bluuebunny/crossref_metadata_2025 huggingface dataset:
Electromagnetic fields induced at the seafloor by Rayleigh‐Stoneley waves
Authors: Webb Spahr , Cox Charles
Published: February 2008

We model oceanic acoustic and seismic disturbances as Rayleigh‐Stoneley waves in the layered medium consisting of ocean, sediment and rock. The waves induce electromagnetic fields because of motion of conducting materials through the geomagnetic field. Low‐frequency disturbances in deep water are efficient electromagnetic generators because the motions are coherent over large volumes of highly conducting seawater. For wave frequencies below 0.1 Hz in deep water, the geomagnetic field is nearly frozen to the moving water. Consequently, the electric field measured by a voltmeter attached to the seabed is approximately Δu × F, where Δu is the difference between the velocity of the water and of the voltmeter itself as it is moved by the motion of the seafloor, and F is the geomagnetic field. This result applied to Love waves suggests that the electric field will result largely from the movement of the detector but detailed calculations have not been made. Although the fields are weak they should be detectable because the main interference is from ionospheric sources and the fields from these sources are greatly attenuated at the seabed by the overlying oceanic shield. The effectiveness of the shield diminishes sharply at frequencies below 0.03 Hz. At high frequencies the principal limitation to detectability is in the inherent noise level of detectors whether electric or magnetic. At present, electric detectors are more effective than magnetic. They appear to be competitive with accelerometers for seismic detection in the deep ocean in a restricted frequency band near 0.05 Hz. A layer of unconsolidated sediment underlying the ocean profoundly affects the dispersion relation of Rayleigh‐Stonely waves and thereby affects the electromagnetic induction process. The shunting effect of a normal deep ocean sediment layer is not large on the electric field, but the low shear modulus of the sediments influences sea bottom motions and therefore greatly changes the fields sensed by bottom mounted instruments especially at higher frequencies.
The seismic reflection character of the continental mohorovicic discontinuity
Authors: Hale Laura D. , Thompson George A.
Published: February 2008

The continental Mohorovicic discontinuity is most often interpreted as a step‐function velocity boundary. However, on deep crustal seismic reflection profiles, reflections, at depths where refraction data places the Moho, have laminated character and are laterally discontinuous. These observations on continental reflection data point to a model of the Moho that has thin layered rather than block structure. Previous workers, who have synthetically generated seismic responses from various crust‐mantle boundary models, have shown that thin layered models of alternating high and low velocity generate responses that best emulate observed Moho arrivals. Geologic interpretations of the thin layering include relatively undeformed metasediments, cumulate layering, tectonic banding, and lenses of partial melt. To obtain more direct geologic evidence, a synthetic seismogram is generated from geologic cross‐sections of the Ivrea‐Verbano Zone, an exposed section through the lower crust and upper mantle. The synthetic shows laminations similar to those observed on reflection data. Without more direct evidence, no single geologic interpretation of the Moho is reasonable. Indeed, the Moho may be a laterally variable boundary, its composition and structure dependent upon the geologic history of the overlying crust.
The quartz ⇆ coesite transformation: A precise determination and the effects of other components
Authors: Bohlen Steven R. , Boettcher A. L.
Published: February 2008

Reaction reversals to within 0.2 kbar locate the quartz ⇆ coesite equilibrium at 24.6, 24.9, 25.5, 26.2, 27.0, 27.9, 28.8, and 29.7 kbar at 350, 400, 500, 600, 700, 800, 900, and 1000°C, respectively, using well‐calibrated (frictionless), 2.54‐cm diameter NaCl furnace assemblies in piston‐cylinder apparatus. The equilibrium is unaffected by the presence of H2O or jadeite, contrary to previous proposals. The positive dP/dT of the reaction at T < 500°C is inconsistent with existing thermochemical data for coesite. Numerous tight reversals provide a sound basis for use of the quartz ⇆ coesite equilibrium as a calibration of pressure in high‐pressure apparatus.
Seismotectonics of the southern boundary of Anatolia, eastern Mediterranean region: Subduction, collision, and arc jumping
Authors: Rotstein Yair , Kafka Alan L.
Published: February 2008

The pattern of seismicity and fault plane solutions of earthquakes are used to outline the tectonic features of the southern boundary of Anatolia in the eastern Mediterranean and southeastern Turkey. The results of this study show that this boundary is composed of two distinct parts. One, in southeastern Turkey and Syria, is a wide and complex zone of continental collision. The other, in the Levantine basin of the eastern Mediterranean, is a zone of oceanic subduction. In the region of continental collision three zones of seismicity are observed. Most of the seismic activity in this region follows the Bitlis zone and is associated with a zone of thrusting and mountain building. This appears to be the zone of most active deformation and plate consumption in the plate boundary region between Arabia and Turkey. A less active zone of seismicity to the north of the Bitlis zone is interpreted to have been more active in the past whereas another active zone of seismicity to the south is interpreted to be a zone which may be more active in the future as the main zone of plate consumption jumps to the south. In the subduction zone of the eastern Mediterranean the depth of the subducted slab and the rate of seismicity generally increase from east to west. The zone of present‐day convergence between Africa and Turkey in the Levantine basin can be best outlined by the northern edge of the Mediterranean ridge. The subduction zone in this area sequentially jumps to the south as small continental fragments collide with existing zones of subduction. Deep seismic activity near the Gulf of Antalya is associated with a detached subducted slab north of the Anaximander Mountains that is distinctly different from the seismic trend which is associated with present‐day active subduction. The plate boundary between Africa and Turkey at the center of the Levantine basin appears to have shifted to the south of the Anaximander Mountains and Florence rise. Most of the focal mechanisms of the earthquakes along the entire southern boundary of Anatolia indicate that N to NNW thrusting is the dominant mode of seismic deformation. The present set of data is inconsistent with a major transform fault in the Cyprean arc east of Cyprus and in southeastern Turkey.
Reinterpretation of seismic reflection data over the East Pacific Rise
Authors: Hale Laura D. , Morton Christopher J. , Sleep Norman H.
Published: February 2008

Multichannel seismic reflection data over the axial region of the East Pacific Rise are depth migrated using detailed velocity information from laboratory measurements of ophiolite samples. The data are from the Lamont Doherty Geological Observatory's line 17 shot across the rise at 9°N. The migrated data are interpreted to show the structure of the seafloor and what we believe is the magma chamber roof. The polarity and apparent root mean squared velocity of the magma chamber roof are asymmetric with respect to the topographic axis of the ridge. The asymmetry is probably real and not an artifact of data collection. Modeling of the magnetic data gathered along the line shows that spreading has not been proceeding normally. A reasonable explanation for the asymmetry is the possibility that the line intersects a transform fault or an abandoned ridge segment near the ridge axis. The shape of the roof reflection is convex upward with an approximate slope of 10° and a width of 4 km. If extrapolated symmetrically to the other side of the ridge axis, the magma chamber roof event is consistent with the funnel‐shaped chamber proposed by Pallister and Hopson (1981) for the Samail ophiolite. If the chamber roof steepens rapidly beyond the extent of the reflection, it would be consistent with the mush‐filled model of Sleep (1975, 1978) and Dewey and Kidd (1977).
Seismicity and crustal structure in the Orozco Fracture Zone: Project Rose Phase II
Authors: Ouchi Toru , Ibrahim Abou‐Bakr K. , Latham Gary V.
Published: February 2008

A total of 301 earthquakes were recorded in the vicinity of the Orozco fracture zone by seven Texas ocean bottom seismograph stations during the 2‐week period of the Rivera Ocean Seismic Experiment (ROSE) (phase II). Using data from the entire ROSE array, hypocenters of 50 earthquakes were determined. These revealed two distinct zones of seismic activity within the fracture zone. In addition to these earthquake families, many very small events were detected by a station located very close to the spreading center of the East Pacific Rise. The magnitudes of these earthquakes, defined by their duration times, were so small that most of them were recorded only at this station (station 14) in continual or swarmlike occurrences. The slope of the frequency‐magnitude distribution of these events is significantly larger than those of other earthquake groups detected during the experiment, i.e., they appear to have an unusually high b value. These results suggest that this new population of earthquakes may be associated with volcanic activity or stress release within highly fractured crustal material. Refraction studies in the fracture zone reveal the presence of a rather high‐velocity crustal layer (6.9–7.0 km/s) beneath the experiment zone. The Moho velocity and the crustal thickness are estimated at 7.8 km/s and 6.2 km, respectively.
Microearthquakes on the East Pacific Rise at 21°N and the Rivera Fracture Zone
Authors: Prothero William A. , Reid Ian D.
Published: February 2008

During March and April of 1974, three ocean bottom seismometer (OBS) capsules were deployed in arrays on the East Pacific Rise (EPR) at 21°N for 8 days, on the Rivera Fraction Zone (RFZ) for 9 days, and at the EPR/RFZ intersection for 12 days. Seismicity on the EPR is characterized by extremely small events, none large enough to trigger all of the instruments simultaneously. A weak 24‐hour periodicity in OBS event trigger rate is observed. This could be related to earth strain tides or a modulation in the trigger sensitivity due to a varying noise level. On the Rivera Fracture Zone, an en echelon pattern of earthquakes with a maximum depth of 10 km was observed. Seismic activity at the junction of the EPR and RFZ becomes more diffuse and terminates abruptly at the EPR. The junction array was slightly offset from the active zone of seismicity, and the depth of only one event is reliably determined at 5.4±2 km. The S wave spectra of microearthquakes at the junction and on the RFZ are computed, and seismic moments and corner frequencies determined. Stress drops are between 0.1 and 10 bars and show no systematic difference at the two deployment sites. Larger earthquakes observed at teleseismic distances do show markedly lower stress drops when they originate near to spreading centers, but these differences are not observed at the microearthquake level in this study. It is suggested that this is due to a natural tendency for small earthquakes to have small stress drops and a lack of large earthquakes during the RFZ deployment.
Tectonic history of the Tharsis Region, Mars
Authors: Plescia J. B. , Saunders R. Stephen
Published: February 2008

The faults of the Tharsis region of Mars have been mapped and delineated on the basis of age. The orientation of each group of faults was analyzed to determine if they were radial to a point. The results indicate that there are at least four discrete centers of faulting within the Tharsis area which are of significantly different age. The four centers proposed are, from oldest to youngest, (1) in the Thaumasia highlands, (2) in northern Syria Planum, and finally, (3) and (4) near Pavonis Mons. While episodes 3 and 4 occupy the same geographic position, they are separated in time by the development of the Tharsis lava plains and shields. Each episode of tectonism was separated from the next by a period of major basaltic volcanism. The tectonic features of the Tharsis area are thus viewed as being the cumulative result of the superposition of several discrete episodes of faulting, rather than the result of one episode about a single point.
A summary of Viking sample‐trench analyses for angles of internal friction and cohesions
Authors: Moore H. J. , Clow G. D. , Hutton R. E.
Published: February 2008

Analyses of sample trenches excavated on Mars, using a theory for plowing by narrow blades, provide estimates of the angles of internal friction and the cohesions of the Martian surface materials.Angles of internal friction appear to be the same as those of many terrestrial soils because they are generally between 27° and 39°. Drift material, at the Lander 1 site, has a low angle of internal friction (near 18°). All the materials excavated have low cohesions, generally between 0.2 and 10 kPa. The occurrence of cross bedding, layers of crusts, and blocky slabs shows that these materials are heterogeneous and that they contain planes of weakness. The results reported here have significant implications for future landed missions, Martian eolian processes, and interpretation of infrared temperatures.
Martian north polar cap 1979–1980
Authors: Iwasaki Kyosuke , Saito Yoshikazu , Akabane Tokuhide
Published: February 2008

A regression curve for the north polar cap of Mars is extracted from the photographs obtained at Kwasan Observatory and at Hida Observatory during the 1979–1980 apparition. The north polar cap was first observed at Ls = 26°, and the latitude of the edge of the cap was about 66°N. The retreat of the cap stopped until Mars reached Ls = 57° when the cap started to shrink. The edge reached a latitude around 80°N at Ls = 85° and then became almost stationary. The roughly constant dimension of the north polar cap before mid‐spring appears to be a regular phenomenon. But the onset time when the cap starts to shrink after the temporary stop appears to be different from year to year. The measurements of the edge of the permanent cap showed an asymmetric shape in relation to the pole. The cap extends farther to the south for longitudes near 0° than for those near 180°. The shape of the permanent cap for Ls = 84°–101° in 1980 agrees very well with that for Ls = 97° in 1972 obtained by Mariner 9.
Seismic constraints on the crustal structure within the Vema Fracture Zone
Authors: Detrick Robert S. , Cormier Marie H. , Prince Roger A. , Forsyth Donald W. , Ambos E. L.
Published: February 2008

Results from recent studies of the Kane and Oceanographer fracture zones suggest that significant variations in crustal thickness are associated with parts of these fracture zones, with total crustal thicknesses in places less than half the thickness of normal oceanic crust. The extent of this anomalous crust and its origin are still very poorly constrained by available geophysical data. Here we report results from a 75 km long refraction line shot in late 1977 over three Hawaii Institute of Geophysics ocean bottom seismometers deployed within the Vema transform valley near its western intersection with the Mid‐Atlantic Ridge (10°53′N, 43°39′W). We show that the apparent crustal thickness beneath this portion of the Vema transform valley is about 5 km, not substantially different from crustal thicknesses typically associated with oceanic crust. However, the Vema fracture zone crust is clearly anomalous and similar in many respects to crust reported from the Kane and Oceanographer fracture zones. It is characterized by low compressional wave velocities, a relatively uniform velocity gradient throughout most of the crust, and the absence of a typical layer 3 refractor. This anomalous crust is interpreted as intensely fractured, hydrothermally altered and serpentinized crustal and upper mantle rocks overlain by locally thick accumulations of basaltic rubble from the adjacent fracture zone walls. The large variation in Moho depths found in these large‐offset, slowly slipping transforms is attributed to differences in the depth and extent of serpentinization of ultramafics lying beneath a thin and highly fractured basaltic and gabbroic layer. One important consequence of the relatively low seismic velocities and densities inferred for the crust in these fracture zones is that transform valleys are not in local isostatic equilibrium with the surrounding seafloor.
H2 generation by reaction between H2O and crushed rock: An experimental study on H2 degassing from the active fault zone
Authors: Kita Itsuro , Matsuo Sadao , Wakita Hiroshi
Published: February 2008

Granite and quartz were crushed under moist conditions in the temperature range of 25–270°C, in order to obtain the information on the mechanism operative for H2 degassing along active faults. It was found that the amounts of H2 gas released by crushing granite and quartz increased with temperature up to around 200°C and then decreased suddenly. The results indicate that H2 gas is generated by a chemical reaction between water and such radicals as Si · and Si—O · formed on the fresh surfaces when the Si—O—Si bonds are destroyed by crushing and that the existence of a maximum for amount of H2 gas is due to the decrease of the lifetime of Si—O · radical at the temperature higher than about 200°C. The results lend credence to the possibility that H2 gas is generated in the temperature range of 25–270°C along the active fault through the reaction between groundwater and the fresh surfaces of underground rocks formed by the fault movements.
Kimberlites in western Liberia: An overview of the geological setting in a plate tectonic framework
Authors: Haggerty Stephen E.
Published: February 2008

The Liberian kimberlite field lies in the southern part of the West African craton. Closely associated kimberlites are present in Sierra Leone, Ivory Coast, Guinea, and Mali and, as a group, are structurally controlled within the Archean basement complex of the Leo uplift. Three major kimberlite dike lineaments are recognized in the northwest sector of Liberia all with prominently developed N‐NE trends. These lineaments are termed the Kumgbo‐Wuese trend, the Border trend, and the Lofa trend. The areas of dike intrusion are covered by the Bopolu geological quadrangle map of Liberia: the Border trend is close to Sierra Leone, the Kumgbo‐Wuese trend is to the east, and the Lofa trend is coincident with the Lofa River. Only one kimberlite pipe has been recognized in Liberia (Mano Godua), but a second may be present, and both lie along the Kumgbo‐Wuese trend. The kimberlite bodies intrude the ∼2700‐m.y.‐old Liberian age province, which is comprised of granitic and granodioritic gneiss complexes. Two prominently developed features dominate the province: a general NE‐SW basement fabric and an en echelon dolerite dike system that parallels the coast, dated at ∼180 m.y. and considered to represent intrusion at the onset of continental fragmentation. Transecting dikes and related faults suggest a relative age for the kimberlites of < 180 m.y., an estimate that corresponds to the 92‐ to 140‐m.y. age for the Sierra Leone field. There is indirect evidence that kimberlites in Guinea and Mali are of a similar age based on the fact these are on strike with kimberlites in Liberia and Sierra Leone, respectively. Trans‐Atlantic correlations between West Africa and northeast South America reveal a cyclic eruption of magma types that were generated in response to plate and continental movements. It is postulated that Liberian kimberlite emplacement was related to postplate motion crustal flexuring that reactivated Precambrian basement sutures. These sutures are parallel to, and correlate with, recognizable paleorift and paleocompressional zones within, and adjacent to, the West African craton. Long sustained periods of tectonic activity can be demonstrated, and it is proposed that, if brittle fractures are continuously reactivated, these will serve as conduits to the upper mantle, tapping kimberlite source regions at depths in excess of 100 km.
Modal petrology of six soils from Apollo 16 Double Drive Tube Core 64002
Authors: Houck Karen J.
Published: February 2008

Detailed petrographic data were collected from six size fractions for six samples from Apollo 16 drive tube section 64002. Soils from the core were found to have similar source rocks (defined as anorthosites, impact melts, and metamorphosed breccia) to Apollo 16 surface soils and core 60009. Two slightly differing sets of source rocks, one with a metamorphosed breccia to crystalline matrix breccia ratio of ∼2:3 and the other with a ratio of ∼ 1:1, were defined for the upper and lower sections of the core, respectively. Analysis of modal data shows that the upper three and the lowermost core soils are mature and have similar maturation histories, whereas the two middle soils are submature and have maturation histories similar to each other but differing from the upper and lowermost soils. In core 64002 soils, mixing has dominated over reworking. Mixing components are thought to be two mature soils distinguished by differing source rocks, and an immature plagioclase‐rich soil. The plagioclase‐rich soil is most prominent in the submature section of the core, and appears to correlate with larger clasts of chalky, friable breccia. The friable breccias and the immature plagioclase‐rich soil are tentatively associated with the Descartes Formation. Supplementation of petrographic data with 53Mn, ferromagnetic resonance (FMR), and core dissection data allows division of the core into three zones. The upper mature zone is fairly homogenous and was recently deposited in a short span of time. The middle submature zone is less homogenous, more plagioclase rich, and may be an ejecta deposit. The lower zone, though mature and homogenous, has the same source rocks as the middle zone.
Elastic anisotropy in marine sedimentary rocks
Authors: Bachman Richard T.
Published: February 2008

Measurements of four of the five elastic stiffnesses of marine calcareous rocks and estimates of the fifth, c13, allow more detailed discussion of elastic wave propagation in these rocks than previously possible. The constant c13, which is seldom measured and was not measured in the rocks of this study, was derived by equating the Gassmann and Hashin‐Shtrikman estimates of the bulk moduli of chalk and limestone and then solving for the single unknown c13. For chalk, the measured constants, in N/m2×1010, are c11 = 1.01, c33 = 0.94, c44 = 0.18, and c66 = 0.21. For limestone, the measured constants, also in N/m2×1010, are c11 = 2.41, c33 = 2.09, c44 = 0.47, and c66 = 0.60. Three physically possible values of c13 were computed for chalk (0.48, 0.52, and 0.63 N/m2 ×1010) and for limestone (0.73, 1.05, and 1.24N /m2×1010). Heretofore, the only statements which could be made about c13 in these rocks are |c13| < 0.87×1010 N/m2 (chalk), and |c13| < 1.94×1010 N/m2 (limestone). The calculations require the assumption that anisotropy in these rocks is caused by mineral alignment and the simplification of monomineralogy (calcite). On the basis of these measurements and estimates, the effect of elastic anisotropy on seismic reflection determinations of vertical compressional wave speed in calcareous rocks below the earth's surface is small ( ±6%), whereas similar determinations of vertically polarized shear wave speeds may be unreliable. Relationships are provided to convert seismic refraction measurements of horizontal compressional wave velocity to vertical velocity. These relationships are independent of the estimated values of c13.
Constraints upon water advection in sediments of the Mariana Trough
Authors: Abbott Dallas H. , Menke William , Morin Roger
Published: February 2008

Thermal gradient measurements, consolidation tests, and pore water compositions from the Mariana Trough imply that water is moving through the sediments in areas with less than about 100 m of sediment cover. The maximum advection rates implied by the thermal measurements and consolidation tests may be as high as 10−5 cm s−1 but are most commonly in the range of 1 to 5×10−6 cm s−1. Theoretical calculations of the effect of the highest advection rates upon carbonate dissolution indicate that dissolution may be impeded or enhanced (depending upon the direction of flow) by a factor of 2 to 5 times the rate for diffusion alone. The average percentage of carbonate is consistently higher in two cores from the area with no advection or upward advection than the average percentage of carbonate in three cores from the area with downward advection. This increase in average amount of carbonate in cores with upward moving water or no movement cannot be attributed solely to differences in water depth or in amount of terrigenous dilution. If the sediment column acts as a passive boundary layer, then the water velocities necessary to affect chemical gradients of silica are in the range 10−9 to 10−10 cm s−l. However, if dissolution of silica occurs within the sediment column, then the advection velocities needed to affect chemical gradients are at least 3×10−8 cm s−l and may be as high as 3×10−6 cm s−l. This order of magnitude increase in advection velocities when chemical reactions occur within the sediments is probably applicable to other cations in addition to silica. If so, then the advection velocities needed to affect heat flow ( >10−8 cm s−1) and pore water chemical gradients are much nearer in magnitude than previously assumed.
A seismic model of a partially molten inner core
Authors: Loper David E. , Fearn David R.
Published: February 2008

Fearn et al. (1981) recently proposed a mechanism whereby the inner core contains a significant fraction of liquid. We construct a model of a partially molten inner core compatible with seismic observation. Specifically, we develop an improved model of bulk anelasticity including the effect of thermal and material diffusion and show that it is quantitatively capable of explaining the observed anelasticity. Also, studies of the reflection of seismic waves from the inner‐core boundary are used to estimate the mass fraction of liquid in the inner core.
Three‐dimensional crust and upper mantle structure at the Nevada Test Site
Authors: Taylor Steven R.
Published: February 2008

The three‐dimensional crust and upper mantle structure at the Nevada Test Site (NTS) is derived by combining teleseismic P wave travel time residuals with Pn source time terms. The NTS time terms and relative teleseismic residuals are calculated by treating the explosions as a network of ‘receivers’ which record ‘shots’ located at the surrounding stations. Utilization of the Pn time terms allows for better crustal resolution than is possible from teleseismic information alone. Average relative teleseismic P wave residuals show a consistent progression of positive (late arrivals) to negative residuals from east to west across the NTS. However, Pn time terms beneath Rainier Mesa are at least 0.3 and 0.5 s less than those beneath Pahute Mesa and Yucca Flat, respectively, indicating the presence of high‐velocity crustal material or crustal thinning beneath Rainier Mesa. The time terms at Pahute Mesa are surprisingly uniform, and the largest time terms and residuals are observed in the northwest and southern parts of Yucca Flat. The Pn time terms show a slight correlation with the working‐point velocity at the shot point for Pahute Mesa and Yucca Flat, indicating that part of the observed lateral variations are caused by shallow effects of the upper crust. Three‐dimensional inversion of the travel time residuals suggests that Yucca Flat is characterized by low‐velocity anomalies confined to the upper crust, Rainer Mesa by very high velocities in the upper and middle crust, and Pahute Mesa by a high‐velocity anomaly extending through the crust and into the upper mantle. Relatively low velocities are observed in the lower crust beneath the Timber Mountain caldera south of Pahute Mesa with no expression in the upper mantle. These observed differences in velocity beneath the Tertiary Silent Canyon and Timber Mountain calderas may be related to their magma volume and mode of enrichment from a mantle‐derived magma source.
The geoid, small‐scale convection, and differential travel time anomalies of shear waves in the central Indian Ocean
Authors: Stark Mitchel , Forsyth Donald W.
Published: February 2008

We have measured 37 differential travel times for the phase pairs SS‐S, sSS‐sS, ScS2‐ScS, ScS2‐S, sScS2‐sScS, and sScS2‐sS. Each pair included a surface bounce point in the central Indian Ocean between 4°N and 21°S and 69° and 85°E. Residuals were calculated relative to both the J‐B travel time tables and the PEM‐C earth model and were corrected for the elevation of the bounce points. The mean J‐B residual associated with bounce points in the Central Indian Basin is −2.8±0.7 s (fast). This contrasts with +4.2 s (slow) reported for the western Pacific by Sipkin and Jordan, and is similar to the −4.0 s found for old continental nucleii. After correcting for known differences between continental and oceanic crustal structure and for differences in measurement technique, we find that travel times in the mantle beneath the Central Indian Basin are midway in character between those beneath pre‐Cambrian shields and those beneath the western Pacific ocean basin. The fastest differential travel times occur along a line passing close to the center of the large Indian Ocean geoidal low. There is a clear increase in travel time with distance from the center of the geoid low, suggesting that the source of the geoid anomaly may also be largely responsible for the difference between the Central Indian and Pacific basins. The Chagos‐Laccadive Ridge seems to coincide with a transition from the fast differential travel times of the Central Indian Basin to slower differential travel times associated with bounce points beneath young seafloor near active spreading centers. There are also variations within the Central Indian Basin that are not related to surface geological features or to the age of the seafloor. These variations are periodic with a wavelength of about 640 km and with a NNE trend, which agrees approximately with the trend that would be expected for linear, convective rolls in the upper mantle.
In situ studies of velocity in fractured crystalline rocks
Authors: Moos Daniel , Zoback Mark D.
Published: February 2008

A study of the effects of macroscopic fractures on P and S wave velocities has been conducted in four wells drilled in granitic rock to depths between 0.6 and 1.2 km. The effect of macroscopic fractures is to decrease both Vp and Vs and increase Vp/Vs. In wells with a relatively low density of macroscopic fractures, the in situ velocity is similar to that of saturated core samples under confining pressure in the laboratory, and there is a clear correlation between zones with macroscopic fractures and anomalously low velocities. In wells with numerous macroscopic fractures, the in situ velocity is lower than that of intact samples under pressure, and there is a correlation between the rate at which in situ velocity increases with depth and the rate at which the velocity of laboratory samples increases with pressure. Differences in in situ P wave velocity between wells cannot be explained solely by differences in the degree of macroscopic fracturing, thus emphasizing the importance of composition and microcracks on velocity. In one highly fractured well the in situ P wave velocity is essentially the same for frequencies ranging from 10 Hz to 20 kHz; this suggests that the macrofractures affect velocity similarly over a broad frequency range. Chemical alteration of rock adjacent to macroscopic fractures appears to play an important role in reducing in situ velocities. Synthetic reflection seismograms generated from the velocity logs suggest that fracture zones are one possible source of deep‐crustal reflectors observed on seismic reflection profiles.
Transport of radon in flowing boreholes at Stripa, Sweden
Authors: Nelson P. H. , Rachiele R. , Smith A.
Published: February 2008

Granitic rock in an underground experimental waste storage site at Stripa, Sweden, is unusually high in natural radioelements (∼40 ppm uranium), with higher concentrations occurring locally in thin chloritic zones and fractures. Consequently, groundwater seeping through fractures into open boreholes is highly anomalous in its radon content, with activity as high as 1 μCi/1. When total count gamma‐ray logs are run in boreholes where groundwater inflow is appreciable, the result is quite unusual: the radon daughter activity in the water adds considerably to the gamma contribution from the rock, and in fact often dominates the log. The total gamma activity increases where radon‐charged groundwater enters a borehole and decays as the water flows along the hole in response to the hydraulic gradient. As a consequence the gamma log serves as a flow profile, locating zones of water entry (or loss) by an increase (or decrease) in the total gamma activity. If mixing within the borehole does not occur, the activity decreases exponentially along the hole away from the entry point because of the steady decay of radon and its daughter products as they migrate with the flow in the water column.This spatial decay rate can be converted to a linear flow rate since the 3.8‐day half‐life of radon governs the response time. For example, if the volumetric flow rate in a 76‐mm hole falls within the range 0.5–50 liters per day and if observations are available from a 10‐m length of hole, then the flow rate can be measured quantitatively. Proportionately higher rates can be measured if longer hole lengths are available for observation. A model for flow through a thin crack emanating radon at a rate E shows that the radon concentration of water entering a hole is E/λh, where λ is the radon decay rate and h the crack aperture, assuming that the flow rate and crack source area are such that an element of water resides within the source area for several radon half‐lives or more. Using this simple relationship, independent measurements of emanation and concentration produce reasonable estimates of fracture aperture. Although uranium concentration values at Stripa are unusually high, neither the emanation coefficients nor the fracture properties appear to be unusual for granitic rock. It therefore seems likely that many granitic sites must exist where the radon content in groundwater is higher than in other geological terranes, although perhaps not as high as the microcurie per liter concentrations found at the Stripa site.
A new rock‐magnetic approach to selecting sediments for geomagnetic paleointensity studies: Application to paleointensity for the last 4000 years
Authors: King John W. , Banerjee Subir K. , Marvin James
Published: February 2008

The hypothesis that the ratio of detrital remanent magnetization to anhysteretic remanent magnetization (DRM/ARM) for sediment samples is a measure of relative geomagnetic paleointensity is critically evaluated by two distinct approaches. One approach is a detailed rock‐magnetic examination of the implicit assumptions of the DRM/ARM method and the construction of a selection process by which to identify sediments that conform to requirements satisfying these assumptions. Sediments are ‘uniform’ with respect to DRM/ARM ratio if they contain magnetite in the 1–15 ym particle size range as the predominant magnetic mineral and have variations in magnetite content of less than 20–30 times the minimum concentration. The DRM/ARM ratios of these sediments should provide estimates of relative geomagnetic paleointensity. Relative particle size variations in magnetite are detected with a plot of anhysteretic susceptibility (XARM) versus low‐field susceptibility (X) and the size range ≃ 1–15 μm is approximately identified by high‐field hysteresis parameters. A rock‐magnetic evaluation of LeBoeuf Lake sediments with these techniques indicates that these sediments are suitable for a relative plaeointensity study. The second approach to evaluating the DRM/ARM ratio as a measure of relative paleointensity is direct comparison with absolute paleointensity data. A comparison between the LeBoeuf Lake estimates and Thellier‐Thellier results from the western United States supports the conclusion that suitable sediments can record geomagnetic paleointensity fluctuations.
Constraints on the structure of the Himalaya from an analysis of gravity anomalies and a flexural model of the lithosphere
Authors: Lyon‐Caen Hélène , Molnar Peter
Published: February 2008

The intracontinental subduction of India beneath the Himalaya presents several similarities to that occurring at island arcs. We study one of those similarities by analyzing gravity anomalies across the Himalaya assuming that the topography is supported by the Indian elastic plate, flexed under the weight of both the overthrust mountains and the sediments in the Ganga Basin. We first examine in detail the effects of each of the following parameters on the configuration of the elastic plate and on the gravity anomalies: the flexural rigidity, the position of the northern end of the elastic plate (the amount of underthrusting of such a plate beneath the range), and the density contrasts between the crust and mantle and between the sediments and the crust. A plate with a constant flexural rigidity of about 0.7×1025N m (between 0.2 and 2.0×1025N m) allows a good fit to the data from the Lesser Himalaya and the Ganga Basin. Such a plate, however, cannot underthrust the entire Himalaya. Instead, the gravity anomalies show that the Moho steepens from only about 3° beneath the Lesser Himalaya to about 15° beneath the Greater Himalaya. This implies a smaller flexural rigidity beneath the Greater Himalaya (0.1 to 1.0×1023N m) than beneath the Ganga Basin and the Lesser Himalaya. Even with a thin, weak plate beneath the Greater Himalaya, the weight of the mountains depresses the plate too much unless an additional force or moment is applied to the plate. The application of a bending moment/unit length to the end of the plate of about 0.6×1018N m is adequate to elevate the Indian plate and to bring the calculated gravity anomalies in agreement with those observed. Both, the smaller flexural rigidity and the bending moment can be understood if we assume that part or all of the Indian crust has been detached from the lower lithosphere that underthrusts the Greater Himalaya. We study the tectonic implications of these results by means of a series of idealized balanced cross sections, from the collision to the present, that reproduce several important features of the geology of the Himalaya and predict an amount of eroded material comparable to that in the Ganga Basin and the Bay of Bengal. These cross sections include high‐grade metamorphic rocks near the Main Central Thrust and a steeper dip of it there than in the Lesser Himalaya. They predict rapid uplift only in the Greater Himalaya and at the foot of the Lesser Himalaya.
Some comments on two‐layer extensional models for the evolution of sedimentary basins
Authors: Hellinger Steven J. , Sclater John G.
Published: February 2008

Simple extensional models that involve listric faulting in the brittle upper crust and plastic flow in the lower lithosphere have been shown to account for the subsidence history of various continental sedimentary basins and continental shelves. In this paper we present structural evidence that extension in the continental lithosphere during rifting is not uniform. We review evidence that the rifting process in major intracratonic rift basins is spasmodic rather than continuous, that extension and subsidence within the rift during a tectonic phase is generally accompanied by uplift and erosion of the rift flanks, that subcrustal heat input during rifting occurs over an area much broader than the rift proper, and that there may be broad regional uplift before and/or during rifting. To explain the uplift of the flanks during rifting and their postrift subsidence, we invoke two mechanisms: subcrustal thinning by extension below the flanks and tilted major normal faults in the crust between graben and flanks. The uplift and erosion of the flanks due to tilted faults probably do not significantly affect estimates of extension from subsidence analysis, but uplift due to both sources can, in certain cases, be incorrectly attributed to regional uplift or doming. We present a simplified analysis of the two‐layer extensional model for the elementary case in which extension is instantaneous and the crust is thinned by a different amount from the subcrustal lithosphere. We provide a simple graphical method of data analysis to obtain the parameters that describe thinning of the crust and lithosphere during extension from the subsidence history. Application of this method to the central part of the Pattani Trough in the intracratonic Gulf of Thailand shows that extension and thinning of the lithosphere of a factor of ∼2 since late Eocene can account for much of the subsidence history and the present‐day heat flow. Our analysis suggests that thinning of the crust in this area is ∼20% greater than thinning of the whole lithosphere. Assuming that none of the lithosphere is removed from the region of extension during rifting, we show by simple geometric examples that a process of nonuniform extension implies that much greater thicknesses of sediment can be deposited in the center of a young basin than in the case of uniform extension of both crust and subcrustal lithosphere. Such an extensional process produces significant uplift of the flanks of the rift, and as a result of erosion of the uplifted areas, the effective cross‐sectional area of the basin can be increased by as much as 10–20%, depending on the rate of erosion, compared to the area that would have been created by a process of uniform extension. Finally, we show that it may ultimately be possible to compute erosion rates from seismic cross sections of basins.
Localized geomagnetic field changes near active faults in California 1974–1980
Authors: Davis Paul M. , Johnston Malcolm J. S.
Published: February 2008

Daily averages of magnetic difference fields for the years 1974–1980 between 23 magnetometers on the San Andreas fault (between San Francisco and the Mexican border) have been analyzed to isolate the geomagnetic variation of crustal origin and to test if it is related to stress. Field changes due to nonuniform secular variation from currents in the core have been identified and removed. External currents in the ionosphere and magnetosphere induce changes in the difference fields due to contrasting impedances at magnetometer sites. These changes are identified and removed by multichannel predictive filtering using Boulder magnetic observatory component fields as inputs to the prediction operator. The remaining fluctuations (up to several nT) are attributed to crustal magnetization changes. Evidence that these changes are stress related includes the variation prior to the Busch fault earthquake (1974) seen at the two stations closest to (within 10 km) the epicenter. Also, stations within 50 km of the 1979 Coyote earthquake undergo localized field change up to a year before that event. If truly precursive, these variations are on a broader spatial and temporal scale than those for the Busch fault event. During the period 1975–1978 the geomagnetic variation is approximately uniform across the array, without the development of any significant anomalies. This period roughly coincides with the 1975.7–1978.7 seismic lull in nearly all of California. It is followed by a marked increase in crustal secular variation. Long‐range coherence between anomalous variation at different stations is not seen, other than the variation before the Busch fault earthquake, suggesting that localized variation occurs on a scale of less than 10 km. Coseismic tectonomagnetic events are not observed at the times of the Busch and Coyote earthquakes, but model calculations suggest that observable events are not expected. If the observed changes are due to stress, they indicate that changes of aseismic stress are significantly larger than coseismic stress release.
A deep crustal electromagnetic sounding in the Georgia Piedmont
Authors: Thompson B. G. , Nekut A. , Kuckes A. F.
Published: February 2008

Electromagnetic soundings in the Georgia Piedmont using a horizontal loop source indicate an upper layer with conductivity variations from less than 2×10−5 to 2.5×10−4 S/m extending to about 15 km, underlain by a zone with conductivity of more than 10−3 S/m. At one site the conductance of any intermediate layer is limited to 2 S to a depth of 15 km. The conductivities of the upper layer agree with those expected from the surface geology, and the increase in conductivity occurs at a depth which correlates generally with reflections detected by the Consortium for Continental Reflection Profiling's seismic profile of the area. A possible source of the high conductivity is water saturated sediments trapped when the region was overthrust in the Appalachian orogeny.
Observations of coupled spheroidal and toroidal modes
Authors: Masters Guy , Park Jeffrey , Gilbert Freeman
Published: February 2008

In low‐frequency seismic spectra the Coriolis force theoretically causes quasi‐degenerate coupling of spheroidal and toroidal modes whose spherical harmonic degrees differ by unity and whose frequencies are close. It has been found that the coupling causes clearly observable effects on seismic spectra below 3.0 mHz. Many fundamental toroidal modes have a spheroidal component and vice versa. For example, the 0Sl modes for l = 8–22 are significantly coupled to the 0Tl modes for l = 9–23. The mean frequencies of pairs of coupled multiplets are repelled, and the mean attenuations are averaged. The shifting of complex frequencies must be adequately encompassed in the construction of spherically averaged and aspherical models of the density and elastic and anelastic structure of the earth. The coupling also causes variations in multiplet amplitudes which result in spectral peaks at toroidal mode frequencies on vertical and radial components and at spheroidal mode frequencies on transverse components. The variations must be taken into account in constructing Green's functions for the study of low‐frequency source mechanisms.
A three‐dimensional study of metal grains in equilibrated, ordinary chondrites
Authors: Willis John , Goldstein Joseph I.
Published: February 2008

Metal particles in Guareña (H6), Colby (L6) and St. Severin (LL6) were studied by optical microscopy and by electron microprobe analysis. Observations from successive polished sections through the metal particles show that kamacite and taenite grains, which often appear to be isolated particles, are connected directly or by intervening sulfides. Also tetrataenite rims are widest when adjacent to sulfide or kamacite. These observations indicate that transfer of Ni during cooling when kamacite‐taenite phase growth takes place does not occur through the silicate phases but proceeds through metal and sulfide phases or along grain boundaries. By utilizing the central Ni content of taenite grains from successive sections, metallographic cooling rates were determined more precisely than by using one arbitrary section. Cooling rates determined in this manner for Guareña, Colby, and St. Severin are 4.3 K m.y.−1, 4.0 K m.y.−1, and 1.0 K m.y.−1, respectively.
Half‐life of 26Al
Authors: Norris T. L. , Gancarz A. J. , Rokop D. J. , Thomas K. W.
Published: February 2008

The half‐life of 26Al has been redetermined because of suggestions of an error in the accepted value based on its use in calculating 21Ne production rates from cosmic rays in meteorites. Two solutions of 26Al were analyzed for the specific radioactivity and mass spectrometric determination of the 26Al concentration. The half‐life obtained for 26Al was 7.05×105 years ±3.4%. This is identical to the accepted value of 7.16×105 years and indicates that problems with the 21Ne production rate is not due to an erroneous half‐life.