bluuebunny/crossref_metadata_2025 huggingface dataset:
Crustal structure along the west flank of the Cascades, western Washington
Authors: Miller Kate C. , Keller G. Randy , Gridley James M. , Luetgert James H. , Mooney Walter D. , Thybo Hans
Published: February 2004

Knowledge of the crustal structure of the Washington Cascades and adjacent Puget Lowland is important to both earthquake hazards studies and geologic studies of the evolution of this tectonically active region. We present a model for crustal velocity structure derived from analysis of seismic refraction/wide‐angle reflection data collected in 1991 in western Washington. The 280‐km‐long north‐south transect skirts the west flank of the Cascades as it crosses three tectonic provinces including the Northwest Cascades Thrust System (NWCS), the Puget Lowland, and the volcanic arc of the southern Cascades. Within the NWCS, upper crustal velocities range from 4.2 to 5.7 km s−1 and are consistent with the presence of a diverse suite of Mesozoic and Paleozoic metasediments and metavolcanics. In the upper 2–3 km of the Puget Lowland velocities drop to 1.7–3.5 km s−1 and reflect the occurrence of Oligocene to recent sediments within the basin. In the southern Washington Cascades, upper crustal velocities range from 4.0 to 5.5 km s−1 and are consistent with a large volume of Tertiary sediments and volcanics. A sharp change in velocity gradient at 5–10 km marks the division between the upper and middle crust. From approximately 10 to 35 km depth the velocity field is characterized by a velocity increase from ∼6.0 to 7.2 km s−1. These high velocities do not support the presence of marine sedimentary rocks at depths of 10–20 km beneath the Cascades as previously proposed on the basis of magnetotelluric data. Crustal thickness ranges from 42 to 47 km along the profile. The lowermost crust consists of a 2 to 8‐km‐thick transitional layer with velocities of 7.3–7.4 km s−1. The upper mantle velocity appears to be an unusually low 7.6–7.8 km s−1. When compared to velocity models from other regions, this model most closely resembles those found in active continental arcs. Distinct seismicity patterns can be associated with individual tectonic provinces along the seismic transect. In the NWCS and Puget Lowland, most of the seismicity occurs below the base of the upper crust as defined by a seismic boundary at 5–10 km depth and continues to 20–30 km depth. The region of transition between the NWCS and the Puget Lowland appears as a gap in seismicity with notably less seismic activity north of the boundary between the two. Earthquakes within the Cascades are generally shallower (0–20 km) and are dominated by events associated with the Rainier Seismic Zone.
Heat flow over Reunion hot spot track: Additional evidence for thermal rejuvenation of oceanic lithosphere
Authors: Bonneville Alain , Von Herzen Richard P. , Lucazeau Francis
Published: February 2004

We present 99 new heat flow determinations obtained with a regular sampling (2–3 km) along two seismic profiles across the Mascarene Ridge, Indian Ocean. This aseismic ridge is volcanic relief due to the activity of a hot spot, now responsible of the active volcanism of Reunion Island. Our sampling technique allows us to identify short‐wavelength processes such as slope‐driven water flows that affect heat flow on the ridge. With the exception of these perturbed sites, heat flow appears to be very uniform across the ridge and significantly higher (6–8 mW m−2) than the reference heat flow predicted for the seafloor age. Although this anomaly is low, it confirms other evidence for thermal rejuvenation of the lithosphere based on gravity and bathymetry. The set of information now available for the Mascarene Ridge seems to indicate a mixed mode of compensation with dynamical support below the front of the swell track and thermal support behind for the older part of the swell.
Techniques and parameters to analyze seismicity patterns associated with large earthquakes
Authors: Eneva Mariana , Ben‐Zion Yehuda
Published: February 2004

A pattern recognition algorithm is developed to provide potential improvements over existing earthquake prediction practices. The parameters employed in the analysis include degree of spatial nonrandomness in two distance ranges, spatial correlation dimension, spatial repetitiveness of earthquakes with a similar size, average depth of events, time interval for the occurrence of a constant number of events, and ratio of the numbers of events in two magnitude intervals. The parameter temporal variations are compared quantitatively with the time series of large events using a technique of association in time. The significance of the association frequencies is evaluated by comparison with chance associations estimated from corresponding simulated random time series. The developed techniques differ from existing approaches in the following aspects. The parameters here emphasize the spatial distribution of earthquakes. Possible correlations among the parameters are evaluated to determine the final set of parameters to be monitored. Threshold values for the assumed anomalies are chosen with consideration of properties of the available earthquake catalogs, such as the number of large events to be retrospectively predicted. Equal weight is given to both locally high and locally low parameter values. Care is taken to distinguish between anomalies preceding large events and those following previous events. It is shown that the relationship between precursory local extrema and precursory trends is nonunique, with precursory local extrema of the same type frequently associated with opposite observable precursory trends. The application of the seismicity parameters and pattern recognition techniques is demonstrated using synthetic earthquake catalogs generated by models of segmented fault systems in a three‐dimensional elastic solid [Ben‐Zion, 1996].
Heat sources and melting in subduction zones
Authors: Iwamori Hikaru
Published: February 2004

In order to understand the mechanism of melt generation and the origin of high heat flow in subduction zones, a series of numerical models of the thermal and flow structures in the mantle have been tested in two‐dimensional boxes with an inclined subducting slab of constant velocity. In contrast to previous models, (1) large convecting cells are used without imposing a high‐temperature profile on the backarc boundary, which enables us to discuss the global heat balance and to seek for the heat sources responsible for the melting and high heat flow observed in subduction zones, and (2) various mechanical conditions (e.g., coupling between the slab and overlying mantle wedge, buoyancy associated with melting) and heat sources (e.g., heat flux from below, internal heat generation, viscous heating) are tested in varying proportions. In all the calculations, steady state or near‐steady state with a small instability periodically occurring at the upper thermal boundary layer is achieved. Under these conditions, the global heat balance can be described by a simple boundary layer argument. The results show that, in order to attain a high enough temperature for melting and a high heat flux, a large amount of internal heating (i.e., more than 2.5×10−7 W/m3) is required if the convection is limited within the upper mantle. The high internal heating required may be explained if the radioactive nuclides in fluids expelled from the subducting slab are added to the wedge and circulate in the convection cell for a sufficient time. Another possible explanation for melting and the high heat flux is that the hot material is supplied from the lower mantle. Based on the thermal and flow structures obtained, melting regimes in subduction zones are discussed, in which the following key processes take place: (1) melting associated with pressure change of a rock packet with its movement, especially compression melting in the downward flow of the mantle wedge along the slab, and (2) melting due to compositional change of the rock packet associated with migration of H2O and melt. To solve these problems in detail, further studies on distribution and migration of the fluids will be required.
Effect of semibrittle deformation on transport properties of calcite rocks
Authors: Siddiqi Gunter , Evans Brian , Dresen Georg , Freund Dieter
Published: February 2004

We investigated the effect of mechanical deformation on transport properties by deforming synthetic calcite/quartz aggregates to strains (ε) up to 5% at confining pressures (Pe) up to 300 MPa, and at temperatures (T) from 300 K to 873 K. Subsequently, we measured permeability (k) using a wide‐range permeameter at effective pressures (Pc) of 15–155 MPa and room temperature. We then measured electrical conductivity (σ) in an impedance‐measuring assembly over the same pressure range at room temperature. The values of permeability and conductivity of the undeformed material at Pe=100 MPa were 4×10−18 m2 and 0.07 S/m. Samples deformed at 673 K and Pc=200 MPa, or at room temperature and Pc = 50–300 MPa, show small variations in permeability and conductivity: k changed only by up to a factor of 3 and σ increased by up to 10%. But, when a sample was deformed at 873 K and Pc=200 MPa, electrical conductivity dropped by 1 order of magnitude and permeability dropped by 2 orders of magnitude. To assess whether changes in length scales of the pore structure owing to deformation may account for large variation in transport properties, we counted cracks and pores, measured their lengths and widths, defined a damage parameter (ξ), and computed effective hydraulic and electrical conductivity using renormalization group methods. The undeformed rocks and the samples deformed at low confining pressure have severe damage, appear to be close to failure, and hence have high transport coefficients. Materials deformed at high pressures and temperatures have lower flaw densities, connectivities, and transport coefficients after deformation. We found that renormalization methods are suitable to model connectivity loss and large changes in transport properties owing to changes in flaw density and length scales. Pore connectivity and transport properties vary strongly during semibrittle deformation.
Interaction of mantle plumes and migrating mid‐ocean ridges: Implications for the Galápagos plume‐ridge system
Authors: Ito Garrett , Lin Jian , Gable Carl W.
Published: February 2004

We investigate the three‐dimensional interaction of mantle plumes and migrating mid‐ocean ridges with variable viscosity numerical models. Numerical models predict that along‐axis plume width W and maximum distance of plume‐ridge interaction xmax scale with (Q/U)½, where Q is plume source volume flux and U is ridge full spreading rate. Both W and xmax increase with buoyancy number ⊓b which reflects the strength of gravitational‐versus plate‐driven spreading. Scaling laws derived for stationary ridges in steady‐state with near‐ridge plumes are consistent with those obtained from independent studies of Ribe [1996]. In the case of a migrating ridge, the distance of plume‐ridge interaction is reduced when a ridge migrates toward the plume because of the excess drag of the faster moving leading plate and enhanced when a ridge migrates away from the plume because of the reduced drag of the slower moving trailing plate. Given the mildly buoyant and relatively viscous plumes investigated here, the slope of the lithospheric boundary and thermal erosion of the lithosphere have little effect on plume flow. From observed plume widths of the Galápagos plume‐migrating ridge system, our scaling laws yield estimates of Galápagos plume volume flux of 5–16×106km3 m.y.−1 and a buoyancy flux of ∼2×103 kg s−1. Model results suggest that the observed increase in bathymetric and mantle‐Bouguer gravity anomalies along Cocos Plate isochrons with increasing isochron age is due to higher crustal production when the Galápagos ridge axis was closer to the plume several million years ago. The anomaly amplitudes can be explained by a plume source with a relatively mild temperature anomaly (50°–100°C) and moderate radius (100–200 km). Predictions of the along‐axis geochemical signature of the plume suggest that mixing between the plume and ambient mantle sources may not occur in the asthenosphere but, instead, may occur deeper in the mantle possibly by entrainment of depleted mantle as the plume ascends from its source.
San Jacinto Fault Zone guided waves: A discrimination for recently active fault strands near Anza, California
Authors: Li Yong‐Gang , Aki Keiiti , Vernon Frank L.
Published: February 2004

We deployed three 350‐m‐long eight‐element linear seismic arrays in the San Jacinto Fault Zone (SJFZ) near Anza, California, to record microearthquakes starting in August through December 1995. Two arrays were deployed 18 km northwest of Anza, across the Casa Loma fault (CLF) and the Hot Springs fault (HSF) strands of the SJFZ. The third array was deployed across the San Jacinto fault (SJF) in the Anza slip gap. We observed fault zone guided waves characterized by low‐frequency, large amplitudes following S waves at the CLF array and the SJF array for earthquakes occurring within the fault zone. However, we did not observe guided waves at the HSF array for any events. The amplitude spectra of these guided waves showed peaks at 4 Hz at the CLF and 6 Hz at the SJF, which decreased sharply with the distance from the fault trace. In contrast, no spectral peaks at frequency lower than 6 Hz were registered at the HSF array. We used a finite difference method to simulate these guided modes as 5 waves trapped in a low‐velocity waveguide sandwiched between high‐velocity wall rocks. The guided mode data are adequately fit by a waveguide on the CLF with the average width of 120 m and S velocity of 2.5 km/s, about 25% reduced from the S velocity of the surrounding rock; this waveguide becomes 40 to 60 m wide with the 5 velocity of 2.8 km/s in the Anza slip gap. On the other hand, there is not a continuous waveguide on the HSF at depth. Locations of the events with guided modes suggest that the fault plane waveguide extends along the CLF between the towns of San Jacinto and Anza, dipping northeastward at 75°–80° to a depth of about 18 km; it becomes nearly vertical in the Anza gap. We speculate that the existence of a continuous low‐velocity waveguide on the CLF can be caused by the rupture of the magnitude 6.9 earthquake on April 21, 1918, occurring near the towns of San Jacinto and Hemet. Further, the lack of a clear waveguide on the HSF suggests that it was not ruptured in this event.
A dynamic balance between magma supply and eruption rate at Kilauea volcano, Hawaii
Authors: Denlinger Roger P.
Published: February 2004

The dynamic balance between magma supply and vent output at Kilauea volcano is used to estimate both the volume of magma stored within Kilauea volcano and its magma supply rate. Throughout most of 1991 a linear decline in volume flux from the Kupaianaha vent on Kilauea's east rift zone was associated with a parabolic variation in the elevation of Kilauea's summit as vent output initially exceeded then lagged behind the magma supply to the volcano. The correspondence between summit elevation and tilt established with over 30 years of data provided daily estimates of summit elevation in terms of summit tilt. The minimum in the parabolic variation in summit tilt and elevation (or zero elevation change) occurs when the magma supply to the reservoir from below the volcano equals the magma output from the reservoir to the surface, so that the magma supply rate is given by vent flux on that day. The measurements of vent flux and tilt establish that the magma supply rate to Kilauea volcano on June 19, 1991, was 217,000±10,000 m3/d (or 0.079±0.004 km3/yr). This is close to the average eruptive rate of 0.08 km3/yr between 1958 and 1984. In addition, the predictable response of summit elevation and tilt to each east rift zone eruption near Puu Oo since 1983 shows that summit deformation is also a measure of magma reservoir pressure. Given this, the correlation between the elevation of the Puu Oo lava lake (4 km uprift of Kupaianaha and 18 km from the summit) and summit tilt provides an estimate for magma pressure changes corresponding to summit tilt changes. The ratio of the change in volume to the change in reservoir pressure (dV/dP) during vent activity may be determined by dividing the ratio of volume erupted to change in summit tilt (dV/dtilt) by the ratio of pressure change to change in summit tilt (dP/dtilt). This measure of dV/dP, when combined with laboratory measurements of the bulk modulus of tholeitic melt, provides an estimate of 240±50 km3 for the volume of Kilauea's magma reservoir. This estimate is much larger than traditional estimates but consistent with seismic tomographic imaging and geophysical modeling of Kilauea's magma system.
Permanent tremor of Masaya Volcano, Nicaragua: Wave field analysis and source location
Authors: Métaxian Jean‐Philippe , Lesage Philippe , Dorel Jacques
Published: February 2004

The Masaya Volcano, Nicaragua, is a basaltic caldera in a subduction zone. The permanent source of the volcanic tremor was located inside Santiago crater, at the lava lake's position and 400 m below the NE rim, and therefore corresponds to superficial magma activity. We used two tripartite arrays (90 m side), one semicircular array (r=120 m) in 1992, and two semicircular arrays (r=60 m) and a 2500 m long linear array radiating out from the source and on the flank of the crater in 1993. We used both a cross‐spectrum method and a correlation method to determine the wave delay time between the reference station and the other stations of an array and to quantify the wave field. Using the delays therefore by intersecting the back azimuth wave directions from the arrays, we could pinpoint the source. Additionally, the correlation coefficients obtained as functions of frequency for the three components of motion confirm the inferred position of the source of tremor. The tremor's wave field is composed of comparable quantities of dispersed Rayleigh and Love surface waves, whose phase velocities lie in the ranges 730–1240 m/s at 2 Hz and 330–550 m/s at 6 Hz. The dispersive phase velocities were inverted to obtain crustal structures with a minimal number of layers. The resulting velocity models are similar for the northern and southern parts of the volcano. After geometrical spreading corrections, Q2Hz=14 and Q3Hz=31 were determined along the northern linear array. The typical low velocities and low Q corresponding to the cone structure and are similar to those of other basaltic volcanoes like Puu Oo, Hawaii, and Klyuchevskoy, Kamchatka.
On the importance of normal vibrations in modeling of stick slip in rock sliding
Authors: Tworzydlo W. Woytek , Hamzeh Osama N.
Published: February 2004

In this paper, numerical studies of friction‐induced dynamic instabilities, particular, stick‐slip sliding of rocks, are presented. Of special interest is the importance of normal compliance of the interface and associated normal vibrations in the occurrence of dynamic instabilities. To clearly illustrate this importance, the interface is represented by the Oden‐Martins constitutive model which does not exhibit explicit velocity or slip dependence of friction. This friction model is utilized in the context of a rather simple rigid body model to perform numerical studies representative of granite blocks in relative sliding at very slow overall velocities. The model represents a common experimental setting that has been used to study stick slip of sliding rock blocks. To capture the stick‐slip phenomenon the detailed dynamic response of this system is modeled numerically. These studies indicate the importance of the normal compliance of the interface on the stability of sliding. In particular, dynamic coupling of the degrees of freedom contributes to dynamic instability of the system and can cause stick‐slip motion. Both stable and unstable sliding are predicted and examined.
Upper mantle S velocity structure of North America
Authors: van der Lee S. , Nolet G.
Published: February 2004

We have inverted fundamental and higher‐mode Rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over North America to obtain an image of the upper mantle S velocity structure down to 660 km. Among the well‐resolved features of the new model are (1) a high‐velocity root beneath the North American craton which extends no deeper than 250 km except near the Archean core of the craton where depths of 350 km are reached, (2) a weak band of low‐velocity along the eastern margin of the North American craton, which reaches into the transition zone, (3) a low velocity slab window beneath the western United States down to a depth of 300 km, (4) areas of low uppermost mantle velocity beneath the Cascade volcanoes, the Yellowstone hotspot track, the Colorado plateau, the Sierra Madre Occidental, and the grabens bordering the Jalisco block, and (5) a pronounced band of high velocities in the transition zone, coinciding with the expected location of the subducted trailing fragments of the Farallon plate. We introduce several improvements to the method of partitioned waveform inversion, which was used to compute the new model: rather than to correct for an estimated depth to the Mohorovicic discontinuity, we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion; we also improve the windowing and filtering operator used to select uncontaminated waveforms.
A continuum mixture model of ice stream thermomechanics in the Laurentide Ice Sheet 2. Application to the Hudson Strait Ice Stream
Authors: Marshall Shawn J. , Clarke Garry K. C.
Published: February 2004

Episodic exportations of ice‐rafted debris to the North Atlantic in the late Pleistocene suggest quasiperiodic ice streaming or surging activity on the northeastern margin of the Laurentide Ice Sheet. Much of this efflux of ice may originate from an ice stream issuing from Hudson Strait and tapping into core regions of the Laurentide in Hudson Bay, Labrador, and the Foxe Basin. Applying the continuum mixture theory outlined by Marshall and Clarke [this issue], we model the thermomechanical evolution of the Hudson Strait Ice Stream in a three‐dimensional finite difference model of the Laurentide Ice Sheet. Our simulations focus on internal dynamics of the ice stream. Under thermal regulation of basal flow we find surge cycles of stream activity interspersed with quiescent periods where the ice stream is frozen to the bed. Modeled surge durations vary from 105 to 3260 years, while surge periodicities range from 585 to 22,410 years. With pervasively warm or cold internal temperature distributions in the ice, ice streams can also establish modes of permanent activity or inactivity under thermal regulation. Our most vigorous ice streams produce peak values of approximately 0.03 Sv of freshwater flux to the North Atlantic from basal meltwater and iceberg production. Associated ice stream velocities in this maximum case approach 6700 m yr−1. The total ice volume mobilized in a single surge event is equivalent to a global sea level rise of 0.04m in the most tranquil surge and almost 0.6m in the most extreme case. These velocities and sea level impacts axe an order of magnitude less than those predicted by MacAyeal [1993a,b], and only our most exuberant streams approach the iceberg flux estimates of Dowdeswell et al. [1995]. We propose that the sediment load of icebergs emanating from Hudson Strait in a surge event may exceed expectations from contemporary icebergs.
Timing of the North‐South China collision from new middle to late Mesozoic paleomagnetic data from the North China Block
Authors: Gilder Stuart , Courtillot Vincent
Published: February 2004

We collected paleomagnetic samples from 43 sites of Middle and Upper Jurassic and Cretaceous rocks along a 250 km wide east‐west trending belt located north of the suture between the North and South China Blocks. In each case, laboratory treatment isolates dual polarity, antipodal magnetizations that are demonstrably pre‐folding. The paleomagnetic poles show very good coherence with coeval poles from other localities from the North China Block (NCB), suggesting the apparent polar wander path (APWP) from the NCB is not distorted by post‐Middle Jurassic local vertical axis block rotations. A comparison of the NCB APWP with that from the South China Block suggests that collision continued after the Early Jurassic and ended no later than the Late Jurassic. A cusp in the NCB APWP coincides with a marked angular unconformity that separates Middle Jurassic from Upper Jurassic and younger sediments. Together, these observations indicate that the scissor‐like collision and associated deformation ceased at the Middle to Late Jurassic boundary (∼159 Ma). The time of rapid (1°/Ma) relative angular velocity between the two plates (225 to 190 Ma) coincides with a peak in U‐Pb and Ar‐Ar dates obtained from metamorphic rocks in the suture. The combined Late Jurassic and Cretaceous paleomagnetic data from both blocks suggest that the united China plate traveled 2000 km north relative to Eurasia during the Late Jurassic before docking in the Cretaceous. This younger collision event is also marked by a cusp on the Cretaceous part of the Eurasian APWP.
Fractal relations for the diameter and trace length of disc‐shaped fractures
Authors: Piggott Andrew R.
Published: February 2004

Analytic relations are developed between the fractal parameters of a random, isotropic population of disc‐shaped fractures and the parameters of the corresponding population of fracture traces expressed in outcrop. These relations indicate that a fractal distribution of fracture diameters translates to a fractal distribution of trace lengths and that the parameters for diameter may be uniquely determined from the parameters for trace length. Probabilistic results demonstrate the accuracy of the analytic relations and identify nonideal behaviors at the limits of the computed trace length data. It is expected that these relations will be useful in the three‐dimensional characterization of fracture systems in rock from in situ trace length data.
Gravitational viscoelastic postseismic relaxation on a layered spherical Earth
Authors: Pollitz Fred F.
Published: February 2004

Viscoelastic relaxation of a ductile asthenosphere underlying a purely elastic plate is a strong candidate process for explaining anomalous rates of crustal deformation observed following large earthquakes. The nongravitational treatment of Pollitz [1992], which is valid on a global scale and includes the effects of compressibility, is here extended to permit the calculation of gravitational viscoelastic relaxation for a specified spherically layered viscoelastic rheology following an earthquake in an elastic layer. The simple approximations we adopt make the resulting treatment particularly suitable for near‐field calculations. For an asthenosphere with a Maxwell rheology, the effect of gravitational coupling is manifested only many relaxation times after the earthquake, as obtained by previous investigators. Its effect is generally to speed up the long‐wavelength component of the relaxation process and attenuate the overall vertical displacement pattern. Several subtle features common to the relaxation behavior from several different fault types (thrust, rift, and strike‐slip) are identified. The effect of gravitational coupling on horizontal displacements is consistent with flexure of the upper elastic plate driven by the corresponding effects on the vertical displacement. Stress diffusion away from the source region generally exhibits pulse‐like behavior which is dispersive in both space and time. If the asthenosphere is confined to a relatively narrow channel, then the dispersion branches governing relaxation are radically altered, and stress diffusion effects far from the coseismic rupture zone exhibit a complicated time dependence reflecting the competing tendencies of toroidal and spheroidal mode relaxation.
Bathymetric roughness of the Southeast Indian Ridge: Implications for crustal accretion at intermediate spreading rate mid‐ocean ridges
Authors: Ma L. Ying , Cochran James R.
Published: February 2004

The nature of the transition from axial highs to axial valleys at mid‐ocean ridges and the physical processes involved in the transition are important for understanding how axial morphology changes with spreading rate, mantle temperature, and lithospheric strength at mid‐ocean ridges. In order to provide observational constraints on the nature of the changes in axial morphology, we examined the regional‐ and segment‐scale variations in axial and flank morphology at the intermediate spreading Southeast Indian Ridge (SEIR) using newly collected geophysical data. An empirical orthogonal function analysis was used to separate regional and local components of the topography field and to estimate bathymetric roughness. Three distinct types of axial morphology were identified from the regional component of ridge topography in our area: axial highs, shallow axial valleys, and “Mid‐Atlantic Ridge‐type” deep axial valleys. Axial depth increases by ∼2100 m from 88°E and 118°E, while off‐axis depth only increases by ∼500 m. In addition, except for one segment with a deep axial valley, there is little change in off‐axis depth within segments, in contrast to the large intrasegment variations in axial depth. These observations indicate that the overall and intrasegment variations in crustal thickness are much smaller than would be predicted from the variations in axial depth and that the major portion of the variations in ridge axis depth are dynamically supported. There are step‐like increases in bathymetric roughness as axial morphology changes from an axial high to a shallow axial valley and from a shallow axial valley to a deep axial valley. The step changes in roughness imply that the change from one mode of axial morphology to another is accompanied by an abrupt change in the strength of the lithosphere. The abrupt changes in lithospheric strength may be due to the existence of a “threshold” mantle temperature or crustal thickness about which the lithospheric strength is very sensitive to small fluctuations. Systematic intrasegment variations in roughness are also observed. Roughness shows V‐shaped patterns within segments with axial highs but no clear pattern within segments with axial valleys. The different patterns in roughness at axial highs and axial valleys on the SEIR may result from the presence or absence of a magma chamber. The presence of a magma chamber at a ridge segment with an axial high implies weaker axial lithosphere and hence lower roughness near the center of segments relative to the segment ends.
Field tests of rolling hinges: Existence, mechanical types, and implications for extensional tectonics
Authors: Axen Gary J. , Hartley John M.
Published: February 2004

In rolling‐hinge models, a locus of isostatic footwall uplift migrates together with the withdrawing hanging wall of a large‐displacement (tens of kilometers) normal fault. A fault‐bend fold that migrates through the footwall results, causing progressive deactivation of the updip part of the fault as it is tilted to a lower dip. Rolling‐hinge migration should leave a discernible geologic record, but compelling tests are difficult to design because predictions of rolling‐hinge models are nonunique and alternative models commonly can explain individual data sets. Field tests based on thermochronology, paleomagnetism, and structural analysis are typically applied to footwall rocks, whereas sedimentology, geochronology of volcanic rocks, and structural analysis are typically applied to upper plate rocks. We review existing theoretical models and case studies in 14 detachment terranes. Some negative results are reasonably compelling, but positive tests as yet tend to permit, rather than demand, a rolling hinge. Locally, both positive and negative conclusions have been reached about the passage of a hinge within the same terrane; such differences in interpretation may depend more on the scale of observation or on the specific model considered than on the geological development of the area. Subvertical simple shear and flexural failure mechanisms, with or without significant induced flow of lower crustal material, are thought to dominate hinge‐related footwall deformation in individual sites. Although rolling‐hinge models offer an alternative to low initial fault dips, hinge‐related deformation does not demand a steep initial fault dip: few areas in which rolling‐hinge tectonics are permissible had initially steep faults (>35°). Rather, where the evidence most strongly favors a rolling hinge, detachment faults had initial dips <35°. In the future, conclusive results are most likely to be obtained if diverse tests are applied to both the upper and lower plates of detachment terranes where other geological sources of ambiguity are minimal. At best, the rolling‐hinge mechanism does not universally explain the observation that earthquakes are rare on low‐angle normal faults nor solve the mechanical problems of frictional slip on such faults.
Large‐scale tectonic features induced by mantle avalanches with phase, temperature, and pressure lateral variations of viscosity
Authors: Brunei David , Machetel Philippe
Published: February 2004

A more general expression for the mantle vorticity equation is proposed for convection using axisymmetrical spherical geometry. Both the main mantle phase changes and radial and lateral variations of viscosity due to temperature and pressure. Four series of computations have been performed with (1) both the latent heat releases of the 400 km exothermic and the 670 km endothermic phase change and uniform and constant mantle viscosity; (2) the 670 km phase change alone and viscosity jumps of 10 or 30 between upper and lower mantle phases; (3) the 670 km endothermic phase change, a viscosity contrast of 30, and temperature and pressure dependent viscosity law; and (4) both 400 km and 670 km phase changes, a viscosity jump of 30, and a temperature and pressure dependent viscosity. The 400 km exothermic phase change modifies the global structure from partly layered to whole mantle convection. This effect is opposite to the effect obtained by increasing the viscosity jump at 670 km. However, both effects induce unrealistic thermal behavior which will not appear with temperature dependent laws for viscosity. The mantle avalanches which suddenly inject huge quantities of cold material into the lower mantle have effects at the surface and at the core‐mantle boundary (CMB). They induced heat flow crises which explain the huge volcanic events, high rates of mid‐oceanic ridge accretion, and periods of low‐frequency magnetic reversal. The surface heat flow proceeds directly from the upper mantle return flow along with the avalanches. The temperature dependent viscosity tends to decrease the strength of the avalanches. The bottom heat flow and the birth of CMB plumes may be considered as the consequences of cold upper mantle material arrival at the CMB. The lower mantle and the upper mantle transit times depend on the thickness of upper and lower mantles but also on the phase changes and on the viscosity. The CMB and surface perturbations may be simultaneous (to a few tens of million years).The temporal evolution of the convection pattern during an avalanche allows us to propose self‐consistent mechanisms for slab migration above the 670 km discontinuity for the birth and disappearance of ridges, the rising of powerful plumes from the CMB, and the creation of low‐viscosity zones which may act as a lubricant under continents for fast migration. These results show that the main mantle phase changes, combined with temperature and pressure dependent viscosity, induce convective behavior which provides an explanation for most of the past and present large‐scale dynamic behavior of the Earth's global tectonics.
Evidence for water influx from a caldera lake during the explosive hydromagmatic eruption of 1790, Kilauea volcano, Hawaii
Authors: Mastin Larry G.
Published: February 2004

In 1790 a major hydromagmatic eruption at the summit of Kilauea volcano, Hawaii, deposited up to 10 m of pyroclastic fall and surge deposits and killed several dozen Hawaiian natives who were crossing the island. Previous studies have hypothesized that the explosivity of this eruption was due to the influx of groundwater into the conduit and mixing of the groundwater with ascending magma. This study proposes that surface water, not groundwater, was the agent responsible for the explosiveness of the eruption. That is, a lake or pond may have existed in the caldera in 1790 and explosions may have taken place when magma ascended into the lake from below. That assertion is based on two lines of evidence: (1) high vesicularity (averaging 73% of more than 3000 lapilli) and high vesicle number density (105–107cm−3melt) of pumice clasts suggest that some phases of the eruption involved vigorous, sustained magma ascent; and (2) numerical calculations suggest that under most circumstances, hydrostatic pressure would not be sufficient to drive water into the eruptive conduit during vigorous magma ascent unless the water table were above the ground surface. These results are supported by historical data on the rate of infilling of the caldera floor during the early 1800s. When extrapolated back to 1790, they suggest that the caldera floor was below the water table.
Spatial distribution of snow in western Dronning Maud Land, East Antarctica, mapped by a ground‐based snow radar
Authors: Richardson Cecilia , Aarholt Eldar , Hamran Svein‐Erik , Holmlund Per , Isaksson Elisabeth
Published: February 2004

During the austral summer 1993/1994, the spatial distribution of snow was mapped by a ground‐based snow radar (800–2300 MHz) in western Dronning Maud Land, East Antarctica. Snow radar soundings were performed along continuous profiles extending from the ice shelf up to the polar plateau, a total distance of 1040 km. The high‐resolution radar registrations revealed subsurface layering in the uppermost 12 m of the snowpack. The travel time record was translated into snow accumulation expressed in water equivalents, based on an empirical relationship between wave speed and firn density. A good knowledge on snow density variations with depth is essential for the variability studies. Generally, the snow layering was well developed in the coastal area and less well developed on the polar plateau. High spatial variability in snow accumulation was observed on a regional as well as on a local scale. The variability was very high in areas with large surface slopes, such as the grounding zone and around nunataks. The highest variability was recorded in the nunatak area, where the standard deviation reached 59% of the spatial average accumulation. On the smooth high‐altitude plateau, variations in accumulation were less pronounced. However, here the standard deviation exceeded 22% of the average accumulation rate. Provided that the snow radar soundings are supported by dating of reference horizons along the travel route, this is a good method to obtain the accumulation rate and pattern for large areas with a high spatial resolution.
Low frictional strength of upper crustal faults: A model
Authors: Streit Jürgen E.
Published: February 2004

Low frictional strength of faults is estimated based on existing models for earthquake recurrence, which account for compaction and hydrothermal lithification of fault rocks, as well as for the buildup of high fluid pressures in faults during interseismic periods. In areas of elevated heat flow and for a geothermal gradient of 30°C/km, fault compaction, lithification, and changes in fluid pressure gradient are inferred to occur at shallow crustal depth (3 to 7 km). Calculated shearing resistance of faults at seismogenic depths is compatible with shear stress drops ascribed to moderate and large earthquakes. The high frequency of microearthquakes, as well as the occurrence of some larger mainshock hypocenters at shallow crustal depth along several strands of the San Andreas Fault system are consistent with a predicted minimum of crustal strength at the top of a high fluid pressure regime. On the basis of field evidence from many exhumed faults for lithification of fault gouge and for faulting at near‐lithostatic fluid pressures, a modified crustal strength profile is suggested to represent the strength of many active faults which involve hydrothermal fluid flow.
Fossil crust‐to‐mantle transition, Val Malenco (Italian Alps)
Authors: Hermann Jörg , Müntener Othmar , Trommsdorff Volkmar , Hansmann Werner , Piccardo Giovanni B.
Published: February 2004

An exhumed, undisturbed fossil lower crust to upper mantle section is preserved in Val Malenco, Italian Alps, and is now exposed along the boundary between Penninic and Austroalpine nappes. Lower‐crustal metapelitic rocks are welded to upper‐mantle ultramafic rocks by a mid‐Permian gabbro intrusion. The tmderplating of gabbro caused granulite metamorphism and partial melting of the metapelites. In the crust‐to‐mantle transition zone of at least 1 km thickness, gabbros, large xenoliths of restitic metapelites and ultramafic rocks occur, with densities of 2.95–3.14, 3.25 and 3.27 g/cm3, respectively. The seismic Moho therefore did not coincide with the boundary between peridotites and crustal rocks but was situated above the upper limit of the peridotitic mantle. The whole complex underwent cooling with only moderate decompression within the kyanite field. This process started at 1 GPa and ∼800°C and ended at 0.85 GPa and 600°C and is interpreted as thermal relaxation after the gabbro intrusion. Later, during Jurassic rifting, the crust‐to‐mantle section was exhumed at the Adria margin of the Tethys ocean.
Fluid flow in synthetic rough fractures and application to the Hachimantai geothermal hot dry rock test site
Authors: Glover P. W. J. , Matsuki K. , Hikima R. , Hayashi K.
Published: February 2004

Fracture profiles from the Hachimantai geothermal hot dry rock (HDR) test site in northern Japan have been measured and analyzed to characterise their geometrical properties. These properties have been used to create a population of numerical synthetic fractures that were tuned to imitate all the geometric and statistical properties of the natural fracture (described in a companion paper). Such fractures have been used as input boundary conditions in three types of modeling. (1) Simple elastic normal closure relating the aperture of the fracture to applied normal load. (2) Hagen‐Poiseuille calculations of fluid transmissivity in the fracture as a function of normal load, fracture fluid pressure, and temperature. (3) Two‐dimensional flow modeling within the rough walled fracture using Reynolds equation. The modeled closure of these fractures provided a realistic relationship between normal and fracture fluid pressure and aperture. When this pressure/aperture relationship was combined with a Hagen‐Poiseuille approach to calculating fluid transmissivity in the fracture, we obtained results which, when compared with data from field transmissivity tests at the Hachimantai site, showed that the functional dependence of fluid transmissivity with fracture fluid pressure was well modeled, but overestimated by a factor of about 2. Reynolds equation flow modeling was carried out in the synthetic fracture to ascertain the extent to which the Hagen‐Poiseuille law was overestimating the transmissivity due to the rough surfaces affecting the fluid flow in the fracture. When the calculations were corrected for this effect, the overestimation in the fluid transmissivity was reduced considerably.
Three‐dimensional gravity and bathymetry analysis of the Mid‐Atlantic Ridge between 20°N and 24°N: Flow geometry and temporal evolution of the segmentation
Authors: Maia Márcia , Gente Pascal
Published: February 2004

Bathymetry and gravity data covering 10 m.y. on both flanks of the Mid‐Atlantic Ridge south of the Kane Fracture Zone reveal a complex segmentation pattern well correlated with short‐scale temporal variations in crustal production. Segment and paleosegment centers are associated with crustal thickening, while their extremities appear as elongated bands of thin crust. Subsidence rates vary between centers and extremities of a segment, with higher than world average values at segmert midpoints and closer to average values at the edges. This is consistent with segment centers being the site of focused mantle upwellings, with associated thin lithosphere, and with thick crust. A pronounced circular minimum in the long wavelength of the Mantle Bouguer Anomaly, associated to a maximum in the long wavelength of the bathymetry, marks the site of a mantle upwelling, associated with a particularly robust segment located north of a small fracture zone (21°20′N). An offset is often observed between extreme values of topography and crustal thickness, resulting in a lack of isostatic equilibrium for off‐axis short‐wavelength features. An obliquity between maxima of crustal thickening associated with active segments and the topographic axial domain suggests that the shift between topography and crustal structure may arise partly from a decoupling between crust and topography formation processes.
Experimental melting curve of iron revisited
Authors: Anderson Orson L. , Duba Al
Published: February 2004

With new experimental data presented in the last 2 years, it becomes possible to resolve conflicts in the data sets used in constructing the melting curve of iron,Tm(P). On the basis of these new data, several data sets have been dropped: theWilliams et al.[1987] melting curve up to 100 GPa and the Bass et al [1987] andYoo et al.[1993] shock‐wave‐derivedTm(P) in the 200–300 GPa range based on light emissivity measurements. TheBoehler[1993]Tm(P) curve to 200 GPa and theBrown and McQueen[1986] shock‐wave‐determinedTm(240) remain, leaving a gap between 240 and 330 GPa. We fill this gap using the Lindemann law of melting. The Lindemann law and the temperature values along theBrown and McQueen[1986] Hugoniot require the value of the Grüneisen ratio, γ; thus γ connectsTmat 330 GPa withTmfound for values of the Hugoniot. It is further shown that the heat of crystallization, ΔHm, is dependent on γ. Thus, through 7, a connection is made between the melting curve and the power generated within the inner core. The effect of all these connections of physical properties through γ leads us to recommend 5600–6500 K as theTmof iron at inner‐outer core boundary conditions.

Though argument continues concerning the amount and nature of alloying elements, there remains little ground for doubting that both inner and outer cores consist mainly of iron.
Earthquakes in the Pamplona zone, Yakutat block, south central Alaska
Authors: Doser Diane I. , Pelton John R. , Veilleux Annette M.
Published: February 2004

The Pamplona zone is a region of complex deformation and moderate seismicity located within the Yakutat block, a region that has been relatively aseismic since a series of large (M>7.8) earthquakes in 1899. In 1970 a sequence of moderate to large sized earthquakes occurred within the Pamplona zone (largest event of Mw=6.7). Together with a Mw=6.1 event in 1958, these events are the only M≥5.5 events known to have occurred in the Pamplona region since 1900. Thus these events give important information on internal deformational processes within the Yakutat block. Waveform modeling of three earthquakes in April 1970, showed rupture complexity along low angle, thrust faults. Focal depths indicate that two of the events occurred above the Wrangell‐Aleutian megathrust, while the largest event may have occurred on the megathrust. Events in 1958 and February 1970 indicate that deformation within the western Pamplona zone is occurring along high angle (>60°) faults with reverse‐oblique motion. We believe the Pamplona spur, the easternmost part of the Pamplona zone, may have behaved as an asperity during the 1899 sequence. The location of the spur may be influenced by a north‐south trending fault zone in the subducting Pacific plate that appears to be responsible for the 1987–1992 Gulf of Alaska sequence, occurring 50 to 200 km south of the Pamplona zone.