bluuebunny/crossref_metadata_2025 huggingface dataset:
A synthetic stratigraphic model of foreland basin development
Authors: Flemings Peter B. , Jordan Teresa E.
Published: February 2008

The processes of erosion and deposition must be included in foreland basin models to predict correctly basin geometry and stratigraphy. We present a synthetic stratigraphic model of the development of nonmarine foreland basins that predicts progressive geometry, topography, and facies patterns. In the model, steady crustal shortening occurs according to a wedge‐thickening model, erosion and deposition follow a diffusive process, and the lithosphere is compensated elastically. Erosion and deposition are controlled by the transport coefficients κ of the diffusion equation. For a range of thrust velocities and lithospheric rigidities, transport coefficients are of order 102–103 m2/yr in the mountain belt; these values are much higher than those derived from the study of scarp degradation. In the sedimentary basin, transport coefficients of order 104 m2/yr are appropriate and are compatible with previous studies of fluvial and deltaic deposition. Rapid thrusting results in a narrow underfilled basin, while slow thrusting results in a wide overfilled basin. In addition, by varying the erosional and depositional transport coefficients while holding other parameters constant, we generate both overfilled and underfilled basins. These results suggest that changes in the rate of thrust loading, the climate, or the source rock lithology can create stratigraphic signatures that have been interpreted to record viscoelastic relaxation of the lithosphere. Clearly, to understand either the long‐term behavior of the lithosphere or to interpret orogenic history from preserved foreland strata, the manner in which a basin was filled must be considered. We apply the model to the evolution of the modern sub‐Andean foreland and find that an erosional transport coefficient of 3000 m2/yr and a depositional transport coefficient of 20,000 m2/yr successfully predict the observed basin geometry.
A theoretical discussion of time domain magnitudes: The Prague formula for Ms and the mantle magnitude Mm
Authors: Okal Emile A.
Published: February 2008

A combination of surface wave theory and phase‐stationary asymptotics is used to relate the time domain amplitude a(t) of a strongly dispersed wave and the moment M0 of the source. This approximation is valid at sufficiently large distances, over 10° for 20‐s Rayleigh waves. We apply this formalism to justify theoretically the Prague formula for Ms. Assuming a Rayleigh Q of 297, we can successfully model the theoretical distance correction as 1.66 log10 Δ in the range 20–160°. We also predict a relation of the form log10 M0 = Ms + 19.46, in good agreement with reported empirical values. Finally, we show that the theory requires Ms to be described by the product (aT); the use of the ratio (a/T) is a partial and ad hoc compensation for a large number of frequency‐dependent terms ignored in the Prague formula. The same formalism can be applied to the inversely dispersed branch of mantle Rayleigh waves, between periods of 60 and 230 s. We provide the theoretical justification for the use of time domain measurements to obtain a mantle magnitude, and in particular for the modeling of the ratio a/A of the time domain and spectral amplitudes as 2/T, at distances ranging from 20 to 120°. At greater distances, and in particular for multiple passages, an additional distance correction must be effected.
Late Mesozoic strike slip movement on the Border Ranges Fault System in the Eastern Chugach Mountains, southern Alaska
Authors: Pavlis Terry L. , Crouse George W.
Published: February 2008

Preliminary studies of mylonitic rocks from the Haley Creek metamorphic assemblage in south‐central Alaska suggest that the Border Ranges fault system was a strike slip, ductile‐shear zone during part of its late Mesozoic history. Evidence for this strike slip event is limited to the hanging wall of the Border Ranges fault system, where deformed crystalline rocks were apparently subjected to a retrograde, mylonitic deformation prior to subduction‐related underthrusting of the latest Cretaceous Valdez Group, but subsequent to a Late Jurassic plutonic event. The strike slip interpretation contrasts with previous interpretations, which related mylonitic fabric development to multiple deformational events during underthrusting of the Chugach terrane, a conflict resulting from the interpretation of lineation as an intersection lineation parallel to the Y axis of strain versus an extension lineation. Evidence for horizontal stretching on a vertical flattening plane includes: (1) a parallelism between all lineations, including intersection lineations, and unequivocal extension lineations developed within systems of strike slip, ductile‐shear zones, (2) extensional fragmentation of feldspars parallel to lineation, (3) local rodding and development of L‐tectonites, (4) boudin axes at a high angle to lineation, (5) crystallographic preferred orientations of quartz, and (6) syn‐kinematic crack‐seal veins with fibers parallel to lineation. The local parallelism between intersection lineations and extension lineation is interpreted as a consequence of sheath folding in a high‐strain zone, and we suggest that it is this complication that is largely responsible for confusion on the kinematic history. Despite evidence for a strain pattern consistent with a strike slip shear zone, evidence for noncoaxial strain and a clear sense of shear is elusive. Nonetheless, coaxial deformation seems unlikely given the structural position of the mylonitic rocks along an ancient plate boundary. Regional correlation of rock types is suggestive of a dextral offset of at least 50 km, although this offset could be due to younger faulting. Thus further work will be needed to clearly distinguish a dextral versus sinistral sense of shear. Important absolute age problems still remain, and thus the tectonic significance of the strike slip event is uncertain. However, the general late Mesozoic age places the event within an important time period in the history of the northern Cordillera. Thus the strike slip event may have (1) played a role in a transform history preceding Early Cretaceous thrusting along the Border Ranges fault system, (2) been a complication from collision of Wrangellia with North America, and (3) been important in the northward transport of outer Cordilleran terranes that is implied by paleomagnetism.
Initial argon in amphiboles from the Chugach Mountains, southern Alaska
Authors: Onstott T. C. , Sisson V. B. , Turner D. L.
Published: February 2008

The 40Ar/39Ar step‐heating analyses of three hornblende samples from the Border Ranges ultramafic‐mafic assemblage yield plateaus with slightly discordant dates of 180.2±0.7, 177.5±0.7, and 181.1±1.7 Ma. A plot of the plateau temperature data for all three hornblende samples on an 36Ar/40Ar versus 39Ar/40Ar diagram reveals the presence of a non‐atmospheric “initial” argon component with an apparent 40Ar/36Ar of 379±16 and yields a date of 175.7±1.0 Ma. A potassium‐poor hastingsite from the same complex yields a complicated spectra with dates as high as ∼900 Ma. These older dates reflect contamination by argon with an isotopic composition similar to that found for the initial argon in the hornblende and do not record a Paleozoic component in the age of the Border Ranges ultramafic‐mafic complex. Hornblende from an Eocene tonalite sill in the nearby Chugach Metamorphic Complex also contains an initial argon component (40Ar/36Ar = 314 ± 4). The composition of this initial argon is presumably dictated by the argon isotopic composition of the host rock fluid at the time of amphibole/fluid equilibration. Approximately 2×10−7 cm3/g of this initial argon is sufficient to perturb the 40Ar*/39ArK “model” plateau dates. Although 40Ar/39Ar stepwise degassing at low temperatures removes an additional “atmospheric” argon component, the initial argon in these samples is released at a rate that closely coincides with the degassing rate of radiogenic argon from the amphibole, and only through application of the 36Ar/40Ar versus 39Ar/40Ar diagram can this nonatmospheric argon component be detected. For the Border Ranges ultramafic‐mafic and Chugach Metamorphic complexes, the range in K‐Ar amphibole dates could be interpreted as recording differential uplift and an extended duration of intrusive activity, respectively. The results suggest, however, that nonatmospheric initial argon contributes significantly to the observed age variations.
Thermal modeling of the Chugach Metamorphic Complex
Authors: James Thomas S. , Hollister Lincoln S. , Morgan W. Jason
Published: February 2008

Heating from hot, young, subducting oceanic lithosphere and from a subducting spreading center has been proposed to explain the metamorphism and associated plutonism observed in former accretionary prism sedimentary rocks in the Gulf of Alaska. We evaluated the thermal effects on the overlying accretionary wedge by heat conduction from a hot, young, subducting slab through numerical modeling. The temperature‐limiting parameter in the modeling is the age of the subducted crust. The rate of convergence, angle of subduction, amount of radiogenic heating, and values of thermal conductivity are of secondary importance in determining temperatures in the accretionary prism. A possible former setting for the subduction of young oceanic crust is recorded in the Valdez metasedimentary rocks in the Chugach Metamorphic Complex of the Chugach terrane in southern Alaska. Petrologic data indicate that rocks now at the surface were at 10‐km depth when they were metamorphosed under greenschist facies conditions (350–400°C) and subsequently intruded by felsic plutons. Using parameter values appropriate for Chugach terrane rocks, our results indicate that, for accretionary prisms less than 20 km thick, oceanic lithosphere aged 1.0 Ma or less must be subducted in order to obtain temperatures greater than 300°C at 10‐km depth within the prism. We also modeled the transient heating of the accretionary prism due to the cessation of subduction of young oceanic lithosphere. Recent seismic refraction surveys from the Trans‐Alaska Crustal Transect have been interpreted to show underplated sedimentary rocks and relict oceanic crust beneath the Chugach Mountains. Transient heating due to cessation of subduction or underplating of hot oceanic crust does not appreciably relax the requirement of subducting oceanic crust aged 1.0 Ma or less in order to attain temperatures greater than 30O°C at 10‐km depth. However, at depths greater than 25 km, it may provide the heat necessary to generate hot fluids and felsic magma, which can augment the temperatures at shallow depths by advection of heat, as well as produce the postmetamorphism or synmetamorphism emplacement of the anatectically derived plutons observed in the Chugach Mountains. Our modeling requires subduction of such young crust that subduction of a spreading ridge was probable, although underplating of extremely young oceanic crust would also have provided sufficient heat to cause the formation of the Chugach Metamorphic Complex.
Primitive calc‐alkaline and alkaline rock types from the Western Mexican Volcanic Belt
Authors: Luhr James F. , Allan James F. , Carmichael Ian S. E. , Nelson Stephen A. , Hasenaka Toshiaki
Published: February 2008

Since the early Pliocene, small volumes of alkaline magmas have erupted in the western Mexican Volcanic Belt (MVB) in close association with the volumetrically dominant calc‐alkaline magmas. Both suites include relatively rare “primitive” types, characterized by high Mg#, Cr, and Ni contents. Primitive hypersthene‐normative basalts, parental to the calc‐alkaline suite, are found along the volcanic front but are absent at greater distances from the trench. The volcanic‐front calc‐alkaline suites are typically hornblende bearing, indicating relatively high water contents. Associated primitive alkaline magmas at volcanic‐front locations also contain hydrous minerals. These nepheline‐normative suites include basanites, phlogopite‐bearing minettes, and other hornblende‐bearing lamprophyres. These are probably the youngest and freshest lamprophyres yet discovered on Earth, and ideal samples for addressing the origin of this exotic class of rocks. On extended MORB‐normalized elemental plots, all lamprophyres show patterns similar to the calc‐alkaline basalts, but have overall enrichments of 2 to 25X in Ti, K, P, Ba, Sr, light rare earth elements, and other incompatible elements. The lamprophyres show the same strong relative enrichments of Ba, K, and Sr, and depletions in Ti and Nb that characterize the calc‐alkaline rocks of western Mexico and all subduction zones. These similarities in relative element abundances support a common source region for all primitive magmas of the western MVB. The primitive calc‐alkaline and alkaline rocks of western Mexico show narrow, overlapping ranges in Sr, Nd, and Pb isotopic ratios, also consistent with derivation of all magmas from a common source. The presence of the relatively cold subducted Cocos and Rivera plates below the volcanic front demands that all primitive magmas originated in the depth range 30 to 75 km. These magmas are enriched in Ba, K, Sr, and other elements, probably transported by hydrous fluids rising from the subducted slab. Several lines of evidence support the presence of phlogopite in the source region of western MVB magmas, probably in the form of phlogopite‐rich veins cutting asthenospheric peridotite. The first liquids formed upon partial melting of this veined source region will concentrate the vein component and generate the lamprophyres. These lamprophyric melts rarely erupt in arc settings, more commonly forming dikes in the arc crust. Eruption of lamprophyric magmas in the western MVB is favored by through‐crustal extensional fracture systems related to active rifting of the Jalisco block from the N. American plate. Larger volumes of calc‐alkaline basalt are generated by melting of the same source which dilutes the vein component with larger proportions of peridotitic wall rock.
Taylor instability in rhyolite lava flows
Authors: Baum B. A. , Krantz W. B. , Fink J. H. , Dickinson R. E.
Published: February 2008

A refined Taylor instability model is developed to describe the surface morphology of rhyolite lava flows. The effect of the downslope flow of the lava on the structures resulting from the Taylor instability mechanism is considered. Squire's (1933) transformation is developed for this flow in order to extend the results to three‐dimensional modes. This permits assessing why ridges thought to arise from the Taylor instability mechanism are preferentially oriented transverse to the direction of lava flow. Measured diapir and ridge spacings for the Little and Big Glass Mountain rhyolite flows in northern California are used in conjunction with the model in order to explore the implications of the Taylor instability for flow emplacement. The model suggests additional lava flow features that can be measured in order to test whether the Taylor instability mechanism has influenced the flow's surface morphology.
Radar imaging of glaciovolcanic stratigraphy, Mount Wrangell Caldera, Alaska: Interpretation model and results
Authors: Clarke Garry K. C. , Cross Guy M. , Benson Carl S.
Published: February 2008

Glaciological measurements and an airborne radar sounding survey of the glacier lying in Mount Wrangell caldera raise many questions concerning the glacier thermal regime and volcanic history of Mount Wrangell. An interpretation model has been developed that allows the depth variation of temperature, heat flux, pressure, density, ice velocity, depositional age, and thermal and dielectric properties to be calculated. Some predictions of the interpretation model are that the basal ice melting rate is 0.64m yr−1 and the volcanic heat flux is 7.0 W m−2. By using the interpretation model to calculate two‐way travel time and propagation losses, radar sounding traces can be transformed to give estimates of the variation of power reflection coefficient as a function of depth and depositional age. Prominent internal reflecting zones are located at depths of approximately 59–91 m, 150 m, 203 m, and 230 m. We attribute these internal reflectors to buried horizons of acidic ice, possibly intermixed with volcanic ash, that were deposited during past eruptions of Mount Wrangell. The depositional dates corresponding to these reflecting zones are 1950–1924, 1853, 1770, and 1728 A.D., and we use this information to propose a volcanic history for Mount Wrangell.
A paleomagnetic constraint on the Late Cretaceous paleoposition of northwestern Baja California, Mexico
Authors: Filmer P. E. , Kirschvink J. L.
Published: February 2008

We present palaeomagnetic data for Cretaceous sedimentary rocks from the El Rosario formation of northwestern Baja California which imply a northward displacement of 15.0±3.8° and a 30.8±4.2° clockwise rotation for the Baja Peninsula relative to the North American craton since Late Cretaceous time. Paired IRM acquisition, X‐ray diffraction, and Mössbauer spectra indicate that fine‐grained magnetite is the dominant magnetic mineral present, with goethite and other more oxidized phases rare to absent. Biochemical evidence constrains the amount of diagenesis undergone by the sediments. We subjected oriented samples to both progressive alternating field and thermal demagnetization, and in general found univectorial magnetizations, yielding a mean inclination of 44.6° and declination of 5.3° with an α95of 2.0°. The corresponding paleolatitude for these values is 26.3±1.6°. Most of the samples were of normal polarity, which is consistent with their deposition within the upper Campanian chron 33N. We collected samples from younger Maastrichtian sediments which yield consistent, reversed directions.
Effects of thermal remanent magnetization on the magnetic anomalies of intrusives
Authors: Arkani‐Hamed J. , Celetti G.
Published: February 2008

Interpretations of magnetic anomalies often assume negligible remanent magnetization because detailed information about magnetic properties is rarely available. This paper attempts to assess the effects of thermal remanent magnetization (TRM) on magnetic anomalies caused by intrusives. Several three‐dimensional axisymmetric and two‐dimensional intrusives of different sizes are emplaced at different depths and at different times, and their thermal evolutions are calculated. On the basis of these thermal evolutions, we determine their TRM patterns, assuming reversals of the Earth's magnetic field have occurred and using typical magnetic blocking temperatures for igneous rocks. Results indicate that an intrusive with a uniform distribution of magnetic minerals acquires a layered pattern of TRM. Near‐surface intrusives acquire stronger magnetization than deep‐seated ones. The contribution of TRM to magnetic anomalies is quite different from the contribution of induced magnetization, especially for near‐surface intrusives. Interpretations of magnetic anomalies caused by deep‐seated intrusives in terms of induced magnetization are more reliable than those of the anomalies which are associated with near‐surface intrusives.
Geoid height versus topography for oceanic plateaus and swells
Authors: Sandwell David T. , MacKenzie Kevin R.
Published: February 2008

Oceanic plateaus and swells are a major component of the seafloor topography, yet they remain among the most poorly understood features. This is especially true of the oceanic plateaus which show large variations in crustal thickness. To determine the depth and mode of compensation for 53 of the largest plateaus and swells, we analyzed the relationship between geoid height and topography in polygonal areas containing each feature. Both geoid height and topography were first band‐pass filtered (400 km < l < 4000 km) to isolate the signal associated with local compensation from flexural and deep mantle signals. The ratio of geoid height to topography was then determined by fitting a straight line to the data. Except for nine of the smaller features there is a high correlation between geoid height and topography that is positive in accordance with Airy and thermal compensation models. Eighteen features have high geoid/topography ratios that cannot be explained by the Airy compensation model of crustal thickening. These features (thermal swells) are partially supported by thermal buoyancy forces in the lower half of the lithosphere. The ratios are highest for active hot spot swells and decay, with the thermal age of the swell, to values consistent with Airy compensation of the enduring volcanic edifice. The remaining features (plateaus) have lower geoid/topography ratios in agreement with the Airy compensation model. Those plateaus with average height greater than 4 km are thought to be continental fragments; the shorter plateaus tend to be volcanic features. Modified continental plateaus, presumably small fragments of extended and intruded continental margin crust, cluster around heights of ∼3 km, overlapping the range associated with oceanic plateaus. Since the origin of many plateaus is poorly understood, this global geoid/topography analysis provides a new technique for comparing the deep structure of oceanic plateaus and swells.
Determination of fault planes at Coalinga, California, by analysis of patterns in aftershock locations
Authors: Fehler Michael , Johnson Paul
Published: February 2008

The May 2, 1983, Coalinga, California, earthquake was not anticipated because it took place along a previously unrecognized fault concealed beneath an anticline, in a region that had little historic seismicity. The main shock hypocenter was at 10 km depth, and faulting did not break the surface. There has been considerable controversy about which of the nodal planes from the main shock fault plane solution was the slip plane. Conflicting results from geodetic, geologic, and aftershock data imply that slip occurred along one or the other or both nodal planes. The aftershock zone is large; several planar features within the aftershock zone indicate that faulting was complex. To delineate the planes of slip, we applied the three‐point method, a statistical technique by which event planes can be determined from aftershock locations. We obtained several important results from our study. First, we identified a number of planes using locations of aftershocks, some of which have not been previously identified from visual inspection of the aftershock locations. Second, the method allows us to choose slip planes by associating three‐point planes with fault plane solutions based on proximity and similar orientation. Finally, three of the planes identified by the method intersect near the main shock hypocenter, making it difficult to determine which plane is the main shock slip plane. Nevertheless, aftershocks during the first 24 hours after the main shock clearly define the high‐angle NE dipping nodal plane, whereas few aftershocks were located along the conjugate plane. Assuming that the aftershocks took place along the rupture zone, we conclude that slip during the main shock occurred predominantly along the high‐angle NE dipping plane from the fault plane solution for this event.
Geochemical and isotopic variations in a bimodal magma System: The Kaiser Spring Volcanic Field, Arizona
Authors: Moyer Thomas C. , Esperança Sonia
Published: February 2008

Volcanism in the Kaiser Spring field of west‐central Arizona postdates detachment faulting and crustal extension in the Basin and Range Province to the south. The volcanic products of this field include a diverse assemblage of rhyolitic and basaltic lavas that include topaz‐ and garnet‐bearing high‐silica rhyolites (HSR), hornblende‐phyric low‐silica rhyolites (LSR), tholeiitic basalts (TB) that vary from subalkaline to alkaline, alkali‐olivine basalts (AOB), and a quartz‐bearing mafic andesite (QMA). Extrusion of HSR lavas clearly preceded eruptions of LSR lava; however, both occurred concurrently with the effusion of basaltic lavas along the periphery of the silicic field. Compared with LSR lavas, the HSR are compositionally evolved, being strongly depleted in feldspar‐compatible elements and having a variable, but high, content of incompatible elements. Rayleigh fractionation calculations and analyses of LSR matrix glass are consistent with derivation of HSR magmas by extensive crystallization of LSR parent melts. Both LSR and HSR lavas are characterized by εNd of −6 to −12, 87Sr/86Sri of 0.7056 to 0.7086, and unradiogenic 206Pb/204Pb of 16.96 to 17.32. Differentiated mafic lavas have Mg values of 54 to 58. The AOB is isotopically similar to basalts from the San Francisco volcanic field, having εNd of +1.2, 87Sr/86Sr of 0.7042, and 206Pb/204Pb of 18.18. Two samples of TB differ markedly from the AOB, having εNd of −6.0 and −7.7, 87Sr/86Sr of 0.7073 and 0.7082, and 206Pb/204Pb of 17.87 and 18.39. With the exception of lower 206Pb/204Pb values, the TB lavas are isotopically similar to lower crustal mafic xenoliths from central Arizona. The QMA has εNd of −2.0 and 87Sr/86Sr of 0.7048, but a Pb isotopic composition that is similar to that of the rhyolites. At least three components are needed to model the combined Sr, Nd, and Pb isotopic compositions of the Kaiser Spring basalts and rhyolites. The isotopic composition of the AOB is consistent with its derivation from an OIB‐like subcontinental mantle reservoir. The rhyolites were derived primarily from a comparatively unradiogenic crustal source, perhaps coupled with subordinate mantle input. The QMA formed by mixing of this same crustal component with mantle‐derived basalt. The isotopic compositions of the TB lavas indicate that a second crustal component contributed to their geochemistry. This component may be mafic lower crust that is similar to amphibolite and eclogite xenoliths from central Arizona. The TB may represent contaminated mantle‐derived melts or melts formed by lower crustal anatexis. The Pb isotopic compositions of the HSR, LSR, TB, and QMA compare favorably with the isotopic composition of local Precambrian granitoids, and indicate that the crustal components of the Kaiser Spring lavas belong to the Mojave crustal terrane.
The dynamics of global positioning system orbits and the determination of precise ephemerides
Authors: Colombo Oscar L.
Published: February 2008

Analytical orbit perturbation theory suggests that the errors in the ephemerides of the Global Positioning System (GPS) satellites should be mostly resonant effects that can be corrected by adjusting a few parameters in a simple empirical acceleration formula, despite the complexity of their causes (mismodeling of gravity, radiation pressure, etc.), at least for arcs free from orbit maneuvers. This theoretical conclusion has been tested with simulations and actual data analyses. For these analyses, data from the Spring 1985 Experiment have been used to calculate improved ephemerides; in turn, these ephemerides have been used in the estimation of the coordinates of stations within the continental United States, previously positioned with very long baseline interferometry (VLBI). The agreement between the VLBI and the GPS results gives a measure of orbit quality. The outcome of this test supports the idea that the errors are mostly of a resonant nature. The principles described here are potentially applicable to the computation of the precise ephemerides of other spacecraft, such as geosynchronous relay satellites and oceanographic satellites, which are usually in highly resonant, nearly circular, repeating orbits.
Effective elastic thicknesses of the lithosphere and mechanisms of isostatic compensation in Australia
Authors: Zuber Maria T. , Bechtel Timothy D. , Forsyth Donald W.
Published: February 2008

The isostatic compensation of the Australian lithosphere is examined using the coherence of the two‐dimensional Fourier transforms of Bouguer gravity and topography. The isostatic model assumes regional compensation of an elastic plate that undergoes flexure in response to surface and subsurface loading. A least squares inversion of the observed coherence for continental Australia yields an effective elastic thickness of 130 km (corresponding flexural rigidity of 2.0×1025 N m). The model indicates that loads with wavelengths of greater than 1500 km are locally compensated, loads with wavelengths in the range 600–1500 km are partially supported by the strength of the lithosphere, and loads with wavelengths of less than 600 km are almost completely supported by lithospheric strength. Regions within the continent with different ages and tectonic histories exhibit effective elastic thicknesses that increase with the time since lithospheric stabilization. Precambrian shields in western, northern, and southern Australia are well fit by an elastic plate with a thickness of the order of 100 km. The Phanerozoic Interior Lowlands have elastic thicknesses that range from 30 to 80 km, and the Eastern Highlands have thicknesses of 15–35 km. The theoretical model of predicted coherence, which assumes statistically independent surface and subsurface loads, does not provide a good fit to the coherence spectrum for the intracratonic basins of central Australia. The disagreement of the model and observations is most simply explained by statistically correlated surface and subsurface loads. Calculations of load distributions show that most of the elevation of the Western Shield can be represented as a locally compensated plateau with wavelengths greater than 1000 km; smaller‐amplitude, shorter‐wavelength relief appears almost entirely as surface loading. Approximately half of the topographic amplitude of the Southeastern Highlands is locally compensated, and both surface and subsurface loading contribute significantly to the regionally compensated topography. The low rigidity and subsurface loading in southeastern Australia are consistent with crustal underplating, igneous intrusion, and thermal perturbation of the upper mantle as mechanisms for the uplift and compensation of the highlands.
The Holocene paleosecular variation record from Elk Lake, Minnesota
Authors: Sprowl Donald R. , Banerjee Subir K.
Published: February 2008

The paleosecular variation record of the geomagnetic field from Elk Lake, Minnesota, is presented and its implications are discussed. The Elk Lake record is compiled from measurements of four parallel cores with a 2 cm sampling interval (≈15 years). Time control is provided by radiocarbon dates and by annual laminations (varves) in the sediment. Stratigraphic control is provided by the annual laminations and by fine scale (2–5 cm) variations in magnetic susceptibility. The varve chronology provides improved time control for the central North American paleosecular variation record back to about 4000 years. The results of this study largely confirm the results of Lund and Banerjee from Lake St. Croix and Kylen Lake and demonstrate the general utility of Holocene sediment paleomagnetic records. Differences between the records appear to be due to experimental procedure so minimal procedures are recommended for future work. Comparison of North American and European records suggests that two drifting, non‐dipole sources, one having its advent during the last 5000 years, can account for much of the secular variation.
A five‐parameter temperature‐corrected Murnaghan equation for P‐V‐T surfaces
Authors: Plymate Thomas G. , Stout James H.
Published: February 2008

We have derived an explicit, empirical V(T,P) equation based on an exponential temperature correction of the isothermal Murnaghan equation. The equation is symmetrical with respect to the first‐ and second‐order derivatives of volume and is consistent with the thermodynamic relationship between (∂KT/∂T)P and (∂αP/∂P)T. The only input data required are the five room temperature, room pressure parameters: Koo, K'oo, αoo, α′oo, and (∂KT/∂T)oo. We have successfully tested our equation against literature pressure‐volume‐temperature (P‐V‐T) data for two materials which exhibit quite different isochore behavior: NaCl and SiO2 (α quartz). For NaCl our equation predicts isochores which are very nearly straight and parallel, reflecting the fact that the product αPKT is approximately constant for this substance. For SiO2 our equation reproduces the strong negative curvature [(∂2T/∂P2)V ≪ 0] of the experimentally determined isochores. These results suggest that our five‐parameter equation can be used to accurately predict the P‐V‐T behavior of a wide variety of solids, including those for which isochore curvature is pronounced.
Verification of airborne positioning using Global Positioning System carrier phase measurements
Authors: Mader Gerald L. , Lucas James R.
Published: February 2008

Two experiments have been conducted to determine the accuracy with which an aircraft can be positioned in flight using carrier phase measurements from satellites of the NAVSTAR Global Positioning System (GPS). In each experiment, a photogrammetric camera was flown over a calibration area providing ground control that enabled aerotriangulation to be performed on ground targets or, conversely, for the camera position to be independently determined. GPS measurements were used to determine the position of an antenna located on the aircraft at the time of each photo and, using the fixed antenna‐camera vector, to determine the positions of the camera. The GPS‐derived camera positions were then used to compute the positions of the ground targets. In an experiment in Texas the differences between the conventional and GPS‐based aerotriangulation display biases of 1–3 cm horizontally and 28 cm vertically and RMS differences of about 2 cm, 3 cm, and 6 cm in north, east, and height, respectively. The camera positions derived from the aerotriangulation showed similar biases when compared to the GPS‐derived camera positions and RMS differences of about 7 cm in the horizontal and 9 cm in height. Because of edge effects in the photography the actual precision is slightly better. A second experiment in Washington revealed biases of several meters, apparently due to unresolved coordinate system differences. However, a Helmert transformation in this case produced RMS differences between GPS‐derived and photogrammetrically determined ground positions of 3–5 cm. While these results demonstrate the need for care in relating GPS results to other geodetic measurements, they also show the great potential of GPS carrier phase measurements for precise kinematic positioning and a variety of remote sensing applications.
The morphology of strike‐slip faults: Examples from the San Andreas Fault, California
Authors: Bilham Roger , King Geoffrey
Published: February 2008

Fault zones are seldom straight nor are fractures within them continuous. Permanent strain fields are imprinted near a fault of complex geometry as a result of continued slip. These strain fields may be manifest as vertical deformation near the fault. Strain fields that increase area result in subsidence, and strain fields that reduce area result in uplift. Although these effects are modified by erosion and deposition, the general features of fault zone morphology are a consequence of the geometry and slip distribution along strike. Graphical views of strain fields associated with simple vertical strike‐slip faults are derived using boundary element methods. If the fault is straight and parallel to the applied shear strain, the hills and valleys resulting from finite fault rupture are separated by the fault trace. This is rarely observed in nature. Instead, strike‐slip faults cut through hills and valleys apparently of their own making. We show this to be a consequence of segments of the fault being nonparallel to each other and to the overall applied shear strain. Coseismic strains are large but transient since they are substantially annulled by rupture on contiguous fault segments. We test our theory with examples from parts of the San Andreas fault system in California. The morphologies of the Parkfield region and the southern San Andreas fault in the Coachella Valley are shown to be consistent with plausible distributions of slip on mapped surface faults. The location of the Salton Sea appears to be a consequence of a reduction of slip on the San Andreas fault toward the Brawley seismic zone.
Streaming potential properties of westerly granite with applications
Authors: Morgan F. D. , Williams E. R. , Madden T. R.
Published: February 2008

This paper reports on laboratory measurements of the streaming potential properties of common minerals and in particular of Westerly granite. Data are presented for these solids in contact with simple electrolytic solutions and in one case for temperature variation in the range 5°–70°C. For 1:1 electrolytes the ratio of the streaming potential coupling coefficient to fluid resistivity was approximately −4 mV/(atm Ωm) and for 2:1 electrolytes −2 mV/(atm Ωm). The effect of temperature was small (0.05 mV/°C temperature increase) and indicates that the zeta potential was independent of temperature. Preliminary experimental results using mixed electrolytic solutions (NaCl + CaCl2) and two‐phase fluid flow are also described. Two‐phase flow can enhance streaming potentials by as much as a factor of 3 or 4. The laboratory data are used to interpret streaming potentials in geological problems, such as earthquake prediction, geothermal prospecting using the self‐potential method, and surface conductivity contribution to the electrical resistivity of rocks.
The ∼60‐year power spectral peak of the magnetic variations around london and the Earth's rotation rate fluctuations
Authors: Jin Rong‐Sheng , Jin Shirley
Published: February 2008

Maximum entropy spectral analysis has been applied to the annual mean magnetic data around London for the years 1839–1974. These data were compiled by Malin and Bullard. The existence of a ∼60‐year spectral line is clearly found. In addition, cross power spectral density analyses using a two‐channel maximum entropy method were applied to both the variations of the annual mean declination around London from 1900 to 1974 and the fluctuations of the Earth's rotation rate from 1876 to 1974. The coherence is high (0.98), and the phase angle indicates that the changes of the magnetic declinations around London lag behind the fluctuations of the Earth's rotation rate by 20±4 years.
Migrating transform zone and lithospheric transfer at the Galapagos 95.5°W propagator
Authors: Kleinrock Martin C. , Hey R. N.
Published: February 2008

At the 95.5°W propagator system along the Cocos‐Nazca plate boundary, the spreading axis is offset left‐laterally about 35 km (∼1 m.y.). This offset migrates westward 52 km/m.y. relative to points along the nearly east‐west trending ridge axis, transferring lithosphere from the Cocos to the Nazca plate as it does so. The required 58 km/m.y. of right‐lateral motion across this migrating transform is apparently not accommodated by a throughgoing north‐south transform fault but rather by an approximately 15‐ to 20‐km‐wide zone of shear deformation dominated by eastward curving of east‐west structures to a northwest‐southeast trend. This pattern of curvature, the coinciding zone of high seismicity, and the abundance of talus in the area suggest that shear is distributed over this entire zone. The deformation is inferred to be taken up by a “bookshelf” mechanism whereby faults oriented at high angles to the shear couple have left‐lateral slip antithetic to the right‐lateral shear couple, rotating individual blocks clockwise. These are interpreted as reactivated abyssal hill‐bounding normal faults probably dipping ∼45°–60°. Northeast‐southwest shortening due to the rotation appears to be accommodated by reverse slip on the faults and may account for the uplift of De Steiguer Ridge in the zone of transferred lithosphere. These faults have apparently had a very complex history: initially, normal faults formed as abyssal hills were created at the spreading axis, and subsequently, they had left‐lateral strike‐slip motion in the transform zone and reverse motion uplifting De Steiguer Ridge. Strain analysis based on observed structural curvature suggests that shear is distributed nonuniformly and lithospheric transfer requires ∼0.4 m.y.
Earthquake source parameters and stress distribution in the Adak Island region of the central Aleutian Islands, Alaska
Authors: Ekström Göran , Engdahl E. R.
Published: February 2008

Source parameters have been systematically determined for all earthquakes with Mw≥5 that occurred between 172°W and 179°W longitude in the Adak Island region of the central Aleutian Islands during 1977‐March 1987. We relocate the events using a plate velocity model developed for the region and use two alternative methods of depth determination. The first method uses arrival times of direct and prominent reflected phases, primarily pwP. The second method uses broadband P wave displacement seismograms in an inversion for source depth. The analysis of these events provides a well‐constrained data set for the study of stress release along an active subduction zone. Three earthquakes, which occurred seaward of the trench axis, are located just below the crust‐mantle interface and show extension nearly perpendicular to the trench axis. Seven events occurred in the Wadati‐Benioff zone. Sixty‐four events are located in the main thrust zone, and, except for five unusual events, are characterized by thrust mechanisms with one nodal plane dipping north at a shallow angle. In cross section the thrust zone appears as a thin (10–15 km thickness) interplate region which extends from 15 to 50 km depth. A 10°–15° average discrepancy between observed slip vector azimuths and the predicted relative motion direction between the Pacific and North American plates exists for these events. The observed slip vectors are oriented more normal to the trench than is predicted by plate motions. Five earthquakes, which occurred as aftershocks to the May 7, 1986, earthquake (Mw = 8.0), in the crust of the overriding plate have strike‐slip mechanisms consistent with right‐lateral motion on arc‐parallel fault planes. Observations of slip vectors along the whole Aleutian arc show a similar trend to that observed in the Adak Island region. The largest differences (∼30°) between observed and predicted slip azimuths occur around 175°E. We propose a model of plate interaction in which a portion of the along‐arc motion occurs along a weak strike‐slip shear zone in the upper plate, near the volcanic line. The slip azimuths in the main thrust zone fit this model well, if the amount of transcurrent slip occurring in the upper plate is ∼60% of the arc‐parallel relative plate motion. A consequence of the model is along‐arc extension of the overriding plate between the accretionary wedge and the volcanic line, especially in the western part of the Aleutian arc. Calculations based on a tectonic model by Geist et al. (1988) for the formation of arc summit basins through block rotation and translation, suggest that along‐arc extension has been significant since late Miocene or early Pliocene.
Acceleration spectra for subduction zone earthquakes
Authors: Boatwright John , Choy George L.
Published: February 2008

We estimate the source spectra of shallow earthquakes from digital recordings of teleseismic P wave groups, that is, P + pP + sP, by making frequency dependent corrections for the attenuation and for the interference of the free surface. The correction for the interference of the free surface assumes that the earthquake radiates energy from a range of depths. We apply this spectral analysis to a set of 12 subduction zone earthquakes which range in size from MS = 6.2 to 8.1, obtaining corrected P wave acceleration spectra on the frequency band from 0.01 to 2.0 Hz. Seismic moment estimates from surface waves and normal modes are used to extend these P wave spectra to the frequency band from 0.001 to 0.01 Hz. The acceleration spectra of moderate subduction zone earthquakes, that is, earthquakes whose seismic moments are less than 1027 dyn cm, exhibit ω‐square or Brune‐type spectra, while the acceleration spectra of large subduction zone earthquakes, that is, earthquakes whose seismic moments are greater than 1027 dyn cm, exhibit intermediate slopes where ü(ω) ∝ ω5/4 for frequencies from 0.005 to 0.05 Hz. For this set of earthquakes, spectral shape appears to be a discontinuous function of seismic moment. Using reasonable assumptions for the phase characteristics, we transform the spectral shape observed for large earthquakes into the time domain to fit Ekström's (1987) moment rate functions for the MS = 8.1 Michoacan earthquake of September 19, 1985, and the MS = 7.6 Michoacan aftershock of September 21, 1985.
Crustal shortening, root spreading, isostasy, and the growth of orogenic belts: A dimensional analysis
Authors: Gratton Julio
Published: February 2008

Scaling laws that describe the growth of mountain belts and the change in their regional topographic profiles with time (t) are obtained by physical arguments that account for crustal shortening, isostasy, and creeping flow at the mountain roots. An average power law rheology characterized by an exponent n is assumed, and different values of n are considered. The model predicts the formation of a localized range characterized by a height (h) and a width (a). As crust on the sides of a range is compressed, the root broadens due to buoyancy‐driven creep. This process in conjunction with isostasy causes h to increase with time as t1/4–t1/3, and a as t3/4–t2/3, as n is varied from 1 (Newtonian rheology) to ∞ (plastic rheology). Accordingly, the aspect ratio a/h increases as t1/2–t1/3. The scaling laws depend on the rheology, but rather weakly; the aspect ratio increases with age but is nearly independent of the rate of shortening. The deformation extends gradually inland with increasing time, so that the age of tectonic deformation decreases with distance from the continental border. Uplift takes place inland from the crest while subsidence occurs at the opposite side. The ratio a/h of the Andes and Tibet is consistent with this model.
Olivine‐modified spinel‐spinel transitions in the system Mg2SiO4‐Fe2SiO4: Calorimetric measurements, thermochemical calculation, and geophysical application
Authors: Akaogi Masaki , Ito Eiji , Navrotsky Alexandra
Published: February 2008

The olivine(α)‐modified spinel(β)‐spinel (γ) transitions in the system Mg2SiO4‐Fe2SiO4 were studied by high‐temperature solution calorimetry. Enthalpies of the β‐γ and a α‐γ transitions in Mg2SiO4 at 975 K and of the α‐γ transition in Fe2SiO4 at 298 K were measured. The γ solid solution showed a positive enthalpy of mixing. Phase relations at high pressures and high temperatures were calculated from these thermochemical data including correction for the effect of nonideality of α, β, and γ solid solutions. The calculated phase diagrams agree well with those determined experimentally by Katsura and Ito very recently. The α − (Mg0.89, Fe0.11)2SiO4 transforms to β through a region of α+β without passing through the α+γ phase field at around 400 km depth in the mantle with an interval of about 18(±5) km. Temperatures at 390 and 650 km depths are estimated to be about 1673 and 1873 K, respectively, assuming an adiabatic geotherm.