bluuebunny/crossref_metadata_2025 huggingface dataset:
Shear wave splitting in refracted waves returned from the upper mantle transition zone beneath northern Australia
Authors: Tong C. , Gudmundsson O. , Kennett B. L. N.
Published: February 2004

The broadband recording site at Warramunga (WRA) in the Northern Territory of Australia provides good coverage of seismic wave propagation through the upper mantle for sources in the earthquake belt through Indonesia and New Guinea. S waves recorded on the radial (SV) and tangential (SH) components are of comparable quality because the hard‐rock recording site minimizes the influence of coupling to P on the radial component. Refracted S waves from the uppermantle transition zone show a clear advance of SH wave arrivals compared with SV. Eleven polarization analyses of waves returned from the transition zone yield an average time shift of 2.3 s with the fast direction scattered about the transverse direction. Nine polarization measurements of waves returned from the top of the lower mantle yield an average time shift of 1.7 s, again with the fast direction near the transverse. No appreciable time differences are observed between the radial and transverse polarizations for paths refracted within the lithospheric lid. Because the observations of shear wave splitting in waves passing through the low‐velocity zone, the transition zone, and the top of the lower mantle are not coherent in their absolute polarization, the cause cannot lie in azimuthal anisotropy at shallow depths under the WRA station. The most plausible explanation is transverse isotropy in shear within the low‐velocity zone under the unusually thick mantle “lid” under Australia. A possible contribution may come from anisotropy in β—olivine at the top of the upper mantle transition zone. Transverse isotropy in the 200‐km‐thick layer below the lithosphere down to the transition zone with a 1% faster shear wave speeds for horizontal polarization compared with vertical polarization will explain the splitting data. For this asthenospheric region the level of anisotropy is quite reasonable and the polarization is consistent with lateral flow. The geometry of the available paths for waves propagating within the mantle lid is not sufficient to place constraints on the anisotropic properties of this heterogeneous and low‐loss region.
Tectonic plates, D″ thermal structure, and the nature of mantle plumes
Authors: Lenardic A. , Kaula W. M.
Published: February 2004

It is proposed that subducting tectonic plates can affect the nature of thermal mantle plumes by determining the temperature drop across a plume source layer. The temperature drop affects source layer stability and the morphology of plumes emitted from it. Numerical models are presented to demonstrate how introduction of platelike behavior in a convecting temperature dependent medium, driven by a combination of internal and basal heating, can increase the temperature drop across the lower boundary layer. The temperature drop increases dramatically following introduction of platelike behavior due to formation of a cold temperature inversion above the lower boundary layer. This thermal inversion, induced by deposition of upper boundary layer material to the system base, decays in time, but the temperature drop across the lower boundary layer always remains considerably higher than in models lacking platelike behavior. On the basis of model‐inferred boundary layer temperature drops and previous studies of plume dynamics we argue that generally accepted notions as to the nature of mantle plumes on Earth may hinge on the presence of plates. The implication for Mars and Venus, planets apparently lacking plate tectonics, is that mantle plumes of these planets may differ morphologically from those of Earth. A corollary model‐based argument is that as a result of slab‐induced thermal inversions above the core mantle boundary the lower most mantle may be subadiabatic, on average (in space and time), if major plate reorganization timescales are less than those required to diffuse newly deposited slab material.
Porosity and permeability evolution during hot isostatic pressing of calcite aggregates
Authors: Zhang Shuqing , Paterson Mervyn S. , Cox Stephen F.
Published: February 2004

Porosity, permeability, and storativity were measured during isostatic hot‐pressing of fine‐grained calcite aggregates at temperatures of 633 to 833 K, confining pressures of 200 to 300 MPa, and argon pore pressures of 100 to 250 MPa. The progressive changes in total porosity were measured in situ by monitoring the sample length changes. The connected porosity and the permeability and storativity were measured in situ by incrementing and oscillating pore pressure techniques, respectively. In a given test, there was a decrease with time in the rate of reduction of porosity, the rates being higher at higher temperature and effective pressure. The permeability k was nonlinearly related to the total porosity ϕ in the form k ∝ ϕn. The exponent n was approximately equal to 3 and thus consistent with the prediction of the “equivalent channel” model, in the range of porosity from 0.18 down to 0.07. Below 0.07, n became much larger (around 14), an effect that can be attributed to loss of connectivity and which is qualitatively similar to that observed by Bernabé et al. [1982] in post‐hot‐pressing measurements. However, a cube law continues to apply below 0.07 total porosity if the permeability is related to the connected porosity itself. The storativity is also nonlinearly related to the porosity. Model analyses of the permeability and storativity results indicate both that the pore apertures decrease and that the pore shapes become more equant as the porosity decreases. The marked downturn in the permeability‐porosity relationship at total porosities below 0.07 appears from microscopical observation to correspond to a change in pore geometry from largely connected, irregular pores between grains to isolated, tubular pores at junctions of several grains. Application of the “Swiss‐cheese” continuum percolation model indicates a percolation threshold of about 0.04 porosity. Microstructural evidence, the apparent activation energy for densification, and the stress dependence of densification rate suggest that porosity reduction has occurred mainly by dislocation creep.
On the vesicularity of pumice
Authors: Thomas Nathalie , Jaupart Claude , Vergniolle Sylvie
Published: February 2004

We develop a physical framework for the evolution of vesicular magma fragments after their formation by fragmentation in explosive eruptions. These fragments are carried by a gaseous jet through the volcanic conduit and into the atmosphere and are thus subjected to a decrease of pressure and temperature. We derive equations for the evolution of void fraction, internal gas pressure and temperature as a function of time and position in the fragment. The driving terms for the evolution are the pressure and temperature history external to the fragment. We show that a magma fragment can expand significantly before being quenched. The influence of melt viscosity is introduced as a dimensionless number that relates melt viscosity, pressure drop between fragmentation within the conduit and expansion in the atmosphere, and duration of the pressure drop. For melt viscosities larger than 109Pa s, corresponding to high‐silica‐content rhyolitic melt with small amounts of dissolved water, expansion of vesicles is negligible so that quenched pumice samples record the state of vesicular magma at the moment of fragmentation. Over the viscosity range of 106to 109Pa s, the amount of vesicularity is a function of melt viscosity and the history of external pressure. For viscosities lower than 106Pa s, melt viscosity is not important, and gas pressure within vesicles is always close to the external pressure. Available data for four Plinian deposits show that pumice vesicularity varies significantly both as a function of time during an eruption and between eruptions. For the magma compositions and volatile contents of the Bishop Tuff, United States; Taupo, New Zealand; and Minoan, Santorini, eruptions, the void fraction of pumice is predicted to be a decreasing function of melt viscosity, in agreement with observations. During an eruption, a change of fragmentation conditions will produce a small difference in pumice vesicularity, so that a systematic change in the average vesicularity within a pumice deposit may record a change of eruption dynamics.
Paleomagnetism, stratigraphy, and petrology of the Upper Cretaceous Roskruge Volcanics at Bell Mountain, southeast Arizona
Authors: Hagstrum J. T. , Lipman P. W. , Sawyer D. A.
Published: February 2004

Reconnaissance geologic mapping and petrologic study of the Upper Cretaceous Roskruge Volcanics at Bell Mountain in the Roskruge Mountains, southeastern Arizona, have identified five compositionally distinct ash flow tuffs, including the basal red Viopuli Tuff, which comprise six or seven separate cooling units. Previous paleomagnetic investigators inferred eleven cooling units at Bell Mountain, but insufficient information was given to determine which tuffs had been sampled. This study was undertaken therefore to establish characteristic remanence directions for each tuff sheet. The available paleomagnetic data corroborate the identification of five map units in the Bell Mountain area, but the number of independent observations is too few for the calculation of a reliable paleopole; paleosecular variation of the geomagnetic field would probably not be averaged out. No correctable tuffs are present at Bell Mountain, so relative vertical axis rotations cannot be determined, although they are probably not large. The absence of regionally correctable tuffs suggests the presence of additional, potentially mineralized, volcanic centers perhaps farther north in the Roskruge Mountains or buried nearby beneath Cenozoic basin fill.
Radial upper mantle attenuation structure of inactive back arc basins from differential shear wave measurements
Authors: Flanagan Megan P. , Wiens Douglas A.
Published: February 2004

We measure differential attenuation between sS‐S and sScS‐ScS phase pairs to characterize the variation of attenuation with depth in the upper mantle of five inactive back arc basins: the Kuril Basin, Sea of Japan, Banda Sea, the Celebes and Sulu Seas, and the Shikoku Basin. A spectral ratio technique is used to measure the differential attenuation operator of the transversely polarized waveforms over a frequency band of 10 to 83 mHz. Two algorithms are employed to compute the vertically averaged attenuation structure: a spectral stacking procedure and a least squares inversion. In the spectral stacking method, the individual spectra are corrected for the elastic structure at the sS or sScS bounce point, and the differential attenuation operator is computed by spectral division. The attenuation operators are then normalized and stacked by source depth to obtain more stable spectra, and an average δt* for sources within a restricted depth range is obtained from the slope of the log‐amplitude spectrum. A model for the depth dependent Q structure is then calculated from the δt* measurements assuming Q is frequency independent. Alternatively, δt* measurements for individual phase pairs are made using a similar technique and analyzed by ray tracing and a least squares inversion to obtain the Q−1 estimates. The Q results obtained from the stacking and inversion methods are generally in good agreement. The Q−1 structures for the various back arc regions are similar to each other within the uncertainties of the derived Q models. In addition to computing Q models for each individual region, we partitioned the data into three tectonic provinces based upon bounce point locations. The resulting average radial Qβ structure for an inactive basin shows a Q of 54 in the uppermost mantle, 115 at intermediate depths, and 173 in the transition zone; we also find a low Q zone of 36 beneath active island arcs. The Q values for inactive back arc basins are lower than the global averages derived from normal modes but are generally consistent with previous body and surface wave studies of other young oceanic regions. The most striking feature of this study is the observation of very strong attenuation concentrated at shallow depths (<160 km) in the upper mantle beneath these basins.
Diurnal and semidiurnal variations in Earth orientation determined from LAGEOS laser ranging
Authors: Watkins Michael M. , Eanes Richard J.
Published: February 2004

Variations in universal time and polar motion due to ocean tides at nearly diurnal and nearly semidiurnal frequencies are determined from analysis of laser ranging to the LAGEOS satellite over the period from 1987 to 1992. The adjusted diurnal tides were K1, S1, P1, O1, and Q1, while the semidiurnal tides were K2, S2, M2, and N2. A formulation was used that explicitly separated prograde and retrograde terms in the polar motion in order to eliminate aliasing from the singularity of retrograde wobble with long period orbit error and nutation. The results are compared to other experimentally derived observations from very long baseline interferometry (VLBI) and with predictions from an ocean tide model. The results of this study agree well with those from the VLBI studies, typically at the 2–3 μs level in universal time and 30–50 microarc sec (μas) level in polar motion. The agreement with the predictions from the ocean tide model were roughly a factor of 3 worse for UT although better for polar motion, particularly in the diurnal band.
Analytical electron microscopy of a synthetic peridotite experimentally deformed in the β olivine stability field
Authors: Dupas Catherine , Doukhan Nicole , Doukhan Jean‐Claude , Green Harry W. , Young Thomas E.
Published: February 2004

A synthetic harzburgite prepared by isostatic hot pressing of finely ground natural olivine admixed with 3% enstatite was deformed at high temperature and high pressure such that the α → β olivine phase boundary was traversed. Both residual and new phases, their grain sizes, their deformation‐induced lattice defects, and their compositions were then characterized by analytical electron microscopy. Owing to large heterogeneities in temperature, pressure, and deformation conditions, contrasting microstructures were observed. Some areas consist essentially of untransformed olivine, while the β polymorph is the main constituent of other areas. Crystals of the β polymorph are generally free of dislocations, except at the grain boundaries of the larger grains where slip bands were nucleated (probably induced by stress concentrations). The activated glide systems are {021} [100]. The orthopyroxene grains have been transformed to very fine grained clinopyroxene and have reacted with chromite to form garnet with a majoritic component. Compared to olivine, crystals of the β phase contain significantly higher concentrations of Fe and trivalent cations. All β crystals are free of inclusions, confirming the conclusion from previous infrared spectroscopic analysis that the high OH concentration in this specimen is dissolved in the β phase. Coupled with these previously measured OH concentrations, our new measurements suggest that incorporation of hydrogen into the β phase and substitution of trivalent cations into the octahedral sites may be coupled. Such increased complexity of mineral chemistry points to a need for detailed investigation of the spectrum of compositions stable in the β structure and coexisting phases in order to better estimate the modal mineralogy of the transition zone.
Scalar magnetic anomaly maps of Earth derived from POGO and Magsat data
Authors: Arkani‐Hamed Jafar , Langel Robert A. , Purucker Mike
Published: February 2004

A new POGO scalar magnetic anomaly map at 400 km altitude is presented which consists of spherical harmonics of degree 15–60. On the basis of the common features of this map with two new Magsat anomaly maps, dawn and dusk, two scalar magnetic anomaly maps of the Earth are presented using two selection criteria with different levels of stringency. These selection criteria suppress the noncrustal components of the original maps by different amounts. The more stringent selection criteria seek to eliminate as much contamination as possible, at the expense of suppressing some anomaly signal. This map is represented by spherical harmonics of degree 15–60. The less stringent selection criteria seek to retain as much crustal signal as possible, at the expense of also retaining some contaminating fields. This map is represented by spherical harmonics of degree 15–65. The resulting two maps are highly correlated with degree correlation coefficients greater than 0.8.
Water circulation in metamorphism
Authors: Rumble Douglas
Published: February 2004

There is “too much” water in a variety of crustal environments, including sedimentary basins and regional metamorphic belts. Regional and contact metamorphic rocks have time‐integrated fluxes of 106 cm3 water/cm2 for pervasive flow and up to 109 cm3 water/cm2 in fracture flow, amounts far greater than can be supplied by dehydration of immediately subjacent rocks. The types of evidence supporting high water fluxes are so numerous and varied, including petrologic, mineralogic, textural, microstructural, and stable isotopic data, that it is unlikely that all of them could have been interpreted erroneously. Thus the question arises as to what mechanisms of permeability enhancement might operate to facilitate large fluid fluxes. Field evidence suggests the operation of reaction enhancement of permeability, grain‐scale dilatancy, and hydraulic fracturing during metamorphism. All these mechanisms are inherently episodic and transient. The transport properties of dynamic permeability enhancement are poorly known in relation to the thermal and deformational cycles of metamorphism. Future research should be directed toward experimental and theoretical simulation of permeability enhancement with close collaboration between isotope geochemists, structural geologists, and petrologists to test models of permeability enhancement in the field.
Silica precipitation behavior in a flow field with negative temperature gradients
Authors: Chigira Masahiro , Watanabe Masakazu
Published: February 2004

To clarify the precipitation behaviour of silica from a hydrothermal solution flowing with decreasing temperature through rock, the effects of temperature, temperature gradient, pH, flow velocity, and solid surface area/fluid mass ratio (A/M ratio) were analyzed by experimentation. Although these effects were not evaluated precisely, we found “threshold conditions” where the solution remains in equilibrium with amorphous silica in a flow field with a negative temperature gradient. The equilibrium conditions are: the mean pore velocity is less than 100 m yr−1, when A/M ratio is more than 700 m2 kg−1, temperature is 80°–120°C, temperature gradient is <50°C m−1, and pH is 6.5 to 8. For the same A/M ratio, temperature gradient, and pH, the mean pore velocity must be less than 5 m yr−1 to keep the solution in equilibrium with the amorphous silica in a flow field at 80°–25°C. These threshold conditions are probably satisfied in many natural geothermal systems and near a repository for high‐level radioactive waste (HLW), suggesting that once groundwater becomes saturated with amorphous silica along the flow path from a geothermal source or HLW repository, it would flow in equilibrium with amorphous silica.
Angular momentum exchange among the solid Earth, atmosphere, and oceans: A case study of the 1982–1983 El Niño event
Authors: Dickey J. O. , Marcus S. L. , Hide R. , Eubanks T. M. , Boggs D. H.
Published: February 2004

The 1982–1983 El Nino/Southern Oscillation (ENSO) event was accompanied by the largest interannual variation in the Earth's rotation rate on record. In this study we demonstrate that atmospheric forcing was the dominant cause for this rotational anomaly, with atmospheric angular momentum (AAM) integrated from 1000 to 1 mbar (troposphere plus stratosphere) accounting for up to 92% of the interannual variance in the length of day (LOD). Winds between 100 and 1 mbar contributed nearly 20% of the variance explained, indicating that the stratosphere can play a significant role in the Earth's angular momentum budget on interannual time scales. Examination of LOD, AAM, and Southern Oscillation Index (SOI) data for a 15‐year span surrounding the 1982–1983 event suggests that the strong rotational response resulted from constructive interference between the low‐frequency (∼4–6 year) and quasi‐biennial (∼2–3 year) components of the ENSO phenomenon, as well as the stratospheric Quasi‐Biennial Oscillation (QBO). Sources of the remaining LOD discrepancy (∼55 and 64 μs rms residual for the European Centre for Medium‐Range Forecasting (EC) and U.S. National Meteorological Center (NMC) analyses) are explored; noise and systematic errors in the AAM data are estimated to contribute 18 and 33 μs, respectively, leaving a residual (rms) of 40 (52) μs unaccounted for by the EC (NMC) analysis. Oceanic angular momentum contributions (both moment of inertia changes associated with baroclinic waves and motion terms) are shown to be candidates in closing the interannual axial angular momentum budget.
Fast P wave propagation in subducted Pacific lithosphere: Refraction from the plate
Authors: Smith Gideon , Gubbins David , Mao Weijian
Published: February 2004

P waves traveling from events in the Tonga‐Kermadec seismic zone to stations in New Zealand are very fast with highly emergent, dispersed waveforms. Ray tracing has shown the waves to travel close to the subducted Pacific plate throughout their length, and synthetic seismogram calculations have shown the dispersion requires a very thin (8–12 km) fast layer. Previous work has been based on data from analogue records and one digital, single‐component short‐period instrument; no polarization analysis was possible, and measurements of dispersion were limited by the bandwidth. From January 1991 to August 1992 we deployed nine broad band, three‐component seismometers in good sites for observing these arrivals; the data are augmented by three‐component, short‐period digital records from new stations of the New Zealand National Network. In this study we analyze 1191 broad‐band and 2076 short period seismograms from 71 events for polarization of the initial P wave. The polarization directions are found to be up to 30° off the great circle path and consistently steep (20° from vertical). They are too steep to be explained by standard ray paths or refraction from a fast horizontal layer. We invert the polarization directions for a tilted interface beneath the array and use arrival times to control the depth to the interface, which is found to lie close to the top of the subducted plate inferred from the seismicity. We conclude that these precursive, emergent P waves have traveled through a fast layer close to the top of the subducted plate and refract upward to the station. A second arrival, with lower dominant frequency near 1 Hz and normal travel time, is occasionally seen on both broad band and short‐period stations. Its polarization direction is similarly steep but difficult to measure; the evidence suggests that it also travels within the plate with similar ray path to the precursor.
Separation of intrinsic and scattering attenuation in southern California using TERRAscope data
Authors: Jin Anshu , Mayeda Kevin , Adams David , Aki Keiiti
Published: February 2004

A multiple lapse time window analysis was applied to three‐component broadband seismograms recorded at five TERRAscope stations in southern California to separate scattering and intrinsic attenuation. Seismic energies were integrated over three consecutive lapse time intervals: 0–15, 15–30, and 30–45 s (measured from the S arrival) for approximately 30 earthquakes with hypocentral distances of less than 70 km from each station. Using the fundamental separability of source, site, and path effects for coda waves, the integrated energies for different magnitude earthquakes were normalized to a common source size at each station, and the effect of near‐site amplification is removed. Subsequently, we constructed a group of geometric spreading‐corrected normalized energy‐distance curves for each station region over frequency bands 0.5–1, 1–2, 2–4, and 4–8 Hz for all five stations. Two more frequency bands, 8–16 and 16–32 Hz, were added at stations PAS and SVD, for which higher sample rate data were available. A theoretical model of body wave energy propagation in a randomly heterogeneous elastic medium was employed to interpret the observation. Two parameters describe the medium in this model. These are the scattering attenuation coefficient ηs and the intrinsic attenuation coefficient ηi. By assuming that scattering is isotropic and including all orders of multiple scattering, this model predicts the spatial and temporal distribution of seismic energy. A two‐step least squares fitting procedure was used to find the best fitting model parameters. The result shows the following: (1) the seismic albedo, B0 = ηs/(ηi + ηs), increases with decreasing frequency for all station regions. The difference in B0 among stations is frequency dependent, and the difference disappears at 6.0 Hz. (2) Significant differences exist for the scattering attenuation coefficient ηs and seismic albedo B0 among stations at lower frequencies. Stations on or close to the fault zone(s), such as PAS, PFO, and SVD, show a stronger scattering at frequencies of 0.75 and 1.5 Hz, whereas station ISA, located far away from any major active faults, shows the lowest scattering. (3) The coda Q−1 determined from the decay rate of coda waves lies, in general, between the intrinsic Q−1 and total Q−1, sometimes closer to the former than the latter, and sometimes opposite, depending on the frequency and station.
Shear stress on a vertical strike‐slip fault in a layered medium due to tectonic loading: San Andreas Fault system
Authors: Rybicki Kacper R.
Published: February 2004

Depth distribution of shear stress at a transform boundary induced by tectonic loading is analyzed. The lithospheric plate is assumed to be made up of two elastic layers and is loaded by the far‐field shear stress. The upper part of the plate boundary is locked. The slipping region that underlies the locked part is modeled by a mode III crack containing a breakdown zone. Owing to the existence of a breakdown zone, the stress possesses a pronounced peak at the tip of the slipping region, and the loading stress takes high values both in front of the slipping region and within the breakdown zone. The stress in the upper part of Earth's crust is little influenced by the form of the slipping region and is mainly affected by elastic properties of the medium. The comparison of the present results with those based on observational data, in particular, the data pertaining to the 1979 Imperial Valley earthquake, allow the formulation of two general suggestions related to the faulting process. First, the depth difference between the point of initiation and the point of deepest penetration of rupture associated with major earthquakes correlates with the size of the breakdown zone. Second, the process of faulting in the top part of Earth's crust is primarily controlled by medium rigidity, which explains why faulting and seismicity are suppressed in sediments.
Linear stability of a layered fluid with mobile surface plates
Authors: Buffett B. A. , Gable C. W. , O'Connell R. J.
Published: February 2004

We develop a general method of calculating the linear stability of a fluid with homogeneous layers that is heated from below. The method employs a propagator technique to obtain expressions for the fluid velocity, stress, and temperature. The principal advantage of the method is the ease with which solutions are adapted to a wide variety of boundary conditions and fluid properties. We demonstrate the utility of the method using three examples which quantify the effects of (1) Theological layering, (2) mobile plates at the surface, and (3) multiple phase transitions. Each example is presented in the context of Earth's mantle. In the first example, we predict that convection becomes confined to the upper mantle once the viscosity increase between the upper and lower mantle exceeds a factor of 2000, consistent with the nonlinear calculations of Davies (1977). In the second example we find that the heat flux variations in a convecting fluid with variably sized, surface plates (Gable et al., 1991) can be attributed, in part, to changes in the critical Rayleigh number. The linear stability of a fluid with multiple phase transitions is significantly affected by the locations of the transitions. We find that phase transitions have their largest effect when they are located at the center of the fluid layer and become much less important when they are located near the exterior boundaries.
Measurements of acoustic surface waves on fluid‐filled porous rocks
Authors: Adler Laszlo , Nagy Peter B.
Published: February 2004

Novel experimental techniques to measure ultrasonic velocity and attenuation of surface waves on fluid‐filled porous natural rocks are presented. Our experimental results are consistent with the theoretical predictions of Feng and Johnson (1983). Depending on the interface conditions, i.e., whether the surface pores are open or closed, pseudo‐Rayleigh, pseudo‐Stoneley, and/or true Stoneley surface waves may exist on fluid‐saturated rocks. We have demonstrated the existence of the “slow” surface wave (true Stoneley mode) on fluid‐filled porous rocks with closed surface pores. The velocity and attenuation of the “slow” surface mode may be used to assess the dynamic permeability of porous formations.
Magnetic properties of dikes from the oceanic upper crustal section
Authors: Johnson H. Paul , Salem Brian L.
Published: February 2004

Recovery of dredge samples of upper crustal dikes from the steep north wall of the Blanco Fracture Zone (West Blanco Deep) allows an evaluation of this rock type as a possible contributor to the magnetic anomaly source layer. Magnetic data from these dike/sill samples indicate that the majority of the rocks meet the essential criteria as a contributor to the source layer, including a high magnetic intensity (2.7 A/m), a ratio of remanent to induced magnetization that is greater than 1, and a relative stability against large‐scale viscous remanent magnetization acquisition over long geological time periods. However, because the natural remanent magnetization intensities of the dike samples are an order of magnitude lower than the extrusive rocks within the axial zone, the presence of abundant near‐surface dikes would lower the net magnetization of a crustal section and may be responsible for the axial magnetic low that has been observed on medium to fast spreading centers. If the magnetization of the dikes does not change with time due to low‐temperature oxidation, then the decreasing ratio of extrusive/intrusive contribution as the pillow basalts undergo systematic oxidation will cause the magnetic layer to appear to thicken and the polarity transition zones to widen as the crust ages and moves off‐axis.
Multiple episodes of dike emplacement along the northwestern margin of the Superior Province, Manitoba
Authors: Zhai Yongjian , Halls Henry C. , Bates Martin P.
Published: February 2004

Three stable magnetization components A, B, and C have been isolated from a set of dikes that have been collectively referred to as the Molson swarm. Positive baked contact tests and square‐shouldered thermal decay curves indicate that these components were acquired during three periods of dike emplacement. Dikes carrying A with a pole at 15.4°N and 96.5°W (A95 = 4.1°, N = 16 sites) have been dated by the U‐Pb method at 1883 Ma and are closely related to the Paleoproterozoic Trans‐Hudson Orogen. Ages of B with a pole at 27.1°N and 140.8°W (A95 = 3.8°, A95 = 3.0°, N = 30) and C with a pole at 40.7°N and 156.3°W (A95 = 9.7°, N = 19) are estimated at 2170–2120 Ma, on the basis of paleomagnetic characteristics that are distinct from the A component, the analysis of metamorphic history of the Pikwitonei Domain, and comparison with well‐dated paleomagnetic poles from mafic igneous rocks in the Superior Province. The associated B and C dikes were emplaced in normal and reversed geomagnetic fields when the northwestern margin of the Superior Province experienced initial rifting prior to the Trans‐Hudson Orogen. These earlier dikes may correlate with known 2094 Ma mafic magmatism documented in the lower Hurwitz Group of the Hearne Province. A stable magnetization component D with a pole at 20.9°N and 133.6°W (A95 = 7.6°, N = 30 samples) was also identified from unbaked granulites throughout the Pikwitonei Domain. This component predates all dike intrusion in the region, has an age range of 2200–2170 Ma, and may have been caused by earlier uplift of the northwestern Superior Province.
Thermal regime of the southern Basin and Range Province: 2. Implications of heat flow for regional extension and metamorphic core complexes
Authors: Lachenbruch Arthur H. , Sass J. H. , Morgan Paul
Published: February 2004

The average heat flow for the presently quiescent southern Basin and Range Province (82±3 mW m−2) is only marginally lower than that for the northern Basin and Range (92±9 mW m−2), even though the former has been relatively inactive over the past 10–15 m.y. This result is consistent with a broad range of simple homogeneous models for province‐wide extension; the high heat flow (roughly equivalent to doubling the stable mantle contribution) can develop quickly from nonconductive processes (extension, magmatism, delamination), but the decay is a conductive process that might have little effect in the first 10–15 m.y. of quiescence. The frequently cited province‐wide estimate of 100% extension (β = 2) in the Basin and Range is consistent with the observed heat flow but not required by it. Extension models involving lithosphere thinning by delamination (and not solely by stretching), and/or magmatic additions, can provide the observed heat flow with less extension. These models are easier to reconcile with the buoyancy requirements of the present high elevation. Loss of buoyancy required by 100% extension poses special problems; either the preextensional terrain was as high as any on Earth today, or a large compensating source of buoyancy developed during the extension process. Possible candidates are magmatic contributions to the crust (which must be very large to be important in this context), trapped melt or lighter phases in the mantle, or a locally hot plume‐fed upper asthenosphere. Superimposed on the regional scale extension are zones where intense local extension has removed much of the upper crust to form metamorphic core complexes. Two thermal observations might bear on their origin: (1) Relatively low heat flow (67±4 mW m−2) is associated with the main trend of metamorphic core complexes (MCCs) in Arizona, and (2) the outcropping rocks in the core complexes have a low radioactive heat production (1.3±.3 μW m−3) compared to the other crystalline rocks in the region (2.1±.2 μW m−3). These observations are consistent with the belief that radioactivity decreases downward in the crust and that MCCs are sites of massive unroofing. In fact, the measured heat flow low could be explained by the removal of 10–20 km of upper crustal radioactivity provided the opposing effects of transient heating from unroofing were short‐lived. According to an idealized model, this would be the case if the compensating mass flow occurred in the lower crust, not in the mantle. In this sense the regional thermal results provide some support for the lower crustal return flow hypothesis for metamorphic core complexes.
Multivariate analysis of waveguide effects on short‐period regional wave propagation in Eurasia and its application in seismic discrimination
Authors: Zhang Tian‐Run , Schwartz Susan Y. , Lay Thome
Published: February 2004

Four data sets characterizing gross crustal waveguide configuration and attenuation properties in Eurasia (surface topography, Moho depth, sediment thickness, and Lg coda Q value) are used to examine path influences on short‐period regional P/Lg ratios. Linear regressions show considerable correlations between log P/Lg and waveguide properties for both earthquake and explosion data. This is of interest because P/Lg ratios are considered to be promising regional discriminants for which it is desirable to reduce scatter caused by propagation effects. To develop a comprehensive and stable model describing the path effects, multivariate regression is applied to the data set. The waveguide properties considered are highly correlated with each other, causing collinearity in the regression process. After backward elimination, we obtain a final model with statistically significant regression coefficients, which involves two independent variables: the path attenuation term γΔ and maximum sediment thickness on each path. These two factors are associated with (1) the overall intrinsic and scattering attenuation in the waveguide and (2) the localized blockage effects caused by waveguide geometry, respectively. Using the final model to correct the observed P/Lg measurements reduces the variances for separate earthquake data and explosion data by 40% and 27%, respectively. This correction slightly enhances the performance of the discriminant for periods near 1 s.
Magmatism in the Garrett transform fault (East Pacific Rise near 13°27′S)
Authors: Hékinian Roger , Bideau Daniel , Hébert Réjean , Niu Yaoling
Published: February 2004

The Garrett transform is characterized by recent (zero age) volcanic activity located within the active tectonic domain of the transform valley at depths greater than the 3500 m. This intratransform volcanic activity contributed to the formation of constructional edifices forming ridges (>300 m in height) and small mounds (<20 m in height) built near slivers of serpentinized peridotites. The erupted lavas are depleted mid ocean ridge basalts (MORBs) with low ratios of K/Ti (0.02–0.11), Zr (30–100 ppm), Y (18–50 ppm), and (La/Sm)N (0.25–0.60). Their more depleted nature and smaller range of variability for the compatible elements (Ni = 70–180 ppm, Mg# = 0.58–0.71, where Mg# is the magnesium number (= Mg+2/Mg+2 + Fe+2)) are the main points of difference between the Garrett intratransform volcanics (GITV) and those from the ultrafast south East Pacific Rise (SEPR). However, ferrobasalts (Mg# = 0.41–0.55) were collected from the intratransform walls as well as at the EPR‐transform intersection. The GITV are even more depleted in incompatible elements than lava from the north East Pacific Rise (21°N‐11°26′N). The Garrett recent lava is believed to have erupted after the successive, incremental partial melting and discontinuous melt extraction of a composite lherzolitic mantle similar to that of the SEPR. The limited range of incompatible element ratios (Zr/Y = 1–2.5, (Ce/Yb)N = 0.4–1) and K/Ti ratios (<0.14) and the samples more porphyritic nature with respect to other SEPR rocks suggest that the intratransform volcanics from the Garrett are extracted from their source and channeled directly toward the surface without extensive mixing in magma chambers. In order to explain the restricted range of compositional variabilities and the absence of the enriched basalts produced by prior melting, we postulate that even though they were produced, these most enriched end‐member lavas did not reach the surface; instead, we propose these melts contribute to the formation of impregnated mantle material in the lithosphere. We suggest that this same petrogenetic style of intratransform volcanism might also characterize other oceanic provinces associated with low magmatic to quasi‐amagmatic regimes.
Attenuation of 1–6 s Lg waves in Eurasia
Authors: Shih X. R. , Chun K.‐Y. , Zhu T.
Published: February 2004

We report our investigation of the Lg attenuation characteristics in Eurasia using digital recordings from the Incorporated Research Institutions for Seismology/Chinese Digital Seismic Network broadband stations. To maximize the utilization of the current station and source distribution in the region, we have derived a new technique known as the source pair/receiver pair (SPRP) method for Lg attenuation measurement. The procedure eliminates seismic source and site effects, leaving behind decontaminated amplitude data from which regional wave attenuation is estimated. For Lg attenuation studies the SPRP method is more flexible and of greater general applicability than its predecessor, the reversed two‐station method (Chun et al., 1987). The application of the SPRP method to our time domain analysis of 33 regional and far‐regional seismic events has provided stable regional average for Lg attenuation in periods of 1–6 s. Our time domain analysis, aided by a damped least squares inversion algorithm, has led to robust measurements of Lg attenuation (Q) and its frequency dependence (η) in nine subregions of tectonic significance. Regions of active tectonics and seismicity generally have lower Q0 than that in the stable continental interior. Subregional Q and η are found compatible with the published results in areas of relatively uniform geology; η values are comparatively poorly determined, suggesting that they are susceptible to the presence of crustal heterogeneities along the wave paths.
Fluid geochemistry at Vulcano island: A change in the volcanic regime or continuous fluctuations in the mixing of different systems?
Authors: Tedesco Dario
Published: February 2004

Increasing fumarolic activity and the rise of outlet fumarole temperatures has occurred at Vulcano island (southern Italy) since September 1987. Regular sampling and analysis have been conducted on the well‐known F5 fumarolic and later on F5HT and FA f umaroles on the Fossa crater plus other gas emanations from different sites on the island. Significant chemical and isotopical oscillations have been recorded over the period 1987–1991. Variations of water vapor in the fumarolic fluid follow a seasonal pattern and can be related to both external (seasonal) and magmatic influences but they suggest a control by a source other than magmatic. Significant variations of H2O, and of some species in the anhydrous gas phase (H2, CO2, SO2, N2, and He), have been related in the past to chemical or dynamic changes in the feeding system at depth. Oscillations of the isotopic composition of helium and carbon suggest possible mixing with a nonsurficial source, possibly crustal in origin. Argon and neon isotopic data support this hypothesis. Several mechanisms are considered for explaining the chemical and isotopic data, in relation to the recent increase in activity. Not all of the chemical and isotopic variations recorded are related to geophysical parameters such as seismicity and deformation. The low CO content of the F5 fumarolic and the apparent constancy of the 3He/4He ratio do not suggest an increase in the input of deep magmatic fluids. The high CO content of the high temperature (650°C) FA fumarole, located at the interior of the crater, along with helium and carbon isotopic ratios similar to other fumarolics is probably due to an easier ascent of these fluids to the surface. Apparent equilibrium temperatures, close to the fumarolic outlet temperature, indicate isothermal expansion of the gas from a shallow equilibration zone for both high‐ and lower‐temperature gases. In particular, the chemical and isotopic features of fluids belonging to both crater and beach fumarole fields are consistent with the existence of two intermediate system, independently feeding the two fields. High 3He/4He ratios for the beach fluids, similar to those of the crater fluids, suggest a large magmatic reservoir at depth, feeding both the crater and the beach intermediate reservoirs.
Seismic refraction measurements within the Peninsular terrane, south central Alaska
Authors: Ambos Elizabeth L. , Mooney Walter D. , Fuis Gary S.
Published: February 2004

We present an interpretation of crustal seismic refraction data from the Peninsular terrane, one of the many exotic terranes that have been accreted to the continental margin of southern Alaska in the past 200 m.y. A seismic refraction line was collected along the Glenn Highway in the Copper River Basin of south central Alaska in 1984 and 1985, as part of the U.S. Geological Survey Trans‐Alaska Crustal Transect (TACT) program. P wave velocities of 2.7–3.5 km/s and thicknesses of 1–2 km characterize post‐Lower Jurassic sedimentary rocks that underlie most of the seismic refraction line. An average crustal velocity structure includes the following five velocity divisions. Beneath the sedimentary rocks lie 1–2 km of 4.0–4.6 km/s materials, correlating with andesitic volcaniclastic sedimentary rocks and lava flows of the Lower Jurassic Talkeetna Formation. Below these rocks, seismic velocity increases rapidly, from 5.0 to 6.1 km/s, in 2–3 km. At 7–8 km depth, velocity jumps to 6.3 km/s and increments to 6.6 km/s by 10–12 km depth. Velocities increase from 6.8 to 7.0 km/s between 12 to 20 km depth. At about 22 km depth, a jump in velocity from 7.0 to 7.4 km/s is inferred but is poorly resolved. Depth to the Moho discontinuity could not be determined from our data. The absence of clear PmP reflections may indicate that Moho is deeper than 40 km. Data from two offset shotpoints northeast of the line and within the Wrangellia terrane constrain the deep structure transition between Peninsular and Wrangellia terranes. The 6.3–6.6 km/s material thickens to the northeast, toward the suture between Peninsular and Wrangellia terranes, but southwest of its mapped trace at the West Fork fault. Peninsular terrane crustal structure appears dissimilar to that of continental interiors. It is similar to velocity structures determined for accreted island arc fragments in California, such as the basement of the Great Valley and the Klamath Mountains.