bluuebunny/crossref_metadata_2025 huggingface dataset:
Morphology and growth history of delgada fan: Implications for the Neogene evolution of Point Arena Basin and the Mendocino Triple Junction
Authors: Drake David E. , Cacchione David A. , Gardner James V. , McCulloch David S. , Masson Douglas
Published: February 2008

Long‐range side scan (GLORIA) sonographs and seismic reflection data acquired during a survey of the western U.S. Exclusive Economic Zone in 1984, coupled with information from Deep Sea Drilling Project sites, provide new insights into the growth and evolution of the Delgada Fan. Construction of the fan commenced in the latest Miocene (∼6 Ma) following the filling of the Neogene Point Arena Basin. The fan presently covers more than 50×103 km2 of the Pacific plate and contains approximately 15×103 km3 of predominantly terrigenous detritus. The large size of the fan is incompatible with the small present‐day supply of sediment to the canyon system. The GLORIA data show the Delgada Fan to be a hybrid‐type fan, exhibiting characteristics of both elongate and radial fans. The morphology and volume of the fan, along with evidence for a decline in accumulation rates on the lower fan during the Quaternary period, suggest that the fan experienced an early growth phase (latest Miocene and Pliocene) characterized by relatively rapid progradation of elongate fan lobes followed by a period (Quaternary) of slower growth that has featured a shift of depocenters to sites closer to the canyons and a transition to distributary channels bordered by less prominent levees and overbank deposits. We examine the growth of Delgada Fan in relation to the Neogene evolution of the North American‐Pacific plate boundary using a series of paleogeographic reconstructions based on recently published time displacement histories of the Mendocino triple junction (MTJ), the San Andreas fault (SAF), and the Pacific plate, upon which the fan rests. The time displacement curves for the SAF and the MTJ suggest that the MTJ and Mendocino Fracture Zone overtook and passed Point Arena Basin at about 10 Ma when the basin lay immediately southwest of the present San Francisco Bay area. We suggest that the MTJ joined the SAF at approximately that time and location, thus making the SAF the master fault in the transform system. This interpretation is compatible with evidence from seismic reflection profiles over the fan, which demonstrate that the fan and the canyon system and therefore Point Arena Basin have moved as a unit since the inception of fan growth (∼6–7 Ma). Point Arena Basin lay southwest of the San Francisco area at 10–12 Ma, and the passage of the MTJ caused the disruption of the forearc shelf and slope and the development of local uplifted and subsiding blocks. In particular, uplift of the “bay block” immediately east of the SAF may have provided the source area for the late Miocene sediments that filled Point Arena Basin and set the stage for the growth of Delgada Canyon and Fan system. The growth rate of the fan has decreased, and the style of deposition has changed as the system was tectonically transported to its present location adjacent to the small youthful drainages of the King Range.
Low‐temperature hydrothermal alteration in near‐surface sediments, Salton Sea Geothermal Area
Authors: Sturz Anne
Published: February 2008

As part of the Lawrence Livermore/Sandia National laboratories shallow drilling (19 wells) into Salton Sea sediments, four 1‐m cores were recovered from 76 m subbottom depth. The cored wells are along a transect from the margin toward the interior of the Mullet Island thermal anomaly. The thermal gradients range from 0.191°C/m near the margin to 0.816°C/m near the interior of the thermal anomaly. Bottom‐hole temperature in a cored well close to the center of the thermal anomaly is 87°C. Pore water chemical composition and bulk and clay size sediment mineralogical and chemical compositions are influenced by hydrothermal reactions which intensify with increasing thermal gradient. X ray diffraction and petrographic microscopic analyses of bulk and clay size fractions of the sediments indicate that the relative proportions of smectite to kaolinite plus illite and of dolomite to calcite increase from the margin toward the interior of the thermal anomaly region. Clay mineral recrystallization and dolomitization are indicated by increased K/Al and decreased Ca/Mg molar ratios in the clay size sediment fraction. Calcium, silica, potassium, lithium, and strontium are released from sediments into pore fluids. These data suggest that even at temperatures less than 100°C, hydrothermal activity significantly alters the pore water chemistry and moderately alters the sediment mineralogical and chemical composition.
Oxygen diffusion in olivine: Effect of oxygen fugacity and implications for creep
Authors: Ryerson F. J. , Durham W. B. , Cherniak D. J. , Lanford W. A.
Published: February 2008

Oxygen self‐diffusion experiments on single crystals of San Carlos olivine (∼Fo92) at 1200° ≤ T ≤ 1400°C, oxygen fugacities (ƒO2) along the Ni‐NiO and Fe‐FeO buffers, and silica activity at the olivine‐orthopyroxene buffer yielded results that follow the relationshipD= 2.6 × 10−10ƒO20.21±0.03exp [−266±11 (kJ mol−1/RT)], whereDis the diffusion coefficient in m2s−1and ƒO2is given in pascals. The activation energy compares reasonably well with results for pure forsterite. The positive dependence of ƒO2implies that the oxygen defect responsible for diffusion is an interstitial rather than a more sterically reasonable oxygen vacancy. Diffusion of oxygen in other close‐packed oxides has also shown a positive dependence on ƒO2. The rate of creep of single‐crystal olivine at fixed orthopyroxene activity also shows a positive ƒO2dependence. If oxygen interstitials should be shown to be unimportant in oxygen diffusion in oxides, then coupled mechanisms such as countervacancy diffusion must be appealed to in order to explain the positive ƒO2dependence. Such processes are rate‐limited by the diffusion of metal vacancies which also display a positive ƒO2dependence in olivine. Compared with data for silicon diffusion in forsterite, our data indicate that oxygen is not the slowest diffusing species in olivine. The activation energy for oxygen diffusion is also low compared to that obtained in a majority of measurements of creep in single‐crystal olivine. Hence if oxygen diffusion contributes to the control of creep in olivine, it must be coupled to another process having a nonzero activation energy. A subinvestigation of the rate of sublimation from polished olivine surfaces, using a high‐resolution thermal balance, showed surface erosion rates at 1300°C of 0.13±0.10 nm/h. Such values are far too slow to noticeably affect our diffusion measurements at temperatures ≤1400°C.
Isotopic and trace element constraints on the origin and evolution of alkaline and calc‐alkaline magmas in the Northwestern Mexican Volcanic Belt
Authors: Verma Surendra P. , Nelson Stephen A.
Published: February 2008

A wide variety of rock types has been produced by Pliocene to Recent volcanism in the northwestern portion of the Mexican Volcanic Belt. Composite cones erupted rocks typical of calc‐alkaline volcanic belts associated with subduction. Monogenetic cinder cones surrounding the composite cones erupted mildly alkaline basalts and related rocks. One larger center, Las Navajas, produced basalts, trachytes, and peralkaline rhyolites. Such alkaline rocks are typically associated with crustal extension related to rifting. Both subduction and rifting appear to be taking place in western Mexico. The most basic calc‐alkaline rocks, although somewhat fractionated, show depletion in high field strength elements (HFSE) and enrichment in alkaline earth elements relative to other incompatible elements, a characteristic typical of magmas in subduction‐related volcanic arcs. The basic alkaline rocks show no depletion in HFSE and show higher concentrations of incompatible elements than the basic calc‐alkaline rocks. The calc‐alkaline rocks show a tight cluster of87Sr/86Sr and143Nd/144Nd ratios, both of which correlate weakly with SiO2. Basic alkaline rocks have lower Sr and higher Nd isotopic ratios, whereas the more differentiated alkaline rocks have isotopic ratios that overlap with and extend beyond the range of values found in calc‐alkaline rocks. Trace element data indicate that calc‐alkaline magmas could not have been derived by crystal fractionation from the alkaline magmas nor by assimilation of crustal materials by alkaline magmas. The evolution of both calc‐alkaline and alkaline rocks requires a process of crystal fractionation accompanied by assimilation (AFC) of crustal rocks to account for changes in isotopic ratios and trace element concentrations in the more differentiated members of each suite. Assuming a contaminant located in the lower or middle crust, AFC modeling shows that the amounts of assimilation required for alkaline magmas are somewhat higher than those for calc‐alkaline magmas. Calc‐alkaline and alkaline rocks appear to have resulted from melting of two distinct sources: alkaline from an OIB‐type source, and calc‐alkaline from a mixed mantle and slab‐derived source.
Chemical evolution of a magmatic system: The Paintbrush Tuff, Southwest Nevada Volcanic Field
Authors: Flood Timothy P. , Vogel Thomas A. , Schuraytz Benjamin C.
Published: February 2008

The chemical and thermal evolution of a single magmatic system is recorded in the Paintbrush Tuff, a series of four ash flow sheets, that were erupted from the same caldera within 0.6 Ma. Glassy pumice samples were collected from the tops and bottoms of the ash flow sheets. Because these samples closely approximate magma compositions, their chemical and mineralogical compositions were used to quantitatively evaluate magma mixing and fractional crystallization as potential differentiation mechanisms. Prior to eruption of the first ash flow sheet, the Topopah Spring Member (>1200 km3), an abrupt compositional interface existed in the magma chamber between high‐silica rhyolite and quartz latite magmas. The subsequent Pah Canyon Member (<40 km3) records mixing of the contrasting magma types represented in the Topopah Spring Member. The magma mixing was most likely due to disruption of the compositional interface during eruption of the Topopah Spring Member. The next unit, the Yucca Mountain Member (<20 km3), represents the reestablishment of high‐silica rhyolite magma in the subcaldera chamber. The origin of this high‐silica rhyolite is best modeled by 15% to 24% fractional crystallization from the Pah Canyon Member. The Tiva Canyon Member is the fourth ash flow sheet (>1000 km3) to be erupted, and it consists of three compositional modes, a higher‐silica rhyolite, rhyolite, and quartz latite. The high‐silica rhyolite of the Yucca Mountain Member is an early eruptive phase of the higher‐silica rhyolite of the Tiva Canyon Member, and their origins are the same. The rhyolite most likely formed by a combination of fractional crystallization and magma mixing of the Pah Canyon magma and the quartz latite of the Tiva Canyon Member. Alternatively, the rhyolite may have formed by fractional crystallization and magma mixing of higher‐silica rhyolite and quartz latite of the Tiva Canyon Member, or by fractional crystallization from the Tiva Canyon Member. The origin of the quartz latite has not been determined. Quantitative modeling indicates that all of the chemical variation within the Pah Canyon, Yucca Mountain, and Tiva Canyon members can be accounted for by fractional crystallization or magma mixing operating alone or in conjunction; no other processes need be invoked. Volume estimates of the magma bodies based on these quantitative models predict much larger volumes than the sizes of the ash flow sheets would indicate. A recurring theme in the evolution of the Paintbrush Tuff is the interaction of quartz latitic magmas with high‐silica magmas in conjunction with crystal fractionation.
Method of resolving radio phase ambiguity in satellite orbit determination
Authors: Counselman Charles C. , Abbot Richard I.
Published: February 2008

For satellite orbit determination, the most accurate observable available today is microwave radio phase, which can be differenced between observing stations and between satellites to cancel both transmitter‐ and receiver‐related errors. For maximum accuracy, the integer cycle ambiguities of the doubly differenced observations must be resolved. To perform this ambiguity resolution, we propose a bootstrapping strategy. This strategy requires the tracking stations to have a wide ranging progression of spacings. By conventional “integrated Doppler” processing of the observations from the most widely spaced stations, the orbits are determined well enough to permit resolution of the ambiguities for the most closely spaced stations. The resolution of these ambiguities reduces the uncertainty of the orbit determination enough to enable ambiguity resolution for more widely spaced stations, which further reduces the orbital uncertainty. In a test of this strategy with six tracking stations, both the formal and the true errors of determining Global Positioning System satellite orbits were reduced by a factor of 2.
Appalachian Stress Study: 2. Analysis of Devonian shale core: Some implications for the nature of contemporary stress variations and Alleghanian Deformation in Devonian rocks
Authors: Evans Keith F. , Oertel Gerhard , Engelder Terry
Published: February 2008

Detailed stress measurements in three boreholes penetrating horizontally bedded Devonian silt‐stones, sandstones, and limestones above a prominent salt decollement in the Appalachian Plateau of western New York have revealed variations in horizontal stress magnitudes which correlate with lithologic and stratigraphic units in all wells. High differential stress levels (up to 20 MPa) were found in shales of very high clay content, contrary to the proposition that such materials have negligible long‐term strength. Elastic modulus data show that stiffer beds generally host higher stress levels and suggest that sand/shale stress contrasts result in large part from elastic shortening of the section in response to regional ENE compression. No correlation between stress and Poisson's ratio was found. However, a major systematic drop in stress level within the generally massive shales, which occurs across a group of sand beds near the base of the Rhinestreet formation, appears to be of different origin. The stress offset corresponds to the top of a section which we conclude, on the basis of local and regional total strain data derived from chlorite fabric measurements, once hosted abnormally high pore pressures. The total strain data also suggest the entire section above the salt has been uniformly shortened during Alleghanian compression. To explain the stress discontinuity, two kinematic patterns for Alleghanian deformation of the Devonian section are proposed, both involving abnormal pore pressure development in the sub‐Rhinestreet section in response to limited drainage of fluid. Drainage of this paleo‐overpressure is the best available explanation of the stress offset, although an additional remnant stress component must also be present to satisfy the stress data precisely.
Elastic constants of corundum up to 1825 K
Authors: Goto Takayasu , Anderson Orson L. , Ohno Ichiro , Yamamoto Shigeru
Published: February 2008

Elastic constants of single‐crystal corundum, α‐Al2O3, up to 1825 K (1.75 times the Debye temperature, θD) have been measured using a new device, the rectangular parallelepiped resonance method. This device uses a long, thin alumina buffer rod, which separates the transducer from the specimen and enables us to pick up the resonance vibration of the specimen at very high temperature. The elastic stiffness constants C11 and C33 show a large decrease, while C12 and C13 only slightly decrease with temperature. C44 decreases linearly with temperature, and C14 is almost constant in the measured range of temperature. The isotropic elastic parameters were calculated by means of Voigt, Reuss, and Hill averaging schemes, enabling us to analyze the high‐temperature elastic properties of corundum. The present elastic data also enable us to determine the thermodynamic parameters of corundum by combining them with the existing data on thermal expansivity and heat capacity. We find that (1) the evidence for anharmonicity in the thermal pressure is quite small, and the volume dependence of the thermal pressure diminishes above 1400 K; (2) the Grüneisen parameter decreases slightly with T at constant P above the Debye temperature; (3) the isothermal Anderson‐Grüneisen parameter approaches the value of the pressure derivative of the isothermal bulk modulus at high temperature; and (4) the parameter q = (∂ℓnγ/∂ℓnV)T also approaches the value 1 at high temperature.
Elasticity of grossular and spessartite garnets by Brillouin spectroscopy
Authors: Bass Jay D.
Published: February 2008

The single‐crystal elastic moduli of natural samples of grossular (99.0 mol % Ca3Al2Si3O12) and spessartite (94.8 mol % Mn3Al2Si3O12) have been measured by Brillouin spectroscopy under ambient conditions. From these results the adiabatic bulk moduli Ks and shear moduli μ are calculated to be Ks = 168.4 ± 0.7, μ = 108.9 ± 0.4 for grossular, and Ks = 178.8 ± 0.8, μ = 96.3 ± 0.5 for spessartite (all in units of gigapascals). A calibration of our spectrometer by determination of the elastic properties of MgO indicates that these modulus values are accurate to within twice the stated rms error. Of the major rock‐forming silicate garnets, spessartite has the largest bulk modulus, and grossular has the largest shear modulus, with the possible exception of majorite (a high‐pressure form of pyroxene). Approximate values for the bulk and shear moduli of common aluminosilicate garnet solid solutions can be obtained with an estimated uncertainty of less than 3% from a linear molar average of the end‐member properties. The elastic properties of pure almandite garnet (Fe3Al2Si3O12), which have thus far not been measured, are estimated as Ks = 177 ± 3 and μ = 97 ± 1 GPa by a linear regression analysis of extant measurements on garnet solid solutions and end‐members. These same data imply, however, that a linear modulus‐composition relationship may not be appropriate for all silicate garnet compositions.
The 1987 Southeastern Alaska Gravity Calibration Range: Absolute and relative gravity measurements
Authors: Sasagawa Glenn S. , Zumberge Mark A. , Stevenson J. Mark , Lautzenhiser Ted , Wirtz Jim , Ander Mark E.
Published: February 2008

In June 1987 a gravimeter calibration range was set up in southeastern Alaska and the Yukon territory, as part of a geophysical determination of the Newtonian gravitational constant. Absolute gravity measurements were made between the range endpoints using the Institute of Geophysics and Planetary Physics absolute gravity meter. The calibration range spans 171.841±0.014 mGal, with a midpoint g value of 9.81746500 ms−2. Relative gravity meters, including a LaCoste and Romberg borehole gravity meter, were read along this range. A scale factor correction (SFC) for borehole meter 14 was found to be (8.1 ± 1.5) × 10−4, and for meter G‐349 the correction was (−3.3 ± 1.7) × 10−4. The SFC for meter D‐85 has an upper bound of ±1.0 × 10−4.
Timing of Tertiary extension in the Railroad Valley‐Pioche Transect, Nevada: Constraints from 40Ar/39Ar ages of volcanic rocks
Authors: Taylor Wanda J. , Bartley John M. , Lux Daniel R. , Axen Gary J.
Published: February 2008

Time‐space relations of extension and volcanism place critical constraints on models of Basin and Range extensional processes. This paper addresses such relations in a 130‐km‐wide transect in the eastern Great Basin, bounded on the east by the Ely Springs Range and on the west by the Grant and Quinn Canyon ranges. Stratigraphic and structural data, combined with 40Ar/39Ar isotopic ages of volcanic rocks, document a protracted but distinctly episodic extensional history. Field relations indicate four periods of faulting. Only one of these periods was synchronous with nearby volcanic activity, which implies that volcanism and faulting need not be associated closely in space and time. Based on published dates and the analyses reported here, the periods of extension were (1) prevolcanic (pre‐32 Ma), (2) early synvolcanic (30 to 27 Ma), (3) immediately postvolcanic (about 16 to 14 Ma), and (4) Pliocene to Quaternary. The break between the second and third periods is distinct. The minimum gap between the first two periods is 2 Ma, but the separation may be much larger. Temporal separation of the last two periods is only suggested by the stratigraphic record and cannot be rigorously demonstrated with present data. The three younger periods of faulting apparently occurred across the entire transect. The oldest period is recognized only at the eastern end of the transect, but appears to correlate about 150 km northward along strike with extension in the Northern Snake Range‐Kern Mountains area. Therefore the oldest period also is regional in extent, but affected a different area than that affected by younger periods. This relation suggests that distinct extensional structures and master detachment faults were active at different times. The correlation of deformation periods of a few million years duration across the Railroad Valley‐Pioche transect suggests that the scale of active extensional domains in the Great Basin may be greater than 100 km across strike.
Regional compositional variations of Late Tertiary bimodal rhyolite lavas across the Basin and Range/Colorado Plateau Boundary in western Arizona
Authors: Moyer Thomas C. , Nealey L. David
Published: February 2008

Bimodal basalt‐rhyolite fields that erupted since 15 Ma form a NE trending line that crosses the boundary between the Basin and Range and the Colorado Plateau in western Arizona. The bimodal volcanism traverses physiographic province boundaries marked by changes in crustal thickness. Silicic volcanism in these fields postdates detachment faulting in the Basin and Range and generally decreases in age from south to north. We distinguish two rhyolite types that occur in these fields and summarize their geology, petrography, and geochemistry. Both high‐silica (HSR) and low‐silica rhyolites (LSR) occur in these fields. HSR lavas contain >75 wt % SiO2 (normalized to 100% volatile free), have high concentrations of incompatible elements (Rb, U, Th, Cs, Ta), low concentrations of compatible elements (Ba, Sr, Eu, Mg, Cr), and relatively flat rare earth element (REE) patterns (La/LuN <7). In contrast, LSR lavas have 68–74.5 wt % SiO2 (normalized to 100% volatile free), low concentrations of incompatible elements, high concentrations of compatible elements, and negatively sloping REE patterns (La/LuN >8). Further distinction of HSR and LSR lavas is made based on phenocryst assemblages. Trace element and isotopic studies suggest that both the HSR and LSR lavas contain a major crustal component and that the HSR lavas may have formed by extensive crystal fractionation of LSR parent magmas. HSR and LSR lavas display systematic compositional variations along the transect; however, their compositions diverge from south to north. HSR lavas erupted on the Colorado Plateau are more differentiated than those erupted in the Basin and Range, perhaps reflecting more extensive crystal fractionation while ascending the thicker crust of the Colorado Plateau province. In contrast, LSR lavas on the Colorado Plateau are more primitive than those in the Basin and Range. This seeming paradox may be related to lateral changes in source composition, possibly caused by a northerly increase in either metamorphic grade or bulk composition. Alternative models for the LSR compositional variations, such as magma mixing and mantle input into lower crustal sources, cannot be tested with our present data.
Isotopic evidence on the structure and origin of subcontinental lithospheric mantle in southern Nevada
Authors: Farmer G. L. , Perry F. V. , Semken S. , Crowe B. , Curtis D. , DePaolo D. J.
Published: February 2008

The trace element and Nd, Sr, and Pb isotopic compositions of extension‐related late Cenozoic basalts in southern Nevada were determined in order to constrain models for the tectonic evolution of the subcrustal mantle in the southern Great Basin since 10 Ma. Basalts in southern Nevada between latitudes 36°N and 37°N have low εNd (−8 to −10), high Δ8/4 (+70 to +107) and Δ7/4 (+8 to +12), high 87Sr/86Sr, and low high field strength element (HFSE) concentrations relative to other Basin and Range basalts. Basalts with these chemical and isotopic traits have erupted semicontinuously in southern Nevada since 10 Ma. During the same period of time, basalts in central Nevada and east central California have shifted toward higher εNd, lower 87Sr/86Sr, lower Δ8/4 and Δ7/4, and higher HFSE contents than observed for the southern Nevada basalts. The latter isotopic characteristics are similar to other basalts in the western United States that are interpreted to have been derived from asthenospheric mantle similar isotopically to the sources of ocean island basalts. The isotopic and chemical characteristics of the southern Nevada basalts are not attributed to asthenospheric mantle but to lithospheric mantle that has been preserved beneath this region despite late Cenozoic extension. The lack of any apparent lithospheric erosion over this time period is attributed to the fact that southern Nevada has been an “amagmatic” zone throughout the Phanerozoic. As a result, the mantle lithosphere may have been more difficult to extend and displace during extension than lithosphere beneath adjacent portions of the Basin and Range. The lack of any extensive Phanerozoic magmatism in southern Nevada also allows the possibility that the lithosphere in southern Nevada originated in the Precambrian. Compared with basalt source regions in Proterozoic lithospheric mantle in northern Arizona and New Mexico, the lithospheric sources for the southern Nevada basalts have lower εNd, higher 87Sr/86Sr, and higher Δ8/4 values. We attribute these isotopic characteristics to lithospheric mantle associated with Proterozoic crust with Nd model ages from 2.0 to 2.3 Ga (province 1), a crustal province that is restricted spatially in the southwest United States to southern Nevada and vicinity.
Crustal extension, crustal density, and the evolution of Cenozoic magmatism in the basin and range of the western United States
Authors: Glazner Allen F. , Ussler William
Published: February 2008

During the Cenozoic Era, volcanism in the Basin and Range Province of the western United States changed from predominantly intermediate (andesitic and dacitic) to predominantly basaltic. Accompanying this shift was a general decrease in the amount of contamination suffered by basalts en route to the surface. Existing models relate these progressive changes to plate tectonic events at the continental margin of western North America, and assume that extensional tectonism favors eruption of basalt. However, in many areas, synextensional rocks are predominantly intermediate or silicic in composition. Here we propose that the nature of volcanism in an area undergoing extension is a strong function of the density contrast between a magma and the overlying crust. Subtle variations in this density difference will determine the level of neutral buoyancy for a magma, and the height to which a column of magma may rise by hydrostatic pressure. Increases in crustal density favor eruption of dense magmas. Contamination of stagnated basalts by crustal material is more efficient than olivine fractionation in reducing the density of residual liquids, and a stagnated magma may ascend buoyantly after relatively small amounts of contamination. A progressive increase in mean crustal density, caused by crustal extension and consequent thinning of the low‐density upper crust coupled with emplacement of mafic plutons into the crust, may account for the observations that (1) the proportion of basalt erupted in the Basin and Range in the Cenozoic has increased with time, (2) younger basalts are generally less contaminated than older basalts, (3) major young silicic centers are apparently restricted to areas of low‐density crust, and (4) midcrustal magma bodies are generally located at prominent P wave velocity discontinuities.
Patterns of volcanism along the southern margin of the Colorado Plateau: The Springerville Field
Authors: Condit Christopher D. , Crumpler L. S. , Aubele Jayne C. , Elston Wolfgang E.
Published: February 2008

The mostly 2.1‐ to 0.3‐m.y.‐old Springerville volcanic field is the southernmost of the Colorado Plateau‐margin fields and encompasses −3000 km2, with a volume of −300 km3. Mapping documents 409 flow units, most emanating from one of the −400 predominantly pyroclastic cones. Alkali olivine basalt constitutes about one‐half the outcrop area, with hawaiite and tholeiite each composing about a quarter of the area, and mugearite and benmoreite each covering less than 1% of the area. The composition of the lavas shows a progression away from tholeiitic and toward alkalic affinities with decreasing age, in marked contrast to that found in both the southeast and western margins of the Colorado Plateau. Estimates of volume effusion rates show an early eruptive episode dominated by large volumes of tholeiitic lavas, a middle period of alkali olivine basalt eruption, and a late pulse of evolved alkalic rocks. Changes in chemistry [Na2O + K2O, MgO/(MgO + FeO*), K/P] with respect to location of lava sources that might reflect thermal and/or chemical differences appear to be minimal. Eastward migration of alkali olivine basalt volcanism within the Springerville volcanic field is essentially identical to that seen in the San Francisco volcanic field for the same time period, supporting the suggestion of Tanaka et al. (1986) that it may reflect the westward motion of the North American continent above relatively fixed mantle sources. The late Cenozoic migration of volcanic fields toward the Colorado Plateau suggested by Best and Hamblin (1978) and by Luedke and Smith (1978) does not seem to be reflected by vent migrations from 2.1 Ma to the present within the Springerville and San Francisco fields.
On the source of cross‐grain lineations in the central Pacific gravity field
Authors: McAdoo David C. , Sandwell David T.
Published: February 2008

Subtle lineations in the marine gravity field of the central Pacific derived from Seasat altimeter data were observed by Haxby and Weissel (1986). They suggested that these “cross‐grain” lineations were evidence of small‐scale convection beneath the Pacific plate. We have examined these features by comparing multiple, collinear gravity and bathymetry profiles in the Fourier transform domain. Our nine gravity profiles were each obtained by stacking (averaging) three or more individual, repeat Geosat/ERM altimeter passes. Prior to stacking, the individual Geosat passes were fit to a cubic spline and then differentiated along track to produce along‐track deflections of the vertical (or horizontal gravity). Corresponding bathymetric profiles were produced by projecting, onto Geosat ground tracks, bathymetric observations from six R/V Thomas Washington legs and three R/V Conrad legs that virtually coincide with these Geosat tracks. After Fourier transforming the resulting gravity and bathymetry profiles, we estimate admittances of gravity to bathymetry. These admittances are generally low; they also tend to be negative at very short wavelengths (λ<50 km). They are consistent with models of flexural isostatic compensation by a very thin lithosphere (approximately 2 km). They are not consistent with models of dynamic compensation. We suggest, therefore, that either (1) these cross‐grain lineations began to form very near the East Pacific Rise or (2) they formed on older, anomalously weak lithosphere. We also suggest that the gravity lineations result primarily from loads beneath the seafloor in combination with, secondarily, loads on the seafloor. Depths of these subseafloor loads appear not to exceed significantly typical Moho depths.
Vertical deformation (1952–1987) in the Salton Trough, California, from water level recordings
Authors: Hudnut K. W. , Beavan J.
Published: February 2008

Analysis of 36 years of water level data from three sites on the shore of the Salton Sea reveals a history of vertical deformation in the Salton Trough. Data from two sites allow estimation of tilt along a 36‐km NW–SE baseline between 1952 and 1970; since 1970 the addition of a 13‐km WNW–ESE baseline allows the full tilt vector to be estimated. We find down to the southeast tilting at an average rate of about 0.01±0.01 μrad/yr from the early 1960s to 1987. An anomalous period of more rapid, 0.2±0.04 μrad/yr, tilting occurs between the mid 1950s and early 1960s, but this coincides with a period from which we have been unable to examine all the original data. A previous analysis of the 1952–1978 data suggested a precursory increase in tilt rate prior to the 1968 Borrego Mountain earthquake, and a tilt reversal in 1972. We show that there was no significant tilt change prior to the 1968 earthquake, and that the apparent 1972 reversal resulted from uncorrected offsets in the water level observations. Tilts derived from water level and from reliable first‐order leveling surveys show limited agreement, but the leveling data contain evidence for nonuniform tilting. Recent analyses of leveling data by others, using different techniques, show 20–30 cm uplift and subsidence events in the Salton Trough between 1968 and 1978. Our data constrain the relative elevations of the three water level sites to be stable within 1–2 cm during these proposed events.
Evolution of the lunar orbit with temperature‐ and frequency‐dependent dissipation
Authors: Ross M. N. , Schubert G.
Published: February 2008

The coupled thermal‐dynamical evolution of the Earth‐Moon system is modeled. The lunar orbit is assumed noncircular and inclined to Earth's equator plane. Solid planet dissipation in the Earth and Moon is assumed to occur by temperature‐ and frequency‐dependent solid friction. The dissipative response of Earth's oceans is represented as an equilibrium ocean with constant phase lag. Subsolidus convection cools the Earth and Moon, removing primordial, radioactive, and (in the Moon) tidal heating. A model without oceanic dissipation results in the Moon moving out to about 50 RE (Earth radii) in less than about 1.5 b.y. Thereafter, dynamic evolution slows considerably because the Earth has cooled and become less dissipative. A model that includes oceanic dissipation shows that about 700 m.y. of large oceanic dissipation (equivalent phase lag equals 1/11) is required to move the Moon to 60 RE. In one interpretation of this result, consistent with variable ocean basin configuration, several episodes of large oceanic dissipation have occurred since the end of the Archean with a cumulative time span of about 700 m.y. In our model, about 90% of the lunar angular momentum received from the Earth is the result of solid friction. Uncertainties in the thermal evolution and dissipation law preclude an unequivocal partitioning of tidal dissipation into solid Earth and oceanic parts. Our model likely underestimates solid Earth dissipation, however, and so emphasizes the proposition that solid Earth tidal friction might have been the main energy sink in the Earth‐Moon system and that oceanic dissipation may have been relatively much less important than solid Earth dissipation over geologic time.
The 1987 Whittier Narrows, California, earthquake: A Metropolitan shock
Authors: Hauksson Egill , Stein Ross S.
Published: February 2008

Just 3 hours after the Whittier Narrows earthquake struck, it became clear that a heretofore unseen geological structure was seismically active beneath metropolitan Los Angeles. Contrary to initial expectations of strike‐slip or oblique‐slip motion on the Whittier fault, whose north end abuts the aftershock zone, the focal mechanism of the mainshock showed pure thrust faulting on a deep gently inclined surface [Hauksson et al., 1988]. This collection of nine research reports spans the spectrum of seismological, geodetic, and geological investigations carried out as a result of the Whittier Narrows earthquake.

Although unseen, the structure was not unforeseen. Namson [1987] had published a retrodeformable geologic cross section (meaning that the sedimentary strata could be restored to their original depositional position) 100 km to the west of the future earthquake epicenter in which blind, or subsurface, thrust faults were interpreted to be active beneath the folded southern Transverse Ranges. Working 25 km to the west, Hauksson [1987] had also found a surprising number of microearthquakes with thrust focal mechanisms south of the Santa Monica mountains, another clue to a subsurface system of thrust faults. Finally, Davis [1987] had presented a preliminary cross section only 18 km to the west of Whittier Narrows that identified as “fault B” the thrust that would rupture later that year. Not only was the earthquake focus and its orientation compatible with the 10–15 km depth and north dipping orientation of Davis' proposed thrust, but fault B appears to continue beneath the northern flank of the Los Angeles basin, skirting within 5 km of downtown Los Angeles, an area of dense commercial high‐rise building development. These results are refined and extended by Davis et al. [this issue].
Coseismic folding, earthquake recurrence, and the 1987 source mechanism at Whittier Narrows, Los Angeles basin, California
Authors: Lin Jian , Stein Ross S.
Published: February 2008

Static deformation associated with the October 1, 1987, Whittier Narrows, California, M = 6.0 earthquake was detected by geodetic elevation changes. The earthquake uplifted a 1.5‐km‐high Quaternary fold (the Santa Monica Mountains anticlinorium) by 50 mm but caused no fault rupture at the ground surface. This suggests that folding and faulting of the Los Angeles basin sediments are coincident and continuing. After correction for surveying errors and nontectonic subsidence, we model the 214 geodetic observations with a simple dislocation in an elastic half‐space. A thrust fault with reverse slip of 1.1±0.3 m and dipping 30°±4°N, with an upper edge at a depth of 12±1 km and a lower edge at 17±1 km, fits the geodetic data best and is consistent with the main shock hypocenter, fault plane solution, and initial aftershock distribution. The upper limit on the static stress drop, Δσ, is 17.5±5.0 MPa (175±50 bar). The lower limit on the geodetic moment is 1.0±0.2 × 1025 dyne cm, in accord with seismic estimates, indicating that most of the slip took place during seismic rupture. If earthquakes of M ≤ 6 characterize the blind thrust fault on which the earthquake occurred, we estimate a 200 year repeat time at Whittier Narrows, and a 5–18 year rate of earthquake occurrence within a 150‐km‐long band in the northern Los Angeles basin and Santa Monica Bay to the west. The 25‐year rate of historical occurrence since 1860 is less than this prediction. The deficit in moment release implies that either vigorous aseismic slip or infrequent larger earthquakes occur here.
Carrier phase ambiguity resolution for the Global Positioning System applied to geodetic baselines up to 2000 km
Authors: Blewitt Geoffrey
Published: February 2008

The Global Positioning System (GPS) carrier phase data are biased by an integer number of cycles. A successful strategy has been developed and demonstrated for resolving these integer ambiguities for geodetic baselines of up to 2000 km in length, resulting in a factor of 3 improvement in baseline accuracy, and giving centimeter‐level agreement with coordinates inferred by very long baseline interferometry in the western United States. For this experiment, a method using pseudorange data is shown to be more reliable than one using ionospheric constraints for baselines longer than 200 km. An automated algorithm exploits the correlations between the many phase biases of a GPS receiver network to enable the resolution of ambiguities for very long baselines. A method called bias optimizing has been developed, which, unlike traditional bias fixing, does not require an arbitrary confidence test. Bias optimizing is expected to be preferable to bias fixing for poorly configured networks. In order to enable ambiguity resolution for long baselines, it is recommended that future GPS networks have a wide spectrum of baseline lengths ranging from <100 to >1000 km and that GPS receivers be used which can acquire dual‐frequency P code data.
A continuum model of continental deformation above subduction zones: Application to the Andes and the Aegean
Authors: Wdowinski Shimon , O'Connell Richard J. , England Philip
Published: February 2008

Large‐scale continental tectonics of back arc (extensional) and Andean‐type (compressional) environments are investigated by using the thin viscous sheet model to calculate the deformation within continental lithosphere that is subjected to horizontal forces on its plate boundaries and to basal drag from the asthenospheric flow beneath. The shear tractions acting on the base of a deformable lithosphere are derived from a corner flow model that assumes a rigid subducting plate and a deformable overlying plate. Because the calculated shear tractions and the deformation within the overlying plate are interdependent, the corner flow and the thin viscous sheet models are solved simultaneously. We use a perturbation method to obtain analytical solutions for the velocity and strain rate fields within the overlying continental lithosphere. The solutions depend on the angle of subduction, the dimensionless thickness of the lithosphere, and the ratio of asthenospheric to lithospheric viscosities, which governs the viscous coupling between the asthenosphere and the lithosphere. Calculations are compared with observations from the Andes and the Aegean; our results explain some of the features of the deformation in these regions that have heretofore not been explained by other models. Our model predicts that in a compressional environment a broad region of uplifted topography will tend to develop above a more steeply dipping slab (30°), rather than above a shallower slab (10°–15°); this is in accord with observations in the various segments of the central Andes. For an extensional environment, the model predicts that a zone of compression can develop near the trench and that extensional strain rate can increase with distance from the trench, as is observed in the Aegean. We also estimate the effective viscosities of ∼1020 Pa s for the asthenosphere, ∼2×l021 Pa s for the Aegean lithosphere, and ∼1022 Pa s for the Andean lithosphere.
Penetrative convective flows induced by internal heating and mantle compressibility
Authors: Machetel Philippe , Yuen David A.
Published: February 2008

The effects of internal heating on anelastic, compressible convection have been investigated in a spherical shell, with the ratio of inner to outer radii appropriate to lower mantle convection. The internal heating and the compressibility are characterized by two dimensionless numbers: R, the ratio of two Rayleigh numbers, and D, the dissipation number. Isothermal, stress‐free boundary conditions are applied at the top and at the bottom of the shell. The equations have been solved, with a time‐dependent finite difference code, in a temperature, vorticity, stream function formulation (axisymmetrical configuration) for Rayleigh numbers Rab ranging from the critical Rc up to 2000 Rc. The neutral surface computed over the (D,R) space leads to an unexpected crest shape which is correlated with a shift toward higher‐degree harmonics of the most unstable modes. The phenomenon of penetrative convection is generated, for the weakly nonlinear cases, by the combination of large internal heating rates R and large dissipation numbers D. Small cells appear at the top of the shell, counterrotating with respect to the surrounding global circulation. The onset of time dependence (at Rab = Rtd) and the evolution of bottom topography have been monitored over the (D,R) space by increasing the Rayleigh numbers gradually from the critical. With no internal heating the stabilizing effect of compressibility delays the transition to time dependence. However, the combined effects of internal heating and compressibility drastically reduce the threshold of time dependence to Rtd = 3 Rc, for R = 16, D = 0.6. The study of the topographic deflection of the bottom interface conducted with the same set of (D, R, Rab) values shows that this deflection always decreases with the increase of the Rayleigh number. To reach situations as close as possible to mantle conditions we have focused on D = 0.3 and R = 16 for Rayleigh numbers up to 2000 Rc. In these highly nonlinear regimes, penetrative convection cells are still generated at the top or at the bottom of the shell and evolve by breaking the surrounding chaotic circulations or by pairing with the closest corotating cells. The destruction of these transient cells by viscous dissipation leaves a persisting hot thermal anomaly. The internal heating combined with compressibility reinforces the chaotic character of convection. As a consequence, the bottom topographic deflections display highly time‐dependent behavior, and the thermal anomaly spectra contain energy up to degree 100. Serious assumptions are imposed on the modeling of the Earth's mantle by the mathematical and numerical difficulties of the problem, in particular by the introduction of an isothermal stress‐free upper boundary. In spite of these limitations we propose that penetrative convection, due to the combination of internal heating and compressibility, could be a mechanism able to generate spontaneously layered convection and local melting in the mantle.
The competition between thermal contraction and differentiation in the stress history of the Moon
Authors: Kirk Randolph L. , Stevenson David J.
Published: February 2008

The scarcity of both extension and compression features on the Moon strongly constrains the history of the lunar radius—to variations of less than ±1 km over the past 3.8 Gyr. This limit has traditionally been interpreted as requiring a delicate balance between thermal contraction of the near‐surface and expansion of a substantial cold interior region. Recent theories of lunar origin (e.g., giant impact), in contrast, favor a “hot” initial state. We propose that a reconciliation may be possible by taking account of the volume change ΔV/V|d due to differentiation. We calculate STP densities based on simplified normative mineralogies for a suite of estimates of the bulk lunar composition, of primary lunar basalt, and of the residuum left when the maximum amount of the latter is extracted from the former. Typically ΔV/V|d ≃ 2 to 5%—an expansion equivalent to heating by ∼103K. Provided the timing of differentiation is correct, one might offset the cooling of a magma ocean as much as 630 km deep by differentiation of the remainder of the Moon (which need not start much below the solidus temperature). A large but not impossible amount of gabbroic melt production is implied: ∼100 times the volume of mare basalts known to have been extruded. We do not address the detailed genetic relationship of this melt to the basalts observed on the lunar surface but point out that it need not have reached the surface directly or even have entered the crust in order for the expansion to have occurred. To assess the timing of melt formation, we investigate a simple conductive lunar thermal model which takes account of both ΔV/V|d and thermal contraction. Our initial state is characterized by a central temperature Tc and a depth Z0 above which the material (derived from the magma ocean) is already at the solidus and is not suceptible to volume changes upon further differentiation. We find a range of models satisfying the limits on radius increase and decrease. The hottest has Tc = 1210 K, Z0 = 400 km; without ΔV/V|d, we would need a larger or colder (or both) core, e.g., Tc ≲ 700 K for Z0 = 200–400 km, in agreement with previous investigators. Our modeling thus lends credence to the idea that the Moon could have been initially ≳50% molten (with the remainder relatively close to the solidus) and yet experienced little volume change over the last 3.8 Gyr.