bluuebunny/crossref_metadata_2025 huggingface dataset:
Paleoseismic investigations in the Santa Cruz mountains, California: Implications for recurrence of large‐magnitude earthquakes on the San Andreas Fault
Authors: Schwartz D. P. , Pantosti D. , Okumura K. , Powers T. J. , Hamilton J. C.
Published: February 2004

Trenching, microgeomorphic mapping, and tree ring analysis provide information on timing of paleoearthquakes and behavior of the San Andreas fault in the Santa Cruz mountains. At the Grizzly Flat site alluvial units dated at 1640–1659 A.D., 1679–1894 A.D., 1668–1893 A.D., and the present ground surface are displaced by a single event. This was the 1906 surface rupture. Combined trench dates and tree ring analysis suggest that the penultimate event occurred in the mid‐1600 s, possibly in an interval as narrow as 1632–1659 A.D. There is no direct evidence in the trenches for the 1838 or 1865 earthquakes, which have been proposed as occurring on this part of the fault zone. In a minimum time of about 340 years only one large surface faulting event (1906) occurred at Grizzly Flat, in contrast to previous recurrence estimates of 95–110 years for the Santa Cruz mountains segment. Comparison with dates of the penultimate San Andreas earthquake at sites north of San Francisco suggests that the San Andreas fault between Point Arena and the Santa Cruz mountains may have failed either as a sequence of closely timed earthquakes on adjacent segments or as a single long rupture similar in length to the 1906 rupture around the mid‐1600 s. The 1906 coseismic geodetic slip and the late Holocene geologic slip rate on the San Francisco peninsula and southward are about 50–70% and 70% of their values north of San Francisco, respectively. The slip gradient along the 1906 rupture section of the San Andreas reflects partitioning of plate boundary slip onto the San Gregorio, Sargent, and other faults south of the Golden Gate. If a mid‐1600 s event ruptured the same section of the fault that failed in 1906, it supports the concept that long strike‐slip faults can contain master rupture segments that repeat in both length and slip distribution. Recognition of a persistent slip rate gradient along the northern San Andreas fault and the concept of a master segment remove the requirement that lower slip sections of large events such as 1906 must fill in on a periodic basis with smaller and more frequent earthquakes.
Analogue models of obliquely convergent continental plate boundaries
Authors: Burbidge David R. , Braun Jean
Published: February 2004

Analogue models are used to examine crustal‐scale faulting at obliquely convergent continental plate boundaries. A uniform Coulomb material is deformed with basal kinematic boundary conditions to model two obliquely convergent lithospheric plates. The mantle part of one plate is assumed to detach from its overriding crust and then be subducted beneath the other plate. The obliquity of the collision is assumed to remain constant throughout the deformation. Experiments are run with obliquities ranging from pure convergence (low obliquity) to pure strike slip (high obliquity). Reverse faults are observed for all obliquities with a nonzero convergent component. By contrast, only collisions with a large amount of strike slip motion exhibit wrench faulting. In experiments dominated by their convergent component, the strike slip motion is totally accommodated by oblique slip along the reverse faults. Strain partitioning between reverse faults and wrench faults is only observed for experiments run above a certain critical partitioning obliquity. Prom the observed initial faults, we can deduce the change in orientation in the principal stress triad as the obliquity is changed. We propose that the initial direction of maximum compressive stress (σ1) rotates horizontally as the obliquity is changed, which in turn affects the geometry of the initial faults formed in the material. In the case of reverse faults, the rotation increases their dip measured along the direction of pure convergence. The relative magnitude of the minimum horizontal stress and the vertical stress determine whether reverse faults or strike slip faults are the first to form. Although long term deformation is more difficult to analyze, a simple relationship for the angle at which strain partitioning occurs is derived.
Precise dating of the Holmatindur cooling event in eastern Iceland: Evidence for mid‐Miocene bipolar glaciation
Authors: Duncan R. A. , Helgason J.
Published: February 2004

A succession of basaltic lavas, volcaniclastic sediments, and lignite beds in the Holmatindur region of eastern Iceland provides the means for tying magnetic reversal stratigraphy and a record of major North Atlantic cooling to a precise radiometric timescale. A prominent part of the section is the Holmatindur clastic bed, which is up to 66 m thick and can be traced 80 km along strike. The geological significance of this unit lies in its composition and associated thin lignite seams. The volcaniclastic composition is basaltic (in contrast to the common rhyolitic/dacitic composition of tuff units in the area), and hyaloclastic material is dominant. Hydroexplosive volcanism most likely produced the hyaloclastite, through subglacial eruptions within the paleovolcanic zone. Pollen assemblages from the lignite beds indicate a dramatic climatic deterioration, from subtropical to cool temperate conditions, through these sediments. Feldspar crystals separated from the Holmatindur clastic bed were determined to be from 10.72 (± 0.16) Ma, from 40Ar‐39Ar incremental heating experiments. We correlate the Holmatindur cooling event with a relative maximum in benthic foraminiferal δ18O (Zone Mi6 of Miller et al. [1991]), which has been interpreted as an episode of ice accumulation within the mid‐Miocene cooling period, as well as with pulses in ice‐rafted debris supplied to deep water sites in the North Atlantic Ocean, a global sea level drop of 70 m, and submarine canyon cutting, all of which occurred within the early part of magnetic anomaly 5 normal (C5n). While strong evidence exists for mid‐Miocene ice sheets in Antarctica, northern hemisphere glaciation is thought to have started much later. The Iceland sites studied here demonstrate that significant ice accumulation occurred, albeit intermittently, as early as late middle Miocene time in the North Atlantic Ocean. From radiometric dating of Tertiary lavas that lie below and above the well‐established lower boundary C5n, we conclude that the onset of this long normal polarity period occurred at 10.94 (± 0.16) Ma. The new data are compatible with the Cande and Kent [1995] interpolated estimate for this boundary (10.95 Ma) but are significantly older, or more precise, than previous direct dating by conventional K‐Ar methods.
Upper plate stressing and seismicity in the subduction earthquake cycle
Authors: Taylor M. A. J. , Dmowska R. , Rice J. R.
Published: February 2004

We investigate upper plate stressing during the earthquake cycle in a subduction segment, using three‐dimensional (3‐D) elastic models to address the effects of strongly heterogeneous coupling along strike of the interplate interface. We show how heterogeneity controls the locations and mechanisms of seismicity in the upper plate. Oblique subduction segments, two from the Aleutians (Andreanof Islands 1986 and Rat Islands 1965) and one from Indonesia (Biak 1996) are studied. All examples of upper plate seismicity from the Aleutians represent events occurring toward the beginning of a new cycle, while in Biak, Indonesia, the examined events occur both toward the end of one cycle and the beginning of the next. In the majority of cases studied, the location and mode of the upper plate seismicity are consistent with space‐ and time‐dependent stressing as predicted by modeling. This confirms earlier observations that seismicity in the vicinity of large/great subduction earthquakes (toward the outer rise, at intermediate depth, and now in the upper plate) depends, in an interpretable manner, on the stage in the earthquake cycle as well as on distribution of coupling along the interplate interface.
Velocity structure in upper ocean crust at Hole 504B from vertical seismic profiles
Authors: Swift Stephen A. , Lizarralde D. , Stephen Ralph A. , Hoskins Hartley
Published: February 2004

Hole 504B provides the only opportunity to directly correlate seismic velocity structure to the lithology and physical properties of upper ocean crust, providing a baseline for comparison with seismic measurements elsewhere. We determine P and S velocities from vertical seismic profiles (VSPs) obtained on Ocean Drilling Program (ODP) Legs 111 and 148. Four issues are considered: the location of the seismic layer 2/3 boundary, P to S wave conversion by scattering, transverse isotropy, and Poisson's ratio as an indicator of lithology, porosity, and structure. (1) In the P velocity profile, the change in slope marking the layer 2/3 boundary coincides with the top of the sheeted dike unit. Seismic layer 2 is composed of the extrusives and the lithologic transition zone, the layer in which flows and dikes interfinger. (2) Even in these normal incident VSPs, several second arrivals with velocities indicative of vertically polarized shear energy are observed. P to S wave conversion within the upper 110 m of basement occurs by scattering from surface roughness and volume heterogeneities and does not depend on angle of incidence as predicted by plane boundary transmission coefficient analysis. (3) Vertical velocities determined from the VSP differ by <10% from horizontal velocities obtained from the oblique seismic experiment (OSE) on Deep Sea Drilling Project (DSDP) Leg 92. The P wave velocity structure is determined by small and intermediate (<1 cm) pore structure with no measurable anisotropy. The large‐scale, well‐oriented vertical fractures, which are formed tectonically, do not have a detectable effect on compressional wave velocities. (4) High Poisson's ratio in the upper 300 m of basement coincides with an extrusive layer composed of pillows and thin flows. Low Poisson's ratio at 850–1150 m below seafloor (mbsf) coincides with the downhole decrease in bulk porosity caused by the transition from extrusives to dikes. Relatively large‐aspect ratio cracks are required to produce such low values of Poisson's ratio. The cracks were likely created by hydraulic fracturing when hot dikes encountered low‐temperature seawater.
Intrinsic and scattering seismic wave attenuation in the Canary Islands
Authors: Canas José Antonio , Ugalde Arantza , Pujades Luis G. , Carracedo Juan Carlos , Soler Vicente , Blanco María José
Published: February 2004

The Canary Islands volcanic complex is studied in terms of coda wave attenuation. A multiple lapse time window method, based on the hypothesis of multiple isotropic scattering with uniform distribution of scatterers, is applied to the available seismic data in order to obtain the intrinsic absorption (Qi−1) and the scattering attenuation (Qs−1) in the Canarian lithosphere. The analysis is performed for two hypocentral distance ranges: from 0 to 80 km and from 0 to 140 km. Results show that in both cases and for all the studied frequency bands (1–2, 2–4,6–8 and 8–10 Hz ) intrinsic absorption dominates. The low albedos found in the region indicate the low degree of heterogeneity in the Canarian lithosphere at the scale length of the studied frequencies. On the other hand the coda attenuation (Qc−1) calculated on the basis of the single‐scattering theory gives values near (Qi−1) for low frequencies and near the total attenuation (Qt−1) for high frequencies. The degree of frequency dependence of the attenuation parameters is strong in all cases. A correlation of the observed attenuation parameters with the geological evidence for a hotspot‐type archipelago and with other geophysical studies is suggested.
An observational test of the critical earthquake concept
Authors: Bowman D. D. , Ouillon G. , Sammis C. G. , Sornette A. , Sornette D.
Published: February 2004

We test the concept that seismicity prior to a large earthquake can be understood in terms of the statistical physics of a critical phase transition. In this model, the cumulative seismic strain release increases as a power law time to failure before the final event. Furthermore, the region of correlated seismicity predicted by this model is much greater than would be predicted from simple elastodynamic interactions. We present a systematic procedure to test for the accelerating seismicity predicted by the critical point model and to identify the region approaching criticality, based on a comparison between the observed cumulative energy (Benioff strain) release and the power law behavior predicted by theory. This method is used to find the critical region before all earthquakes along the San Andreas system since 1950 with M≥6.5. The statistical significance of our results is assessed by performing the same procedure on a large number of randomly generated synthetic catalogs. The null hypothesis, that the observed acceleration in all these earthquakes could result from spurious patterns generated by our procedure in purely random catalogs, is rejected with 99.5% confidence. An empirical relation between the logarithm of the critical region radius (R) and the magnitude of the final event (M) is found, such that log R∝0.5M, suggesting that the largest probable event in a given region scales with the size of the regional fault network.
Dynamics of subduction initiation at preexisting fault zones
Authors: Toth John , Gurnis Michael
Published: February 2004

We show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces, consistent with what is observed in the western Pacific. The dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method. We investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion. The viscosity of the medium is non‐Newtonian and temperature‐dependent. We also examine the influence of a perfectly plastic yield stress. A new method to model the preexisting weak zone as a graded fault with remeshing is introduced. The first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics. Initially, topography across the plate boundary is characterized by over 1000 m of uplift close to the trench. Subsequently, a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m/Myr for a slab subducting at 2 cm/yr. Tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration. For a plate force of 4×1012 N/m, subduction initiates even with a fault shear stress of 3 MPa, the resistance most consistent with seismological observations. However, subduction will not initiate when this fault shear stress is >5 MPa. Our results are consistent with plate reconstructions, which predict the initiation of subduction across preexisting weak zones within oceanic basins.
Flow directions in dikes from anisotropy of magnetic susceptibility data: The bootstrap way
Authors: Tauxe L. , Gee J. S. , Staudigel H.
Published: February 2004

One of the first applications of anisotropy of magnetic susceptibility (AMS) was an attempt to determine flow directions from mafic dikes [Khan, 1962]. Since the seminal work of Knight and Walker [1988] defining the expected behavior of AMS in response to magma flow, there has been increasing interest in using AMS for this purpose. Here we present a quantitative method for interpretation of AMS data from dikes, using a parametric bootstrap. First, dikes must be sampled with at least five (and preferrably more) samples from within 10 cm of the dike margin. The distributions of the eigenvalues and eigenvectors of the AMS tensor are delineated by calculating eigenparameters of many bootstrapped paradata sets. We generate paradata sets by first selecting a sample at random, then calculating a replacement set of data by drawing tensor elements from normal distributions with the mean and standard deviation of the entire site. The bounds containing 95% of the eigenparameters of the bootstrapped data serve as confidence limits for the parameter of interest. Classification of dikes proceeds as follows: Sites whose maximum and intermediate eigenvalues could not be distinguished are deemed uninterpretable. In addition, sites with principal eigenvectors with angles >45° away from the dike margin (inverse) or with markedly different directions on either side of the dike (scissored) are excluded. The remaining dikes are classified as having unique flow direction information if the principal eigenvectors from at least one side are distinct from the dike plane based on the distribution of the bootstrapped principal eigenvectors. If neither side has principal eigenvectors distinct from the dike plane, the dikes are classified as having lineation information only. A study comprising 251 dikes from the Troodos ophiolite has 151 sites with directional data, 38 sites with lineations only, 7 inverse sites, 5 scissored sites, and 55 sites not fitting into any other category. The flow directions interpreted from the data were generally southerly, toward a fossil transform zone.
INDEPTH (International Deep Profiling of Tibet and the Himalaya) multichannel seismic reflection data: Description and availability
Authors: Alsdorf D. , Makovsky Y. , Zhao W. , Brown L. D. , Nelson K. D. , Klemperer S. , Hauck M. , Ross A. , Cogan M. , Clark M. , Che J. , Kuo J.
Published: February 2004

Project INDEPTH (International Deep Profiling of Tibet and the Himalaya) has collected over 300 km of multichannel, deep seismic reflection data using explosive sources as part of a multidisciplinary effort to image the structure of the crust and uppermost mantle of the Tibetan plateau. The reflection profiles lie within the Yadong‐Gulu rift and were acquired in the summers of 1992 and 1994. Data processing utilized typical industry tools, and a new method was used to migrate the data. Both unmigrated and migrated sections are presented here in large format to facilitate further interpretations.
Synergy of active and passive airborne thermal infrared systems for surface compositional mapping
Authors: Hook Simon J. , Cudahy Thomas J. , Kahle Anne B. , Whitbourn Lewis B.
Published: February 2004

NASA thermal infrared multispectral scanner (TIMS) and Commonwealth Scientific Industrial Research Organisation mid‐infrared airborne CO2 laser spectrometer (MIRACO2LAS) data were acquired over the Mount Fitton area, South Australia, in order to evaluate their combined use for geological mapping and mineral exploration. TIMS is a passive, imaging system with six spectral bands in the thermal infrared wavelength region (8–12 μm), whereas MIRACO2LAS is an active, profiling system with ∼100 spectral bands in the 9–11 μm spectral range. The TIMS and CO2 laser data (emissivity variations for TIMS and apparent reflectance for MIRACO2LAS) were processed to enhance spectral information related to the surface composition. This spectral information was compared with existing geological maps and field emissivity spectra. Known geological units were well discriminated in the TIMS imagery, including a range of quartz‐rich and carbonate‐rich sedimentary units, as well as several previously unmapped areas of alteration in the carbonate rocks. However, the broadband spectral resolution of TIMS did not allow identification of discrete mineral constituents. In contrast, the high spectral resolution MIRACO2LAS data provided diagnostic spectral information about a range of minerals present including quartz, dolomite, talc, and tremolite, albeit, along discrete profiles. The widths of some of these diagnostic spectral features were less than 0.2 μm wide, which is half the resolution of the TIMS band passes (0.4 μm). The MIRACO2LAS spectra closely matched the shape, location, and depth of spectra of field samples measured by both a field emission spectrometer and a laboratory laser spectrometer. Pure mineral spectra measured by the (bidirectional) laboratory laser spectrometer also closely matched those measured by a conventional laboratory spectrometer measuring directional hemispherical reflectance. These results indicate that future remote thermal infrared systems designed for improved geological mapping and mineral exploration should incorporate both an imager for mapping lithological units and a high spectral resolution profiler for identifying dominant mineral constituents. Recently, a hyperspectral imaging thermal infrared instrument has been developed termed Spatially Enhanced Broadband Array Spectrograph System (SEBASS) [Hackwell and Warren, 1997]. This instrument should allow spectral identification of an entire scene [Gillespie et al., 1997], although the swath width is currently limited to 128 pixels as opposed to the 752 pixels of TIMS after panoramic correction.
A two‐dimensional P‐SV hybrid method and its application to modeling localized structures near the core‐mantle boundary
Authors: Wen Lianxing , Helmberger Donald V.
Published: February 2004

A P‐SV hybrid method is developed for calculating synthetic seismograms involving two‐dimensional localized heterogeneous structures. The finite difference technique is applied in the heterogeneous region and generalized ray theory solutions from a seismic source are used in the finite difference initiation process. The seismic motions, after interacting with the heterogeneous structures, are propagated back to the Earth's surface analytically with the aid of the Kirchhoff method. Anomalous long‐period SKS‐SPdKS observations, sampling a region near the core‐mantle boundary beneath the southwest Pacific, are modeled with the hybrid method. Localized structures just above the core‐mantle boundary, with lateral dimensions of 250 to 400 km, can explain even the most anomalous data observed to date if S velocity drops up to 30% are allowed for a P velocity drop of 10%. Structural shapes and seismic properties of those anomalies are constrained from the data since synthetic waveforms are sensitive to the location and lateral dimension of seismic anomalies near the core‐mantle boundary. Some important issues, such as the density change and roughness of the structures and the sharpness of the transition from the structures to the surrounding mantle, however, remain unresolved due to the nature of the data.
Paleomagnetism of the Middle Proterozoic Electra Lake Gabbro, Needle Mountains, southwestern Colorado
Authors: Harlan Stephen S. , Geissman John W.
Published: February 2004

The Electra Lake Gabbro is a small 1.435 Ga pluton that intrudes 1.7 to 1.6 Ga gneisses and schists of the Needle Mountains in southwestern Colorado. Paleomagnetic samples were collected from the main phases of the gabbro, diabase dikes, granite, and alaskite dikes that cut the gabbro and from a partially melted zone in gneiss along the southern margin of the pluton. Gabbro, diabase, and some melt zone samples have a single‐polarity characteristic magnetization of northeast declination (D) and moderate negative inclination (I). Demagnetization behavior and rock magnetic characteristics indicate that the remanence is carried by nearly pure magnetite. After correction for the minor west dip of overlying Paleozoic strata, we obtain a mean direction of D = 32.1°, I = −41.9° ( k: = 94, α95 = 3.3°, N = 21 sites) and a paleomagnetic pole at 21.1°S, 221.1°E, (K = 89, A95 = 3.4°). This pole is similar to poles from the Middle Proterozoic Belt Supergroup but is located at a higher southerly latitude than poles from other 1.47–1.44 Ga plutons from North America, most of which plot at equatorial latitudes. The reason for this discrepancy is not clear but may result from a combination of factors, including unrecognized tilting of the gabbro, the failure of this relatively small pluton to fully average paleosecular variation, and uncertainties in the overall reliability of other 1.5–1.4 Ga poles of the North American apparent polar wander path.
Magnetization of 0–29 Ma ocean crust on the Mid‐Atlantic Ridge, 25°30′ to 27°10′N
Authors: Tivey Maurice A. , Tucholke Brian E.
Published: February 2004

A sea‐surface magnetic survey over the west flank of the Mid‐Atlantic Ridge from 0 to 29 Ma crust encompasses several spreading segments and documents the evolution of crustal magnetization in slowly accreted crust. We find that magnetization decays rapidly within the first few million years, although the filtering effect of water depth on the sea‐surface data and the slow spreading rate (<13 km/m.y.) preclude us from resolving this decay rate. A distinctly asymmetric, along‐axis pattern of crustal magnetization is rapidly attenuated off‐axis, suggesting that magnetization dominated by extrusive lavas on‐axis is reduced off‐axis to a background value. Off‐axis, we find a statistically significant correlation between crustal magnetization and apparent crustal thickness with thin crust tending to be more positively magnetized than thicker crust, indicative of induced magnetization in thin inside corner (IC) crust. In general, we find that off‐axis segment ends show an induced magnetization component regardless of polarity and that IC segment ends tend to have slightly more induced component compared with outside corner (OC) segment ends, possibly due to serpentinized uppermost mantle at IC ends. We find that remanent magnetization is also reduced at segment ends, but there is no correlation with inside or outside corner crust, even though they have very different crustal thicknesses. This indicates that remanent magnetization off‐axis is independent of crustal thickness, bulk composition, and the presence or absence of extrusives. Remanence reduction at segment ends is thought to be primarily due to alteration of lower crust in OC crust and a combination of crustal thinning and alteration in IC crust. From all these observations, we infer that the remanent magnetization of extrusive crust is strongly attenuated off‐axis, and that magnetization of the lower crust may be the dominant source for off‐axis magnetic anomalies.
Two‐ and three‐dimensional modeling of hydrothermal convection in the sedimented Middle Valley segment, Juan de Fuca Ridge
Authors: Rabinowicz M. , Boulègue J. , Genthon P.
Published: February 2004

The heat flow map derived from 550 measurements collected in a the southern portion of the sedimented rift in Middle Valley, northern Juan de Fuca Ridge, displays kilometer‐sized quasi‐circular regions of very high heat flow. Some of these domains, explored during Ocean Drilling Program (ODP) leg 139, are thought to be discharge zones of large‐scale hydrothermal plumes. To understand this unique data set, we modeled the kilometer‐scale hydrothermal circulation within both the sedimentary and the igneous crust, using a set of two‐ and three‐dimensional models that use a numerical technique based on horizontal spectral decomposition of the flow equations. These models include variations in the viscosity and density of the hydrothermal fluids with temperature. We examine the variations in flow patterns due to different permeability‐versus‐depth distribution within sediment and pillow layers. Models with the same permeability in both layers do not match the seafloor heat flow field in Middle Valley. When the permeability decreases from the bottom to the top of the simulation domain by a factor greater than 20, convection assumes a plume pattern to produce surface heat flow comparable to that observed in Middle Valley. Within the models the ratio of the heat flux above the recharge and discharge domains is directly related to the vertical harmonic mean of the permeability field. A value of 7×10−16m2provides a good match to the heat flow observations. The Darcy velocities of the hydrothermal fluids in the discharge areas approach 16 cm/yr while in the recharge areas they are lower than 3 cm/yr. These rates and the temperature inside the plumes are sufficiently high to produce the observed massive sulfide deposits and mineral alterations in 1–2 × 105years. The dynamic pressure produced by the hydrothermal flow matches the pressure measured in drill sites. This process may play a major role in compaction, fracturing, and uplift of the sediment cover. For example, the dynamic pressure in the ascending plume equals the lithostatic pressure at a depth of 50 m. Resulting hydrofracturing could explain the genesis of the vent fields associated with the hydrothermal discharge.
Thermal convection in a volumetrically heated, infinite Prandtl number fluid with strongly temperature‐dependent viscosity: Implications for planetary thermal evolution
Authors: Grasset O. , Parmentier E. M.
Published: February 2004

Parameterized models of the thermal evolution of planets are usually based on the assumption that the lithosphere‐convecting mantle boundary can be defined by an isotherm at a temperature below which viscosity is infinite on geologic timescales. Recent experimental results argue against this assumption. We have investigated both the definition of the lithosphere‐convecting mantle boundary and the power law relation describing convecting heat transfer, based on numerical experiments of thermal convection in a volumetrically heated fluid with temperature‐dependent viscosity. Other recent studies have treated only the heating from below, but volumetric heating is likely to be the dominant mode of heating in planetary mantles, either as a consequence of radioactive heating or as a proxy for secular cooling. Convection can occur either in the whole box or be located under a stagnant lid. In the lid regime, convection is driven by a temperature contrast depending on the rheology of the fluid and the interior temperature. This result, in agreement with experimental studies, indicates that boundary between the stagnant lid and the convecting layer (similar to the lithosphere‐convecting mantle boundary) cannot be defined as a fixed isotherm. During thermal evolution of planets, the viscosity contrast in the convecting mantle remains constant, not the temperature at the bottom of the lithosphere. We present an example showing that the evolution of planets is strongly dependent on the criterion chosen to define the lithosphere‐convecting mantle boundary. For reasonable values of the activation energy for thermally activated creep, the temperature defining the lithosphere‐convecting mantle boundary, the mantle temperature, and the thickness of the lithosphere could be larger than expected from previous models which treat the base of the lithosphere as a fixed isotherm.
Plume‐ridge interaction in the Easter‐Salas y Gomez seamount chain‐Easter Microplate system: Pb isotope evidence
Authors: Kingsley Richard H. , Schilling Jean‐Guy
Published: February 2004

Lead isotope ratios are reported on fresh basalt glasses from 28 seamounts located along the western end of the Easter‐Salas y Gomez seamount chain (ESC), southeast Pacific for the purpose of testing the mantle plume source‐migrating ridge sink (MPS‐MRS) model for the origin of this extensive intraplate neovolcanic seamount chain. Most of these basalts west of 103°W are less than 1.5 m.y. old. Results reveal a 1100‐km‐long westward gradient decreasing in radiogenic Pb from Salas y Gomez to the Easter microplate (EMP). The results confirm the previously established four‐point average trend based on data from Salas y Gomez, Easter Island, the east rift, and the west rift of the Easter microplate [Hanan and Schilling, 1989]. Tight linear arrays in Pb isotope space of the combined EMP and ESC data sets indicate binary mixing between the radiogenic isotope‐rich Salas y Gomez plume and the depleted asthenosphere source is the dominant factor in controlling Pb isotopic variation in the region. Mapping of the plume component mass fraction reveals not only a westward dilution of the plume component from Salas y Gomez to the EMP but also dilution with depleted material across the ESC on both flanks. The mapping is consistent with the MPS‐MRS model which calls for a subhorizontal, sublithospheric plume channel connecting the mantle plume conduit near Salas y Gomez with the East Pacific Rise, where plume flow is confined along a thermal groove. Mixing by entrainment of surrounding depleted material takes place progressively downstream. A numerical thermal lithosphere cooling model reveals a rheological boundary layer (i.e., the base of the lithosphere) shoaling by 1.3° to 3.5° from Salas y Gomez to about 100 km east of the EMP. The slope magnitude is consistent with those suggested by laboratory and numerical fluid dynamic models for hot buoyant plume‐ridge channeled flow to develop and be maintained.
The 1994 Cacoosing Valley earthquakes near Reading, Pennsylvania: A shallow rupture triggered by quarry unloading
Authors: Seeber Leonardo , Armbruster John G. , Kim W.‐Y. , Barstow Noel , Scharnberger Charles
Published: February 2004

A mb(Lg) 4.6 mainshock on January 16, 1994, in the Cacoosing valley, 10 km west of Reading, southeastern Pennsylvania caused modified Mercalli VI‐VII intensities and the highest earthquake damage (∼$2 million) in the eastern United States since 1944. Aftershock hypocenters from a temporary local seismic network are confined to the upper 2.5 km of the crust. They are clustered around the periphery of a tabular 3 × 3 km zone that is interpreted to outline the mainshock rupture. This zone matches the nodal plane with reverse and left‐lateral slip (strike 135°, dip 54° southwest, and rake 55°) of a focal mechanism obtained from aftershock first motions and from mainshock waveforms. This rupture does not correlate with any of the faults mapped in the epicentral area, but it is parallel to the most prominent fracture set, including joints and small faults. Maximum possible strike‐slip accumulated on the causative fault is no more than several tenths of meters. A large carbonate rock quarry is centered above the rupture. We calculate a small, but significant (0.13 MPa) Coulomb stress increase caused by the quarry on the shallow portion of the rupture. Most of this increase was caused by pore pressure rise after the quarry was abandoned in December 1992 and flooded. Seismicity started may 1993. We conclude that the 1993–1997 Cacoosing Valley sequence is probably triggered by the quarry. About 200 km northeast of Cacoosing, another quarry in early Paleozoic carbonate rocks triggered the 1974 mb(Lg) 3.0 Wappingers Falls earthquake.
Introduction to Special Section: Frontiers in Rock Magnetism and Domain Theory—Basic and Applied
Authors: Banerjee Subir K.
Published: February 2004

Kittel [1946], then of the erstwhile Bell Laboratories, predicted that magnetization reversal in small uniformly magnetized, or single‐domain particles will occur only if a large negative magnetic field (∼2×102 mT) was applied. In other words, once magnetically saturated and left in the ambient Earth's magnetic field, the single‐domain (SD) particles would exhibit high values of magnetic remanence, as well as high coercive forces. The terrestrial geomagnetic “recording media,” sediments and rocks with strongly ferrimagnetic particles in them, owe their magnetic stability or “memory” over geological timescales to such high coercive forces. Stacey (1963) applied Kittel's theory to calculate SD size thresholds for magnetite and hematite, the two most common iron oxide minerals in the Earth's crust.
Helium and carbon isotope systematics at Ontake volcano, Japan
Authors: Sano Yuji , Nishio Yoshiro , Sasaki Sho , Gamo Toshitaka , Nagao Keisuke
Published: February 2004

We have measured chemical compositions, He/Ne ratios, and He and C isotopic compositions of 47 gas samples collected from crater fumaroles and hot springs associated with Ontake volcano, Japan. Geographical and temporal variations of chemical compositions, 3He/4He ratios, and δ13C values of CO2 are reported here. Between 1981 and 1993, 3He/4He ratios decreased with increasing distance from the central cone of the volcano, while δ13C values increased with the distance. Magmatic He and CO2 are probably carried by a fluid flowing through the volcanic edifice and are diluted by crustal He and CO2. A negative correlation of 3He/4He ratios and δ13C values suggests two‐component mixing between magmatic and crustal end‐members. At the sampling sites close to the fault formed by a M6.8 earthquake that occurred in September 1984, 3He/4He ratios have increased significantly with time from 1984 to 1993, while, except for three sites close to the fault, no systematic change of δ13C values is found. On the other hand, He and C isotopes have been constant at the sites distant from the fault. The temporal variations of 3He/4He ratios may be related to the recent reactivation of Ontake volcano.
Three long lava flows in north Queensland
Authors: Stephenson P. J. , Burch‐Johnston A. T. , Stanton D. , Whitehead P. W.
Published: February 2004

The Kinrara, Toomba, and Undara basaltic lava flows are from 55 to 160 km long and range in age from 13 to 190 ka. The lavas were emplaced down low gradients (0.2° to 0.4°) with volumes ranging up to 30 km3. They were not unusually hot at eruption (1130°–1160°) nor unusually fluid. Gentle topography controlled the flows, and shallow drainage lines captured them. Lava tubes operated in places, and some drained to form caves. Injection under surface crust was widespread, producing inflation features ranging from tumuli and low plateaus to extensive ridges. Sustained eruption was essential for the development of the long flows, but each is composite, with pauses between successive pulses that partially covered the earlier, longer flows. The lava structures are mainly pahoehoe but some 'a'a lavas are present. Of the three volcanoes involved, Undara is a simple low‐angle lava cone with a 200‐m‐wide crater, Toomba is a low‐angled cone with several eruption centers, and Kinrara has a deep crater with evidence of strong fountaining. Effusion rates are not known but may have been relatively low, similar to those observed in Hawaiian volcanoes. Lava tubes, most of which remained undrained, are believed to have been of major importance in flow emplacement. Given the evidence of successive flows and the time needed to develop widespread inflation, it is suggested that the two long flows over 100 km involved many decades of eruption.
Fracturing of ice under compression creep as revealed by a multifractal analysis
Authors: Weiss Jérôme , Gay Michel
Published: February 2004

Fracturing of freshwater granular ice up to failure under uniaxial compression creep was investigated from series of interrupted creep tests and from a multifractal analysis of the corresponding fracture patterns. At the early stages of damage corresponding to primary and secondary creep, the fracturing process is dominated by the nucleation of microcracks from stress concentrations within the material (unlike rocks, artificial freshwater ice does not contains starter flaws). Because of the crack nucleation mechanisms, the microstructure of the material (e.g., the nonfractal grain size distribution) strongly influences the organization of fracturing which is therefore nonfractal. As fracturing proceeds during tertiary creep, a hierarchical (fractal) organization of the fracturing emerges progressively over a wider scale range. At failure, this fractal organization is fully developed without detectable lower or upper bound, and the role of the initial microstructure has completely disappeared. Similarly, cracks are preferentially oriented along the compression axis at the early stages of damage, but this anisotropy vanishes as failure is approached. The simultaneity between the onset of tertiary creep and the emergence of fractal organization suggests that the acceleration of the deformation during tertiary creep is due to the cataclasis of a material which becomes granular. An important consequence of the fractal organization of fracturing is that homogenization procedures, as well as damage mechanics, developed to study the behavior of damaged materials, cannot be used to describe tertiary creep and failure.
Nonlinear refraction and reflection travel time tomography
Authors: Zhang Jie , ten Brink Uri S. , Toksöz M. Nafi
Published: February 2004

We develop a rapid nonlinear travel time tomography method that simultaneously inverts refraction and reflection travel times on a regular velocity grid. For travel time and ray path calculations, we apply a wave front method employing graph theory. The first‐arrival refraction travel times are calculated on the basis of cell velocities, and the later refraction and reflection travel times are computed using both cell velocities and given interfaces. We solve a regularized nonlinear inverse problem. A Laplacian operator is applied to regularize the model parameters (cell slownesses and reflector geometry) so that the inverse problem is valid for a continuum. The travel times are also regularized such that we invert travel time curves rather than travel time points. A conjugate gradient method is applied to minimize the nonlinear objective function. After obtaining a solution, we perform nonlinear Monte Carlo inversions for uncertainty analysis and compute the posterior model covariance. In numerical experiments, we demonstrate that combining the first arrival refraction travel times with later reflection travel times can better reconstruct the velocity field as well as the reflector geometry. This combination is particularly important for modeling crustal structures where large velocity variations occur in the upper crust. We apply this approach to model the crustal structure of the California Borderland using ocean bottom seismometer and land data collected during the Los Angeles Region Seismic Experiment along two marine survey lines. Details of our image include a high‐velocity zone under the Catalina Ridge, but a smooth gradient zone between Catalina Ridge and San Clemente Ridge. The Moho depth is about 22 km with lateral variations.
Seismic attenuation in upper ocean crust at Hole 504B
Authors: Swift Stephen A. , Lizarralde D. , Stephen Ralph A. , Hoskins Hartley
Published: February 2004

Seismic attenuation and its relationship to borehole stratigraphy in the upper 1.8 km of ocean basement at Hole 504B are determined from analysis of vertical seismic profile (VSP) data. VSP data provide unambiguous measurements of seismic amplitude decay along a vertical propagation path through the crust, and ancillary borehole measurements enable detailed modeling of the relative contributions from geometrical spreading, scattering, and intrinsic loss mechanisms to this decay. About 60% of the total observed amplitude decay occurs in the pillow basalt section and is due mostly to geometrical spreading and scattering from impedance contrasts. The remaining amplitude decrease is concentrated in two layers, at 500–650 and 800–900 meters below seafloor (mbsf) (225–375 and 525–625 m below basement), across which amplitude rapidly decays and the frequency characteristics of the downgoing wave field are significantly and permanently modified. Attenuation in these layers is not due to scattering but rather to an intrinsic mechanism that can be characterized by Q of 10 and 8, respectively. It is likely that the Q structure of both of these intervals is formed with the crust near the ridge and thus related to fundamental ocean crust forming processes. The shallow interval coincides with a change in alteration mineralogy deposited by late‐stage fluid flow and may separate lower lavas that were emplaced within the rift zone from upper lavas that were emplaced by off‐axis flow through large lava tubes. Intrinsic attenuation in the deeper horizon is probably due to an increase in porosity and cracking associated with either intracrustal deformation or subhorizontal faulting. There is negligible attenuation of seismic frequencies in the dikes below 1000 mbsf (∼725 m subbasement).
Evolution of the Gorda Escarpment, San Andreas Fault and Mendocino triple junction from multichannel seismic data collected across the northern Vizcaino block, offshore northern California
Authors: Godfrey Nicola J. , Meltzer Anne S. , Klemperer Simon L. , Tréhu Anne M. , Leitner Beate , Clarke Samuel H. , Ondrus Amy
Published: February 2004

The Gorda Escarpment is a north facing scarp immediately south of the Mendocino transform fault (the Gorda/Juan de Fuca‐Pacific plate boundary) between 126°W and the Mendocino triple junction. It elevates the seafloor at the northern edge of the Vizcaino block, part of the Pacific plate, ∼1.5 km above the seafloor of the Gorda/Juan de Fuca plate to the north. Stratigraphy interpreted from multichannel seismic data across and close to the Gorda Escarpment suggests that the escarpment is a relatively recent pop‐up feature caused by north‐south compression across the plate boundary. Close to 126°W, the Vizcaino block acoustic basement shallows and is overlain by sediments that thin north toward the Gorda Escarpment. These sediments are tilted south and truncated at the seafloor. By contrast, in a localized region at the eastern end of the Gorda Escarpment, close to the Mendocino triple junction, the top of acoustic basement dips north and is overlain by a 2‐km‐thick wedge of pre‐11 Ma sedimentary rocks that thickens north, toward the Gorda Escarpment. This wedge of sediments is restricted to the northeast corner of the Vizcaino block. Unless the wedge of sediments was a preexisting feature on the Vizcaino block before it was transferred from the North American to the Pacific plate, the strong spatial correlation between the sedimentary wedge and the triple junction suggests the entire Vizcaino block, with the San Andreas at its eastern boundary, has been part of the Pacific plate since significantly before 11 Ma.
Precipitation sealing and diagenesis: 2. Theoretical analysis
Authors: Aharonov Einat , Tenthorey Eric , Scholz Christopher H.
Published: February 2004

We propose a new model to describe precipitation‐induced permeability reduction during chemical diagenesis of sandstones. The model has two parts: (1) geochemical model for determining the expected mass of precipitates and (2) physical model relating how a given mass of precipitates affects permeability. The model is used to study the experiments reported by Tenthorey et al. [this issue], where reactions involving dissolution of quartz and feldspar and precipitation of authigenic minerals were observed to cause drastic permeability reduction in feldspathic sandstone. We find that the geochemical evolution of the system dictates permeability evolution, and thus permeability dependence on temperature and stress is related to geochemical responses to these variables. The model predictions for the experimental system of Tenthorey et al. provide a very good fit to the experimental permeability curves over all times, stresses, and temperatures within the experimental range. Following the experiments and model, we suggest that precipitation of secondary mineral phases is an extremely efficient method for permeability reduction but one that depends critically on the system's approach to equilibrium. The reaction products of chemical diagenesis observed in rocks are the consequence of chemical reactions that occur in natural systems as they approach equilibrium, and we predict extensive and rapid permeability reduction as a result of such reactions.