bluuebunny/crossref_metadata_2025 huggingface dataset:
Preseismic and coseismic magnetic field measurements near the Coyote Lake, California, earthquake of August 6, 1979
Authors: Johnston M. J. S. , Mueller R. J. , Keller V.
Published: February 2008

The epicenter of the Coyote Lake earthquake (ML = 5.9 ± 0.2) of August 6, 1979, is located within an array of recording magnetometers which has been in operation since 1974. The nearest instrument, COY, was within 5 km of the epicenter. It was installed in October 1978 and is located on sedimentary rock, although volcanic and ultramafic rocks with magnetizations of up to 1 A/m outcrop 2 km to the west. A second recording magnetometer was operated for 18 days, beginning 4 days after the main event, to record the latter stages of the aftershock activity. Although longer‐term magnetic field variations were recorded at station COY early in 1979 relative to other sites in the area, no anomalous changes within the two months prior to the earthquake were observed outside the present measurement uncertainty of 0.8 nT for hourly average differences. During the late aftershock stage, no magnetic field change greater than 0.25 nT occurred for more than a day. We conclude that in contrast to the 2‐nT change observed before a previous M = 5.2 earthquake near Hollister, California, no demonstrable preseismic, coseismic, or postseismic tectonomagnetic effect was detected. A reasonable seismomagnetic model of the earthquake indicates that station COY was poorly located to detect stress‐generated magnetic perturbations from this earthquake. Using a magnetization distribution indicated by modeling the aeromagnetic data over the area, we have calculated that homogeneous shear stress changes of about 5 MPa or greater would have been necessary to produce any observable effect at COY. This change in stress is precluded by geodetic data from over the area. However, COY is ideally situated for detection of electrokinetically generated magnetic anomalies. This initial null observation indicates that the assumptions used in the calculation of electrokinetic effects have, in this case, not been satisfied.
An elastic rebound model for normal fault earthquakes
Authors: Koseluk Richard A. , Bischke Richard E.
Published: February 2008

A viscoelastic finite element method is used to formulate an elastic rebound model for normal fault earthquake cycles. Model generated displacements correlate well with geodetic survey data collected before and after the 1954 (M = 7.1) Fairview Peak, Nevada, and the 1959 (M = 7.1) Hebgen Lake, Montana, earthquakes. Model coseismic deformation results in an uplift of the footwall block and a depression of the hanging wall block as has been reported by Savage and Hastie (1966). The interseismic phase of the deformation is characterized by a regional doming of the crust in the vicinity of the modeled normal faults. Doming observed before and after and adjacent to the faults which ruptured during the Hebgen Lake earthquake, indicate that areas which are experiencing doming may be the future sites of normal fault earthquakes. Two areas which are presently experiencing crustal doming and are in the vicinity of a normal fault are the Socorro, New Mexico, and Diablo plateau, Texas (Brown et al., 1978), regions. The interseismic doming will not occur if the modeled fault ruptures through the entire lithosphere. If a single normal fault is active, then model generated coseismic and interseismic deformations result in (sum to) a tilt block topography. If two normal faults are active, one master fault and a minor fault which dips toward the master fault, then the resultant deformation is a horst and graben type of topography. The rate at which the doming proceeds is primarily a function of the seismic stress drop and the effective viscosity of the asthenosphere. Doming rates following the Hebgen Lake earthquake indicate an asthenospheric viscosity of about 2 × 1021 P. Model recurrence intervals for the Fairview Peak and Hebgen Lake earthquakes are on the order of 103 yr.
Frequency modulation of the Chandlerian component of polar motion
Authors: Carter William E.
Published: February 2008

Detections of variations in the frequency of the Chandlerian component of polar motion have been reported by several investigators, starting at least as early as Chandler's (1892) report. The concept has never gained general acceptance because no credible cause has been identified. Investigations reported in this paper suggest that the Chandlerian component may be frequency modulated as a linear function of the polar motion magnitude. The variation in frequency is estimated to be ∼0.03 to 0.06 cycles per year increase per 0.1 second of arc decrease in the polar motion magnitude. Nonequilibrium response of the oceans to polar motion, which takes the form of a pole tide that varies with time in enhancement and phase lag, is suggested as a possible cause of the frequency modulation.
Heat flow from Io (JI)
Authors: Matson Dennis L. , Ransford Gary A. , Johnson Torrence V.
Published: February 2008

We have reexamined the existing ground‐based measurements of Io's thermal emission at infrared wavelengths of 8.4, 10.6, and 21 μm. Present in these data is the signature of ‘hot spots,’ presumably similar to the hot spots seen by the IRIS experiment on Voyager. It is possible to extract from these data the total amount of power radiated. Since the hot spots are believed to be a result of deep‐seated activity in Io and since the remainder of Io's surface is an extraordinarily poor thermal conductor, the power radiated by the hot spots is essentially the total heat flow. Our analysis yields a heat flow of 2 ± 1 W m−2. This value is tremendously large in comparison to the average heat flow of the Earth (0.06 W m−2) and the moon (0.02 W m−2), but it is characteristic of active geothermal areas on the Earth. A heat flow this large requires that the interior of Io be at least partially molten on a global scale.
Source characterization of two Reykjanes Ridge earthquakes: Surface waves and moment tensors; P waveforms and nonorthogonal nodal planes
Authors: Tréhu Anne M. , Nábělek John L. , Solomon Sean C.
Published: February 2008

Well‐constrained fault plane solutions from P wave first motions for mid‐ocean ridge normal faulting earthquakes usually require nonorthogonal nodal planes. Local structural effects and/or departures from a double‐couple source mechanism have been invoked to explain this phenomenon. In order to obtain an independent determination of the source mechanisms for the April 24, 1970, and April 3, 1972, events on the southern Reykjanes Ridge, we invert the Rayleigh wave radiation pattern to obtain the source moment tensor. The moment tensor formulation should be particularly well suited to this problem because it is not restricted a priori to a double‐couple source mechanism. A potential drawback of the technique, however, is the requirement that phase velocities along the earthquake‐station paths be known very accurately in order to obtain the source phase from the observed phase, and an objective of this study was to determine whether a regionalized phase velocity model compiled from published dispersion curves is adequate. The results of the moment tensor inversion for both events indicate shallow normal faulting with the tension axis approximately horizontal and perpendicular to the local strike of the ridge. Apparent departures from a pure double‐couple source seem to result from errors in the data and the poor resolution of the Mxz and Myz components of the moment tensor for shallow sources. After performing the inversion under a series of increasingly more stringent constraints we conclude that the data for both events are compatible with pure double‐couple sources with moments of 4.8 and 7.5 × 1024 dyn cm, respectively. We then show that interference between P, pP, and sP due to shallowness of the source can account for the observed nonorthogonality and match the observed P waveforms for the April 3, 1972, event with theoretical seismograms calculated for a shear fault whose orientation is consistent with the surface wave solution. The best fit to the data is obtained for a long, narrow fault (13 km by 3 km), with rupture initiating near the seafloor. The moment indicated by the P waves is 7.5 × 1024 dyn cm. These source parameters give an average displacement of about 60 cm and a stress drop of 30–60 bars, taking into account various uncertainties. Although we might expect attentuation to be high in the mid‐ocean ridge environment, the average attenuation required to fit the teleseismic data is not higher than normal (t* = 1 s). The P waves from the April 24, 1970, earthquake were too small to be suitable for quantitative modeling by synthetic seismograms but are qualitatively consistent with a shallow fault model similar to that for the larger event. We conclude that the faulting process described by these two earthquake mechanisms is directly related to the formation of rift valley topography.
Widespread remagnetization of Helvetic limestones
Authors: Kligfield Roy , Channell J. E. T.
Published: February 2008

The rock and paleomagnetism of shallow water Mesozoic limestones from the Helvetic nappes, the autochthonous cover sediments of the Aiguilles Rouges and Aar massifs, and the Chaine Subalpine of Switzerland and NE France have been studied. Fold tests indicate that the magnetization after alternating field (af) and thermal cleaning postdates Neogene folding; no reversed directions were observed. Acquisition of viscous remanent magnetization (VRM) in the laboratory shows that the natural remanent magnetization (NRM) intensity can be accounted for by viscous build‐up during the Brunhes epoch. The alternating magnetic field required to destroy the VRM produced in various times (t) is proportional to log t. The behavior of the magnetization at low temperatures indicates that the VRM is probably held by very fine grains whose room temperature state is close to superparamagnetism. The blocking temperatures of the NRM are mainly below 350°C. Thermomagnetic experiments, coercivity spectra, rotational hysteresis, and optical examination suggest that pyrrhotite, occurring as minute inclusions within pyrite grains, is responsible for the VRM.
Geology and seismic structure of the northern section of the Oman ophiolite
Authors: Christensen Nikolas I. , Smewing John D.
Published: February 2008

In the north Oman mountains, a continuous ophiolite succession is exposed, from tectonized harzburgites and dunites at the base, through layered gabbros and peridotites, high‐level gabbros and plagiogranite, to a dike swarm and pillowed volcanics overlain by pelagic shales. The upper part of this sequence possesses a static metamorphic overprint, which passes downward from greenschist facies in the lowermost volcanics and most of the dike swarm to amphibolite facies in the lowermost dike swarm and in the high‐level intrusives. These pervasively altered rocks are underlain by layered gabbros and peridotites, where hydration is restricted to fractures. Compressional and shear wave velocities have been measured to confining pressures of 6 kbar for 139 water‐saturated cores from this sequence. The samples were field‐oriented and collected from known stratigraphic levels, which allows the construction of velocity‐depth profiles and an examination of anisotropy within the ophiolite. Compressional wave velocities measured for the basalt section are quite variable, ranging from 4.5 to 6.0 km/s at in situ pressures. Within the sheeted dike section, Compressional and shear wave velocities vary from 5.4 to 6.3 km/s and from 2.9 to 3.6 km/s, respectively. From the basalt‐sediment contact through the dike section, velocities increase rapidly with depth. This gradient is related to increasing metamorphic grade and a decrease in grain boundary porosity. The top of seismic layer 3 is located near the lower boundary of the sheeted dike swarm and marks the transition from greenschist facies to amphibolite facies metamorphics. The dikes decrease in abundance downward into the high‐level intrusives: coarse‐grained gabbroic and granitoid rocks, which often contain abundant hornblende, with isotropic textures and lacking compositional layering. Mean compressional and shear wave velocities increase uniformly from 6.7 and 3.6 km/s, respectively, in the highlevel intrusives, to 7.5 and 3.9 km/s, respectively, at the base of the layered sequence. Measured velocities of the ultramafic tectonites are highly variable because of the anisotropy and serpentinization. Petrofabrics of relic olivine in the Wadi Ragmi region show strong preferred fabrics for the tectonites, and calculated compressional wave velocities for the serpentine‐free rocks range from 7.8 to 8.5 km/s, with the fast direction parallel to the direction of spreading inferred from dike orientations, in excellent agreement with observed upper oceanic mantle seismic anisotropy. The crust‐mantle boundary, which is defined by the abrupt increase in seismic velocities, coincides with the petrological contact between layered gabbros and peridotites and tectonized harzburgite and dunites.
Alkaline ring complexes in Egypt: Their ages and relationship in time
Authors: Serencsits C. McC. , Faul H. , Foland K. A. , Hussein A. A. , Lutz T. M.
Published: February 2008

Potassium‐argon ages are presented for nine alkaline ring complexes in the Southeastern Desert of Egypt, with two or more dates for six of them. The ages range from Cambrian to Cretaceous. This contrasts with the previously held opinion that the ages of these complexes were Mesozoic‐Cenozoic. A range in emplacement ages within any particular complex could not be discerned. This limits the time span of emplacement of any complex to less than a few million years. There is no obvious tectonic significance in the overall distribution of the ages. The intrusions happened periodically in time. Their ages are multiples of 52 m.y. plus 38 m.y.
Cesium chloride versus nickel arsenide as possible structures for (Mg, Fe)O in the lower mantle
Authors: Navrotsky A. , Davies P. K.
Published: February 2008

Thermochemical systematics are used to estimate the pressure for the transition from rocksalt to cesium chloride and from rocksalt to nickel arsenide structures. For MgO, both transitions are predicted to occur at pressures greater than 3000 kbar, which implies that neither transition is likely for MgO in the lower mantle. For FeO, the transition to nickel arsenide is predicted to occur at much lower pressure (∼300 kbar) than that to cesium chloride (3000 kbar), and it is suggested that the transition observed by Jeanloz and Ahrens in shock experiments near 700 kbar may be to the nickel arsenide structure. If a (Mg, Fe) O solid solution with NiAs structure exists in the lower mantle as a minor phase, it will be strongly enriched in iron and other transition metals relative to a (Mg, Fe) SiO3 perovskite phase. This enrichment in the heavier elements, as well as the slightly higher density of nickel arsenide than rocksalt phases, would accentuate the density difference between MSiO3 and MO phases and enhance gravitational settling and lower mantle heterogeneity.
Marine geophysical study of the Comorin Ridge, North Central Indian Basin
Authors: Kahle Hans‐Gert , Naini Bhoopal R. , Talwani Manik , Eldholm Olav
Published: February 2008

Marine geophysical data are used to interpret the continental margin structure south of India and west of Sri Lanka in the northern Central Indian Basin. Free‐air and isostatic anomaly maps are presented. They show the prominent Indian Ocean gravity low with values more negative than −100 mgal south‐west of Sri Lanka. A steep gradient in the free‐air and isostatic anomaly forms a NNW‐SSE trending line extending south and striking parallel to the southwest coast of India. This line marks the eastern boundary of a topographic and basement high here named the ‘Comorin Ridge’ which is also clearly seen on a sediment isopach map. The ridge seems to act as a barrier to the passage of Ganges Cone sediments to the west. A comparison of the eastern (landward) edge of the Comorin Ridge with similar structures of other passive continental margins such as those off southern South Africa (Agulhas Fracture zone), off the Falkland Plateau and off western Norway is made. It is suggested that the eastern edge of the Comorin Ridge may mark a significant structural crustal boundary.
Analysis of peak ground motion in terms of a model of inhomogeneous faulting
Authors: McGarr A.
Published: February 2008

Measurements of peak ground velocity v and acceleration a for earthquakes and mine tremors, with local magnitudes ranging from −0.8 to 6.4, have been analyzed in terms of a model of inhomogeneous faulting. The fault model involves the failure of a circular ‘asperity’ of radius ri surrounded by a previously faulted annular region of outer radius r0. The failure of the asperity results in seismic radiation with a characteristic frequency proportional to 1/ri followed by lower‐frequency radiation, proportional to 1/r0, as static equilibrium is regained over the larger region. For r0/ri ≫ 1 both v and a are associated primarily with the failure of the asperity but also depend on the large‐scale source parameters. Specifically, v = (βΔτr0/μR) (0.10 r0/ri+ 0.15), and a = (Δτ/ρR) [0.30(r0/ri)2 + 0.45], where β is the shear wave velocity, Δτ is the overall stress drop, μ is the modulus of rigidity, and ρ is density. The terms involving r0/ri, correspond to the high‐frequency radiation associated with the failure of the asperity, and the other terms indicate the peak parameters due to the broad‐scale readjustment. Observations of peak ground motion for events with seismic moments ranging from 5 × 1016 to 1026 dyn cm indicate that r0/ri is normally in the range of 1–10 and appears to be independent of earthquake size. The inhomogeneous fault model also yields convenient expressions for the small‐scale displacement and stress drop of the asperity failure as well as for the level of regional stress available to cause slip.
Paleomagnetism of the Roskruge and Gringo Gulch Volcanics, southeast Arizona
Authors: Vugteveen Rob Wm. , Barnes Arthur E. , Butler Robert F.
Published: February 2008

Paleomagnetic data were obtained from nine cooling units of the Late Cretaceous (∼72 Ma) Roskruge Volcanics and from 25 flows in the lower Paleocene Gringo Gulch Volcanics, both from southeastern Arizona. Alternating field demagnetization successfully erased the infrequent secondary components of magnetization. The paleomagnetic pole position obtained from the Roskruge Volcanics is at 73.6°N, 176.0°E with dp = 6.2° and dm = 8.8°. The Gringo Gulch Volcanics pole position is at 77.0°N, 201.0°E with dp = 1.2° and dm = 1.7°. In conjunction with other recently published Paleocene and Eocene paleomagnetic poles, these data provide details of North American apparent polar wander during the Laramide orogeny (∼80 to ∼40 Ma). An episode of rapid apparent polar wander from the Cretaceous pole position in the Bering Strait to the Eocene pole position near the present rotation axis occurred during that interval of time. The initiation of this episode of apparent polar wander appears to be coincident with the major plate reorganizations which occurred at the onset of the Laramide orogeny.
Nonuniform seismic slip rates along the Middle America Trench
Authors: McNally Karen C. , Minster J. Bernard
Published: February 2008

Revised estimates of seismic slip rates along the Middle America Trench are lower on the average than plate convergence rates but match them locally (for example, Oaxaca). Along the Cocos‐North American plate boundary this can be explained by nonuniformities in slip at points of aseismic ridge or fracture zone subduction. For at least 81 yr (and possibly several hundred years), no major (Ms≥ 7.5) shallow earthquake is known to have occurred near the Orozco Fracture Zone and Tehuantepec Ridge areas. Compared with the average recurrence periods for large earthquakes (33 ± 8 yr since 1898 and 35 ± 24 yr between 1542 and 1979), this suggests that either a large (M≥ 8.4) event may be anticipated at such locations, or that these are points of aseismic subduction. Large coastal terraces and evidence suggesting tectonic uplift are found onshore near the Orozco Fracture zone. The larger discrepancy between plate convergence and seismic slip rates along the Cocos‐Carribbean plate boundary is more likely due to decoupling and downbending of the subducted plate. We used the limited statistical evidence available to characterize both spatial and temporal deficiencies in recent seismic slip. The observations appear consistent with a possible forthcoming episode of more intense seismic activity. Based on a series of comparisons with carefully delineated aftershock zones, we conclude that the zones of anomalous seismic activity can be identified by a systematic, automated analysis of the worldwide earthquake catalog (mb≥ 4).
Seismic images of the deep structure of the San Andreas Fault System, Central Coast Ranges, California
Authors: Zandt George
Published: February 2008

Three‐dimensional inversion of teleseismic P wave travel time residuals recorded at the U.S. Geological Survey central California array has resolved small‐scale (∼tens of kilometers) crustal and upper mantle heterogeneity down to depths of 90 km beneath the California coast ranges. Upper crustal lateral velocity variations of ±8% correlate closely with surface geology. Lower‐than‐average velocities are associated with thick Tertiary sedimentary fill and higher‐than‐average velocities with basement exposures. Lower crustal velocity heterogeneity of ±4% appear to reflect crustal thickness variations. A thinner crust is indicated southwest of the San Andreas fault and northwest of San Pablo Bay. A linear zone of low‐velocities (0 to −4%) subparallel to the San Andreas fault was resolved in the upper mantle. The preferred interpretation is that the low‐velocities indicate a narrow upwarp of asthenosphere to unusually shallow depths (∼45 km) beneath the coast ranges. Such an unusual upper mantle structure may have been produced by the northwestward migration along the California coast of a transiently unstable Mendocino triple junction. The inversion results also indicate the possibility of partial decoupling of the crust from the upper mantle.
The effect of hot spots on the oceanic age‐depth relation
Authors: Heestand Richard Lee , Crough S. Thomas
Published: February 2008

Hot spots located both midplate and on midocean ridges can bias the empirical age‐depth relation. Ocean floor bathymetry for seafloor greater than 80 m.y. B.P. in age has been previously used to determine if the oceanic lithosphere behaves as a cooling half space or as a cooling plate. However, large areas of the ocean basins do not fit a simple age‐depth relation. Anomalously shallow swells, approximately 1000 km wide and 1 km high surround hot spots such as Iceland, the Azores, and the Cape Verde Islands. By sorting seafloor depth data by distance to the nearest hot spot track, we show that the along‐isochron variation in depth in the North Atlantic correlates with distance from a hot spot track. Due to the absence in the North Atlantic of seafloor both old and far from hot spots, we compare depth versus age at constant distance and do not observe flattening of the age‐depth relation in any distance range. The age‐depth relation, depth (meters) = 2700 + 295 t(m.y.)1/2, derived for seafloor 1200 to 1800 km from a hot spot track represents our best estimate of the age‐depth relation appropriate to seafloor unaffected by a hot spot track. In the North Atlantic the previously inferred flattening of the age‐depth relation appears to be caused by the inclusion of hot spot swells. This implies that hot spots may be the primary means of heat transfer from the deeper mantle to the base of the lithosphere.
Stress diffusion along rupturing plate boundaries
Authors: Lehner F. K. , Li V. C. , Rice J. R.
Published: February 2008

An analysis is made of viscoelastic lithosphere/asthenosphere coupling in the time‐dependent redistribution of stress along plate boundaries or other seismic lineaments following great earthquakes. The study is based on a generalization by Rice of Elsasser's model of stress‐diffusion, in which general elastic plane stress deformations are allowed in lithospheric plates which are coupled in an elementary way to a (Maxwellian) viscoelastic asthenosphere. Solutions are developed which describe the large‐scale quasi‐static distribution of thickness‐averaged stresses in the lithosphere at or near stationary or travelling rupture zones, modeled here by either crack‐like zones of fixed stress drop or dislocation‐type slip zones. Sudden ruptures shed load onto the asthenosphere which is gradually transferred back to the lithosphere by a slow relaxation process. The spatial and temporal characteristics of the predicted stress alterations suggest a significant role of lithosphere/asthenosphere coupling effects in triggering interactions of great earthquakes, patterns of prolonged aftershock activity, and the breaking of barriers or gaps by time‐dependent stressing.
Memory, relaxation, and microfracturing in dilatant rock
Authors: Holcomb David J.
Published: February 2008

When Palisades diabase and Westerly granite are made dilatant by cyclic differential stress not exceeding ≅ 85% of fracture strength, they behave as nonelastic, energy‐absorbing media. However, in some ways, their properties show reversibility more typical of elastic media. The quasi‐elastic behavior is characterized by the property ‘memory’. Memory in dilatant rock is manifested by the closure of material property hysteresis loops when the sample is returned to certain previous differential stress states. Simple examples of memory have been observed before in Westerly granite. The present work confirms the existence of memory for strain and compressional wave velocity in Westerly granite and Palisades diabase at confining pressures between 250 and 1500 bars. Memory of maxima or minima of the stress difference can be developed, with multiple points remembered simultaneously. A theory of memory based on reversible tensile Griffith cracks was developed which predicts a variety of phenomena, many of which have been observed. A property that is not predicted is the healing which occurs when unloading is stopped and the load is held constant. A time‐dependent increase in velocity and decrease in sample volume is observed, consistent with the healing of cracks. Relaxation ceases after approximately 2 min. After relaxation, a further decrease in load produces an initial elastic response followed by a return to the previous nonelastic behavior. Perfect memory is lost when microfracturing occurs as a result of either exceeding the previous absolute maximum stress or extensive cycling. Acoustic emissions are observed for conditions where memory is not perfect and are not observed when memory is perfect. Using travel time data, a new, low value was found for the dependence of dilatancy onset on pressure.
Strain accumulation in southern California, 1973–1980
Authors: Savage J. C. , Prescott W. H. , Lisowski M. , King N. E.
Published: February 2008

Frequent surveys of seven trilateration networks in southern California over the interval 1973–1980 suggest that a regional increment in strain may have occurred in 1978–1979. Prior to 1978 and after late 1979 the strain accumulation has been predominantly a uniaxial north‐south compression. This secular trend was interrupted sometime in 1978–1979 by an increment in both north‐south and east‐west extension in five of the seven networks. The onset of this change appears to have occurred first in the networks farthest south. The changes occurred without any unusual seismicity within the networks, but the overall seismicity in southern California was unusually low prior to and has been unusually high since the occurrence. The average principal strain rates for the seven networks in the 1973–1980 interval are 0.17 μstrain/yr north‐south contraction and 0.08 μstrain/yr east‐west extension. Although the observed increment in strain could be related to unidentified systematic error in the measuring system, a careful review of the measurements and comparisons with three other measuring systems reveal no appreciable cumulative systematic error.
One‐dimensional model of vapor‐dominated geothermal systems
Authors: Straus Joe M. , Schubert Gerald
Published: February 2008

Vapor‐dominated geothermal systems similar to the Geysers in northern California and Kawah Kamojang, West Java, are modeled as one‐dimensional structures in a porous medium saturated with water and steam. The model consists of a near‐surface condensate layer lying above a two‐phase counter‐flow region with rising steam and descending water. Real systems have condensate layers several hundred meters thick and vapor‐dominated two‐phase reservoir temperatures near 513 K. Models which reproduce these general characteristics have net mass flow rate M to thermal conductivity k ratios smaller than about 2.5 × 10−7 K s2 m−3 and permeability K to thermal conductivity ratios larger than 4.5 × 10−17 m s3 K kg−1. The mobile fluids in the main reservoirs of these models are essentially all steam. However, the reservoirs of real systems can contain substantial amounts of residual water. Temperature T and pressure p data in the main reservoir of the Kamojang system are best fit by models with K/k between 10−15 and 10−14 m s3 K kg−1. Thus the permeability of the main reservoir is between 4 × 10−15 and 4 × 10−14 m2 (4 and 40 mdarcy) for k = 4 J m−1 s−1 K−1. These p and T data are not diagnostic of the overall flow rate through the Kamojang system. A relatively low permeability cap is required to stabilize the water layer above the steam region.
First‐order major element variation in basalt glasses from the Mid‐Atlantic Ridge: 29°N to 73°N
Authors: Sigurdsson Haraldur
Published: February 2008

Basaltic glasses from 29°N to 73°N on the Mid‐Atlantic Ridge define two geographic and compositional groups that are characterized by different major element concentrations, phase assemblages and partition coefficients. Group A glasses occur on the ridge segment north of Gibbs Fracture Zone, from 54°N, along the Reykjanes Ridge, the western volcanic zone of Iceland and Kolbeinsey Ridge, up to 70°N. Similar glasses occur also on the ridge from 29°N to 34°N. Group A is characterized by pronounced alumina depletion and iron‐enrichment trends, lower silica and alkalis and higher iron and high CaO/Al2O3. Clinopyroxene is only present in the more evolved group A glasses (Mg‐value < 58). Olivine‐glass partition coefficients for Mg and Fe are systematically lower in group A than group B (Schilling and Sigurdsson, 1979), reflecting effects of melt composition on structure of the liquids. Group B glasses define the ridge segment north of 35°N over the Azores platform and up to Gibbs F. Z., at 53°N. They are also present on the Mohns Ridge north of the Jan Mayen F. Z. at 71°N and at least as far north as 73°. Group B glasses are characterized by higher silica and alkalis, low iron and lack of alumina depletion and iron‐enrichment trends. They contain calcic clinopyroxene (Cpx) throughout the compositional range, whereas the sub‐calcic augite is absent. The major‐element variation within each group can be modeled quantitatively by fractional crystallization involving the phenocryst phases. In group A, the model involves only olivine (Ol) and plagioclase (Pl) extraction in the early stages, but a total of 60% crystallization of the most primitive magma is required, yielding a final solid extract of Ol‐Pl‐Cpx in the proportions 1:3.3:1.5. Group B fractionation models require clinopyroxene extraction throughout the compositional range, and 45% crystallization of the primitive magma is required to account for the most evolved glasses giving a total solid extract of Ol‐Pl‐Cpx in proportions 1:5:3.9. The range of glass compositions in the two groups forms two distinct but parallel cotectic trends within the basalt tetrahedron, which are believed to define the quaternary univariant line Ol‐Pl‐Cpx‐Liq along which the magmas evolved during fractionation at low to intermediate pressure. It is proposed that the shift in phase boundaries and univariant lines reflects differences in melt and source compositions of the two groups. The distribution of the two groups is thus taken to indicate two major petrographic provinces of the Mid‐Atlantic Ridge.
Possible isotopic fractionation effects in material sputtered from minerals
Authors: Haff P. K. , Watson C. C. , Tombrello T. A.
Published: February 2008

We discuss in detail a model which makes definite predictions for the fractionation of isotopes in sputtered material. The fractionation patterns can be nonlinear, and the pattern for a particular set of isotopes depends on the chemical matrix within which those isotopes are contained. Calculations are presented for all nonmonoisotopic elements contained in the minerals perovskite, anorthite, ackermanite, enstatite, and troilite. All isotopes are fractionated at the level of approximately 4–6‰ per atomic mass unit. O is always positively fractionated (heavier isotopes sputtered preferentially), and heavier elements are generally negatively fractionated (lighter isotopes sputtered preferentially). The value of δ(18O:16O) is always less by about 1.8‰ than a linear extrapolation based upon the calculated δ(17O:16O) value would suggest. The phenomenon of both negative and positive fractionation patterns from a single target mineral can be used to make an experimental test of the proposed model.
Phase relationships of I‐type granite with H2O to 35 kilobars: The Dinkey Lakes biotite‐granite from the Sierra Nevada Batholith
Authors: Stern Charles R. , Wyllie Peter J.
Published: February 2008

The Dinkey Lakes biotite‐granite from the Sierra Nevada batholith was reacted (with varying percentages of H2O in sealed platinum capsules) in a piston‐cylinder apparatus between 10 and 35 kbar. The results were combined with the results from previously published experiments to provide comprehensive phase relationships for an I‐type granite: a P‐T diagram with excess H2O; isobaric T‐XH2O diagrams at 25, 30, and 35 kbar showing H2O‐undersaturated relations; the H2O‐undersaturated liquidus surface mapped with contours for constant H2O contents and fields for near‐liquidus minerals; and the solubility of H2O in granite liquids to 35 kbar. Results and their implications show: (1) The solidus temperature decreases from 680°C at 2 kbar to 620°C at 10 kbar, then increases to 700°C at 35 kbar because of changes from less dense to more dense subsolidus mineral assemblages. (2) The melting interval with excess H2O, which is only 35°C at 2 kbar, increases to 105°C at 10 kbar and 150°C at 35 kbar because the liquidus minimum in the complex rock system departs from granite composition with increasing pressure. (3) The solubility Of H2O in granite liquid is 27 ± 2.5 weight percent at 35 kbar and 850°C, indicating that a miscibility gap persists between H2O‐saturated silicate magmas and aqueous vapor phase, at least to pressures corresponding to 120‐km depth in the mantle. Dissolution of alkali feldspar (20% of rock) in the subsolidus aqueous vapor phase indicates that deep‐seated aqueous fluids are concentrated solutions. (4) Quartz and coesite are the liquidus minerals at mantle pressures for all H2O contents, indicating that granites and rhyolites cannot be primary magmas from mantle peridotite or subducted oceanic gabbroic crust. (5) The liquidus surface at crustal pressures, with plagioclase and quartz as primary minerals, indicates that primary liquids of granite composition with moderate H2O contents can be generated in the crust at reasonable temperatures; these liquids could rise to near surface levels without vesiculation. Granite liquid together with residual crustal minerals could constitute plutonic magmas of intermediate composition.
Petrology and geochronology of metamorphosed volcanic rocks and a Middle Cretaceous volcanic neck in the east‐central Sierra Nevada, California
Authors: Kistler Ronald W. , Swanson Samuel E.
Published: February 2008

Metamorphosed Mesozoic volcanic rocks from the east‐central Sierra Nevada range in composition from basalt to rhyolite and have ages, based on whole rock Rb‐Sr and U‐Pb zircon dating, of about 237–224, 185, 163, 134, and 100 m.y. The major plutons of the batholith in this area are of Triassic (215–200 m.y.) and Cretaceous (94–80 m.y.) ages. Initial 87Sr/86Sr values for the metamorphosed volcanic rocks of the area are in the range from 0.7042 to 0.7058 and are generally different from the values for the surrounding batholithic rocks (0.7056–0.7066). A circular, zoned granitic pluton, with an outcrop area of 2.5 km2, similar in appearance to a ring dike complex, was apparently a conduit for some or possibly all of the middle‐Cretaceous metamorphosed volcanic rocks exposed about 5 km to the south in the western part of the Ritter Range. Samples from the metamorphosed volcanic rocks and the pluton yield a Rb‐Sr whole rock isochron age of 99.9 ± 2.2 m.y. with an initial 87Sr/86Sr of 0.7048 ± 0.0001. Major element variation diagrams of the pluton and volcanic rocks define coincident compositional trends. The pluton, intruded into Early Jurassic metamorphosed volcanic rocks (Rb‐Sr whole rock isochron age of 185 ± 6 m.y., initial 87/86Sr = 0.70474 ± 0.00009), is composed of a central body of granite that is intruded into and almost completely surrounded by a crescent‐shaped outer rim of quartz monzodiorite. Contact relations between the granitic rocks, intrusive in some places and completely gradational at other places, indicate that the quartz monzodiorite was not completely crystallized prior to the intrusion of the granite. Modal minerology within the pluton is also consistent with this inferred crystallization history. The granitic rock textures are cataclastic, characterized by a groundmass of feldspar and quartz that surrounds larger grains of feldspar, hornblende, and biotite. Widely spaced aplite dikes from the granite intrude the quartz monzodiorite along the contact between the granitic rocks. The aplite dikes and interstitial alkali feldspar associated with myrmekite in the pluton, being undeformed, indicate crystallization of a small amount of silicate liquid after cataclasis. Aplite compositions suggest a minimum crystallization pressure of 0.5 kb (1–2 km); this depth is consistent with crystallization near the base of a large stratovolcano. The pluton is characterized by steep, linear features, defined by mineral clots and inclusions, that probably formed during the upward movement of the nearly crystallized magma in the volcanic conduit. The ages of volcanic events relative to the ages of the major intrusive epochs and the major element and isotopic compositions of the volcanic rocks relative to the major plutons indicate that the volcanic rocks are not simply or directly related to the major plutons in the Sierra Nevada.
Intermediate wavelength magnetic anomalies over ocean basins
Authors: Harrison C. G. A. , Carle H. M.
Published: February 2008

We have examined three very long magnetic field profiles taken over ocean basins for the presence of intermediate wavelength magnetic anomalies. One profile was from the Atlantic Ocean in the Transatlantic Geotraverse area, one ran along latitude 35°S in the SE Pacific, and one ran along 150°W in the Pacific. All three profiles show the presence of intermediate wavelength (65–1500 km) magnetic anomalies generated in the crust or upper mantle. The analysis of magnetic field power spectra shows that the core field becomes unimportant at about a wavelength of 1500 km. Sea floor spreading anomalies should produce a maximum in power at about a wavelength of 65 km. Between these two wavelengths there should be a minimum in power which is not seen on observed records. Inverting the anomalous field to obtain some idea of the magnetization necessary to explain these intermediate wavelength magnetic anomalies shows that values of magnetization in excess of 1 A m−1 are needed if the magnetized layer is as thick as the oceanic crust. Alternatively, rather large thicknesses of upper mantle material with lower intensities of magnetization need to be used. The reason why such magnetization variations exist is not known. It can be shown that upward continuation of the magnetic anomaly signature to an altitude of 350 km (about the perihelion altitude of MAGSAT) will produce anomalies up to 10 nT in amplitude. These should be capable of being seen by MAGSAT, and thus allow us to determine the spatial arrangement of the intermediate wavelength anomalies and hence, hopefully, a clue as to their origin.
Constraints on geological strain rates: Arguments from finite strain states of naturally deformed rocks
Authors: Pfiffner O. A. , Ramsay J. G.
Published: February 2008

A summary of known finite strain states is presented; longitudinal strains (1 +e) as measured in many rocks often range from 1 to 40 and 1 to 0.025. The time span available to produce such measurable strains in young orogenic zones seems to be less than 10 m.y., possibly less than 1 m.y., which constrains conventional strain rates into the range of 10−13s−1to 10−15s−1. For both pure and simple shear (the most efficient way and a much less efficient way to accumulate incremental strains, respectively) the ellipticity of the finite strain ellipse increases in a nonlinear manner. Finite strain variations in adjacent layers, which give rise to features such as cleavage refraction, arise with only slight differences in the strain rates within these layers.
Fine structure of the dipping seismic zone and subduction mechanics in the Shumagin Islands, Alaska
Authors: Reyners Martin , Coles Kenneth S.
Published: February 2008

Microearthquake data from a permanent, telemetered array have been used to elucidate the structure and tectonics of the subduction zone in the Shumagin Islands, Alaska.The shallow microseismicity is characterized by active interplate thrusting in the 25–45 km depth range. Arcward of this interplate activity, microearthquakes in the overlying plate have a strike slip mechanism. The near‐horizontal P axis of this mechanism is oriented in the direction of plate convergence, suggesting that the plate interface at shallow depths is currently efficiently coupled. Below 45 km depth, the dipping seismic zone appears to be double‐planed. The upper plane dips at 32° from 45 to 100 km depth, where it exhibits a kneelike bend below the volcanic front. The lower plane begins at about 65 km depth, where it is separated from the upper plane by some 25 km, and appears to converge with the upper plane at about 120 km depth. The inferred geometry of the plate interface suggests that the double‐planed portion of the dipping seismic zone is a product of elastic unbending of the subducted plate. A composite fault plane solution for events in the upper plane indicates, however, that downdip tension is currently present rather than the downdip compression that would be expected from unbending. It is proposed that because of locking of the plate interface at shallower depths, slab pull is currently overprinting the unbending stresses which predominate at times when the plates are unlocked. This model is consistent with previous interpretations that there is a high probability of a great earthquake in the Shumagin Islands seismic gap within the next decade or two.
A schematic model of crater modification by gravity
Authors: Melosh H. J.
Published: February 2008

Models are proposed to account for the formation of slump terraces, central peaks, peak rings, and concentric rings in large impact structures. Crater slumping and central peak formation is attributed to the process of acoustic fluidization, which endows a volume of the target surrounding the crater with a Bingham plastic rheology for a short time after the impact. The parameters of this Bingham plastic model are remarkably constant throughout the solar system the Bingham yield stress ranges from a maximum of 80 bars on Mercury to a minimum of 7 bars on Callisto. The Bingham viscosity is between 109 and 1010 P at the onset of central peak formation for all bodies studied, although it appears to rise to 1011 P as crater diameter grows to a few hundred kilometers. Asymmetric (or scarp‐bounded) concentric rings are due to a fundamentally different process. They form when the target planet's lithosphere fractures under stresses imposed by the flow of the asthenosphere toward the transient crater cavity. Lithosphere yield strengths of less than a few hundred bars are implied by observed ring development on the moon and Callisto.
Reconstructed seismic velocity structure of the Lewis Hills Massif and implications for oceanic fracture zones
Authors: Karson J. A.
Published: February 2008

The Lewis Hills Massif of the Bay of Islands Ophiolite Complex provides a lower crustal to upper mantle cross section of an oceanic fracture zone segment and adjacent oceanic lithosphere. The structural and petrologic character of the fracture zone region is distinctly different from the comparatively simple layered structure of the rest of the Bay of Islands complex. Results of detailed field mapping, petrography, and laboratory seismic velocity measurements have been combined in order to reconstruct the seismic velocity structure of this slice of oceanic lithosphere. Velocities and velocity anisotropies of each of the major rock units in the massif are restored to their preobduction geometries relative to different stratigraphic levels in the complex and proximity to the fracture zone region. Although all oceanic fracture zones probably have unique structural and petrologic characteristics in detail, from the perspective of the present study it is suggested that, in general, seismic properties differing from that of ‘normal’ oceanic crust and upper mantle may be expected within approximately 10 km of the axis of fracture zones. In these regions, lower crustal as well as upper mantle anisotropy is expected. The Moho might be expected to shallow due to the presence of abundant ultramafic cumulates, resulting in crustal thinning, and significant velocity inversions may occur with depth where large cyclic mafic layers occur within cumulate ultramafics. In oceanic lithosphere which has passed or is passing through a transform fault domain, enhanced mantle anisotropy might occur. The middle to lower crust in these regions should be composed of highly anisotropic metamorphic rocks, which grade upward into serpentinites and highly fractured gabbros, diabases, and basalts with relatively low velocities. It is suggested that velocities as well as velocity anisotropies produced by these types of structures might be determined in situ by seismic refraction experiments and may be used to map the extent of deformation in the oceanic lithosphere near fracture zones.
Directional hypothesis: Late stage preearthquake loading may often be asymmetric
Authors: Kelleher John , Isacks Bryan
Published: February 2008

Spatial patterns of seismicity before some mainshocks appear skewed to one side or toward one end of the pending surface of rupture. Several examples of thrust‐type earthquakes, for example, indicate stronger patterns of prior seismicity within the overthrusting block. Similarly some aftershock patterns appear skewed to one side of the rupture surface or in a direction away from the location of prior seismicity. For some earthquakes, therefore, the zones of most significant activity before and after the mainshock do not coincide and, thus, are difficult to explain in terms of loading processes or physical properties that are completely symmetric about the plane of rupture. Furthermore, there are some indications that these asymmetric phenomena may repeat in the same or in nearby locations. These observations appear to be explainable in terms of either: (1) an asymmetric distribution of rock properties about the fault zone with consequent asymmetric seismic response to stress accumulation, or (2) a fundamental directionality to the process of tectonic loading resulting possibly from earlier crustal or plate movement in a nearby region. These two explanations are not incompatible and may in fact be related.