bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismic signature and geochemistry of an island arc: A multidisciplinary study of the Kohistan accreted terrane, northern Pakistan
Authors: Miller D. Jay , Christensen Nikolas L.
Published: February 2004

Systematic sampling and mapping in the Kohistan accreted arc terrane of northern Pakistan has provided a sample suite representing the lithologic diversity of the section from its base along the Main Mantle Thrust upward through several stacked intrusions and their metamorphosed equivalents into the Kohistan batholith. A new lithologic column for the terrane has been developed during the course of this study using geothermobarometry based on elemental exchange reactions calculated from electron microprobe analyses of mineral assemblages. The pressures calculated from the chemical analyses of various mineral assemblages are constrained by recently published activity‐composition models and an internally consistent thermodynamic data base. Compressional and shear wave velocities and densities have been measured on samples that represent the diverse lithologies in this section and have been correlated with the new lithologic column. This data set has been augmented with published compositions and values for the uppermost part of the arc, not sampled in this study. Compressional wave velocities range from 4.3 km s−1 in supracrustal volcanogenic sediments to 7.5 km s−1 in lower crustal mafic cumulates. Underlying ultramafic rocks with velocities of 8.0 to 8.4 km s−1 define a sharp seismic Moho. The strong inflection in the velocity profile to greater than 8.0 km s−1 is due to the transition from plagioclase‐bearing to plagioclase‐free cumulate rocks in a continuous ultramafic‐mafic intrusion. The base of the crust lies at least four kilometers below this transition. Correlation of these laboratory‐measured values and the new lithologic column has allowed development of a velocity profile comparable to profiles developed through more conventional field seismic methods. Geochemical indices of fractionation exhibit a reasonable correlation with Compressional and shear wave velocities. An estimate of mean crustal Vp is remarkably similar to models for other cordilleran terranes at 6.7 plus or minus 0.05 km s−1. An estimate of the bulk chemical composition of the Kohistan terrane does not compare favorably with most published assessments of bulk continental crustal chemical composition, but is significantly more mafic than the latter. The striking resemblance of our reconstructed velocity profile to models generated from field seismic studies not only addresses the veracity of these models, but suggests that the Kohistan arc is a superbly well‐preserved analog for other arc terranes.
Chemical evolution of a large mafic intrusion in the lower crust, Ivrea‐Verbano Zone, northern Italy
Authors: Sinigoi Silvano , Quick James E. , Clemens‐Knott Diane , Mayer Adriano , Demarchi Gabbriella , Mazzucchelli Maurizio , Negrini Luisa , Rivalenti Giorgio
Published: February 2004

The Ivrea‐Verbano and adjacent Strona‐Ceneri zones have been described collectively as a section through the continental crust. While resident in the lower crust, amphibolite to granulite‐facies paragneiss of the Ivrea‐Verbano Zone was intruded by huge volumes of mafic to intermediate plutonic rocks grouped as the Mafic Complex. Growth of the Mafic Complex involved hypersolidus deformation in an extensional environment. Isotopic and trace element variations close to the axis of this structure indicate crystallization from mantle‐derived melts that were extensively contaminated by crustal material. Previous investigations determined that the contaminant was fingerprinted by 87Sr/86Sr > 0.71, δ18O = 10–12.5‰, and a positive Eu anomaly. In the present study, the contaminant is also shown to have been enriched in Ba with respect to Rb and K. Charnockites associated with paragneiss septa in the lower part of the Mafic Complex have the appropriate chemistry to be samples of the contaminating material. These chemical features can be explained by melting of granulite‐facies paragneiss, which had previously been depleted in K and Rb by an earlier melting event. The Ba enrichment in the core of the Mafic Complex can be modeled by a replenishment‐tapping‐fractional‐crystallization (RTF) process operating within a small magma chamber is repeatedly replenished by mantle melts and contaminated by Ba‐rich charnockite. Very high Ba/K in the lower part of the complex are tentatively attributed to chemical exchange between the cumulate framework and infiltrating anatectic melts from underlying paragneiss septa. In contrast to the Mafic Complex, the chemistry of coeval granites in the adjacent Strona‐Ceneri zone reflect a component derived from crustal rocks that had not been significantly depleted by a previous melting event. Significantly, the incompatible trace element abundances in the Mafic Complex and Strona‐Ceneri granites are similar to model compositions for the lower and upper crust, respectively.
Fluid expulsion sites on the Cascadia accretionary prism: Mapping diagenetic deposits with processed GLORIA imagery
Authors: Carson Bobb , Seke Erol , Paskevich Valerie , Holmes Mark L.
Published: February 2004

Point‐discharge fluid expulsion on accretionary prisms is commonly indicated by diagenetic deposition of calcium carbonate cements and gas hydrates in near‐surface (<10 m below seafloor; mbsf) hemipelagic sediment. The contrasting clastic and diagenetic lithologies should be apparent in side scan images. However, sonar also responds to variations in bottom slope, so unprocessed images mix topographic and lithologic information. We have processed GLORIA imagery from the Oregon continental margin to remove topographic effects. A synthetic side scan image was created initially from Sea Beam bathymetric data and then was subtracted iteratively from the original GLORIA data until topographic features disappeared. The residual image contains high‐amplitude backscattering that we attribute to diagenetic deposits associated with fluid discharge, based on submersible mapping, Ocean Drilling Program drilling, and collected samples. Diagenetic deposits are concentrated (1) near an out‐of‐sequence thrust fault on the second ridge landward of the base of the continental slope, (2) along zones characterized by deep‐seated strikeslip faults that cut transversely across the margin, and (3) in undeformed Cascadia Basin deposits which overlie incipient thrust faults seaward of the toe of the prism. There is no evidence of diagenetic deposition associated with the frontal thrust that rises from the dècollement. If the dècollement is an important aquifer, apparently the fluids are passed either to the strike‐slip faults which intersect the dècollement or to the incipient faults in Cascadia Basin for expulsion. Diagenetic deposits seaward of the prism toe probably consist dominantly of gas hydrates.
Valley incision and the uplift of mountain peaks
Authors: Montgomery David R.
Published: February 2004

Simple end‐member models describe the effect of isostatically compensated valley incision on the elevation of mountain peaks. These models allow calculation of the maximum portion of mountain peak elevations attributable to the development of observed relief and predict general downstream patterns of ridgetop uplift in response to erosion by fluvial processes. Analyses of topographic profiles across major drainages of the central Sierra Nevada and the Tibetan Plateau indicate that isostatically compensated valley incision could account for at most 5–10% of the present elevation of mountain peaks. In contrast, analyses of topographic profiles across the Himalaya show that as much as 20–30% of the present elevation of Himalayan peaks could be explained by isostatically compensated valley incision. The degree to which valley incision is compensated locally, however, depends on the flexural rigidity of the lithosphere, the size of the range, and its tectonic boundary conditions. These results show that while a tectonic control is required for late Cenozoic surface uplift of the Sierra Nevada, a significant portion of the elevation of Himalayan peaks may reflect late Cenozoic valley incision on the margin of the Tibetan Plateau.
A statistic for testing the elevation characteristics of landscape simulation models
Authors: Willgoose Garry
Published: February 2004

There is an urgent need to test recently constructed conceptual models of landscape evolution and geomorphology against field data. Many statistics that have commonly been used, such as Strahler statistics, are poorly suited to this testing being unable to potentially falsify the models when applied in controlled conditions. The relations between these statistics and catchment mass movement processes are, in many cases, indirect. Moreover, they are sensitive to unknowable inputs (e.g., initial conditions). One of the main differences among the competing landscape evolution models is their representation of mass movement and erosion processes, so that statistics that are directly linked to these processes, or their geomorphic signature, are needed. This paper examines the use of a log‐log linear relation between catchment area, slope, and elevation. This relation has been previously derived quantitatively from the catchment erosion and hydrologic processes for dynamic and declining geomorphic equilibria. Using a catchment evolution model, this paper shows that the relation is robust against deviations from the assumptions of spatial and temporal homogeneity of erosion, climate, and tectonic uplift made in its derivation. The sensitivity of the geomorphology to time‐varying tectonic uplift and climate and spatially variable soil erodability is examined. It is thus asserted that the relation between area, slope, and elevation is suitable for testing of models against field data, where spatial and temporal homogeneity of erosion, climate, and tectonic uplift are only approximately true. In the case of variable soil erodability, planar drainage patterns exhibit self‐organization with the high elevation regions of the catchment having low soil erodability. Consequently, a downstream increasing erodability of the soil material, consistent with a downstream fining type of behavior, is observed. It is further argued that the equilibria relation is fundamental and uniquely defines the elevation form of catchments so that all other elevation statistics are derived from it.
Structure of the subducting Nazca Plate beneath Peru
Authors: Norabuena Edmundo O. , Snoke J. Arthur , James David E.
Published: February 2004

Arrival times from intermediate‐depth (110–150 km) earthquakes within the region of flat subduction beneath the subandean zone and foreland basins of east‐central Peru provide constraints on the geometry and velocity structure of the subducting Nazca plate. Hypocentral locations and origin times for these events were determined using observations from a 15 station digitally recording locator array deployed in the epicentral region of eastern Peru. Observed P wave arrival times for coastal stations in Peru, some 3–6° from the epicenters, are up to 4 s early relative to predicted arrival times based on the best fit velocity‐depth model used for hypocenter locations. These large negative time residuals appear to be the result of propagation paths which have long segments in the colder, higher‐velocity subducting plate, P wave travel times were modeled for the effects of the slab using three‐dimensional (3‐D) ray tracing. Computed ray paths show that travel times to coastal stations for the eastern Peru events can be satisfactorily modeled with average velocities relative to the surrounding mantle 6% lower within the uppermost slab (assumed on the basis of other studies to be unconverted basaltic oceanic crust 6 km thick) and 8% higher within the cold uppermost mantle of the slab. Ray tracing for this plate model shows that P wave “shadow zones” can occur if the source‐slab‐receiver geometry results in seismic rays passing through regions in which the dip angle of the slab changes significantly. Such geometries exist for seismic waves propagating to some coastal stations from intermediate‐depth earthquakes located east of the Andes. Observed first‐arrival times for such cases do in fact have smaller negative residuals than those for geometries which allow for “direct” paths at similar distances. Modeling such arrivals as internally reflected waves propagating through the high‐velocity part of the plate produces a significant improvement in the travel time residuals. For the slab velocities given above, we obtain a model thickness of approximately 36 km for the cold slab interior and a slight northwest component of dip in the region of subhorizontal subduction.
Evolution of the Precambrian lithosphere: Seismological and geochemical constraints
Authors: Durrheim Raymond J. , Mooney Walter D.
Published: February 2004

Several recent models of crustal evolution are based on the belief that the thickness of the continental crust is proportional to its age, with ancient crust being the thickest. A worldwide review of seismic structure contradicts this belief and falsifies these models, at least for the Archean. Proterozoic crust has a thickness of 40–55 km and a substantial high‐velocity (>7 km/s) layer at its base, while Archean crust is only 27–40 km thick (except at the site of younger rifts and collisional boundaries) and lacks the basal high‐velocity layer. Seismology also provides evidence that the lithosphere is thickest beneath Archean cratons, while diamond ages show that this lithospheric keel must have already existed in the Archean. Geochemical data also indicate significant differences between Archean and Proterozoic lithosphere. Major and trace element studies of sediments show a change in upper crustal composition between the Archean and Proterozoic. Archean rocks are depleted in Si and K and enriched in Na, Ca, and Mg. There is also a marked change in the Eu/Eu*ratio. Mantle xenoliths and continental flood basalts show that the mantle lithosphere beneath Archean crust is ultradepleted in FeO compared to that beneath post‐Archean crust. The secular change in the crust‐forming process is attributed to a decline in mantle temperature, leading to a change in the composition of the lithospheric mantle. The higher temperature of the Archean mantle led to the eruption of komatiitic lavas, producing a refractory lithospheric mantle which is ultradepleted in FeO and volatiles. The resultant lithospheric keel is intrinsically less dense than the surrounding mantle and thus not susceptible to delamination. It was sufficiently thick and cool for diamonds to form during the Archean. In contrast, Proterozoic crust developed above fertile mantle. The eruption of continental flood basalts and underplating of basaltic sills is attributed to subsequent heating and partial melting of the lithospheric mantle. Consequently, Proterozoic crust is thickened and has a high‐velocity basal layer.
Modeling of periodic great earthquakes on the San Andreas Fault: Effects of nonlinear crustal rheology
Authors: Reches Ze'ev , Schubert Gerald , Anderson Charles
Published: February 2004

We analyze the cycle of great earthquakes along the San Andreas fault with a finite element numerical model of deformation in a crust with a nonlinear viscoelastic rheology. The viscous component of deformation has an effective viscosity that depends exponentially on the inverse absolute temperature and nonlinearly on the shear stress; the elastic deformation is linear. Crustal thickness and temperature are constrained by seismic and heat flow data for California. The models are for anti plane strain in a 25‐km‐thick crustal layer having a very long, vertical strike‐slip fault; the crustal block extends 250 km to either side of the fault. During the earthquake cycle that lasts 160 years, a constant plate velocity νp/2 = 17.5 mm yr−1 is applied to the base of the crust and to the vertical end of the crustal block 250 km away from the fault. The upper half of the fault is locked during the interseismic period, while its lower half slips at the constant plate velocity. The locked part of the fault is moved abruptly 2.8 m every 160 years to simulate great earthquakes. The results are sensitive to crustal rheology. Models with quartzite‐like rheology display profound transient stages in the velocity, displacement, and stress fields. The predicted transient zone extends about 3–4 times the crustal thickness on each side of the fault, significantly wider than the zone of deformation in elastic models. Models with diabase‐like rheology behave similarly to elastic models and exhibit no transient stages. The model predictions are compared with geodetic observations of fault‐parallel velocities in northern and central California and local rates of shear strain along the San Andreas fault. The observations are best fit by models which are 10–100 times less viscous than a quartzite‐like rheology. Since the lower crust in California is composed of intermediate to mafic rocks, the present result suggests that the in situ viscosity of the crustal rock is orders of magnitude less the rock viscosity determined in the laboratory.
A geological model for the structure of ridge segments in slow spreading ocean crust
Authors: Tucholke Brian E. , Lin Jian
Published: February 2004

First‐order (transform) and second‐order ridge‐axis discontinuities create a fundamental segmentation of the lithosphere along mid‐ocean ridges, and in slow spreading crust they commonly are associated with exposure of subvolcanic crust and upper mantle. We analyzed available morphological, gravity, and rock sample data from the Atlantic Ocean to determine whether consistent structural patterns occur at these discontinuities and to constrain the processes that control the patterns. The results show that along their older, inside‐corner sides, both first‐and second‐order discontinuities are characterized by thinned crust and/or mantle exposures as well as by irregular fault patterns and a paucity of volcanic features. Crust on young, outside‐corner sides of discontinuities has more normal thickness, regular fault patterns, and common volcanic forms. These patterns are consistent with tectonic thinning of crust at inside corners by low‐angle detachment faults as previously suggested for transform discontinuities by Dick et al. [1981] and Karson [1990]. Volcanic upper crust accretes in the hanging wall of the detachment, is stripped from the inside‐corner footwall, and is carried to the outside comer. Gravity and morphological data suggest that detachment faulting is a relatively continuous, long‐lived process in crust spreading at <25–30 mm/yr, that it rnay be intermittent at intermediate rates of 25–40 mm/yr, and that it is unlikely to occur at faster rates. Detachment surfaces are dissected by later, high‐angle faults formed during crustal uplift into the rift mountains; these faults can cut through the entire crust and may be the kinds of faults imaged by seismic reflection profiling over Cretaceous North Atlantic crust. Off‐axis variations in gravity anomalies indicate that slow spreading crust experiences cyclic magmatic/amagmatic extension and that a typical cycle is about 2 m.y. long. During magmatic phases the footwall of the detachment fault probably exposes lower crustal gabbros, although these rocks locally may have an unconformable volcanic carapace. During amagmatic extension the detachment may dip steeply through the crust, providing a mechanism whereby upper mantle ultramafic rocks can be exhumed very rapidly, perhaps in as little as 0.5 m.y. Together, detachment faulting and cyclic magmatic/amagmatic extension create strongly heterogeneous lithosphere both along and across isochrons in slow spreading ocean crust.
Early Cretaceous Sr‐rich silicic magmatism by slab melting in the Kitakami Mountains, northeast Japan
Authors: Tsuchiya Nobutaka , Kanisawa Satoshi
Published: February 2004

Early Cretaceous plutonic rocks in the Kitakami Mountains, northeast Japan, related to the westward subduction along the eastern margin of the East Asian continent, intruded the Jurassic accretionary complex and a microcontinental block. These plutonic rocks are classified into types A and B. The former is further subdivided into high‐Sr, low‐Sr, and intermediate series. High‐Sr series rocks occur as the central parts of three zoned plutons in the North Kitakami belt; named the Hashigami, Tanohata, and Miyako plutons. The high‐Sr series rocks show distinct chemical characteristics compared with those of the low‐Sr series. In particular, the high‐Sr series is characterized by SiO2 greater than 61 wt %, higher Na2O and lower K2O concentrations, and slightly higher Al2O3 and lower total FeO and MgO concentrations. In trace element chemistry, the high‐Sr series rocks are richer in Sr and Ga, and poorer in Rb, Pb, and Y. These characteristics resemble those of the high‐Al2O3 trondhjemite, tonalite, and dacite (high‐Al TTD). Mass balance calculation indicates that the high‐Sr series magma can be derived by partial melting of hypothetical oceanic crust (mid‐oceanic ridge basalt (MORB), plus 5% altered MORB, plus 5% sediments) leaving garnet, clinopyroxene, quartz, rutile, and apatite as restite. On the basis of the phase relations for tholeiitic basalt and the estimated geothermal gradient of the subducted slab, slab melting is considered to have occurred at a depth of 2.0 to 2.2 GPa (about 70 to 80 km), and the age of subducted oceanic crust may be younger than 10 to 25 Ma. The coexistence of the magma derived from the subducted slab with that derived from the mantle wedge is a particularly important characteristic of the Early Cretaceous magmatism in the Kitakami Mountains. This suggests that the mantle beneath the Cretaceous forearc was warmer than for normal forearcs. Subduction of the Farallon‐Izanagi ridge may have caused the warm mantle.
Seismic decoupling with chemical and nuclear explosions in salt
Authors: Glenn L. A. , Goldstein P.
Published: February 2004

An extensive series of simulations was performed of underground explosions in salt, using both chemical and nuclear explosives. In both cases, the radius of the initial emplacement cavity was varied from the fully tamped configuration to as large as 80 m/kt⅓; when not fully tamped, the cavity was assumed initially to contain air at ambient temperature and pressure. In the nuclear source case, the simulations are shown to be in good agreement with the Salmon/Sterling events conducted by the United States and with recently released Russian data on a similar pair of explosions in an Azgir salt dome. Simulation of the U.S. Cowboy series of chemical explosions in a Louisiana salt mine are also shown to be in very good agreement with the experimental data; however, the constitutive model for the salt that best explains these data is different from that derived for Salmon; both salt models are amply supported by laboratory and field data. The main result of these simulations is that cavity decoupling with chemical explosives is much less efficient than with nuclear explosives. Although maximum decoupling factors, ƒmax, near 200 may be attainable with either of the two sources, the cavity size required to achieve this value appears to be >40 m/kt⅓. For cavity radii half as large, ƒmax is roughly 4 times lower with nuclear explosives, and lower by another factor of 4 with chemical sources. Moreover, if the initial cavity radius is a more modest 10 m/kt⅓, ƒmax < 3 even with a nuclear source.
Qs estimation for unconsolidated sediments using first‐arrival SH wave critical refractions
Authors: Wang Z. , Street R. , Woolery E. , Harris J.
Published: February 2004

On the basis of the assumptions of multilayered and homogenous media with constant velocities and Q values, and plane seismic waves, the seismic pulse‐broadening equation, τ = τ0 + Ct/Q (C = 0.5), is used to estimate the shear wave quality factor (Qs) of unconsolidated sediments from SH wave critical refractions in the upper Mississippi embayment. The principal advantage of using the SH wave pulse‐broadening technique instead of other methods for deriving Qs is that only one‐fourth to one‐half wavelength of signal is needed. Results from the study show (1) Qs varies from 10 to 60 in the unconsolidated sediments studied, (2) Qs is independent of frequency in the range of 10 to 70 Hz, and equivalent to the constant(Q0) of power law attenuation (Q = Q0ƒγ, 0 < γ < 1.0), and (3) Qs, to the first approximation, is related to the shear wave velocities (Vs) of the unconsolidated sediments by the relationship Qs = 0.08 Vs + 6.99 ± 12.10.
A numerical assessment of slab strength during high‐ and low‐angle subduction and implications for Laramide orogenesis
Authors: Spencer Jon E.
Published: February 2004

A finite element computer program for one‐dimensional heat flow was modified to apply to two‐dimensional subduction where conductive heat transport parallel to the upper and lower plates was neglected. One‐dimensional finite element columns were cut, displaced, and reassembled to model heat advection. Calculated temperature, depth‐dependent pressure, stress levels associated with dislocation creep in olivine at constant strain rate, and a 1‐GPa deviatoric stress maximum were used to calculate the form of the brittle‐plastic failure envelope for profiles across subducted slabs. The failure envelopes were then used to calculate maximum sustainable compression parallel to the slab. Model results suggest the following: (1) In high‐angle subduction without frictional heating and horizontal heat flow, temperatures along the interplate contact zone remain below about 120°C, which is far below temperatures necessary for olivine plasticity. Without substantial frictional heating or lateral heat flow from the adjacent magmatic arc, the subducted slab will gain rather than lose strength before its upper surface contacts asthenosphere. In addition, viscous coupling due to olivine plasticity and associated ablation of the upper plate will be unimportant. Slab strength reduction due to heating occurs rapidly after the top of the slab comes into contact with asthenosphere. (2) In low‐angle subduction the top of the slab remains in contact with overlying lithosphere and does not contact asthenosphere. Slab strength reduction due to conductive heating from above occurs slowly because the upper plate is cooled by the lower plate and is the source of progressively less heat. Temperatures along the interplate contact zone may be sufficient for olivine plasticity at large distances from the trench, and viscous coupling between plates and associated ablation of the upper plate may be important in this tectonic setting. (3) Low‐angle subduction for distances of over 1500 km during the Laramide orogeny in southwestern North America appears viable. At 50 Ma, slab strength beneath the Black Hills of South Dakota is predicted to have been comparable to, or greater than, strength at the trench.
Postseismic deformation following the 1989 (M = 7.1) Loma Prieta, California, earthquake
Authors: Savage J. C. , Lisowski M. , Svarc J. L.
Published: February 2004

Postseismic deformation along a 90‐km profile bisecting the projected surface trace of the coseismic rupture of the 1989 Loma Prieta earthquake has been monitored by frequent GPS surveys for 3.3 years following the earthquake. In addition to the expected deformation associated with secular strain accumulation on the San Andreas and Calaveras faults, deformation associated with postseismic readjustment has been detected. Most of that deformation can be attributed to 1.5 m right‐lateral and 0.9 m reverse postseismic slip on a 5‐km‐wide downdip extension of the Loma Prieta rupture. In addition, there seems to be a 0.1 m postseismic collapse of the Loma Prieta rupture zone in the direction perpendicular to the plane of the rupture. The fault‐normal (N48°E) surface displacements plotted as a function of time exhibit a curvature suggesting a relaxation time of about 1.4 years. Similar plots of the fault‐parallel (N42°W) displacement components do not exhibit significant curvature. Presumably, the deformation shown in those plots is dominated by secular strain accumulation along the San Andreas and Calaveras faults rather than postseismic relaxation.
Role of plates and temperature‐dependent viscosity in phase change dynamics
Authors: Zhong Shijie , Gurnis Michael
Published: February 2004

The effects of plates and slabs on phase change dynamics have been investigated with convection models. Two complementary methods to simulate plates are used: material property and imposed surface velocity methods with temperature‐dependent viscosity. For a wide range of model parameters, plates and slabs exert a significant control on phase change dynamics. As plate length (and hence plate age and convection cell aspect ratio) increases, both the propensity for slab penetration and the mass flux across an endothermic phase change increase. When cold downwellings are stiffened with a temperature‐dependent rheology, slab penetration is enhanced, but total mass flux changes little. Plates organize large‐scale flow and thermal structure and thereby affect phase change dynamics. As plates become larger, the resulting largerscale structures are influenced less by endothermic phase changes, thus reducing the degree of layering. A model showing completely layered convection for a plate of unit length becomes unlayered when the plate is 3 or 5 times longer. For a given Clapeyron slope, the proportion of time for slab penetration increases from zero for cases with small plates to more than 0.5 for cases with large plates. The degree of layering, plate velocity, and mass flux are controlled by large‐scale structures, while slab penetration may be more related to small‐scale features. Therefore, whether or not subducted slabs penetrate the phase change may not necessarily indicate that convection is entirely layered or entirely unlayered. The episodicity of convection induced by an endothermic phase change strongly depends on plate length, rheology, and Clapeyron slope. A large plate and a stiff slab both weaken the episodicity of convection. Only for a certain range of Clapeyron slopes can the phase change induce a strong episodic thermal convection.
Deep fault plane geometry inferred from multiplet relative relocation beneath the south flank of Kilauea
Authors: Got J.‐L. , Fréchet J. , Klein Fred W.
Published: February 2004

Dense microearthquake swarms occur in the upper south flank of Kilauea, providing multiplets composed of hundreds of events. The similarity of their waveforms and the quality of the data have been sufficient to provide accurate relative relocations of their hypocenters. A simple and efficient method has been developed which allowed the relative relocation of more than 250 events with an average precision of about 50 m horizontally and 75 m vertically. Relocation of these events greatly improves the definition of the seismic image of the fault that generates them. Indeed, relative relocations define a plane dipping about 6° northward, although corresponding absolute locations are widely dispersed in the swarm. A composite focal mechanism, built from events providing a correct spatial sampling of the multiplet, also gives a well‐constrained northward dip of about 5° to the near‐horizontal plane. This technique thus collapses the clouds of hypocenters of single‐event locations to a plane coinciding with the slip plane revealed by previous focal mechanism studies. We cannot conclude that all south flank earthquakes collapse to a single plane. There may locally be several planes, perhaps with different dips and depths throughout the south flank volume. The 6° northward‐dipping plane we found is too steep to represent the overall flexure of the oceanic crust under the load of the island of Hawaii. This plane is probably an important feature that characterizes the basal slip layer below the upper south flank of Kilauea volcano. Differences in seismicity rate and surface deformations between the upper and lower south flank could be related to the geometry of this deep fault plane. The present work illustrates how high precision relative relocations of similar events in dense swarms, combined with the analysis of geodetic measurements, can help to describe deep fault plane geometry. Systematic selection and extensive relative relocation of similar earthquakes could be attempted in other well‐instrumented, highly seismic areas to provide reliable basic information, especially useful for understanding of earthquake generation processes.
Constraints on Himalayan deformation inferred from vertical velocity fields in Nepal and Tibet
Authors: Jackson Michael , Bilham Roger
Published: February 2004

Spirit leveling data from the Nepal Himalaya between 1977 and 1990 indicate localized uplift at 2–3 mm/yr in the Lesser Himalaya with spatial wavelengths of 25–35 km and at 4–6 mm/yr in the Greater Himalaya with a wavelength of ≈40 km. Leveling data with significantly sparser spatial sampling in southern Tibet between 1959 and 1981 suggest that the Himalayan divide may be rising at a rate of 7.5±5.6 mm/yr relative to central Tibet. We use two‐dimensional dislocation modeling methods to examine a number of structural models that yield vertical velocity fields similar to those observed. Although these models are structurally nonunique, dislocation models that satisfy the data require aseismic slip rates of 2–7 mm/yr on shallow dipping faults beneath the Lesser Himalaya and rates of 9–18 mm/yr on deep thrust faults dipping at ≈25°N near the Greater Himalaya. Unfortunately, the leveling data cannot constrain long‐wavelength uplift (>100 km) across the Himalaya, and unequivocal estimates of aseismic slip in central Nepal are therefore not possible. In turn, the poor spatial density of leveling data in southern Tibet may inadequately sample the processes responsible for the uplift of the Greater Himalaya. Despite these shortcomings in the leveling data, the pattern of uplift is consistent with a crustal scale ramp near the Greater Himalaya linking shallow northward dipping thrust planes (3–6°) beneath the Lesser Himalaya and southern Tibet. Aseismic slip on the potential rupture surface of future great earthquakes beneath the Nepal Himalaya south of this ramp appears not to exceed 30% of the total convergence rate between India and southern Tibet resulting in an accumulating slip deficit of 13±8 mm/yr.
Kinematics of arc‐continent collision in the eastern Central Range of Taiwan inferred from geomorphic analysis
Authors: Willemin James H. , Knuepfer Peter L. K.
Published: February 2004

Active arc‐continent collision in Taiwan is an example of a typical collisional orogen, with a deformed continental margin (Foothills Belt, Hsueshan Range, and Central Range) separated from an accreted island arc (Coastal Range) by a collapsed forearc basin (Longitudinal Valley). The geomorphology of the eastern Central Range reflects spatial and temporal variations in uplift rate and pattern along the orogen during late Quaternary time. Digitized drainage basin perimeters, stream channels, and the mountain‐piedmont junction of a 200‐km‐long segment of the eastern Central Range provide data for calculating quantitative geomorphic parameters. Statistical analyses of these parameters define four domains of distinct landscape character. The northern domain extends northward along the coast from the northern end of the Longitudinal Valley (LV) and has landscape characteristics suggesting uplift near the mountain front. The central two domains are adjacent to the LV and show evidence of uplift concentrated in the interior of the range, away from the front. The boundary between the two central domains occurs near the middle of the LV. The southern domain extends southward along the coast from the southern end of the LV and has characteristics suggesting a transient response to increased uplift rate. Analysis of fluvial terrace profiles in the Central Range adjacent to the southern half of the LV indicates a change in uplift pattern through time, with migration of maximum uplift rate from the mountain front to the interior of the range. This change probably occurred just prior to late Pleistocene time.
Aeromagnetic evidence for a buried Early Cretaceous magmatic arc, northeast Japan
Authors: Finn Carol
Published: February 2004

Positive aeromagnetic anomalies, recent drilling, and models constructed from these data delineate the plutonic roots of the Early Cretaceous Kitakami magmatic arc in northeast Japan. Buried plutons, mostly offshore, produce belts of positive magnetic anomalies. These anomalies and magnetotelluric data suggest that the plutons form a batholith 70–120 km wide, nearly 800 km long, and 10–15 km thick. The batholith may mark the location of the main Kitakami arc. Most of the exposed Kitakami plutons are 2–20 km in diameter; some are 3 km thick. The small plutons line up along NW trending faults; some may have been satellite vents that tapped into the magma supply of the main arc. The batholithic roots of the main arc now compose almost half of the modern Japan forearc basement. Steep magnetic gradients, offset anomalies, and basin stratigraphy portray extensive faulting of the Kitakami batholith during oblique subduction in the Late Cretaceous and rifting in the Miocene. The eastern boundary of the Kitakami batholith lies between 90 and 140 km west of the modern trench, much closer than the 300‐km distance between the active arc and trench. The Early Cretaceous forearc basin and accretionary prism may underlie the modern forearc basin east of the batholith, but clear evidence is lacking. Much of the Early Cretaceous margin, including most of its forearc therefore is missing. How the material was removed is unknown: it could have been strike‐slip faulted, eroded by subduction‐related processes, or both.
Long river profiles, tectonism, and eustasy: A guide to interpreting fluvial terraces
Authors: Merritts Dorothy J. , Vincent Kirk R. , Wohl Ellen E.
Published: February 2004

Along three rivers at the Mendocino triple junction, northern California, strath, cut, and fill terraces have formed in response to tectonic and eustatic processes. Detailed surveying and radiometric dating at multiple sites indicate that lower reaches of the rivers are dominated by the effects of oscillating sea level, primarily aggradation and formation of fill terraces during sea level high stands, alternating with deep incision during low stands. A eustasy‐driven depositional wedge extends tens of kilometers upstream on all rivers (tapering to zero thickness). This distance is greater than expected from studies of the effects of check dams on much smaller streams elsewhere, due in part to the large size of these rivers. However, the change in gradient is nearly identical to other base level rise studies: the depositional gradient is about half that of the original channel. Middle to upper reaches of each river are dominated by the effects of long‐term uplift, primarily lateral and Vertical erosion and formation of steep, unpaired strath terraces exposed only upstream of the depositional wedge. Vertical incision at a rate similar to that of uplift has occurred even during the present sea level high stand along rivers with highest uplift rates. Strath terraces have steeper gradients than the modern channel bed and do not merge with marine terraces at the river mouth; consequently, they cannot be used to determine altitudes of sea level high stands. Strath formation is a continuous process of response to long‐term uplift, and its occurrence varies spatially along a river depending on stream power, and hence position, upstream. Strath terraces are found only along certain parts of a coastal stream: upstream of the aggradational effects of oscillating sea level, and far enough downstream that stream power is in excess of that needed to transport the prevailing sediment load. For a given size river, the greater the uplift rate, the greater the rate of vertical incision and, consequently, the less the likelihood of strath terrace formation and preservation.
Integrated lithospheric modeling combining thermal, gravity, and local isostasy analysis: Application to the NE Spanish Geotransect
Authors: Zeyen Hermann , Fernàndez Manel
Published: February 2004

A two‐dimensional algorithm to determine the steady state thermal structure of the lithosphere that integrates thermal, gravity, and local isostasy analyses is presented. Gravity analyses together with seismic data are used to constrain spatial variations in density and crustal structure, while absolute elevation is used to determine the lithospheric mantle thickness. The calculation is performed using a finite element technique that links the different physical equations. The program optionally calculates the temperature at any material boundary and, with given rheological parameters, the strength distribution and the total lithospheric strength in selected columns. We apply the algorithm to the Northeastern Spanish Geotransect which extends from the Pyrenees to the Balearic Promontory and along which a strong variation in crustal and lithospheric thickness is evident. We assess the use of two different inferred density models for the lithospheric mantle: The first assumes a linear decrease in density with increasing temperature using the asthenospheric density as a reference; the second model assumes a constant density for the whole lithospheric mantle. Although conceptually the two hypotheses differ substantially, the results obtained do not show significant differences. Lithospheric thicknesses of 120–130 km below the Pyrenees, 60–65 km in the Valencia Trough, and 65–75 km below the Balearic Promontory are deduced. In all cases the mean lithospheric mantle density has to be 40–60 kg m−3 higher than the asthenospheric density. The algorithm is shown to be a powerful tool in lithospheric thermal modeling especially in areas where surface heat flow is poorly constrained because of the temperature‐density‐elevation relationship.
Large‐scale landslide simulations: Global deformation, velocities and basal friction
Authors: Campbell Charles S. , Cleary Paul W. , Hopkins Mark
Published: February 2004

The cause of the apparent small friction exhibited by long runout landslides has long been speculated upon. In an attempt to provide some insight into the matter, this paper describes results obtained from a discrete particle computer simulation of landslides composed of up to 1,000,000 two‐dimensional discs. While simplified, the results show many of the characteristics of field data (the volumetric effect on runout, preserved strata, etc.) and with allowances made for the two‐dimensional nature of the simulation, the runouts compare well with those of actual landslides. The results challenge the current view that landslides travel as a nearly solid block riding atop a low friction basal layer. Instead, they show that the mass is completely shearing and indicate that the apparent friction coefficient is an increasing function of shear rate. The volumetric effect can then be understood. With all other conditions being equal, different size slides appear to travel with nearly the same average velocity; however, as the larger landslides are thicker, they experience smaller shear rates and correspondingly smaller fractional resistance.
Elastic behavior of contacting surfaces under normal loads: A computer simulation using three‐dimensional surface topographies
Authors: Yoshioka Naoto
Published: February 2004

It is very important to understand the potential and limitation of the contact theories in order to apply them to geophysical problems. Using three‐dimensional topographies, a computer simulation concerning the behavior of contacting surfaces under normal loads was performed to examine the contact models previously proposed. Two models, the Brown and Scholz (B&S model) and the Yamada et al. (Y model), were compared in detail. It is found that the Y model neglects the contacts between asperities and valleys or shoulders, which results in a smaller area of contact and smaller stiffness than with the B&S model. The contact area formed between asperities and shoulders or valleys is estimated at about 20% of the total contact area, a value that does not depend on either normal stress or surface roughness. The real area of contact is proportional to normal load even under purely elastic conditions, as originally proposed by Archard, and it is much smaller than 1% of the total area of contact up to the normal stress of 30 MPa. Average area of contact, contact density, mean contact pressure, and maximum contact pressure were also calculated as a function of normal stress, which revealed the detailed process of contact mechanics under normal loads.
Stress and the spatial distribution of seismicity in the central Aleutians
Authors: Gross S. J. , Kisslinger C.
Published: February 2004

Changes in static stress caused by 52 earthquakes of average mb 4.7 are modeled and compared to changes in the spatial distribution oi mostly smaller surrounding earthquakes recorded by the Central Aleutian Seismic Network. The comparisons have been used to evaluate possible fault failure criteria and background stress states appropriate for a subduction zone. Statistical measures were developed to assess the significance of changes in spatial distribution of seismicity. The significance of changes in the spatial distribution of seismicity occurring near the times of target earthquakes was evaluated by comparing them to changes occurring at other times in the same catalog. The most successful model includes background stresses caused by viscous drag forces exerted on the subducting plate as it moves through the mantle and assumes that surrounding seismicity occurs on strong faults in all orientations. This stress state suggests that the main thrust zone between the subducting and overriding plates is weak, because otherwise forces from the collision of the two plates would dominate the stress field. The best‐fitting model is preferred over three other combinations of background stress state and fault friction which fit the data equally well because it more strongly correlates with seismicity near the larger earthquakes in more active areas.