bluuebunny/crossref_metadata_2025 huggingface dataset:
Fabric evolution during high shear strain deformation of magnesiowüstite (Mg0.8Fe0.2O)
Authors: Heidelbach Florian , Stretton Iona , Langenhorst Falko , Mackwell Stephen
Published: May 2003

High‐strain deformation experiments have been performed on aggregates of magnesiowüstite to investigate textural evolution and associated changes in mechanical behavior. The experiments were performed using a torsional configuration in a gas‐medium deformation apparatus at a confining pressure of 300 MPa and temperatures of 1300 and 1400 K. Samples were deformed at constant angular velocities equivalent to shear strain rates in the range 10−4 to 10−3 s−1 and an oxygen fugacity fixed by the iron jacket. In all cases, little change in the flow stress of the samples was observed after the initial yield, even to shear strains as large as γ = 15.5. Significant changes, however, were observed in the microstructures and textures of the samples as analyzed with the scanning electron microscope. Initial grain elongation and shearing were followed at higher shear strains by grain refinement due to subgrain rotation recrystallization. The initial deformation texture, compatible with dislocation glide on all three probable slip systems ({111}, {110}, {100} in the 〈110〉 direction), is transformed into a recrystallization texture. Investigations of the dislocation microstructure in the transmission electron microscope confirm the assignment of slip systems. The change in crystallographic preferred orientation due to recrystallization is not large enough to cause a significant weakening of the sample, which is in accordance with the mechanical data and with subgrain rotation as the recrystallization mechanism. The formation of subgrains at these relatively low homologous temperatures (<0.5Tm) is possibly promoted by cross slip between the different glide planes.
Development and stability of multiple groundwater flow systems near cooling plutons
Authors: Roselle Gregory T. , Bowman John R. , Forster Craig B.
Published: May 2003

In volcanic terrains, igneous plutons are emplaced beneath topographic highs resulting in two distinct groundwater flow systems: topography‐ and buoyancy‐driven. The interface between these systems is a potential site of ore deposition. This study investigates the interaction between topographic and hydrothermal flow systems using a 2D time‐transient solution to the heat, fluid, and 18O/16O mass‐transport equations. The goal of this study is to explore factors controlling the geometry, lateral extent, and stability of this interface. Results show that the interface geometry and stability (temporal and spatial) are most influenced by pluton depth and lateral extent of the topographic high. More laterally extensive topography produces a near horizontal interface whose location is stationary in space (±500 m) and nearly constant in temperature (≥200°C) for times >40,000 years. Less‐extensive topography produces spatially and temporally stable interfaces, but at high angles to the surface. Shallow pluton emplacement results in a transient mixing zone. Many authors have suggested that ore formation occurs first and then stabilizes the interface by formation of a caprock. However, this study shows that under certain conditions, the interface can be stabilized first and that this may then lead to a caprock formation. An 18O/16O transport simulation shows that steep chemical gradients can be maintained nearly coincident to this interface for 10,000s of years. The spatial and temporal stability of the mixing interface, and maintaining steep chemical gradients along that interface, are three criteria necessary for mineral (ore) deposition involving fluid mixing at this hydrologic interface.
Shear heating of a fluid‐saturated slip‐weakening dilatant fault zone 1. Limiting regimes
Authors: Garagash Dmitry I. , Rudnicki John W.
Published: May 2003

The one‐dimensional model of Rudnicki and Chen [1988] for a slip‐weakening dilating fault is extended to include shear heating. Because inertia is not included, instability (a seismic event) corresponds to an unbounded slip rate. Shear heating tends to increase pore pressure and decrease the effective compressive stress and the resistance to slip and consequently tends to promote instability. However, the decrease of effective compressive stress also reduces the magnitude of shear heating. Consequently, in the absence of frictional weakening and dilation, there exists a steady solution for slip at the tectonic rate in which the pressure does not change and the shear heating is exactly balanced by heat flux from the fault zone. In the absence of shear heating, dilatancy tends to decrease pore pressure and inhibit instability; more rapid slip weakening promotes instability. Analysis of undrained, adiabatic slip (characteristic of rapid slip or hydraulically and thermally isolated faults) reveals that the interaction of these effects can cause increased slip weakening to be stabilizing and increased dilatancy to be destabilizing. These counterintuitive effects are due to the dependence of the shear heating on the total shear stress (not just its change). They occur for small thermal expansivity and for material parameters within a plausible range for 0–10 km depth.
Importance of a stochastic distribution of floods and erosion thresholds in the bedrock river incision problem
Authors: Snyder Noah P. , Whipple Kelin X. , Tucker Gregory E. , Merritts Dorothy J.
Published: May 2003

Fluvial erosion of bedrock occurs during occasional flood events when boundary shear stress exceeds a critical threshold to initiate incision. Therefore efforts to model the evolution of topography over long timescales should include an erosion threshold and should be driven by a stochastic distribution of erosive events. However, most bedrock incision models ignore the threshold as a second‐order detail. In addition, climate is poorly represented in most landscape evolution models, so the quantitative relationship between erosion rate and measurable climatic variables has been elusive. Here we show that the presence of an erosion threshold, when combined with a well‐constrained, probabilistic model of storm and flood occurrence, has first‐order implications for the dynamics of river incision in tectonically active areas. First, we make a direct calculation of the critical shear stress required to pluck bedrock blocks for a field site in New York. Second, we apply a recently proposed stochastic, threshold, bedrock incision model to a series of streams in California, with known tectonic and climatic forcing. Previous work in the area has identified a weak relationship between channel gradient or relief and rock uplift rate that is not easily explained by simpler detachment‐limited models. The results with the stochastic threshold model show that even low erosion thresholds, which are exceeded in steep channels during high‐frequency flood events, fundamentally affect the predicted relationship between gradient and uplift rate in steady state rivers, in a manner consistent with the observed topography. This correspondence between theory and data is, however, nonunique; models in which a thin alluvial cover may act to inhibit channel incision in the low uplift rate zone also provide plausible explanations for the observed topography. Third, we explore the broader implications of the stochastic threshold model to the development of fluvial topography in active tectonic settings. We suggest that continued field applications of geomorphic models, including physically meaningful thresholds and stochastic climate distributions, are required to advance our knowledge of interactions among surficial, climatic, and crustal processes.
Equation of state of stishovite and interpretation of SiO2 shock‐compression data
Authors: Panero Wendy R. , Benedetti Laura Robin , Jeanloz Raymond
Published: May 2003

We present an isothermal equation of state for the high‐pressure stishovite phase of SiO2 based on synchrotron X‐ray diffraction at 300 K to 60 GPa. The inferred equation of state is the same for stishovite synthesized from pure SiO2 glass as from a natural, basalt glass that forms stishovite in association with other mineral phases, yielding a zero‐pressure bulk modulus K0T = 312.9(±3.4) GPa and pressure derivative K′0 = 4.8(±0.2). Our measurements of stishovite at static, high pressures predict a shock‐compressed (Hugoniot) state that is 2.4 (±1.6)% denser than observed for SiO2, suggesting that an amorphous phase, rather than stishovite, is produced under impact loading.
Vp and Vp/Vs images in the Mw 6.0 Colfiorito fault region (central Italy): A contribution to the understanding of seismotectonic and seismogenic processes
Authors: Chiarabba C. , Amato A.
Published: May 2003

In 1997, the Umbria‐Marche Apennines (central Italy) was struck by a series of moderate magnitude normal faulting earthquakes, accompanied by a prolonged sequence of aftershocks. Seismological analysis revealed a ∼40‐km‐long NW trending structure composed by several low‐angle normal fault segments. In this paper we present the three‐dimensional (3‐D) Vp and Vp/Vs structure of the fault zone determined by the inversion of P and S wave arrival times from 1982 aftershocks. The high‐resolution tomographic images reveal strong heterogeneity along and across the fault zone and allow us, along with the 3‐D location of earthquakes, to define the fault geometry at depth and to improve our understanding on the seismotectonics of this part of the northern Apennines thrust‐and‐fold belt. The relocated aftershocks suggest that the main ruptures occurred on planes slightly steeper than previously hypothesized and at depths shallower than those evidenced by 1‐D locations. We find that the preexisting structure, which developed during the Neogene compressional tectonics, plays an important role in the earthquake generation and rupture evolution of the normal faulting events. We propose that the main shocks, whose hypocenters are all located at ∼6 km depth, originated along a shallow decollement of the Meso‐Cenozoic cover, thrust over the metamorphic basement. Geometrical discontinuities and lateral steps along the inherited structures control the lateral fault extent and determine fault segmentation. This study offers a good example of how lithological heterogeneities and the complexity of the upper crust structure strongly affect the faulting process.
Seismic and seafloor evidence for free gas, gas hydrates, and fluid seeps on the transform margin offshore Cape Mendocino
Authors: Tréhu Anne M. , Stakes Debra S. , Bartlett Cindy D. , Chevallier Johanna , Duncan Robert A. , Goffredi Shana K. , Potter Susan M. , Salamy Karen A.
Published: May 2003

Seismic data and seafloor samples indicate the presence of free gas, gas hydrate, and fluid seeps south of the Gorda Escarpment, a topographic feature that marks the eastern end of the Gorda/Pacific transform plate boundary southwest of Cape Mendocino, California. In spite of high sedimentation rates and high biological productivity, direct or indirect indicators of gas hydrate presence had not previously been recognized in this region, or along transform margins in general. Gas is indicated by a bottom simulating reflection (BSR) observed near the Gorda Escarpment, by “bright spots” and “gas curtains” scattered throughout the sedimentary basin to the south, and by δ13C and δ18O isotopes of carbonates, which are similar to those recovered from other hydrate‐bearing regions. The BSR reflection coefficient of −0.13 ± 0.04 and interval velocities as low as 1.38 km/s indicate that free gas is present beneath the BSR. Local shallowing of the BSR toward the north facing Gorda Escarpment and beneath a channel near the crest suggests fluid flow toward the seafloor. Integrating these various observations, we suggest a scenario in which methane is formed in thick Miocene and Pliocene deposits of organic‐rich sediments that fill the marginal basin south of the transform fault. Dissolved and free gas migrates toward the escarpment along stratigraphic horizons, resulting in hydrate formation and in channels, slumps and chemosynthetic communities on the face of the escarpment. We conclude that the BSR appears where hydrate‐bearing sediments are uplifted because of current triple junction tectonics.
Tide gauge records of uplift along the northern Pacific‐North American plate boundary, 1937 to 2001
Authors: Larsen Christopher F. , Echelmeyer Keith A. , Freymueller Jeffrey T. , Motyka Roman J.
Published: May 2003

Vertical crustal motions at 15 sites along the northern Pacific‐North America plate boundary are determined using relative sea level changes from tide gauge records. Our analysis is based on monthly mean sea levels, from which barometric pressure and seasonal effects are removed. The records are corrected for common‐mode oceanographic variations. These records are statistically examined for non‐linear behavior related to glacial isostatic, tectonic and postseismic effects. To estimate land uplift rates, the local effect of global sea level rise is removed from the relative sea level rates. Slow rates of vertical motion are observed along the southern strike‐slip plate boundary. The extremely rapid uplift of the northern strike‐slip boundary can be attributed entirely to viscoelastic postglacial rebound associated with tidewater‐glacier retreat in Glacier Bay and regional post‐Little Ice Age deglaciation. Isostatic modeling indicates a mantle viscosity of ∼2 × 1019–5 × 1019 Pa s, similar to that found elsewhere along the Pacific‐North America plate margin. At Yakutat, near the transition of plate motion from strike‐slip to subduction, complex non‐linear behavior is evident, with a significant change in uplift rate following the 1979 St. Elias earthquake. Non‐linear uplift rates are predominant within the 1964 Great Alaskan earthquake near‐field. Rapid uplift at Kodiak during a 3.5 year period starting mid‐1964 totaled 47 ± 8 cm. Anchorage, Seward and Seldovia exhibited oscillatory uplift in the period immediately following the earthquake until mid‐1972. Since mid‐1972, uplift rates have increased steadily at Anchorage, Seward, Cordova, and Valdez. During this period Nikiski and Kodiak show decreasing uplift rates.
Sediment transport by dry ravel
Authors: Gabet Emmanuel J.
Published: May 2003

Dry ravel is a general term that describes the rolling, bouncing, and sliding of individual particles down a slope and is a dominant hillslope sediment transport process in steep arid and semiarid landscapes. During fires, particles can be mobilized by the collapse of sediment wedges that have accumulated behind vegetation. On a daily basis, particles may be mobilized by bioturbation and by small landslides. Experiments on a dry ravel flume indicate that a basic expression of the momentum equation predicts the distance traveled by particles propelled down a rough surface. This equation is further elaborated to produce a nonlinear slope‐dependent transport equation for dry ravel that represents the rate at which sediment crosses a contour width of slope. Sediment traps installed on two hillslope transects near Santa Barbara, California, measured the flux from dry ravel initiated by bioturbation, and the data support the form of the equation. Additionally, a physical model, based on the infinite slope stability analysis, is proposed for the initiation of dry ravel by landsliding. The analytical result from this model is supported by experiments and field data reported by others.
Deep‐ocean temperature variations and implications for errors in seafloor heat flow determinations
Authors: Davis Earl E. , Wang Kelin , Becker Keir , Thomson Richard E. , Yashayaev Igor
Published: May 2003

The accuracy with which seafloor heat flow is determined depends on the temporal stability of bottom water temperature. Indirect tests for stability are provided most commonly by observing the uniformity of heat flow with depth. This criterion is met to a high degree of certainty at two sites in the eastern North Pacific Ocean, where colocated high‐quality probe and borehole heat flow data can be compared. A more direct test for stability is provided by long‐term observations of bottom water temperature. Previously published records and new data show temperature variations of only a few hundredths of a degree at sites in the central and eastern North Atlantic and the eastern North Pacific. Resultant gradient perturbations are geothermally insignificant (<5 mK m−1) at depths greater than 1–2 m below the seafloor, consistent with the uniformity of heat flow with depth observed in these areas. Geothermally problematic bottom water temperature variations are observed or inferred in the western North and South Atlantic and western South Pacific. Variations range up to ±0.15 K and are capable of producing gradient perturbations of up to 5 mK m−1 at depths as great as 5 m below the seafloor. While these data are instructive, their distribution is not adequate to provide general guidelines for estimating geothermal gradient perturbations. Data from shallower sites are needed in all oceans to define depth limits of acceptable bottom water temperature variability, and from other deep‐ocean locations where near‐source bottom water transients or vigorous deep‐water circulation dynamics are likely to be present.
Rayleigh wave tomography of China and adjacent regions
Authors: Huang Zhongxian , Su Wei , Peng Yanju , Zheng Yuejun , Li Hongyi
Published: May 2003

This paper presents a tomographic study on the S wave velocity structure of China and adjacent regions. Group velocity dispersions of fundamental Rayleigh waves along more than 4000 paths were determined with frequency‐time analysis. The study region was divided into a 1° × 1° grid, and velocities in between grid nodes were calculated by bilinear interpolation. The Occam's inversion scheme was adopted to invert for group velocity distributions. This method is robust and allows us to use a fine grid in model parameterization and thus helps to restore a more realistic velocity pattern. Checkerboard tests were carried out, and the lateral resolution was estimated to be 4°–6° in China and its eastern continental shelves. The resulting group velocity maps from 10 to 184 s showed good correlation with known geological and tectonic features. The pure path dispersion curves at each node were inverted for shear wave velocity structures. The three‐dimensional velocity model indicates thick lithospheres in the Yangtze and Tarim platforms and hot upper mantles in Baikal and western Mongolia, coastal area and continental shelves of eastern China, and Indochina and South China Sea regions. The Tibetan Plateau has a very thick crust with a low‐velocity zone in its middle. Beneath the crust a north dipping high‐velocity zone, mimicking a subducting plate, reaches to 200 km in depth and reaches to the Kunlun Mountains northward. In northern Tibet a low‐velocity zone immediately below the Moho extends eastward then turns southward along the eastern edge of the plateau until it connects to the vast low‐velocity area in Indochina and the South China Sea.
Physical properties of the P96 ice core from Penny Ice Cap, Baffin Island, Canada, and derived climatic records
Authors: Okuyama Junichi , Narita Hideki , Hondoh Takeo , Koerner Roy M.
Published: May 2003

Ice cores from nonpolar environments are being developed as valuable paleoclimatic resources. However, the recognition of a climatic signal and the development of timescales in those ice cores are both difficult due to heavy summer melting. A 177.91‐m‐long ice core (P96) was drilled in such a high‐melt area from Penny Ice Cap, Baffin Island, Canada, in 1996. The core covers the Holocene period and has pre‐Holocene ice near the bed. The purpose of this work is to investigate the physical properties of the ice and to show how these properties can be interpreted in terms of climatic signals. Visual‐stratigraphic observations and measurements of crystal size and c axis orientation were performed using conventional techniques. Melt features provided a clear picture of Holocene summer climatic changes: a warmer period prior to 2 ka, the Little Ice Age, and the modern warming trend. The crystalline structure displayed normal grain growth down to 50 m and recrystallization below this. In the 0–50 m depth interval, periodical variations in crystal size were observed and are attributed to the difference of time for which the ice was soaked in meltwater every season. These variations made it possible to develop an annual record. The c axis fabric evolution displayed a change in the ice flow pattern from divergent, above 140 m depth, to shear‐dominated flow below that. The initial fabric of superimposed ice shows a preferred c axis distribution normal to the surface. The fabric is shown to be important for reconstructing mass balance chronologies from ice cores drilled in areas of superimposed ice formation. These physical properties provide information on climatic change.
Merapi Volcano, Central Java, Indonesia: A case study of radionuclide behavior in volcanic gases and its implications for magma dynamics at andesitic volcanoes
Authors: Le Cloarec Marie‐Françoise , Gauthier Pierre‐J.
Published: May 2003

For more than 20 years, volcanic gases have been regularly collected at Merapi Volcano, Indonesia, and have been subsequently analyzed for their (210Pb), (210Bi), and (210Po) activities and SO2 contents. These new results show the high volatility of the three radionuclides in andesitic gases, although their emanation coefficients (0.94%, 3.5%, and ≤53% for 210Pb, 210Bi, and 210Po, respectively) are significantly lower than those observed at basaltic volcanoes. This emphasizes the major role of magma temperature on the degassing of these metals, which are mainly transported in volcanic gases as Pb‐chloride compounds and as Bi‐ and Po‐metallic species. Radioactive disequilibria between 210Pb, 210Bi, and 210Po in the gas phase are characteristic of degassing processes and gas paths within the edifice. Gases released at both Gendol and Woro fumarolic fields are of magmatic origin, but their radionuclide content is strongly altered by secondary processes (chemical reactions with surrounding brines and deposition of sublimates in the ground). High‐temperature gases collected in the summit gas plume are of pure primary magmatic origin and are likely directly tapped in the degassing reservoir. Gases arising from the growing domes are strongly depleted in the most volatile isotopes and gas species. This suggests that magma mostly degasses at depth in open system, explaining why the volcano usually undergoes no explosive eruptions. The gas flux at Merapi is sustained by a deep magma supply which is twice the extrusion rate of lava. Unerupted degassed magma thus progressively accumulates beneath the summit, yielding an overpressure which can trigger dome collapse without precursor.
Frequency of large crustal earthquakes in Puget Sound–Southern Georgia Strait predicted from geodetic and geological deformation rates
Authors: Hyndman R. D. , Mazzotti S. , Weichert D. , Rogers G. C.
Published: May 2003

Earthquake hazard in the Cascadia subduction zone forearc comes from three sources: great subduction earthquakes, Wadati–Benioff slab events, and earthquakes in the forearc crust. This study deals with a concentration of forearc crustal earthquakes in the Puget–Georgia region of southwestern Canada and northwestern United States. These earthquakes are due to margin‐parallel shortening, rather than compression in the direction of plate convergence. The frequency of large earthquakes has previously been based mainly on extrapolation of the statistics of smaller events from the short instrumental record. In this study, an independent estimate has been obtained through the seismic moment rate required to accommodate current rates of deformation from GPS and geological data. The catalogue statistics to Mx = 7.5 give a moment rate of 4.0 × 1017Nm/yr and a shortening rate of 2.9 mm/yr if all deformation is seismic. GPS data indicate local current N–S shortening of 3 ± 1 mm/yr. For a seismogenic thickness of 12 km, this deformation represents a moment rate of 4.1 × 1017Nm/yr. The predicted occurrences are 0.022/yr (45 years) forM> 6, and 0.0025/yr (400 years) forM> 7, within the uncertainties of the catalogue statistics. Large characteristic earthquakes are not required. Forearc long‐term average motion based upon larger‐scale GPS, paleomagnetic, and geological data ranges from 5 to7 mm/yr. The estimated long‐term occurrence of large events thus is approximately double the current rate, and we infer that the extra earthquakes could follow abrupt N–S shortening in this part of the forearc associated with the oblique rupture motion of great subduction events.
Fluidization and melting of fault gouge during seismic slip: Identification in the Nojima fault zone and implications for focal earthquake mechanisms
Authors: Otsuki Kenshiro , Monzawa Nobuaki , Nagase Toshiro
Published: May 2003

The record of physical processes that occur during seismic slip events is well preserved in fault rocks from the active Nojima fault in Japan. The fault rocks formed at about 3 km depth, and comprise thin alternating layers of very fine gouge and pseudotachylyte derived from granite. Each layer is thinner than a few millimeters, and corresponds to one seismic slip event. The very thin slip zone width suggests that some mechanisms of slip weakening operated, and our studies of the fault rocks suggest that fluidization and melting of gouge were particularly important. Fluidized and nonfluidized gouges were distinguished using the detection probability of fragmented counterparts method. It is known from granular material science that the phase transition from a grain friction regime to fluidization of granular materials can occur only by a very small decrease in volume fraction of solid grains. Once fluidization occurs, the frictional resistance decreases abruptly to nearly zero even before thermal pressurization reaches its extreme state. When fault gouge is melted, frictional resistance is governed by the viscosity of melt which depends mainly on temperature, chemical composition, H2O concentration and the volume fraction of unmelted grains. For each pseudotachylyte layer, we estimated the temperature of melt using various temperature indices, and measured volume fraction of unmelted grains. We synthesized these data to reconstruct the change in viscosity during seismic slip events. The melt viscosity is very high (107–9 Pas) during initial melting due to the combined effect of low temperature (750°–800°C) and large volume fraction of solid grains. Thus seismic slip is inevitably restrained instantaneously. Once this mechanical barrier is overcome, the viscosity reduces continuously and dramatically as slip increases. At 1000°C, the viscosity reduces to 104Pas and eventually to 10Pas at 1280°C. Thus stress drops almost completely and rupturing tends to run away.
Catastrophic 1638 earthquakes in Calabria (southern Italy): New insights from paleoseismological investigation
Authors: Galli Paolo , Bosi Vittorio
Published: May 2003

The reanalyses of all the primary historical sources of the catastrophic March–June 1638 Calabrian earthquakes permitted to split up the seismic sequence in different events that occurred in conterminous areas and to direct our geological surveys of the June shock inside the Sila massif (northern Calabria). We carried out paleoseismological analyses along the Lakes fault (LF), a previously unknown fault. LF (trending NW‐SE) cuts the eastern sector of the Sila massif and dips southwestward, damming the drainage network that runs toward the Ionian Sea. We opened four trenches close to or inside some of the small marshes and ponds created by the footwall uplift, finding evidence of several displacement events, the last one being the June 1638 earthquake (M = 6.7). The fault location fits with the historical description of a long fracture opened during the event, our trenching area being still named by the natives “the earthquake sag.” The young geomorphic expression of LF recalls that of the 1980 Irpinia fault, suggesting its relatively recent activation within the extensional regime characterizing the Calabrian arc. Conversely, the northern continuation of LF (Cecita Lake fault), which we argue to be active too, is not associated to any historical earthquake of the Italian seismic catalogue, this fact evidencing a long, at least 1000 years, elapsed time. Our study permits (1) the relocation of the June 1638 earthquake inside the Sila massif, the epicenter being shifted with respect to the macroseismically derived one, (2) the revision of the whole 1638 sequence, (3) the discovery of the LF, (4) the recognition of the evidence of the June 1638 surface faulting plus four previous paleoearthquakes, and (5) the evaluation of its slip rates and return interval. Finally, the relationship of LF with the neighboring faults casts light to the seismotectonic of the region, providing a new insight to the seismic hazard analysis.
Exhumation of the central Wasatch Mountains, Utah: 2. Thermokinematic model of exhumation, erosion, and thermochronometer interpretation
Authors: Ehlers Todd A. , Willett Sean D. , Armstrong Phillip A. , Chapman Davis S.
Published: May 2003

The Wasatch fault is a ∼370 km long normal fault in Utah that marks the boundary between the stable Colorado Plateau to the east and the extending Basin and Range to the west. Understanding the thermokinematic evolution of this fault can provide insights into intracontinental extensional tectonics and deformation processes in other rift zones (e.g., East Africa Rift, Transantarctic Mountains). We explore the thermokinematics of footwall exhumation and erosion in the Cottonwood Intrusive Belt of the central Wasatch Mountains. Emphasis is placed on using low‐temperature thermochronometers to quantify (1) the spatial and temporal variability of exhumation and erosion rates, (2) the geometry of footwall tilt, (3) the fault dip angle, and (4) the magnitude and duration of exhumation. These processes are investigated using two‐dimensional (2‐D) thermal‐kinematic models coupled with cooling‐rate‐dependent kinetic models which predict exhumed apatite fission track (AFT) and (U‐Th)/He ages. The range of model parameters considered includes footwall exhumation and erosion rates at the fault between 0.2 and 2.0 mm yr−1, footwall tilt hinge positions between 15 and 40 km distance from the fault, a single planar normal fault with dip angles of 45° and 60°, and exhumation magnitudes of up to 15 km at the fault. Simulations include the formation of a low thermal conductivity sedimentary basin and erosion of heat‐producing layers. Erosion maintains a constant topographic profile. The kinematic and exhumation history of the Wasatch Mountains is investigated by comparing model predicted thermochronometer ages to observed AFT, ZFT, and (U‐Th)/He ages. Predicted and observed ages are compared using a reduced chi‐square analysis to determine a best fit kinematic model for the Wasatch Mountains. The preferred model includes exhumation occurring on either a 45° or 60° dipping fault, a footwall hinge located a minimum of 20–25 km from the fault, and a step decrease (deceleration) in the footwall exhumation rate at the fault from 1.2 to 0.8 mm yr−1 at around 5 Ma. The model also suggests an exhumation duration of ∼12 Myr ± 2 Myr).
Physical properties of upper oceanic crust: Ocean Drilling Program Hole 801C and the waning of hydrothermal circulation
Authors: Jarrard Richard D. , Abrams Lewis J. , Pockalny Robert , Larson Roger L. , Hirono Tetsuro
Published: May 2003

The hydrologic evolution of oceanic crust, from vigorous hydrothermal circulation in young, permeable volcanic crust to reduced circulation in old, cooler crust, causes a corresponding evolution of geophysical properties. Ocean Drilling Program (ODP) Hole 801C, which obtained the world's oldest section of in situ, normal oceanic crust, provides the opportunity to examine relationships among hydrologic properties (porosity, permeability, fluid flow), crustal alteration, and geophysical properties, at both core plug and downhole log scales. Within these upper crustal basalts, fluid flux in zones with high porosity and associated high permeability fosters alteration, particularly hydration. Consequently, porosity is correlated with both permeability and a variety of hydration indicators. Porosity‐dependent alteration is also seen at the log scale: potassium enrichment is strongly proportional to porosity. We extend the crustal alteration patterns observed at Hole 801C to a global examination of how physical properties of upper oceanic crust change as a function of age based on global data sets of Deep Sea Drilling Project and ODP core physical properties and downhole logs. Increasing crustal age entails macroporosity reduction and large‐scale velocity increase, despite intergranular velocity decrease with little microporosity change. The changes in macroporosity and velocity are significant for pillows but minor for flows. Matrix densities provide the strongest demonstration of systematic age‐dependent alteration. On the basis of observed decreases in matrix density that are proportional to the logarithm of age, approximately half of all intergranular‐scale crustal alteration occurs after the first 10–15 Myr. Apparently, crustal alteration continues, at a decreasing rate, throughout the lifetime of oceanic crust.
Simulation of tensile crack generation by three‐dimensional dynamic shear rupture propagation during an earthquake
Authors: Dalguer L. A. , Irikura K. , Riera J. D.
Published: May 2003

As there is no evidence that cracks are created directly in modes II and III, shear cracks probably occur only along a weakness plane, as in a preexisting fault. Mode I fracture therefore may be an important factor in crack formation during shallow earthquakes. A three‐dimensional shear dynamic rupture process was simulated on the assumption that shear slip occurs only in a preexisting fault and the possibility of introducing new internal cracks that propagate under tensile stress as a consequence of the dynamic process of shear slip propagation. The discrete element method (DEM) was used to solve this problem because it can introduce internal tensile cracks. The simple slip‐weakening model was used as the friction law on the preexisting fault for shear rupture propagation. For new tensile cracks, fracture follows classical Griffith theory when the critical value for tensile fracture surface energy is reached. The proposed model was used to simulate the rupture process of a strike‐slip shallow fault. Results show that the generation of new cracks is affected by rupture directivity in terms of the hypocenter and asperity location as well as by fault geometry with respect to the free surface. Cracks develop a flower‐like structure that surrounds the preexisting fault. When the asperity is located at less than a certain depth, the flower‐like structure that originates from the top of the fault reaches the free surface. We consider that this is the mechanism for forming the flower structure near surface during a strike‐slip shallow earthquake.
Seismic and laboratory constraints on crustal formation in a former continental arc (ACCRETE, southeastern Alaska and western British Columbia)
Authors: Morozov Igor B. , Christensen Nikolas I. , Smithson Scott B. , Hollister Lincoln S.
Published: May 2003

The ACCRETE project studies continent formation by assemblage of exotic terranes and arc magmatism. A marine‐land seismic profile crosses the terranes with dense sampling and remarkably strong S waves as well as P waves. Unique, high‐quality S‐wave seismic data provide further constraints on interpretation and are used together with new laboratory P‐ and S‐wave velocity measurements (corrected for high temperatures within and at the base of the crust) to make a lithologic and petrologic interpretation. VP/VS ratios indicate three distinct terranes that were assembled to form continental crust. These terranes are the outboard, accreted Alexander‐Wrangellia terrane, the Coast Mountains Batholith (CMB) magmatic arc, and the inboard Stikinia terrane. Below the CMB, VP and VP/VS to a depth of 10–15 km are appropriate for tonalite and diorite. The VP and VP/VS increase with depth indicating that the rocks become more mafic, but VP/VS is not high enough for gabbro. The VP and VP/VS of the lower crust are consistent with a mixture of mafic garnet granulite and restite, whose quartz content lowers the VP/VS ratio below that of gabbro. When corrected for high temperature and uplift due to exhumation, the crust under the CMB exhibits seismic properties of an average continental crust. These results suggest that gabbro could have intruded a metasedimentary pile in the deep crust to cause melting of the metasediments to form intrusions of the CMB, leaving behind a mixture of mafic garnet granulite and sillmanite‐garnet‐quartz restite and generating the deeper part of what becomes an average continental crustal section.
Landscape evolution at extensional relay zones
Authors: Densmore Alexander L. , Dawers Nancye H. , Gupta Sanjeev , Allen Philip A. , Gilpin Ruth
Published: May 2003

It is commonly argued that the extensional relay zones between adjacent crustal‐scale normal fault segments are associated with large catchment‐fan systems that deliver significant amounts of sediment to hanging wall basins. This conceptual model of extensional basin development, while useful, overlooks some of the physical constraints on catchment evolution and sediment supply in relay zones. We argue that a key factor in the geomorphic evolution of relay zones is the interplay between two different timescales, the time over which the fault array develops, and the time over which the footwall catchment‐fan systems are established. Results of numerical experiments using a landscape evolution model suggest that, in isolated fault blocks, footwall catchment evolution is highly dependent on the pattern and rate of fault array growth. A rapidly linked en echelon fault geometry gives rise to capture of relay zone drainage by aggressive catchment incision in the relay zone and to consequent increases in the rate of sediment supply to the hanging wall. Capture events do not occur when the fault segments are allowed to propagate slowly toward an en echelon geometry. In neither case, however, are large relay zone catchment‐fan systems developed. We propose several physical reasons for this, including geometric constraints and limits on catchment incision and sediment transport rates in relay zones. Future research efforts should focus on the timescales over which fault array development occurs, and on the quantitative variations in catchment‐fan system morphology at relay zones.
Observation and modeling of the winter coastal oceanic circulation in the Gulf of Lion under wind conditions influenced by the continental orography (FETCH experiment)
Authors: Estournel C. , Durrieu de Madron X. , Marsaleix P. , Auclair F. , Julliand C. , Vehil R.
Published: May 2003

Hydrological and currentmeter observations were collected on the continental shelf and slope of the Gulf of Lion during the FETCH experiment (13 March to 15 April 1998). Results from the first part of the cruise, characterized by strong northern winds, are presented. The hydrological structures evidence well‐mixed water masses on the eastern and western ends of the shelf. In the central part, the situation is more complex, with the influence of the Rhône river's freshwater plume in the first 40 m of the water column and, closer from the bottom, with the confrontation of downwelled coastal cold water and upwelled warmer and saltier slope water. Current measurements show the path of the cyclonic circulation along the slope, which is part of the general circulation of the western Mediterranean, and suggest the presence of large and temporary eddies on the shelf. This oceanic circulation is simulated with a free surface three‐dimensional model using realistic forcing. The model outputs are in agreement with the main hydrological and circulation patterns. The results further indicate that coastal eddies are generated by the mesoscale structure of the wind field.
Impacts of salinity on the eastern Indian Ocean during the termination of the fall Wyrtki Jet
Authors: Masson Sébastien , Menkes Christophe , Delecluse Pascale , Boulanger Jean‐Philippe
Published: May 2003

The impact of salinity and the barrier layer on eastern Indian Ocean dynamics is investigated using the OPA ocean general circulation model for 1979–1993. The study focuses on processes involved in the eastern Indian Ocean circulation during the period of the fall Wyrtki Jet. An exploration of interactions between salinity and ocean dynamics is made using a set of salinity sensitivity experiments. The inclusion of salinity favors a stronger Wyrtki Jet that extends further eastward in two steps. First, a sporadic barrier layer along the jet increases the jet speed by trapping wind momentum in a thinner mixed layer. Second, zonal advection transports the current maximum eastward and participates in the creation of a strong current front at the eastern edge of the jet during the following month. Off Sumatra, the sea surface salinity gradient, bordering the Indian Ocean fresh pool, strengthens when the Wyrtki Jet reaches this region. The pressure gradient associated with the enhanced salinity front then becomes larger than the eastward wind stress forcing in the zonal momentum equation. This initiates a westward surface current opposed to the wind stress. The later penetration of the Wyrtki Jet into the Indian warm and fresh pool modifies the sea level and favors geostrophic westward currents around 4°N. A combination of these mechanisms additionally favors a northward shift of the Wyrtki Jet. All these phenomena affect the zonal displacement of the Indian fresh pool and create SST anomalies of 0.5°C, in a region of high ocean‐atmosphere coupling.
Mesoscale variability of Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) satellite ocean color: Global patterns and spatial scales
Authors: Doney Scott C. , Glover David M. , McCue Scott J. , Fuentes Montserrat
Published: May 2003

Using the semivariogram approach from geostatistics, we characterize for the first time the global patterns of mesoscale (approximately 10–200 km and days to a few weeks) ocean biological variability. The magnitude of the variability and spatial length scale fields are reported for a full year of Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) ocean color data. The analysis shows a number of coherent geographical patterns across a wide range of biological and physical environments. Submonthly, small spatial scale variance dominates ocean color variability in oligotrophic regimes and is about a third of the total variance even in regions with strong seasonality. When normalized to the mean chlorophyll as a coefficient of variation, the resolved mesoscale variance (relative sill) is approximately uniform in the open‐ocean tropics and subtropics (0.1–0.2), increasing to 0.3–1.0 in western boundary current, upwelling (Equatorial Pacific, coastal) and subpolar/polar regions, approximately scaling with the mean chlorophyll concentration. The unresolved variance (nugget), which includes submesoscale geophysical variability as well as instrument/algorithm noise, is inversely correlated with mean chlorophyll concentration in the oligotrophic subtropical gyres, reducing to a low background value (0.1) elsewhere. The high fraction of unresolved variance in the subtropics is consistent with other studies showing substantial speckling due to noise in the atmospheric correction methods. The mesoscale spatial scale (range) is approximately zonal, with values of 200–350 km near the Equator to less than 50 km close to the poles similar to the physical scales estimated from altimeter sea surface height variability data and the first baroclinic Rossby deformation radius. Our results demonstrate the global generality of previous local and regional findings that small‐scale variability in ocean biology and physics occurs on comparable spatial length scales. Regional deviations from these large‐scale variance and spatial length scales trends, for example, lower range and higher resolved variance in coastal upwelling regimes, are explicable by the characteristics of the biological‐physical interaction in those environments.
Effects of temperature on the microstructure of first‐year Arctic sea ice
Authors: Light B. , Maykut G. A. , Grenfell T. C.
Published: May 2003

While the apparent optical properties of sea ice vary with ice type and temperature throughout the annual cycle, they depend more fundamentally on how inclusions of brine, gas, precipitated salts, and other impurities are distributed within the ice. Since little is known about these distributions or about how they evolve with temperature, experiments were designed to collect detailed information on the microstructure of Arctic sea ice over a wide range of temperatures. An imaging system, capable of resolving inclusion sizes of less than 0.01 mm in diameter, was used to examine the microstructure of first‐year ice in a temperature‐controlled laboratory. Experiments were initially carried out at −15°C to obtain size distributions for brine inclusions and gas bubbles in cold ice. Brine inclusion dimensions were found to range from less than 0.01 mm to nearly 10 mm, with number densities averaging about 24 pockets per mm3. This is an order of magnitude larger than number densities previously reported. Gas bubbles in the samples were generally smaller than 0.2 mm and had number densities of approximately 1 per mm3, also an order of magnitude larger than previously reported. Large changes in microstructure were observed as samples were cooled to −30°C and subsequently warmed to −2°C. Observational results document the thermal evolution of the ice, as well as interactions between brine inclusions, gas bubbles, and precipitated salts. The link between the structural and optical properties of sea ice is closely tied to the total cross‐sectional area of the inclusions. We show that this quantity increases dramatically when the ice cools below −23°C or warms above −5°C, but because changes in brine inclusions offset changes in precipitated salts, it remains surprisingly constant between these temperatures.
A comparison between radar imagery and coupled wave‐current model results for a study of northwest Pacific seamount trapped waves
Authors: Lin R.‐Q. , Chubb S. R.
Published: May 2003

This paper demonstrates that a coupled new coastal wave model [Lin and Huang, 1996a, 1996b; Lin and Perrie, 1997, 1999] combined with a coastal current circulation model [Haidvogel and Aike, 1999], accurately predicts the surface features of seamount trapped waves. The environmental conditions, such as wind, buoyancy frequency, bottom topography, latitude, and tidal current are the model‐input parameters. We have tested the applicability of the coupled model, using a set of trapped waves in the vicinity of two seamounts in the Northwest Pacific ocean. Our motivation for doing this is to see if it is possible to understand the origin of sea surface manifestations of these seamounts that appear to be present in images derived using a synthetic aperture radar (SAR), from the Russian Kosmos 1870 satellite. Each seamount is about 0.7 km in height and has a 20 km semidiameter width, and the ocean is about 5 km in depth. The model results suggest it is possible to detect these kinds of sea surface signatures from even such small seamounts in deep ocean by radar imagery, directly, at low wind speeds (<1 m/s), and indirectly, at higher wind speeds, from wave‐breaking that occurs in the shorter wavelength portions of the spectrum.