bluuebunny/crossref_metadata_2025 huggingface dataset:
Interferometric Global Positioning System: Three‐dimensional positioning source for airborne gravimetry
Authors: Brozena J. M. , Mader G. L. , Peters M. F.
Published: February 2008

The Naval Research Laboratory (NRL) and the National Geodetic Survey recently collaborated on an experiment to test interferometric mode Global Positioning System (GPS) tracking as a source of altitude and vertical acceleration for airborne gravity measurement reduction. A Texas Instruments TI‐4100 GPS receiver and an extremely precise radar altimeter were installed in an NRL P‐3 Orion aircraft. A second TI‐4100 was located at a fixed base station of known location. Carrier phase data from four GPS satellites were recorded from the fixed and airborne receivers and postprocessed to produce three‐dimensional displacements of the airborne station relative to the base station. Differences in GPS and radar altimeter coordinate systems were modeled and removed: the resulting GPS and radar altitudes were compared. The two unfiltered 1‐Hz data sets differed by an rms of 20 cm when aircraft dynamics were controlled by autopilot in straight and level flight. During hand‐flown maneuvers such as turns, climbs, and descents, rms differences varied from 40 to 80 cm. A significant portion of the differences can be attributed to remaining coordinate system discrepancies and to uncompensated geometric effects due to aircraft motion. We have also examined vertical acceleration time series produced by doubly differencing both data sets. By applying a low‐pass filter which allows resolution of features with wavelengths of 20 km and greater (at nominal aircraft speeds of 100 m/s) the rms difference between the radar and GPS derived vertical accelerations was less than 2 mGal (2×10−5m/s2) during straight and level flight. It should be possible to improve this accuracy and wavelength resolution with future improvements in measurement and processing techniques. The results of this experiment show that the incorporation of interferometric mode GPS tracking into the airborne gravity measurement system will make possible rapid and relatively inexpensive gravity surveys over any type of terrain while simultaneously providing accurate measurements of ground height profiles beneath the aircraft.
The influence of formation material properties on the response of water levels in wells to Earth tides and atmospheric loading
Authors: Rojstaczer Stuart , Agnew Duncan Carr
Published: February 2008

The water level in an open well can change in response to deformation of the surrounding material, either because of applied strains (tidal or tectonic) or surface loading by atmospheric pressure changes. Under conditions of no vertical fluid flow and negligible well bore storage (static‐confined conditions), the sensitivities to these effects depend on the elastic properties and porosity which characterize the surrounding medium. For a poroelastic medium, high sensitivity to applied areal strains occurs for low porosity, while high sensitivity to atmospheric loading occurs for high porosity; both increase with decreasing compressibility of the solid matrix. These material properties also influence vertical fluid flow induced by areally extensive deformation and can be used to define two types of hydraulic diffusivity which govern pressure diffusion, one for applied strain and one for surface loading. The hydraulic diffusivity which governs pressure diffusion in response to surface loading is slightly smaller than that which governs fluid flow in response to applied strain. Given the static‐confined response of a water well to atmospheric loading and Earth tides, the in situ drained matrix compressibility and porosity (and hence the one‐dimensional specific storage) can be estimated. Analysis of the static‐confined response of five wells to atmospheric loading and Earth tides gives generally reasonable estimates for material properties.
Laboratory electromagnetic model results for the EMSLAB region
Authors: Chen J. , Dosso H. W. , Nienaber W.
Published: February 2008

Apparent resistivities and vertical to horizontal magnetic field ratios for the Lincoln Line traverse in the EMSLAB project region are studied using a scaled laboratory analogue model. The model includes a simulation of the Juan de Fuca Plate subducting the Canadian‐U.S. Pacific Northwest region. In the model, the subducting plate is taken to be horizontal offshore, dipping at 10° beneath the coast, then inland bending farther to dip at 45° in the Washington‐Oregon region and at 30° in the British Columbia region. The model results show that for inland sites the apparent resistivity for the telluric field component parallel to the coast, and thus also parallel to the strike of the dipping conductive substructure, depends strongly on period, while the resistivity for the telluric field perpendicular to the coast is largely a galvanic effect and changes less with period. The amplitude of the vertical to horizontal magnetic field ratio along the Lincoln Line shows a saddle shaped response, with the coastal maximum followed by a second maximum inland due to the dipping substructure. Further, rather than decreasing rapidly with distance inland as expected for a simple coast effect, the field ratios retain significant values large distances inland for periods of 3–120 min. Model results for 120‐min period are compared with one example of field results for the EMSLAB project region.
Range front faulting and volcanism in the Mono Basin, eastern California
Authors: Bursik Marcus , Sieh Kerry
Published: February 2008

The spatial and temporal pattern of range front normal faulting and volcanism in the Mono Basin of eastern California suggests that dikes are being intruded underneath the Mono Craters in response to crustal stretching and are now accommodating strain that was once taken up by range front faulting. The section of the Sierra Nevadan range front near the craters accommodated as much as 1 mm/yr of extension as recently as about 40,000 years ago. For the past 40,000 years, this section of range front has been inactive, even though range front extension to the north and south has continued at up to 0.9 mm/yr. For the past 40,000 years, dikes, intruding underneath the Mono Craters, seem to have been accommodating the 1 mm/yr of extension that was previously taken up by faulting. Since the basin is extending obliquely to the trend of the frontal faults, there is a component of dextral shear to their motion, so that the Mono Craters may be forming on an extensional boundary of a pull‐apart basin. If the craters represent incipient caldera formation, then calderas such as Long Valley may also have formed in pull‐apart zones.
The multiphase grain control percolation: Its implication for a partially molten rock
Authors: Nakano Tsukasa , Fujii Naoyuki
Published: February 2008

A model of percolation process named the multiphase grain control percolation (MGCP) is proposed in order to estimate the connectivity of the melt phase in a partially molten rock in which spatial melt distribution is determined by the combination of surrounding grain species. According to the stability of the melt phase at the grain boundaries, three types of percolation model are introduced: the corner percolation, the edge percolation and the corner‐edge (CE) percolation. The CE percolation can be reduced to the corner or edge percolation. The connectivity of the melt phase is determined as a function of modal composition in a multiphase grain system. For various combinations of grain species surrounding the corner or edge occupied by the melt phase, we obtain the critical grain fraction at which the connectivity changes abruptly from 0 to 1. The critical grain fraction estimated from the MGCP does not coincide with that from the ordinary percolation model in which random distribution of the melt phase is assumed. As a geophysical application of the MGCP, we construct the connectivity diagrams (modal composition diagram of the connectivity of the melt phase) for two partially molten peridotites by using the experimental results of dihedral angle measurements. From these diagrams, we predict that the melt phase is connected extensively if fraction of olivine exceeds 63 vol. % for a natural peridotite composed of olivine, orthopyroxene and clinopyroxene, and 38 vol. % for a synthetic peridotite composed of olivine and orthopyroxene. For the former peridotite, we further estimate critical melt fraction. It is shown that the melt phase can be connected extensively when the melt fraction exceeds 0.8 vol. % in the peridotite composed of more than 63 vol. % olivine.
Crustal strain near the Big Bend of the San Andreas Fault: Analysis of the Los Padres‐Tehachapi Trilateration Networks, California
Authors: Eberhart‐Phillips Donna , Lisowski Michael , Zoback Mark D.
Published: February 2008

In the region of the Los Padres‐Tehachapi geodetic network, the San Andreas fault (SAF) changes its orientation by over 30° from N40°W, close to that predicted by plate motion for a transform boundary, to N73°W. The strain orientation near the SAF is consistent with right‐lateral shear along the fault, with maximum shear rate of 0.38±0.01 μrad/yr at N63°W. In contrast, away from the SAF the strain orientations on both sides of the fault are consistent with the plate motion direction, with maximum shear rate of 0.19±0.01 μrad/yr at N44°W. The strain rate does not drop off rapidly away from the fault, and thus the area is fit by either a broad shear zone below the SAF or a single fault with a relatively deep locking depth. The fit to the line length data is poor for locking depth d less than 25 km. For d of 25 km a buried slip rate of 30 ± 6 mm/yr is estimated. We also estimated buried slip for models that included the Garlock and Big Pine faults, in addition to the SAF. Slip rates on other faults are poorly constrained by the Los Padres‐Tehachapi network. The best fitting Garlock fault model had computed left‐lateral slip rate of 11±2 mm/yr below 10 km. Buried left‐lateral slip of 15±6 mm/yr on the Big Pine fault, within the Western Transverse Ranges, provides significant reduction in line length residuals; however, deformation there may be more complicated than a single vertical fault. A subhorizontal detachment on the southern side of the SAF cannot be well constrained by these data. We investigated the location of the SAF and found that a vertical fault below the surface trace fits the data much better than either a dipping fault or a fault zone located south of the surface trace.
Color and albedo of the south polar layered deposits on Mars
Authors: Herkenhoff Ken E. , Murray Bruce C.
Published: February 2008

Five color/albedo units, including polar frost, have been recognized and mapped in the southern layered deposits on Mars. Atmospheric dust scattering was measured in shadows and modeled in order to remove the component of brightness in Mars images due to the atmosphere and quantify the albedo and color of the surface. The layered deposits appear to be mantled by red dust, except where eolian stripping has exposed the underlying bedrock. Frost and bare ground are mixed below the resolution of the images in many areas adjacent to the polar cap, some of which appear to be younger than the surrounding layered terrain. Dark material has been deposited in topographic depressions in much of the south polar region, including the layered deposits. The available observational data suggest that the layered deposits are composed of bright dust, ice, and a small amount of dark material. If the dark material is sand, a periodic change in polar winds seems required in order to transport the sand poleward into the layered terrain. In any case, the observations are not consistent with the layered deposits being composed only of bright dust and ice. The Mars observer camera and infrared instruments should be particularly useful in this investigation.
Mars atmospheric D/H: Consistent with polar volatile theory?
Authors: Jakosky Bruce M.
Published: February 2008

Preferential escape of H relative to D will cause an enrichment of the atmospheric abundance of the latter over geological time. Measurement of atmospheric HDO and H2O provides an estimate of this enrichment (Owen et al., 1988). By assuming an initial H/D ratio similar to the Earth's, Yung et al. (1988) constructed a photochemical model based on the current atmosphere to determine the relative escape rates; they concluded that regular exchange of atmospheric water with a nonatmospheric reservoir containing about 3 m of water was required and that all but 0.2m had escaped to space. This reservoir size appears to be inconsistent with current theories for evolution of the polar deposits, which suggest that they contain the equivalent of tens of meters of water and that this water exchanges on time scales which are short compared to the age of the planet. Solar forcing throughout the obliquity cycling may cause the time‐averaged atmospheric water content to be larger than the current value, significantly increasing the escape rates and increasing the required size of the nonatmospheric reservoir to a value consistent with the content of the polar deposits. If an early, wet atmosphere is present, as discussed by Owen et al., the requirements for polar cap exchange are eased. It is premature to conclude that the escape theories are inconsistent with polar cap behavior; better observations and models may, in turn, constrain the content and history of the polar caps. The models do suggest, however, that at least 60 m of water has escaped from the surface and atmosphere to space over geologic time.
Seismic potential of the Queen Charlotte‐Alaska‐Aleutian Seismic Zone
Authors: Nishenko S. P. , Jacob K. H.
Published: February 2008

The 5000 km long Queen Charlotte‐Alaska‐Aleutian seismic zone is subdivided into 17 unequally sized segments. Their boundaries are delineated based on the prior distribution of large and great earthquakes. The 17 segments are chosen to represent areas likely to be ruptured by “characteristic” earthquakes. This term usually implies repeated breakage of a plate boundary segment by either a large or great earthquake, whose source dimensions remain consistent from cycle to cycle. This definition does not exclude the possibility that occasionally adjacent characteristic earthquake segments may break together in a single “giant” event that is larger than the characteristic size outlined. Conversely, a segment can also sometimes break in a series of smaller ruptures. Formal computations of the conditional probabilities for future large and great earthquakes in the 17 segments of the Queen Charlotte‐Alaska‐Aleutian seismic zone are based on the following data sets and findings: (1) recurrence intervals from historic and geologic data; (2) direct recurrence time estimates based on rates of relative plate motion and the size or displacement of the most recent characteristic event in each segment; and (3) the application of a lognormal distribution of recurrence times for large and great earthquakes. Results of these computations indicate seven areas that have high (i.e., ≥60%) conditional probabilities for the recurrence of either large or great earthquakes within the next 20 years (1988–2008). These areas include Cape St. James, Yakataga, the Shumagin Islands, Unimak Island, and the Fox, Delarof, and Near Islands segments of the Aleutian arc. When a shorter time interval is considered (1988–1998), those segments more likely to rupture in large (MS 7–7.7) rather than great earthquakes have a high conditional probability. These areas include the Unimak, Fox, and Delarof Islands segments. The largest uncertainties in these forecasts stem from the short historic record (providing a single recurrence time estimate for some segments, or widely varying estimates for others); from the unknown importance of aseismic slip; and from a vague definition of “characteristic” earthquake size. In fact, characteristic earthquake size may not be a time‐invariant quantity.
Chemical constraints on lithosphere composition and evolution beneath the Colorado Plateau
Authors: Roden Michael F. , Smith Douglas , Murthy V. Rama
Published: February 2008

We have measured Sr and Nd isotopic composition and rare earth element (REE) abundances in a variety of inclusions and host minettes from the Navajo Volcanic Field of the Colorado Plateau in order to develop geochemical constraints on the composition and evolution of the mantle beneath the relatively undisturbed Proterozoic crust of the Colorado Plateau. Spinel and garnet peridotites and eclogites derived from less than 80 km in depth have geochemical signatures (e.g., light REE (LREE) depletion) similar to oceanic lithosphere; we interpret these rocks to be fragments of lithosphere, possibly part of an island arc which was accreted to the continent in the Proterozoic. Isotopic evidence suggests that some of the peridotites were hydrated by an incompatible element‐poor, H2O‐rich fluid, whereas the eclogites were metasomatized by a Na2O‐rich phase with high 87Sr/86Sr. Some of the peridotites have highly radiogenic Nd isotopic ratios which reflect long‐term isolation from mantle convection and show that they probably are not samples of recently subducted lithosphere. Garnet peridotites derived from at or near the base of the lithosphere are isotopically distinct from the shallow xenoliths and similar to ocean island basalts. Many of these same peridotites are enriched in the LREE. An inverse correlation between temperature of equilibration and 87Sr/86Sr in these garnet peridotites may reflect infiltration of relatively low 87Sr/86Sr melts into refractory, relatively high 87Sr/86Sr wall rock (lithosphere?). These high‐temperature, low 87Sr/86Sr melts were possibly parental to the megacrystalline inclusions associated with the garnet peridotites. Overall, our data for the Colorado Plateau suggest that highly depleted, shallow lithosphere is underlain by material enriched in incompatible elements. This latter material was located near the inferred Oligocene base of the lithosphere and its composition may reflect mixing between asthenosphere and lithosphere. Melting of this material as a consequence of warming of the Colorado Plateau lithosphere during the late Tertiary may explain the origin of the Hopi Buttes alkaline magmas. The host minettes are isotopically similar to the bulk earth and their major element chemical variation can be explained by fractional crystallization. However, isotopic heterogeneity in the same eruptive center and decreasing Nb and Ta abundances with magmatic evolution indicate a complex petrogenesis involving exotic phases. The variation of isotopic composition with trace element ratios is inconsistent with bulk assimilation of crust, and we suggest that it reflects melting of a veined source containing phlogopite, apatite, and possibly a Nb‐Ta‐rich phase in addition to normal peridotite minerals.
Internal field mapping in single‐domain and multidomain grains
Authors: Dunlop David J. , Enkin Randolph J. , Tjan Eugenie
Published: February 2008

We have calculated the vector internal field Hi within a ferromagnetic cube with a one‐dimensional domain structure, consisting of lamellar domains separated by Bloch‐type walls. The internal field is not purely a demagnetizing field antiparallel to the spontaneous magnetization Ms. It has components perpendicular to Ms as well. These transverse fields are most conspicuous at grain surfaces, particularly along edges and at corners. They may act as centers for nucleation of non‐uniform magnetization structures in large single‐domain (SD) grains. An approximate picture of leakage of fringing fields is obtained by replacing the sheets of + and − magnetic poles that are the actual source of Hi by single + and − poles with a larger spacing. Thus Hi within an SD grain is approximately “bipolar,” a two‐domain (2D) grain has a “tetrapolar” field and so on. The full diagonalized “demagnetizing” tensor has been calculated for points inside a SD cube. Only at points along (100) axes through the cube center are the off‐diagonal elements zero. At the center point, Hi = −Ms/3, as for a sphere, and averaged over the cube, the demagnetizing factor is 1/3. Two‐domain grains with and without finite‐width walls have almost identical internal field patterns, except that finite‐width walls produce strong leakage fields along transverse faces which should be well imaged by the Bitter colloid technique. In a 2D cube with equal domains, the demagnetizing factor near the center of either domain is 0.12, close to the grain‐average N of 0.135 determined from the overall demagnetizing energy. When the domains are of unequal size, demagnetizing fields increase rapidly with wall displacement in the larger domain and decrease in the smaller domain, favoring a return to the demagnetized state. We demonstrate that, as expected, the field within either domain in a 2D cube is the superposition of Hi due to that domain alone plus the exterior field He of the other domain. The ability to calculate exterior as well as interior fields is vital in understanding magnetic particle interactions. Finally, we find that the rotation of the magnetization within rock bodies in response to self‐demagnetizing fields is well predicted by using the demagnetizing tensor for uniform magnetization, provided M ≤ 15 A/m. Deflection of the natural remanent magnetization due to specimen shape and the shape anisotropy contribution to magnetic anisotropy measurements could be determined by this method.
A simple statistical model for geomagnetic reversals
Authors: Constable Catherine
Published: February 2008

The diversity of paleomagnetic records of geomagnetic reversals now available indicate that the field configuration during transitions cannot be adequtely described by simple zonal or standing field models. A new model described here is based on statistical properties inferred from the present field and is capable of simulating field transitions like those observed. Some insight is obtained into what one can hope to learn from paleomagnetic records. In particular, it is crucial that the effects of smoothing in the remanence acquisition process be separated from true geomagnetic field behavior. This might enable us to determine the time constants associated with the dominant field configuration during a reversal.
Earthquake depths and the relation to strain accumulation and stress near strike‐slip faults in southern California
Authors: Sanders Christopher O.
Published: February 2008

In this paper I report and attempt to interpret several observations about the depth distribution of earthquakes in the region of southern California containing the southern San Andreas and San Jacinto fault zones. These observations are as follows: (1) Earthquakes in the major fault zones are predominantly deep (maximum, 22 km; characteristic, 11–18 km). (2) Earthquakes in the crustal blocks bounding the fault zones are predominantly shallow (maximum, 17 km; characteristic, 1–6 km). (3) In the San Jacinto fault zone, maximum earthquake depths correlate with surface heat flow. These relations together with focal mechanisms, geodetic strain measurements, and fault zone models are consistent with the following ideas: (1) Interseismic plate motion is accommodated by aseismic slip along an extension of the major fault zone below a brittle zone that is locked between large earthquakes. (2) The aseismic slip in a narrow fault zone in the brittle‐plastic transition region concentrates strain at the base of the brittle fault zone. (3) Deep earthquakes occur in the lower part of the brittle fault zone due to stick‐slip failure of highly stressed patches. (4) Background earthquakes and aftershocks that occur several kilometers deeper than large earthquake hypocenters suggest that a zone of mixed slip behavior may exist between the stable sliding (deep) and stick‐slip (shallow) regions of the fault zone. This zone of mixed slip may fail predominantly by stable sliding, analogous to the central San Andreas and Calaveras faults of California. Seismic precursors to large earthquakes may occur in this zone, and large earthquakes may nucleate near the upper edge of this zone. Furthermore, the difference in seismicity between the San Jacinto and southern San Andreas faults suggests that the nature of this mixed zone may evolve as total displacement in the fault zone increases. (5) Shear stress may be less in the crustal blocks than in the deep brittle fault zones and generally at a level sufficient to cause brittle failure only shallow in the crustal blocks. (6) In the stress field produced by plate motion and slip in the deep fault zone, the upper brittle fault zone is not oriented favorably for shear failure. Lack of shallow earthquakes in the fault zones and the predominance of shallow earthquakes on favorably oriented fractures in the adjacent crustal blocks suggest that either stress in the upper brittle fault zone is relatively low or the upper fault zone is effectively strong due to its orientation.
Late Quaternary tectonic deformation in the Cape Arago‐Bandon Region of coastal Oregon as deduced from wave‐cut platforms
Authors: McInelly Galan W. , Kelsey Harvey M.
Published: February 2008

The Cape Arago region of south central Oregon sits on the upper plate of the Cascadia subduction zone about 80 km east of the base of the continental slope. The style of late Pleistocene deformation along the Cascadia forearc near Cape Arago is well expressed by the altitudinal variation of a set of five uplifted wave‐cut platforms. These platforms record open folding, with a half wavelength of about 6–7 km, as well as late Quaternary offset on flexural‐slip reverse faults that parallel bedding in the underlying bedrock. The folds have produced both landward and seaward tilts to the uplifted wave‐cut platforms. Because the folds cut obliquely across the coastline, the magnitude of coastal uplift is variable. In the case of the lowest, 80 ka wave‐cut platform, this variable uplift has resulted in coastal deformation ranging from subsidence to a maximum uplift rate of 0.8 m/kyr. Quantitative analysis of the mechanism of flexural slip folding for the South Slough syncline near Cape Arago indicates that the late Quaternary strain rate has decreased in the last 200 kyr. Evidence of past great subduction‐style earthquakes, such as regional uplift and regional landward tilting of wave‐cut platforms, or regional submergence of coastlands, is lacking in the Cape Arago region. Instead, the deformational style is controlled by folding. Though localized folding is dominant, the occurrence of great subduction‐style earthquakes is not precluded because localized folding could occur concurrently with regional coseismic deformation.
Effects of dynamic long‐period ocean tides on changes in Earth's rotation rate
Authors: Nam Young S. , Dickman S. R.
Published: February 2008

As a generalization of the zonal response coefficient first introduced by Agnew and Farrell (1978), we define the zonal response function κ of the solid earth‐ocean system as the ratio, in the frequency domain, of the tidal change in Earth's rotation rate to the tide‐generating potential. Amplitudes and phases of κ for the monthly, fortnightly, and 9‐day lunar tides are estimated from 2½ years of very long baseline interferometry UT1 observations (both 5‐day and daily time series), corrected for atmospheric angular momentum effects using NMC wind and pressure series. Using the dynamic ocean tide model of Dickman (1988a, 1989a), we also predict amplitudes and phases of κ for an elastic earth‐ocean system. The predictions confirm earlier results which found that dynamic effects of the longer‐period ocean tides reduce the amplitude of κ by about 1%. However, agreement with the observed κ is best achieved for all three tides if the predicted tide amplitudes are combined with the much larger satellite‐observed ocean tide phases; in these cases the dynamic tidal effects reduce κ by up to 8%. Finally, comparison between the observed and predicted amplitudes of κ implies that anelastic effects on Earth's rotation at periods less than fortnightly cannot exceed 2%.
The bootstrap for magnetic susceptibility tensors
Authors: Constable Catherine , Tauxe Lisa
Published: February 2008

In studies of the anisotropy of susceptibility or remanence of paleomagnetic samples it is conventional to specify the anisotropy in terms of the parameters of the anisotropy ellipsoids, namely the directions of the principal axes of the ellipsoid and their associated eigenvalues. Confidence intervals for these parameters have in the past often been estimated by using a linearization scheme to propagate the effect of small changes through the eigenvalue decomposition. The validity of these approximations is explored using a Monte‐Carlo simulation from measurements that are presumed normally distributed, showing that there are circumstances in which the linearization scheme gives confidence intervals that are much too small. Q‐Q plots indicate that the common assumption that the noise in the measurements is Gaussian does not always hold. Because of these shortcomings in the conventional technique we propose using a bootstrap resampling scheme to find empirically the distribution of uncertainties in the results. Confidence intervals for the eigenvalues are found directly from their empirical distributions. For the principal axes, approximate elliptical regions of confidence on the unit sphere are parameterized in terms of the Kent orFB5distribution. The number of modes observed in the distribution of eigenvalues obtained by bootstrapping is used to classify the shape of the susceptibility ellipsoid as spherical, oblate, prolate or triaxial. The empirical nature of the bootstrap technique allows the extension of the analysis of uncertainties to parameters derived from the principal susceptibilities, such as percentage anisotropy or shape factor.
Differential stress, strain rate, and temperatures of mylonitization in the Ruby Mountains, Nevada: Implications for the rate and duration of uplift
Authors: Hacker Bradley R. , Yin An , Christie John M. , Snoke Arthur W.
Published: February 2008

Knowledge of the magnitude of the differential stress during the formation of mylonitic rocks provides constraints on mechanical and thermal models for the exhumation of the metamorphosed foot walls of major low‐angle detachment faults. We have analyzed the differential flow stress during the mylonitization of quartzose rocks in the Ruby Mountains, Nevada, using grain‐size piezometers and kinetic laws for grain growth. Quartzites from mylonitic shear zones in Lamoille Canyon and Secret Creek gorge have grain sizes of 91–151 μm and 42–64 μm, respectively. The peak temperature during mylonitization was 630°±50°C, and analysis of grain‐growth kinetics indicates that mylonitization continued during cooling to temperatures ≤450°C. Quartz grain‐size piezometers suggest that the mylonitization occurred under differential stresses (σ1‐σ3) of 38–64 MPa, or maximum shear stresses of 19–32 MPa. Extrapolation of quartzite flow laws indicates that the mylonitization occurred at strain rates between 10−10and 10−13s−1; arguments presented in the paper suggest that the likely range of strain rates is 10−11to 10−12s−1. These strain rates are compatible with displacement rates of the order of 23 mm yr−1along a 1.5‐km‐thick simple shear zone. Such a shear zone dipping 15° would produce an uplift rate of 5.8 km m.y.−1and a horizontal extension rate of 22 km m.y.−1. This uplift rate indicates that midcrustal mylonitic rocks could have been lifted up along a 1.5‐km‐thick simple shear zone dipping 15° in 2.6 m.y.
A note on the propagation behavior of a crack nucleated by a dislocation pileup
Authors: Wong Teng‐Fong
Published: February 2008

When an operating slip band encounters a microstructural obstacle, nucleation of tensile microcracking can occur by several mechanisms. Linear elastic fracture mechanics is used to analyze one such semibrittle cracking mechanism involving a dislocation pile‐up. After crack nucleation, the dislocations move in and coalesce at one end of the wedge crack. Under overall compressive loading, the wedge crack propagation is stable and arrest occurs at an equilibrium length. Analytic estimates of the equilibrium length and crack orientation are obtained, and are compared with experimental observations of a peridotite and a quartzite. The critical driving shear stress (i.e. the resolved shear stress minus the lattice “friction”) for wedge crack nucleation is relatively small in comparison with the macroscopic yield stress, implying that a significant portion of the applied deviatoric stress is used to overcome the shear resistance due to the lattice “friction” or Peierls stress. The equilibrium wedge crack length depends sensitively on the confining stress, and also depends on the differential stress and the Peierls stress.
Volume loss and defluidization history of Barbados
Authors: Speed R.
Published: February 2008

The basal complex of Barbados island is a tectonic amalgam of Eocene strata accreted at an outer deformation front at the end of Eocene time. The structures and porosity‐depth function of such sediments give clues to the succession of pore volume loss episodes, each of which must have been accompanied by defluidization. The basal complex evolved as a perpetually shallow, low‐temperature mass which underwent little or no unroofing in its 36 m.y. history in the accretionary prism. In its early phase of deformation, the accretionary structure formed by thrust imbrication, perhaps with little concurrent volume loss. Burial compaction followed tectonic thickening during accretion, causing probable large pore volume loss within limits of 0–70% of the initial volume of the imbricate stack. A late phase of tectonics has affected Barbados and a wide region of the structural high since 20 Ma. Such tectonics reactivated faults in the basal complex which acted as channelways for deep‐seated and perhaps shallow fluids to emerge from the structural high. The late stage tectonics also caused local basins for deposition of sediment cover and emplacement of thrust nappes of forearc basin strata above the basal complex. Such top loading caused a renewed phase of burial compaction and loss of about 37% or more of the pore volume that existed at the onset of late stage tectonics.
Observation of shear wave splitting from natural events: South Moat of Long Valley Caldera, California, June 29 to August 12, 1982
Authors: Shih Xiao R. , Meyer Robert P.
Published: February 2008

Three‐component seismograms from local earthquakes recorded by three University of Wisconsin stations in the south moat of Long Valley caldera in 1982 have been analyzed for shear wave splitting. The average polarization direction of the first‐arriving shear waves observed at the two stations south of the resurgent dome is NNW–SSE. Time delays between the fast and slow shear waves are proportional to hypocentral distances, and the maximum velocity anisotropy inferred is 6.4%. The average shear wave polarization observed at the station above the resurgent dome is north‐south. The time delays there are not a function of hypocentral distance, and the normalized time delays (ms/km) from events at the southwest boundary of the caldera are larger than those at other azimuths, indicating maximum 9.6% velocity anisotropy. The east‐west to ENE‐WSW variation of the tensional stress fields implied by the shear wave polarizations indicates the interaction between the inflating caldera and the Sierra Nevada range‐front faults under the generally WNW–ESE tension expected from the regional tectonics.
Effectiveness of natural zeroing of the thermoluminescence in sediments
Authors: Berger Glenn W.
Published: February 2008

The last exposure to sunlight of detrital feldspars and quartz grains in Quaternary sediments can be dated directly by thermoluminescence (TL) methods, but the effectiveness of zeroing of the light‐sensitive TL signal by daylight varies widely, depending upon details of the sedimentation processes. To document this variability and to help develop objective criteria useful for future sample selection and laboratory procedures, I have collected and analyzed several eolian, fluvial, marine deltaic, and lacustrine samples from less‐than‐100‐year‐old deposits as well as suspensions from sediment plumes. Also, underwater spectral irradiance was measured during recovery of suspensions. TL properties of polyrnineralic (feldspar dominated) 2–4 μm and 4–11 μm size fractions were measured in the laboratory. Unexpectedly, some light‐sensitive TL was found to remain in all samples. However, the light‐sensitive TL was more effectively zeroed in some samples (a proximal loess, a sandy braided‐channel deposit, tidal flat and upper‐delta‐front muds, and thin clayey lacustrine laminae) than in others (a silty braided‐channel deposit, submarine fan muds, and silt‐bearing lacustrine laminae). For the first group the minimum relict paleodose obtained with the partial‐bleach technique is about 0.4±0.3 Gy, corresponding to a relict TL age estimate of about 200 years for these sediments. Thus similar sediments (excluding fluvial deposits) as young as about 2 ka can be accurately and routinely dated without significant correction for a zero‐point error if one uses the partial‐bleach technique coupled with integrated optical‐bleaching energy fluences of less than ≃ 1 J cm−2and wavelengths only above about 550 nm. Also, some simple optical‐bleaching tests are reported that have a potential use in sample selection and in provenance studies of some surficial detrital sediments. Finally, a simple mathematical model is presented that helps account for the gross features of the observed bleaching‐response curves.
Distribution of attenuation in the Kaoiki, Hawaii, source volume estimated by inversion of P wave spectra
Authors: Scherbaum Frank , Wyss Max
Published: February 2008

A new method to simultaneously invert for Q structure and source parameters was used on a set of 635 microearthquakes (0.9 < M < 2.0) in the Kaoiki area of southern Hawaii. Approximately 2800 signals were analyzed which had been recorded by 6 short period vertical seismographs at epicentral distances of a few to 10 km. The hypocentral depths ranged from O to 14 km, with the bulk of the sources in the 7.5–10.5 km range. The hypothesis to be tested was that the source volume of the M = 6.6 Kaoiki main shock of November 16, 1983, may be heterogeneous in attenuation distribution. We assumed that the observed P wave displacement spectra could be modelled by a source spectrum with an ω−2 high‐frequency decay, a single‐layer resonance filler to account for local site resonances and whole path attenuation along the ray path. In a next step the attenuation factor Q was constrained by tomographically reconstructing the three‐dimensional Q structure for the source region and using it as starting model for a nonlinear inversion of the corner frequency, the seismic moment M0, and a new Q value. This process was iterated until the results changed less than 0.1% and were accepted as final. The average Q was approximately constant and very low (105<Q<115) between O and 7 km depth. It increased to approximately 160 between 9 and 11 km. In the depth ranges were the spatial coverage was good (3–9 km) the Q values in the NW part of the 1983 aftershock volume were larger by approximately 10–15% compared to those in the SE part. This 12% contrast correlates with other evidence for heterogeneity. The NW region shows high coda‐Q, no surface faulting, medium seismicity rate, and medium precursory quiescence, while the SE region shows low coda‐Q, pervasive faulting, high seismicity rate, strong precursory quiescence, and possibly low b values. We conclude that the Kaoiki source volume is strongly heterogeneous with the contrasting density of cracks controlling the difference in crustal parameters. The new simultaneous inversion technique is well suited to derive detailed Q structure and source parameter information from microeaithquake spectra recorded at close epicentral distances.
Evidence of active ground deformation on the mid‐ocean ridge: Axial Seamount, Juan de Fuca Ridge, April‐June 1988
Authors: Fox Christopher G.
Published: February 2008

Since September 1987 a precision bottom pressure recorder (BPR) has been deployed within the summit caldera of Axial Seamount on the central Juan de Fuca Ridge. The instrument is capable of measuring pressure to 1 mbar resolution and recording these measurements at 64 samples per hour for up to 15 months. After removal of oscillatory signals due to tidal and oceanographic effects and linear trends caused by sensor drift, any significant change in the pressure record should indicate a change of depth associated with vertical ground movement. In subaerial volcanic systems, such ground movements commonly indicate active inflation or deflation of underlying magma bodies and are frequently coincident with rift eruptions. Axial Seamount was selected for this pilot study for three primary reasons: (1) its tectonic setting, (2) the presence of a well‐formed summit caldera, and (3) observational evidence of geologically recent volcanic activity. Results from the first 9 months of the BPR deployment revealed a significant change in pressure, which is interpreted to represent a 15‐cm subsidence of the caldera floor during two 2‐ to 3‐week periods in April–June 1988. Also during these periods, an anomalous decline in temperature at the site was recorded that is correlated with an apparent increase in current velocity at the Axial Seamount Hydrothermal Emissions Study (ASHES) vent field, suggesting vigorous advection of cold water into the caldera. Concurrent oceanographic data from Geosat and from current meter arrays do not indicate any large‐scale oceanographic phenomena capable of generating these simultaneous events. One mechanism to explain simultaneous ground subsidence and temperature decline at the caldera center and increased bottom current at the caldera margin is the generation of a buoyant parcel of heated water in response to the intrusion or the eruption of magma associated with volcanic deflation. Such a parcel must have been sufficiently buoyant and proximal to induce horizontal entrainment currents capable of producing the bottom water conditions that were observed at both sites. Similar volcanic events also may have generated large midwater plumes that have been described previously along the southern Juan de Fuca Ridge.
High‐resolution reflectance spectra of Mars in the 2.3‐μm region: Evidence for the mineral scapolite
Authors: Clark Roger N. , Swayze Gregg A. , Singer Robert B. , Pollack James B.
Published: February 2008

Spatially resolved reflectance spectra of Mars in the 2.2‐ to 2.4‐μm spectral region were obtained in August 1988 using the NASA 3‐m Infrared Telescope Facility. The spectra show weak absorption features due to Martian atmospheric carbon monoxide and a surface mineral. Both CO and the mineral absorptions are composed of overlapping narrow features, but in many locations, such as Hellas, Chryse, Eden, and Moab, the mineral absorptions are quite strong, at least 3 times stronger than at the most absorbing wavelengths of CO near 2.33 μm. Therefore CO complicates the analysis of the surface mineral but does not always overwhelm its signature. Model removal of the Martian atmospheric CO has been performed, and the remaining absorption bands are identified as scapolite. Relatively strong absorptions that match bands in the spectrum of scapolite and have little or no CO absorption interference are seen near 2.41, 2.39, and 2.29 μm. Absorption also occurs at the scapolite bands at 2.36 and 2.33 μm, but the analysis is complicated by uncertainty in the atmospheric CO removal at these wavelengths. Weaker scapolite bands are seen at 2.44 and 2.23 μm where there is virtually no atmospheric interference. The scapolite bands observed on Mars are due to HCO3− and HSO4− ions in the scapolite structure. The bicarbonate and bisulfate contents appear to vary with location: the scapolite in Hellas is more bisulfate‐rich relative to that in the Chryse/Moab/Eden area. Other locations contain little (Arabia, Syrtis Major, Hellespontica, and Isidis) or no scapolite (e.g., Margaritifer, Ausonia, and Erythraeum). The calculated abundances are unconstrained because the amounts of HCO3− and HSO4− in the Martian scapolites as well as their grain sizes are not known. If the scapolites contain about 3 wt % of each, near the maximum possible, the scapolite abundances probably range from about 5 wt % scapolite at Eden and Hellas; 3–5% at Chryse, Moab, and Oxia Palus; 2–3% at Arabia, Syrtis Major, and Isidis; to less than 2% at Hellespontica, Syrtis Minor, and Margaritifer, assuming a relatively large grain size of 50–100 μm. If the characteristic grain sizes are smaller or the HCO3− and HSO4− contents are lower, the scapolite abundances required to match the observed band depths would be higher. The mineral bands are apparent in many of the Mars spectra measured, so it appears to be widely but not uniformly distributed. The newly observed fine structure also varies greatly in both depth and spectral detail with location on Mars. Thus there appears to be regional variations in composition. The mineral phases appear to reflect local or regional geology and are not primarily contained in the homogeneous, globally redistributed aeolian dust. Higher spectral resolution Martian spectra in the 2.3‐μm region as well as at 3.9 μm are needed to confirm the scapolite identification and to constrain its abundance.
High‐resolution topography and albedo of the south polar layered deposits on Mars
Authors: Herkenhoff Ken E. , Murray Bruce C.
Published: February 2008

Using a new photoclinometric technique with high‐resolution Mariner 9 images, maximum slopes of 10°–20° are found to occur on an exposure of layered deposits within the south polar residual cap of Mars. Stereophotogrammetry is used to constrain the photoclinometric solutions, which resolve layer thicknesses of 100–300 m. Albedo variations are correlated with slope, indicating that frost is present on level areas. There is evidence for temporal changes in frost distribution in the 7 days (4° of LS) between the two images used in this study. The magnitude of the slopes derived here and consideration of the stability of water ice at the surface of the layered deposits strongly suggest the presence of a competent weathering rind. The weathered surface may be composed of dark filamentary sublimation residue particles that protect the underlying ice from solar heating. This hypothesis is consistent with previous studies of the regional color and albedo of the layered deposits, which indicate that the deposits are slightly darker and less red than the bright dust that mantles much of the south polar region. Furthermore, the proposed weathering mechanism provides a plausible source of dark, saltating material for the Martian polar dune fields.