bluuebunny/crossref_metadata_2025 huggingface dataset:
Paleomagnetic evidence for post‐2.55‐GA tectonic tilting and 1.1‐Ga Reactivation in the Southern Kapuskasing Zone, Ontario, Canada
Authors: Costanzo‐Alvarez Vincenzo , Dunlop David J.
Published: February 2008

We report paleomagnetic results from two traverses in the southern part of the Kapuskasing structural zone (KSZ) of north‐central Ontario, which is hypothesized to be an oblique cross section through upper and middle Archean crust exposed by uplift and tilting. Anorthosites and granulite‐grade gneisses were sampled to the east in the Shawmere traverse, while amphibolite‐grade tonalitic gneisses were sampled to the west in the Racine traverse. Three distinct remanent magnetization (RM) components were isolated by alternating field and thermal demagnetization. The A RM has mean directions D = 50°, I = +62° (k = 138, α95 = 5°, N = 5 sites (23 samples)) for the Shawmere traverse and D = 23° I = +53° (k = 49, α95 = 7°, N = 8 sites (46 samples)) for the Racine traverse. Because of the high (>500°C) laboratory unblocking temperatures of the A RM in the Shawmere granulite‐grade rocks, we believe this magnetization was acquired in early slow uplift, without tilting, from about 24‐km to about 16‐km depths, and has survived the later main KSZ uplift and tilting event. The Racine A RM has laboratory unblocking temperatures down to ≈400°C and was blocked prior to uplift from a shallower crustal level (about 8 km). With tilt corrections of 30° and 15°, respectively, about the 35° striking axis of the KSZ, the Shawmere and Racine A paleomagnetic poles fall on the Laurentian apparent polar wander path around 2.55 Ga, dating the time of initial uplift. Crustal tilting in the KSZ occurred after 2.55 Ga and was greater in the east than in the west. The B RM, found mainly in Shawmere anorthosites, has D = 115°, I = −68° (k = 17, α95 = 17°, N = 4 sites (14 samples)), with a paleopole at 144°W, 49°N, near 1.1 Ga Keweenawan reverse poles and the normal pole of the nearby 1.0 Ga Nemegosenda carbonatite. The B RM is probably a thermochemical overprint related to reactivation of the KSZ at the time of Keweenawan rifting and volcanism in the Lake Superior region. A novel C RM, with D = 64°, I = +2° (k = 18, α95 = 13°, N = 6 sites (10 samples)), is either an unresolved hybrid of the A and B RMs or an earliest Keweenawan (1.3 Ga) overprint.
Warm fluid migration along tectonic melanges in the Kodiak Accretionary Complex, Alaska
Authors: Vrolijk Peter , Myers Georgianna , Moore J. Casey
Published: February 2008

Analyses of methane + water fluid inclusions from syntectonic quartz veins in melange boudins of the Kodiak accretionary complex, Alaska, indicate fluid temperatures ranging from 215° to 290°C at depths between 10 and 14 km. These fluid temperatures are significantly higher than those predicted by conductive heat flow models of subduction zones. Given the available geological constraints, this discrepancy is interpreted as the result of migration of warm fluids from deeper structural levels along active fault zones during subduction‐related underthrusting of these units. This interpretation requires more careful consideration of how migrating fluids affect the general thermal structure of subduction zones.
Ocean bathymetry and mantle convection: 2. Small‐scale flow
Authors: Davies Geoffrey F.
Published: February 2008

Numerical models demonstrate that a boundary layer instability of the lower lithosphere does not stop the lithosphere from subsiding, and so it cannot explain the so‐called flattening of old seafloor. The slower subsidence of the old seafloor is in any case better seen as a perturbation, probably due to plumes. Furthermore, if a boundary layer instability, or any mode of upper mantle convection, transported most of the heat emerging from old seafloor, there would be pervasive, large‐amplitude local bathymetrie, gravity, and geoid anomalies. Such anomalies have never been convincingly differentiated from the effects of the cooling lithosphere and hotspot tracks, except for some low‐amplitude, linear geoid anomalies near spreading centers. The only modes of mantle convection with horizontal scales less than those of plates for which there is reasonably clear evidence are hotspot plumes and some possible near‐ridge boundary layer instabilities. Hotspots transport less than about 10% of the Earth's heat flux, and other small‐scale modes are probably less important.
Formation of curved seafloor fabric by changes in rift propagation velocity and spreading rate: Application to the 95.5°W Galapagos propagator
Authors: Acton Gary , Stein Seth , Engeln Joseph F.
Published: February 2008

The geometry of oceanic spreading centers can change by rift propagation, in which a new ridge segment propagates and preempts a preexisting one. As the locus of relative motion between the two plates shifts, the dying and growing ridge segments are joined by a boundary, usually a transform, which migrates with time, causing seafloor to be transferred from one plate to the other. As a result, originally ridge‐parallel isochrons in a zone between the growing and failed rifts can be reoriented to trends oblique to both the ridge and spreading direction. An attractive feature of this model is that the reorientation occurs by strictly rigid plate tectonics, since relative motion occurs only between points on different plates. Recent surveys of the Galapagos propagating rift system at 95° W show seafloor fabric curving away from the growing rift and toward the failed rift. These observations have been interpreted as requiring a zone of distributed simple shear between the two rifts. Such behavior would represent a local deviation from rigid plate tectonics, since relative motion would be distributed over a finite region not part of either of the two plates. Here we explore possible rigid plate models for the formation of curved seafloor lineaments by rift propagation. We find that such fabric can be generated by changes either in the velocity (rate or direction) of rift propagation, or in the spreading rate during rift propagation. The curvature of various tectonic elements is diagnostic of the different possibilities. The geometry observed at the Galapagos can result from either rift propagation acceleration, or a spreading rate decrease, during the last few hundred thousand years. The reverse curvature could result from either deceleration of rift propagation or an increase in spreading rate. Propagator acceleration and spreading rate slowing, which produce similar structural trends, could in principle be distinguished using spreading rate data. At Galapagos, however, since either would have occurred within the present period of normal polarity, both are consistent with the magnetic anomaly data. We thus conclude that the data interpreted as requiring a shear zone are equally consistent with two distinct models based on rigid plate tectonics. It may, however, be possible to discriminate between rigid and shear models using fabric observations at other propagators. The shear model requires that the sense of curvature at Galapagos be a ubiquitous feature of active propagators. In contrast, the rigid plate tectonic model allows for fabric with a variety of orientations, including reverse curvature formed by either propagation deceleration or increasing spreading rate and linear fabric formed by constant propagation and spreading velocities.
Implications of magma convection for the size and temperature of magma chambers at fast spreading ridges
Authors: Wilson Douglas S. , Clague David A. , Sleep Norman H. , Morton Janet L.
Published: February 2008

We present a new thermal model that resolves the discrepancy between observations of narrow (∼4 km) magma chambers on the northern East Pacific Rise (EPR) and predictions based on previous thermal models of wide (∼20 km) chambers by including cooling of the lower crust by convection within the magma chamber. For this cooling to produce a solid crust within a few kilometers of the axis the temperature of the magma chamber must be significantly below the solidus temperature of the lower crust. This model is internally consistent for an average temperature of 1240°C for primary magmaleaving the mantle and an average magma chamber temperature of 1150°C. These temperatures imply ∼10% MgO for primary magma, which is at the low‐Mg end of the range of proposed compositions, and ∼6% MgO for magma residing in the chamber which is comparable to the most evolved basalts commonly erupted along the EPR. There is a trade‐off in the inputs to the model such that raising the primary magma temperature requires a lower magma chamber temperature to maintain the same maximum temperature of the lower crust. It is reasonable for the magma chamber to be more evolved than most erupted lavas if magma mixing exerts substantial control over the composition of the erupted lavas. Aphyric 10% MgO basalt should be buoyant in 6% MgO basalt, therefore the mixing need not involve the entire chamber prior to eruption.
Large‐Scale waveform inversions of surface waves for lateral heterogeneity: 2. Application to surface waves in Europe and the Mediterranean
Authors: Snieder Roel
Published: February 2008

Linear surface wave scattering theory is used to reconstruct the lateral heterogeneity under Europe and the Mediterranean using surface wave data recorded with the Network of Autonomously Recording Seismographs (NARS). The waveform inversion of the phase and the amplitude of the direct surface wave leads to a variance reduction of approximately 40% and results in phase velocity maps in the period ranges 30–40 s, 40–60 s and 60–100 s. A resolution analysis is performed in order to establish the lateral resolution of these inversions. Using the phase velocity perturbations of the three period bands, a two‐layer model for the S velocity under Europe and the Mediterranean is constructed. The S′ velocity perturbations in the deepest layer (100–200 km) are much more pronounced than in the top layer (0–100 km), which confirms that the low‐velocity zone exhibits pronounced lateral variations. In both layers the S velocity is low under the western Mediterranean, while the S velocity is high under the Scandinavian shield. In the deepest layer a high S velocity region extends from Greece under the Adriatic to northern Italy. Several interesting smaller features, such as the Massif Central, are reconstructed. One of the spectacular features of the reconstructed models is a sharp transition in the layer between 100 and 200 km near the Tornquist‐Tesseyre zone. This would indicate that there is a sharp transition at depth between Central Europe and the East European platform. The waveform inversion of the surface wave coda leads to good waveform fits, but the reconstructed models are chaotic. This is due both to a lack of sufficient data for a good imaging of the surface wave energy on the heterogeneities and to an appreciable noise component in the surface wave coda.
Nonuniform magnetization of Jasper Seamount
Authors: Gee J. , Tauxe L. , Hildebrand J. A. , Staudigel H. , Lonsdale P.
Published: February 2008

Paleopoles derived from seamounts have been used to reconstruct the tectonic history of ocean basins; however, the interpretation of seamount magnetization models and the validity of seamount paleopoles may be affected by inhomogeneous magnetization. Multibeam bathymetric data, sea surface and deep‐tow magnetic field data, and paleomagnetic analyses of dredged samples were used to examine the origin of nonuniform magnetization within Jasper Seamount (30°27′N, 122°44′W). Models indicate that the seamount is predominantly reversely magnetized with local zones of normal polarity as corroborated by deep‐tow measurements. Lithologies likely to be volumetrically important in a seamount edifice show highly variable magnetic properties. Basalts have high intensities (0.5–27.0 A/m), high Koenigsberger ratios (Q) and low viscous remanence (VRM) acquisition. Low Q ratios and high VRM acquisition coefficients of coarse‐grained material and volcaniclastics suggest that they may have substantial viscous and induced components. Models for Jasper are characterized by low uniform intensities and far‐sided paleopoles. The shallow model inclinations may be attributed to nondipolar components in the time‐averaged geomagnetic field. The low intensities of the uniform models and the large nonuniform component in the seminorm solutions imply a complex distribution of magnetization sources within Jasper. This nonuniformity may result from either lithological variability or construction of the seamount spanning two or more polarity intervals.
NATURE AND SIGNIFICANCE OF IGNEOUS ROCKS CORED IN THE STATE 2–14 RESEARCH BOREHOLE: SALTON SEA SCIENTIFIC DRILLING PROJECT, CALIFORNIA
Authors: Herzig Charles T. , Elders Wilfred A.
Published: February 2008

The State 2–14 research borehole of the Salton Sea Scientific Drilling Project penetrated 3.22 km of Pleistocene to Recent sedimentary rocks in the Salton Sea geothermal system, located in the Salton Trough of southern California and northern Baja California, Mexico. In addition, three intervals of igneous rocks were recovered; a silicic tuff and two sills of altered diabase. The chemical composition of the silicic tuff at 1704 m depth suggests that it is correlative with the Durmid Hill tuff, cropping out 25 km NW of the geothermal system. In turn, both of these tuffs may be deposits of the Bishop Tuff, erupted from the Long Valley caldera of central California at 0.7 Ma. The diabases are similar to basaltic xenoliths found in the nearby Salton Buttes rhyolite domes. These diabase are interpreted as hypabyssal intrusions resulting from magmatism due to rifting of the Salton Trough as part of the East Pacific Rise/Gulf of California transtensional system. The sills apparently intruded an already developed geo‐thermal system and were in turn altered by it.
The Growth of Geological Structures by Repeated Earthquakes 1. Conceptual Framework
Authors: King Geoffrey C. P. , Stein Ross S. , Rundle John B.
Published: February 2008

In many places, earthquakes with similar characteristics have been shown to recur. If this is common, then relatively small deformations associated with individual earthquake cycles should accumulate over time to create geological structures. Following this paradigm, we show that existing models developed to describe leveling line changes associated with the seismic cycle can be adapted to explain geological features associated with a fault. In these models an elastic layer containing the fault overlies a viscous half‐space with a different density. Fault motion associated with an earthquake results in immediate deformation followed by a long period of readjustment as stresses relax in the viscous layer and isostatic equilibrium is restored. Deformation is also caused as a result of the loading and unloading due to sediment deposition and erosion. In this paper, the parameters that control the growth of dip‐slip structures are identified. We find that the flexural rigidity of the crust (or the apparent elastic thickness) provides the main control of the width of a structure. The loading due to erosion and deposition of sediment determines the ratio of uplift to subsidence between the two sides of the fault. The flexure due to sediment load is much more important in this respect than whether the fault is normal or reverse in character. We find that, in general, real structures are associated with apparent elastic thicknesses of 4 km or less and thus with very low flexural rigidities.
Waveform Inversion of Expanding Spread Profile 5 From the North Atlantic Transect
Authors: Cary Peter W. , Chapman Christopher H.
Published: February 2008

WKBJ waveform inversion of Expanding Spread Profile 5 from the North Atlantic Transect is presented. The inversion consists of a Monte Carlo search to determine the major features of the model, followed by a gradient descent to the global minimum. Nonlinear error analysis completes the analysis. Modifications to existing algorithms to allow the search for low velocity zones and interfaces are described. The results agree closely down to midcrustal depths with results from trial‐and‐error inversions. Previous interpretations have concluded that a low velocity zone exists in the lower crust. Our waveform analysis shows that this feature may exist but is not necessarily needed to satisfy the data.
Signals and Noise in Measurements of Low‐Frequency Geomagnetic Fields
Authors: Nichols E. A. , Morrison H. F. , Clarke J.
Published: February 2008

The apparent magnetic noise, obtained from the coherency function for two parallel magnetic sensors, generally overestimates sensor noise because the sensors do not measure the same signal. The different signals result from the nonparallel alignment of the sensors and from the additional magnetic signal induced in each sensor by its motion in the Earth's magnetic field. A magnetometer array experiment was completed in Grass Valley, Nevada, to determine the minimum magnetic signal that could be detected in the presence of background natural field variations and motion of the sensor. Superconducting quantum interference device (SQUID) magnetometers with internal biaxial tiltmeters were used to record the magnetic fields and the motion of the sensors. A least squares fitting program enabled the field at one site in the array to be predicted from a remote site and to produce a residual field with a standard deviation of 8 pT over a 1‐hour period with a low‐pass filter at 0.3 Hz. Consistent field‐to‐residual ratios of 40–60 dB were achieved, with some ratios exceeding 70 dB. The least squares fit uses only a linear combination of the magnetic and tilt fields at a remote site to predict the observed magnetic field. This procedure allows for correction of calibration and orientation errors as well as the removal of the apparent fields originating from sensor movement. Misalignment and motion of the sensor are shown to be the major sources of magnetic field noise. The orientation error is typically of the same magnitude as the noise induced by sensor motion. In order to achieve ratios better than 20–40 dB one must include both the orthogonal fields and the tiltmeter outputs. Inclusion of a frequency‐dependent transfer function should increase the prediction ability of the least squares model, as evidenced by the improvement to 70 dB obtained with simple band limiting of the original data. These techniques should be applicable to any type of artificial source survey where natural field fluctuations are the noise‐limiting factor. The ability to describe the observed signals should allow a dramatic increase in one's ability to detect an artifically generated signal, allowing signal‐to‐noise improvements of 40–60 dB without increasing transmitter power or the averaging time.
The growth of AA lava flow fields on Mount Etna, Sicily
Authors: Kilburn Christopher R. J. , Lopes Rosaly M. C.
Published: February 2008

The historical lavas on Mount Etna provide a large body of data for examining the growth of aa flow fields. A flow field, the final product of an eruption, is composed of one or more flows. Flow generation depends on the ratio SR/SD of crustal retaining strength (SR) to stress driving the lava outward (SD), rate of flow thickening h, and the time for which h is sustained. At low SR/SD, lava advances by simple spreading, and new flows are formed by subdividing about topographic highs (bifurcation). At low h, growing crusts can contain thickening at all values of SR/SD. Overflow of chilled margins can occur at higher sustained h. Increasing SR/SD favors flow propagation by breaching of crusts. Flow growth is examined by extending the Bingham model of Hulme (1974). Cooling and discharge rate along a flow appear to be the main factors controlling the generation of new flows, with the result that long durations and low average effusion rates favor flow fields with large width/length ratios. Bifurcation generates flows randomly. Breaching and overflow generate flows systematically because of their dependence on cooling. With time, therefore, flow field growth evolves from predominant lengthening to widening. Such behavior is evident for Etnean flow fields, which can also be grouped into two populations on the basis of their width/length ratios. These populations may be related to two distinct mechanisms of magma ascent.
Bayes classification of terrain cover using normalized polarimetric data
Authors: Yueh H. A. , Swartz A. A. , Kong J. A. , Shin R. T. , Novak L. M.
Published: February 2008

The normalized polarimetric classifier which uses only the relative magnitudes and phases of the polarimetric data is proposed for discrimination of terrain elements. For polarimetric data with arbitrary probability density function, the distance measures of the normalized polarimetric classifier based on a general class of normalization functions are shown to be equivalent to one another. When the system absolute calibration factors are common to all polarimetric channels, the normalized polarimetric classifier derived is shown to be optimal. Further assuming a complex Gaussian distribution of the unnormalized data, the distance measure of the normalized polarimetric classifier is given explicitly and is shown to be independent of the number of scatterers illuminated. The usefulness of the normalized polarimetric classifier is demonstrated by the classification of grass and tree regions in experimental data obtained from the Massachusetts Institute of Technology Lincoln Laboratory. The classification errors generated using the optimal normalized polarimetric classifier are shown to be smaller than those generated using other types of classifiers which employ only magnitude ratios or phase differences to classify radar images.
The rise flank trails left by migrating offsets of the equatorial East Pacific Rise axis
Authors: Lonsdale Peter
Published: February 2008

Sea Beam and magnetic surveys of the young (<1 Ma) rise flank around nontransform rise crest offsets at 5.5°N, 4.9°N, 3.4°N, 2.0°N, and 2.8°S mapped distinctive trails of obliquely lineated, highly magnetic crust that spreads from the characteristically curving overlapping rift zones. A Deep Tow survey around the small (1.5 km) 3.9°N offset found only normal rise flank terrain. Near‐bottom Deep Tow observations at the medium size (9 km) 5.5°N offset and the large (27 km) 2.0°N offset provided more detailed information on the formation of crust in the offset trails, and of the structure of tectonically modified crust at interrift rise crest sites. Patches of interrift crust, which forms deep overlap basins at medium size offsets and basins and plateaus of rotated abyssal hills at large offsets, are periodically shed from the axial zone, together with abandoned portions of overlapped rift zone, to become important elements of the rise flank trails. The oblique azimuths of the paired “fracture zone” trails that diverge from the rise crest offsets show that most have migrated along the rise crest at average speeds of 25–125 mm/yr. Net movement of the 27‐km 2.8°S offset has been slower, with alternating periods of rapid northward and southward migration. Information on the migration history of the surveyed offsets was supplemented by inferring the migration direction of another eight offsets on the equatorial East Pacific Rise (EPR) whose trails were less thoroughly mapped. Most offsets of the southern Pacific‐Cocos boundary have migrated south, and most on the northern Pacific‐Nazca boundary have migrated north; the patterns are not readily explained by the inferred distribution of mantle upwellings along the EPR axis.
A Bayesian approach to the detection of temporal changes in P wave velocity
Authors: Hurukawa Nobuo , Imoto Masajiro
Published: February 2008

On the basis of Akaike's Bayesian information criterion (ABIC), a new method is proposed for detecting a temporal change in a seismic velocity in a source region. The method of joint hypocenter determination was modified in order to determine a seismic velocity in a source layer as a function of time together with hypocenters and station corrections. Arrival times of initial waves of shallow earthquakes in a small area are analyzed in this method. The smoothness of the estimated temporal variation in the velocity is guaranteed by the introduction of a prior distribution of the parameter. The hyperparameter of the prior distribution of the velocity, the reading error of arrival times, and the initial velocity in the source layer are chosen to minimize ABIC. This procedure was applied to the 1983 eastern Yamanashi M = 6.0 earthquake in central Japan. We analyzed P arrival times of 374 earthquakes observed at 12 stations in the network of the National Research Center for Disaster Prevention by dividing the whole period (from October 1981 to May 1987) into 12 six‐month subperiods. Calculating ABICs for different combinations of the three parameters above, we searched for the minimum value of ABIC and found two minima. The first one corresponds to a model of a constant velocity in time, and the other corresponds to a model of a variable velocity with 5% velocity change at maximum. However, since ABIC in the former is 10 smaller than that in the latter, the former constant velocity model is statistically more suitable than the latter. Furthermore, generating artificial data with the same reading errors as the actual data, we used computer simulation to examine the lower limit of the velocity change detectable for this data set. In conclusion, the velocity in the source region is 6.24±0.18 km/s, and the velocity change exceeding 6–7% at maximum did not exist during the 6 years before and after the M = 6.0 earthquake.
Displacement of surface monuments: Vertical motion
Authors: Wyatt Frank K.
Published: February 2008

Several measurements of vertical ground motion at Piñon Flat Observatory, California, indicate the overall weakness and instability of the Earth's weathered surface with respect to the underlying rock. Cumulative long‐period motions of order 0.5 mm per year dominate these records, though smaller elastic deformations caused by precipitation loading, atmospheric loading, and tidal strains are evident at higher frequencies; all of these help to characterize the near‐surface material. The long‐period records suggest that near‐surface weathering is the dominant influence on monument motion, at least at this site on crystalline rock in a semi‐arid environment. Rainfall loading gives an average vertical modulus of 2.6 GPa for the material in the uppermost 26 m of the ground, compared with 88 GPa for granite under moderate confining stress; atmospheric loading gives similar results but indicates the ground is permeable to airflow at periods longer than a few hours. Earth tide records show Poisson's ratio to be 0.09 in contrast to the normative range of 0.2–0.25, establishing that horizontal strains couple only weakly into vertical ones, so that vertical strains near the surface are a poor measure of areal strain. The form of the ground‐surface displacement power spectrum indicates that analyses of geodetic surveys would be improved with the inclusion of a monument‐positioning error budget that increases with time. Because of ground instability, and the generally small rate of crustal tectonic motions, deeply emplaced monuments will be needed for observational programs designed to detect short‐term changes in crustal deformation over baselengths of order 10 km and less.
Timing of mantle upwelling: Evidence for a passive origin for the Red Sea rift
Authors: McGuire Anne V. , Bohannon Robert G.
Published: February 2008

Mantle xenoliths from western Saudi Arabia, some 220 km east of the Red Sea margin, indicate temperatures as high as 900°C in the mantle at about 40 km depth beneath an area of the Arabian Shield where surface heat flow is lower than the world average. This suggests that there has been a large increase in the mantle temperature that has not had time to equilibrate at the surface. The amount of uplift and the difference between the observed xenolith temperature and that obtained by projecting the surface heat flow to depth suggests that the lithosphere has been thinned by 30 to 100 km to its present calculated thickness of about 80 km. Calculations of the time necessary for thermal reequilibration indicate that the lithospheric thinning probably occurred less than 20 m.y. ago, so it postdates rifting which started about 30 Ma. If there was a prolonged upwelling of deep mantle material, resulting in a thinner lithosphere prior to rifting, there should have been enough time for some reequilibration at the surface, and the observed heat flow should be higher over the broad area of uplift adjacent to the Red Sea. Our calculations are consistent with a postrift age of uplift adjacent to the Red Sea as determined from regional geologic evidence and fission track ages on apatites from shield rocks. The Red Sea appears to have begun with alkaline volcanism on the Arabian Shield, followed by rifting. Uplift and lithospheric thinning adjacent to the Red Sea rift began 5–10 m. y. after rifting. This suggests that the Red Sea is a passive rift that formed because of two‐dimensional plate motions, rather than as a consequence of mantle upwelling. A possible cause for the stress in the plates might have been the change of plate motions in the Indian Ocean that resulted from the collision of India with Eurasia.
Thermal structure of the lithosphere in Fujian, China
Authors: Wan Tianfeng , Tong Yongfu , Zheng Wiewen
Published: February 2008

The principal thermal parameters of the lithosphere (average reduced heat flow, heat production, and heat production depth parameter) have been determined from an analysis of silica heat flow estimates in Fujian, China. Silica data indicate that relative to western Fujian, heat flow is higher in eastern Fujian, with thinner lithosphere, a shallower depth to the Curie isotherm (about 27 km), and slightly higher geothermal activity. However, even in eastern Fujian, the thermal anomaly is small, no anomalous heat source is indicated, and conditions are not thought to be favorable for geothermal resources related to magma chambers, hydrothermal reservoirs, or hot dry rock. Widespread thermal springs in this region are heated by deep circulation of groundwater along faults in association with a slightly elevated geothermal gradient.
Theory of thermal recovery from a spherically stimulated hot dry rock reservoir
Authors: Elsworth Derek
Published: February 2008

A conceptual model is presented to describe thermal recovery from a semi‐infinite hot dry rock (HDR) geothermal reservoir containing an equidimensional permeable zone. Transient behavior may be represented uniquely by five dimensionless parameters. Variation in production temperature TD with time tD is influenced by reservoir throughput QD thermal porosity ϕD and depth ratio a/z. Of these, only throughput QD exercises significant control on transient performance, the parameter being directly proportional to reservoir circulation rate and inversely proportional to the effective radius of the stimulated zone. Steady production temperature TD is indexed to throughput QD and depth ratio a/z, only. Steady production temperatures are always highest for a host medium bounded by a proximal constant temperature surface and lowest for an insulated boundary. Boundary effects are insignificant for reservoir burial depths up to an order of magnitude greater than the reservoir radius. A threshold behavior in time (tDQD) is evident for very large reservoir throughput (QD). This bounding behavior describes, in dimensionless time, the maximum rate at which thermal depletion may occur. This state is evident for large dimensionless throughput magnitudes (QD) corresponding directly with high circulation rates within the reservoir. Predictions compare favorably with results from a 300‐day circulation test at the Fenton Hill Geothermal Energy Site, New Mexico.
Seismic boundary conditions for porous media
Authors: de la Cruz V. , Spanos T. J. T.
Published: February 2008

Boundary conditions for fluid‐filled porous media are derived in a form independent of the choice of equations of motion. A definition of the boundary surface, together with the equation of continuity for a porous medium, leads to some of the boundary conditions. The others are suggested by the physical meaning of the various stress tensors and the porosities and the concept of the alignment. For problems of seismic transmission and reflection it is demonstrated that these boundary conditions are complete. When one of the media degenerates to an elastic solid or a fluid continuum, the boundary conditions are specialized accordingly.
Temporal changes in geothermal gradient, Franciscan Subduction Complex, northern California
Authors: Underwood Michael B.
Published: February 2008

The Franciscan Complex comprises several tectonostratigraphic terranes that were accreted to coastal northern California during late Mesozoic to Miocene time. Estimates of peak paleotemperature (based upon measurements of vitrinite reflectance) reveal spatial and temporal patterns which, when integrated with pressure‐depth limits imposed by stratigraphic relations and inorganic metamorphic phases, enable reconstruction of past geothermal gradients. Geothermal gradients during metamorphism of matrix to the Mesozoic melange (the Central Belt) may have been unusually small; this condition can be explained by the depression of isotherms during subduction of old oceanic crust. However, the gradients offshore northern California increased during the Paleogene period (the Yager and Coastal terranes). During the middle Miocene epoch a pulse of hydrothermal discharge produced an overprint across the entire King Range terrane, and high regional heat flow continued through the remainder of the Cenozoic. This change in thermal regime must have been influenced by the proximity of the Pacific‐Farallon ridge‐and‐transform system to the Franciscan‐Cascadia subduction front. The thermal structure of the subduction complex was modified because of conductive and advective heat flux from the subducting oceanic lithospheric into the overriding accretionary wedge.
Excess argon in amphiboles from fluid interaction and short intrusion interval at the Epizonal Marangudzi Complex, Zimbabwe
Authors: Landoll John D. , Foland K. A. , Henderson C. M. B.
Published: February 2008

The epizonal Marangudzi complex of the Karoo‐associated Nuanetsi igneous province in southern Zimbabwe was emplaced into Precambrian gneisses and consists of a gabbro mass intruded in turn by ring dikes of quartz and nepheline syenites. Amphibole and biotite from various lithologies have been examined using 40Ar/39Ar techniques in order to investigate the temporal distribution of intrusions and the possible occurrence of excess Ar. With one important exception, samples give internally concordant incremental‐release spectra; average 40Ar/39Ar dates range from 177.6 to 181.3 Ma. Three amphiboles give dates which are significantly older than the other samples and contain small yet discernible quantities of excess 40Ar. Excess Ar is restricted to amphiboles displaying substantial Fe/Mg zoning which is interpreted as reflecting late magmatic or early sub‐solidus reaction with fluid phases. Argon was apparently acquired from the fluid during alteration and, ultimately, could have been magmatic or could have come from degassing of the country rocks. The remaining samples (both amphibole and biotite) show only a restricted range (∼1 m.y.) in apparent age and are essentially analytically indistinguishable, implying rapid cooling of the complex. The average for these analyses, 178.1 Ma, reflects the mean age of emplacement of cogenetic intrusions within a short interval ≤1 m.y. in duration. This revised age of the Marangudzi complex fits well within the temporal framework of Karoo province magmatism in the region.
Shoshonitic volcanism in the Northern Mariana Arc: 1. Mineralogic and major and trace element characteristics
Authors: Bloomer Sherman H. , Stern Robert J. , Fisk Elisha , Geschwind C. H.
Published: February 2008

Petrographic and geochemical characteristics of samples dredged from 23 submarine volcanic edifices in the northern Mariana and southern Volcano arcs define two distinct rock series. The central and northern Mariana Arc (to 23°N) and the northern Volcano Arc (north of Iwo Jima) are characterized by plagioclase‐clinopyroxene‐orthopyroxene‐titantomagnetite bearing subalkaline rocks, including both low‐K and medium‐K series. The northern Mariana Arc and southern Volcano Arc, from 23°N to Iwo Jima, are erupting rocks of a shoshonitic series with phenocrysts of plagioclase, clinopyroxene, olivine, and biotite. These rocks are less saturated than those of the subalkaline provinces and are substantially enriched in Ba (400–900 ppm), Sr (600–1000 ppm), K2O (1–4.5%), and K2O/Na2O (0.4–1.2) relative to the subalkaline lavas. Ba/Y and Ba/Zr increase by a factor of 3 to 4 in the shoshonitic rocks, but K/Rb, Ba/Sr, and K/Ba are relatively constant throughout the arc. There is no relationship between degree of enrichment and volcano volume or degree of fractionation: seamounts and islands within each province have the same range of compositions. Much of the intra‐edifice variation in lava composition can be modeled by 10–70% crystallization of plagioclase and clinopyroxene, with smaller amounts of olivine, orthopyroxene, and titanomagnetite, or by accumulation of 10–40% phenocrysts, dominantly plagioclase. The differences between parental lavas in the alkalic and subalkalic provinces require melting of two distinct mantle sources. The occurrence of this enriched mantle in the northern Marianas may be a consequence of the propagation of the Mariana Trough spreading center into the Volcano Arc.
Early rifting of an oceanic island arc (Fiji) produced shoshonitic to tholeiitic basalts
Authors: Gill James , Whelan Peter
Published: February 2008

Shoshonitic volcanism began in Fiji within 50 km of and 0.5 m.y. after formation of a transverse rift which broke the oceanic Vitiaz Arc across strike. Shoshonitic basalts erupted from 10 volcanoes along three lineaments for up to 225 km away from the rift along strike, from 5.5 to 3.0 Ma, during Fiji's “incipient rifting stage.” Three shoshonitic suites are distinguished based on differentiation‐normalized alkali contents, but K‐Rb‐Ba‐Sr, P‐rare earth element (REE), and Ti‐Zr‐Hf enrichments occur independent of each other. Enrichments are extremes of typical arc geochemical signatures, and are superimposed on high field strength elements and HREE concentrations and ratios like those of arc tholeiites. Medium‐ and high‐K calcalkaline volcanics are interspersed with shoshonites. Still farther from the transverse rift, coeval tholeiitic basalts erupted along the volcanic front. Decreasing large ion lithophile element enrichments from shoshonitic through calcalkaline to tholeiitic are attributed to increased percent fusion away from the transverse rift. All three suites are basalt‐dominated despite thickened crust, due to regional tension associated with fragmentation of the oceanic arc.
Chemical geodynamics in a back arc region around the Sea of Japan: Implications for the genesis of alkaline basalts in Japan, Korea, and China
Authors: Nakamura Eizo , Campbell Ian H. , McCulloch Malcolm T. , Sun Shen‐Su
Published: February 2008

Major and trace element have been analyzed from alkaline basalts from southwestern Japan, Korea, and northeastern China. No significant differences were found in the immobile incompatible element ratios, such as Zr, Y, Hf, Th, and Ti. These ratios, as well as normalized incompatible element patterns, resemble those of continental and oceanic island alkaline basalts. However, southwestern Japanese alkaline basalts show evidence of K, Ba, and Rb enrichment and a slight depletion in Ta relative to La, implying a weak island arc signature. Korean and Chinese alkaline basalts do not have such a signature. Rare earth elements (REE) show near‐constant La/Sm ratios and a crossover at the high REE end of patterns for each areas studied. The parallelism in light REE can be derived if the magmas are mixtures formed by (1) relatively large degrees of partial melting of an enriched mantle plume from deeper in the mantle and (2) a small degree of partial melting of a depleted mid‐ocean ridge basalt (MORB)‐type source. These observations when combined with seismic results suggest that the upper mantle beneath southwestern Japan has been weakly affected by metasomatism caused by dehydration and/or partial melting of subducted Pacific plate (not Philippine Sea plate). The mantle plume may have reacted with weakly metasomatized MORB‐type depleted mantle to produce alkaline basalt magmas retaining mild island arc characteristics in southwestern Japan. However, the metasomatism by the subduction of the Pacific plate has not affected the mantle beneath Korea and northeastern China. Here the interaction between plume and MORB‐type mantle produced alkaline basalt magma similar to normal continental and oceanic alkaline basalts.
Elasticity of single‐crystal forsterite measured to 1700 K
Authors: Isaak Donald G. , Anderson Orson L. , Goto Takayasu , Suzuki Isao
Published: February 2008

Measurements of the nine adiabatic elastic constants of single‐crystal forsterite are reported for the temperature range 300–1700 K at ambient pressure. The measured room temperature Cijs and their first temperature derivatives at ambient conditions are generally consistent with previous measurements, which were taken over a narrower temperature range. Slight nonlinear behavior is observed in the temperature dependence of the C11s, C22s, and C33s moduli. The resulting Hill‐averaged bulk and rigidity moduli Ks and G are lower at 1700 K than predicted by linear extrapolation of low‐temperature data. At 1700 K the new data give Ks = 103.8 ± 1.1 GPa and G = 61.3 ± 0.5 GPa. Isotropic elastic wave velocities have been calculated using the Hill averaging scheme over the range of temperatures measured. At T = 1700 K and ambient pressure we find vp = 7.79 ± 0.04 km s−1 and vs = 4.48 ± 0.03 km s−1. The parameter (∂ℓnvs/∂ℓnvp)P is near to 1.2 and independent of temperature. Calculations are presented for elastic properties and other parameters important for the equations of state and anharmonicity of forsterite. Recent high‐temperature thermal expansion measurements on a forsterite specimen from the same boule are used in this analysis. Effects of uncertainties in high‐temperature thermal expansion data on these parameters have been evaluated and are discussed.
Petrologic evolution of divergent peralkaline magmas from the Silent Canyon Caldera Complex, Southwestern Nevada Volcanic Field
Authors: Sawyer David A. , Sargent K. A.
Published: February 2008

The Silent Canyon volcanic center consists of a buried Miocene peralkaline caldera complex and outlying peralkaline lava domes. Its location has been corroborated by geophysical data and more than 50 drill holes. Two widespread ash flow sheets, the Tub Spring and overlying Grouse Canyon members of the Miocene Belted Range Tuff, were erupted from the caldera complex and have volumes of 60–100 km3 and 200 km3, respectively. Eruption of the ash flows was preceded by widespread extrusion of precaldera comendite domes and was followed by extrusion of postcollapse peralkaline lavas and tuffs within and outside the caldera complex. Lava flows and tuffs were also deposited between the two major ash flow sheets. Rocks of the Silent Canyon center vary significantly in silica content and peralkalinity. The most mafic rocks are precollapse and postcollapse trachytes (65–69% SiO2). Low‐silica comendites (69–73% SiO2) were erupted as the mafic upper part of the chemically zoned Grouse Canyon Member and as postcollapse lavas. The lower part of the Grouse Canyon Member and the underlying rhyolite of Split Ridge are moderately peralkaline comendite (PI is molar ratio Na + K/Al is 1.17–1.26). These comendites have major element characteristics and trace element enrichments approaching those of pantellerites. The Tub Spring Member, by contrast, is a weakly peralkaline chemically unzoned silicic comendite (75–76% SiO2) ash flow tuff. Weakly peralkaline silicic comendites (PI 1.0–1.1) are the most abundant precaldera lavas. Postcollapse lavas range from trachyte to silicic comendite; some have anomalous light rare earth element (LREE) enrichments. Silent Canyon rocks follow a common petrologic evolution from trachyte to low‐silica comendite; above 73% SiO2, compositions of the moderately peralkaline comendites diverge from those of the weakly peralkaline silicic comendites. These contrasting differentiation paths are shown in the behavior of Fe and other transition metals, Al, Na, K; the trace elements Ba, Zr, Nb; and probably F and Cl. Weakly peralkaline silicic comendites show a LREE/heavy REE crossover in early erupted/late erupted rocks; moderately peralkaline comendites are enriched in all REE. The development of divergent peralkaline magmas, toward both pantelleritic and weakly peralkaline compositions, is unusual in a single volcanic center.
Classification of Earth terrain using polarimetric synthetic aperture radar images
Authors: Lim H. H. , Swartz A. A. , Yueh H. A. , Kong J. A. , Shin R. T. , van Zyl J. J.
Published: February 2008

Supervised and unsupervised classification procedures are developed and applied to synthetic aperture radar (SAR) polarimetric images in order to identify their various Earth terrain components. For supervised classification processing, the Bayes technique is used to classify fully polarimetric and normalized polarimetric SAR data. Simpler polarimetric discriminates, such as the absolute and normalized magnitude response of the individual receiver channel returns, in addition to the phase difference between the receiver channels, are also considered. Another processing algorithm, based on comparing general properties of the Stokes parameters of the scattered wave to that of simple scattering models, is also discussed. This algorithm, which is an unsupervised technique, classifies terrain elements based on the relationship between the orientation angle and handedness of the transmitting and receiving polarization states. These classification procedures have been applied to San Francisco Bay and Traverse City SAR images, supplied by the Jet Propulsion Laboratory. It is shown that supervised classification yields the best overall performance when accurate classifier training data are used, whereas unsupervised classification is applicable when training data are not available.
Borehole wave particle motion in anisotropic formations
Authors: Leveille J. P. , Seriff A. J.
Published: February 2008

Recent observations of particle motion in vertical seismic profile surveys suggest the existence of borehole waves for which the formation horizontal particle motion is polarized in a specific azimuthal direction, possibly the direction of a local principal axis of stress. From an analysis of the static displacement of a pressurized vertical cylindrical borehole in a transversely isotropic formation with a horizontal axis of symmetry, we conclude that horizontal particle motion should not be azimuthally polarized for “tube waves” (lowest mode symmetric borehole wave) even in an ideal azimuthally anisotropic medium. Azimuthally polarized particle motion does exist for the “bending mode” of the borehole (lowest antisymmetric mode) even in isotropic formations; the polarization direction being determined by the source direction. Finite element calculations for a vertical borehole in a transversely isotropic formation with a horizontal axis of symmetry show that two bending modes with different velocities exist. The horizontal particle motion of one is polarized along the symmetry axis and the other is polarized orthogonal to that axis. At low frequencies the velocity of each approaches the velocity of a vertically propagating formation body shear wave with the same polarization direction. The recent experimental observations include one borehole arrival with a propagation velocity and a ratio of horizontal to vertical formation particle motion consistent with the hypothesis that it is a bending mode in an azimuthally anisotropic medium. The polarization direction and excitation of such a mode do not seem unreasonable. Other polarized arrivals cannot be easily explained.