bluuebunny/crossref_metadata_2025 huggingface dataset:
Permeability models for heated saturated igneous rocks
Authors: Le Ravalee Mickaële , Guéguen Yves
Published: February 2004

This paper is concerned with the heating effects in saturated, cracked granitic rocks. A model is constructed which permits permeability to be derived as a function of temperature. Granites have generally a low permeability so that the fluid response is considered to be undrained. Heating produces a fluid pressure rise. As long as the resulting deformation is reversible, cracks open elastically, and the rock behavior is described by thermoporoelasticity. If the confining pressure is constant, the fluid pressure derivative dp/dT depends essentially on the solid bulk modulus K5, on the thermal expansion coefficient αm, and on the initial crack aspect ratio A0. Irreversible deformations, i.e., crack propagation, take place when tensional stresses at the crack tip exceed the rock strength. This occurs at a critical temperature Tc which is controlled by dp/dT. Tc depends (1) on boundary conditions (constant confining pressure or constant bulk deformation), (2) on rock properties (solid bulk modulus Ks and thermal expansion coefficient αm) and most of all on crack aspect ratio. Tc is rather high for very thin cracks and low for high aspect ratio cracks. A model is developed in order to describe permeability and connected porosity evolution as temperature increases: at temperatures higher than Tc, cracks propagate step by step. The results of the modeling suggest that permeability is controlled by several effects. The balance of these is variable. On one hand, when crack propagation occurs, crack closure occurs simultaneously so that permeability decreases. On the other hand, connectivity is improved by the crack propagation process and this produces a permeability increase. The overall permeability increase is expected to cover at the most 2 orders of magnitude.
Source time functions of nuclear explosions and earthquakes in central Asia determined using empirical Green's functions
Authors: Li Yingping , Toksöz M. Nafi , Rodi William
Published: February 2004

Relative source time functions (RSTF) have been estimated for four underground nuclear explosions and seven earthquakes in Central Asia using broadband P waveforms of nearby smaller events as empirical Green's functions (EGF). RSTFs of the four explosions (mb = 5.3 to 6.5) are each dominated by a simple pulse with a source duration of 0.4 to 0.8 s. RSTFs for two of the explosions show a significant secondary pulse with a pulse width similar to that of the first pulse. We conclude that the secondary phases are most likely associated with the spall slapdown phenomenon. Seismic moment releases by the spall phases are less than one third of those by the first explosion pulses. Elastic radii of the explosions are estimated to be about 0.25 to 0.5 km, and stress drops of the explosions range from 13 to 52 MPa. In contrast, RSTFs of earthquakes studied (mb = 5.5 to 6.6) typically comprise multiple source pulses with a total source duration from a few to several tens of seconds, indicating that the complex source process involves a fault dimension of several tens of kilometers. Stress drops of the earthquakes are much smaller than those of the nuclear explosions, ranging from 0.5 to 3.5 MPa. Our study demonstrates the power of the EGF method for retrieving RSTFs and reveals that differences in RSTFs and source parameters can be used to distinguish large nuclear explosions from moderate to large earthquakes (mb ≥ 5.5).
Lithospheric bending at subduction zones based on depth soundings and satellite gravity
Authors: Levitt Daniel A. , Sandwell David T.
Published: February 2004

A global study of trench flexure was performed by simultaneously modeling 117 bathymetrie profiles (original depth soundings) and satellite‐derived gravity profiles. A thin, elastic plate flexure model was fit to each bathymetry/gravity profile by minimization of the L1 norm. The six model parameters were regional depth, regional gravity, trench axis location, flexural wavelength, flexural amplitude, and lithospheric density. A regional tilt parameter was not required after correcting for age‐related trend using a new high‐resolution age map. Estimates of the density parameter confirm that most outer rises are uncompensated. We find that flexural wavelength is not an accurate estimate of plate thickness because of the high curvatures observed at a majority of trenches. As in previous studies, we find that the gravity data favor a longer‐wavelength flexure than the bathymetry data. A joint topography‐gravity modeling scheme and fit criteria are used to limit acceptable parameter values to models for which topography and gravity yield consistent results. Even after the elastic thicknesses are converted to mechanical thicknesses using the yield strength envelope model, residual scatter obscures the systematic increase of mechanical thickness with age; perhaps this reflects the combination of uncertainties inherent in estimating flexural wavelength, such as extreme inelastic bending and accumulated thermoelastic stress. The bending moment needed to support the trench and outer rise topography increases by a factor of 10 as lithospheric age increases from 20 to 150 Ma; this reflects the increase in saturation bending moment that the lithosphere can maintain. Using a stiff, dry‐olivine rheology, we find that the lithosphere of the GDH1 thermal model (Stein and Stein, 1992) is too hot and thin to maintain the observed bending moments. Moreover, the regional depth seaward of the oldest trenches (∼150 Ma) exceeds the GDH1 model depths by about 400 m.
Geologic investigations of a “slip gap” in the surficial ruptures of the 1992 Landers earthquake, southern California
Authors: Spotila James A. , Sieh Kerry
Published: February 2004

A 3‐km‐long gap in the dextral surficial rupture of the 1992Mw= 7.3 Landers earthquake occurs at the north end of a major fault stepover between the Johnson Valley and Homestead Valley faults. This gap is situated along a segment of the Landers rupture that has been modeled geophysically as having a deficit in average slip at depth. To better evaluate the nature of the slip gap, we document in detail the character and distribution of surficial rupture within it. Along the gap, is a northwest trending thrust fault rupture with an average of less than 1 m of northeast directed reverse‐slip and nearly no oblique right slip. We interpret this rupture to be limited to the shallow crust of the northern end of the stepover and to have been the secondary result of dextral shear, rather than a mechanism of rigid‐block slip‐transfer from the Landers‐Kickapoo fault. A zone of en echelon extensional ruptures also occurs along the slip gap, which we interpret as the secondary result of diffuse dextral shear that accommodated less than 0.5 m of west‐northwest extension. These secondary ruptures represent a discontinuity in the surficial dextral rupture of the Landers earthquake, which we propose resulted from the lack of a mature fault connection between the Johnson Valley and Homestead Valley faults. The rupture pattern of the slip gap implies a significant deficit in net surficial slip, which compares favorably with some geophysical models. Aspects of this rupture pattern also suggest a temporal sequence of rupture that compares favorably with geophysical interpretations of the dynamic rupture propagation.
Systematic stress variations in the southern San Joaquin Valley and along the White Wolf fault: Implications for the rupture mechanics of the 1952 Ms 7.8 Kern County earthquake and contemporary seismicity
Authors: Castillo David A. , Zoback Mark D.
Published: February 2004

The southern San Joaquin Valley (SSJV) stress state is characterized by systematic variations in the regional principal stress directions and relative magnitudes, as well as local and possibly temporal variations that appear to be the result of the 1952 Ms 7.8 Kern County earthquake. The regional maximum horizontal principal stress (SHmax) orientation in the San Joaquin Valley systematically rotates from ∼NE‐SW compression along the western margins of the valley to ∼N‐S compression in the SSJV. This ∼N‐S SHmax stress direction in the SSJV is consistent with active development of a ∼E‐W trending structural fabric of fold axes and shallow thrust, and the reverse dip‐slip motion observed during the Kern County earthquake on the south‐southeast dipping White Wolf fault. Contemporary seismicity (30–40 years after the mainshock) clusters in two separate areas along the White Wolf fault: the southwest region where the 1952 earthquake nucleated, and the northeast region near where the earthquake rupture terminated. Earthquakes in the southwest and northeast regions show a diversity of focal mechanisms that include strike‐slip, reverse, oblique, and, to a lesser extent, normal slip. Inversion of earthquake focal mechanisms for in situ stress in the southwest region indicates a strike‐slip/reverse stress regime with S1 oriented approximately perpendicular to the ruptured fault plane implying low frictional strength in the nucleation zone of the 1952 earthquake. Inversion of focal plane mechanisms in the northeast region indicates a strike‐slip stress regime with S2 nearly perpendicular to secondary 1952 rupture planes also implying low frictional strength. These results indicate near‐complete stress drops for fault planes associated with the 1952 earthquake (and some of the contemporary earthquakes), implying fault surfaces which are frictionally weak (i.e., slip planes subparallel to principal stress planes). Based on the observed stress state in the southern San Joaquin Valley, much of the seismicity may be the result of elevated fluid pressures within these active fault zones.
Transient stresses at Parkfield, California, produced by the M 7.4 Landers earthquake of June 28, 1992: Observations from the UPSAR dense seismograph array
Authors: Spudich Paul , Steck Lee K. , Hellweg Margaret , Fletcher J. B. , Baker Lawrence M.
Published: February 2004

The M 7.4 Landers earthquake triggered widespread seismicity in the western United States. Because the transient dynamic stresses induced at regional distances by the Landers surface waves are much larger than the expected static stresses, the magnitude and the characteristics of the dynamic stresses may bear upon the earthquake triggering mechanism. The Landers earthquake was recorded on the UPSAR (U.S. Geological Survey Parkfield Small Aperture Array) array, a group of 14 triaxial accelerometers located within a 1‐square‐km region 10 km southwest of the town of Parkfield, California, 412 km northwest of the Landers epicenter. No triggered earthquakes were observed at Parkfield. Multiple filter analysis shows that the displacements, obtained by double integration, are dominated by the fundamental mode Love and Rayleigh modes, with some higher‐mode contributions for periods shorter than 10 s. Most of the surface waves propagated along the great circle path from Landers, but a late arriving surface wave appears to have been scattered from the Sierra Nevada Mountains. We used a standard geodetic inversion procedure to determine the surface strain and stress tensors as functions of time from the observed displacements. Peak dynamic strains and stresses at Earth's surface are about 7 μstrain and 0.035 MPa, respectively, and they have a flat amplitude spectrum between 2‐s and 15‐s period. These stresses agree well with stresses predicted from a simple equation using the ground velocity spectrum observed at a single station. Peak stresses ranged from about 0.035 MPa at the surface to about 0.12 MPa between 2 and 14 km depth, with the sharp increase of stress away from the surface resulting from the rapid increase of rigidity with depth and from the influence of mode shapes. Because of the free‐surface boundary conditions, the horizontal components of the stress tensor tend to dominate in the top 5–6 km of the crust, which might cause triggered seismicity to have strike‐slip or normal mechanisms. Comparison of dynamic stresses induced by the Landers, Loma Prieta, and Petrolia earthquakes at a variety of sites indicates that the Landers stresses were not spectacularly larger than those induced by the other sources. Landers dynamic stresses were comparable to Coalinga static stresses at Parkfield. The effective strain caused by Landers at Parkfield, where no earthquakes were triggered, are the same amplitude as those at some sites in Nevada where earthquakes were triggered. Comparing various authors' observations of dynamic stresses, there is no obvious characteristic of these stresses that correlates with the triggered seismicity.
Shear and compression waves in shocked calcium carbonate
Authors: Aidun John B. , Gupta Y. M.
Published: February 2004

Transverse and longitudinal particle velocity histories were measured in Carara marble subjected to compression shear loading in plate impact experiments. Peak compressive stresses ranged between 0.5 GPa and 6.3 GPa (0.8% and 13.8% density compression). These measurements permitted a direct determination of both shear and longitudinal wave speeds in a material undergoing a shock‐induced phase transformation. Such data provide an improved characterization of the shocked state and permit a direct comparison of the density variations of the elastic moduli under shock and hydrostatic loading without invoking assumptions regarding material strength. The present measurements have led to a revised assessment of the shock response of calcite rocks. In agreement with ultrasonic data, the measured shear wave speeds under shock loading undergo a rapid decrease associated with the transformation to CaCO3(II). Subsequently, the shear modulus increases continually up the highest compressions measured, contrary to earlier conjectures. Qualitative evaluation of shear wave amplitudes demonstrates that calcite rock retains strength after yielding and undergoing multiple phase transformations. Comparison of bulk modulus values determined from the present shock measurements with those from ultrasonic measurements indicate that the former are frozen‐phase‐composition values when the sample is a mixed‐phase aggregate. Because of the transformations, mean stresses cannot be determined directly from the bulk modulus values. Under shock loading, polycrystalline calcite samples transform to CaCO3(II) at approximately 2.3 GPa longitudinal stress. The bulk modulus in the shocked samples indicate that by 4.7 GPa peak longitudinal stress (11% compression) another phase is produced. Previous, higher‐stress Hugoniot data for calcite and aragonite samples suggest that this phase is aragonite. CaCO3(III) may not occur at all, in which case the bulk modulus values indicate that aragonite may be produced at a peak longitudinal stress of only 3.8 GPa. In contrast to the direct comparisons afforded by the bulk modulus values, comparison of the Hugoniot with the hydrostat in this stress range leads to incorrect conclusions concerning the shock response of calcite rocks. Also, speculation is offered on the role of phase transformation‐enhanced plasticity in the dynamic response of calcite rocks.
A simple model for the fault‐generated morphology of slow‐spreading mid‐oceanic ridges
Authors: Thatcher Wayne , Hill David P.
Published: February 2004

We postulate that fluctuations in magmatic activity at mid‐oceanic ridges perturb the horizontal least principal stress across rift‐bounding normal faults, leading to alternating phases of magmatic accretion, which increases valley width, and tectonic extension, which results in the growth of inner rift wall topography. Fine‐scale bathymetrie surveys and earthquake fault plane solutions show that active normal faults at slow‐spreading ridges are moderately dipping (approximately 45°) planar features throughout the seismogenic oceanic lithosphere. A simple quantitative model that includes flexural deformation of a 10‐km‐thick elastic plate by slippage on 45° dipping normal faults can match the bathymetrie profiles across several slow‐spreading ridge segments. Comparison among dip distributions of normal‐faulting earthquakes at mid‐ocean ridges, in the trench‐outer rise region, and on continents suggests that most events from these three tectonic environments initiated at dips close to 45°, raising unanswered questions about the mechanical conditions under which the faults originated.
Fluid‐rock reaction weakening of fault zones
Authors: Wintsch R. P. , Christoffersen R. , Kronenberg A. K.
Published: February 2004

The presence of weak phyllosilicates may explain the low shear strengths of fault zones if they define well‐developed fabrics. The growth of phyllosilicates is favored in meteoric water‐dominated granitic fault systems, where mineral‐aqueous fluid equilibria predict that modal phyllosilicate will increase via feldspar replacement reactions. In deeper, more alkaline, rock‐dominated regimes, the reactions reverse, and feldspars tend to replace phyllosilicates. In Mg‐rich mafic rocks, however, phyllosilicates (chlorite, biotite) can replace stronger framework and chain silicates in both shallower (<∼10 km) meteoric H2O‐dominated and in deeper, alkaline, rock‐dominated regimes. Where these phyllosilicates precipitate in active fault zones, they contribute directly to reaction softening. Because low‐temperature deformation of phyllosilicates is not governed by factional processes alone but can occur by pressure‐independent dislocation glide, the strength of phyllosilicate‐rich fault rocks can be low at all depths. Low strain rate creep during interseismic periods can align phyllosilicate grains in foliated gouge and phyllonites. Where preferred orientations are strong and contiguity of phyllosilicates is large, strengths of rocks within fault zones may approach minimum strengths defined by single phyllosilicate crystals. Fault zones containing localized high concentrations of phyllosilicates with strong preferred orientations in well‐defined folia can exhibit aseismic slip, especially where mafic Mg‐rich rocks occur along the fault (like parts of the San Andreas Fault).
Seismic cycles along the Aleutian arc: Analysis of seismicity from 1957 through 1991
Authors: Boyd Thomas M. , Engdahl E. Robert , Spence William
Published: February 2004

We catalog and relocate Aleutian arc seismicity. Between 1957 and 1991, two great earthquakes ruptured the same 250‐km‐long portion of the central Aleutian arc: the 1957 Aleutian Islands earthquake and the 1986 Andreanof Islands earthquake. Because accurate estimates of the moment distribution of the 1957 earthquake are not available, the spatial distribution of aftershocks for each of these events is compared and tested against models describing the modes of occurrence of great subduction zone earthquakes. Earthquake relocations are based on P wave arrival times published in the International Seismological Summary, the Bureau Central International Seismologique, and the International Seismological Centre bulletins and include corrections for the near‐source velocity structure associated with the down‐going slab. Magnitude estimates are extracted from bulletins and prior to 1964 are estimated by us from microfilmed records. Our catalog is complete above magnitude 5.5. Aftershocks associated with the 1957 and 1986 earthquakes appear to occur in different areas. East of the main shock epicenters, aftershock locations are anticorrelated. West of the main shock epicenter, aftershocks of the 1986 earthquake tended to concentrate along the updip edge of aftershock clusters associated with the 1957 earthquake. If we assume aftershocks rim the distribution of seismic moment release associated with each event, these observations imply that the moment distribution of the 1986 earthquake was different from that of the 1957 earthquake. This suggests that we should use caution in identifying mechanically strong portions of a fault, asperities, by simply mapping the moment distribution of a single great earthquake. A fundamental tenet of the asperity model, that rupture always occurs on the strongest portions of the fault with weaker portions rupturing either aseismically or dynamically as a result of rupture on a strong fault patch, may in the case of the central Aleutian arc not be correct. Thus observing the moment distribution from a single great earthquake may tell us little about what the distribution of moment release will look like during the next earthquake.
Limit cycles in the basal thermal regime of ice sheets
Authors: Payne A. J.
Published: February 2004

This paper investigates the thermodynamic consequences of introducing basal sliding into a two‐dimensional ice sheet model. The coupling of ice sheet form, flow field, and temperature evolution gives rise to limit cycles in the basal thermal regime of the ice sheet. Typical periods are 4000 to 5000 years and up to 45% of the ice sheet length can be affected. The limit cycles are caused by the switching on and off of sliding as basal ice reaches the pressure melting point. An analysis of the various heat fluxes at the ice sheet base stresses the roles of vertical diffusion and the heat generated by internal deformation and sliding in controlling the period and extent of the cycles. Limit cycles are found to persist in the face of various climatic (snow accumulation rate and air temperature) parameterizations, as well as for different basal sliding parameterizations. The type of oscillation change produced by varying accumulation rate is particularly interesting. Several distinct patterns of behavior are observed. For low accumulation rate ice sheets, a period of 6000–7000 years is dominant; for medium accumulation rate ice sheets, the 3000 to 4000 year period is dominant; while for larger accumulation rates, the oscillations become highly irregular with long (>10,000 years) periods.
Crustal and upper mantle contribution to the axial gravity anomaly at the southern East Pacific Rise
Authors: Magde Laura S. , Detrick Robert S.
Published: February 2004

This paper reassesses the crustal and upper mantle contribution to the axial gravity anomaly and isostatic topography observed at two segments (14°S and 17°S) of the southern East Pacific Rise (SEPR) in order to determine what constraints these data place on the amount of melt present in the underlying mantle. Gravity effects due to seafloor topography and relief on the Moho (assuming a constant crustal thickness and density) overpredict the amplitude of the gravity high at the EPR by 8–10 mGal. About 70% of this mantle Bouguer anomaly (MBA) low (6–7 mGal) can be explained by a region of partial melt and elevated temperatures in the mid‐to‐lower crust beneath the rise axis. Compositional density reductions in the mantle due to melt extraction are shown to make a negligible contribution to the amplitude of the observed MBA. Temperature‐related mantle density variations predicted by a simple, plate‐driven, passive flow model with no melt retention can adequately account for the mantle contribution to the observed MBA within the experimental uncertainty (±1 mGal). However, the retention of a small amount of melt (≤1–2% at 14°S; ≤4% at 17°S) in a broad region (tens of kilometers wide) of upwelling mantle is also consistent with the observed gravity data given the uncertainty in crustal thermal models. The anomalous height of the narrow, topographic high at the EPR provides the strongest evidence for the existence of significant melt fractions in the underlying mantle. It is consistent with the presence of a narrow (∼10 km wide) partial melt conduit that extends to depths of 50–70 km with melt concentrations up to 2% higher than the surrounding mantle. Along‐axis variations in mantle melt fraction that might potentially indicate focused upwelling are only marginally resolvable in the gravity data due to uncertainties in crustal thermal models. The good correlation between along‐axis variations in depth, and changes in axial volume and gravity, argue against the mantle melt conduit as being the major source of this along‐axis variation. Instead, this variability can be adequately explained by a combination of along‐axis changes in crustal thermal structure and/or along‐axis crustal thickness changes of a few hundred meters.
Chemical transport and dissolution/precipitation of crystalline solution during hydrothermal convection
Authors: Rabinowicz M. , Dandurand J. L. , Schott J.
Published: February 2004

A mathematical formalism is developed to compute the aqueous species transport coupled to reactions forming crystalline solutions during hydrothermal circulation. The formalism takes into account that, during convection in a fracture network at temperatures from 0°C to 200°C, dissolution/precipitation reactions between the fluids and crystalline solutions do not reach a “true” equilibrium at the local fluid temperature; rather a “pseudo‐equilibrium” is reached locally either with the dissolving or with the last precipitated crystalline solution. These assumptions permit the explicit solutions of the mass transfer equations during simple convective loops. Two examples of reaction associated with convective flow are given: (1) 16O and 18O partitioning between quartz and an aqueous fluid and (2) compositional variations in the celestite‐barite (Sr,Ba)SO4 solid solution. Computations show that after several convective cycles, an asymptotic precipitation regime is reached which is independent of the initial composition of the fluids percolating in the fracture network. Also, for most crystalline solutions, the compositions of the precipitated solids in the asymptotic precipitation regime are not affected by the fact that the “pseudo‐equilibrium” is reached with the dissolving or with the last precipitated crystalline solution. Thus, explicit relations are derived giving the composition of the precipitated products as a function of the convective fluid temperature and the reacting crystalline solution. These relations are suggested as possible geothermometers to study paleohydrothermal systems.
Closure domains in magnetite
Authors: Özdemir Özden , Xu Song , Dunlop David J.
Published: February 2004

We have observed clear arrays of closure domains at internal and external boundaries, such as crystal edges, cracks, and chemically altered regions, in a single crystal of magnetite, using the Bitter colloid technique with a {110} viewing plane. The common occurrence of closure domains in magnetite confirms the controlling role of magnetostatic energy in determining domain structures in strongly magnetic materials, first postulated by Landau and Lifschitz (1935). Closure domains also reconcile a long‐standing discrepancy between the numbers of body domains observed in magnetite and the numbers predicted, which we show are much reduced when closure domains are taken into account. We verify experimentally that closure domains change their shape and internal structure depending on the crystallographic orientation of the boundary at which they form. When the boundary intersects the {110} viewing plane along a <111> easy axis, for example, at favorably oriented internal boundaries or cracks, the closure domains are asymmetric in shape and are bounded by 71° and 109° walls. However, when the boundary is a {111} crystal face, containing no <111> easy axis, the closure domains are symmetric in form and bounded by ≈90° walls. Closure domains of this latter type collect colloid densely along one of the two bounding walls, suggesting concentrations of magnetic poles, and the two walls are at angles of 70°–80° to each other, rather than the expected 90°. Several possible models are proposed to explain this behavior. Our study shows that internal domain structures in magnetite are relatively simple. Much more complicated structures appear on viewing surfaces that do not contain two sets of <111> easy axes. The lack of orientation of the viewing surface in most previous studies may account for the reported rarity of closure domains in magnetite. Closure domain arrays forming at the margin of a chemically altered area are the first reported evidence for a direct link between chemical alteration and domain structures that could result in remagnetization.
Neotectonic activity on continental fragments in the Southwest Indian Ocean: Agulhas Plateau and Mozambique Ridge
Authors: Ben‐Avraham Z. , Hartnady C. J. H. , le Roex A. P.
Published: February 2004

The Agulhas Plateau and Mozambique Ridge are composed in part of continental fragments embedded within oceanic crust of the southwest Indian Ocean. Recent studies of the plateaus and their vicinity have discovered significant evidence for neotectonic activity. In both areas, newly obtained seismic reflection profiles indicate possibly young basaltic intrusions in the northern, oceanic parts of the plateaus. Small rock fragments recovered from the southern Mozambique Ridge comprise metamorphic and volcanic lithologies. The volcanic rocks are made of extremely fresh quenched glasses. Although no radiometric dates are available for the volcanic glasses, their lack of any significant alteration suggests that eruption took place in the last few tens of thousands of years, supporting the seismic reflection evidence for magmatic activity in this region. The active crustal stretching and tensional stresses implied by this relatively recent tectonism probably cannot be generated by distantly applied plate‐driving torques, such as ridge push, but appear to require buoyancy‐related forces originating in the underlying upper mantle.
Trace element and Sr and Nd isotopic composition of mantle xenoliths from the Big Pine Volcanic Field, California
Authors: Beard Brian L. , Glazner Allen F.
Published: February 2004

Mantle xenoliths from the Quaternary Big Pine volcanic field of eastern California provide direct evidence for the presence of isotopically enriched mantle beneath the Sierran Province of the western United States. Sr and Nd isotopic compositions determined on five bulk rock and five clinopyroxene separates are negatively correlated; εNd values range from +13.8 to −3.4, and 87Sr/86Sr ratios range from 0.7022 to 0.7065. The measured Sr and Nd contents and Ce/Yb ratios of mantle xenoliths in the Big Pine volcanic field are well correlated with Sr and Nd isotopic compositions, and these xenoliths define an 820‐m.y.‐old Sm‐Nd isochron. The Big Pine mantle xenoliths have similar textures and mineral compositions, and we interpret the large range in isotopic and trace element compositions to be a result of cryptic mantle metasomatism. The timing of this mantle metasomatic event is unclear but a maximum Late Proterozoic age can be inferred from the 820 Ma Sm‐Nd isochron. If the Sm‐Nd isochron is interpreted as a result of recent metasomatism then the metasomatising agent was similar in composition to the Big Pine basalts. If the isochron is interpreted to have age significance then this mantle metasomatism was associated with late Proterozoic rifting of the western margin of North America. Although production of isotopically enriched mantle is generally attributed to ancient subduction, these data are inconsistent with such an interpretation.
Equation of state for lambda transition in quartz
Authors: Dorogokupets Peter I.
Published: February 2004

An equation of state has been developed for quartz, a substance with a lambda‐type phase transition. It has the properties of the Murnaghan equation of state in the normal region and reflects the fundamental behavior of thermodynamic functions in the anomalous region. The term jT/(i‐T)1/2 has been introduced in the temperature dependent equations of isobaric heat capacity and the volume coefficient of thermal expansion, where i and j are fitting coefficients. The i coefficient is numerically close to the temperature of the phase transition Tc and is usually in the 0.1<|i‐Tc|<20 K interval. The range of applicability of the Murnaghan equation is delineated in the PT stability field of quartz. In the lambda range, the derivative of thermal expansion with respect to temperature is pressure dependent according to rules that allow integration of the thermal expansion and molar volume for calculation of the change in thermodynamic functions from room pressure to any given pressure. Available phase equilibria and thermochemical data have been used with the proposed equation of state to calculate the standard thermodynamic properties for coesite and the SiO2 phase diagram including alpha quartz, beta quartz, and coesite.
The San Andreas Fault system through the Transverse Ranges as illuminated by earthquakes
Authors: Seeber Leonardo , Armbruster John G.
Published: February 2004

The pattern of seismogenic faulting in the San Gorgonio Pass‐San Bernardino basin area of the San Andreas fault (SAF) zone is mapped from about 7000 quality‐selected focal mechanisms (1981‐Nov '92). They are derived from phase data of the southern California network by a relocation procedure based on location‐dependent station‐corrections and by a grid‐search procedure. About 3/4 of the mechanisms have been interpreted as planes of rupture, or “slip planes”, that delineate many distinct faults. One of these faults is right‐lateral, dips steeply northeast and is continuous trough the San Gorgonio Pass area; we interpret it as the main branch of the SAF. A very large earthquake seems possible on this portion of the SAF because of its continuity and because of the unusually deep reach of the seismicity (23 km). Southwest of this fault, a volume of diffuse and persistent seismicity has a sharp downward cut‐off which may reflect a basal detachment dipping 20° northeast and intersecting the SAF along the deepest seismicity. Northeast of the SAF, the floor of the seismicity is shallower by as much as 10 km. Stress characteristics are derived from slip planes in 10 selected subregions. The regime is transpressional near the constricting bend of the SAF at San Gorgonio Pass; it is dominated by horizontal extension in the San Bernardino Basin area; and it is again transpressional in the eastern San Gorgonio Mountains north of the Cucamonga thrust. The broad features of this zonation may be related to the intersection of the SAF with the Pinto Mountain fault and with the San Jacinto fault. A change of fault kinematics in the Yucaipa cluster coincides with the 1992 Landers and Big Bear main shocks and may be a manifestation of static stress change.
Localization and fault growth in layered brittle‐ductile systems: Implications for deformations of the continental lithosphere
Authors: Davy P. , Hansen A. , Bonnet E. , Zhang Shou‐Zhu
Published: February 2004

We present experiments and numerical simulations dealing with the growth of faults in thin brittle/ductile systems to understand deformation modes in the continental lithosphere. Experiments were uniaxial shortening of layers of dry sand and silicone putties of various viscous resistances. For large strength ratios between the brittle and ductile layers (R> 5–10), the deformation localizes into two shear bands; the fault pattern is created before reaching 10% shortening, and has fractal dimensions varying between 1.6 and 1.8. For small strength ratios (R< 5–10), deformation never localizes; the fault pattern is homogeneous with a trivial dimension of 2, and grows continuously during deformation. The transition between localized and homogeneous deformation occurs when the mechanical resistance of brittle layers is 5–10 times larger than the resistance of ductile layers. This transition was also investigated by means of electrical analog simulations. A fuse network, which represents an elasto‐brittle layer, is coupled with a capacitor layer which models strain‐rate dependent fluids. An AC potential is applied and the fuses progressively burned out until they form a connected network. The AC‐potential frequency, ƒ, is a tuning parameter similar to the applied strain rate in experiments. A critical frequency is obtained marking a transition between a localization mode where the density of burned fuses decreases as the system size increases, and a delocalization mode where the density of burned fuses remains constant with increasing system size. The scaling dependency of the fracture process, as well as the critical frequency, are consistent with experimental results. Available information on the rheology of the continental lithosphere shows that this mechanical transition is bracketed by the possible range of brittle‐to‐ductile strength ratios.
Rheology of crystal‐bearing silicate melts: An experimental study at high viscosities
Authors: Lejeune Anne‐Marie , Richet Pascal
Published: February 2004

The viscosity of partially crystallized Mg3Al2Si3O12 melts has been measured under uniaxial compression in the interval 1010 ‐ 1013 poise as a function of the volume fraction of crystals. These inclusions are well‐rounded spherulites of aluminous enstatite, having the same composition as the melt, and whose growth rate is negligible at the temperature of the measurements. The viscosity increases by less than 1 order of magnitude for crystal fractions ϕ of 40 vol % and remains Newtonian up to the maximum stress exerted, namely 1 kbar. The Einstein‐Roscoe equation, η = η0 (1 ‐ ϕ/ϕm)−n, provides very good fits to the measurements only if either the ϕm or n parameter is allowed to depend on temperature. For modeling of magmatic processes, however, the widely recommended constant values ϕm = 0.6 and n = 2.5 should be adequate. The rheology changes abruptly when the clustered spherulites begin to oppose shear deformation, at a crystal fraction of about 40 vol %. The viscosity becomes non‐Newtonian, with yield strengths of a few tens of bars at temperatures at which the viscosity of the melt is higher than 1010 poise. As long as the crystal fraction remains lower than 70 vol %, the deformation proceeds in an irregular manner with a nonuniform distribution of crystals and melt. The deformation becomes again regular at low stresses with lower melt fractions, but samples undergo extensive fracturation along the direction of uniaxial stress. Similar rheology changes have been observed during the isothermal crystallization of Li2Si2O5 melts, which produces small ellipsoidal inclusions. These results suggest that the influence of solid suspensions on the rheology of magmas is primarily determined by the crystal fraction, even though additional measurements would be useful to determine the possible influence of other factors such as the size distribution or the shape of the inclusions.
Origin of Kenya Rift Plateau‐type flood phonolites: Integrated petrologic and geophysical constraints on the evolution of the crust and upper mantle beneath the Kenya Rift
Authors: Hay David E. , Wendlandt Richard F. , Keller G. Randy
Published: February 2004

A gravity profile across the Kenya Rift at the equator was modeled with recent results of petrologic and seismic investigations as constraints. This profile is dominated by a 600‐km‐wide gravity low of 100 × 10−5 m/s2 (100 mGal). The model incorporates the important concept of modification of the lower crust by magmatic processes. Alkali basaltic magmas are believed to have modified the lower crust in southern Kenya during early rift‐related basaltic volcanism (23–14 Ma) on the basis of (1) synchronous voluminous eruption of alkali basalt in northern Kenya; (2) Kenya Rift International Seismic Project 1990 (KRISP 90) seismic lines which delineate an ∼10‐km‐thick layer of basal lower crust in southern Kenya which is absent or very thin to the north; (3) results from high pressure/temperature experiments on a late Miocene Plateau phonolite and related geochemical modeling which indicate a lower crustal origin for these voluminous lavas (14–11 Ma) by fusion of alkali basaltic material; and (4) Plateau phonolite distributions that are conspicuously limited to the southern Kenya Rift above this anomalous lower crust. The gravity model features a lens of mafic intrusives (3000 kg/m3; constrained by petrologic arguments) at the base of the crust. It extends from 25 to 34 km depth beneath the rift valley and pinches out at a distance of about 250 km on each side of the rift. Surrounding lower crust (2850 kg/m3) is bounded by upper crust (2700 kg/m3) at about 15 km depth. The gravitational effect of the positive density contrast in the lower crust due to the lens of mafic intrusives is offset by an underlying wedge of anomalously low density mantle (3150 kg/m3). This wedge is about 300 km wide at the Moho and is relatively steep sided, in agreement with KRISP teleseismic results. East of the rift, this anomalous mantle is bounded by normal upper mantle (3260 kg/m3) extending to 100 km depth. West of the rift, normal upper mantle extends to 90 km depth. Within the rift valley, shallow horst and graben are indicated by the gravity data.
Microplate versus continuum descriptions of active tectonic deformation
Authors: Thatcher Wayne
Published: February 2004

Whether deformation of continents is more accurately described by the motions of a few small rigid plates or by quasi‐continuous flow has important implications for lithospheric dynamics, fault mechanics, and earthquake hazard assessment. Actively deforming regions of the western United States, central Asia, Japan, and New Zealand show features that argue for both styles of movement, but new observations are necessary to determine which is most appropriate and at what scale the description applies. Geologic, geodetic, seismic, and paleomagnetic measurements tend to sample complementary aspects of the deformation field, so an integrated observation program can utilize the strengths of each method and overcome their separate spatial or temporal biases. Provided the total relative motion across each region is known and the distribution of active faults is well mapped, determination of fault slip rates can provide potentially decisive constraints. Reconnaissance geological studies supply useful slip rate estimates, but precise values depend upon detailed intensive investigation of individual sites. Geodetic survey measurements can determine the spatial pattern of contemporary movements and extract slip rate information, but the sometimes elusive effects of cyclic elastic strain buildup and relief must be accounted for in relating current movements to the long‐term deformation pattern. Earthquake catalogs can be applied to determine seismic strain rates and relative velocities but must be averaged over large regions and are usually limited by the inadequate duration of historical or instrumental seismicity catalogs. Paleomagnetic determinations of vertical axis rotations provide estimates of block rotation rates but are often locally variable and averaged over many millions of years. Which of the two descriptions of continental tectonics is more nearly correct depends on the local rheological stratification of the lithosphere, especially the strength and thickness of the elastic crust relative to the ductile lithosphere, and dynamical models can provide contrasting forecasts of observable features with testable consequences.
Monte Carlo sampling of solutions to inverse problems
Authors: Mosegaard Klaus , Tarantola Albert
Published: February 2004

Probabilistic formulation of inverse problems leads to the definition of a probability distribution in the model space. This probability distribution combines a priori information with new information obtained by measuring some observable parameters (data). As, in the general case, the theory linking data with model parameters is nonlinear, the a posteriori probability in the model space may not be easy to describe (it may be multimodal, some moments may not be defined, etc.). When analysing an inverse problem, obtaining a maximum likelihood model is usually not sufficient, as we normally also wish to have information on the resolution power of the data. In the general case we may have a large number of model parameters, and an inspection of the marginal probability densities of interest may be impractical, or even useless. But it is possible to pseudorandomly generate a large collection of models according to the posterior probability distribution and to analyse and display the models in such a way that information on the relative likelihoods of model properties is conveyed to the spectator. This can be accomplished by means of an efficient Monte Carlo method, even in cases where no explicit formula for the a priori distribution is available. The most well known importance sampling method, the Metropolis algorithm, can be generalized, and this gives a method that allows analysis of (possibly highly nonlinear) inverse problems with complex a priori information and data with an arbitrary noise distribution.
Three‐dimensional modeling of pull‐apart basins: Implications for the tectonics of the Dead Sea Basin
Authors: Katzman Rafael , ten Brink Uri S. , Lin Jian
Published: February 2004

We model the three‐dimensional (3‐D) crustal deformation in a deep pull‐apart basin as a result of relative plate motion along a transform system and compare the results to the tectonics of the Dead Sea Basin. The brittle upper crust is modeled by a boundary element technique as an elastic block, broken by two en echelon semi‐infinite vertical faults. The deformation is caused by a horizontal displacement that is imposed everywhere at the bottom of the block except in a stress‐free “shear zone” in the vicinity of the fault zone. The bottom displacement represents the regional relative plate motion. Results show that the basin deformation depends critically on the width of the shear zone and on the amount of overlap between basin‐bounding faults. As the width of the shear zone increases, the depth of the basin decreases, the rotation around a vertical axis near the fault tips decreases, and the basin shape (the distribution of subsidence normalized by the maximum subsidence) becomes broader. In contrast, two‐dimensional plane stress modeling predicts a basin shape that is independent of the width of the shear zone. Our models also predict full‐graben profiles within the overlapped region between bounding faults and half‐graben shapes elsewhere. Increasing overlap also decreases uplift near the fault tips and rotation of blocks within the basin. We suggest that the observed structure of the Dead Sea Basin can be described by a 3‐D model having a large overlap (more than 30 km) that probably increased as the basin evolved as a result of a stable shear motion that was distributed laterally over 20 to 40 km.