bluuebunny/crossref_metadata_2025 huggingface dataset:
Geoid and depth anomalies over ocean swells and troughs: Evidence of an increasing trend of the geoid to depth ratio with age of plate
Authors: Cazenave Anny , Dominh Kien , Rabinowicz Michel , Ceuleneer Georges
Published: February 2008

Medium‐wavelength (1000–4000 km) geoid and depth anomalies are positively correlated over a number of oceanic features. We have computed the geoid to topography ratio (or admittance) over 28 areas covering midplate swells, deep‐sea troughs, and aseismic plateaus after correcting the data for lithospheric cooling effects and for sediment loading. The observed admittances range from ∼ 1 to ∼ 5 m km−1 depending upon the area considered. Aseismic plateaus have a low admittance, consistent with Airy compensation by crustal thicknening. Larger admittances are observed above mid‐ocean swells and ocean deeps. For these latter features a linearly increasing trend with the square root of plate age of is observed. Linear regression indicates an admittance increase of 0.53 m km−1 per (m.y.)1/2. Observed admittances are consistent with dynamic support by convective upwellings and downwellings, provided that a shallow, low‐viscosity layer exists beneath the lithosphere. According to recent convection modeling, the observed admittance increase with age may be understood either in terms of viscosity, increase of the low‐viscosity layer, from ∼ 10−2 at mid‐ocean ridges to ∼ 1 in old basins (mean upper mantle viscosity taken as reference) or in terms of thickness increase of the lithosphere at the expense of a low‐viscosity layer of constant viscosity.
Variations of elastic plate thickness at continental thrust belts
Authors: McNutt M. K. , Diament M. , Kogan M. G.
Published: February 2008

After examining 15 reliable estimates of the elastic thickness of the lithosphere where it underthrusts mountain belts in Europe, Asia, and the Americas, we conclude that the strength of continental lithosphere as a function of depth must be very different from that of oceanic lithosphere. Depending on the state of stress, continental lithosphere may be either very much stronger or very much weaker than would be predicted for oceanic lithosphere of the same thermal age. We explain this observation as the result of two competing effects: a thicker thermal plate beneath the continents which allows a deeper elastic/ductile transition for plates more than 100 million years old, and a low activation energy for ductile creep in the continental crust which leads to massive failure of the upper elastic lithosphere at high bending stresses. The 100+ ‐km values for elastic plate thickness of billion‐year‐old lithosphere require a thermal plate thickness of at least 250 km. The magnitude of apparent thinning of the elastic plate at high stress is consistent with that calculated from a simple yield envelope which includes a ductile zone in the lower crust. The low yield strength of continental materials leads to correlations between effective elastic plate thickness and all of the following: dip of the underthrust plate; radius of curvature of the thrust front in map view; and total length of the mountain belt. Using insight derived from laboratory experiments on the buckling of spherical shells and drawing analogies with the geophysical characteristics of oceanic subduction zones, we arrive at the following sequence of causes and effects. The primary variable is the dip of the downgoing plate, presumably prescribed by its own excess mass and pressure exerted by dynamic flow in the mantle. Where this dip is steep, the rigidity of the plate is weakened due to inelastic yielding. The weakened plate can then buckle at small wavelengths into short, arcuate mountain belts. Thus we believe that the mantle ultimately controls the pattern of surface tectonics, but that the nonelastic rheology of the lithosphere is the mechanism by which that control is imposed.
Transform fault earthquakes in the North Atlantic: Source mechanisms and depth of faulting
Authors: Bergman Eric A. , Solomon Sean C.
Published: February 2008

We have determined the centroid depths and source mechanisms of 12 large earthquakes on transform faults of the northern Mid‐Atlantic Ridge from an inversion of long‐period body waveforms. The earthquakes occurred on the Gibbs, Oceanographer, Hayes, Kane, 15°20′, and Vema transforms. We have also estimated the depth extent of faulting during each earthquake from the centroid depth and the fault width. For five of the transforms, earthquake centroid depths lie in the range 7–10 km beneath the seafloor, and the maximum depth of seismic faulting is 14–20 km. On the basis of a comparison with a simple thermal model for transform faults, this maximum depth of seismic behavior corresponds to a nominal temperature of 900° ± 100°C. In contrast, the nominal temperature limiting the maximum depth of faulting during oceanic intraplate earthquakes with strike‐slip mechanisms is 700° ± 100°C. The difference in these limiting temperatures may be attributed to the different strain rates characterizing intraplate and transform fault environments. Three large earthquakes on the 15°20′ transform have shallower centroid depths of 4–5 km and a maximum depth of seismic faulting of 10 km, corresponding to a limiting temperature of 600°C. The shallower extent of seismic behavior along the 15°20′ transform may be related to a recent episode of extension across the transform associated with the northward migration of the triple junction among North American, South American, and African plates to its present position near the transform. The source mechanisms for all events in this study display the strike‐slip motion expected for transform fault earthquakes; slip vector azimuths agree to within 2°–3° of the local strike of the zone of active faulting. The only anomalies in mechanism were for two earthquakes near the western end of the Vema transform which occurred on significantly nonvertical fault planes. Secondary faulting, occurring either precursory to or near the end of the main episode of strike‐slip rupture, was observed for five of the 12 earthquakes. For three events the secondary faulting was characterized by reverse motion on fault planes striking oblique to the trend of the transform. In all three cases the site of secondary reverse faulting is near a compressional jog in the current trace of the active transform fault zone. We find no evidence to support the conclusions of Engeln, Wiens, and Stein that oceanic transform faults in general are either hotter than expected from simple thermal models or weaker than normal oceanic lithosphere.
Effect of borehole deviation on breakout orientations
Authors: Mastin Larry
Published: February 2008

Well bore breakouts are zones of spalling and fracture that form on opposite sides of a well bore and tend to change the cross‐sectional shape of the borehole from circular to roughly elliptical. In vertical holes drilled in areas where one principal stress (Sv) is vertical, breakouts tend to form at opposite ends of the borehole diameter parallel to the least compressive horizontal principal stress direction (Sh). This paper uses an analytical elastic solution for stress at the wall of a borehole to analyze the rotation of breakout orientations away from the direction of Sh as the borehole deviates from the direction of the vertical principal stress. The calculated orientations of breakouts in deviated boreholes depend on the type of faulting regime in which the well was drilled (i.e., normal, strike‐slip, or thrust), on the deviation angle ø of the borehole axis from vertical, on the angle θ between the horizontal projection of the borehole axis and the direction of Sh, and on the relative magnitudes of the three principal stresses (Sh; SH, the greatest compressive horizontal stress; and Sv). In the strike‐slip faulting regime, regardless of the values of θ, Sv, SH, and Sh, a borehole must deviate at least 35° from vertical before the horizontal projection of the breakout orientation differs by more than 10° from Sh. In the normal and thrust faulting regimes, however, the borehole deviation angle øcrit required to rotate projected breakout orientations by 10° from Sh approaches zero as the value of Sh approaches that of SH. In the thrust faulting regime, only about 3% of all combinations of θ, Sv, SH, and Sh will produce øcrit values less than 10°, while about 12% of all such combinations will yield øcrit < 10° in the normal faulting regime. About 12% and 33% of such combinations will yield øcrit < 20° in the thrust and normal faulting regimes, respectively. Careful study of changes in breakout orientation as a function of borehole deviation may improve the resolution of inferred stress directions when studying breakouts in deviated boreholes in these two faulting regimes.
Water ice phases II, III, and V: Plastic deformation and phase relationships
Authors: Durham W. B. , Kirby S. H. , Heard H. C. , Stern L. A. , Boro C. O.
Published: February 2008

Several of the high‐pressure crystalline forms of ice exist in the interiors of the larger icy moons of the outer planets. Guided by the phase relations established by Bridgman 75 years ago, we have transformed ordinary water ice I to ices II, III, V, and perhaps VI. We subsequently deformed those phases in a gas deformation apparatus designed for these experiments. We found ice II to be the strongest of the high‐pressure phases, having a strength roughly comparable to that of ice I. Ice III is very weak: at fixed levels of stress, ice III flows 102‐103 times faster than ice II. We confirm Bridgman's (1912) findings that ices III and V can exist metastably within the ice II field and, in addition, find that these phases may be deformed plastically within much of the metastable region without reverting to ice II. The most persistent metastable existence of ice III occurs in the warmer portions of the ice II field, through which historical and present‐day temperature‐pressure profiles in the icy moons pass. Thus, although the stability field of ice III is small, the weakness of this phase may have profoundly influenced the evolution and present‐day behavior of the icy moons.
A Model for transitional field geometries involving low‐order zonals and drifting nondipole harmonics
Authors: Weeks R. J. , Fuller M. , Williams I.
Published: February 2008

It is now clear that the transitional fields during reversals are not dominated by dipole fields nor are they purely zonal harmonics. Zonal fields generate paleomagnetic reversal records in which inclination reverses by a gradual change through a vertical inclination and the declination instantaneously switches from 0 to 180° or vice versa. In terms of VGP paths these directions define either a “near sided” path, in which the pole passes through the site, or a “far sided” path, in which the pole passes through the sites antipode. Although some records approximating this character have been found, other types of records reveal that the transitional field cannot be purely zonal. An earlier zonal model (Williams and Fuller, 1981) has been modified to include a drifting nondipole field and is successful in simulating many features of the paleomagnetic reversal records. A prediction of this model is that paleomagnetic reversal records at particular sites depend upon the phase relation between the decay and reversal of the low order harmonics and the passage of drifting anomalies of the nondipole field. Therefore, records obtained from different latitudes and longitudes will have quite different appearances. Only by looking at multiple records of a single reversal will the nature of the field become apparent. The model is nonunique, but is, at least, consistent with the data for the Matuayama/Brunhes transition. We suggest that it supports the idea that zonal harmonics dominate transition fields, but they are accompanied by secular variation which may involve drifting nondipole fields.
Evidence for a new geomagnetic reversal from Lava flows in Idaho: Discussion of short polarity reversals in the Brunhes and late Matuyama polarity chrons
Authors: Champion Duane E. , Lanphere Marvin A. , Kuntz Mel A.
Published: February 2008

K‐Ar ages and paleomagnetic data for basalt samples from a new core hole (site E) at the Idaho National Engineering Laboratory (INEL) indicate that the age of the reversed polarity event recorded in Snake River Plain lavas is older than 465±50 ka (1000 years before present) reported previously by Champion et al. (1981). Nine basalt flows, eight with normal polarity and one with reversed polarity, were recognized in the site E core hole. The flows above and below the reversed flow have ages of 491±80 ka and 580±93 ka, respectively. The inclination of the paleomagnetic field direction of the reversed flow at site E agrees with the inclination of reversed flows elsewhere at INEL which have an age of 565±14 ka. These reversed flows were previously thought to be correlative with the Emperor event. We suggest that this polarity event is an older event which we name the Big Lost Reversed Polarity Subchronozone and Subchron. A review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the Brunhes and two besides the Jaramillo in the late Matuyama. These 10 short subchrons begin to indicate the many short events that Cox (1968) hypothesized must exist if polarity interval lengths have a Poisson distribution. These events are true subchrons, not excursions, and may or may not have low associated paleointensities, although low field strengths might explain why the reversal process aborts. The mean sustained polarity interval length since late Matuyama Chron time is 90,000 years. The similarity of this number with the 105‐year period of the Earth's orbital eccentricity suggests anew that linkage between geomagnetic, paleoclimatic, and possible underlying Earth orbital parameters should be evaluated.
Tectonic history of the Juan de Fuca Ridge over the last 40 million years
Authors: Wilson Douglas S.
Published: February 2008

Interpretation of a new compilation of magnetic anomaly data for north of the Mendocino fracture zone and younger than chron 15 (39 Ma) indicates a transition from a pattern of stable transform faults to a pattern of persistent ridge propagation. Reorganizations of the propagation pattern occurred at 30 Ma in response to a counterclockwise change in direction of relative motion, and at 19 Ma in response to a clock‐wise change. In addition, there are significant changes in the gradient of the spreading rate, apparently without major changes in motion direction, at 27 and 24 Ma. The change at 30 Ma is apparently the result of the separation of the Juan de Fuca plate from the rest of the Farallon plate along the Pioneer fracture zone. The pattern of absolute motion after 30 Ma does not generally support Menard's pivoting subduction model and suggests that Juan de Fuca plate motion is being significantly affected by forces transmitted across the Mendocino transform fault.
Extension tectonics: The Neogene opening of the north–south trending basins of central Thailand
Authors: McCabe Robert , Celaya Michael , Cole Jay , Han Hyun‐Chul , Ohnstad Tiffany , Paijitprapapon Vivat , Thitipawarn Veeravat
Published: February 2008

Paleomagnetic samples were collected from late Neogene basalt flows from Thailand. All of these flows are horizontal and are relatively unaltered in thin section. These rocks possess a stable magnetization which is believed to be primary. Samples from 48 lava flows were collected from sites located within the Khorat Plateau, the Chao Phraya‐Phitsanulok Basin, and the mountainous terrane west of the Chao Phraya‐Phitsanulok Basin. These data were combined with previously reported late Neogene data from five flows from western Thailand. Although the average inclination from the 53 sites is indistiguishable from the expected dipole inclination, the average declination has a net clockwise rotation of 13.5±5.8 from the geocentric dipole field. Furthermore, the mean declination values from the 29 flows from the Khorat Plateau are indistinguishable from the present dipole field direction (Dm = 4.3°±7.5°) and indistinguishable from the mean declination from 28 late Neogene volcanic flows from Vietnam. In contrast, the mean declinations from 24 flows collected from central and western Thailand are deflected significantly clockwise (Dm = 24.4°±7.7°) from the geocentric dipole field direction. The differential rotation between western and central Thailand versus the Khorat Plateau suggests that Indochina is composed of at least two structural blocks which underwent a different rotational history. These observations, when combined with geologic and geophysical data from the Chao Phraya‐Phitsanulok Basin, Gulf of Thailand, and the intermontane basins of western Thailand, suggest that the rotations are recording a late Neogene phase of E–W extension of these basins. We suggest that the formation of these basins and the related basaltic volcanism developed in reponse to subduction of the Indian plate under western Burma. We envision the tectonics of this region is similar in style to the Basin and Range region of the western United States. Last, we have observed field relationships from some of the rhyolites located in the central basin. Although these rhyolites are reported to be Mesozoic or Paleozoic in age, our field observations and a K‐Ar age date show that at least some of these rhyolites are younger than the basalts. We suggest that the rhyolites form a bimodal suite with the basaltic rocks which were erupted in the later stages of the extension.
The magnetic properties of metaperidotitic rocks as a function of metamorphic grade: Implications for crustal magnetic anomalies
Authors: Shive Peter N. , Frost B. Ronald , Peretti A.
Published: February 2008

Seventeen samples from the Malenco serpentinite in the Swiss Alps, representing systematic prograde metamorphic conditions from prehnite‐pumpellyite to upper amphibolite, are moderately to strongly magnetic (average susceptibility of 6.61×10−2 SI units) due predominantly to the presence of a magnetite‐rich spinel. Some greenschist facies samples contain the metal alloy awaruite (Ni3Fe); these contain in addition much greater amounts of magnetite. Small amounts of pyrrhotite in about half the samples complete the contributions to the total magnetization. Susceptibilities generally decrease with increasing metamorphic grade because of the production of increasing amounts of chromerich spinel which dilutes the magnetite component. These results indicate that significant magnetic anomalies should be generated in suture zones containing metamorphosed serpentinite and in areas where serpentinite is being actively subducted.
Borehole Gravity Measurements in the Salton Sea Scientific Drilling Project Well State 2–14
Authors: Kasameyer Paul W. , Hearst Joseph R
Published: February 2008

Borehole gravity measurements over a depth range from 1737 to 1027 m and the vertical gradient of gravity above ground were measured at the Salton Sea Scientific Drilling Project well State 2–14. Uncorrected borehole gravimetric densities match values from gamma‐gamma logs, indicating that the high densities seen in State 2–14 in the depth range 0.5–3 km extend for a few kilometers from the well. The aboveground gradient was found to be 4.1 µGal/m higher than expected; correcting for this value increases the gravimetric density in the borehole. Combining the borehole gravity and estimated vertical gravity gradients on the surface, we find that this densified zone coincides with much of a broad thermal anomaly that has been found to the northeast of the Salton Sea geothermal field.
Laboratory‐Determined Transport Properties of Core From the Salton Sea Scientific Drilling Project
Authors: Lin Wunan , Daily William
Published: February 2008

Two cores from the Salton Sea Scientific Drilling Project have been studied in the laboratory to determine electrical resistivity, ultrasonic velocity, and brine permeability at pressures and temperatures close to estimated borehole conditions. Both samples were siltstones; the first sample was from 1158‐m depth, and the other was from 919‐m depth. A synthetic brine with 13.6 weight percent NaCl, 7.5 weight percent CaCl2, and 3.2 weight percent KCl was used as a pore fluid. The dry bulk density of the first sample was 2.44 Mg m−3 with an effective porosity of 8.7%. The second sample had a dry bulk density of 2.06 Mg m−3 with an effective porosity of 22.2%. At the midplane of the first sample, electrical impedance tomography was used to map the spatial variation of resistivity during the experiment. Also, at the midplane of both samples, ultrasonic tomography was used to map the spatial variation of P wave velocity. In addition, resistivity was measured with six pairs of electrodes along the sample axis. Both samples showed a strong anisotropy in resistivity and ultrasonic velocity measured perpendicular and parallel to the sample axis. The brine permeability of the first sample decreased from 5 μD to about 1.6 μD during the experiment. The second sample permeability had the same trend, but the permeability values were about 3 orders of magnitude larger. The second sample was subjected to temperatures and pressures exceeding those experienced in situ. Permeability, resistivity, and ultrasonic velocity of this sample showed a discontinuous change just beyond these in situ conditions. This discontinuity implies a structural change in the rock under conditions which would be found below its origin depth in the borehole. A model is proposed to explain the observed velocity anisotropy and variations in velocity, electrical resistivity anisotropy, and permeability with effective depth. When in situ stress is released, microcracking may occur along grain boundaries preferentially oriented parallel to bedding. This microcracking controls velocity and resistivity anisotropy at room conditions. When pressure and temperature are restored, competing effects of compaction and thermal softening of the minerals cause a reversal in the anisotropy. At temperatures and pressures above those at in situ conditions, thermal fracturing or geochemical alteration along grain boundaries causes a discontinuous change in sample physical properties.
Basement Structure and Implications for Hydrothermal Circulation Patterns in the Western Moat of Long Valley Caldera, California
Authors: Suemnicht Gene A. , Varga Robert J.
Published: February 2008

Detailed surface mapping, subsurface drill hole data, and geophysical modeling are the basis of a structural and hydrothermal model for the western part of Long Valley caldera. Six fault zones are recognized in the western caldera with dominant orientations of north, northwest, and northeast, sub‐parallel to regional fault trends in the surrounding Sierran basement. The internal structural geometry of the cores of 12 exogenous domes inside and outside the caldera suggests that the domes erupted at the intersections of these principal fault trends rather than along the axis of a single dike. Gravity modeling and subsurface data from deep geothermal wells indicate that the floor of the caldera is segmented into a number of discrete fault blocks with varying offsets. One of the northeast trending fault zones, designated Discovery fault in this paper, appears to be part of the original Sierran embayment that existed before caldera collapse. Recent hydrothermal alteration occurs along Discovery fault strands and composite vertical offset of intracaldera volcanic units across the entire fault zone may be as much as 400 m. Field relationships, geophysical interpretations, and interpretaions of drill hole data suggest that this fault is a fundamental flaw in the western caldera. The preexisting tectonic framework of the basement rocks controlled the configuration of the western caldera floor and, through it, the location of postcollapse eruptive centers. These deep basement structures may also provide the high fracture density which controls circulation in the present geothermal system of Long Valley.
Centroid Depths of Mid‐Ocean Ridge Earthquakes: Dependence on Spreading Rate
Authors: Huang Paul Y. , Solomon Sean C.
Published: February 2008

From inversions of teleseismic P and SH waveforms we have determined the source parameters of 50 large earthquakes that occurred during 1962–1983 on slowly spreading mid‐ocean ridges. All events are characterized by predominantly normal faulting on planes that dip at approximately 45° and strike parallel to the local trend of the ridge axis. Centroid depths range from 1 to 6 km beneath the seafloor. The P waves from these earthquakes show strong water column reverberations, suggesting that fault rupture extended to the seafloor. Under the assumption that the centroid depth marks the mean depth of fault slip, earthquake faulting extended to a depth of 2–10 km for these earthquakes. The water depths inferred from the predominant periods of the water column reverberations constrain the epicenters of these earthquakes to lie within the relatively deep inner floor of the median valley. The maximum centroid depths of ridge crest earthquakes decrease with increasing spreading rate, and the maximum seismic moment may also decrease with increasing spreading rate. These results indicate a general decrease with spreading rate in maximum thickness of the mechanically strong layer beneath the ridge axis region. The concentration of large earthquakes within the deepest parts of the median valley and the depth extent of seismic faulting support lithospheric necking models for the origin of the median valley.
HENGILL TRIPLE JUNCTION, SW ICELAND 1. TECTONIC STRUCTURE AND THE SPATIAL AND TEMPORAL DISTRIBUTION OF LOCAL EARTHQUAKES
Authors: Foulger G. R.
Published: February 2008

The Hengill area is an unstable ridge‐ridge‐transform triple junction in SW Iceland. It contains the active central volcano Hengill, associated with the present accretionary zone, and the extinct Grensdalur central volcano, associated with an accretionary zone that became inactive at about −0.7 m.y. when the locus of spreading migrated about 5 km W to the Hengill zone. The dominant tectonic trend of the area is N25°E, parallel to the accretionary zones, but transverse tectonic/topographic features have also developed in the neighborhood of the two central volcanoes. These result from local modification of the regional stress field associated with the spreading by radially symmetric stress fields associated with the volcanoes, and the migration of volcanism. A double‐high temperature geothermal system mirrors the double volcanic system. The transform branch of the triple junction is represented by an EW striking zone of historic destructive earthquakes in the S of the area. A seismological study was conducted with the aims of studying the tectonic structure and evaluating the passive seismic method as a geothermal prospecting tool. The seismicity may be broadly divided into two groups. First, infrequent intense episodes of crustal movement occur that are associated with the release of tectonic stress along the plate boundary. Second, continuous small‐magnitude activity occurs on a day to day basis. This activity is mostly associated with the extinct Grensdalur central volcano, not the present plate boundary. Statistically significant temporal variations in b accompanied a swarm in the transform zone. The continuous small‐magnitude activity may be geothermal in origin, releasing, thermal stress, and not associated with plate boundary tectonics.
FISSION TRACK THERMOCHRONOLOGIC ANALYSIS OF THE RYOKE BELT AND THE MEDIAN TECTONIC LINE, SOUTHWEST JAPAN
Authors: Tagami Takahiro , Lal Nand , Sorkhabi Rasoul B. , Nishimura Susumu
Published: February 2008

Fission track (FT) ages were determined on 27 apatite and 21 zircon samples collected from three traverses across the eastern part of the Ryoke belt. FT zircon ages are reasonably consistent, averaging 58 Ma in the three traverses, suggesting uniform uplift and cooling of the region through the zircon isotherm (∼240°C). FT apatite ages range from 60 to 10 Ma and are divided into two groups: “old” apatites, located far from the Median Tectonic Line (MTL), cluster around 54 Ma in the three traverses, also indicating uniform uplift and cooling through the apatite isotherm (∼105°C); and “young” apatites, located near the MTL, show age differences between the three traverses. FT length data of nine apatites reveal later thermal overprinting events near the MTL, probably due to shear heating induced by fault movements. The observed uplift and cooling rates of the Ryoke belt and the timing of motion along the MTL are discussed in the tectonic framework.
Yarkovsky Thermal Drag on LAGEOS
Authors: Rubincam David Parry
Published: February 2008

Thermal drag accounts for about 70% of the observed average drag on LAGEOS. This estimate is based on a new thermal model which includes the radiative heat transfer between the retroreflector, its mounting rings, and the aluminum cavity in which it sits. The new estimate is a significant increase over the 47% of the old model, which did not include these interactions. Neutral particle drag accounts for about 13% of the average drag, and charged particle drag, at least 5%. Thus approximately 88% of the observed average drag can be reasonably explained. The rest can probably be attributed to more charged particle drag than the lower limit given above. A better thermal model would probably alter the above figures only slightly. The thermal drag calculation assumes that the LAGEOS spin axis has remained fixed in space and is where the telemetry data say it was after apogee kick motor separation. Ground measurements of the spin axis direction would be desirable.
Three Hawaiian calderas: An origin through loading by shallow intrusions?
Authors: Walker George P. L.
Published: February 2008

The calderas of Kilauea and Mauna Loa are highly dynamic structures, and in the <200‐year historic period have varied in volume by a factor of 2, and gained or lost 1 km3 per century. The deeply eroded caldera of the extinct Koolau Volcano in Oahu is wider than active Hawaiian calderas, and its lavas have a strong centripetal dip and funnel structure not evident at Kilauea or Mauna Loa. The differences can be attributed to the different erosion depths, and the time integrated subsidence profile of Kilauea is also a stepped funnel (having its apex at Halemaumau). Koolau caldera is the focus of an extraordinarily intense dike complex, and an intriguing feature is the great diminution in dike concentration into the caldera. It is thought that dike injection in any part of the complex generally continued until it reached 50% to 65%. In outer parts of the caldera, the complex was maintained at or rebuilt to this value despite subsidence. In the center of the caldera (where the positive Bouguer anomaly is centered), subsidence evidently greatly outpaced the capacity of dike injections to rebuild the complex. Assuming the same dike injection rate as Kilauea and Mauna Loa yields a volumetric subsidence rate in Koolau caldera exceeding 1 km3 per century. Hawaiian calderas are much more dynamic than calderas of silicic volcanoes, shaped by frequent small events instead of a few great ones. The temporal and volumetric correspondence of historical subsidence events with eruptions is poor, and this and the high subsidence rates argue for a caldera‐forming mechanism that consumes the subsided rocks. It is suggested that subsidence is caused by the great localized excess load of intrusive rocks, carrying the center of the volcano into the thermally weakened lithosphere above the Hawaiian hot spot. It is envisaged that under steady state conditions the magma chamber rises, as the injection of intrusions causes the level of neutral buoyancy (at which the chamber is located) to ascend, and keeps pace with subsidence.
Dikes and the petrology of Waianae Volcano, Oahu
Authors: Zbinden Elizabeth A. , Sinton John M.
Published: February 2008

We summarize the distribution, orientation, and composition of the dikes of Waianae Volcano and attempt to relate this summary to the evolution of the volcano. Dikes are located in the caldera, in a well‐defined northwest rift zone, and in less well‐defined south and northeast rift zones. The active axis of the south rift zone may have shifted with time. The dikes generally are parallel to their rift zones and are oriented in many directions in the caldera area. They commonly dip about 70° to 80° and are generally less than a meter thick. Most are virtually identical in texture, mineralogy, and composition to nearby extrusive rocks. A few have unusual low‐magnesium silicic compositions not yet observed in flows; these may be classified as icelandites. Trends defined by the compositions of tholeiitic dikes are similar to, but extend further than, crystal fractionation trends of tholeiitic flows of Waianae and other Hawaiian volcanoes. The basalts can be divided into a more silica saturated strongly tholeiitic subgroup and a less saturated mildly tholeiitic subgroup. The icelandites lie on trend with the second group. Most of the analyzed rift zone dikes are strongly tholeiitic; most of the analyzed caldera dikes are mildly tholeiitic; all of the differentiated high‐silica low‐magnesium dikes found are inside the caldera near the eruptive center.
Active tectonics of the Eastern Sunda and Banda Arcs
Authors: McCaffrey Robert
Published: February 2008

The tectonics of the eastern Sunda and Banda arcs of eastern Indonesia are inferred from the centroid depths, seismic moments, and fault plane solutions of 63 large earthquakes that occurred between 1962 and 1984. Source parameters are estimated by inverting long‐period, teleseismic P and SH waves. Earthquakes show that the collision of the Australian continent with the Banda arc shortens the upper plate beneath the forearc and back arc basin over the broad area between the Timor and Seram troughs. Along the eastern end of the Sunda arc and along the southern Banda arc, earthquakes reveal that the arc and forearc respond to collision by shortening in the direction of convergence, by elongating in the direction perpendicular to convergence, and by thrusting over the back arc basin. Shallow thrust and strike‐slip earthquakes beneath the Banda Basin (the back arc basin) demonstrate that deformation within the back arc accommodates some of the northward motion of the Australian continent. The rate of north‐south shortening of the entire upper plate at the longitude of Timor calculated from the seismic moments of the earthquakes over the 22‐year period is roughly 20% of the predicted convergence rate between Australia and southeast Asia. Strike‐slip faulting within the Banda Basin results also in eastward motion of the Banda arc with respect to western Indonesia and Australia. Eastward extrusion of the Banda arc results in thrusting at the Aru trough. Earthquakes due to thrusting of the back arc basin beneath the volcanic arc are too shallow to corroborate that the arc's polarity has reversed although the Wetar back arc thrust zone at present has a high seismic slip rate. Earthquakes and their mechanisms provide evidence that the lithosphere of the Weber Basin, a 7500‐m‐deep forearc basin, rests on the subducting lithosphere, without intervening asthenosphere, so that the Weber Basin subsides in response to sinking of the subducting lithosphere. Closure of the Banda Basin is geometrically incompatible with the Australian plate subducting simultaneously beneath both the Timor and Seram troughs. Instead, the Bird's Head, which subducts beneath Seram, is decoupled from the Australian plate in western New Guinea and probably moves west or southwest with respect to Australia. In the eastern Indonesian example, strike‐slip faulting plays a major role in the evolution of the collision zone and clearly demonstrates the complexity in three dimensions of continental accretion and mountain building by arc‐continent collision.
Relative dating of Hawaiian lava flows using multispectral thermal infrared images: A new tool for geologic mapping of young volcanic terranes
Authors: Kahle Anne B. , Gillespie Alan R. , Abbott Elsa A. , Abrams Michael J. , Walker Richard E. , Hoover Gordon , Lockwood John P.
Published: February 2008

The weathering of Hawaiian basalts in arid and semiarid environments is accompanied by change in their thermal infrared emittance spectra. The spectral differences can be measured and mapped with multispectral imaging systems. The differences appear to be related to the degree of development, preservation, and alteration of glassy crusts; the oxidation of iron; and the accretion of silica‐rich surface veneers. Because the measurements are quantitative and in image format, they are useful for estimating relative ages in geologic mapping of lava flows. In Hawaii this technique is most diagnostic for distinguishing among sparsely vegetated flows less than 1.5 ka in age.
Optimal polarizations for achieving maximum contrast in radar images
Authors: Swartz A. A. , Yueh H. A. , Kong J. A. , Novak L. M. , Shin R. T.
Published: February 2008

There is considerable interest in determining the optimal polarizations that maximize contrast between two scattering classes in polarimetric radar images. In this paper a systematic approach is presented for obtaining the optimal polarimetric matched filter, i.e., that filter which produces maximum contrast between two scattering classes. The maximization procedure involves solving an eigenvalue problem where the eigenvector corresponding to the maximum contrast ratio is optimal polarimetric matched filter. To exhibit the physical significance of this filter, it is transformed into its associated transmitting and receiving polarization states, written in terms of horizontal and vertical vector components. For the special case where the transmitting polarization is fixed, the receiving polarization which maximizes the contrast ratio is also obtained. Polarimetric filtering is then applied to synthetic aperture radar images obtained from the Jet Propulsion Laboratory. It is shown, both numerically and through the use of radar imagery, that maximum image contrast can be realized when data is processed with the optimal polarimetric matched filter.
Experimental studies of particle‐laden plumes
Authors: Carey S. N. , Sigurdsson H. , Sparks R. S. J.
Published: February 2008

Experiments are described in which buoyant plumes of fresh water and solid particles are generated in a tank of salt water. Dependence of the behavior of the plumes on particle concentration and grain size was studied. At low concentrations, behavior was similar to single‐component plumes, and the principal effect of the particles was to reduce the buoyancy flux. Measurements of particle concentration across the plumes demonstrated that there were substantial amounts of particle reentrainment from an outer sedimenting veil. Reentrainment caused the plume margins to become denser than the ambient fluid. Total particle flux in the plumes increased with height and can be several times greater than the flux at the plume source. Plumes with high particle concentration exhibit several kinds of convective instability, which generated gravity currents and rapid sedimentation. Gravity currents were generated at the margins of concentrated plumes. These flows are attributed to reentrainment, causing the flow margins to have excess density. When the plume density was close to but less than the density of the ambient fluid, a fountain forms high in the plumes which collapsed asymmetrically to generate a gravity current on one side of the plume. When the density of the particle‐laden flow was greater than the ambient fluid, a symmetrical fountain formed, and gravity currents moved across the tank floor, depositing sediment. At a well‐defined distance from the source, buoyant fluid ascended from the flow fronts to form a ring of plumes around the central fountain. These plumes coalesced to form a broad buoyant cloud. The plumes are generated by the sedimentation from the gravity currents, which resulted in the fresh water and some entrained particles being released. All the experimental plumes ascended to the top of the tank and spread out as gravity currents beneath the upper surface. Particles often fall from this layer in discrete parcels of particle‐rich fluid with shapes similar to thermals. The fall velocities were much greater than the terminal velocities of individual particles and the sedimenting region is inhomogeneous. These observations indicate that sedimentation is enhanced by convective instabilities when particle‐rich fluid is emplaced above particle‐free fluid. The experiments indicate that there may be several different mechanisms of generating pyroclastic flows and surges from the collapse of volcanic eruptions columns. The experiments also demonstrate that the sedimentation of particles from a plume has an important influence on dynamical behavior.
Single zircon dating constraining the maximum age of the Barberton Greenstone Belt, southern Africa
Authors: Kröner Alfred , Todt Wolfgang
Published: February 2008

One of the central issues in early Precambrian crustal evolution is the age and genetic relationship between greenstone belt supracrustal assemblages and nearby high‐grade gneiss terranes. The crucial question, in most cases, is which of the two is older. We report zircon ages, using the new single grain evaporation technique, from a metaquartzite and a felsic volcanic flow of the Onverwacht Group in the Barberton greenstone belt (BGB), South Africa and Swaziland, that constrain the age of this basal part of the greenstone sequence between 3451 ± 15 and 3438 ± 6 Ma. This is almost 100 Ma younger than previous age estimates assigned to the Onverwacht and agrees well with recent Sm‐Nd, Pb‐Pb and zircon ion‐microprobe dating. Tonalitic orthogneisses of the Ancient Gneiss Complex (AGC) adjacent to the BGB have ages up to 3644 ± 4 Ma and are thus demonstrably older than the greenstones; also, they cannot be derived from them by partial melting as has been proposed previously. This age relationship as well as the presence of metaquartzites and thrust slices of pregreenstone tonalite in the Onverwacht tectonic sequence rule out an entirely oceanic evolution for the early history of the BGB and suggest that continental crust was either below or nearby.
Tilt observations using borehole tiltmeters: 1. Analysis of tidal and secular tilt
Authors: Levine Judah , Meertens Charles , Busby Robert
Published: February 2008

We have designed a borehole tiltmeter using two horizontal pendulums which have periods of 1 s. We have installed the instruments at seven sites in Colorado and Wyoming to evaluate the secular tilt, the tides, and the coherence between nearby instruments. Using 28 days of data from Boulder, Colorado, the standard deviations of the tidal estimates are 5% for the semidiurnal component M2 and 9% for the diurnal component O1. The estimates agree with models that include the body tide, the ocean load, and the topographic correction to better than the estimated uncertainty. Tidal measurements at Erie, Colorado, have larger, possibly nonrandom variability that may be caused by a coupling between the tides and long‐period tilts. The coherence between nearby instruments is ≳0.5 for frequencies ranging from 0.5 to 2 cycles per day, but drops to a much smaller value outside of this range. The secular tilt rate is quite variable and ranges from 0.1 to 1 μrad/yr. Measurements at Erie, Colorado, and in Yellowstone National Park, Wyoming, exhibit an annual or biannual periodicity with an amplitude of about 1 μrad, which is consistent with a 1/f2 extrapolation of the diurnal noise power to longer periods.
Tilt observations using borehole tiltmeters: 2. Analysis of data from Yellowstone National Park
Authors: Meertens Charles , Levine Judah , Busby Robert
Published: February 2008

We have installed borehole tiltmeters at five sites in Yellowstone National Park, Wyoming, and have used these instruments to measure the spatial variation of the amplitude and phase of the principal semidiurnal tide. The measured tides vary both with position and azimuth and differ from the sum of the body tide and the ocean load by up to 50%. The difference predicted by a finite element model constructed from seismic, refraction, and gravity data has a maximum value of only 12%, although the discrepancy between our observations and the model is only marginally significant at some sites. The disagreement between the model and our observations is much larger than we observed using the same instruments at other sites and cannot be attributed to an instrumental effect. We have been unable to modify the model to explain our results while keeping it consistent with the previous observations.
Low‐frequency source characteristics of the great 1960 Chilean earthquake
Authors: Cifuentes Inés L. , Silver Paul G.
Published: February 2008

The low‐frequency nature of the great 1960 Chilean earthquake, the Earth's largest recorded event, is characterized, and a source model composed of three events with a combined seismic moment of 5.5 × 1023 N m and a duration of 1500 s is presented. The results show conclusively that large‐scale slip, which is only observed at low frequencies, preceded the main shock. Complex amplitude measurements of normal mode data in the band 1.0–5.0 mHz, recorded at eight vertical component stations, comprise the data set. In this band the earthquake source is assumed to be described by a line source propagating at a constant rupture velocity. The data set is the basis for a sequence of least squares inversions to determine the seismic moment, rupture velocity, spatial slip distribution, and the temporal slip function. The data are best explained by a source model that begins 1150 s before the main shock with an event of moment 1.9 × 1023 N m and rise time of 300 s, a main shock with moment of 3.2 × 1023 N m and for which most of the low‐frequency energy was released beginning about 50 s after the short‐period origin time, and an event 350 s later with moment of 0.4 × 1023 N m. The uncertainty in the moment estimate of the great 1960 Chilean earthquake is about 50% and is due primarily to the probable range of fault plane dip, which is not well constrained at this time.
Source type plot for inversion of the moment tensor
Authors: Hudson J. A. , Pearce R. G. , Rogers R. M.
Published: February 2008

Seismic signals provide information about the underlying moment tensor which, in turn, may be interpreted in terms of source mechanism. This paper is concerned with a two‐dimensional graphical display of all possible relative sizes of the three principal moments; it provides a method of representing the probability density of these relative sizes deduced from a given set of data. Information provided by such a display, together with that relating to the orientation of the principal moments, provides as full a picture of the moment tensor as possible apart from an indication of its absolute magnitude. As with the compatibility plot, which was previously introduced to portray probability measures for different forms of P wave seismogram given a presumed source type, this “source type plot” for display of the principal moments is constructed to be “equal area” in the sense that the a priori probability density of the moment ratios is uniform over the whole plot. This a priori probability is based on the assumption that, with no information whatsoever concerning the source mechanism, each principal moment may independently take any value up to some arbitrary upper limit of magnitude, with equal likelihood. Although we have in mind the study of teleseismic relative amplitude data, the ideas can, in principle, be applied quite generally. The aim is to be able to display the degree of constraint imposed on the moment tensor by any set of observed data; estimates of the sizes of the principal moments together with their errors, when displayed on the source type plot, show directly the range of moment tensors compatible with the data.