bluuebunny/crossref_metadata_2025 huggingface dataset:
The effect of fluid chemistry on wetting textures in novaculite
Authors: Lee V. W. , Mackwell S. J. , Brantley S. L.
Published: February 2008

We have performed two suites of experiments at 600°C and 200 MPa on samples of Arkansas Novaculite to investigate the effect of fluid chemistry on the local distribution of fluids amid grains of solid rock under crustal conditions. Analysis of texture before and after these experiments indicates that fluid chemistry has an effect on both equilibrium wetting angles and the degree of fluid connectivity along triple junctions. The first suite of experiments shows that triple grain junctions in low‐porosity (ϕ < 1%) novaculite have dihedral angles (Θ) less than 60° in the presence of 1 m MgCl2, 3 m KCl, and 3 m, 4 m, or 6 m NaCl solutions. In these samples wetted triple junctions observed under a scanning electron microscope in polished section were approximately 0.15 μm from corner to corner. In fracture surfaces the triple junctions appear to form an interconnected network of very fine tubules. Novaculite in the presence of CaCl2 solutions also contains wetted triple junctions with Θ < 60° but appears to be less wetting than the previous fluids. Novaculite in the presence of H2O and CO2‐H2O equilibrates to nonwetting textures (Θ ≥ 60°) and lacks interconnected tubules as observed in fracture surfaces. In the second set of experiments samples were equilibrated sequentially in two different fluids. After this treatment, samples exhibited textures characteristic of the second pore fluid, attesting to the reversibility of the textural equilibration process and suggesting that reversion of textures occurs rapidly (<24 hours) under these conditions.
Shear wave travel time, amplitude, and waveform analysis for earthquakes in the kurile slab: Constraints on deep slab structure and mantle heterogeneity
Authors: Schwartz Susan Y. , Lay Thorne , Beck Susan L.
Published: February 2008

Shear wave travel times, amplitudes and broadband waveforms from 16 events in the Kurile slab are analyzed to place further constraints on the seismic velocity heterogeneity associated with the subducted oceanic lithosphere. Several previous studies have proposed that the Kurile slab penetrates into the lower mantle with little deformation other than a steepening of the dip angle, while other studies suggest that strong slab deformations occur with perhaps negligible lower mantle penetration. In an effort to detect any deep slab effects, we compare anomaly patterns covering a 60° azimuthal range straddling the strike of the slab for events in three different depth intervals (> 500 km, 350–500 km, and 100–200 km). S and ScS pulse broadening is used as a measure of waveform complexity. Complexity of broadband shear waves does not vary systematically across our azimuthal range as the propagation direction changes with respect to the postulated lower mantle slab model for this region. Strong residual sphere travel time variations are found for shear waves from events at all depths; however, these variations do not display the depth dependence apparent in previous P wave analysis that has been interpreted as requiring a change in slab geometry and lower mantle penetration. The same S wave residual pattern is apparent in sS depth phases, and is well predicted by a recent model of shear velocity structure beneath North America. A combination of deep mantle and near‐receiver aspherical structure can explain most of our observed shear wave travel time patterns, with only a minor source depth‐dependent component remaining after correction. Individual amplitude and travel time measurements are poorly correlated; however, ScS/S amplitude ratios and ScS‐S differential times are significantly correlated, consistent with geometric focussing and defocussing effects somewhere along the raypaths. The travel time behavior suggests that the responsible velocity heterogeneity is not located near the source. The absence of a strong deep slab effect on the shear waves indicates that deep slab heterogeneity is less pronounced for shear waves than expected on the basis of existing P wave models derived from travel time residual sphere analysis. Various interpretations include: estimates of δVs/δT and compositional effects previously used to predict shear velocity structure of the deep Kurile slab are in error, with actual lower mantle slab anomalies being much weaker than proposed; the Kurile slab is so distorted in the lower mantle that no source depth dependent trends are produced for our limited focal sphere coverage; or the Kurile slab does not penetrate into the lower mantle.
Geophysical log responses and their correlation with bed‐to‐bed stress contrasts in Paleozoic rocks, Appalachian Plateau, New York
Authors: Plumb Richard A. , Evans Keith F. , Engelder Terry
Published: February 2008

A 1‐km profile of in situ stress and geophysical log data was acquired in the Wilkins well to study the relationship between rock properties and in situ stress contrasts. The Wilkins well penetrates Devonian clastic rocks on the Appalachian Plateau near the town of South Canisteo, New York. Open hole hydraulic fracture stress measurements were made in stratigraphic sequences where geophysical logs indicated significant bed‐to‐bed variations in elastic and lithologic properties. Analysis of stress magnitudes and interval‐averaged geophysical data shows that principal horizontal stress magnitudes correlate directly with elastic stiffness and inversely with clay content. A similar relation is found for older Paleozoic strata penetrated by a well at Auburn, New York. Correlations between stress magnitude and geophysical properties observed in the Wilkins and Auburn wells provide strong evidence that bed to bed stress variations arise from a uniform ENE‐WSW directed strain acting on beds of different Young's modulus rather than from variations in rock shear strength. Because of their high Young's modulus, sandstones, siltstones, and limestones in the northern Appalachian Basin are likely to be stronger barriers to hydraulic fracture propagation than shales. Porosity logs in the Wilkins well show that the large decrease in horizontal stress found at the base of the Rhine street Formation occurs where shales are less compacted. The correlation with undercompaction is consistent with a paleo‐overpressure drainage mechanism as the cause for the stress decrease.
Shear wave splitting and subcontinental mantle deformation
Authors: Silver Paul G. , Chan W. Winston
Published: February 2008

We have made measurements of shear wave splitting in the phases SKS and SKKS at 21 broadband stations in North America, South America, Europe, Asia, and Africa. Measurements are made using a retrieval scheme that yields the azimuth of the fast polarization direction ϕ and delay time δt of the split shear wave plus uncertainties. Detectable anisotropy was found at most stations, suggesting that it is a general feature of the subcontinental mantle. Delay times range from 0.65 s to 1.70 s and average about 1 s. Somewhat surprisingly, the largest delay time is found in the 2.7 b.y.‐old Western Superior Province of the Canadian Shield. The splitting observations are interpreted in terms of the strain‐induced lattice preferred orientation of mantle minerals, especially olivine. We consider three hypotheses concerning the origin of the continental anisotropy: (1) strain associated with absolute plate motion, as in the oceanic upper mantle, (2) crustal stress, and (3) the past and present internal deformation of the subcontinental upper mantle by tectonic episodes. It is found that the last hypothesis is the most successful, namely that the most recent significant episode of internal deformation appears to be the best predictor of ϕ. For stable continental regions, this is interpreted as “fossil” anisotropy, whereas for presently active regions, such as Alaska, the anisotropy reflects present‐day tectonic activity. In the stable portion of North America there is a good correlation between delay time and lithospheric thickness; this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant. The acceptance of this hypothesis has several implications for subcontinental mantle deformation. First, it argues for coherent deformation of the continental lithosphere (crust and mantle) during orogenies. This implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent “underplating” or conductive growth of the thermal boundary layer. One of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes. Most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed (by convective instability or delamination) and replaced by warm asthenosphere. These models, however, are incompatible with the evidence for preserved coherent lithospheric deformation; rather, the deformed mantle appears to have been heated in place. We suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation.
North American Plate Dynamics
Authors: Richardson Randall M. , Reding Lynn M.
Published: February 2008

Deformation within the North American plate in response to various tectonic processes is modeled using an elastic finite element analysis. A coarse grid contains 328 elements and 190 nodes, while a fine grid contains 718 elements and 396 nodes for a spatial resolution of 2°–3°. Abundant information about the present‐day state of intraplate stress constrains the modeling. The dominant pattern that must be fit by all acceptable models is an ENE trend for the maximum compressive stress for most of the plate east of the Rocky Mountains. The tectonic processes considered in the modeling include ridge forces associated with the normal thermal evolution of oceanic lithosphere, shear and normal stresses transmitted across transforms, normal stresses transmitted across convergent boundaries, stresses due to horizontal density contrasts within the continent, and shear tractions applied along the base of the plate. Model stresses are calculated with respect to a lithostatic reference stress state. Distributed ridge forces of magnitude 2×1012 N/m predict deviatoric stresses of the order of 20–40 MPa that are capable of accounting for the dominant observed ENE stress trend. Driving drag models also fit the trend but are not preferred because of a predicted tenfold increase in compressive stress magnitudes from east to west across the plate. Assuming that ridge forces account for the dominant ENE stress trend, bounds may be placed on other possible forces. For example, shear stresses transmitted across transform boundaries along the San Andreas and Caribbean are small, of the order of 5–10 MPa. Also, compressive stresses of the order of 5–10 MPa transmitted across the major transforms improve the fit to the data. Compressive stresses across convergent margins along the Aleutians and the Middle America trench are important.
Regional patterns of tectonic stress in Europe
Authors: Müller Birgit , Zoback Mary Lou , Fuchs Karl , Mastin Larry , Gregersen Soren , Pavoni Nazario , Stephansson Ove , Ljunggren Christer
Published: February 2008

Nearly 1500 stress orientation determinations are now available for Europe. The data come from earthquake focal mechanisms, overcoring measurements, well bore breakouts, hydraulic fracturing measurements, and young fault slip studies and sample the stress field from the surface to seismogenic depths. Three distinct regional patterns of maximum compressive horizontal stress (SHmax) orientation can be defined from these data: a consistent NW to NNW SHmax stress orientation in western Europe; a WNW‐ESE SHmax orientation in Scandinavia, similar to western Europe but with a larger variability of SHmax orientations; and a consistent E‐W SHmax orientation and N‐S extension in the Aegean Sea and western Anatolia. The different stress fields can be attributed to plate‐driving forces acting on the boundaries of the Eurasian plate, locally modified by lithospheric properties in different regions. On average, the orientation of maximum stress in western Europe is subparallel to the direction of relative plate motion between Africa and Europe and is rotated 17° clockwise from the direction of absolute plate motion. The uniformly oriented stress field in western Europe coincides with thin to medium lithospheric thickness (approximately 50–90 km) and high heat flow values (>80 m W/m2). In western Europe a predominance of strike‐slip focal mechanisms implies that the intermediate principal stress is vertical. The more irregular horizontal stress orientations in Scandinavia coincide with thick continental lithosphere (110–170 km) and low heat flow (<50 m W/m2). The cold thick lithosphere in this region may result in lower mean stresses associated with far‐field tectonic forces and allow the stress field to be more easily perturbed by local effects such as déglaciation flexure and topography. The stress field of the Aegean Sea and western Anatolia is consistent with N‐S extension in a back arc setting behind the Hellenic trench subduction zone. The stress field is influenced in places by regional geologic structures, e.g., in the Western Alps, where SHmax directions show a slight tendency toward a radial stress pattern. Not all major geologic structures, however, appear to affect the SHmax orientation, e.g., in the vicinity of the Rhine rift system horizontal stress orientations are continuous.
The dynamics of motion of the South American Plate
Authors: Meijer P. Th. , Wortel M. J. R.
Published: February 2008

Observations of the intraplate stress field, as compiled and evaluated in the context of the World Stress Map Project, can be compared with stress modelling results to increase our understanding of the dynamics of motion of the lithospheric plates. In this paper we investigate the driving and resistive plate tectonic forces acting on the South American plate in order to establish a detailed basis for future stress modelling. Our basic assumption is that the plate is in mechanical equilibrium; that is, the net torque of the forces acting on the plate vanishes. The complexity of the forces exerted along the margins of the South American plate requires the dynamical analysis to be performed step‐wise. Incorporating the forces which are expected to be of major importance, a first‐order model is defined. The effect of additional plate boundary forces is subsequently examined in terms of superpositions onto the first‐order model. With the first‐order model the relation between three resistive forces and shear stress on the base of the lithosphere is derived. The ridge push force, driving the motion of the South American plate, can be quantified independently and is included in the calculations as a known quantity. Solutions are obtained using four different determinations of the Euler vector of absolute motion of the South American plate, which is taken to define the orientation of the basal shear stress. The results demonstrate that the concept of a basal shear stress actively driving plate motion cannot be discarded on the basis of a dynamical analysis. Further, with the assumption that basal drag is entirely concentrated below the continental pan of the lithosphere, estimates of the maximum amount of continental basal shear resistance are derived. Evaluation of an alternative model for the South American plate, excluding the northernmost pan of the Andes, demonstrates the significance of the resistive force exerted on the South American plate due to convergence with the Caribbean plate. It is to be expected that comparison of theoretical distributions of intraplate stress, computed on the basis of the present dynamical analysis, with observations of the actual state of stress will further constrain the range of feasible force models.
Properties of aftershock sequences in southern California
Authors: Kisslinger Carl , Jones Lucile M.
Published: February 2008

The temporal behavior of 39 aftershock sequences in southern California, 1933–1988, was modeled by the modified Omori relation. Minimum magnitudes for completeness of each sequence catalog were determined, and the maximum likelihood estimates of the parametersK,p, andc, with the standard errors on each, were determined by the Ogata algorithm. Thebvalue of each sequence was also calculated. Many of the active faults in the region, both strike slip and thrust, were sampled. Thepvalues were graded in terms of the size of the standard error relative to thepvalue itself. Most of the sequences were modeled well by the Omori relation. Many of the sequences hadpvalues close to the mean of the whole data set, 1.11±0.25, but values significantly different from the mean, as low as 0.7 and as high as 1.8, exist. No correlation ofpwith either thebvalue of the sequence or the mainshock magnitude was found. The results suggest a direct correlation ofpvalues is with surface heat flow, with high values in the Salton Trough (high heat flow) and one low value in the San Bernardino Mountains and on the edge of the Ventura Basin (both low heat flow). The large fraction of the sequences withpvalues near the mean are at locations where the heat flow is near the regional mean, 74 mW/m2. If the hypothesis that aftershock decay rate is controlled by temperature at depth is valid, the effects of other factors such as heterogeneity of the fault zone properties are superimposed on the background rate determined by temperature. Surface heat flow is taken as an indicator of crustal temperature at hypocentral depths, but the effects on heat flow of convective heat transport and variations in near‐surface thermal conductivity invalidate any simple association of local variations in heat flow with details of the subsurface temperature distribution. The interpretation is that higher temperatures in the aftershock source volume caused shortened stress relaxation times in the fault zone materials, leading to a faster decay rate (higherpvalue). In 1967, Mogi proposed such a relation on the basis of Japanese data. Seismic velocity distributions published for southern California generally support the hypothesis, with low velocities (higher temperatures) corresponding to highpvalues, and vice‐versa.
Crustal subsidence, seismicity, and structure near Medicine Lake Volcano, California
Authors: Dzurisin Daniel , Donnelly‐Nolan Julie M. , Evans John R. , Walter Stephen R.
Published: February 2008

The pattern of historical ground deformation, seismicity, and crustal structure near Medicine Lake volcano illustrates a close relation between magmatism and tectonism near the margin of the Cascade volcanic chain and the Basin and Range tectonic province. Between leveling surveys in 1954 and 1989 the summit of Medicine Lake volcano subsided 389±43 mm with respect to a reference bench mark 40 km to the southwest (average rate = 11.1±1.2 mm/yr). A smaller survey across the summit caldera in 1988 suggests that the subsidence rate was 15–28 mm/yr during 1988–1989. Swarms of shallow earthquakes (M ≤ 4.6) occurred in the region during August 1978, January–February 1981, and September 1988. Except for the 1988 swarm, which occurred beneath Medicine Lake caldera, most historical earthquakes were located at least 25 km from the summit. The spatial relation between subsidence and seismicity indicates (1) radially symmetric downwarping of the volcano's summit and flanks centered near the caldera and (2) downfaulting of the entire edifice along regional faults located 25–30 km from the summit. We propose that contemporary subsidence, seismicity, and faulting are caused by (1) loading of the crust by more than 600 km3 of erupted products plus a large volume of mafic intrusives; (2) east‐west extension in the western Basin and Range province; and, to a lesser extent, (3) crystallization or withdrawal of magma beneath the volcano. Thermal weakening of the subvolcanic crust by mafic intrusions facilitates subsidence and influences the distribution of earthquakes. Subsidence occurs mainly by aseismic creep within 25 km of the summit, where the crust has been heated and weakened by intrusions, and by normal faulting during episodic earthquake swarms in surrounding, cooler terrain.
The effects of grain‐scale deformation on the Bloomsburg Formation Pole
Authors: Stamatakos John , Kodama Kenneth P.
Published: February 2008

Previous paleomagnetic results from the Silurian Bloomsburg Formation in the central Appalachian Valley and Ridge Province suggest that the Bloomsburg's characteristic magnetization is a synfolding magnetization acquired during the Acadian Orogeny. However, there is little geological evidence to support extensive Acadian folding in the central Appalachians. A more favorable explanation is that the characteristic magnetization originated as a prefolding Silurian magnetization and has been altered by grain‐scale deformation to mimic a synfolding Devonian remagnetization. In order to investigate whether penetrative deformation has altered the Bloomsburg Formation's characteristic signal, the relationship between strain and remanence was examined around three central Appalachian Valley and Ridge folds. Structural analysis of mesoscopic and microscopic features along with center‐to‐center finite strain analysis at these three folds indicates a deformation sequence that includes prefolding layer‐parallel shortening overprinted by top‐to‐the‐foreland, bedding‐parallel shear and flexural slip/flow folding. Inclination of the characteristic magnetization varies systematically with the magnitude of finite strains. This relationship between strain and remanence suggests that the characteristic magnetization has been progressively rotated toward shallower inclinations by penetrative deformation analogous to rigid particle rotation in a viscously deforming matrix. Using directions from sites with relatively low finite strain values, a new Bloomsburg Formation pole is calculated at 18.7°N, 107.4°E (K = 58.4, A95 = 8.2°). This new pole falls on the Silurian track of the North American apparent polar wander path between the Silurian Wabash Reef pole and the Silurian‐Devonian Andreas pole. In addition, these results suggest that previously reported 20°–30° difference in declination of site mean directions north arid south of the Pennsylvania salient may also be the result of grain‐scale reorientation of the remanence‐carrying grains and not oroclinal bending.
New paleomagnetic data from Thurston Island: Implications for the tectonics of west Antarctica and Weddell Sea opening
Authors: Grunow A. M. , Kent D. V. , Dalziel I. W. D.
Published: February 2008

Paleomagnetic data from three West Antarctic crustal blocks (Antarctic Peninsula (AP), Thurston Island‐Eights Coast (TI), and the Ellsworth‐Whitmore Mountains (EWM) indicate that there has been motion between the individual blocks and motion relative to East Antarctica during the Mesozoic. A Triassic paleomagnetic pole from the TI block (116°E, 61°S, A95 = 19.4°, N = 3 VGPs) appears to indicate that the block has rotated ∼90° relative to East Antarctica between 230 Ma and 110 Ma. Our previously reported Middle Jurassic paleomagnetic pole from the EWM block indicates that a 90° rotation relative to East Antarctica occurred sometime between the Cambrian and 175 Ma. We believe that the 90° counterclockwise EWM rotation occurred between ∼220 Ma and 175 Ma related to the development of post‐Gondwanide Orogeny shear zones. The motion of the AP, TI, and EWM blocks appears to be linked during the mid‐to late Mesozoic to three major events in the evolution of the southern ocean basins. Opening in the Mozambique‐Somali‐Weddell Sea basins may have produced major counterclockwise rotation of the TI block with respect to East Antarctica between the Jurassic and Early Cretaceous based on new Late Jurassic (145°E, 64.5°S, A95 = 7°,N = 5 VGPs) poles. We believe that the TI rotation, as well as deformation in the southern AP block, was caused by collision and shearing of the EWM block against the other two as the EWM block moved southward with East Antarctica. An Early Cretaceous paleomagnetic pole (232°E, 49°S, A95 = 7.9°, N = 5 VGPs) from the TI block requires that between the Early and mid‐Cretaceous there was clockwise rotation, with respect to East Antarctica, of the AP‐TI‐EWM blocks (an entity we call Weddellia). A change in the opening history of the Weddell Sea basin caused by initiation of spreading in the South Atlantic ocean basin at ∼130 Ma probably started Weddellia's clockwise rotation. Two new ∼110 and ∼90 Ma poles from the TI block (210°E, 73°S, A95 = 7.6°,N = 7 VGPs and 161°E, 81°S, A95 = 3.9°,N = 18 VGPs, respectively) are similar to equivalent age poles from the AP block and East Antarctica and indicate that Weddellia was at or near its present‐day position with respect to East Antarctica by ∼110 Ma. This corresponds to a time of major plate reorganization in the South Atlantic and southeast Indian Oceans. Based on both the new TI paleomagnetic data and previously reported data from Marie Byrd Land (MBL), dextral shearing would be expected to have occurred between MBL and Weddellia since the mid‐Cretaceous. Pine Island Bay, the area between the TI and MBL blocks, marks a fundamental and complex tectonic boundary in West Antarctica that we propose has largely been a zone of transcurrent shearing.
The regional intraplate stress field in South America
Authors: Assumpcao Marcelo
Published: February 2008

A compilation of lithospheric stress directions for continental South America and the inferred major patterns of the regional intraplate stress field are presented. Stress orientations are based primarily on earthquake focal mechanisms and Quaternary fault slip inversion published in the literature. Four new focal mechanisms based on short‐period P wave modeling, and selected centroid‐moment‐tensor solutions (published by U.S. Geological Survey) consistent with P wave first motions at South American and other World‐Wide Standard Seismograph Network stations are also included in the data base. The observed patterns of intraplate stresses may be useful in constraining numerical models of the plate driving forces in the South American plate. In the Andean plateau (altitudes greater than 3000 m), N‐S extensional stresses predominate. E‐W compressional stresses are observed in the sub‐Andean and platform regions extending up to about 1000 km east of the Andes. The maximum horizontal stress (SHmax) is uniformly oriented in the E‐W direction throughout western South America. Averages of the SHmax direction at grid points spaced 2.5° were taken as representing the “regional” field. The E‐W direction of this regional field is not affected by the change of strike of the Andean chain nor by the contact of a flat subducted slab underneath. The regional SHmax direction is oriented 15° more clockwise than the direction of the Nazca plate convergence. The difference between the regional SHmax orientations and the absolute plate motion may be only about 6°. This may indicate that contact forces with the Nazca plate may not be the only major contributor to the intraplate stresses in western South America. The eastern limit of this Andean stress province seems to coincide with aseismic regions in the Upper Amazon basin and in the Parana basin. In the central Amazonian region, seismicity and stress data suggest a seismic province with N‐S compressional stresses. The origin of these stresses is not yet clearly understood but could possibly be related to lower crustal loading along the middle Amazon basin. In northeastern Brazil, seismicity is characterized by upper crustal strike‐slip earthquakes bordering the Potiguar marginal basin. A model is proposed for this region in which the stress field is the result of a superposition of regional E‐W compressional stresses and local extensional stresses (oriented perpendicular to the continental margin) possibly related to density contrasts and sediment loading in the continental shelf.
Late Cretaceous paleomagnetism of the Tucson Mountains: Implications for vertical axis rotations in south central Arizona
Authors: Hagstrum Jonathan T. , Lipman Peter W.
Published: February 2008

The Tucson Mountains of southern Arizona are the site of an Upper Cretaceous caldera from which the rhyolitic Cat Mountain Tuff was erupted at about 72 Ma. Two magnetic units within the Cat Mountain Tuff are distinguished by paleomagnetic data in both the northern and southern Tucson Mountains. The resurgent Amole pluton (≃72 Ma) in the northern Tucson Mountains was emplaced soon after eruption of the Cat Mountain Tuff but cooled and was magnetized after northeastward tilting (50°–85°) of the adjacent caldera‐fill sequence. Petrologic and paleomagnetic data indicate that the lower magnetic unit of the Cat Mountain Tuff caps the Silver Bell Mountains to the northwest. A previous paleomagnetic investigation (N= 34) indicates that the Silver Bell Mountains have been rotated clockwise 30°±16° (95% confidence level) about a vertical axis relative to cratonic North America. A similar paleomagnetic study of Upper Cretaceous volcanic, volcaniclastic, and intrusive units in the Tucson Mountains (N= 26) indicates that these rocks have been rotated 7°±14° clockwise relative to stable North America. A direct comparison of paleomagnetic directions for the lower unit of the Cat Mountain Tuff shows a 17°±10° clockwise rotation between the Silver Bell Mountains and the Tucson Mountains which supports the relative accuracy of the absolute rotations determined for these two mountain ranges. Preliminary paleomagnetic directions for middle Tertiary units from the Silver Bell and Tucson Mountains are consistent with clockwise rotation having occurred prior to deposition of these rocks. Clockwise rotation of crustal blocks in southern Arizona likely was associated with strike‐slip movement on major northwest trending faults in the region, and this movement may have been related to oblique subduction of oceanic plates along the western continental margin in Late Cretaceous and early Tertiary time. The available paleomagnetic data indicate that rocks in southern Arizona have not remained unrotated with respect to North America since Late Cretaceous time and that vertical axis rotations may have played an important role in the region during Laramide deformation.
Late Paleozoic paleomagnetic results from the Junggar block, northwestern China
Authors: Li Yianping , Sharps Robert , McWilliams Michael , Li Yongan , Li Qiang , Zhang Wei
Published: February 2008

We have obtained middle Carboniferous to Permian paleomagnetic results from the Junggar block in order to test its tectonic coherence and to reconstruct its late Paleozoic position. From Hoboksar, on the northwestern margin of Junggar, a paleomagnetic pole from basic/silicic flows yields ϕp = 299°E, λp = 70°N (N = 15 sites, K/ A95 = 36/6.4°). In the same general area, a pole from lava flows, plutons and as well as from remagnetized limestones/sandstones, is at ϕp = 350°E, λp = 69°N (N = 26 site, K/A95 =15/7.5°). Farther to the southwest in Keramayi, a pole from late Paleozoic dike swarms lies at ϕp = 230°E, λp = 80°N (N= 23 sites, K/A 95 = 16/7.8°). In the southern margin near Urumqi, from sedimentary rocks and volcaniclastics of the Middle to Upper Carboniferous Qijiagou and Aoertu formations, a pole is produced at ϕp = 7°E, λp = 72°N (N = 15 sites, K/ A95 = 28/7.4°). From the Middle Carboniferous Liushugou Formation and dike swarms intruding into the formation, two poles are located at ϕp =231°E, λp = 69°N (N =4 sites, K/A95 = 46/13.7°) and ϕp = 172°E, λp = 55°N (N = 7 sites, K/A95 = 17/ 15.1°), respectively, though reliabilities of these two poles are in question. Magnetic polarities of all the poles in our study are exclusively reversed. Based on positive fold tests and a baked contact test, we believe that these magnetizations were acquired during the Kiaman reversed interval (250–320 Ma). More accurate ages of the magnetizations cannot be resolved at present. Despite widely separate locations, the pole positions are roughly located on a small circle segment around the Junggar block. Similar inclinations suggest a coherent Junggar block since Permian time. Based on the results of this study and other published paleomagnetic data, a mean middle Carboniferous to Permian paleomagnetic pole is calculated at ϕp = 329°E, λp = 82°N (N = 6 poles, K/A95 = 26/13.4°). Recognition of late Paleozoic apparent polar wander for Junggar is obscured by a lack of detailed geochronologic data and possible relative horizontal rotations around vertical axis. The paleolatitude for the southern Junggar predicted by this pole is only 4° south of the present latitude. Difference exists between the mean pole from Junggar and Permian poles from other tectonic units in Asia (Tarim, Siberia, Kazakhstan, and North and South China), indicating post‐Permian relative motions. At the boundary between Tarim and Junggar, this motion is likely to have a left‐lateral sense.
Self‐similar distribution and properties of macroscopic fractures at depth in crystalline rock in the Cajon Pass Scientific Drill Hole
Authors: Barton Colleen A. , Zoback Mark D.
Published: February 2008

This study was conducted in order to characterize the frequency, orientation, and aperture of macroscopic fractures in the crust and their effect on physical properties over an appreciable depth interval (1829–3450 m). The following are our major findings: (1) Over the range of apparent apertures measured with confidence, the frequency of fractures with a given aperture decreases as aperture increases. With applied corrections for sampling bias, the observed distribution of fracture aperture has a power law form providing evidence of the self‐similar nature of fractures in the crystalline crust. Fractal analysis of the fracture aperture data yields a fractal dimension of 1.4 over the range of reliable aperture measurements in this study from 15 to 100 mm. (2) Fracture frequency does not systematically decrease with depth in the study interval. (3) No significant correlation was found between fracture occurrence and lithology, and both fracture spacing and aperture are uncorrelated with fracture orientation or depth. (4) The majority of fractures encountered in the well strike NNW‐SSE and dip steeply to the west. One set of steeply dipping fractures appears to be related to the NW striking San Andreas fault and appears to be related to steeply dipping, NW striking shear fractures observed in nearby outcrops that are characterized by laumontitic alteration. (5) The fractures bear no obvious relation to the current northeast direction of maximum horizontal compression but do correlate with anomalies in physical properties measurements of compressional and shear velocity, porosity, and resistivity. (6) Macroscopic fractures strike in a direction nearly orthogonal to the fast propagation direction of seismic wave anisotropy determined from vertical seismic profiling experiments in the well. These fractures appear to be unrelated to the observed seismic anisotropy. (7) Hydraulically conductive fractures and major faults indicate that fluid‐conducting fractures are a subset of the overall statistically significant population and not related to the San Andreas fault or to the orientation of SHmax in an obvious way.
Dynamic modeling of explosively driven hydrofractures
Authors: Nilson R. , Rimer N. , Halda E.
Published: February 2008

The long‐range extension of explosively driven fractures depends upon the flow of gas into the fractures as well as the nonlinear stress wave dynamics of the surrounding solid. These two interactive processes are, for the first time, modeled in a simultaneous fashion which includes solid dynamic as well as fluid dynamic considerations. At each time step in the numerical procedure, a fluid dynamics algorithm is first used to calculate the pressure distribution along the crack(s) and the rate of fluid advance, after which a solid dynamics algorithm calculates the incremental motion of the solid, the new stress field, and the opening displacement of the crack(s). To gain generality and simplicity, gas‐filled fractures are introduced using a crack porosity formulation in which local crack strains are adjusted to achieve a balance between the internal gas pressure and the local compressive stress. Comparisons with exact self‐similar solutions include cases in which the fracture speed is controlled by solid dynamic as well as fluid dynamic considerations. Example calculations include single or multiple fractures with individually calculated pressure distributions as well as more pervasive fracture swarms which are presumed to be fully pressurized. A broad range of results are presented for underground nuclear explosions which are detonated in a premined spherical cavity. As the initial radius of the cavity is increased, the explosion becomes progressively decoupled, and the primary constraint on fracture propagation changes from solid dynamics to fluid dynamics, thereby exercising the full range of the controlling physics.
Preparation and deformation of synthetic aggregates of quartz
Authors: Luan F. C. , Paterson M. S.
Published: February 2008

Synthetic quartz aggregates of low porosity have been fabricated from natural quartz powder, impure silica gel, and high‐purity silicic acid by hydrothermal isostatic pressing at 300 MPa. The resulting specimens have water contents comparable to those of natural quartzites and have been used for deformation tests. The flow stress at 1200–1300 K was found to vary in an inverse relationship to the water pressure estimated under the assumption that the measured water content homogeneously filled the measured porosity. It is questionable that the natural‐quartz‐origin specimens were fully equilibrated in respect to the activity of the water, but equilibration has probably been achieved in the amorphous‐silica‐origin specimens, which crystallized under the experimental conditions. In the deformation of the latter materials, the experimental activation energy at water pressures approaching the confining pressure was found to be about 150 kJ mol−1 for both. However, the impure gel‐origin material gave a stress exponent of about 2.3, whereas the high‐purity silicic‐acid‐origin material gave a stress exponent of about 4, in spite of the grain sizes being about 90μm and 20 μm, respectively. It is concluded that while the impure specimens have higher intragranular strengths, there is also a significant contribution of grain boundary processes to the strain in them which is absent in the high‐purity specimens.
Global upper mantle tomography of seismic velocities and anisotropies
Authors: Montagner Jean‐Paul , Tanimoto Toshiro
Published: February 2008

A data set of 2600 paths for Rayleigh waves and 2170 paths for Love waves enabled us to retrieve three‐dimensional distributions of different seismic parameters. Shallow layer corrections have been carefully performed on phase velocity data before regionalization and inversion at depth. The different seismic parameters include the five parameters of a radially anisotropic medium and the eight azimuthal anisotropic parameters as defined by Montagner and Nataf. It is found that the lateral heterogeneities of velocities and anisotropies in the upper mantle are dominated down to 250–30 km by plate tectonics with slow velocities below ridges, high velocities below continents and a velocity increasing with the age of the seafloor. Anisotropy is present in this whole depth range and the directions of maximum velocities are in good agreement with absolute plate velocities. Below 300 km, there is a sharp decreasing of the amplitude of lateral heterogeneities of seismic velocities and anisotropies. Below 450 km, lateral heterogeneities display a degree 2 and to a less extent a degree 6 pattern. Therefore, between 250 km and 450 km, there is a transition region where vertical circulation of matter is possible as shown by subducted slabs and “plumes” of slow velocities but which probably separates two types of convection. The first one is closely related to plate tectonics and to the distribution of continents. The second one dominates below 450 km and is characterized by two downgoing and two upgoing flows.
The use of glass for dating volcanic ash by thermoluminescence
Authors: Berger Glenn W.
Published: February 2008

The development of a new physical dating method for late Pleistocene tephrochronometry is described here. Tests of laboratory procedures applied to glass from independently dated samples show that the thermoluminescence (TL), additive‐dose dating method can yield accurate ages for both proximal and distal tephra from a few hundred years to at least 400 ka. The technique involves purification of 4‐ to 11‐μm glass by heavy liquid centrifugation, pre‐TL readout heating at 50°–75°C for 8 days, and careful regression analysis of TL growth curves. Several unknown‐age tephra are dated directly by this method for the first time. These include five ash beds from Summer Lake, Oregon (67–200 ka), a Sheep Creek Valley ash from Alaska (81±9 ka), an ash from Nevada (200±50 ka) correlated to the Wadsworth Tephra, an unknown ash (210±30 ka) from Rye Patch Dam in Nevada previously correlated to the 400‐ to 600‐ka Dibekulewe tephra, and ash 9M from Washtucna in eastern Washington (250±30 ka).
Abnormal tsunamis caused by the June 13, 1984, Torishima, Japan, earthquake
Authors: Satake Kenji , Kanamori Hiroo
Published: February 2008

An earthquake with an MS = 5.6 (mb=5.5) which occurred near Torishima, Japan, on June 13, 1984, generated abnormally large tsunamis (tsunami magnitude Mt = 7.3) for its relatively small earthquake magnitude. The maximum amplitude of the tsunamis is not a simple function of distance as the magnitude formula implies. In order to quantify the abnormal tsunamis, we modeled the tsunami source using a finite difference computation on the actual bathymetry. The source of these abnormal tsunamis is modeled as a water surface displacement, with a radius of 12 km and maximum water height of 13 cm. The displaced water volume is 4×1013 cm3, and the potential energy is calculated as 2×1017 ergs. If this water displacement is due to a fault motion at 5 km depth, a moment magnitude Mw is estimated to be 6.3. This value is larger than Mw estimated from seismic waves by 0.7, but smaller than Mt by 1.0. The former difference suggests that the earthquake source is very different from an ordinary faulting. If we assume the source is a horizontally lying tensile crack at shallow depth (about 2 km), that explains both the water surface displacement and the seismic moment tensor. The tensile crack is interpreted as an injection of material, as suggested by a seismological study. The discrepancy between Mw and Mt indicates that the propagation path is responsible for the observed large tsunamis. The numerical computation shows a focusing of energy during the propagation of the tsunami along the shallow Izu‐Bonin ridge system. Thus the abnormally large tsunamis from the Torishima earthquake are a result of both source and path effects.
Four thousand years of seismicity along the Dead Sea Rift
Authors: Ben‐Menahem Ari
Published: February 2008

A study of major earthquake occurrence along the Dead Sea transform (35.5°–36.5° E; 27.2°–37.5° N) during the past four millenia has been attempted. Geological, archaeological, biblical, historical, and seismological evidence were integrated in an effort to quantify the space‐time distribution of seismicity in the said province. The overall earthquake activity in the conterminous Near East indicates a stable pattern and appeared to have been stationary over the examined time window. About 110 earthquakes in the magnitude range 6.7 ≤ ML ≤ 8.3 affected the area during the past 2500 years. Of these, 42 originated along the Dead Sea fault system itself, while 68 were imported from the Helenic‐Cyprian arcs and the Anatolian‐Elburz‐Zagros fault systems. These events were responsible for the repeated destruction of many cultural centers. In the Dead Sea region proper, the major seismic activity since 2100 B.C.E. (Before Christian Era), has been confined to the vicinity of its eastern shore with extremal seismicity at its southern tip near the prehistorical site of Bab‐a‐Dara'a (31° 15'N, 35° 32'E). This may constitute the first solid evidence that the Biblical “cities of the Plain” (Sodom, Gommorah, etc.) were located there. Recent studies of earthquake deformations in the Lisan deposits near Bab‐a‐Dara'a, agree with our findings. At the present time, a magnitude 6¾ earthquake is pending at the northern edge of the Levant rift, with its average recurrence interval (83 years) exceeded by one standard deviation (32 years).
Error variances‐covariances of GEM‐T1: Their characteristics and implications in geoid computation
Authors: Haagmans R. H. N. , van Gelderen M.
Published: February 2008

GEM‐TI is the first Earth potential model for which a full error variance‐covariance matrix of its coefficients was made available. This matrix offers the opportunity to look into the properties of the error covariance functions for geoid heights from GEM‐T1. They appear to be rather anisotropic and inhomogeneous. Geoid error covariance functions are used in regional geoid computations, where usually a set of potential coefficients is combined with gravity anomalies. In order to see the effect of simplification of the error variance‐covariance propagation in geoid computations, three cases were considered: the full matrix, the diagonal elements only, and the limitation to the use of error degree variances. The resulting variations in the estimated geoid errors are shown in two test areas for different cap sizes. They can reach 50 cm for a zero cap size. Finally, test computations are presented of geoid determination by least squares collocation. Since the error covariance of the observations is included in the collocation operator, the geoid heights depend upon the choice of the type of error covariance propagation of the Earth potential coefficient set, as it is a part of the observation. For the three considered cases, variations in the geoid heights amount up to 45 cm.