bluuebunny/crossref_metadata_2025 huggingface dataset:
A mantle melting profile across the Basin and Range, SW USA
Authors: Wang K. , Plank T. , Walker J. D. , Smith E. I.
Published: October 2002

The major and trace element composition of late Cenozoic basalts (0–10 Ma) across the Basin and Range province (B&R) preserve a clear signal of mantle melting depth variations. FeO, Fe8.0, and Tb/Yb increase, whereas Si8.0 and Al8.0 decrease, from west to east across the B&R along a profile at 36°–37°N. These variations are qualitatively consistent with shallower melting beneath the Western Great Basin (WGB) than in the central B&R. In order to quantify the depth range and percent of decompression melting, we invert primary Na2O and FeO contents of basalts using a melting model based on the partitioning of FeO and MgO in olivine and Na2O in clinopyroxene. An independent inversion, using the rare earth elements (REE), corroborates the melting depths obtained from the major element model and places most of the melting beneath the central B&R in the garnet‐peridotite stability field. We find that the shape of the melting region across the B&R closely mimics the shape of the mantle lithosphere, as inferred from geological and geophysical observations. Melting across the study area occurs largely within the asthenosphere and generally stops at the base of the mantle lithosphere. In the WGB, melting paths are shallow, from 75 to 50 km, and in some cases extend almost to the base of the crust. These melting paths are consistent with adiabatic melting in normal‐temperature asthenosphere, beneath an extensively thinned (or absent) mantle lithosphere. Shallow melting is consistent with geobarometry and isotopic compositions of local mantle xenoliths. Lithospheric thinning was caused by thermal erosion during Mesozoic subduction and/or simple shear or foundering during Cenozoic extension. In contrast, melting beneath the central B&R occurs beneath thick mantle lithosphere and requires mantle potential temperatures 200°C hotter than normal (melting paths from 140 to 100 km). The excess temperature beneath the central B&R is consistent with active upwelling of hot mantle in this region.
New evidence for syntectonic fluid migration across the hinterland‐foreland transition of the Canadian Cordillera
Authors: Knoop Stuart R. , Kennedy Lori A. , Dipple Gregory M.
Published: October 2002

Oxygen isotope data from syntectonic veins, thrust faults, and wall rocks suggest that fluids infiltrated the Western Ranges of the Rocky Mountain foreland from deeper rocks of the Dogtooth Range during Mesozoic contraction. This signifies the first such evidence for kilometer‐scale fluid migration at the hinterland‐foreland transition of the Canadian Cordillera. Fluid infiltration resulted in isotopic depletion in wall rocks and isotopic disequilibrium between veins (fluid) and their host rocks downstream of a lithologic, structural, and isotopic shift between predominantly siliciclastic (Dogtooth Range) and calcareous (Western Ranges) sequences. The shift corresponds with an average δ18O that is 3.0‰ higher in the Western Ranges. In the Dogtooth Range, atypically low carbonate values (15.6‰ ± 0.8‰ (VSMOW)), and the consistency with which vein signatures approach the average bulk rock δ18O of individual outcrops indicate fluid buffering by a siliciclastic reservoir and outcrop‐scale (10–100 m) fluid circulation. In contrast, rocks and veins in the Western Ranges exhibit a gradational increase in δ18O that extends ≥14 km eastward and up section from the isotopic shift and possess values that are below or at the low end of typical carbonate compositions (16.2–20.9‰ (VSMOW)). Furthermore, veins systematically exhibit lower oxygen values than their host rocks; this implies that the fluids that entered the Western Ranges were in modest disequilibrium with the rocks through which they flowed. We use a one‐dimensional reactive transport model to infer that rocks in the Western Ranges experienced a time‐integrated fluid flux of 1.1 × 105 mol H2O cm−2 (≈2.4 × 106 cm3 cm−2 ) and sluggish reaction kinetics during the flow event.
New paleomagnetic data from the northern Argentine Puna: Central Andes rotation pattern reanalyzed
Authors: Prezzi Claudia B. , Alonso Ricardo N.
Published: October 2002

Along the central Andes a pattern of vertical axis rotations (CARP) has been paleomagnetically identified. These rotations, which are counterclockwise north of Arica and clockwise to the south, have been detected in Mesozoic and Cenozoic rocks cropping out in the Andean forearc and the Altiplano‐Puna. Different models have been proposed to explain the geodynamic evolution of the central Andes, but the origin of these rotations remains controversial. Such models involve oroclinal bending and/or local block rotations. This lack of agreement is partly due to the scarce paleomagnetic database, particularly for the Argentine Puna. Here we report new paleomagnetic data obtained in the northern Argentine Puna from 57 sites drilled from Miocene volcaniclastic rocks and 12 sites from the Cretaceous‐Eocene Salta Group. It was determined that (1) the Morro Blanco area did not undergo significant vertical axis rotation since at least the middle Miocene; (2) the Loma Blanca mine area underwent a clockwise vertical axis rotation of ∼10° later than the late Miocene; and (3) the Coranzulí area underwent a clockwise vertical axis rotation of ∼40° later than the Cretaceous‐Eocene. The central Andes database was analyzed, and some features of the CARP were determined. These features were explained considering “domino”‐style models for the forearc and a local block rotation model in response to penetrative shear for the Altiplano‐Puna. Penetrative shear would occur along a high electrical conductivity zone. However, the possibility of the existence of an oroclinal bending of <5° cannot be discarded.
Crustal construction of a volcanic arc, wide‐angle seismic results from the western Alaska Peninsula
Authors: Lizarralde Daniel , Holbrook W. Steven , McGeary Susan , Bangs Nathan L. , Diebold John B.
Published: October 2002

Results from the 1994 Aleutian Seismic experiment delineate basic oceanic arc crustal architecture, constrain magmatic flux rates and bulk arc composition, and address questions of continental crustal genesis. Here we present results from a transect across protocontinental crust of the westernmost Alaska Peninsula (line A3) and compare this structure to a purely oceanic arc transect farther west. Arc crustal structure is similar along these two transects. Magmatic accretion occurs at the top and bottom of preexisting oceanic crust as a 5‐ to 10‐km‐thick upper crustal carapace of low velocity (2–5.8 km s−1) volcaniclastics, flows and small plutons, and a mafic lower crustal underplate (∼7.0 km s−1) of variable thickness, for a maximum arc crust thickness of ∼25–30 km. Lateral lower crustal velocity gradients and high velocities (>7.5 km s−1) beneath the forearc suggest dominantly vertical lower crustal accretion above a focused melt source and a forearc underlain by little magmatic crust but rather partially intruded and/or serpentinized mantle. The ratio of upper to lower crustal volume is ∼1, and the total arc crust volume implies a magmatic flux of ∼67 km3 km−1 m.y.−1, more than twice previous estimates for this arc and global productivity. The crust is thinner and more mafic than continental crust, and it lacks a massive tonalitic upper crust characteristic of the continents. An interpreted accumulation of upper crustal carapace material at midcrustal depths on line A3 has a velocity of ∼6.4 km s−1, suggesting an intermediate composition. Accretionary complex terranes consisting of accumulations of this type material would thus have bulk compositions similar to continental crust.
Paleoseismology along the Cittanova fault: Implications for seismotectonics and earthquake recurrence in Calabria (southern Italy)
Authors: Galli Paolo , Bosi Vittorio
Published: October 2002

This paper focuses on the seismogenic structure responsible for the 5 February 1783 earthquake (M> 7, Calabria, southern Italy), in particular the slip rate and recurrence time along the Cittanova fault, thus giving new insights on the seismic hazards and seismotectonics of the region. The 1783 earthquake, one of the strongest earthquake of the Mediterranean area, caused more than 30,000 casualties and radically changed the physiography of the local landscape. Until now, the Cittanova fault (or Aspromonte fault) has been generally identified as the surficial expression of the seismogenetic structure, even though its youthful activity has never been demonstrated. Recently, the shortage of geologic evidence of middle‐upper Pleistocene activity on this fault led some authors to locate the 1783 fault elsewhere. In our study we used a multidisciplinary approach (paleoseismology, geomorphology, historical seismology, archaeoseismology); our analysis of contemporary literary sources strengthened our interpretation of the coseismic origin of many geological features and, sometimes, oriented our field research toward specific areas. In particular, we excavated trenches and pits at three different sites along the trace of the Cittanova fault for paleoseismological analysis and14C dating. Our data document the Holocene and present activity of the Cittanova fault, providing a minimum vertical slip rate of 0.44 mm/yr and a possible return time of 1350–1800 years for earthquakes similar to 1783. The penultimate event has been dated to the 4th century A.D., a time when southern Calabria and eastern Sicily were devastated by an earthquake documented by archaeoseismic analyses.
Fabric development with nearest‐neighbor interaction and dynamic recrystallization
Authors: Thorsteinsson Throstur
Published: October 2002

Polycrystals undergoing ductile deformation develop lattice‐preferred orientation (fabric) as a result of intracrystalline slip. A consequence of fabric development is that bulk physical properties become anisotropic. Fabric development and macroscopic deformation are studied by examining three effects: nearest‐neighbor interaction (NNI) among crystals, polygonization, and migration recrystallization. The effects of NNI are modeled by arranging the crystals on a three‐dimensional cubic grid and assigning six neighbors to each crystal. The “strength” of interaction can vary from no interaction (homogeneous stress) to “strong” interaction (significant stress redistribution). Increasing the NNI leads to a more homogeneous strain. Fabric varies in both strength and symmetry at a given bulk strain, depending on the strength of NNI. For a prescribed fabric the strain rate increases as the NNI increases. Recrystallization is modeled from energy balance considerations, and polygonization is formulated in terms of stress differences. Both processes require knowledge of dislocation density, which is calculated in the model as a function of crystal strain and grain size, both of which vary with time. Models of fabric development in ice that include NNI lead to more realistic fabric evolution than models with homogeneous stress. However, available data on fabric evolution are inadequate to determine quantitatively the strength of NNI acting in ice.
Lithospheric deformation during the early stages of continental collision: Numerical experiments and comparison with South Island, New Zealand
Authors: Pysklywec Russell N. , Beaumont Christopher , Fullsack Philippe
Published: October 2002

The nature of lithospheric deformation during continental plate collision still remains unresolved. While it has often been proposed that the mantle lithosphere is accommodated by distributed thickening of a viscous root, an alternate hypothesis suggests that significant portions of convergent mantle lithosphere essentially undergo underthrusting or subduction. To further consider this issue, we model the thermochemical evolution of the lithosphere‐mantle system using arbitrary Lagrangian‐Eulerian finite element techniques. We incorporate a mix of viscous (thermally activated power law creep) and plastic (frictional Coulomb with strain softening) rheologies in the numerical experiments to treat disparate composition in the crust and mantle. A range of rheological and mechanical parameters is explored to determine controls on the style of lithospheric deformation. The models suggest that during the initial stages of plate collision the mantle lithosphere is characterized by plate‐like behavior and underthrusting/subduction of the upper region in conjunction with distributed thickening and Rayleigh‐Taylor type viscous instability of the lower portion. Depending on the material rheology, temperature regime, and imposed convergence velocity, the deforming mantle lithosphere demonstrates various combinations of these “end‐member” behavioral modes. The modeling results are interpreted in the context of observed lithospheric deformation across South Island, New Zealand. A combined style of underthrusting and distributed thickening is consistent with the observed crustal structure in the young collisional orogen as well as seismic imaging of the geometry of the underlying lithosphere.
The Q structure of the upper mantle: Constraints from Rayleigh wave amplitudes
Authors: Selby Neil D. , Woodhouse John H.
Published: October 2002

The three‐dimensional shear attenuation, or Qμ, structure of the upper mantle is still poorly understood despite the increasing accuracy and sophistication of velocity models of the same part of the Earth. Here we present a set of three‐dimensional Qμ models of the upper mantle based on amplitude measurement of minor and major arc Rayleigh waves in the period range 70–170 s. The models show that areas of low attenuation underlie continents to a depth of around 300 km and areas of high attenuation are associated with oceans and plate boundaries, particularly in the top 200 km. Below ∼350 km, the sensitivity of the data to attenuation structure decreases rapidly. It is well known that the amplitudes of surface waves can be strongly influenced by factors other than attenuation, in particular, focusing due to elastic structure and errors in source mechanisms. We consider the effects of focusing by discussing the implications of the results of Selby and Woodhouse [2000]. Comparisons with focusing predictions using phase velocity maps allow us to identify features due to focusing in maps of attenuation distribution. The influence of the source on the observed attenuation distribution is investigated by including a source term in the model inversion. We find that successfully combining the data at each frequency into one three‐dimensional model requires that this source term must vary with frequency, suggesting that a simple correction to the scalar moment of each event cannot explain the observations. Although the effects of focusing and the source can be significant, we find that the observed attenuation pattern is robust up to degree 8; however, shorter‐wavelength structure can be strongly influenced by other controls. Finally, we use one of the models to predict the degree 2 attenuation pattern for 250‐s Rayleigh waves. The observed pattern suggests that existing normal mode observations may be explainable in terms of structure above 350 km depth.
A deep strong reflector in the Nankai accretionary wedge from multichannel seismic data: Implications for underplating and interseismic shear stress release
Authors: Park Jin‐Oh , Tsuru Tetsuro , Takahashi Narumi , Hori Takane , Kodaira Shuichi , Nakanishi Ayako , Miura Seiichi , Kaneda Yoshiyuki
Published: October 2002

We present the results of structural and stratigraphic interpretations for the multichannel seismic profiles to reveal the structure of the Nankai accretionary wedge off Shikoku Island. On the basis of reflection characteristics we identify three major seismic units that correspond to a unit consisting of both Oligocene‐Miocene accretionary prism and the sedimentary cover (unit A), a unit of underthrust Miocene‐Pliocene hemipelagic sediments (unit B), and a unit of subducting pre‐Miocene oceanic crustal rocks (unit C). According to relative strength of the topmost reflector of unit B the Nankai accretionary wedge can be divided into three different structural zones along the cross‐strike direction: apparent décollement reflector zone, low‐amplitude reflector (LAR) zone, and deep strong reflector (DSR) zone. Despite the velocity uncertainty involved in the depth conversion and therefore thickness estimation error, the décollement step‐down and the possible duplex structures in the LAR and DSR zones support that the landward thickness variation of unit B is caused by the structural thickening such as the underplating process. Structural features and reverse polarity suggest that the DSR as an upper interface of the underplated layer (unit B) corresponds to a shear plane or a fault zone with high pore fluid pressures. We found that most of the DSR was observed in a region which has not suffered from the 1946 coseismic dislocation even though it was locked during the interseismic period. The DSR as a shear plane or a fault zone constrains a regional variety of the coseismic dislocation around the updip limit of the Nankai seismogenic zone. Considering all of the DSR characteristics, we propose a hypothesis for the DSR's development and role. The DSR may play an important role as a possible releaser of the shear stress energy stored at the plate boundary around the updip limit of the Nankai seismogenic zone. A good correlation between the theoretical vertical displacement and seafloor topographic feature support the possible slip along the DSR as a shear plane.
Envelope broadening of spherically outgoing waves in three‐dimensional random media having power law spectra
Authors: Saito Tatsuhiko , Sato Haruo , Ohtake Masakazu
Published: October 2002

High‐frequency S wave seismogram envelopes are broadened with increasing travel distance due to diffraction and scattering. The basic mechanism of the broadening has been studied on the basis of the scattering theory with the parabolic approximation for the scalar wave equation in random media. However, conventional models are not realistic enough since the plane wave modeling is too simple and the Gaussian autocorrelation function (ACF) is far from the reality to represent the inhomogeneity in the Earth. Focusing on the early part of envelopes, we formulated the envelope broadening of spherically outgoing scalar waves in three‐dimensional von Kármán‐type random media, of which the spectra decay according to a power law at large wave numbers. Random media are characterized by three parameters: RMS fractional velocity fluctuation ε, correlation distance a, and order κ that controls the gradient of the power law spectra. This model predicts that the envelope duration increases with both travel distance and frequency when short‐wavelength components are rich in random media, while the duration is independent of frequency when short‐wavelength components are poor. Introducing phenomenological attenuation Q, we developed a method for estimating the parameters of inhomogeneity and attenuation from the envelope duration. Applying this method to S wave seismogram envelopes for the frequency range from 2 to 32 Hz in northeastern Honshu Japan, we estimated the random inhomogeneity parameters as κ = 0.6, ε2.2a−1 ≈ 10−3.6 [km−1] and f/Q = 0.0095 [s−1], where f is frequency. The power law portion of the estimated power spectral density function is P(m) ≈ 0.01 m−4.2 [km3], where m is wave number.
The near‐coastal microseism spectrum: Spatial and temporal wave climate relationships
Authors: Bromirski Peter D. , Duennebier Fred K.
Published: October 2002

Comparison of the ambient noise data recorded at near‐coastal ocean bottom and inland seismic stations at the Oregon coast with both offshore and nearshore buoy data shows that the near‐coastal microseism spectrum results primarily from nearshore gravity wave activity. Low double‐frequency (DF), microseism energy is observed at near‐coastal locations when seas nearby are calm, even when very energetic seas are present at buoys 500 km offshore. At wave periods >8 s, shore reflection is the dominant source of opposing wave components for near‐coastal DF microseism generation, with the variation of DF microseism levels poorly correlated with local wind speed. Near‐coastal ocean bottom DF levels are consistently ∼20 dB higher than nearby DF levels on land, suggesting that Rayleigh/Stoneley waves with much of the mode energy propagating in the water column dominate the near‐coastal ocean bottom microseism spectrum. Monitoring the southward propagation of swell from an extreme storm concentrated at the Oregon coast shows that near‐coastal DF microseism levels are dominated by wave activity at the shoreline closest to the seismic station. Microseism attenuation estimates between on‐land near‐coastal stations and seismic stations ∼150 km inland indicate a zone of higher attenuation along the California coast between San Francisco and the Oregon border.
Fabric of the Rio Ceará–Mirim mafic dike swarm (northeastern Brazil) determined by anisotropy of magnetic susceptibility and image analysis
Authors: Archanjo Carlos J. , Araújo Marcelus G. S. , Launeau Patrick
Published: October 2002

Anisotropy of magnetic susceptibility (AMS) and petrofabric studies of the Early Cretaceous Rio Ceará–Mirim dike swarm reveal that shape‐preferred orientation of opaque grains controls the magnetic fabric. The 350‐km‐long swarm is characterized by relatively thick dikes, of the order of 20 m up to 150 m in width. The strong magnetic susceptibility of these rocks, around 5 × 10−2 SI, is attributed to magnetite with low Ti content. The magnetite usually forms euhedral to subhedral, equant to skeletal grains. The resulting AMS has a large proportion of abnormal fabric types, as observed in 58% of the 50 dikes studied. The remaining 42% of the dikes show a “normal” fabric regionally characterized by steep magnetic foliations and subhorizontal lineations. In the central eastern part of the swarm, however, the lineation plunges downdip, suggesting a magmatic feeder zone. The shape alignment of the plagioclase supports the inferred regional flow pattern, indicating a primary origin for the normal AMS. Inverse to highly imbricate magnetic fabrics have lineations perpendicular to, or at a high angle to, the dike wall. A shape lineation of opaque grains lies generally close to the magnetic lineation, while the foliation is provided by the oblate‐shaped plagioclase fabric. Petrofabric studies indicate that the abnormal magnetic fabric is unrelated to flow. Competition of interstitial magnetite grains crystallizing either along, as well as perpendicular to the magma stretching direction seems to account for the weak magnetic anisotropy and the irregular distribution of the AMS types on the swarm.
Paleomagnetism and geochronology of the Ecstall pluton in the Coast Mountains of British Columbia: Evidence for local deformation rather than large‐scale transport
Authors: Butler Robert F. , Gehrels George E. , Baldwin Suzanne L. , Davidson Cameron
Published: October 2002

Samples for geochronologic, geobarometric, and paleomagnetic analyses were collected across the northern portion of the Ecstall pluton southeast of Prince Rupert, British Columbia. Al‐in‐hornblende geobarometry indicates pressures from 740 ± 10 to 840 ± 30 MPa corresponding to crystallization depths of ∼25 to ∼30 km. U/Pb analyses of zircons from western, central, and eastern localities within the pluton yield crystallization ages of 91.5 ± 1.0 Ma, 90.8 ± 1.0 Ma, and 90.5 ± 1.0 Ma, respectively. Rock magnetic experiments, reflected light microscopy, and thermal demagnetization behavior suggest that natural remanent magnetism is carried by low‐Ti titanohematite. Unblocking temperatures of the characteristic remanent magnetization (ChRM) are dominantly in the 560°C to 630°C range, with age of magnetization approximated by the40Ar/39Ar hornblende ages of 84.2 ± 0.10 Ma on the western margin and 76.4 ± 0.6 Ma in the center of the pluton. Site‐mean ChRM directions were isolated for paleomagnetic samples from 23 sites and are distributed along a small circle with subhorizontal axis at ∼340° azimuth. ChRM directions from the central portion of the pluton are concordant with the expected Cretaceous magnetic field direction, while ChRM directions from the western margin are discordant by >70°. Folding of the Ecstall pluton, either during Late Cretaceous west directed thrust transport above the convex upward Prince Rupert Shear Zone or during younger deformation of the pluton and underlying shear zone, can account for the paleomagnetic data and is consistent with the geochronologic, geobarometric, and structural geologic observations.
Impact of the intermediate principal stress on the strength of heterogeneous rock
Authors: Fjær Erling , Ruistuen Helge
Published: October 2002

A numerical model simulating a granular material has been used to study the impact of the intermediate principal stress on rock strength. While maintaining the minimum principal stress at zero, the model predicts strength to increase with increasing constant ratios of σ2/σ1. This trend was found to reverse as the stress ratio exceeded 0.5, and for the case of σ2 = σ1 the predicted compressive strength nearly equaled the one obtained under uniaxial loading conditions. This impact of the intermediate principal stress indicates that not only the stress level but also the stress symmetry has an impact on the rock strength. If the rock is heterogeneous, which rocks always are at some scale, there is a different probability for failure at different orientations of the sample, relative to the orientation of the stresses (irrespective of any anisotropy effects). If σ2 = σ3or σ2 = σ1, there are many equivalent orientations of the macroscopic failure plane once the failure criterion is fulfilled, and the failure plane will take the orientation for which the rock fails most easily. If σ2 is truly intermediate, only two potential orientations of the failure plane fulfils the failure criterion initially. As a consequence, the expectation value for the rock strength is higher when σ2 is truly intermediate. A numerical model has been developed, which incorporates rock heterogeneity through a smoothened failure criterion. The model quantitatively couples the impact of the intermediate principal stress on rock strength to the natural variation in experimental tests results.
An experimental study of the kinetics of decompression‐induced crystallization in silicic melt
Authors: Hammer Julia E. , Rutherford Malcolm J.
Published: October 2002

Experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression. Pinatubo dacite was held at 780°C, 220 MPa, fO2 = NNO + 2, H2O‐saturated conditions for an equilibration period, decompressed to final pressures, Pf, ranging from 175 to 5 MPa, and then held for 0.3–931 hours. According to the plagioclase liquidus curve in PH2O‐T space for the relevant melt composition, these decompressions impose effective undercoolings, ΔTeff, of 34–266°C. Growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 ≤ Pf < 150 MPa (ΔTeff = 34–93°C), and equilibrium crystal modes are achieved in <168 hours. Microlite nucleation is the dominant transformation process for 10 < Pf < 50 MPa (ΔTeff = 125–241°C), and chemical equilibrium is not attained by 168 hours under these conditions. Slow, steady decompressions typically produced normally zoned, euhedral, and planar‐faceted feldspar crystals, although anhedral morphologies were produced at very low Pf. Contrary to expectation, slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure. Although counterintuitive, these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth (controlled by ΔTeff and melt viscosity) experienced during each decompression path. Analysis of liquid to solid transformation kinetics using TTT‐style diagrams shows that crystallization occurs most rapidly at ∼100 MPa by a crystal growth mechanism. The next most efficient crystallization conditions are at 25 MPa, in a crystal nucleation‐dominated regime.
Thermal constriction and slab tearing at the origin of a superinflated spreading ridge: Marsili volcano (Tyrrhenian Sea)
Authors: Marani Michael P. , Trua Teresa
Published: October 2002

Recently acquired swath bathymetry of the Marsili basin has offered an unprecedented opportunity to study the processes of back arc ocean basin development in the Tyrrhenian Sea. In particular, the detailed morphology of Marsili seamount, a large, strongly elongated volcano located in an axial position within the <2 Ma ocean crust floored Marsili basin, is a key to understanding the mechanisms governing lithosphere formation in this young basin. The basin is near circular in shape with diameter on the order of 120 km and is positioned in the southern Tyrrhenian Sea, above the steeply dipping Ionian oceanic slab of Mesozoic age. It is bounded southward by the Aeolian volcanic arc and the Calabrian accretionary wedge, surface evidence of the northwesterly directed subduction. The most outstanding feature of the basin is the elongated, 3000‐m‐high Marsili volcano which reveals distinctive morphology strikingly akin to the high‐order segmentation and volcanic landforms described in mid‐ocean slow spreading ridges. On the basis of its distinctive morphology and incremental growth relationship we propose that Marsili volcano represents a superinflated spreading ridge resulting from a distinct thermal pulse of increased melt production occurring within the young and immature Marsili basin. Surrounding cooler continental lithosphere thermally constricts ridge propagation and crust production in Marsili basin to the finite scale of Marsili volcano. Increased melt production to feed the superinflated Marsili ridge is generated by deep, lateral asthenospheric mantle flow produced at the edges of tears that bound the subducting ocean crust of the Ionian plate. Slow spreading plate separation, outpaced by the increase in magma generation, results in vertical accretion to produce the superinflated ridge. The existence of dip‐directed tears delimiting the narrow Ionian slab is supported by the geological evolution of the surrounding foreland and Apennine/Maghrebid mountain belt during early/middle Pleistocene, i.e., the time of formation of the Marsili volcano. Present‐day structure and volcanism furnish direct and indirect surface evidence of the presence and location of the slab tears.
Homogeneous bubble nucleation in rhyolitic magmas: An experimental study of the effect of H2O and CO2
Authors: Mourtada‐Bonnefoi C. C. , Laporte D.
Published: October 2002

Rapid decompression experiments were performed to study homogeneous bubble nucleation in a crystal‐free rhyolitic liquid at 800°C. Bubble nucleation was produced by lowering the pressure at 1–10 MPa s−1 from an initial value between 200 and 295 MPa to a final value below the volatile saturation pressure PSat. Six volatile compositions with 4.1–7.7 wt % H2O and 10–1200 ppm CO2 were investigated. For each composition we determined the critical pressure PHoN below which homogeneous nucleation can proceed. The samples quenched below PHoN showed a nucleated core with a large number of uniformly spaced bubbles. With decreasing pressure, bubble number densities increased from <1011 m−3 (for samples quenched just below PHoN) to >1015 m−3. The degree of supersaturation required for homogeneous nucleation, ΔPHoN (= PSat − PHoN), increased with decreasing H2O content or increasing CO2 content. Huge values of ΔPHoN, ≥135 to 310 MPa, were measured in the H2O‐poor compositions (4.1–4.6 wt % H2O; 50–1100 ppm CO2); much lower values from ≈60 to 160 MPa were obtained in the H2O‐rich compositions (7.0–7.7 wt % H2O; 10–630 ppm CO2). The high ΔPHoN in liquids with 4–5 wt % H2O should result in the buildup of large degrees of supersaturation during magma ascent, a very late nucleation event, and a rapid (explosive) vesiculation. By contrast, rhyolitic liquids with much larger water contents have higher saturation pressures and much lower ΔPHoN: bubble nucleation may therefore occur at depth in the volcanic conduit favoring a subsequent near‐equilibrium degassing.
Crustal structure of central Lake Baikal: Insights into intracontinental rifting
Authors: ten Brink Uri S. , Taylor Michael H.
Published: October 2002

The Cenozoic rift system of Baikal, located in the interior of the largest continental mass on Earth, is thought to represent a potential analog of the early stage of breakup of supercontinents. We present a detailedPwave velocity structure of the crust and sediments beneath the Central Basin, the deepest basin in the Baikal rift system. The structure is characterized by a Moho depth of 39–42.5 km; an 8‐km‐thick, laterally continuous high‐velocity (7.05–7.4 km/s) lower crust, normal upper mantle velocity (8 km/s), a sedimentary section reaching maximum depths of 9 km, and a gradual increase of sediment velocity with depth. We interpret the high‐velocity lower crust to be part of the Siberian Platform that was not thinned or altered significantly during rifting. In comparison to published results from the Siberian Platform, Moho under the basin is elevated by <3 km. On the basis of these results we propose that the basin was formed by upper crustal extension, possibly reactivating structures in an ancient fold‐and‐thrust belt. The extent and location of upper mantle extension are not revealed by our data, and it may be offset from the rift. We believe that the Baikal rift structure is similar in many respects to the Mesozoic Atlantic rift system, the precursor to the formation of the North Atlantic Ocean. We also propose that the Central Baikal rift evolved by episodic fault propagation and basin enlargement, rather than by two‐stage rift evolution as is commonly assumed.
Frictional‐viscous flow of phyllosilicate‐bearing fault rock: Microphysical model and implications for crustal strength profiles
Authors: Bos Bart , Spiers Christopher J.
Published: October 2002

It is widely believed that around the brittle‐ductile transition, crustal faults can be significantly weaker than predicted by conventional two‐mechanism brittle‐ductile strength envelopes. Factors contributing to this weakness include the polyphase nature of natural rocks, foliation development, and the action of fluid‐assisted processes such as pressure solution. Recently, ring shear experiments using halite/kaolinite mixtures as an analogue for phyllosilicate‐rich rocks for the first time showed frictional‐viscous behavior (i.e., both normal stress and strain rate sensitive behavior) involving the combined effects of pressure solution and phyllosilicates. This behavior was accompanied by the development of a mylonitic microstructure. A quantitative assessment of the implications of this for the strength of natural faults has hitherto been hampered by the absence of a microphysical model. In this paper, a microphysical model for shear deformation of foliated, phyllosilicate‐bearing fault rock by pressure solution‐accommodated sliding along phyllosilicate foliae is developed. The model predicts purely frictional behavior at low and high shear strain rates and frictional‐viscous behavior at intermediate shear strain rates. The mechanical data on wet halite + kaolinite gouge compare favorably with the model. When applied to crustal materials, the model predicts major weakening with respect to conventional brittle‐ductile strength envelopes, in particular, around the brittle‐ductile transition. The predicted strength profiles suggest that in numerical models of crustal deformation the strength of high‐strain regions could be approximated by an apparent friction coefficient of 0.25–0.35 down to depths of 15–20 km.
Physical conditions, structure, and dynamics of a zoned magma chamber: Mount Pelée (Martinique, Lesser Antilles Arc)
Authors: Pichavant Michel , Martel Caroline , Bourdier Jean‐Louis , Scaillet Bruno
Published: October 2002

Experimental results and petrologic observations of the eruptive history of Mount Pelée are integrated, and a model for the magma storage system is presented. Recent (stage 3) Plinian and Pelean activity (P1, 650 years B.P.; 1902, 1929) erupted relatively homogeneous andesites (average 62 wt % SiO2). They are porphyritic (35–58 vol % crystals) and contain phenocrysts of plagioclase (Plag) (An50–90), orthopyroxene (Opx) (En52–60), and magnetite (Mt) (∼Mt70). Glasses (both interstitial and trapped) are rhyolitic (74–77 wt % SiO2). Clinopyroxene (Cpx), ilmenite (Ilm), amphibole (Amph) (mostly resorbed pargasitic hornblendes), and olivine (Ol) are present as minor phases. Products of 1902 and 1929 contain mafic enclaves (51–59 wt % SiO2) with compositions similar to basaltic andesite lavas erupted during stage 2 (40,000–19,500 years B.P.). Conditions in the andesitic part of the magma chamber, as determined from experimental phase equilibria, do not differ between the P1, 1902, and 1929 eruptions (875–900°C, 2 ± 0.5 kbar, ΔNNO = +0.4–0.8, melt H2O content of 5.3–6.3 wt %). New experimental data on a basaltic andesite composition (53 wt % SiO2) from stage 2, at 4 kbar, 950–1025°C, for melt H2O concentrations from 8.3 to 2.6 wt %, andfO2between NNO and NNO + 4 simulate crystallization in the mafic part of the chamber. Liquidus or near‐liquidus Ol, An‐rich Plag, Al‐ and Fe3+‐rich salite and augite, pargasitic hornblende, and Al‐ and Mg‐rich Mt have compositions close to phenocrysts in mafic products from stages 2 and 3. Experimental liquids range from basaltic andesite to dacite. Application of experimentally derived mineral‐melt Al/Si and Fe/Mg partition coefficients to mineral compositions from mafic lavas and cumulates from stage 2 shows that the chamber is fed by relatively evolved parental basaltic liquids (Mg # ∼ 55–60)(Mg # = Mg/(Mg + FeT). They have low temperatures (≤1050°C), high melt H2O contents (>5–6 wt %), andfO2(ΔNNO mostly between +1 and +2) and crystallize an Ol + Cpx + Mt assemblage followed by Plag + Amph, although Amph may have started to crystallize with Ol and Cpx. Compositions of natural glasses and amphibole in mafic cumulates and lavas record a continuous evolution from basaltic‐basaltic andesite to basaltic andesite‐dacite liquids. Crystal fractionation of basaltic magmas is the main process controlling the chemical diversity at Mount Pelée. Crystallization in the mafic part produces an andesitic‐dacitic residual liquid which subsequently evolves to produce the andesitic part. The present‐day situation is typical of low fluxes of mafic magmas in comparison with stage 2.
A reversal of the Earth's magnetic field recorded in mid‐Miocene lava flows of Gran Canaria: Paleodirections
Authors: Leonhardt R. , Matzka J. , Hufenbecher F. , Soffel H. C. , Heider F.
Published: October 2002

A paleomagnetic investigation has been carried out on a continuous series of 87 volcanic lava flows from the mid‐Miocene shield basalts of Gran Canaria. The sequence covers a reversal of the Earth's magnetic field from reverse to normal. Intermediate directions of the field are recorded in 34 lava flows. During the reversal the virtual geomagnetic poles (VGPs) show an accumulation in three consecutive areas: southeast of South America, east of India, and in the central Pacific. In addition to the reversal three excursions are recorded in the sequence, two preceding and one succeeding the reversal. The intermediate directions of the excursion just before and after the transition have the same VGP positions as the first and the last accumulation zone, respectively. From radiometric age constraints this transition corresponds very likely to a ∼14.1 Ma field reversal. The remarkable returns to the same transitional positions near South America and the central Pacific indicate preferred locations of the VGP during the transitional stages. Only the South America cluster can be correlated to VGP clusters observed in other reversals of the last 10 Myr and to lower mantle seismic anomalies. The VGPs in the Pacific track instead along the antipode of the site longitude. This indicates that mechanisms other than lower mantle heterogeneities additionally affect the transitional record of the sampled sequence and zonal harmonics are plausible candidates.
Whole mantle shear structure beneath the East Pacific Rise
Authors: Melbourne Timothy I. , Helmberger Donald V.
Published: October 2002

We model broadband seismograms containing triplicated S, S2, and S3 along with ScS to produce a pure path one‐dimensional model extending from the crust to the core‐mantle boundary beneath the East Pacific Rise. We simultaneously model all body wave shapes and amplitudes, thereby eliminating depth‐velocity ambiguities. The data consist of western North American broadband recordings of East Pacific Rise (EPR) affiliate transform events that form a continuous record section out to 82° and sample nearly the entire East Pacific Rise. The best fitting synthetics contain attenuation and small changes in lithospheric thickness needed to correct for variation in bounce point ages. The 660‐km discontinuity is particularly well resolved and requires a steep gradient (4%), extending down to 745 km. We find no discernible variation in apparent depths of the 405‐ and 660‐km discontinuities over ridge‐orthogonal distances on the order of 1000 km (or 20 Ma lithosphere). Body waveform comparisons indicate that we can resolve discontinuity depths to less than ±10 km, providing an upper limit to transition zone topography. These depth estimates, in conjunction with the fan shot nature of the ray paths, lower the detection limit from S2 precursor analysis of the lateral length scale over which short‐wavelength topographic variation could occur and indicate the sub‐EPR Transition Zone and upper mantle are remarkably homogeneous. The lower mantle beneath the East Pacific Rise is well modeled by PREM, with the greatest variation occurring in ScS, reflecting strong heterogeneity along the core‐mantle boundary. Together, these observations require that the East Pacific Rise spreading ridge cannot be actively supplied from the local lower mantle and that tomographically imaged lateral variation beneath the ridge likely reflects lateral smearing of outlying velocity gradients. Dynamically, the transition zone therefore appears vertically decoupled from the overlying East Pacific Rise spreading system.
Oceanic torques on solid Earth and their effects on Earth rotation
Authors: Fujita Masayuki , Chao Benjamin F. , Sanchez Braulio V. , Johnson Thomas J.
Published: October 2002

We calculate that the oceanic torque vectors (x, y, z components) acting on the solid Earth using the Parallel Ocean Climate Model (POCM) output data at 3‐day intervals for the period 1988–1995. This study examined three distinct types of torque, with different physical mechanisms: pressure, gravitational, and frictional. The pressure torque is further divided into two parts: that due to Earth's ellipticity and that due to the ocean bottom topography. According to resultant time series the ellipticity pressure torque causes the largest effect for the x and y components. The gravitational torque and the ellipticity pressure torque are shown numerically to be exactly proportional by a factor of −0.49. The topographic pressure torque is somewhat smaller in overall amplitude than the ellipticity‐induced torque (the sum of the ellipticity pressure torque and gravitational torque). The ocean bottom frictional torque is found to be negligible in all three components. To validate the torque calculations, we compare the calculated oceanic torques with effective torque derived from the ocean angular momentum (OAM) variation, both from POCM. It is shown that the effective torque associated with the mass term of the OAM agrees remarkably well with the ellipticity‐induced torque as required by theory. For the motion term the effective torque can be explained mostly by the topographic pressure torque in the intraseasonal periods, but the seasonal and longer‐period components show significant differences between the two. The difference should theoretically be attributed to the exclusion of the wind frictional torque on the ocean. However, a comparison based on the NASA GEOS‐1 atmospheric model shows that the wind frictional torque obtained is insufficient in power in the x and y components. Finally, in the application to the Earth rotation excitation it is found that both the ellipticity and topographic torques have comparable contributions to polar motion excitation for both seasonal and nonseasonal variations. For the length of day variation, our torque approach is presently inadequate because the oceanic contribution is relatively too small to be estimated in the presence of data and model errors.
Lower crustal deformation beneath the central Transverse Ranges, southern California: Results from the Los Angeles Region Seismic Experiment
Authors: Godfrey Nicola J. , Fuis Gary S. , Langenheim Victoria , Okaya David A. , Brocher Thomas M.
Published: October 2002

We present a P wave velocity model derived from active source seismic data collected during the 1994 Los Angeles Region Seismic Experiment. Our model extends previously published upper crustal velocity models to mantle depths. Our model was developed by both ray tracing through a layered model and calculating travel times through a gridded model. It includes an 8‐km‐thick crustal root centered beneath the surface trace of the San Andreas fault, north of the highest topography in the San Gabriel Mountains. A simple mass balance calculation suggests that ∼36 km of north‐south shortening across the San Andreas fault in the central Transverse Ranges could have formed this root. If north‐south compression began when the “Big Bend” in the San Andreas fault formed at ∼5 Ma, 36 km of shortening implies a north‐south contraction rate of ∼7.1 mm/yr across the central Transverse Ranges. If, instead, north‐south compression began when the Transverse Ranges formed at 3.4–3.9 Ma, 36 km of shortening implies a contraction rate of 9.2–10.6 mm/yr. North of the San Andreas fault, the Mojave Desert crust has a low‐velocity (6.3 km/s) mid and lower crust and a 28‐km‐deep Moho. South of the San Andreas fault, beneath the Los Angeles and San Gabriel Valley basins, there is a fast (6.6–6.8 km/s), thick (10–12 km) lower crust with a 27‐km‐deep Moho. Farther south still, the lower crust of the Continental Borderland is fast (6.6–6.8 km/s) and thin (5 km) with a shallow (22 km deep) Moho.