bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of differentiation in magma reservoirs
Authors: Jaupart Claude , Tait Stephen
Published: February 2004

In large magma chambers, gradients of temperature and composition develop due to cooling and to fractional crystallization. Unstable density differences lead to differential motions between melt and crystals, and a major goal is to explain how this might result in chemical differentiation of magma. Arriving at a full description of the physics of crystallizing magma chambers is a challenge because of the large number of processes potentially involved, the many coupled variables, and the different geometrical shapes. Furthermore, perturbations are caused by the reinjection of melt from a deep source, eruption to the Earth's surface, and the assimilation of country rock. Physical models of increasing complexity have been developed with emphasis on three fundamental approaches. One is, given that large gradients in temperature and composition may occur, to specify how to apply thermodynamic constraints so that coexisting liquid and solid compositions may be calculated. The second is to leave the differentiation trend as the solution to be found, i.e., to specify how cooling occurs and to predict the evolution of the composition of the residual liquid and of the solid forming. The third is to simplify the physics so that the effects of coupled heat and mass transfer may be studied with a reduced set of variables. The complex shapes of magma chambers imply that boundary layers develop with density gradients at various angles to gravity, leading to various convective flows and profiles qf liquid stratification. Early studies were mainly concerned with describing fluid flow in the liquid interior of large reservoirs, due to gradients developed at the margins. More recent work has focused on the internal structure and flow field of boundary layers and in particular on the gradients of solid fraction and interstitial melt composition which develop within them. Crystal settling may occur in a surprisingly diverse range of regimes and may lead to intermittent deposition events even with small crystal concentrations. Incorporating thermodynamic constraints in the study of the dynamics of settling has only just begun. Many dynamical phenomena have been found using theoretical arguments, laboratory experiments on analog systems, and numerical calculations on simplified chemical systems. However, they have seldom been applied to natural silicate melts whose phase diagrams and important physical properties such as thermal conductivity and chemical diffusion coefficients remain poorly known. There is a gap between model predictions and observations, as many models are designed to explain large‐scale features and many observations deal with the local texture and mineral assemblages of the rocks. This review stresses the relevance to the geological problem of the work carried out in parallel in other disciplines, such as physics, fluid dynamics, and metallurgy.
Laboratory measurements of a complete set of poroelastic moduli for Berea sandstone and Indiana limestone
Authors: Hart David J. , Wang Herbert F.
Published: February 2004

Measurements have been completed for eight different poroelastic moduli of water‐saturated Berea sandstone and Indiana limestone as a function of confining pressure and pore pressure. The poroelastic moduli for Indiana limestone are generally consistent to ±10%, which was verified by a formal inversion procedure for independent moduli from the eight measurements. For Indiana limestone, best fit values were drained bulk modulus, 21.2 GPa; the undrained bulk modulus, 31.7 GPa; drained Poisson's ratio, 0.26; undrained Poisson's ratio, 0.33; and pore pressure buildup coefficient, 0.47 at 20–35 MPa effective stress. The poroelastic moduli for Berea sandstone are generally consistent to ±20%. The greater inconsistency is most likely caused by the nonlinear variation of the moduli at different strains. For Berea sandstone, best fit values were drained bulk modulus, 6.6 GPa; undrained bulk modulus, 15.8 GPa; drained Poisson's ratio, 0.17; undrained Poisson's ratio, 0.34; and pore pressure buildup coefficient, 0.75 at 10 MPa effective stress.
The accommodation of Arabia‐Eurasia Plate convergence in Iran
Authors: Jackson James , Haines John , Holt William
Published: February 2004

Continental convergence between Arabia and Eurasia is taken up by distributed deformation in Iran. At wavelengths large compared with the thickness of the lithosphere this deformation is best described by a continuous velocity field. The only quantitative source of information on the spatial distribution of strain rates within Iran is the record of earthquakes. We find that we can reproduce the style of deformation observed in the seismicity by simply minimizing the rate of work in a continuous viscous medium that has to accommodate the Arabia‐Eurasia plate motion between the defined shapes of Iran's rigid borders. When, in addition, we specify central Iran, Azerbaijan, and the southern Caspian basin to be relatively rigid blocks within the deforming zone, then the fit to the style of the observed strain rate distribution is even better. We conclude that much of the pattern of deformation in Iran is predetermined by the shape of its rigid borders and by the disposition of relatively rigid blocks within it. This is likely also to have been a common occurrence in older orogenic belts. We confirm earlier suggestions that earthquakes between 1909 and 1992 can account for only a small part (∼10–20%) of the total deformation required by the convergence between the Arabia and Eurasia plates. We then show that the whole plate motion can be accommodated by a velocity field with the same orientations and relative magnitudes of principal strain rates seen in the earthquakes but with larger absolute magnitudes. There is therefore no requirement that the large proportion of aseismic deformation in Iran is substantially different in style, orientation, or distribution from that released seismically in the earthquakes.
Frictional weakening and slip complexity in earthquake faults
Authors: Shaw Bruce E.
Published: February 2004

Previous work has shown that velocity‐weakening friction produces slip complexity in simple dynamical models of earthquake faults (Carlson and Langer, 1989). Here I show that a different type of dynamical instability, caused by slip‐weakening friction, also produces slip complexity. The deterministically chaotic slip complexity produced by slip‐weakening friction in a simple one dimensional model is studied and the scaling of the distribution of sizes of events with the parameters in the model examined. In addition, a possible physical origin of frictional weakening is examined, through a very simplified mathematical representation of a physical process proposed by Sibson (1973), whereby frictional heating causes an increase in pore fluid temperature and pressure, thereby reducing the effective normal stress and friction. The two different types of frictional weakening are derived from two opposing limits, with slip weakening occurring when the dissipation of heating is slow compared to the rupture timescale, as Lachenbruch (1980) has shown, while velocity weakening is shown to occur when the dissipation is fast compared to the rupture timescale. Since both end‐member cases of frictional weakening are seen to produce slip complexity, slip complexity is argued to be a generic feature of frictional weakening and elastodynamics on a fault.
The Mount Perkins block, northwestern Arizona: An exposed cross section of an evolving, preextensional to synextensional magmatic system
Authors: Faulds James E. , Feuerbach Daniel L. , Reagan Mark K. , Metcalf Rodney V. , Gans Phil , Walker J. D.
Published: February 2004

The steeply tilted Mount Perkins block, northwestern Arizona, exposes a cross section of a magmatic system that evolved through the onset of regional extension. New 40Ar/39Ar ages of variably tilted (0–90°) volcanic strata bracket extension between 15.7 and 11.3 Ma. Preextensional intrusive activity included emplacement of a composite Miocene laccolith and stock, trachydacite dome complex, and east striking rhyolite dikes. Related volcanic activity produced an ∼18–16 Ma stratovolcano, cored by trachydacite domes and flanked by trachydacite‐trachyandesite flows, and ∼16 Ma rhyolite flows. Similar compositions indicate a genetic link between the stratovolcano and granodioritic phase of the laccolith. Magmatic activity synchronous with early regional extension (15.7–14.5 Ma) generated a thick, felsic volcanic sequence, a swarm of northerly striking subvertical rhyolite dikes, and rhyolite domes. Field relations and compositions indicate that the dike swarm and felsic volcanic sequence are cogenetic. Modes of magma emplacement changed during the onset of extension from subhorizontal sheets, east striking dikes, and stocks to northerly striking, subvertical dike swarms, as the regional stress field shifted from nearly isotropic to decidedly anisotropic with an east‐west trending, horizontal least principal stress. Preextensional trachydacitic and preextensional to synextensional rhyolitic magmas were part of an evolving system, which involved the ponding of mantle‐derived basaltic magmas and ensuing crustal melting and assimilation at progressively shallower levels. Major extension halted this system by generating abundant pathways to the surface (fractures), which flushed out preexisting crustal melts and hybrid magmas. Remaining silicic melts were quenched by rapid, upper crustal cooling induced by tectonic denudation. These processes facilitated eruption of mafic magmas. Accordingly, silicic magmatism at Mount Perkins ended abruptly during peak extension ∼14.5 Ma and gave way to mafic magmatism, which continued until extension ceased.
Paleozoic plate dynamics
Authors: Jurdy Donna M. , Stefanick Michael , Scotese Christopher R.
Published: February 2004

Current plate motions can be accounted for by a balance of active forces, slab pull, ridge push, and, for continental plates, trench suction, with drag beneath the plate as a resistive force. If we assume that the same forces have acted through time, we can reconstruct plate motions from the geometry of past plate boundaries. Paleozoic reconstructions are made with paleomagnetic, tectonic, climatic, and biogeographic data, as no ocean floor remains. PALEOMAP reconstructions are used to estimate past plate speeds and to test simple dynamical models in order to determine which ranges of forces best accounts for the observations. Over the last 600 m.y., plate speeds averaged over 40‐ to 100‐m.y. intervals show considerable variation; Gondwana's speed oscillates from 20 to 60 km/m.y. over a long timescale (200–400 m.y.) with considerable noise superposed. Over the Paleozoic Era motions for large continental regions average 28 km/m.y.; force balance models based on present‐day observations suggest that continental regions without a large attached slab would move 30 mm/yr. The opening and closing of the ocean between Laurentia and Gondwana 560–400 Ma is used to test dynamical models and the parameter values assumed. In the late Precambrian, Laurentia rifted away from Gondwana. In the earliest Cambrian it was near 40°S; by Late Cambrian and Ordovician it had moved to the equator. During the Silurian and Devonian, Laurentia reversed direction and later collided with Gondwana at 40°S. In a model of the forces acting on the plates, slab pull, ridge push, and trench suction are assumed to balance plate drag. Only certain ranges of ridge‐push and trench parameters can model both the opening and subsequent closing of the ocean. The dynamic models, with parameter values inferred from present rates, bracket the rates required by the reconstructions.
Along‐axis segmentation and isostasy in the Western rift, East Africa
Authors: Upcott N. M. , Mukasa R. K. , Ebinger C. J. , Karner G. D.
Published: February 2004

Structural variations along the southern sectors of the Western rift, East Africa, have previously been described, but subsurface structures in the northern sector (Uganda, Zaire) are virtually unknown. Our aims are to investigate the along‐axis segmentation of the northern sector, thereby adding to the structural picture of the Western rift, and to study the isostatic compensation of the varying rift morphology along the sector's length. This study describes the first gravity survey to be carried out on the shallow Lake Albert, forward models of these and existing gravity data, and the results from inverse modeling of existing aeromagnetic data designed to delimit border and transfer fault systems. Our tectonic model shows that the northern rift sector is segmented along‐axis into five 25 to 65‐km‐wide, 80 to 100‐km‐long rift segments, characterized by closed‐contour Bouguer anomaly lows, and bounded by steep gravity, aeromagnetic, and topographic/bathymetric gradients. Werner and Euler deconvolution results and gravity anomaly data reveal that some faulted basins are separated by structural highs and cross‐rift ramps or faults and suggest sedimentary basin depths of 4–6 km. Forward modeling of structural and free‐air gravity profiles across individual basins and flanks using a model that assumes flexural compensation also suggests sediment thicknesses of up to 5.5 km, similar to the estimates from magnetic data. The basin and flank morphology can be explained by 6–9 km of extension of a lithosphere with an effective elastic thickness (Te) of 25 km (equivalent to a flexural rigidity of 1.4 × 1023 N m), similar to results in other Western rift basins. Potential field data and lithospheric strength estimates in the Western rift system show small along‐axis variations in lithospheric structure, regardless of the presence or absence of Cenozoic magmatism.
A history of Pacific Northwest earthquakes recorded in Holocene sediments from Lake Washington
Authors: Karlin Robert E. , Abella S. E. B.
Published: February 2004

Large earthquakes can trigger slumping of the steep walls of lake basins and landslides in the drainage area, resulting in turbidite deposition in the lake and increased detrital flux from inlets. Holocene sediments in piston cores from Lake Washington contain a series of terrigenous layers that were episodically deposited in the lake. Sedimentological, geochemical, and paleomagnetic analyses on nine piston cores show that the detrital layers are temporally and areally correlatable, indicating basinwide disruptions. These layers are opaque on X‐radiographs, are coincident with magnetic susceptibility peaks, have abundant aluminosilicate minerals, are relatively coarse grained, and have low organic carbon and biogenic silica contents. The thicknesses and geographic distributions of the layers suggest that they are not due to floods or delta destabilization. Side scan swath imagery and subbottom profiling show that large slumps, subaqueous landslides, and debris flows are common along the margins of the lake. A detailed chronology, established from 21 radiocarbon ages on five cores, show that a prominent turbidite was deposited about 1000–1100 years ago. This turbidite apparently was triggered by a large earthquake that probably occurred on the Seattle fault. Other depositional events in the sediment record at 1500–1700, 2400–2500, and 2800–3200 years ago coincide with periods of landsliding that have been previously inferred from the dating of drowned trees in the lake. More than 30 depositional events have occurred in the last 12,000 years and 21 disturbances have occurred since the deposition of the Mazama ash about 7,600 years ago. If all the events are due to earthquakes, the Puget Sound region has been subject to major shaking every 300 to 400 years. The strong intensities needed to trigger subaqueous slides may not be generated by just local sources such as the Seattle fault but could also be caused by great subduction earthquakes occurring along the coast.
On the formation of eruption columns following explosive mixing of magma and surface‐water
Authors: Koyaguchi Takehiro , Woods Andrew W.
Published: February 2004

When magma vents into the sea or a crater lake, the ensuing magma‐water interaction can affect the style of eruption dramatically. If the mass of surface water incorporated into the erupting material is small, (<15% of the total mass), then typically, this water vaporizes, and the density and temperature of the erupting mixture decreases. As a result, the minimum eruption velocity for which a Plinian‐style eruption column may develop decreases. If a larger mass of cold surface water is added to the mixture, then part of this water may not vaporize, the initial mixture has the saturation temperature, and the initial density increases again. For sufficiently large masses of surface water mixed into the erupting magma, the ascending mixture cannot become buoyant. Instead, relatively cold, wet and dense ash flows spread laterally from a collapsing fountain. With a small or moderate eruption rate, <108 kg/s, the height of rise of a buoyant column does not vary significantly with the surface water content. However, for very large eruption rates, >108 kg/s, the height progressively decreases with surface water content. This occurs when the magma and surface water begin to constitute a significant fraction of the mass at the top of the column, so that an increasing fraction of the initial magmatic thermal energy is converted to the surface water rather than the entrained air. The transitions in eruption style which result from changes in the mass of surface water mixing with the magma may account for observations of both buoyant plumes and wet surge during the eruptions of Taal in 1965 and Miyake‐jima in 1983 and for the changes in the eruptive activity at Surtsey in 1963–1964 as the access of seawater to the vent became more restricted. We also present calculations which suggest that the accretionary lapilli, which are often found in wet flow deposits, may result from condensation of vapor in both the cold, wet collapsing fountains and in the flows themselves.
Comparison of strain and stress tensor orientation: Application to Iran and southern California
Authors: Gillard Dominique , Wyss Max
Published: February 2004

Strain and stress tensors along plate boundaries in Iran and southern California are oriented coaxially if they are estimated based on small to moderate earthquakes. If large earthquakes (M>7) are included in the analysis, the estimate of stress directions does not change, but the orientation of the released strain rotates significantly. Thus the direction of the total released strain is different from the stress by about 20° in both Iran and southern California, and by about 45° in central California. We interpret this observation to indicate that the smaller earthquakes occur on faults favorably oriented for failure in the prevailing stress field, in which they are possibly generated. The larger events rupture near‐vertical strike‐slip faults that are unfavorably oriented but weak. This weakness causes failure under low ratio of shear to normal stress. This view is supported by the fact that, in the southern California data set, the average angle between the fault planes and the greatest principal stress is about 40°, whereas it forms an angle of 70° with the San Andreas fault (nearly 90° in central California). For estimating the seismic strain release by Kostrov's method along about a 10‐km‐wide section of the San Andreas fault zone, we used the data set from which Jones determined the stress directions for earthquakes in the range of 2.6<M≤4.3. We estimated the directions of principal strain for Iran based on 111 focal mechanisms and the scalar moment for earthquakes in the range of 4.4<M <8. We inverted regional subsets of these data for volumes with dimensions of a few hundred kilometers for the stress directions using the method of Gephart and Forsyth. The average misfits for these inversions ranged from 2° to 5°. Based on test inversions of synthetic data with known misfits, we found that these misfits can be attributed solely to inaccuracies of less than 10° in the fault plane solutions. Therefore the results for Iran are most likely obtained for volumes with uniform stress directions. We propose that stress inversions with misfits smaller than 6° can be accepted as meaningful, as an approximate general guideline, because our tests for Iran agree with those for Hawaiian data sets performed by Wyss.
Crustal thickness beneath the Queen Charlotte Basin, Canada: Results of a seismically constrained gravity inversion
Authors: Lowe C. , Dehler S. A.
Published: February 2004

A gravity inversion routine was used to calculate crustal thickness variations in the Queen Charlotte Basin, western Canada. First, a “crustal” Bouguer anomaly was derived using a digital, well‐constrained depth‐to‐basement map and densities determined from well logs and rock samples. This anomaly was subsequently inverted for crustal thickness assuming a single density contrast at depth and an average crustal thickness provided by seismic refraction data. The results, restricted to that portion of the basin for which there are good quality data, show crustal thickness variations of the order of 12 km. Regions of thin crust are located off northern Vancouver Island (∼20 km), along the axis of Hecate Strait and in northern Queen Charlotte Sound (∼ 24 km). Thicker crust is associated with Central Queen Charlotte Ridge and northeastern Hecate Strait. Tests show these trends in crustal thickness prevail for all reasonable ranges of input parameters. Calculated residual gravity anomalies contain information useful for mapping the distribution of volcanics interbedded with sediments in subbasins and for refining estimates of depth‐to‐basement derived from seismic data alone. Our results are, for the most part, in excellent agreement with seismically derived estimates of Moho depth. They show minimum crustal thickness beneath regions of maximum sediment accumulation, supporting an extensional origin for the basin. They indicate that this extension was quite heterogeneous and yield minimum estimates which vary from ∼13% in some parts of the basin to as much as 65% in others. The results can be used to discriminate between models on the Tertiary evolution of the basin; the observed trends are not predicted by the lithospheric flexure or simple margin‐perpendicular extension models but are consistent with a model of crustal thinning by distributed shear in a transtensional regime.
Segmentation and Holocene surface faulting on the Median Tectonic Line, southwest Japan
Authors: Tsutsumi Hiroyuki , Okada Atsumasa
Published: February 2004

The Median Tectonic Line (MTL) active fault system is a 300‐km‐long, arc‐parallel, right‐lateral strike‐slip fault related to oblique subduction of the Philippine Sea plate beneath the Eurasian plate at the Nankai trough, southwest Japan. The fault on Shikoku Island has a slip rate of 5–10 mm/yr, one of the highest in onshore Japan. We identified 12 geometric segments along the MTL separated by discontinuities such as en echelon steps, bends, changes in strike, and gaps in the surface trace. A chronology of latest Holocene surface faulting on the MTL was constructed based on previously published and new work including trenching, geomorphic mapping, and acoustic profiling and piston coring of the submarine extension of the fault zone. The MTL probably does not rupture along its entire length in a single earthquake but instead consists of multiple “earthquake segments” that rupture independently of one another. The recurrence interval and surficial offset for surface‐rupturing earthquakes at four individual sites on the MTL in Shikoku Island are 1000–3000 years and 5–8 m, respectively. The relatively long recurrence interval and large surficial offset per earthquake suggest large seismic moment release during surface‐rupturing earthquakes on the MTL, as commonly observed for Japanese moderate to large intraplate earthquakes. Because the predicted rupture lengths based on empirical relations between surface displacement and rupture length are considerably larger than those of individual segments, large earthquakes on the MTL probably rupture multiple geometrically defined segments. The fault was previously thought to be unruptured at least in the past 1000 years based on the absence of historical evidence for destructive earthquakes. However, trench excavations show that part of the MTL ruptured most recently during or after the 16th century A.D. with at least 6.9 ± 0.7 m of slip along the main fault trace. This rupture may be correlated to the 1596 Keicho‐Kinki earthquake, but further geological and historical investigations are needed to confirm this hypothesis.
Diffusion transport model for pelagic sediments on the Mid‐Atlantic Ridge
Authors: Mitchell Neil C.
Published: February 2004

The diffusion model is potentially useful for quantifying the effect of downslope gravity transport on sedimentation rate variations, which are commonly found between Deep Sea Drilling Project and Ocean Drilling Program sites. If appropriate, the model and stratigraphy could be used, for example, to constrain the amount and timing of fault block rotation. Deep Tow profiler records from the French‐American Mid‐Ocean Undersea Study area of the Mid‐Atlantic Ridge are examined to determine whether they are consistent with a diffusion model. Sedimentary contacts with basement show that a variety of processes may be operating, some of which are inconsistent with the diffusion model. For example, there are moats around some contacts which are typical of scouring or nondeposition due to high current velocities. More than half of the contacts, however, show onlapping, which is qualitatively consistent with the diffusion model. Sediments generally fill low areas of the topography and have smooth surfaces, as expected from a diffusion model. Where the fluxes of sediment entering a basin are constant, the diffusion model predicts that the sediment surface should evolve to a parabola (the steady state solution). Some 20 curved surfaces in the profiler data were digitized and least squares parabolas fitted to them with rms errors of less than 1 m. The slopes of the model parabolas provide values for the ratio of sediment flux to diffusivity at the edges of the basins (steep surfaces are produced by low diffusivity or high fluxes). This ratio is combined with estimates of the fluxes to determine the apparent sediment diffusivity, Kapp, for eacri basin. Flux is estimated by assuming the abyssal hill topography acts as a simple sediment trap, so that the lateral flux equals the width of a basin's pelagic catchment area times the area's mean sedimentation rate S. Using this method, median Kapp is 0.04–0.11 m2 yr−1 (assuming S = 10–30 m m.y.−1). Variations in Kapp and the assumptions behind the diffusion model are discussed, and alternative causes of sediment surface tilts are considered, in particular differential compaction, which is modelled using a simple porosity‐depth function.
Oceanic versus emplacement age serpentinization in the Josephine ophiolite: Implications for the nature of the Moho at intermediate and slow spreading ridges
Authors: Coulton A. J. , Harper G. D. , O'Hanley D. S.
Published: February 2004

We present field, petrographie, and geochemical evidence for oceanic serpentinization in the 162 Ma Josephine ophiolite of NW California and SW Oregon. Undeformed and unrodingitized dikes that intruded into serpentinized shear zones provide time markers for serpentinization and deformation. The dikes intruding serpentinites are of two types: (1) Fe‐Ti enriched dikes with normal mid‐ocean ridge basalt (NMORB) magmatic affinity which are geochemically linked to the uppermost lavas and a late Fe‐Ti dike within the crustal sequence of the ophiolite; and (2) hornblende‐bearing, calc‐alkaline dikes intruded during ophiolite emplacement between 150 and 146 Ma. Based on crosscutting relationships between dikes and serpentinites, serpentinization of upper mantle peridotites took place at or near the ridge axis, during periods of amagmatic structural extension in the absence of a magma chamber. Lizardite‐bearing seipentinites in the northern peridotite are constrained to be oceanic and indicate temperatures of <350°C within the oceanic upper mantle. Oceanic, lizardite‐bearing, serpentinized shear zones, from the ultramafic cumulate section to the basal sole, indicate that the entire Josephine peridotite may have been transected by serpentinized shear zones prior to ophiolite emplacement The ultramafic cumulate sequence was completely serpentinized prior to ophiolite emplacement, and we suggest that the paleo‐Moho in the Josephine ophiolite is a serpentinization boundary. The basal sole is interpreted to be a reactivated oceanic fault along which antigorite mylonites formed from a preexisting serpentinite during ophiolite emplacement. Oceanic serpentinization in the Josephine ophiolite took place beneath a 2–3 km crustal sequence and suggests that serpentinization may be an important feature at intermediate spreading rate mid‐ocean ridges, as well as at slower spreading ridges.
Mechanics of dip‐slip faulting in an elastic half‐space
Authors: Rudnicki J. W. , Wu M.
Published: February 2004

The mechanics of dip‐slip faulting is examined using a model of a crack in a plane strain elastic half‐space loaded by remote stresses: vertical stresses are equal to the overburden and horizontal stresses are the sum of a constant (c) times the overburden and a depth‐independent term σtect, which may be compressive or tensile. Opening is allowed in response to local tension, and slip on closed regions is governed by Mohr‐Coulomb conditions. Because regions of opening and slip, in general, must be determined as part of the solution, relative displacements and stress intensity factors are calculated by iterative solution of integral equations. If the fault is not too shallow (h/L > 0.3, where h is the depth of the upper end of the slip zone and L is the length along dip) and the Coulomb stress (the shear stress minus the friction coefficient ƒ times the effective normal stress) is of one sign over most of the fault, simple approximations suffice. Use of these approximations and the criterion that the energy release rate equals a critical value demonstrates that updip propagation is unstable if the slip zone length exceeds a minimum value. For typical values of ƒ dip angle, and c, the slip zone will be locked below a certain depth. In general, the depth at which the Coulomb stress changes sign underestimates the locking depth, but the magnitude of the difference decreases as the depth of the sign change approaches the lower end of the slip zone. For shallow slip zones, h/L < 0.3, slip induces changes in both shear and normal stresses on the slip surface. For reverse (normal) slip, this coupling reduces (increases) the compressive normal stress and amplifies (diminishes) slip near the upper end of the slip zone surface. For extensional loading (σtect < 0), opening can occur for shallow embedded zones and, more prominently, for surface breaking zones. Although the shear stress drop is nonuniform in this model, calculations reveal that unless fault opening occurs, the surface displacement is well‐approximated by that due to a uniform shear stress drop equal to the average.
A Pan African origin and uplift for the gneisses and peridotites of Zabargad Island, Red Sea: A Nd, Sr, Pb, and Os isotope study
Authors: Brueckner H. K. , Elhaddad Mervet A. , Hamelin Bruno , Hemming Sidney , Kröner Alfred , Reisberg Laurie , Seyler Monique
Published: February 2004

A Sr, Nd, Pb, and Os isotopic study of peridotites and granulite‐facies gneisses from Zabargad Island in the Red Sea suggests that the tectonothermal, petrogenetic, and geochemical evolution of these rocks occurred largely during the Pan African Orogeny rather than the recent opening of the Red Sea. Sm‐Nd model ages and whole rock errorchrons indicate that spinel Iherzolites and gneisses differentiated from a common depleted mantle source about 700 Ma. The Iherzolites were mylonitized, metasomatized, and amphibolitized during a structural event that juxtaposed the peridotites with the gneiss complex and uplifted the gneiss/peridotite complex to relatively shallow crustal levels. Most radiometric dating schemes suggest a Pan African age for this event. The gneisses generally have lower 143Nd/144Nd, 87Sr/86Sr, 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios than the peridotites. They extend linear trends defined by the spinel and amphibole peridotites on Sr‐Nd, Sm‐Nd, and Pb‐Pb diagrams, suggesting the gneisses were either the source or buffering medium for the Pan African metasomatism. Only one post‐Pan African event had a significant effect on the geochemistry of the gneiss/peridotite complex: shallow level metasomatism by ultrahot (750–900°C) hypersaline solutions with high 87Sr/86Sr (≈0.710) ratio led to the development of gem‐quality olivine crystals as well as low‐pressure mineral assemblages in the peridotites, gneisses and younger rocks. Plagioclase‐rich assemblages with apparent igneous textures (“troctolites”) that are most common in the southern peridotite body may have formed by interaction of these fluids with peridotite (i.e., are “pseudo‐troctolites”). Metasomatism changed the 87Sr/86Sr, Sm/Nd, and Re/Os ratios of the plagioclase peridotites making them unsuitable representatives of the Pan African mantle.
Relationship between axial morphology, crustal thickness, and mantle temperature along the Juan de Fuca and Gorda Ridges
Authors: Hooft Emilie E. E. , Detrick Robert S.
Published: February 2004

We analyze gravity data over the Juan de Fuca and northern Gorda Ridges to understand the lithospheric structure of two ridges with contrasting axial morphologies spreading at the same intermediate rate (28 mm/yr half rate). The Cleft Segment, at the south end of the Juan de Fuca Ridge, has an axial high morphology while the northern segment of the Gorda Ridge has a rift valley. Residual mantle Bouguer anomalies (RMBA) on the northern Gorda Ridge are elevated relative to the Cleft Segment by 10–20 mGal, indicating thinner crust and/or a colder mantle. The minimum value (−50 mGal) of the RMBA along the Juan de Fuca Ridge is over Axial Seamount and gradually increases south toward the Blanco Transform. The observed RMBA are interpreted to result from along axis variations in crustal thickness and mantle density, both of which are controlled by temperatures in the upper mantle where decompression melting occurs. We estimate that mantle temperatures are elevated by 30°–40°C beneath Axial Seamount, resulting in an excess crustal thickness of ∼1.7 km. The Cleft Segment is associated with crust that is estimated to be only 300–700 m thicker, and mantle temperatures are only 10°–15°C higher than beneath the northern Gorda Ridge. However, even these small differences in crustal thickness and mantle temperature appear to be sufficient to produce a major change in lithospheric strength and axial morphology. These results are consistent with the predicted sensitivity of recent thermo‐mechanical models for rift valley formation to small changes in crustal thickness and mantle temperature at intermediate spreading rates. We attribute the systematic differences in axial morphology, crustal thickness, mantle temperature, and lithospheric strength along the Juan de Fuca/Gorda ridge system to the presence of the Cobb thermal anomaly at Axial Seamount. The hotter mantle beneath the Juan de Fuca Ridge results in greater amounts of decompression melting and the formation of a thicker crust and a thinner, weaker lithosphere than along the Gorda Ridge.
Using differential waveform data to retrieve local S velocity structure or path‐averaged S velocity gradients
Authors: Passier M. L. , Snieder R. K.
Published: February 2004

An algorithm is presented which uses the surface waveforms originating from one event recorded in two stations which are located close to each other. The waveforms are inverted for local laterally homogeneous S velocity structure or for path‐averaged horizontal S velocity gradients. In the first case the stations are situated on the same great circle to the source. In the second case the two stations lie at approximately the same epicentral distance, but with a slightly different azimuth. In both cases, the frequency‐dependent phase differences between the two recordings are used as data for an inversion for S velocity values. No a priori knowledge about the source parameters and the medium between the source and the nearest receiver is needed. These factors are estimated in the first step of the inversion (linear). In the second step of the inversion (nonlinear) the S velocity information is obtained. Synthetic tests show that the algorithm is a powerful tool to image structure with only two fundamental mode surface wave recordings as data. In addition, they show that for the case where a horizontal gradient is determined, higher modes can be included in the analysis to display gradients at larger depths, without the need to identify the individual modes separately. The power of the method is illustrated with some real data examples that are all related to a pronounced tectonic boundary in western Eurasia: the Tornquist‐Teisseyre zone. The results of the experiments with real data coincide well with previously obtained models for this area.
Quantitative analysis of abyssal hills in the Atlantic Ocean: A correlation between inferred crustal thickness and extensional faulting
Authors: Goff John A. , Tucholke Brian E. , Lin Jian , Jaroslow Gary E. , Kleinrock Martin C.
Published: February 2004

A recent cruise to the Office of Naval Research Atlantic Natural Laboratory obtained ∼100% Hydrosweep bathymetrie coverage, >200% Hawaii MRl (HMRl) side scan coverage, gravity and magnetics over an area spanning three ridge segments along axis (∼25°25′N to ∼27°10′N), and crustal ages from 0 to 26–30 Ma (∼400 km) on the west flank of the Mid‐Atlantic Ridge. This data set represents a first opportunity for an extensive regional analysis of abyssal hill morphology created at a slow spreading ridge. The primary purpose of this work is to investigate the relationship between abyssal hill morphology and the properties of the ridge crest at which they were formed. We apply the method of Goff and Jordan [1988] for the estimation of two‐dimensional statistical properties of abyssal hill morphology from the gridded Hydrosweep bathymetry. Important abyssal hill parameters derived from this analysis include root‐mean‐square (rms) height, characteristic width, and plan view aspect ratio. The analysis is partitioned into two substudies: (1) analysis of near‐axis (< 7 Ma) abyssal hills for each of the three segments and (2) analysis of temporal variations (∼2–29 Ma) in abyssal hill morphology along the run of the south segment. The results of this analysis are compared and correlated with analysis of the gravity data and preliminary determination of faulting characteristics based on HMRl side scan data. Principal results of this study are: (1) Abyssal hill morphology within the study region is strongly influenced by the inside‐outside corner geometry of the mid‐ocean ridge segments; abyssal hills originating at inside corners have larger rms height and characteristic width and smaller plan view aspect ratio than those originating at outside corners. (2) The residual mantle Bouguer gravity anomaly is positively correlated with intersegment and along‐flow‐line variations in rms height and characteristic width, and it is negatively correlated with plan view aspect ratio. From this result, we infer that lower‐relief, narrower, and more elongated abyssal hills are produced when the crust being generated is thicker. (3) Intersegment variations in near‐axis rms height negatively correlate with average fault density as determined from analysis of HMRl side scan imagery.
Failure of columnar saline ice under biaxial compression: Failure envelopes and the brittle‐to‐ductile transition
Authors: Schulson E. M. , Nickolayev O. Y.
Published: February 2004

Experiments were performed on the compressive failure of columnar saline ice loaded biaxially at temperatures from −40°C to −50°C and at strain rates from 10−6 s−1 to 10−1 s−1. The major stress (σ11) was applied across the columns, and the minor stress was applied proportionally either across the columns (σ22) or along the columns (σ33). The results show that the macroscopic behavior changes from brittle to ductile upon reducing the strain rate to a critical level and that the transition strain rate first increases and then decreases with increasing across‐column confining stress. The brittle strength increases sharply under low degrees of across‐column confinement and then decreases upon increasing σ22 further. This behavior is manifested in a truncated, Coulombic‐type of failure envelope where failure on the rising branch is by splitting (zero confinement) and by shear faulting (moderate confinement) within the loading plane and on the descending branch by spalling out of the loading plane. The ductile strength increases monotonically with increasing confining stress and with both decreasing temperature and increasing strain rate. The ductile failure envelope is semielliptical in shape and can be described by Hills criterion for the yielding of plastically orthotropic materials. Correspondingly, the components of the inelastic “strain vector” in the X1‐X2 plane are reasonably normal to the failure envelope. Neither the brittle nor the ductile strength is affected by a confining stress along the columns (σ33). Brittle behavior is explained in terms of the frictional crack‐sliding wing crack mechanism, and ductile behavior is related to both basal (under low confinement) and nonbasal (under high confinement) dislocation processes. The brittle‐to‐ductile transition is attributed to the relaxation of crack tip tensile stresses and is modeled by incorporating the crack size and the resistance to creep, crack propagation, and frictional sliding.
Piezornagnetic field for Parkfield fault model
Authors: Stuart William D. , Banks Philip O. , Sasai Yoichi , Liu Shu‐Wang
Published: February 2004

A theoretical piezomagnetic field is calculated as a function of time and ground position for an earthquake instability model applied to moderate (M ∼ 6) earthquakes on the San Andreas fault near Parkfield, California. The instability model simulates fault slip and stress for all parts of a 32‐year earthquake cycle, including unstable (mainshock) slip. Mainshock slip occurs when a nearly locked patch of the fault becomes sufficiently loaded by surrounding aseismic slip that it reaches a condition of rapid failure. The piezomagnetic field is calculated directly from fault slip using analytic solutions based on piezomagnetic nuclei that are analogous to strain nuclei. During the interseismic interval, increasing stress concentration on the locked patch is accompanied by a gradual evolution of the magnetic anomaly field, leading to maximum cumulative changes of about ±2 nT in the vicinity of the pending epicenter. Mainshock slip cancels about half of the interseismically accumulated changes, and rapid post seismic relaxation cancels most of the rest. At certain sites near the locked patch the rate of change of the field progressively increases during the interseismic interval, while at others it progressively decreases. This effect is most pronounced during the last 3–4 years before the mainshock and, at suitably chosen pairs of sites, results in reversal of the temporal trend of the differential field. Such reversals, if separable from geomagnetic variation due to other sources, would constitute precursory indicators of approaching failure.
Hillslope development in areas of active tectonics
Authors: Arrowsmith J. Ramón , Pollard David D. , Rhodes Dallas D.
Published: February 2004

Tectonic and geomorphic displacements of the Earth's surface control topographic profile development; therefore, their analysis should be combined. In the model presented here, transient finite difference solutions to the continuity equation for material transport determine geomorphic displacements. The material transport rate is a function of distance from the divide to the power m, local slope to the power n, and a rate constant. Values of m and n may be adjusted to simulate processes varying from rainsplash and soil creep (i.e., diffusive; m = 0, n = 1) to slope wash and river flow (m > 0, n > 0). The actual geomorphic displacements may be transport or weathering‐limited, depending on soil profile development. Superimposed edge dislocations in an elastic half‐plane are used to model tectonic displacements. Slip along a normal or reverse fault of any dip, depth and down‐dip length may be incremental (earthquake) or continuous (aseismic creep). Considering climate and material properties constant, the ratio of the transport capacity rate constant to the fault slip rate roughly determines form. This model extends existing morphologic diffusion erosion analyses to include other geomorphic conditions and processes (transport‐ or weathering‐limited conditions, material flux boundary conditions, and the development of gullies and knickpoints) and more heterogeneous spatial and temporal distributions of tectonic displacement (such as those due to slip along buried thrust faults). We advocate calibration of these parameters and processes to provide a quantitative approach to modeling landform development, determining deformation rates, and inferring earthquake hazards.
Evidence for climatic warming in northcentral Oklahoma from analysis of borehole temperatures
Authors: Deming David , Borel Robb A.
Published: February 2004

Temperatures were measured in six boreholes (∼380 m deep) in northcentral Oklahoma (36.36 °N, 96.70 °W) in May 1993. Temperatures in the upper 150 m of the boreholes appeared to be anomalously warm when the average thermal gradient was extrapolated from below 150 m depth. The average thermal gradient below 150 m depth was 37.5°C/km; the average thermal gradient from 20 to 150 m depth was 27.4°C/km. Several hundred laboratory measurements of thermal conductivity were made on cores collected at the study site; 65 specific heat capacity measurements were also made. In situ porosity was estimated from density logs and measurements of rock matrix density. Average thermal diffusivity of the upper 110 m of the stratigraphic section was estimated to be 20.3±2.0 m2/yr (6.43×10−7 m2/s). Heat flow was estimated to be 52±6 mW/m2 from 20 to 110 m depth and 69±7 mW/m2 from 277 to 305 m depth. The observed energy imbalance and anomalously warm temperatures in the upper 150 m of the boreholes could not be explained solely by hypotheses related to topographic gradients, vegetation, heat refraction, groundwater flow, or land use changes. The only hypothesis which satisfactorily explained all of the observations was an apparent increase (1.25–1.50 ±0.5°C) in ground surface temperature (GST) related to a climatic warming starting in the middle to early 19th century or before (1835, +50, −150 years). When constraints from surface air temperatures (SATs) were used to interpret borehole temperatures, a better match to observations was obtained, suggesting that changes in SATs at the study site were tracked by changes in GSTs.