bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismic reflection images of the crust of the northern part of the Chugach Terrane, Alaska: Results of a survey for the Trans‐Alaska Crustal Transect (TACT)
Authors: Fisher Michael A. , Brocher Thomas M. , Nokleberg Warren J. , Plafker George , Smith Gregory L.
Published: February 2008

Deep crustal seismic reflection data show strong reflections from the middle and lower crust of the convergent continental margin near the eastern end of the Aleutian trench. These data were collected across the Border Ranges fault system, a major suture zone that separates the Peninsular and Chugach tectonostratigraphic terranes. The Chugach terrane consists of metamorphosed, strongly deformed flysch, subduction melange as well as blueschist‐ and greenschist‐facies rocks. Even so, highly reflective rocks lie at great depth within or beneath this terrane. The shallow part of the seismic section (0–5 s) reveals only a few reflections, which undulate and interweave, indicating that some of them originate outside of the plane of the seismic section. Other undulating events could reveal early Cenozoic granitic plutons or antiformal stacks of rock imbricated along thrust faults. In contrast to this shallow reflection pattern, three reflection bands contain most of the events on the middle part of the seismic section (5–12 s or 12–34 km). We interpret divergent reflections within the uppermost band (5–5.5 s) as gently north dipping thrust faults, and subparallel reflections truncated at the boundaries of the upper and middle bands may image a metamorphic foliation or thrust faults. If the truncated reflections represent thrust faults, then the reflection geometry suggests that subparallel roof and floor thrust faults bound imbricated rocks, forming duplex structures. The upper and middle bands are separated by an area on the seismic section that reveals few reflections. This area correlates approximately in depth with a high velocity (7.5 km/s) layer evident in refraction models. The poorly reflective, high velocity unit might be volcanic rocks like those exposed in the structurally lowest part of the Chugach terrane. If so, then the bottom of the poorly reflective rocks indicates the base of the Chugach terrane. Rocks that cause the upper and middle reflection bands and the intervening, poorly reflective volcanic rocks may form a midcrustal shear zone that is about 10 km thick. The deepest band of subparallel reflections (30–35 km) correlates closely in depth with the top of the Wadati‐Benioff zone associated with the underthrusting Pacific plate. The reflections probably stem from the decollement that separates lithospheric plates within the subduction zone. This reflection band increases abruptly in thickness, possibly indicating local underplating of subducted sediment. No events from the oceanic Moho have been recognized. Neither the Border Ranges fault system, the suture between the Chugach and Peninsular terranes, nor deep crustal layers of the Peninsular terrane are evident in these seismic reflection data.
Dynamic source parameters of the March‐May 1969 Serdo Earthquake Sequence in central Afar, Ethiopia, deduced from teleseismic body waves
Authors: Kebede Fekadu , Kim Won‐Young , Kulhánek Ota
Published: February 2008

Long‐period teleseismic P and S waveforms for the largest four earthquakes in the Serdo sequence are compared with synthetic seismograms to infer mechanisms, focal depths, source time functions, and dynamic energy release. For all four events studied, the mechanisms are predominantly strike slip. Depth estimates obtained for the four earthquakes by matching synthetic seismograms to the observations are found to lie between 5 and 8 km. The azimuthal variation of observed body wave duration for the main event indicates that the rupture propagated from NE to SW. Average seismic moments obtained from body wave synthetics vary from 4.6×1017 to 2.5×1018 N m. Average final displacements, stress drops, and lengths of the fault are found to lie between 0.16 and 0.56 m, 0.45 and 1.56 MPa, and 10 and 16 km, respectively. The low stress drop value obtained may indicate the presence of softer materials near the source. The energy release shows that the mode of energy release during the sequence took place in two steps, mainly by the two largest shocks.
Anelasticity and thermal structure of the oceanic upper mantle: Temperature calibration with heat flow data
Authors: Sato Hiroki , Sacks I. Selwyn
Published: February 2008

The thermal structure of the oceanic upper mantle is examined on the basis of seismic attenuation, surface heat flow, and mid‐ocean ridge basalt (MORB) composition. The comparison of seismic Q with laboratory Q determined at sufficiently high pressure and temperature allows us to constrain temperature in the Earth. Since the inferred absolute temperature relies on knowledge of the frequency dependence of Q (if any), we calibrate an unknown frequency effect on Q by comparing the derived temperatures with geotherms calculated from surface heat flow and MORB composition. This comparison yields a correction factor (found to be small) for the geotherm derived from laboratory (ultrasonic) Q. Examples of thermal structure are presented for the Iceland Plateau and the Pacific Ocean. Under the Iceland Plateau, we expect partial melt for the asthenosphere only in the vicinity of the spreading ridge. In asthenosphere older than ∼5 m.y. of the Iceland Plateau and older than ∼40 m.y. of the Pacific Ocean, temperatures are below the dry peridotite solidus, and no partial melt is expected. These results are consistent with the thermal structure derived from a comparison of seismic and laboratory velocities. It is shown that there is a good agreement between temperatures determined from seismic attenuation, heat flow, and MORB chemistry. Since the laboratory measurements were made on a dry peridotite and we employ a dry peridotite solidus for determining the geotherm, the consistency in temperatures derived from three independent data strongly suggests that the upper mantle (remote from subduction zones) is essentially dry. It appears that the spreading process in mid‐ocean ridge causes asthenosphere material to ascend from greater depths where it was at higher temperature. This gives rise to pressure release melting.
Tomographic inversion for three‐dimensional velocity structure at Mount St. Helens using earthquake data
Authors: Lees Jonathan M. , Crosson Robert S.
Published: February 2008

Tomographic inversion is applied to 17,659 P phase observations at 21 stations from 2023 earthquakes in the vicinity of Mount St. Helens to study the three‐dimensional velocity structure. Block size for the inversion is 2 km horizontally and 2 km or more vertically. Locations of hypocenters are assumed known and are based on a reference one‐dimensional, layered velocity structure. A conjugate gradient technique (LSQR) is used to invert the large sparse system of equations, augmented by regularization with a Laplacian roughening matrix. Resolution is estimated by computing the impulse response of the inversion for various critical locations, and uncertainties of the estimates are determined by a jackknife approach. The results of the inversion show a remarkable correlation with known geological and geophysical features. The Spirit Lake and Spud Mt. plutons are characterized by high‐velocity regions extending to approximately 9 km depth. The St. Helens seismic zone, a band of diffuse seismicity extending NNW from the volcano is evident as a prominent low‐velocity lineation. The change in character of the velocity anomalies south of St. Helens corresponds well with the near cessation of seismic activity there. A low‐velocity anomaly beneath the crater from 6 to 16 km depths may represent modern magma accumulations.
Basaltic volcanism in Ethiopia: Constraints on continental rifting and mantle interactions
Authors: Hart William K. , WoldeGabriel Giday , Walter Robert C. , Mertzman Stanley A.
Published: February 2008

Middle to late Cenozoic mafic lavas from the Ethiopian volcanic province exhibit considerable chemical and isotopic diversity that is linked to eruption age and eruption location. These variations provide a geochemical framework in which continental rifting can be examined. Trace element and Sr, Nd, and Pb isotopic data are interpreted to indicate involvement of up to two depleted and two enriched mantle reservoirs throughout Cenozoic rift development in Ethiopia. Superimposed on the characteristics imparted by varying degrees of melting of these distinct reservoirs are the effects of crystal fractionation and, in some instances, crustal contamination. Initial stages of Oligocene rifting and volcanism, as manifested by the rift‐bounding plateau flood basalts, are attributed to asthenospheric upwelling and melting of a heterogeneous, enriched subcontinental lithospheric mantle. Mildly alkaline lavas were produced from an enriched source with characteristics similar to those of the inferred source of other mantle‐derived lavas and xenoliths from east Africa (LoNd array, EMI to HIMU). Contemporaneous tholeiitic lavas were derived from a source similar to that producing oceanic basalts from Samoa and the Society Islands (EMII). As lithospheric thinning and rifting continued into the Miocene, upwelling depleted asthenosphere (depleted OIB reservoir, PREMA) interacted with the lithospheric sources producing lavas with hybrid elemental and isotopic characteristics (11–6 Ma plateau and rift margin basalts). Crustal contamination is most evident in the Oligocene to Miocene plateau basalts and is suggested to have taken place primarily at middle to lower crustal levels during initial stages of continental rifting. By 4–5 Ma b.p. continental breakup had begun in Afar, with basalts during this period being derived almost entirely from a depleted PREMA‐type reservoir. In the Main Ethiopian Rift, where continental breakup is less advanced, young rift basalts retain a geochemical signature consistent with enriched (LoNd)‐depleted (PREMA) mantle hybridization. During the Holocene, proto‐oceanic crust and oceanic crust characterize the Afar and Red Sea/Gulf of Aden, respectively, and input from a depleted MORB source first becomes apparent. Chronologic and tectonic control on mantle melting, mantle reservoir interactions, and crust‐mantle interactions is a theme common to many extensional regions. Another common feature is the apparent role of a depleted PREMA‐type reservoir in these regions, supporting the idea that this reservoir is located at depth within the convecting asthenosphere. Involvement of enriched mantle during continental extension‐related magmatism is also prevalent, but the geochemical signature of this component varies from region to region, suggesting a strong link to local crust formation history and local enrichment events such as subduction‐driven lithospheric recycling.
Constraints on magma genesis behind the Neogene Cascade Arc: Evidence from major and trace element variation of high‐alumina and tholeiitic volcanics of the Bear Creek Area
Authors: Brandon Alan D.
Published: February 2008

Lava flows and dikes from the Bear Creek area within the Neogene Blue Mountains Province (BMP) were erupted in an extensional tectonic environment. Major and trace element variations in the Bear Creek suite constrain models for the petrogenesis of these rocks. Bear Creek basalts, basaltic andesites, and andesites fall into two compositional groupings, high alumina and tholeiitic. The most primitive flow is high alumina in composition with an Mg # = 0.69. Various phase equilibria tests suggest that magma of this composition may be a hybrid of an eclogite‐derived melt and a peridotite contaminant, or it could be a melt derived wholly from a peridotite source. Large ion lithophile element enrichment and high field strength element depletion (especially Nb) in this sample are consistent with its derivation from a fluid metasomatized peridotite source. An alternative source is required for three Bear Creek tholeiites and high‐alumina basalts (HABs) that differ from the other Bear Creek samples primarily in having larger Nb contents relative to other incompatible trace elements. This source can be modeled as partial melts of a peridotite which has been contaminated by a pelagic sediment, presumably from the subducted slab. Least squares mixing calculations with high‐pressure mineral phases from peridotites indicate that high‐pressure fractionation may have generated the more primitive high‐alumina types in the Bear Creek suite. However, using the computer program SILMIN (Ghiorso, 1985) to semiquantitatively model shallow magmatic processes demonstrates that assimilation of greywacke and rhyolite (as a partial melt of the greywacke) and fractional crystallization (AFC) at 1 kbar pressure better explain the major element variation in the high‐alumina types. The tholeiites appear to be derived from high‐alumina parents via fractional crystallization at shallower levels in the crust. Semiquantitative modeling using trace element distributions supports a model in which the HAB suite was derived from peridotite and underwent AFC in the upper crust and the tholeiites were derived via Rayleigh fractionation from HAB parents. The trace element modeling also indicates that the range of trace element abundances cannot be explained by mixing of melts derived from eclogite and peridotite. Therefore a model in which dehydration of the Farallon plate metasomatizes mantle wedge peridotite and where pelagic sediment is incorporated into the peridotite, which subsequently melt to yield magmas that are modified by contamination and fractionation in the upper crust, explains all of the geochemical variation in the Bear Creek area rocks. This model may in part be applicable to the origin of the Picture Gorge and Slide Creek volcanic sequences in the BMP.
Mapping small elevation changes over large areas: Differential radar interferometry
Authors: Gabriel Andrew K. , Goldstein Richard M. , Zebker Howard A.
Published: February 2008

A technique that uses synthetic aperture radar (SAR) images to measure very small (1 cm or less) surface motions with good resolution (10 m) over large swaths (50 km) is presented along with experimental results. The method could be used for accurate measurements of many geophysical phenomena, including swelling and buckling in fault zones, residual displacements from seismic events, and prevolcanic swelling. The method is based on SAR interferometry, where two images are made of a scene by simultaneously flying two physically separated antennas. Then the phases of corresponding pixels are differenced, and altitude formation is deduced from some simple computation and image rectification. It is also possible to use one antenna flown twice over the same scene; then, if the second flight exactly duplicates the track of the first, an interesting possibility occurs. There would be no phase changes between the images at all unless there was a physical change in the scene, such as ground swelling, that would alter the distance from some resolution element to the antenna. Since the phase changes all occur at the short carrier wavelength, the basic limitation on sensitivity is only the phase noise in the system. When the two imaging passes are made from flight tracks that are separated (which is the case with the Seasat images used here), it is no longer possible to distinguish surface changes from the parallax caused by topography. However, with some additional computation, a third image made at some other baseline may be used to remove the topography and leave only the surface changes. This method was applied using Seasat data to an imaging site in Imperial Valley, California, where motion effects were observed that were ascribed to the expansion of water‐absorbing clays. Phase change images of this area are shown, along with associated ground truth about the presence of water. Problems with the technique are explored, along with a discussion of future experimental possibilities on upcoming SAR missions like Earth Observing System (EOS), Earth Resources Satellite (ERS 1), SIR‐C, and the Venus imaging radar, Magellan.
Source parameters of the October 1, 1987 Whittier Narrows Earthquake from crustal deformation data
Authors: Linde Alan T. , Johnston M. J. S.
Published: February 2008

Offsets in the regional strain field, generated by the October 1, 1987, Whittier Narrows earthquake, were recorded with large amplitudes on two deep‐borehole dilational strainmeters at distances of 46.7 and 65.5 km from the hypocenter and marginally on instruments at greater distances in the Parkfield area and at Pinon Flat, where laser extensometers also recorded small offsets. These data are insufficient to solve for the location and physical parameters of the earthquake, but by also using the measured elevation changes in the epicentral area, we are able to invert for source models consistent with all available observations of crustal deformation. The source models obtained indicate that slip extends to a depth of about 30 km, well below the recorded aftershock zone. The requirement for deeper (and presumably aseismic) slip derives from the large negative dilatation experienced by the nearest strainmeter (PUBS), but high‐frequency data from the same site exclude any significant slow component of moment release. By ignoring PUBS we obtain a moment of about 0.7 × 1018 N m on a fault that has a strike and dip of about N60°W and 40° down to the north, respectively, and extends to a depth of about 16 km. The most likely reason for the anomalous offset at PUBS appears to be sympathetic slip triggered on a nearby fault by the main shock. Precursive strain during the month prior to the earthquake is not apparent at the nanostrain level in the data from the closest instrument, but the event was accompanied by a change in strain rate at the two nearer sites.
A new siluro‐Devonian paleopole from Early Paleozoic rocks of the Franklinian basin, North Greenland Fold Belt
Authors: Stearns Carola , Van der Voo Rob , Abrahamsen Niels
Published: February 2008

Demagnetization of dolomitic shales, dolomites, and carbonate breccias of the Cambro‐Ordovician Kap Stanton Formation within the Franklinian basin sequence of the North Greenland Fold Belt yields a characteristic direction (declination/inclination is 16/−16, tilt corrected) from 14 of the 28 sites. A negative breccia test and positive local and regional fold tests bracket the age of the magnetization as postdepositional but pre‐Ellesmerian (pre‐Carboniferous). After rotating the corresponding paleopole (1°N, 302°E) to account for the Cretaceous‐Tertiary opening of the Labrador Sea, the Greenland pole remains removed by approximately 20° from the existing North American Apparent Polar Wander Path (APWP). However, the rotated paleopole (2°S, 284°E in North American coordinates) is similar to British Silurian and Devonian paleopoles in a Bullard et al. (1965) reconstruction (pole is 2°S, 322°E in British coordinates). We interpret our new paleopole as Late Silurian to Early Devonian in age, requiring a modification of previously published APWPs for Laurentia. If the age assignment is correct, the new paleopath for North America/Greenland and that for Great Britain can be brought into good agreement by closing the Atlantic Ocean, thus indicating that North America, Greenland, and Great Britain were assembled in a supercontinent in Late Silurian to Early Devonian time.
Pleistocene deglaciation and the global gravity field
Authors: Mitrovica J. X. , Peltier W. R.
Published: February 2008

The present‐day global geoid and free air gravity anomaly signals induced by Pleistocene deglaciation have been computed, along with their secular variations. A revised Green function has been derived for the free air gravity anomaly, and we find that an Earth model with a lower mantle viscosity moderately larger than the upper mantle value satisfies observations of the anomaly over both Hudson's Bay and Fennoscandia. Using this “preferred” Earth model, the predicted global geoid anomaly map is characterized by peak negative values of −36.7, −9.4, and −21.9 m over Hudson's Bay, northern Europe, and Antarctica, respectively, and by a smooth, small amplitude (≲2.5 m) upwarping over the major ocean basins. Although the field has an order of magnitude less power than the observed, there is a significant spatial correlation between the two. Inferences of mantle viscosity based upon comparisons between the observed geoid and the computed deglaciation‐induced anomaly are hampered by the large contribution from the convective circulation in the Earth's mantle. We have subtracted a predicted convection signal (based on seismic tomography) from the observed field; however, the “residual” geoid thus obtained is still too inaccurate to allow a useful comparison. In contrast, a filtered version of the observed geoid over Canada, which retains harmonic coefficients in the degree range 10–22, provides a relatively uncontaminated (by convection) datum that is indeed satisfied by the preferred Earth model. Secular variations in the Earth's gravitational field are naturally dominated by the signal from Pleistocene deglaciation. In this respect, we compute zonal harmonics in the expansion of the present day rate of change of the geopotential, and in particular, we predict amplitudes comparable to that for the previously analysed J2 for J4,J5,J7, and J9, all of which reflect, in large part, the response due to the melting of the massive Laurentide ice sheet. This analysis suggests that measurement of these higher‐order secular variations of the zonal harmonics based upon analysis of long time series of LAGEOS ranging data could provide very useful additional constraints on the radial variation of mantle viscosity. These constraints, along with all others described in this paper, are shown to be relatively insensitive to assumptions concerning the deflection, during the glacial isostatic adjustment process, of the 400‐ and 670‐km density discontinuities within the mantle.
Petrologic and geologic variations along the Southern Explorer Ridge, northeast Pacific Ocean
Authors: Michael Peter J. , Chase Richard L. , Allan James F.
Published: February 2008

Mid‐ocean ridge basalts (MORB) from the well‐defined Southern Explorer Ridge segment (SER) in the northeast Pacific Ocean are moderately to strongly enriched in incompatible elements. Enriched MORB were erupted at the highest and widest part of the segment (culmination) where the magma supply may be the greatest and also at the northern and southern ends of the SER. Variations in basalts' incompatible element enrichment occur over short distances and suggest that the underlying mantle is heterogeneous on a small scale. The variations also preclude the existence of a long‐lived, well‐mixed magma chamber beneath the robust culmination. Instead, magma chambers are probably temporally and spatially isolated, as evidenced by the presence of highly differentiated lavas. Less enriched (transitional) MORB were erupted along the central part of the SER, 11–27 km south of the culmination, and were supplied to the ridge separately from a less enriched pan of the mantle. A more continuous magma chamber cannot be ruled out for this section on geochemical grounds. All of the MORB from the SER have undergone significant amounts of fractional crystallization. Unlike certain segments of the East Pacific Rise, the most evolved MORB were erupted near the culmination of the ridge segment. Aphyric lavas have been recovered only from the vicinity of the culmination, while phyric lavas have erupted along the entire length of the SER. There is no correlation between composition and total crystal abundances. Despite the requirement of multiple parental magmas, most of the lavas from the culmination and the distal ends of the ridge fall on a common liquid line of descent, indicating that they formed at similar depths and extents of partial melting. The transitional MORB from the central part of the ridge have probably formed by lesser extents of partial melting and separated from their mantle source at greater depths based on abundances of Na and moderately incompatible elements such as Sm and Zr. Enriched, high‐melt fraction lavas at the magma‐starved southern end of the SER closely resemble lavas from the culmination. Either the southernmost lavas have flowed laterally within the crust from near the culmination, or there is another melting anomaly that supplies the southern end of the SER. If the latter case is true, correspondence between ridge morphology and mantle thermal state is poor.
An analysis of split shear waves observed above crustal and uppermost mantle earthquakes beneath Shikoku, Japan: Implications in effective depth extent of seismic anisotropy
Authors: Kaneshima Satoshi , Ando Masataka
Published: February 2008

Shear wave splitting has been detected in three‐component seismograms from a number of small earthquakes taking place either within the upper crust or the uppermost mantle beneath the Shikoku area, Japan. The faster shear waves are polarized nearly in E‐W at the stations in the central part of Shikoku, and in NEN‐SWS at the southern stations. The arrivals of slower shear waves polarized orthogonally to the faster ones are also clearly recognized in many seismograms at one of the stations. The faster shear wave polarizations at the stations in the central part of Shikoku and the pattern of travel time differences between split shear waves are successfully explained in terms of crustal anisotropy generated by vertical alignment of stress‐induced microcracks. The difference in polarization directions suggests that the tectonic stress state beneath the southern Shikoku region is significantly different from that beneath the central part of Shikoku. A quantitative analysis of travel time differences between split shear waves suggests that the presence of seismic anisotropy which causes the observed shear wave splitting is limited to the upper 10 or 15 km of the crust. Nevertheless, the possibility that anisotropy is concentrated in a much thinner surface layer cannot be ruled out. The average aspect ratio of crack might be greater than of the order of 10−2, or the cracks may not be purely vertical but have some fluctuations in the dip angle.
Observations of motional electromagnetic fields during EMSLAB
Authors: Chave Alan D. , Filloux Jean H. , Luther Douglas S. , Law Lawrie K. , White Antony
Published: February 2008

The long‐period (>1 day) behavior of the seafloor electromagnetic fields during EMSLAB is considered in detail with an emphasis on interpretation in terms of oceanic motions. The study begins with a summary of the physics of motional electromagnetic induction, in which the seawater conductivity‐weighted, vertically integrated velocity measured by the horizontal electric field is emphasized. Using frequency‐domain methods, it is shown that seafloor and terrestrial magnetic variations have similar spectral shapes, indicating a common origin, but the seafloor electric field is not consistent with either at periods longer than 4 days, suggesting an oceanic source. The magnetic field variations are highly coherent across the EMSLAB array at periods shorter than 9 days, but the similarity decreases at longer periods, probably due to long‐term instrumental drift. The seafloor horizontal electric field data exhibit the broadband coherence characteristic of ionospheric sources only at periods shorter than 1–2 days and are essentially incoherent between 4 days and a week except in association with certain propagating wave phenomena. The distinction between the depth‐averaged velocity inferred from the horizontal electric field and its point measurement by the vertical electric field is demonstrated with data. Some specific properties of the seafloor electric fields are then considered in detail. The first of these is a 4‐day wave observed to propagate from north‐to‐south along the east flank of the Juan de Fuca Ridge. The wavelength, propagation sense, and association with the ridge are all consistent with topographically trapped Rossby wave behavior at a zero in the group velocity. A second north‐to‐south propagating wave is seen at periods of 8–12 days in the middle of the EMSLAB area with a probable wavelength of order 1000 km. Unresolved low‐frequency structures in the electric field are also mentioned. These observations clearly demonstrate the power of electromagnetic array methods for the study of long‐period oceanic behavior.
A comparison of techniques for magnetotelluric response function estimation
Authors: Jones Alan G. , Chave Alan D. , Egbert Gary , Auld Don , Bahr Karsten
Published: February 2008

Spectral analysis of the time‐varying horizontal magnetic and electric field components yields the magnetotelluric (MT) impedance tensor. This frequency dependent 2×2 complex tensor can be examined for details which are diagnostic of the electrical conductivity distribution in the Earth within the relevant (frequency dependent) inductive scale length of the surface observation point. As such, precise and accurate determination of this tensor from the electromagnetic time series is fundamental to successful interpretation of the derived responses. In this paper, several analysis techniques are applied to the same data set from one of the EMSLAB Lincoln Line sites. Two subsets of the complete data set were selected, on the basis of geomagnetic activity, to test the methods in the presence of differing signal‐to‐noise ratios for varying signals and noises. Illustrated by this comparison are the effects of both statistical and bias errors on the estimates from the diverse methods. It is concluded that robust processing methods should become adopted for the analysis of MT data, and that whenever possible remote reference fields should be used to avoid bias due to uncorrelated noise contributions.
The system Mg2SiO4‐Fe2SiO4 at high pressures and temperatures: Precise determination of stabilities of olivine, modified spinel, and spinel
Authors: Katsura Tomoo , Ito Eiji
Published: February 2008

High‐pressure and high‐temperature phase relations in the system Mg2SiO4‐Fe2SiO4 were thoroughly reinvestigated at pressures up to 21 GPa and at temperatures of 1200°C and 1600°C using a uniaxial split‐sphere apparatus. Both normal and reverse experiments were performed on the univariant reactions. Both exsolving and dissolving experiments were performed to determine the divariant loops at 1600°C. The run products were examined using microfocused X ray diffractometry, scanning electron microscopy, and electron probe microanalysis. The phase diagrams thus constructed were compared with those previously proposed using an experimental or thermochemical method. Phase changes in mantle olivine with Mg number of 89 occur in the following order: (α) − (α+β) − (β) − (β+γ) − (γ). The α+β loop narrows with increasing temperature. The applicability of an isochemical peridotitic mantle model is discussed in the light of the location and thickness of the 400‐km discontinuity.
Summer earthquakes in southern California related to pressure patterns at sea level and aloft
Authors: Namias Jerome
Published: February 2008

This paper attempts to confirm with an expanded data set the author's earlier conjecture that pressure and wind patterns over a large portion of the northern hemisphere play a role in triggering moderate size earthquakes in southern California. Various indices of atmospheric pressure and geostrophic wind over the eastern North Pacific, western North America, and eastern North America suggest a connection between seasonal anomalous pressure patterns and moderate size quakes (or their absence) over southern California in summer. Composite charts of seasonal pressure anomalies indicate that summer quakes are more likely when the North Pacific high, the western North American low‐pressure, and the continental upper level anticyclone over eastern United States are well developed, and conversely, quakes are inhibited when these centers of action are poorly developed. A low‐frequency signal shows up in both the summer earthquake and pressure anomaly data. The above conclusions are statistically significant at the 90% confidence level or above. These empirical findings appear significant enough to warrant further testing and the development of a physical hypothesis. Some speculations are offered.
Pb, Sr, and Nd isotopes in seamount basalts from the Juan de Fuca Ridge and Kodiak‐Bowie Seamount Chain, northeast Pacific
Authors: Hegner E. , Tatsumoto M.
Published: February 2008

Pb, Sr, and Nd isotopic ratios and their parent/daughter element concentrations for 28 basalts from 10 hotspot and nonhotspot seamounts are reported. Nd and Sr isotopic compositions (143Nd/144Nd = 0.51325–0.51304; 87Sr/86Sr = 0.70237–0.70275) plot in the envelope for Juan de Fuca‐Gorda ridge basalts with tholeiitic basalts showing more depleted sources and a better negative correlation than transitional to alkalic basalts. Pb isotopic ratios in tholeiitic and alkalic basalts overlap (206Pb/204Pb = 18.29–19.44) and display a trend toward more radiogenic Pb in alkalic basalts. The isotopic data for hotspot and nonhotspot basalts are indistinguishable and correlate broadly with rock composition, implying that they are controlled by partial melting. The isotopic variation in the seamount basalts is about 60% (Nd‐Sr) to 100% (Pb) of that in East Pacific Rise basalts and is interpreted as a lower limit for the magnitude of mantle heterogeneity in the northeast Pacific. The data indicate absence of a chemically distinct plume component in the linear seamount chains and strongly suggest an origin from mid‐ocean ridge basalt‐like east Pacific mantle.
Microstructural evidence for northeastward movement on the Chocolate Mountains Fault Zone, southeastern California
Authors: Simpson Carol
Published: February 2008

Microstructural analysis of rocks from the Chocolate Mountains fault zone, Gavilan Hills area, southeastern California, shows unequivocal evidence for northeast directed transport of the upper plate gneisses over lower plate Orocopia schists. Samples were taken from transects through the fault zone. Prefaulting fabrics in upper plate gneisses show a strong component of northeast directed rotational deformation under lower amphibolite facies conditions. In contrast, prefaulting lower plate Orocopia schists show strongly coaxial fabrics (minimum stretch value of 2.2) formed at greenschist grade. Mylonitic fabrics associated with the Chocolate Mountains fault are predominantly northeast directed shear bands that are unidirectional (northeastward) in the gneisses but bi‐directional in the schists, suggesting a significant component of nonrotational deformation occurred in the Orocopia schists during and after emplacement of the upper plate. The kinematic findings are in agreement with Dillon et al. [1989], who found that the vergence of asymmetrical folds within the fault zone indicates overthrusting to the northeast, toward the craton, in this region. The available evidence favors a single protracted northeastward movement on the Chocolate Mountains fault zone with temperatures waning as deformation proceeded.
Smoothing and extrapolation of crustal stress orientation measurements
Authors: Hansen Katherine M. , Mount Van S.
Published: February 2008

The theory and mechanics of a statistical smoothing algorithm for estimating stress fields based on observed data are described. Incorporated into the algorithm are tunable parameters which allow the user to adjust the smoothness and fidelity of the fitted stress field. In addition, the user has the option of using robustness weights, or to weight the predicted stress field by assigning quality ranks to each data point. Fitted stress fields can be displayed as either a gridded map (axes of maximum, or minimum, horizontal stress are shown as short bars located at each point in an evenly spaced grid), or as a stress trajectory map. Examples of predicted stress fields for southern California and western Canada, based on borehole elongation data, are displayed as stress trajectory maps. A predicted paleostress field for the Spanish Peaks intrusion complex in Colorado, based on vertical dike orientations, is displayed as a gridded map. The smoothing algorithm is not limited to the analysis of stress orientation data. Any data consisting of a set of undirected lines measured at discrete locations are appropriate for input; possible candidates include strain data, mineral or intersection lineations, and lineaments.
Evolution of Mauna Kea Volcano, Hawaii: Petrologic and geochemical constraints on postshield volcanism
Authors: Frey F. A. , Wise W. S. , Garcia M. O. , West H. , Kwon S.‐T. , Kennedy A.
Published: February 2008

All subaerial lavas at Mauna Kea Volcano, Hawaii, belong to the postshield stage of volcano construction. This stage formed as the magma supply rate from the mantle decreased. It can be divided into two substages: basaltic (∼240–70 ka) and hawaiitic (∼66–4 ka). The basaltic substage (Hamakua Volcanics) contains a diverse array of lava types including picrites, ankaramites, alkalic and tholeiitic basalt, and high Fe‐Ti basalt. In contrast, the hawaiitic substage (Laupahoehoe Volcanics) contains only evolved alkalic lavas, hawaiite, and mugearite; basalts are absent. Sr and Nd isotopic ratios for lavas from the two substages are similar, but there is a distinct compositional gap between the substages. Lavas of the hawaiitic substage can not be related to the older basalts by shallow pressure fractionation, but they may be related to these basalts by fractionation at moderate pressures of a clinopyroxene‐dominated assemblage. We conclude that the petrogenetic processes forming the postshield lavas at Mauna Kea and other Hawaiian volcanoes reflect movement of the volcano away from the hotspot. Specifically, we postulate the following sequence of events for postshield volcanism at Mauna Kea: (1) As the magma supply rate from the mantle decreased, major changes in volcanic plumbing occurred. The shallow magma chamber present during shield construction cooled and crystallized, and the fractures enabling magma ascent to the magma chamber closed. (2) Therefore subsequent basaltic magma ascending from the mantle stagnated within the lower crust, or perhaps at the crust‐mantle boundary. Eruptions of basaltic magma ceased. (3) Continued volcanism was inhibited until basaltic magma in the lower crust cooled sufficiently to create relatively low‐density, residual hawaiitic melts. Minor assimilation of MORB‐related wall rocks, reflected by a trend toward lower 206Pb/204Pb in evolved postshield lavas, may have occurred at this time. A compositional gap developed because magma ascent was not possible until a low‐density hawaiitic melt could escape from a largely crystalline mush. Eruption of this melt created aphyric hawaiite and mugearite lavas which incorporated cumulate gabbro, wherlite, and dunite xenoliths during ascent.
Simulations of the general circulation of the Martian atmosphere: 1. Polar processes
Authors: Pollack James B. , Haberle Robert M. , Schaeffer James , Lee Hilda
Published: February 2008

We have conducted numerical simulations of the general circulation of the Martian atmosphere with a three‐dimensional model based on the primitive equations of meteorology. The radiative effects of atmospheric dust on solar and thermal radiation have been incorporated into the model. It has 13 vertical layers that span the altitude range from the surface to approximately 47 km and a horizontal resolution of 7.5° latitude by 9° longitude. A large number of numerical experiments were conducted for alternative choices of seasonal date and dust optical depth. During each experiment the dust optical depth was kept constant in time, and the amount of dust was constant in space as well, except for the effects of topography. Carbon dioxide condensed in the atmosphere as well as at the ground in the winter polar regions over the entire range of dust optical depths considered (0–5). However, the rate of atmospheric CO2 condensation increased sharply as the dust content of the winter polar region increased. The simulated properties of atmospheric CO2 condensation imply that CO2 ice clouds are chiefly responsible for the occurrence of anomalously low brightness temperatures in the winter polar regions and that they manifest themselves at optical wavelengths as polar hoods. A number of hemispherical asymmetries may ultimately be due to the strong seasonal variation in the atmospheric dust content and to feedback relationships between the dust optical depth on the one hand and the magnitude of poleward heat transport and the rate of atmospheric CO2 condensation on the other hand. These asymmetries include the greater prevalence of polar hoods in the northern polar region during winter, the lower albedo of the northern polar cap during spring, and the total dissipation of the northern CO2 ice cap during the warmer seasons.
Seismicity and crustal structure of the Fiji Platform, southwest Pacific
Authors: Hamburger Michael W. , Everingham Ian B. , Isacks Bryan L. , Barazangi Muawia
Published: February 2008

Data from a telemetered seismic network in Fiji provide new constraints on the crustal structure and active deformation within and around the Fiji Platform, an unusual fragmented arc complex situated within the Pacific/Indo‐Australian plate boundary. Seismic refraction data indicate that crustal thickness in Fiji is typical of island arcs, averaging about 15–20 km, with relatively low midcrustal seismic velocities (Vp = 6.0 km/s), characteristic of felsic arc‐related rocks. The anticipated higher velocity lower crustal layer is either thin or absent. Mantle velocity beneath the platform is low (7.6 km/s), in the same range as that observed beneath the neighboring marginal basins and island arcs. The record of crustal Seismicity, based on historical, teleseismic, and microearthquake catalogs, indicates significant activity both within and along the boundaries of the Fiji Platform. Earthquakes located along the edges of the platform are associated with major zones of back arc deformation of the North Fiji and Lau basins: the Fiji Fracture Zone to the north, the Hunter Fracture Zone to the south, and a deformed zone in the eastern portion of the North Fiji Basin to the west. Earthquakes within the platform (“intraplate events”) occur more sporadically than those along its margins; areas of significant intraplate activity include the Taveuni/northeastem Vanua Levu area, Koro Sea, the Baravi Basin, and central and south‐eastern Viti Levu. Earthquake depths within the platform are concentrated in the upper 13–16 km of lithosphere, as in other areas of extensional/strike‐slip tectonics. In Fiji, however, the seismicity apparently extends through the entire crustal section. The large 1953 intraplate Suva earthquake (Ms = 6.75) was caused by strike‐slip faulting, under the same east‐west extensional stress field that is deforming the North Fiji and Lau basins. Microearthquakes within and around the island of Viti Levu appear to occur as a result of the same stresses. These earthquakes occur in discrete nests that are tightly clustered in space and time; they are related to late Cenozoic faults that cut the Mio‐Pliocene rocks of Viti Levu. The characteristics of its crustal structure, the anomalous mantle velocity, and the active deformation within the Fiji Platform distinguish it from microcontinental blocks that have been observed elsewhere. The seismicity and focal mechanisms within the Fiji Platform demonstrate that it is not technically isolated from the active deformational processes that pervade the neighboring arcs and back arc basins.
Geodetic measurement of tectonic deformation in the Santa Maria Fold and Thrust Belt, California
Authors: Feigl Kurt L. , King Robert W. , Jordan Thomas H.
Published: February 2008

We have analyzed geodetic observations to resolve tectonic deformation across the Santa Maria Fold and Thrust Belt, northwest of Santa Barbara, California. The geodetic network forms a braced quadrilateral with ∼40 km sides whose southwest corner is the Vandenberg very long baseline interferometry station. The data include (1) historical triangulation from the 1880s, 1920s and 1950s; (2) electronic distance measurement trilateration from 1971 and 1985; and (3) Global Positioning System observations from 1986 and 1987. We combine the three different types of data to estimate two‐dimensional station positions and strain rate parameters simultaneously. Using a model which constrains the relative velocity field to be linear in space and constant in time, we find significant strain rates. The maximum compressive strain is oriented N17°E±5°E, and the compressive strain rate in this direction is 0.13±0.03 μstrain/yr. Under the assumption that the unresolved rotational component of the velocity field is zero, we estimate that the integrated rate of deformation across the basin is 7±1 mm/yr oriented at N03°E±13°. This vector can be decomposed into 6±2 mm/yr of crustal shortening on the general structural trend of N30°E and 3±1 mm/yr of right‐lateral shear across this axis. On the basis of these values and earthquake focal mechanisms in the area, we infer that the deformation occurs on northwest trending folds and thrusts within the belt. These results are consistent with the rate and direction of deformation across the central California Coast Ranges inferred by balancing Pacific‐North America plate motion against San Andreas slip and Basin and Range extension. They imply that the Santa Maria Fold and Thrust Belt is the primary active element in transforming motion from the Coast Ranges to the western Transverse Ranges and the Santa Barbara Channel.
Seismic imaging of upper crustal structure using travel times from the PASSCAL Ouachita Experiment
Authors: Lutter W. J. , Nowack R. L. , Braile L. W.
Published: February 2008

A first arrival travel time inversion applied to the 1986 PASSCAL Ouachita experiment has been used to image upper crustal structure. Twenty‐one shot points every 10 km were recorded along two 100‐km‐long segments with 400 seismographs on each segment. A splined velocity model parameterization allowed for the inversion of laterally inhomogeneous models but not velocity discontinuities. Over 800 travel times were used in the inversion with a variable data covariance weighting. The multiple source nature of the experiment design allowed the use of reciprocity relationships among the 21 shot points to enhance data correlation. A succession of models with velocities specified at 30, 90, 171, and 221 nodes were inverted for whose horizontal node spacing decreased from 100 to 12.5 km in the final 221 node model. Vertical node spacings of 3.0 and 1.5 km were used. The results of the inversions indicate changes in depth of isovelocity lines which are interpreted as Triassic fault structure based on nearby well data. The final RMS travel time residual is 0.045 s. Resolution using first arrivals deteriorates rapidly below 9 km depth. Three distinct velocity gradients can be correlated with geologic units. Surface exposed Cretaceous and Jurassic sediments with velocities from 2.0 to 3.5 km/s extend to 1.5 km depth. Triassic rift fill sediments, which do not outcrop, have inverted velocities ranging from 3.5 to 4.75 km/s between depths of 1.5–3.0 km. Paleozoic sediments are imaged by a decreased velocity gradient below a depth of 3.0 km. Lack of resolution at depths greater than 9 km necessitates detailed modeling of wide‐angle reflected phases to further constrain deeper crustal structure.