bluuebunny/crossref_metadata_2025 huggingface dataset:
A high‐resolution seismic reflection/refraction study of the Chugach‐Peninsular Terrane Boundary, southern Alaska
Authors: Brocher Thomas M. , Fisher Michael A. , Geist Eric L. , Christensen Nikolas I.
Published: February 2008

We present results from a high‐resolution seismic refraction analysis of the shallow (approximately 2 km) crustal structure along the 107‐km‐long Trans‐Alaska Crustal Transect Chugach reflection line in southern Alaska and a comparison with laboratory measurements of field samples. The refraction analysis includes the two‐dimensional interpretation of several thousand first‐ and secondary‐arrival travel times digitized from 1024‐channel split‐spread common shot gathers. The velocity model derived from this analysis better defines the location and geometry of terrane boundaries than does the normal incidence reflection section and agrees well with surface mapping of lithologies. Furthermore, the model predicts travel times within 100 ms of the reflection times recorded from the base of the Quaternary on the Chugach reflection section. Thicknesses of Quaternary deposits, with velocities between 1.1 and 2.0 km/s, correlate inversely with the quantity of observed lower crustal reflections on the Chugach section, suggesting that the presence or absence of these sediments in sufficient thickness exerted primary control on the quality of the deeper portion of the section. There is a significant velocity contrast between crystalline rocks across the Border Ranges fault (5.0 versus 5.6 km/s), the major contact between the Chugach and Peninsular terranes, in agreement with laboratory measurements of field specimens. In the Peninsular terrane the modeling indicates that an unnamed fault delimiting the southern flank of the Copper River Basin dips steeply northward at 50° and has about 1300 m of vertical offset. Laboratory measurements document a maximum velocity anisotropy of 20% for phyllitic schists of the Valdez Group in the Chugach terrane. In agreement with the observed E‐W strike and near‐vertical dip of the Valdez Group, we determined a significant (14%) velocity anisotropy for ray paths oriented N‐S versus NE‐SW.
Introduction to Special Section on Alkaline Arc Magmatism
Authors: Box Stephen E. , Flower Martin F. J.
Published: February 2008

Alkaline magmas are an uncommon but intriguing member of many of the Earth's volcanic arc systems. This peculiar magmatism is generally associated with perturbations to the “normal” tectonic setting of arc magmatism. In December 1986, we convened a symposium at the AGU Fall Meeting to explore connections between the tectonic environments of alkaline arc magmatism and specific petrologic processes associated with their genesis. Twenty‐five contributions reported occurrences of alkaline arc magmas, including modern and ancient examples from both intraoceanic and continental margin volcanic arcs.
Migration of fluid phases and genesis of basalt magmas in subduction zones
Authors: Tatsumi Yoshiyuki
Published: February 2008

Dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of H2O in the production of magmas at convergent plate boundaries. The subduction of oceanic lithosphere, occurring with increasing pressures and rising temperatures, causes liberation of H2O from the slab. Amphibole, which can be stable to the highest PT conditions among hydrous phases in the slab, decomposes at around 90 km depth. It follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab‐derived H2O is not a direct trigger for the production of arc magmas. Instead, the H2O released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals (serpentine, talc, amphibole, chlorite, and phlogopite). This hydrated peridotite is dragged downward on the slab toward higher PT regions and releases H2O to shallower potential magma source regions in the mantle wedge. Combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge, it is concluded that those minerals decompose beneath the forearc region. On the other hand, high PT experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front. Phlogopite in the peridotite decomposes to release H2O at a deeper level (about 200 km). H2O liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites. Along with the migration of H2O through the above processes, subduction components, especially large ion lithophile elements, can be overprinted on the magma source region, which governs the geochemical characteristics of arc magmas.
Anelasticity of the Iceland Plateau from surface wave analysis
Authors: Chan W. Winston , Sacks I. Selwyn , Morrow Richard
Published: February 2008

We study the anelasticity of the asthenosphere beneath the youngest seafloor of the Iceland Plateau region using spectral analyses of surface waves. The epicenters and station geometry naturally define a fan‐shooting experiment with pure path propagation in regions with different seafloor ages. Both Rayleigh and Love waves are used in this study to help constrain the source parameters needed for a reliable assessment of Q. The inversion results indicate an approximately 130‐km‐thick layer of Qβ of about 7 for the asthenosphere of the youngest region of 0–5 Ma. For the region of age 5–10 Ma, Qβ is about 50 and increases to over a hundred for the oldest zone (10 Ma). These attenuation values, when combined with laboratory experiments, indicate a partial melt of 3% in the asthenosphere near the spreading ridge.
Stable isotope signatures of water‐rock interaction in mid‐ocean ridge hydrothermal systems: Sulfur, oxygen, and hydrogen
Authors: Bowers Teresa Suter
Published: February 2008

Reaction path models of seawater‐basalt interaction at mid‐ocean ridges similar to those presented by Bowers and Taylor (1985) are extended to include sulfur isotopic exchange equilibria. The effects of variable contribution of sulfur from basalt and of high‐ and low‐temperature reduction of sulfate in the fluid are considered. Results of the model calculations are compared with δ34S analyses of disseminated sulfides in altered basalts, chimney sulfides, and fluids collected from mid‐ocean ridge hot springs. The calculations indicate that under the physical conditions of water‐rock interaction in mid‐ocean ridge circulation systems, the overwhelming influence on the sulfur isotope signature of exiting fluids is basaltic sulfur. No trace of δ34S enrichment by reduction of seawater sulfate is recorded in hot spring fluids unless the reduction occurs at high temperature (350°C) or near the venting areas. A comparison of recent oxygen and hydrogen isotope data from several vent sites suggests that contributions of altered basalt and fluid with a significant history of low‐temperature interaction with basalt can be qualitatively assessed.
Evidence for dynamic withdrawal from a layered magma body: The Topopah Spring Tuff, southwestern Nevada
Authors: Schuraytz Benjamin C. , Vogel Thomas A. , Younker Leland W.
Published: February 2008

The Topopah Spring Tuff is a classic example of a compositionally zoned ash flow sheet resulting from eruption of a compositionally zoned magma body. Geochemical and petrographic analyses of whole rock tuff samples indicate that the base of the ash flow sheet and the predominant volume of erupted material consist of crystal‐poor high‐silica rhyolite, with a gradational transition into overlying crystal‐rich quartz latite. However, major and trace element analyses of glassy pumice lumps and microprobe analyses of their silicate and oxide phenocrysts provide closer approximations of the chemical and thermal gradients within the magma body. The gradients inferred from these data indicate that the transition from high‐silica rhyolitic to quartz latitic magma within the chamber was abrupt, rather than gradational, with a distinct liquid‐liquid interface separating the contrasting magma layers. Compositionally and texturally distinct pumice lumps are present throughout the ash flow sheet. The degree of heterogeneity within and among pumice lumps increases with stratigraphic height, becoming most pronounced in the uppermost quartz latite, where the chemical variability among pumice lumps is as great as that of the entire ash flow sheet. These observations are consistent with fluid dynamic models in which the velocity field developed near the entrance region of the vent(s) results in simultaneous withdrawal of magma from all points of a continuously expanding lateral and vertical region within the chamber. The abrupt transition to chemically bimodal pumice types near the top of the ash flow sheet, dominated by those of quartz latitic composition, implies that the interface between the magma layers remained relatively stable until draw‐down breached the interface and preferentially erupted hotter, more mafic magma along with lesser amounts of the remaining high‐silica rhyolitic magma.
Long‐wavelength magnetic anomalies in the Caribbean: Plate boundaries and allochthonous continental blocks
Authors: Counil Jean‐Louis , Achache Jose , Galdeano Armand
Published: February 2008

Vector magnetic anomaly maps are derived at satellite altitude from Magsat data over the Caribbean and Middle America. To minimize smoothing of the anomaly field caused by averaging the data over the whole altitude range of Magsat measurements, two maps are derived in two nonoverlapping altitude ranges: 350–420 and 420–550 km. A composite map of the vertical component of the anomaly field is then computed at ground level. This map displays large positive anomalies associated with fast subduction zones. These anomalies define discontinuities in the downgoing slab associated with seismic gaps. Positive anomalies are also consistently observed above allochthonous blocks of continental origin in the Caribbean area and suggest that the average magnetization is larger in the continental lithosphere. Elongated magnetic lows are observed above transform boundaries of the Caribbean plate. The Cayman Trough anomaly is consistent with magnetization of the order of 1 A/m in the upper mantle. Demagnetization of the lithosphere along the Bocono‐El Pilar fault system which bounds the Caribbean plate to the south may be accounted for by frictional heating along shear zones. The pattern of anomalies along the southern margin of the United States is related to its accretionary tectonic history. Various estimates of the vertically integrated magnetization of both oceanic and continental lithospheres are computed using these anomalies and provide constraints on the vertical distribution of lithospheric sources of Magsat anomalies.
A correlation between the b value and the fractal dimension of earthquakes
Authors: Hirata Takayuki
Published: February 2008

Seismicity has fractal structures in space, time, and magnitude distributions, as expressed by the fractal dimension D, Omori's exponent p, and the b value, respectively. We expect that there is correlation among these scaling parameters. Aki (1981) speculated that there is a relation D = 3b/c (c = 1.5) between the b value and the fractal dimension D of fault planes. We point out that Aki's fractal dimension corresponds to the capacity dimension D0 and may be compared with the correlation dimension D2, obtained from the spatial distribution of earthquakes. By analyzing the actual earthquake catalogue, we calculated the fractal dimension D2 and the b value. Our result does not support Aki's speculation that D0 = 3b/c, but shows, on the contrary, that there is a negative correlation (D = 2.3 − 0.73b) between the b value and the fractal dimension of the spatial distribution of earthquakes in the Tohoku region.
Geochemistry of exposed granulite facies terrains and lower crustal xenoliths in Mexico
Authors: Roberts Sheila J. , Ruiz Joaquin
Published: February 2008

The trace element compositions of 13 samples from exposed granulite facies terrains and 17 granulitic lower crustal xenoliths were determined using an inductively coupled plasma‐mass spectrometer to constrain the composition and evolution of the Mexican lower crust. Most xenoliths found are pyroxeneplagioclase orthogneisses, although quartzo‐feldspathic paragneisses form up to 50% of the xenolith population in some areas. Lithologies in the exposed terrains consist of pelitic and carbonate‐rich paragneisses, charnockites, and less abundant basic, intermediate, and felsic orthogneisses. Both xenoliths and exposed granulites generally have low abundances of incompatible elements such as K, Rb, Th, and U. The xenoliths, however, often have lower abundances of these elements than the exposed granulites. High K/Rb ratios, coupled with an inverse correlation between K2O and K/Rb, indicate that most samples from the Mexican lower crust are depleted in both K and Rb. Furthermore, Th/U ratios similar to upper crustal materials and high La/Th ratios suggest depletion in both Th and U relative to average crustal material. Removal of a melt phase, either after the crystallization of cumulates or following a partial melting event, can explain many of the chemical features observed and may be responsible for the elemental depletions. Some samples, however, appear to have been unaffected during their residence in the lower crust and chemically resemble their presumed protoliths. Available geobarometric data suggest that xenoliths equilibrated near the crust/mantle boundary (∼10 kbar) and represent the lowermost crust, while the exposed granulites equilibrated at shallower crustal levels (∼7 kbar). Thus the differences observed between the xenoliths and exposed granulites indicate that there may be chemical and lithologic zonation in the Mexican lower crust. The lowermost crust in Mexico consists of crystallized basaltic liquids, cumulates, and/or residue after the removal of a partial melt as well as garnet‐rich metasediments.
Experimental constraints on the generation of FeTi basalts, andesites, and rhyodacites at the Galapagos Spreading Center, 85°W and 95°W
Authors: Juster Thomas C. , Grove Timothy L. , Perfit Michael R.
Published: February 2008

One‐atmosphere experiments conducted on a synthetic glass similar to Galapagos Spreading Center (GSC) FeTi basalt (POO.82N2), (Byerly et al., 1976) define liquid lines of descent at fO2 values between the quartz‐fayalite‐magnetite (QFM) buffer and 2 log units more oxidizing than the nickle‐nickle oxide (NNO) buffer. The experiments provide a framework for understanding the development of FeTi basalts by fractionation at near‐ocean floor conditions. GSC lavas from near 85°W initially follow a compositional trend, distinguished by FeO° (= FeO + 0.9Fe2O3) enrichment and SiO2 depletion, which is nearly identical to the trend observed in experiments at QFM to which olivine seeds were added. This compositional trend can be produced by crystallization along an olivine → pigeonite reaction boundary in a shallow crystal‐rich magma reservoir. In contrast, GSC lavas from 95°W do not mimic the 1‐atm liquid line of descent, but appear to have fractionated at somewhat higher pressure. Basaltic liquids from 95°W underwent fractional crystallization at 1–2 kbar, did not experience FeO° enrichment along an olivine → low‐Ca pyroxene reaction boundary, and developed FeO° enrichment concomitant with SiO2 enrichment. This compositional variation is consistent with a differentiation process in which crystals are continually removed from contact with liquid. Rhyodacites from 95°W cannot be related to the basalts and FeTi basalts recovered at 95°W by shallow‐level crystal fractionation. Instead, rhyolite liquids were formed either by fractionation of similar parents at greater depth and higher PH2O, or formed by fractionation of different parents. Andesite formed by mixing between basaltic and rhyodacitic liquids. As a consequence, mixed andesites define a trend of decreasing P2O5 which has been previously interpreted to represent apatite saturation at approximately 0.22 wt% P2O5, significantly earlier than at 85°W (where P2O5 decreases at approximately 0.7 wt% P2O5). Our experiments suggest that the fO2 when titanomagnetite first saturates at the GSC was approximately at the NNO buffer. Together with the Fe2O3/FeO data of Byers et al. (1983, 1984) and Christie et al. (1986), this requires an increase in fO2 during crystallization in excess of that produced during closed‐system fractional crystallization. We suggest that this increase in fO2 results from interaction with oxidizing surroundings in an open‐system process.
Seismic velocity structure at Deep Sea Drilling Project site 504B, Panama Basin: Evidence for thin oceanic crust
Authors: Collins John A. , Purdy Michael G. , Brocher Thomas M.
Published: February 2008

We present an analysis of wide‐angle reflection/refraction data collected in the immediate vicinity of Deep Sea Drilling Project hole 504B in the Panama Basin, currently the deepest drill hole (1.288 km) into oceanic crust. The data were acquired with a 1785 inch3 air gun array and fixed‐gain sonobuoy receivers and consist of four intersecting profiles shot along three different azimuths. Near‐normal‐incidence, multichannel seismic (MCS) reflection data were acquired simultaneously. Observed P and S wave arrivals out to maximum ranges of 30 km provide constraints on the velocity structure of the middle and lower crust and on total crustal thickness. Comparison of the travel times and amplitudes of the P and S wave arrivals on all four profiles revealed important similarities which were modeled using the reflectivity synthetic seismogram method. Forward modeling shows that in contrast to standard oceanic velocity models, a velocity‐depth profile that better explains the observed data is characterized by high‐velocity gradients (up to 0.6 km/s/km) in the middle crust, a 1.8‐km‐thick low‐velocity zone (Vp = 7.1–6.7 km/s) immediately above Moho, and a total crustal thickness of only 5 km. Interpretation of the high‐velocity gradients in the middle crust is constrained by the observation of P wave amplitude focusing at ranges of 16–19 km. Although not as well developed in comparison to the P wave arrivals, S wave arrivals show similar focusing. Total crustal thickness is constrained by the combined interpretation of a P wave, wide‐angle reflection event observed at a range of 16–28 km and an MCS reflection event with a crustal travel time of 1.4–1.5 s. Although these events cannot be directly correlated, their travel times are consistent with the assumption that both have a common origin. Amplitude modeling of the wide‐angle event demonstrates that these events are generated at the Moho.
Evidence for the variation of stress drop between normal and thrust faulting earthquakes in Italy
Authors: Cocco Massimo , Rovelli Antonio
Published: February 2008

The horizontal components of 45 strong motion accelerograms recorded in Friuli, Italy, from May to September 1976, have been analyzed in order to estimate the stress drop of eight thrust faulting earthquakes. Two different stress parameters, the Brune stress drop Δσ and the apparent stress σa, have been considered. The Friuli main shock has a strong stress drop (Δσ ≈ 800 bars), while the aftershocks have stress drops which range from 250 to 400 bars. Five accelerograms written by the Montenegro, Yugoslavia, earthquake of April 1979 have also been analyzed. This thrust faulting earthquake had a stress drop of Δσ ≈ 900 bars. The stress drops of these thrust faulting earthquakes are compared to the stress drops determined by Rovelli et al. (1988a) for nine normal faulting earthquakes which occurred in the Apennines region from 1979 to 1984. The radiated energies and apparent stresses have also been calculated for the set of Apennine earthquakes. The thrust faulting earthquakes in the Alpine‐Dynaride region exhibit higher values for both the Brune and the apparent stress than the normal faulting earthquakes in the Apennines region. Excluding the Friuli and Montenegro main shocks, the Brune stress drops of the thrust faults are approximately 3 times those of the normal faults, while the apparent stresses are twice as large. This difference is statistically significant at a confidence level of 95%. The same difference also emerges from the scaling of the peak ground motions recorded in these two regions, suggesting an enhanced severity of the seismic input in the Alpine‐Dynaride environment compared with the Apennines.
Seismically induced water level fluctuations in the Wali Well, Beijing, China
Authors: Liu Lan‐Bo , Roeloffs Evelyn , Zheng Xiang‐Yuan
Published: February 2008

Water level records from a high‐speed recorder in the Wali well have been compared with long‐period seismograms to study the response of the well to seismic waves. Water level oscillations caused by two teleseismic events have been studied. The data show that the peak gain of well water level relative to aquifer pressure occurred at periods of 19–23 s for both events, in contrast with previous theory that predicted peak gain at periods of 35–39 s for a well with 565 m open to the aquifer. We present a more exact analysis of the vertical flow field in the open part of the well bore, which yields a theoretical response with peak gain much closer to the observed periods. The more exact analysis shows that during the seismically induced oscillations little flow takes place more than 200 m below the top of the aquifer.
Source complexity of the October 1, 1987, Whittier Narrows earthquake
Authors: Bent Allison L. , Helmberger Donald V.
Published: February 2008

Long‐ and short‐period body waveform data recorded at regional and teleseismic distances are modeled to obtain the source parameters of the Whittier Narrows earthquake of October 1, 1987. Although the long‐period data can be satisfied by a simple thrust earthquake on a plane striking 280° at a depth of 14 km, we require a double source solution to satisfy both the long‐ and short‐period teleseismic data. The first subevent has a focal mechanism of strike (θ) = 250°, dip (δ) = 30°N, and a slip vector (λ) = 90°; the second is θ = 280°, δ = 40°N, and λ = 98°. The second event occurred 1.0 s after the first, was 5 times larger (1.2 × 1025 dyn cm for the second, 0.23 × 1025 for the first) in terms of seismic moment and was also shallower (12 km, whereas the first subevent was at 15 km). The two subevents probably occurred on a north dipping listric fault but may have occurred on two subparallel faults. These features were determined by studying a variety of phases including teleseismic P and SH and regional Pnl. The Whittier Narrows earthquake was a high stress drop event, but a single value for stress drop cannot be obtained because of the complex nature of the source. If we define the fault area as the aftershock area, we obtain a stress drop of about 750 bars.
Linear perturbation analysis of a shear‐loaded asperity
Authors: Gao Huajian
Published: February 2008

The elastic problem of a shear‐loaded asperity on an infinite fault plane is studied based on a linear perturbation approach developed by Rice (1985). The fault plane is broken everywhere except at an asperity whose shape differs modestly from a circle. The stress concentration along the bounding edge of the asperity due to remotely applied displacements and/or forces is analyzed in terms of in‐plane shear mode stress intensity factor K2 and antiplane shear intensity factor K3. The asperity configuration corresponds to an external crack which is of importance in understanding the general shear mode fracture of elastic connections. Solutions by Fourier series are developed for asperities subjected to remotely applied displacements and forces. Assuming the fracture process on the fault plane is governed by the intensity of maximum shear stress concentration, we use the perturbation results to study the configurational stability of a circular asperity. It is found that the circular shape is configurationally stable when rigid rotations are fully suppressed at remote field, and the crack front is thus expected to remain circular during quasi‐static growth.
Thermal development of low‐pressure metamorphic belts: Results from two‐dimensional numerical models
Authors: Hanson R. Brooks , Barton Mark D.
Published: February 2008

We have used two‐dimensional numerical models and analytical solutions to heat flow equations to investigate the mechanisms and the spatial and temporal development of regionally extensive low‐pressure facies‐series metamorphism (LPM). Two‐dimensional models are necessary for describing the evolution of isotherms in the crust where lateral heat flow and local time‐dependent heat sources are important. The models demonstrate that felsic intrusions can produce regionally extensive LPM where their abundance through time in the upper crust exceeds approximately 50%. Evenly spaced intrusions elevate vertical metamorphic gradients in regions between intrusions by ∼5° to 10°C km−1 over background values at ∼40% abundance and by ∼15° to >30°C km−1 at ∼70% abundance. Other mechanisms, such as intrusion of mafic magmas in the lower crust, crustal extension, or aqueous fluid advection, cannot independently produce maximum temperatures in such regions. If coeval with intrusion, these other processes will contribute to maximum temperatures; however, the distribution of isotherms and the timing of metamorphism in the upper crust are governed by the shallow intrusions. Intrusion width, abundance, and composition, and timing of emplacement of nearby intrusions have the dominant influences on maximum temperatures between intrusions. For 10‐km‐wide intrusions, abundances necessary for production of regionally extensive LPM are ∼15% less for gabbros than for granites and ∼20% less when intrusions are emplaced simultaneously than when complete cooling occurs between intrusive events. Intrusion shape, height, convection (magmatic and hydrothermal), and recharge (open system magma chambers) have a significantly less effect on thermal evolution. The total area affected by intrusions depends on their final distribution, not the magmatic flux. In the development of low‐pressure metamorphic terranes with abundant intrusions, temperatures in the upper crust are significantly elevated only near recently emplaced intrusions. Consequently, the final distribution of metamorphic grades has little resemblance to the distribution of isotherms at any time—a fundamental difference with LPM in regions lacking abundant shallow intrusions, where metamorphism likely occurs concurrently over large regions. This model of the development of widespread LPM by the juxtaposition through time of local, short‐lived thermal maxima should be considered when interpreting temperature‐dependent phenomena (for example, deformation) in terranes with abundant intrusions.
Structure of the lower crust beneath the Carolina trough, U.S. Atlantic continental margin
Authors: Tréhu Anne M. , Ballard A. , Dorman L. M. , Gettrust J. F. , Klitgord K. D. , Schreiner A.
Published: February 2008

Data from three large‐offset seismic profiles provide information on the crustal structure beneath the Carolina trough. The profiles, obtained by the U.S. Geological Survey, the Naval Oceanographic Research Development Agency, and the Scripps Institution of Oceanography in 1985, were oriented parallel to the trough and were located (1) seaward of the East Coast Magnetic Anomaly (ECMA), which is generally thought to represent the boundary between oceanic and continental crust; (2) along the axis of the trough between the ECMA and the hinge zone, which is thought to reflect the landward limit of highly stretched and altered transitional crust; and (3) along the Carolina platform landward of the basement hinge zone on crust thought to have been thinned only slightly during rifting. These data constrain the velocity structure of the lower crust and provide evidence for a thick lens of high‐velocity (>7.1 km/s) lower crustal material that extends beneath the Carolina trough and the adjacent ocean basin. This lens reaches a maximum thickness of about 13 km beneath the deepest part of the trough, thins to about 5 km seaward of the ECMA, and is either very thin or absent landward of the hinge zone. It is interpreted to represent material that was underplated beneath and/or intruded into the crust during the late stage of continental rifting and that led to an anomalously thick plutonic layer during the early seafloor spreading phase. These data thus support the recent conclusions of White et al. (1987b) and Mutter et al. (1988) that the initiation of seafloor spreading is attended in many, if not most, cases by the generation of an anomalously large volume of melt.
Dynamic rupture modeling with laboratory‐derived constitutive relations
Authors: Okubo Paul G.
Published: February 2008

A laboratory‐derived state variable friction constitutive relation is used in the numerical simulation of the dynamic growth of an in‐plane or mode II shear crack. According to this formulation, originally presented by J. H. Dieterich, frictional resistance varies with the logarithm of the slip rate and with the logarithm of the frictional state variable as identified by A. L. Ruina. Under conditions of steady sliding, the state variable is proportional to (slip rate)−1. Following suddenly introduced increases in slip rate, the rate and state dependencies combine to produce behavior which resembles slip weakening. When rupture nucleation is artificially forced at fixed rupture velocity, rupture models calculated with the state variable friction in a uniformly distributed initial stress field closely resemble earlier rupture models calculated with a slip weakening fault constitutive relation. Additional rupture models are calculated in which rupture nucleation is achieved naturally, with numerical simulations of the quasi‐static response of the fault leading to the onset of unstable, dynamic rupture. When rupture nucleation with the state variable friction law takes place naturally, a large fraction of the fault accelerates before accelerating slip is concentrated in what ultimately becomes the rupture nucleation patch. The state evolution accompanying this accelerating slip leads to higher average rupture speeds or a more rapid rupture acceleration to nearPwave rupture speeds. Rupture models are also calculated for the seismological asperity problem, that is, the failure of a highly stressed fault patch surrounded by a region of zero stress drop. Dynamic overshoot of slip into the region of zero stress drop roughly agrees with a simple energy balance analysis; the final size of the rupture is proportional to the square of the size of the high stress patch. Earlier frictional stability analyses have led to the definition of a critical fault patch size for rupture nucleation. This critical patch size is generally different from critical crack lengths determined from crack tip energy balance considerations applied to a simpler slip weakening law. In the model calculations, dynamic rupture does not nucleate if the starting patch size is less than the critical patch size. This is consistent with the frictional stability analyses. Thus these model calculations suggest that dynamic rupture following a state variable friction relation is similar to that following a simpler fault slip weakening law. However, when modeling the full cycle of fault motions, rate‐dependent frictional responses included in the state variable formulation are important at low slip rates associated with rupture nucleation. The critical rupture nucleation dimension appropriate for a slip weakening fault does not predict the critical nucleation dimension for a state variable fault.
Free thermohaline convection in sediments surrounding a salt column
Authors: Evans David G. , Nunn Jeffrey A.
Published: February 2008

Complex groundwater convection patterns are possible near salt domes because groundwater is subject to both lateral heat and salinity gradients. In order to assess the mechanisms responsible for driving convection near salt domes we use dimensional analysis and numerical simulations to investigate coupled heat and salt transport in homogeneous sediments surrounding a cylindrical salt column. The dimensional analysis does not require the Boussinesq assumption. The coupled heat, solute, and groundwater transport equations are controlled by three dimensionless parameters: the Rayleigh number, the Lewis number, and the buoyancy ratio. The buoyancy ratio is the ratio of salinity to temperature effects on groundwater density, and it directly affects the groundwater flow equation. A finite difference numerical multigridding algorithm is used to iteratively solve the coupled transport equations. The multigridding technique was about 3 times faster than a point‐wise successive overrelaxation solution. Boundary conditions for the numerical simulations were adjusted to represent different contrasts in the thermal gradient between the salt and the overlying sediments. The contrast in thermal gradient is parameterized by the thermal conductivity ratio and is responsible for isotherms being elevated near the salt. The analysis suggests that a wide range of convective flow patterns are possible, with flow occurring either up or down along the salt flank. The sense of convection is dependent mainly on the value of the buoyancy ratio and how sharply isotherms are pulled up near the salt. These factors in turn depend on the regional salinity variation, the time since diapirism, and the thermal conductivity of water saturated sediments. While this analysis provides useful insight into the mechanisms driving free convection near salt domes, the assumptions about medium and fluid properties may limit the applicability of dimensional analysis in simulating flow in specific geologic settings.
Thermal conductivity measured with a line source between two dissimilar media equals their mean conductivity
Authors: Lee Tien‐Chang
Published: February 2008

Apparent thermal conductivity ka, measured with a line source lying between two dissimilar media, equals the mean conductivity of the two media, ka = (k1+k2)/2. This theoretically derived formula is substantiated by data in the literature. With reference to a medium of known conductivity k1, the relation justifies the development of an inexpensive, simple, rapid, and reliable method to determine conductivity k2 of samples which cannot be penetrated with a conventional needle probe. Measurements by methods of half‐space needle probe or half‐space line source have been made in the past by discarding the contribution of k1, resulting in overestimation of conductivity if correction through calibration is not made. For example, with respect to an epoxy half‐space probe, a sample of fused silica (k2 = 1.36 W m−1 K−1) could be overestimated by 6.5% if the finite conductivity of epoxy (k1 = 0.087 W m−1 K−1) is neglected.
Evolution of the eastern Indian Ocean since the Late Cretaceous: Constraints from Geosat altimetry
Authors: Royer Jean‐Yves , Sandwell David T.
Published: February 2008

We propose a new model for the tectonic evolution of the eastern Indian Ocean from the Late Cretaceous to the present. Two types of data are used to improve previously published reconstructions. First, recent reinterpretations of seafloor magnetic anomalies, between Australia and Antarctica and in the Wharton Basin, provide new constraints on spreading rates and the timing of major reorganizations. Second, vertical deflection profiles (i.e., horizontal gravity anomaly), derived from 22 repeat cycles of Geosat altimeter data, reveal the tectonic fabric associated with fracture zones. These new Geosat data provide tight constraints on paleospreading directions. For example, three prominent fracture zones can be traced from south of Tasmania to the George V Basin, Antarctica, providing an important constraint on the relative motions of Australia and Antarctica through the Late Eocene. In addition, the Geosat profiles are used to locate the conjugate continental margins and continent‐ocean boundaries of Australia and Antarctica, as well as the conjugate rifted margins of Kerguelen Plateau and Broken Ridge. Based on a compilation of magnetic anomaly data from the Crozet Basin, the Central Indian Basin, the Wharton Basin and the Australian‐Antarctic Basin, ten plate tectonic reconstructions are proposed. Reconstructions at chrons 5 (11 Ma), 6 (21 Ma), 13 (36 Ma) and 18 (43 Ma) confirm that the Southeast Indian Ridge behaved as a single plate boundary since chron 18. The constraints from the Geosat data provide an improvement in the fit of the Kerguelen Plateau and Broken Ridge at chron 20 (46 Ma). To avoid overlaps between Broken Ridge and the Kerguelen Plateau prior to their breakup, our model includes relative motions between the northern and southern provinces of the Kerguelen Plateau. Finally, we examine the implications of our model for the relative motions of India, Australia and Antarctica on the tectonic evolution of the Kerguelen Plateau and Broken Ridge, and the adjacent Labuan Basin and Diamantina Zone, as well as the emplacement of the Ninetyeast Ridge and the Kerguelen Plateau over a fixed hot spot.
Effect of water on the rheology of experimentally deformed quartzite
Authors: Koch Philip S. , Christie John M. , Ord Alison , George Richard P.
Published: February 2008

Empirical flow laws have been determined for Simpson quartzite samples deformed to mechanical steady state in the α‐quartz stability field using Griggs‐Blacic solid‐medium deformation apparatus. Experiments were conducted on samples both as received (“dry”) and with water added via the dehydration of a talc confining medium (“wet”). Best fits of the power law type yield a stress exponent of 2.72±0.19, an activation enthalpy of 134±32 kJ mol−1, and a preexponential constant of 1.16 [+1.15, −0.58] × 10−7 MPa−2.72 s−1 for the dry quartzite law; and are 2.61±0.15, 145±17 kJ mol−1, and 5.05 [±5.00] × 10−6 MPa−2.61 s−1 for the wet quartzite. The enhanced hydrolytic weakening of the wet experiments appears to affect the flow laws mostly in the preexponential constants, possibly as a defect concentration term that is higher in the wet than in the dry law. Over the range of experimental conditions (750°–900°C, 10−7 to 10−4 s−1, and 1.0–1.25 GPa confining pressure), dry specimens are 1.5–2.5 times stronger than wet samples deformed at the same conditions. The microstructures produced are analogous to those observed in many natural quartz tectonites. Recrystallization is well developed in the wet but not the dry specimens. At flow stresses below 500 MPa, microstructures indicative of recovery are well developed. A comparison of our creep activation enthalpies with those determined from diffusion experiments are consistent with deformation of wet Simpson quartzite that is limited by oxygen self‐diffusion. Inconsistencies between the present study and previous studies are probably the result of differences in sample material, sample treatment, and sample assemblies (chemical environment).
Local Sq response functions from EMSLAB data
Authors: Bahr Karsten , Filloux Jean H.
Published: February 2008

Magnetic signals from six seafloor stations on the Juan de Fuca plate and one land station are examined for information content about the structure of the regional lithosphere and mantle. Single‐site Z/H responses for the first four Sq harmonics are calculated from 24 quiet days. Two corrections are applied to the data: (1) to the usual strictly Pm+1m source morphology, we also include the contribution of the Pm+3m spherical harmonic to the magnetic potential of Sq; and (2) the coast effect is removed by use of two‐dimensional model calculations with a simulated nonuniform source field. The model enables us to separate the anomalous vertical magnetic field attributable to the lateral conductance change from the normal vertical magnetic field in the absence of lateral conductance changes. The normal vertical field is calculated from the vertical conductivity distribution and the particular form of the source field. The sum of both normal and anomalous fields reproduces the observations. The anomalous part has been used to correct the observed vertical to east magnetic component ratio for the effect of the land‐ocean transition. The corrected ratio has been interpreted with Schmucker's “equivalent conductor” method, yielding estimates of the impedance at Sq frequencies. These estimates are used as an independent check on magnetotelluric (MT) impedances in the Sq frequency range, in particular for the purpose of identifying the local static distortion of the electric field. No significant distortion seems detectable at the outermost seafloor stations. The Sq and MT impedances are consistent with a single model.
Multivariate analysis of geomagnetic array data: 1. The response space
Authors: Egbert Gary D. , Booker John R.
Published: February 2008

In this paper we develop a new approach to the multiple‐station analysis of geomagnetic array data. Our approach is based on the observation that the space of external sources, and hence the space of observable electromagnetic fields at the Earth's surface (the “response space”), can often be well approximated by spaces of very low dimensionp. We demonstrate that the simplifying assumption of a finite dimensional external source space is required for a rigorous justification of transfer function (TF) methods in general and show that whenp= 2, the response space is equivalent to all possible interstation and intercomponent TFs. Heuristically, estimation of the response space by thepdominant eigenvectors of the spectral density matrix (i.e., the frequency domain matrix of averaged cross products for all components measured in the array) can be easily justified. A more rigorous statistical treatment is also discussed. Our statistical model (a complex errors‐in‐variables model) allows for (potentially correlated) errors in all measured field components and treats all stations in a symmetric fashion. The model is thus substantially more reasonable than the usual statistical model used in TF estimation which assumes that the fields at a reference site are noise free and “normal.” We illustrate our approach with a small five‐station magnetotelluric array which was run as part of the EMSLAB experiment. For this small array,p= 2 is a good approximation, and the two dominant eigenvectors provide an estimate of the response of the Earth to (approximately) uniform sources. There is, however, substantial additional structure in the spectral density matrix (SDM), and further analysis of the eigenvalues and eigenvectors of the SDM reveals that the “noise” (i.e., the portion of the data which does not fit a uniform source model) is dominated by source effects. These results illustrate a significant advantage of multiple‐station techniques: they allow us to test model assumptions and to separate noise into coherent and incoherent components, allowing a clearer understanding of the problems, and possibilities, to be found in the interpretation of geomagnetic induction data.
Earthquakes as a self‐organized critical phenomenon
Authors: Bak Per , Tang Chao
Published: February 2008

The Gutenberg‐Richter power law distribution for energy released at earthquakes can be understood as a consequence of the earth crust being in a self‐organized critical state. A simple cellular automaton stick‐slip type model yields D(E) ≈ E−τ with τ≈1.0 and τ≈1.35 in two and three dimensions, respectively. The size of earthquakes is unpredictable since the evolution of an earthquake depends crucially on minor details of the crust.
Chronology of volcanism and rift basin propagation: Rungwe Volcanic Province, East Africa
Authors: Ebinger C. J. , Deino A. L. , Drake R. E. , Tesha A. L.
Published: February 2008

Continental rifts are segmented along their lengths into 50 to 100 km‐long extensional basins, suggesting a genetic relationship with regularly segmented oceanic rifts. To investigate the spatial and temporal development of along‐axis segmentation in youthful continental rifts, field, remote sensing, and K‐Ar geochronology studies were conducted in four Western rift (East Africa) basins. Volcanism within the Rungwe region began in late Miocene time prior to or concurrent with the development of high‐angle border faults bounding the then isolated Karonga and Rukwa basins. The Usangu and Songwe border fault segments located between the Rukwa and Karonga basins developed in mid‐Pliocene and Pleistocene time, respectively. Within the northern Karonga basin, approximately 2–4 km of WSW‐ENE directed “thick‐skinned” extension is estimated both from extrapolation of late Pleistocene slip rates to 5 Ma and from surface fault geometries. Differential strains between these extensional basins of different ages are transmitted to adjoining segments by NW and ENE striking oblique‐slip transfer faults within comparatively high‐strain accommodation zones. Throws along transfer faults also accommodate 2 km (minimum) variations in depth to basement beneath adjacent basins, whereas monoclines and ramps accommodate differential uplift along the flanks of basins. The observed geometry of en echelon border fault segments linked by shorter oblique‐slip transfer faults is similar to patterns predicted in numerical models of en echelon normal fault interactions [e.g., Aydin and Pollard, 1982]. Eruptive centers for alkali olivine basalt and phonolite flows within the Rungwe region coincide with the tips of en echelon border fault segments and with high‐angle transfer faults linking discrete border fault segments. The locations of eruptive centers appear to be controlled by faults, and these centers generally have propagated northward along transfer faults during Plio‐Pleistocene time. Historic eruptions near late Miocene centers located at the southern end of the transfer fault system, however, argue against a northward migration of the magma source. Thus, an along‐axis propagation of “thick‐skinned” lithospheric extension links once isolated basins and produces an along‐axis segmentation of this youthful rift. Propagation of magmatism along transfer faults linking en echelon border faults locally contributes to the regular segmentation.