implies an increase in eruption rates.

Journal of Geophysical Research Solid Earth↗
USGS2012

Global petrologic variations on the moon: A ternary-diagram approach.

A ternary-diagram approach for determination of global petrologic variations on the lunar surface is presented that incorporates valuable improvements in our previous method of using geochemical variation diagrams. This new approach uses a ternary diagram that is subdivided into equally spaced segments along each of its three sides and has a triangular area in its center. The segments are assigned distinct colors that cover the range of the visible spectrum. The apexes are assigned the three primary colors, and the center triangle, which represents equal proportions of the three apexes, is assigned the color gray. The apexes are assigned the average Fe and (Th/Ti) c (ratio normalized to chondrites) compositions of KREEP/Mg-suite rocks, mare basalts, and ferroan anorthosites. The composition of each picture element (pixel) within these Apollo orbital gamma ray databases is used to determine its position within the ternary diagram. The color corresponding to this ternary position is then placed on a classification map at that pixel's position within the orbital databases. Error databases produced for the Fe and (Th/Ti) c data were used to exclude pixels having high errors. The resultant classification map shows spatial transitions among petrologic units, allows direct determination of the relative proportions of each end-member composition in a pixel, and increases the geologic interpretability of these data over that of the individual elemental databases. The classification units correspond remarkably well to observed geologic units, when we consider the spatial resolution (100 km) of the gamma ray spectrometer. Our results are as follows: (1) The highlands contain large areas of relatively pure ferroan anorthosite. (2) The average composition of the upper lunar crust is represented by an “anorthositic gabbro” composition, which supports the “magma-ocean” hypothesis for lunar crustal genesis. (3) KREEP/Mg-suite rocks are a minor fraction of the upper lunar crust. (4) Within the farside highlands, areas of KREEP/Mg-suite rocks coincide mostly with areas of crustal thinning, which are probably areas of KREEP basalt extrusions or localized excavations of Mg-suite rocks or KREEP-rich rocks. (5) Portions of the east limb and farside highlands have considerable amounts of a mafic, chondritic Th/Ti component (like mare basalt) whose occurrences coincide with mapped concentrations of light plains that display dark-halo craters; the presence of this component supports the hypothesis that mare volcanism had occurred within this highland region before the end of final heavy bombardment.

Journal of Geophysical Research Solid Earth↗
USGS2012

Character and distribution of borehole breakouts and their relationship to in situ stresses in deep Columbia River basalts

The character and distribution of borehole breakouts in deeply buried basalts at the Hanford Site in south central Washington State are examined in light of stress indicator data and hydraulic-fracturing stress data by means of acoustic televiewer and acoustic waveform logging systems. A series of boreholes penetrating the Grande Ronde Basalt of the Columbia River Basalt Group were logged to examine the extent of breakouts at depths near 1000 m. Breakouts occur discontinuously throughout the interiors of most flows. In some boreholes the distribution of borehole wall breakouts closely correlates with the incidence of core disking. Differences in the distribution of breakouts and disking are attributed to differences in failure mechanisms. A thin interval of breakout-free basalt occurs near the upper and lower limits of flow interiors, with many intervals of breakouts terminating at the intersection of oblique fractures with the borehole. Hydraulic-fracturing stress measurement results obtained from four deep boreholes indicate anisotropic horizontal principal stresses, with maximum principal stress along an approximate north trending axis, consistent with the east trending orientation of breakouts. Acoustic waveform logs indicate that there is no measurable difference between the seismic properties of breakout-free flow tops and flow interiors. The highly coherent waveforms obtained in almost all flow interiors indicate that damage to the borehole wall associated with breakout formation remains confined to the thin annulus of stress concentration.

Journal of Geophysical Research Solid Earth↗
USGS2012

On predicting changes in the geomagnetic field

The present method of using constant secular variation rates to forecast magnetic components at a given site or to forecast spherical harmonic coefficients is known to be inaccurate. A new predictive method using trend and trigonometric functions fitted to known past values is used to extrapolate for a few years into the future. This provides an improvement over the usual linear extrapolation method. This method applied to the spherical harmonic coefficients (SHC) in the series of International Geomagnetic Reference Fields (IGRF) provides an analytic function for each SHC so that charts can be compiled for a few years in the future without using the idea of a constant secular variation. In this case, the trend and trigonometric functions utilize the same number of Fourier and trend coefficients as are now present in the IGRF models on which they are based so that they can completely replace the IGRF information.

Journal of Geophysical Research Solid Earth↗
USGS2012

Reflections from midcrustal rocks within the Mesozoic subduction complex near the eastern Aleutian Trench

Seismic reflection data collected in 1973 by Western Geophysical Company show that highly reflective rocks make up the midcrust of the convergent margin adjacent to the eastern Aleutian Trench. These rocks form an arch that strikes obliquely across the strongly expressed northeast-southwest structural grain of exposed Mesozoic rocks. In an earlier report we proposed that the deep events mark the location of either the presently subducting plate or the top of underplated rocks. However, the short radius of curvature of this arch precludes the possibility that the deep events indicate the igneous oceanic crust. Instead, the deep reflections could be from underplated rocks that have been arched by the imbrication or underplating of strata below the reflective rocks. High-amplitude water layer and interbed multiples prevent precise connection of shallow and deep geology. For example, the Border Ranges fault, a suture between major tectonostratigraphic terranes, is not revealed on any of our seismic sections. We speculate, however, that one band of reflections that rises toward but does not reach the surface is from the Eagle River thrust fault, which separates Late Cretaceous melange from deformed turbidite sequences of the same age.

Journal of Geophysical Research Solid Earth↗
USGS2012

Stress rotation during the Coalinga aftershock sequence

This study considers spatial and temporal changes of the stress regime during the 1983 Coalinga aftershock sequence. In both cases the observed changes manifest themselves as rotations of the most compressive stress axis. Inversion of 165 M ≥3 aftershocks shows that in the southern aftershock zone the azimuth of the most compressional stress axis is oriented 24°E of north, while in the northern and central zones it is oriented 51°E of north. Statistical analysis by use of non-parametric bootstrap resampling shows that this rotation is significant at above the 95% confidence level. Inverting the focal mechanisms of 122 M ≥3 aftershocks in the central and northern aftershock zones reveals a rotation of the principal stress axes during the first 500 days after the main shock. The compressional axis rotates from an azimuth of 62°E of north to 47°E of north. Statistical analysis by use of nonparametric bootstrap resampling shows that this rotation is just significantly different from zero at the 95% confidence level, subject to the assumption that 80% of the fault planes can be correctly picked from the two nodal planes. As the number of fault planes picked correctly varies from 50% (random choices) to 100% the significance of the rotation varies from 87% to 98%. Further investigation fails to find the signature of the stress changes predicted by dislocation theory in the focal mechanisms of the aftershocks. Active folds in the area suggest that the pre-main shock stress regime had a compressional axis with a NE-SW orientation. Thus the rotation during the aftershock sequence is thought to be a rebound from a stress anomaly induced by the main shock. This stress anomaly may be causally related to the occurrence of the aftershocks.

Journal of Geophysical Research Solid Earth↗
USGS2012

Columbia Glacier, Alaska: Changes in velocity 1977-1986

The Columbia Glacier, a grounded, iceberg-calving tidewater glacier near Valdez, Alaska, began to retreat about 1977. Drastic retreat occurred in 1984, and by early 1986, retreat amounted to 2 km. The glacier has thinned more than 100 m since 1974 at a point 4 km behind the 1974 terminus position. Between 1977 and 1985 the lower glacier ice velocity increased from 3–8 m/d to 10–15 m/d. Ice velocity in the region 0.5 km above the terminus was highest near the time the glacier was most receded (late fall), and lowest near the time of maximum length (early summer), for years 1977–1982. Velocity in the region 52–57 from the head of the glacier was highest in mid-spring, and lowest in early fall from 1977 to 1985. Through the years 1983–1985, the dates of maximum and minimum velocities within 0.5 km of the receding terminus tended toward the dates of the 52–57 km maximum and minimums. This occurred because as the terminus receded, it was no longer strongly influenced by the reverse slope of the terminal moraine shoal. Velocities near the terminus fluctuated by 2–3 m/d during summer and fall, when liquid water input was variable, and were relatively constant during winter. Hourly variations in ice velocities are controlled by liquid water input to the glacier hydraulic system and tide stage. Velocity increases near periods of high surface water input and decreases during periods of high tide as a result of hydrostatic back pressure.

Journal of Geophysical Research Solid Earth↗
USGS2012

Analysis of time series of glacier speed: Columbia Glacier, Alaska

During the summers of 1984 and 1985, laser measurements were made of the distance from a reference location to markers on the surface of the lower reach of Columbia Glacier, Alaska. The data have numerous gaps, mostly because of inclement weather. The laser measurements were corrected for variations in atmospheric temperature and pressure and for the trajectory of the marker. The marker speed was calculated by using a combination of cubic splines and digital filters to arrive at a data set with 1-hour time intervals. The speed varies from 7 to 15 m/d and has three noteworthy components: (1) a low-frequency perturbation in speed with a time scale of days related to increased precipitation, (2) semidiurnal and diurnal variations related to sea tides, and (3) diurnal variations related to glacier surface melt. The low frequency and tidal period variations are separated by low-pass filtering and subtracting the filtered data from the original data. The variations from melt runoff and tides have the same frequency range and are examined using harmonic analysis and multiple regression with the various forcing functions. The high-frequency portion of the ice speed signal is dominated by tidal influences and to a lesser extent by meltwater influences. The low-frequency portion of the signal is dominated by the effects of precipitation.

Journal of Geophysical Research Solid Earth↗
USGS2012

Gravity anomaly at a Pleistocene lake bed in NW Alaska interpreted by analogy with Greenland's Lake Taserssauq and its floating ice tongue

A possible example of a very deep glacial excavation is provided by a distinctive gravity low located at the front of a valley glacier that once flowed into glacial Lake Aniuk (formerly Lake Noatak) in the western Brooks Range. Geologic and geophysical data suggest that sediments or ice filling a glacially excavated valley are the most probable cause of the 30–50 mGal anomaly. Reasonable choices of geometric models and density contrasts indicate that the former excavation is now filled with a buried-ice thickness of 700 m or sediment thicknesses greater than 1 km; comparable anomalies are not known for other glaciated lacustrine valleys. However, many fiords do exceed 1 km in depth, and Crary found one nearly 2 km deep in Antarctica. In studying this fiord, he suggested the probable increased efficiency of excavation directly behind the point where an outlet glacier becomes afloat to form the Ross Ice Shelf and where it thus has a vertical component of motion and a mechanism for debris removal. Floating glacier ice tongues are now rare in the Arctic, but they exist in maritime parts of northern Ellesmere Island and Greenland. Studies of ice movement, environment, and morphology of another large floating glacier tongue in a perennially frozen lake in the Angiussaq Mountains of northern Greenland suggest that Pleistocene Lake Aniuk could have had a similar environment, water temperature, and near-stable water level and that it could have maintained both a floating polar glacier tongue and a perennial ice cover. No direct evidence of efficient excavation was observed in Greenland, but efficient glacial erosion behind a floating polar ice tongue could explain the excavation that caused the Alaskan gravity anomaly.

Journal of Geophysical Research Solid Earth↗
USGS2012

Analysis of two-color geodimeter measurements of deformation within the Long Valley caldera: June 1983 to October 1985

Line length changes from several baselines in a trilateration network within the Long Valley caldera clearly define a decrease in strain rate from June 1983 through October 1985. The data consist of more than 1600 length measurements on 23 baselines using a two-color geodimeter, which has a precision of 0.2 ppm of the line length. Initial measurements made during the summer of 1983 show extension rates were as high as 6 ppm/yr on some baselines. The rates subsequently decreased steadily, to less than 1 ppm/yr during the summer of 1985. Ih contrast, the strain rates on a few other baselines were less than 0.2 ppm/yr averaged over the 2.3-year interval. A model is constructed using these observations as well as yearly observations of a trilateration network within and near the caldera. The model contains two points of inflation located at 5 and 10 km beneath the resurgent dome of the caldera plus dextral slip on a fault plane within the south moat within an elastic half-space. To mimic the observed time dependence in the two-color geodimeter data, we conclude that the rates of both slip and inflation have steadily decreased over the survey interval. Because the two-color geodimeter network is closer to all three sources than the single-color geodimeter network, these data are important for defining the factor of 2–3 rate decrease of slip and inflation. Comparing our estimates of slip and inflation with similar estimates for the year prior to July 1983, we conclude that the rate decrease is probably a result of postseismic and postinflation relaxation following the earthquake swarm in the south moat in January 1983.

Journal of Geophysical Research Solid Earth↗