bluuebunny/crossref_metadata_2025 huggingface dataset:
Tomographic imaging of the Alaska subduction zone
Authors: Zhao Dapeng , Christensen Douglas , Pulpan Hans
Published: February 2004

The Alaska subduction zone is characterized by the Pacific plate descending beneath the North American plate, causing abundant seismic activity in the crust and along the Wadati‐Benioff zone down to a depth of approximately 200 km, We have used 142,908 P wave arrival times from 12,237 shallow‐ and intermediate‐depth earthquakes recorded by the Alaska Earthquake Information Center jointly run by the Geophysical Institute, University of Alaska Fairbanks and U.S. Geological Survey in the period from January 1977 to November 1991, to investigate the three‐dimensional (3‐D) P wave velocity structure beneath central and southern Alaska. Travel times and ray paths are accurately calculated by using an efficient 3‐D ray‐tracing technique. The nonlinear tomographic problem is solved by iteratively conducting linear inversions, and the velocity structure and hypocentral locations are simultaneously determined. We conducted two types of inversions. One is an inversion with a laterally homogeneous starting model. The others are what we call slab inversions in which we introduce into the initial model the high‐velocity subducting Pacific plate as a priori information. We found that the slab inversions gave a seismologically more plausible result and a final root‐mean‐square travel time residual significantly smaller than that of the inversion with the homogeneous starting model. Detailed P wave tomographic images are obtained for the crust and upper mantle down to a depth of 200 km with spatial resolutions of 30–60 km. The tomographic image of the upper crust correlates well with the major surface geological features, such as slow sedimentary basins and fast ultramafic bodies. Prominent low‐velocity anomalies exist in the crust and upper mantle beneath active volcanoes. In the mantle wedge the low‐velocity anomalies dip toward the continental side and extend to a depth of about 150 km, which are considered to be associated with the active volcanism in the Alaska subduction zone. The results suggest that the subducting Pacific plate has a thickness of 45–55 km and a P wave velocity 3–6% higher than that of the surrounding mantle.
Deformation of Kilauea volcano during 1982 and 1983: A transition period
Authors: Wallace Michelle Hall , Delaney Paul T.
Published: February 2004

A major transition in the style of deformation of Kilauea volcano occurred during the period January 1982 through November 1983. Prior to 1983, the deformation was dominated by magma intrusions, rapid extension of the summit, and seaward movement of the south flank. The pattern of activity changed to magma eruption and slowed deformation of the volcanic edifice in January 1983. An earthquake swarm in the southwest rift zone in June 1982 and an intrusion of a large dike in the east rift zone in January 1983 prepared the volcano for the eruptive sequences that continue today. We estimate the sources of surface displacements associated with these events using a least squares nonlinear inversion technique and planar dislocations. The models are constrained with measurements of line length changes and of surface height changes from tide gauges, water wells, and spirit‐level tilt data. Deformation associated with the 1982 earthquake swarm is best modeled as the emplacement of a steeply dipping dike in the upper and middle southwest rift zone extending from 2–3 km to 10–11 km depth with a width of ∼1–2 m. Tilt measurements gathered at the time of the earthquake swarm record a small deflation of Kilauea's summit; however, trilateration data indicate the summit was rapidly extending in 1982. The extension and our models are consistent with seaward slip of the south flank on a low‐angle basal thrust fault. Dislocation models of the January 1983 dike intrusion yield a 14‐ to l5‐km‐long dike along the zone of observed ground rupture. The dike extends from just below the surface to the region of the most intense rift zone seismicity at 4–5 km depth. A strong nonlinear relationship among height, dip, and width results in a suite of models that explain the data equally well. Tests of parameter sensitivity indicate that the dike width and height are the least constrained parameters; the best fit average width ranges from 1.7 to 2.8 m. Our models also include a source of contraction at ∼5 km depth that produces 40 cm of ground subsidence near Makaopuhi, the point of initiation of the intrusion. This location is consistent with geochemical and seismic evidence that indicate magma is stored in the rift zone. Residual errors for all models indicate that substantial line‐length changes are unexplained. Seaward motions of the south flank during 1982 and 1983 are not well modeled by simple dikes and summit deflations; however, attempts to constrain low‐angle faults in the south flank were only partly successful due to the limited extent of the geodetic networks.
Depth dependence of anisotropy of Earth's inner core
Authors: Song Xiaodong , Helmberger Don V.
Published: February 2004

Both body wave (PKP) travel times (Creager, 1992; Song and Helmberger, 1993a; McSweeney and Creager, 1993; Shearer, 1994) and fits to the splitting of core modes (Tromp, 1993) show general agreement that the top 300 km of inner core is very anisotropic. The anisotropy displays axial symmetry around the Earth's spin axis, with the polar direction 3% faster than the equatorial direction. One key problem now is the depth dependence of the inner core anisotropy. Here we extend our polar path studies to include both long‐period and short‐period modeling for the PKP phases at ranges 120° to 173°. Arrivals from the top of the inner core (PKIKP) and reflections from the inner core boundary (PKiKP) can be observed distinctly in short‐period records at ranges 130° to 140° and as waveform distortions in the long‐period records at ranges 130° to 146°. These waveforms provide a new set of data for examining the topmost 150 km of the inner core, which is not well sampled by the previous body wave travel times. Record sections of waves traversing the inner core nearly parallel to the Earth's spin axis (polar paths) from three events, two beneath the South Sandwich Islands and one along the Macquarie Ridge, recorded at World Wide Standardized Seismograph Network, Canadian Network, and Long Range Seismic Measurements stations are analyzed. Our results suggest that the top 150 km of the inner core is only weakly anisotropic (less than 1%), with strong evidence indicating that the top 60 km is not anisotropic at all.
A rheologic model for wet crust applied to strike‐slip faults
Authors: Chester Frederick M.
Published: February 2004

The strength and stability of crustal faults are not adequately addressed by the widely used two‐mechanism rheologic models of the crust based on Byerlee's law and power law creep. The models neglect the fluid‐assisted mechanisms of deformation that are important to strength of the crust and do not describe the variation in rate dependence of friction that governs the stability of fault slip. Several distinct mechanisms of frictional slip are defined on the basis of variations in frictional behavior and microfabric of simulated fault gouge in laboratory experiments. State variable constitutive relations are used in a multiple‐mechanism formulation to describe the rate and temperature dependence of three friction mechanisms in wet quartz gouge at elevated temperatures. This multimechanism description of friction is substituted for Byerlee's law in the two‐mechanism models to generate a multimechanism rheologic model for the crust. The multimechanism model is used to predict the rate and temperature dependence of crustal strength and the slip rate and depth (temperature) conditions over which each mechanism dominates. Application of the model to strike‐slip faults in the crust illustrates that the thickness of the actively shearing zone within faults is a critical parameter governing fault strength and stability. The model predicts that frictional strength at midcrustal depths is significantly reduced relative to Byerlee's law only for thick fault zones. The reduction in strength is due to the operation of a strain rate sensitive friction mechanism involving combined cataclasis and solution transfer. The rheologic model predicts that only very thin faults display the rate dependent characteristics necessary for initiation of seismic slip to any significant depth in the crust.
Detection of geomagnetic jerks using wavelet analysis
Authors: Alexandrescu Mioara , Gibert Dominique , Hulot Gauthier , Le Mouël Jean‐Louis , Saracco Ginette
Published: February 2004

Wavelet analysis is applied to detect and characterize singular events, or singularities, or jerks, in the time series made of the last century monthly mean values of the east component of the geomagnetic field from European observatories. After choosing a well‐adapted wavelet function, the analysis is first performed on synthetic series including an “internal”, or “main”, signal made of smooth variation intervals separated by singular events with different “regularities”, a white noise and an “external” signal made of the sum of a few harmonics of a long‐period variation (11 years). The signatures of the main, noise, and harmonic signals are studied and compared, and the conditions in which the singular events can be clearly isolated in the composite signal are elucidated. Then we apply the method systematically to the real geomagnetic series (monthly means of Y from European observatories) and show that five arid only five remarkable events are found in 1901, 1913, 1925, 1969, and 1978. The characteristics of these singularities (in particular, homogeneity of some derived functions of the wavelet transform over a large range of timescales) demonstrate that these events have a single source (of course, internal). Also the events are more singular than was previously supposed (their “regularity” is closer to 1.5 than to 2., indicating that noninteger powers of time should be used in representing the time series between the jerks).
Patterns of seismic release in the southern California region
Authors: Press Frank , Allen Clarence
Published: February 2004

Southern California experiences earthquakes on the San Andreas system of vertical right‐lateral predominantly strike‐slip faults and on a second system of faults that includes thrusts, oblique‐slip, left‐lateral, and other faults. Pattern recognition and cluster analysis are used to analyze the catalog of earthquakes with magnitudes ≥5.5 from 1915 to 1994. We use pattern recognition to find a suite of traits that would characterize each of these two systems and distinguish them from each other. Both pattern recognition and cluster analysis show that epochs of seismic release occur in which one or the other system is the predominant form of earthquake activity. For the past 2 decades the second system has been the active one. Small changes in the direction of plate movements could account for this phenomenon. Seismic release on the San Andreas system is preceded by episodes of activity in the Great Basin or in the Gulf of California. Presumably, these episodes would represent extension in the former region and spreading and slip on transform faults in the latter.
Estimation of rod scale errors in geodetic leveling
Authors: Craymer Michael R. , Vaníček Petr , Castle Robert O.
Published: February 2004

Comparisons among repeated geodetic levelings have often been used for detecting and estimating residual rod scale errors in leveled heights. Individual rod‐pair scale errors are estimated by a two‐step procedure using a model based on either differences in heights, differences in section height differences, or differences in section tilts. It is shown that the estimated rod‐pair scale errors derived from each model are identical only when the data are correctly weighted, and the mathematical correlations are accounted for in the model based on heights. Analyses based on simple regressions of changes in height versus height can easily lead to incorrect conclusions. We also show that the statistically estimated scale errors are not a simple function of height, height difference, or tilt. The models are valid only when terrain slope is constant over adjacent pairs of setups (i.e., smoothly varying terrain). In order to discriminate between rod scale errors and vertical displacements due to crustal motion, the individual rod‐pairs should be used in more than one leveling, preferably in areas of contrasting tectonic activity. From an analysis of 37 separately calibrated rod‐pairs used in 55 levelings in southern California, we found eight statistically significant coefficients that could be reasonably attributed to rod scale errors, only one of which was larger than the expected random error in the applied calibration‐based scale correction. However, significant differences with other independent checks indicate that caution should be exercised before accepting these results as evidence of scale error. Further refinements of the technique are clearly needed if the results are to be routinely applied in practice.
Liquid segregation parameters from amphibolite dehydration melting experiments
Authors: Wolf Michael B. , Wyllie Peter J.
Published: February 2004

We have experimentally obtained some of the parameters necessary for understanding the segregation of silicate liquid from amphibolite undergoing dehydration melting at 1 GPa and 750–1000°C. The solidus for this calcic amphibolite (68% Mg‐Hb, 32% Pl (An90)) is <750°C. Amphibolites begin to melt at relatively high temperatures in the garnet‐absent field, but the solidus appears to backbend at ∼1 GPa, coincident with the garnet‐in boundary. Hornblende breakdown due to garnet formation releases H2O and causes melting. Thus, in the garnet‐present field (≥1 GPa), the amphibolite dehydration melting solidus may be coincident with the H2O‐saturated solidus. Liquid interconnectivity may be achieved at <900°C and <5 vol % liquid, based on both physical and chemical data from solid rock runs. Mass balance calculations from powdered rock runs suggest that small amounts (∼5–15 vol %) of hydrous (≥4 wt % H2O), low‐viscosity (103–104Pa s), heavy rare earth element‐depleted, felsic liquid may be segregated during amphibolite dehydration melting at ≥875°C. The rapid breakdown of coarse‐grained Hb cores may lead to the formation of transient H2O‐saturated liquids with even lower viscosities (∼102 Pa s). Although comprising only a small portion of the melting cycle, these H2O‐rich conditions may enhance the segregation of liquid by reduction of liquid viscosities and by mechanical effects on the restite (e.g., increased deformation and liquid fracturing). During anatexis of coarse‐grained (natural) rocks, transient conditions may control the initial stages of liquid segregation.
The elastic modulus of media containing strongly interacting antiplane cracks
Authors: Davis Paul M. , Knopoff Leon
Published: February 2004

We calculate the elastic modulus for up to 10000 randomly oriented, strongly interacting, nonintersecting, antiplane cracks that have a logarithmic size distribution for a range of concentrations c from 0 to 2. An antiplane boundary integral method is used to compute elemental dislocations on the crack faces. The ratio of slips in the cases of interacting cracks to those for isolated cracks has a nearly unit average. The effective modulus is well fit by a mean‐field model in which the cracks do not interact. Prom Gauss's theorem in two dimensions, we can show that the mean‐field approximation is appropriate for the problem of the modulus of a high concentration of randomly distributed cracks. The mean total field is the external stress field. The modulus as a function of concentration is then simply <μ> = μ0/(1 + c/2). This expression differs from the self‐consistent result in that the modulus does not become zero at finite concentration. It also differs from modifications to the self‐consistent method which predict an exponential decay of modulus with c.
Accurate fault mechanism determinations for a 1984 earthquake swarm at Campi Flegrei caldera (Italy) during an unrest episode: Implications for volcanological research
Authors: De Natale Giuseppe , Zollo Aldo , Ferraro Anna , Virieux Jean
Published: February 2004

We have analyzed 42 microearthquakes (1<ML <3) that occurred at Campi Flegrei volcanic area during an intense swarm‐like activity (more than 500 events in half a day) in the last period of strong ground uplift (1982–1984). Focal mechanisms of these earthquakes have been computed by a probability method, which uses P wave polarity and S wave polarization data. Composite focal mechanisms for different groups of earthquakes have also been computed by the same technique. Results show a well‐defined fault plane, in agreement with the elongation of the epicentral pattern. The orientation and dip of this fault plane, together with other observations about hypocenter locations and focal mechanisms of the whole seismicity accompanying the ground uplift episode, suggest the presence of an elliptical fracture system at the caldera center. This fracture system, dipping toward the center of the caldera, should represent the limit of the most recent caldera collapse, which is also indicated by a negative Bouguer gravity anomaly. Moreover, an important differential feature among various groups of earthquakes has been shown, namely, a rotation up to about 30° of the strike of the compressive stress axis, between the easternmost and the westernmost events. The resulting pattern indicates that the axis is always oriented towards the town of Pozzuoli, which roughly represents the very center of the caldera, and also the center of symmetry of the ground deformation. These observations strongly suggest that earthquakes are generated by the variation of the stress field associated with the deformation, along prefractured zones, because of their greater weakeness. This study provides, then, the first direct evidence that both seismicity and ground deformations have a common origin.
Introduction to Special Section: Magmatism and Extension
Authors: Metcalf Rodney V. , Smith Eugene I.
Published: February 2004

The relationship between magmatism and the formation of continental rift zones is the subject of much controversy. In particular, the cause and effect relationships between magmatism and extension and the mode of generation of magma during the process of extension are still hotly debated. This controversy served as the theme of a symposium on “Cenozoic Magmatism in the Colorado River Extensional Corridor and Adjacent Areas” and a field trip held as part of the Geological Society of America Cordilleran/Rocky Mountain Section meeting in Reno, Nevada [Metcalf et al., 1993]. It was clear from data presented at the symposium that a considerable amount of new information has become available regarding magmatism and extension since the last special section on this topic published by the Journal of Geophysical Research (June 1989).
Syntectonic generation and segregation of tonalitic melts from amphibolite dikes in the lower crust, Striding‐Athabasca mylonite zone, northern Saskatchewan
Authors: Williams M. L. , Hanmer S. , Kopf C. , Darrach M.
Published: February 2004

Vapor‐absent melting in a swarm of amphibolite dikes in the lower crust has produced segregations of tonalitic and trondhjemitic composition in a variety of textures and structures that dramatically illustrate the mechanisms of melt segregation during deformation. The 3.2 Ga Chipman dikes intrude the mylonitized Chipman tonalite within the Striding‐Athabasca mylonite zone of northern Saskatchewan. Dike emplacement spans a major sinistral transpressive ductile deformation and granulite facies metamorphic event. Older dikes are intensely folded and sheared; younger dikes are undeformed. Major and trace element analyses indicate that the dikes are tholeiitic basalts, similar in composition to midocean ridge basalt. Thermobarometry finds conditions of 750–850°C, 1.0 GPa. The youngest, most pristine dikes contain hornblende and plagioclase with minor clinopyroxene and garnet. Older, migmatitic dikes have tonalitic to trondhjemitic segregations spatially associated with garnet crystals. Where small, the segregations occupy tails or strain shadows on every garnet crystal. Where garnets and segregations are large, leucosomes form an interconnected network that extends into the host tonalite. Tonalitic pools, probably of dike origin, collect in boudin necks and fold hinges. The Chipman dikes are interpreted to have been emplaced, solidified, and partially melted during ductile shearing in the lowermost crust, perhaps near the base of an Archean island arc. They appear to offer an exceptional view of magma genesis where underplated mantle‐derived mafic magmas provide not only heat but also a component of material to new felsic magmas. Deformation is essential to the process, allowing access of mafic magma into the lower crust and facilitating the segregation of the new felsic melts from their source rocks.
Source parameters of the volcanic eruption earthquakes at Mount Tokachi, Hokkaido, Japan, and a magma ascending model
Authors: Nishimura Takeshi
Published: February 2004

We estimate the source parameters of the eruption earthquakes observed at Mount Tokachi, Hokkaido, Japan, in 1989, on the basis of the single force model. By fitting the theoretical source spectra with the observed ones, the peak amplitude of force and the source duration were estimated to be 107–1010 N, and 0.5–0.8 s, respectively. For the explosion earthquakes, which have the largest magnitudes among the eruption earthquakes, the peak amplitude of force is proportional to the square of source duration suggesting constant temperature inside the magma just beneath a crater. For small eruption earthquakes, which are small events accompanying weaker eruptions (not explosive), the source duration has a constant value independent of the peak amplitude of force; this suggests that the temperature inside the magma is proportional to the square of radius of the magma body. To give a physical explanation of the empirical relations of source parameters, we present a magma ascending model as a source of eruption earthquakes; a spherical batch of gas‐rich magma vertically migrates from a deep‐seated magma reservoir to the surface and is cooled by heat conduction. When the magma reaches the ground surface, it erupts and causes an eruption earthquake. This model predicts that the ascending magma has a same temperature at the surface when the magma radius exceeds a “critical magma radius” (CMR) which is determined from the physical properties of the magma or volcano. On the other hand, when the magma radius is less than CMR, the temperature increases with the radius. The value of CMR for Mount Tokachi is estimated at about 10 m. Comparing the empirical relations derived from seismic analysis with the magma ascending model, we conclude that the radius of an ascending batch of magma plays an important role in the generation of eruption earthquakes. The magma batch whose radius is larger than CMR causes an explosion earthquake, and the magma smaller than CMR causes a small eruption earthquake.
Three nonhomogeneous Poisson models for the probability of basaltic volcanism: Application to the Yucca Mountain region, Nevada
Authors: Connor Charles B. , Hill Brittain E.
Published: February 2004

The distribution and timing of areal basaltic volcanism are modeled using three nonhomogeneous methods: spatio‐temporal nearest neighbor, kernel, and nearest‐neighbor kernel. These models give nonparametric estimates of spatial or spatio‐temporal recurrence rate based on the positions and ages of cinder cones and related vent structures and can account for migration and shifts in locus, volcano clustering, and development of regional vent alignments. The three methods are advantageous because (1) recurrence rate and probability maps can be made, facilitating comparison with other geological information; (2) the need to define areas or zones of volcanic activity, required in homogeneous approaches, is eliminated; and (3) the impact of uncertainty in the timing and distribution of individual events is particularly easy to assess. The models are applied to the Yucca Mountain region (YMR), Nevada, the site of a proposed high‐level radioactive waste repository. Application of the HopkinsFtest, Clark‐Evans test, and K function indicates volcanoes cluster in the YMR at the >95% confidence level. Weighted‐centroid cluster analysis indicates that Plio‐Quaternary volcanoes are distributed in four clusters: three of these clusters include cinder cones formed <1 Ma. Probability of disruption within the 8 km2area of the proposed repository by formation of a new basaltic vent is calculated to be between 1 × 10−4and 5 × 10−4in 104years (the kernel and nearest‐neighbor kernel methods give a maximum probability of 5 × 10−4in 104years), assuming regional recurrence rates of 5–10 volcanoes/m.y. An additional finding, illustrating the strength of nonhomogeneous methods, is that maps of the probability of volcanic eruption for the YMR indicate the proposed repository lies on a steep probability gradient: volcanism recurrence rate varies by more than 2 orders of magnitude within 20 km. Insight into this spatial scale of probability variation is a distinct benefit of application of these methods to hazard analysis in areal volcanic fields.
Mössbauer spectroscopy analysis of iron‐containing minerals in the Chinese loess
Authors: Eyre John K. , Dickson Domonic P. E.
Published: February 2004

Chinese loess has been analyzed in bulk, using Mössbauer spectroscopy, in order to investigate the effects of climate on the overall iron‐containing mineralogy. Strongly weathered loess is found to differ from weakly weathered loess in two significant respects. It contains a much larger content of fine‐grained (superparamagnetic) hematite and the ratio of paramagnetic Fe3+/Fe2+ is substantially increased. The suggestion is that weathering results in the liberation of Fe from silicate minerals and the formation of fine‐grained hematite. Measurements of total iron content by atomic absorption spectrometry highlight that the strongly weathered loess has a higher content of iron than the weakly weathered loess, and that this is too large an effect to be explained by the leaching of carbonate. The indication is that the mineralogy of the dust input to the loess plateau varies as a function of climate. One possibility is that the grain size (and hence mineralogy) of the dust fluctuates in response to vegetational cover in proximal source areas. During interglacial times the vegetational cover would expand to stabilize these proximal sources, resulting in a bias to finer dust from more distant sources. Thus the mechanisms linking climate and mineralogy in the Chinese loess may be more complex than previously thought.
Experimental constraints on the dynamics of the partially molten upper mantle: 2. Deformation in the dislocation creep regime
Authors: Hirth Greg , Kohlstedt David L.
Published: February 2004

Experiments have been conducted to investigate the effect of melt on the creep behavior of water‐free olivine aggregates deformed in the dislocation creep regime. The influence of the melt phase is modest at melt fractions less than ∼0.04. However, at melt fractions > 0.04, the creep rate of melt‐added samples is enhanced by more than an order of magnitude relative to melt‐free aggregates. This unexpectedly large influence of melt on strain rate arises because deformation occurs by grain boundary sliding (GBS) accommodated by a dislocation creep process. Four observations support this hypothesis. (1) The strain rate enhancement observed in the dislocation creep regime can be related to the stress concentration caused by the reduction in the solid‐solid grain boundary area. (2) Both melt‐free and melt‐added samples exhibit strain rates indicating that deformation is limited by slip on (010)[100], the easiest slip system in olivine. (3) The GBS mechanism occurs near the transition between diffusion and dislocation creep. (4) Grains in specimens deformed in the GBS regime are not significantly flattened, even after ∼50% shortening. In melt‐free aggregates, a transition from the GBS mechanism to dislocation creep limited by slip on (010)[001], the hardest slip system, is observed with an increase in grain size. A transition to (010)[001] limited creep was not observed for partially molten aggregates because grain growth was inhibited by the presence of melt. The results of this study indicate that the viscosity of the upper mantle may decrease by at least an order of magnitude if the retained melt fraction exceeds 0.04 or if the onset of melting results in a reduction in grain size and a concomitant transition from (010)[001] to (010)[100] limited creep.
Comparative geothermometry of recent Hawaiian eruptions
Authors: Helz Rosalind Tuthill , Banks Norman G. , Heliker Christina , Neal Christina A. , Wolfe Edward W.
Published: February 2004

In this paper we compare lava temperatures measured using Cr‐Al thermocouples or infrared spectrometry with estimated quenching temperatures based on the glass geothermometry calibration of Helz and Thornber (1987). Comparative data are available for the April 1982 and September 1982 summit eruptions, the Pu'u O'o east rift eruption (1983–1986), all three eruptions at Kilauea, and the 1984 Mauna Loa eruption. The results show that quenching temperatures, based on the MgO contents of Kilauean glasses (TMgO), lie within ±10°C of field measurements using the infrared spectrometer for 85% of the samples. Where a Cr‐Al thermocouple was used, 90% of the field measurements lie within +1° to −11°C of TMgO for samples with Tfield > 1130°C. Samples where Tfield < 1130°C show larger divergence. The uncertainty in TMgO by itself is ±10°C, so the level of agreement between field measurements and TMgO is very good for Kilauean lavas. Systematic comparison of field measurements of temperature with glass geothermometry for the 1984 Mauna Loa eruption suggests that, although the field and glass temperatures lie within ±10°C of each other, the Kilauean TMgO calibration is nevertheless not appropriate for Mauna Loa glasses and that actual quenching temperatures for Mauna Loa samples will lie 10°–20°C higher than would be predicted from the Kilauea calibration curve. Consideration of possible effects of variable volatile content suggest that in most cases these are small. Samples erupted early in an eruption may reflect preeruptive water contents different enough to affect TMgO significantly, but later spatter samples and all flow samples appear to have equilibrated at low enough water contents for the calibration to be applicable. We conclude that the MgO‐based geothermometer can be applied to glassy Kilauean samples to give temperatures that generally will lie within ±10°C of a field measurement. Plots of glass MgO content versus time, if a suitable sample base is available, should give a thorough, quantitative record of the thermal history of any Kilauean eruption.
Strength of the lithosphere: Constraints imposed by laboratory experiments
Authors: Kohlstedt D. L. , Evans Brian , Mackwell S. J.
Published: February 2004

The concept of strength envelopes, developed in the 1970s, allowed quantitative predictions of the strength of the lithosphere based on experimentally determined constitutive equations. Initial strength envelopes used an empirical relation for frictional sliding to describe deformation along brittle faults in the upper portion of the lithosphere and power law creep equations to estimate the plastic flow strength of rocks in the deeper part of the lithosphere. In the intervening decades, substantial progress has been made both in understanding the physical mechanisms involved in lithospheric deformation and in refining constitutive equations that describe these processes. The importance of a regime of semibrittle behavior is now recognized. Based on data from rocks without added pore fluids, the transition from brittle deformation to semibrittle flow can be estimated as the point at which the brittle fracture strength equals the peak stress to cause sliding. The transition from semibrittle deformation to plastic flow can be approximated as the stress at which the pressure exceeds the plastic flow strength. Current estimates of these stresses are on the order of a few hundred megapascals for relatively dry rocks. Knowledge of the stability of sliding along faults and of the onset of localization during brittle fracture has improved considerably. If the depth to the bottom of the seismogenic zone is determined by the transition to the stable frictional sliding regime, then that depth will be considerably more shallow than the depth of the transition to the plastic flow regime. Major questions concerning the strength of rocks remain. In particular, the effect of water on strength is critical to accurate predictions. Constitutive equations which include the effects of water fugacity and pore fluid pressure as well as temperature and strain rate are needed for both the brittle sliding and semibrittle flow regimes. Although the constitutive equations for dislocation creep and diffusional creep in single‐phase aggregates are more robust, few data exist for plastic deformation in two‐phase aggregates. Despite the fact that localization is ubiquitous in rocks deforming both in brittle and plastic regimes, only a limited amount of accurate experimental data are available to constrain predictions of this behavior. Accordingly, flow strengths now predicted from laboratory data probably overestimate the actual rock strength, perhaps by a significant amount. Still, the predictions are robust enough that uncertainties in geometry, mineralogy, loading conditions and thermodynamic state are probably the limiting factors in our understanding. Thus, experimentally determined rheologies can be applied to understand a broad range of topical problems in regional and global tectonics both on the Earth and on other planetary bodies.
Constructional features of Troodos type oceanic crust: Relationships between dike density, alteration, magnetization, and ore body distribution and their implications for in situ oceanic crust
Authors: Hall James M. , Yang Jing‐Sui
Published: February 2004

Quantitative relationships are demonstrated between dike density and hydrothermal alteration, magnetization, location of volcanic massive sulfide (VMS) ore bodies and a number of other properties in a 40 by 15 km segment of the Extrusive Series of the Troodos Ophiolite. Low‐temperature alteration extends from the sediment‐extrusive interface to close to the 25% dike density surface, while the onset of greenstone type alteration is closely associated with the 50% dike density surface. These associations are explained in terms of the regional scale variations in the dike density contour surfaces following the form of the top of the gabbroic layer of the ophiolite. An upper magnetic zone, characterized by stable high remanence, terminates just below the 25% dike density surface. A deeper magnetic zone, characterized by high induced magnetization, straddles the top of the Sheeted Complex. Volcanic massive sulfide bodies are concentrated in a narrow depth range centered about 0.1 km above the 25% dike density surface. A model for the postridge crest evolution of ore bodies accounts for this optimum depth range for preservation. Physical property changes with dike density, and the depth distribution of VMS ore bodies and prospects for the ophiolite as a whole support this model. Similar relationships appear to be present in the Samail, Macquarie Island, Newfoundland and Chilean ophiolites and Icelandic crust. As a result of relationships found in the Troodos ophiolite, an alternative is proposed to the current lithologic profile for Ocean Drilling Program Hole 504B.
Paleomagnetic analysis of fold kinematics and implications for geological models of the Cantabrian/Asturian arc, north Spain
Authors: Stewart Simon A.
Published: February 2004

The Ponga Unit lies in the core of the Cantabrian/Asturian arc, a thrust system which was emplaced and then noncoaxially refolded during the Variscan Orogeny. Alternative models for thrust system geometry prior to refolding of the Ponga Unit have been proposed. Although the Ponga Unit becomes populated with steeply plunging folds under each model, the alternative models imply differing kinematic histories for these folds. Paleomagnetic data were acquired from 22 sites in grey and red Carboniferous limestones around these folds in order to determine the rotational histories of the structures and hence to identify the appropriate geological model for initial thrust system geometry. Low anisotropy of magnetic susceptibility indicated that the samples were suitable for rigid‐body rotation studies. Four remanence components were identified on stepwise demagnetization: A very low temperature, randomly orientated component V; a low‐temperature component L present between 50°C and 200°C (64/349, α95 = 4.4°), clustered around the Recent field in geographic coordinates; an intermediate‐temperature component I present between 200°C and 400–600°C, of reverse polarity and shallow inclination and showing significant between‐site dispersion; and a high‐temperature component H (30/342, α95 = 21°), lying close to the Eocene reference direction in geographic coordinates. The I component fails a fold test based upon a first deformation phase (D1) hanging wall anticline and is therefore a remagnetization. The I component consistently gives positive fold tests of the steeply plunging fold structures when the structures are unfolded about their plunging hinges. This demonstrates that these folds result from north‐south shortening of essentially homoclinal westerly dipping D1 hanging wall ramps and that alternative models involving steeply dipping interference folds arising from complex hanging wall deformation during D1 are invalid. A partial restoration of the thrust system based on the former model with the paleomagnetic data pinned to a calculated Stephanian reference direction shows that the thrust front was approximately north‐south in trend in the Ponga Unit, as it was farther south in the Esla Unit, where anticlockwise rotations and dextral wrench faulting have subsequently occurred. The data also record a tilt to the north of up to 30°, which has occurred incrementally since the end of Variscan deformation.
Lithospheric deformation inferred from teleseismic shear wave splitting observations in the United Kingdom
Authors: Helffrich George
Published: February 2004

Observations of splitting in teleseismic shear waves recorded at stations in the United Kingdom (UK) seismic network operated by the British Geological Survey are related to the tectonic structures and history in the UK. The combination of network density and tectonic variability in the UK permits an assessment to be made of the relationship between splitting and geology. Fast polarization directions are most commonly parallel to geologic features resulting from compressional or strike‐slip events, and splitting delay times mirror the increasing involvement of basement in deformation as observed through field investigation. Different orogenic episodes may be distinguished in the splitting orientations. A peak in delay times is suggested in the observations at the Iapetus Suture, a site of Paleozoic subduction now in the UK interior. These observations are consistent with the idea that continental anisotropy arises through coherent deformation of the lithosphere and is preserved long after orogenic deformation ceases.
Shear fracture patterns and connectivity at geometric complexities along strike‐slip faults
Authors: Du Yijun , Aydin Atilla
Published: February 2004

Shear fracture patterns and connectivity at geometric complexities such as echelon fault steps and fault bends along strike‐slip faults are investigated by simulating shear fracture propagation paths using a displacement discontinuity boundary element method together with a maximum distortional strain energy density criterion. It is shown that shear fracture patterns and connectivity depend on the specific geometric configurations, the applied stress orientations and the coefficient of friction. For a given initial configuration of the major faults, the subsequent patterns of the associated shear fractures and their linkage can be predicted if the remote state of stress and the coefficient of friction are known. The analyses of fault geometries commonly encountered in nature have important implications for evaluating the effect of faults on fluid flow and earthquake hazards associated with the geometric complexities along strike‐slip faults in the upper crust.
Thermoluminescence dating of late Pleistocene loess and tephra from eastern Washington and southern Oregon and implications for the eruptive history of Mount St. Helens
Authors: Berger Glenn W. , Busacca Alan J.
Published: February 2004

The first numeric dating of late Pleistocene loess from the Palouse region of eastern Washington is reported. This loess is interbedded with thin, distal tephra layers, some identified as Mount St. Helens products. Both the interbedded tephra layers and thick paleosols that have developed in the loess have previously been correlated across the region. A thicker correlative of one of these tephra layers (Mount St. Helens set C) was collected from Summer Lake, Oregon, for direct dating by thermoluminescence (TL). Polymineral, fine grain TL ages for loess from three chosen sections in Washington range from 17 to 83 ka. These imply ages for some tephra beds that are significantly older than implied by earlier radiocarbon (14C) dating. For example, TL ages of 17–21 ka for loess bracketing a tephra layer correlated to Mount St. Helens set S suggests a somewhat older age for this ash than the previously accepted 14C age of ∼15.5±0.5 ka cal yr (corrected to calendar years). TL ages of 46±6 (1σ) ka and 57±7 ka for loess above, and of 53 ± 7 ka and <75±6 ka below a tephra bed correlated to a Mount St. Helens set Cw layer, along with a TL age of 46±5 ka for fine‐silt‐sized glass from a Mount St. Helens set Cy bed in Oregon, revise upward from ∼38 ka cal yr (14C age is ∼36 ka B.P.) to ∼50 ka the age of this regionally important marker ash set. Several TL ages, taken together, imply a much longer eruptive history for Mount St. Helens than previously thought. A TL age of 83±8 ka just below a previously undated tephra layer from Mount St. Helens (EMSH ash) suggests that the eruptive history of the mountain extends back to at least 80 ka. Furthermore, age depth extrapolation through our sections to an ash (WA‐5C) probably correlated to Mount St. Helens suggests an eruptive history of at least 120 ka.
On the correlation of electrical conductivity and heat flow in Middle Valley, Juan de Fuca Ridge
Authors: Webb Spahr C. , Edwards R. Nigel
Published: February 2004

The near‐surface electrical conductivity has been mapped within an area of Middle Valley, a sediment‐filled axial valley at the northern end of the Juan de Fuca ridge. The conductivity in the uppermost 20 m of sediment was determined by analyzing the magnetic field signal transmitted between a source coil and a receiver that were towed along the seafloor. The instrument is a version of a time domain electromagnetic (TDEM) sounding system. The heat flow pattern within Middle Valley, with a large central anomaly above a basement high, is reproduced by the conductivity measurements, the result of enhanced pore fluid electrical conductivity due to higher near‐surface temperatures in the high heat flow regions. The high correlation between heat flow and conductivity requires that porosity as inferred from Archie's law must be nearly uniform in the central part of the study area. Porosities derived from the conductivity measurements are in close agreement with measurements from the Ocean Drilling Project (ODP) boreholes. Higher near‐surface porosities are required in the eastern part of the valley to match the observed heat flow, consistent with the higher porosity seen at ODP site 855. A small region of apparently lower porosity was detected to the west of the center of the valley.
Seismic moment and duration of recent large and great earthquakes
Authors: Cifuentes Inès Lucía
Published: February 2004

The methodology developed to verify the existence of a low frequency precursor to the great 1960 Chilean earthquake is applied to International Deployment of Accelerometers (IDA) station data in the 1–6 mHz band to determine the seismic moment and temporal centroid of 22 recent large and great earthquakes, including the 1989 Macquarie Ridge earthquake which has been reported to have a low‐frequency precursor (Imhlé et al., 1993). No clear evidence of precursors to any of these events is observed in the 1–6 mHz band. The seismic moment of large unilateral earthquakes of a given type of faulting is shown to be approximately proportional to the square of the duration. These data are observed to be well fit by this relation.
Initiation and propagation of shear zones in a heterogeneous continental lithosphere
Authors: Tommasi Andréa , Vauchez Alain , Daudré Bertrand
Published: February 2004

Continental plates represent rheologically heterogeneous media in which complex finite strain fields may develop due to interaction of plate tectonic processes with intraplate heterogeneities. Such a deformation pattern is displayed by the Borborema shear zone system in northeastern Brazil. It involves continental‐scale, curvilinear, E‐W trending right‐lateral transcurrent shear zones that branch off from a major NE trending, right‐lateral strike‐slip deformation zone formed at the northern termination of the São Francisco craton and that finally terminate in transpressive metasedimentary belts. We suggest that this strain field results from the compressional deformation of a highly heterogeneous continental lithosphère composed of a stiff domain (craton) and rheologically weaker domains (basins). The effect of a low‐viscosity domain on the deformation of a continental plate and, in particular, the perturbation induced by this domain on the development of a shear zone formed at the termination of a stiff block were investigated using numerical modeling. The low‐strength domain induces an enhanced strain localization, and the geometry of the shear zone is significantly modified. It is either split, forming a branched shear zone system in which one branch maintains its original orientation while the other rotates toward the low‐viscosity domain, or completely rotated toward the weak block. The perturbation of the finite strain field depends on the ability of the weak domain to accommodate deformation, which is controlled by its initial viscosity contrast relative to the surrounding lithosphere, its orientation relative to the convergence direction, and its distance from the shear zone initiated at the termination of the stiff block. The interaction between imposed boundary conditions (tectonic forces and plate geometry) and the internal structure of the plate may give rise to highly heterogeneous strain fields, as exhibited by the Borborema shear zone system, in which intraplate rheological heterogeneities induce the development of branched or sinuous shear zones. A heterogeneous continental plate subjected to a normal convergence may therefore display significant lateral variations in strain intensity, with shear zones bordering nearly undeformed blocks, and in deformation regimes and vertical strains that would result in contrasting metamorphic and uplift histories.