bluuebunny/crossref_metadata_2025 huggingface dataset:
Equilibrium geometry of a fluid phase in a polycrystalline aggregate with anisotropic surface energies: Dry grain boundaries
Authors: Laporte Didier , Provost Ariel
Published: February 2004

The equilibrium distribution of a fluid phase in a rock is controlled by the ratio of grain boundary energy to solid‐fluid interfacial energy (“surface energy”). A strong dependence of interfacial energy on interface orientation (anisotropy) is the rule for solid‐fluid surfaces of geological interest. We investigated the effects of surface energy anisotropy on the equilibrium interface configuration at the junction of two crystals with a fluid in a two‐dimensional solid‐fluid system. The two major effects are to promote the development of planar solid‐fluid interfaces parallel to crystallographic planes of minimum energy and to lead to large variations of the dihedral angle from one triple junction to the other (as a function of the orientation of crystalline lattices relative to the grain boundary). Despite these large variations, the frequency distributions of equilibrium dihedral angles remain unimodal; also the relationship between the mean fluid dihedral angle and the ratio of grain boundary to surface energy is very close to the isotropic law. Even in the most anisotropic systems, the fluid dihedral angle is therefore a parameter of primary importance for predicting the grain‐scale geometry of a low‐volume fraction of fluid and its mobility.
Quantifying mixing and age variations of heterogeneities in models of mantle convection: Role of depth‐dependent viscosity
Authors: Hunt D. L. , Kellogg L. H.
Published: February 2004

Using a two‐dimensional finite element model of mantle convection containing over a million tracer particles, we examine the effects of depth‐dependent viscosity on the rates and patterns of mixing. We simulate the processes of recycling crust at subduction zones and the homogenization of recycled material (by dispersion and by melting at mid‐ocean ridges). Particles are continually introduced at downwellings and destroyed when they either are so thoroughly dispersed that it would be impossible to measure their presence in the geochemical signature of mid‐ocean ridges or oceanic islands, or when they are close to spreading centers, at which point melting would “reset” the geochemical clock. A large number of factors influence the flow pattern and thus the rate at which heterogeneities are dispersed by convection. We examine the effect of increasing the viscosity with depth, and determine how both the residence time of heterogeneities and the extent of lateral mixing and exchange between the upper and lower mantle vary with the viscosity profile of the mantle. We determine the particle distribution resulting from convection models with three viscosity profiles: uniform viscosity, a smooth increase of viscosity with depth, and an abrupt jump in viscosity between the upper and lower mantle. We characterize the resulting distribution of heterogeneities in space and time by examining the age distribution of particles and their locations relative to others introduced into the flow at separate downwellings. Mixing rates in the three models are calculated as a function of the number of particles removed from the flow through time. We found that an increase of viscosity at depth does not induce age stratification in which older particles stagnate in the lover mantle, and it does not produce an upper layer (the source of mid‐ocean ridge basalt) that is well‐mixed compared to the deeper regions. However, pronounced lateral heterogeneity is evident in the distribution of particles of different ages and starting locations that is not apparent from the particle positions alone.
Crustal structure of the Southwest Indian Ridge at the Atlantis II Fracture Zone
Authors: Muller M. R. , Minshull T. A. , White R. S.
Published: February 2004

The Southwest Indian Ridge is a slow spreading end‐member of the mid‐ocean ridge system. The deepest borehole penetrating the lower oceanic crust, Ocean Drilling Program hole 735B, lies on the eastern transverse ridge of the Atlantis II Fracture Zone at 57°E. A wide‐angle seismic survey in the vicinity of the borehole reveals a crustal structure that is highly heterogeneous. To the east of Atlantis Bank, on which hole 735B is located, the crust consists of a 2–2.5 km thick high‐velocity‐gradient oceanic layer 2 and a 1–2 km thick low‐velocity‐gradient layer 3. The transform valley has a 2.5–3 km thick crust with anomalously low velocities interpreted to consist largely of highly serpentinized mantle rocks. The seismically defined crust is thickest beneath the borehole, where layer 2 is thinner and the lower crust is inferred to contain 2–3 km of partially serpentinized mantle. The seismic velocity models are consistent with gravity data which show weak residual mantle Bouguer anomalies because the regions of thinner crust have lower crustal densities. Stress variations deduced from mass balances between the transform valley floor and the adjacent transverse ridges are much larger than the likely threshold for lithospheric failure and therefore indicate that the relief is supported dynamically. The variation of crustal thickness with spreading rate defined by data from the Southwest Indian Ridge and elsewhere is consistent with models of melt generation in which the upwelling mantle is cooled by conductive heat loss at very slow spreading rates, resulting in reduced melt generation under the spreading axis. Large segment‐scale variations in crustal thickness suggest subcrustal along‐axis migration of melt toward segment centers.
Transition from circular to stellate forms of submarine volcanoes
Authors: Mitchell Neil C.
Published: February 2004

Large volcanic islands and guyots have stellate forms that reflect the relief of radiating volcanic rift zones, multiple volcanic centers, and embayments due to giant flank failures. Small mid‐ocean ridge volcanoes, in contrast, are commonly subcircular in plan view and show only embryonic rift zones. In order to characterize the transition between these two end‐members the morphology of 141 seamounts and guyots was studied using the shape of the depth contour at half the height of each edifice. Irregularity was characterized by measuring perimeter distance, elongation, and moment of inertia of the contours, assuming an “ideal” edifice is circular. The analysis reveals a general transition over 2–4 km edifice height (best transition estimate 3 km), while some large edifices 4–5 km high show no major embayments or ridges, suggesting considerable variation in the effectiveness of mechanisms that cause flank instability and growth of rift zones. The various origins of the transition are discussed, and the upper limit of magma chambers, many of which lie above the basement of the larger edifices, is proposed to affect the morphologic complexity via a number of mechanisms and is an important factor affecting the mode of growth. The origins of the truncated cone shape of mid‐ocean ridge volcanoes are also discussed. Of the eruption mechanisms that have been proposed to explain their flat summits, the most likely mechanisms involve eruption from small ephemeral magma bodies lying within the low‐density upper oceanic crust. The discussion includes speculations on factors affecting the depths of magma chambers beneath oceanic volcanoes. Supporting table is available via Web browser or via Anonymous FTP from ftp://kosmos.agu.org, directory “append” (Username = “anonymous”, Password =“guest”); subdirectories in the ftp site are arranged by paper number. Information on searching and submitting electronic supplements is found at http://www.agu.org/pubs/csupp_about.html.
Deformation properties of subfreezing glacier ice: Role of crystal size, chemical impurities, and rock particles inferred from in situ measurements
Authors: Cuffey K. M. , Conway H. , Gades A. , Hallet B. , Raymond C. F. , Whitlow S.
Published: February 2004

To improve understanding of the deformation properties of subfreezing polycrystalline glacier ice and, in particular the role of crystal size, chemical impurities, and rock particle impurities, we analyze in situ strain rates of the basal layers of Meserve Glacier, Antarctica. Strain rates were monitored on the walls of a subglacial tunnel (where down flow shear stress is relatively uniform) and ice properties were measured (texture, fabric, and impurity content). We propose a simple empirical model describing strain rate variations due to variations in crystal size and impurity content, and we use all relevant Meserve data to constrain model parameters. We conclude that there is a direct dependence of strain rate on crystal size, which reflects an important role for a grain‐size‐sensitive deformation mechanism such as grain boundary sliding or diffusion. Chemical impurities are found to enhance the grain‐size‐sensitive deformation and are found to be an important control on strain rate variations in the very impure ices of Meserve Glacier. However, the per molar sensitivity of strain rate to chemical impurity content is shown to be very low, such that in the ice age ices of the Greenland ice sheet there is probably an immeasurable contribution of chemical impurities to strain rate enhancement, though we cannot exclude chemical enhancements as high as 1.3 there. Our analyses detect no direct rheologic effect of rock particles in the Meserve ices, which suggests that rock content is not directly responsible for the low viscosity of dirty basal layers.
Pelagic sedimentation on rough seafloor topography 2. Inversion results from the North Atlantic Acoustic Reverberation Corridor
Authors: Webb Helen F. , Jordan Thomas H.
Published: February 2004

Seafloor slope distributions are sensitive to basement topography, the surface shape of sediment distributions, and the average thickness of sediment within a region. By comparing slope distributions computed from bathymetric data to distributions from realistic models of sedimented topography, we determine how characteristic values of average sediment thickness (L), RMS basement variability (H), and apparent diffusivity (κ) vary throughout the Office of Naval Research Acoustic Reverberation Corridor, located on the western flank of the Mid‐Atlantic Ridge north of the Kane Fracture Zone (25.5°–27.25°N, 44.5°–49°W). Values of L from basin‐centered subregions show thinner sediments near the axis, thickening westward with maximum values found on 10–20 Ma seafloor. Lower values of L are found for older (late Oligocene/early Miocene) seafloor. Values of L on young seafloor suggest a sediment accumulation rate of 6.5±2.3 m/m.y. since ∼10 Ma. Values of H computed from hill‐centered regions range from 217 to 306 m, with an average of 267±27 m. Hills are generally larger to the north of a low running through the study area. Values of κ range from 0.12 to 0.77 m2/yr, with an average of 0.32±0.19 m2/yr. Lower values of Kbetter describe the surface shapes of individual high‐standing ponds, while higher values are needed to model the surface shapes of thick deposits in regional lows. This difference in κ with geological setting is probably due to variations in sediment transport mechanisms.
Crustal composition and mantle heat flow: Implications from surface heat flow and radiogenic heat production in the Variscan Erzgebirge (Germany)
Authors: Förster Andrea , Förster Hans‐Jürgen
Published: February 2004

From an enlarged data set of temperature logs and thermal conductivity measurements, surface heat flow (qs) in the Erzgebirge was determined to range from 61 to 112 mW m−2. U‐Th‐K2O data show that the heat flow pattern is controlled to first order by the occurrence of high heat production Variscan granites within a metamorphic basement. Highest heat flow correlates with granite plutons and sharply decreases off granite. These granites display variable but typically high radiogenic heat production (A), usually between 4 and 10 μW m−3, depending on their chemical type and degree of fractionation. U accounts for 40–90% of the total heat production in the granites, whereas U and Th contribute equally to the radioactivity in the metamorphic basement. Compositional heterogeneity in the upper crust, owing to variation of conductive heat transfer, required corrections of measured surface heat flow up to 20 mW m−2, depending on location of the heat flow site. Heat budget calculations, considering crustal models derived from seismic and gravimetric surveys, define the thickness (D) of crust enriched in radioactive elements to 15 km, the rate of reduced heat flow (q*) to 30–34 mW m−2, and the mantle heat flow to 20–30 mW m−2, in contradiction to what is implied from qs ‐ A plots. In fact, D (5–8 km) and q* (45–52 mW m−2) from these plots represent the thickness of, and the heat flow beneath, the (granite) layer that is most enriched in heat‐ producing elements. For a crustal section strongly heterogeneous in upper crustal heat production, traditional interpretation of qs ‐ A plots is misleading and should be supplemented routinely by heat budget calculations.
Comparisons of gravity anomalies at pseudofaults, fracture zones, and nontransform discontinuities from fast to slow spreading areas
Authors: Kruse Sarah E. , Tebbens Sarah F. , Naar David F. , Lou Qingyuan , Bird Robert T.
Published: February 2004

Published mechanisms for rift tip propagation at spreading centers include extensional deformation and an initial period of slow spreading. We investigate whether the gravity signal and inferred crustal structure at pseudofaults formed in medium to superfast spreading environments resemble the gravity signal at fracture zones or nontransform discontinuities formed in slow spreading environments. We find that altimetry‐based gravity anomalies on the Mathematician, Bauer, Easter, Juan Fernandez, and northern Chile Ridge pseudofaults, located in 75–150 mm/yr (full rate) seafloor spreading environments, are similar in amplitude and form to Atlantic fracture zones with 20–30 mm/yr spreading rates. A 5–15 mGal positive mantle Bouguer anomaly is observed on the pseudofault bounding the eastern Juan Fernandez microplate, comparable to those at some similar age‐offset nontransform discontinuities in slow spreading environments. Our results suggest that the deeps associated with active propagating rift tips result from both a dynamic mantle component and anomalous crust, the latter of which remains frozen at pseudofaults. We predict that any pseudofaults with age offsets more than ∼1 m.y. and not coincident with hotspot volcanism will be associated with thin (and possibly unusually dense) crust, even in superfast seafloor spreading environments.
Broadband source modeling of the November 8, 1997, Tibet (Mw= 7.5) earthquake and its tectonic implications
Authors: Velasco Aaron A. , Ammon Charles J. , Beck Susan L.
Published: February 2004

We studied the source process of a large (Ms= 7.9) intraplate earthquake that occurred on November 8, 1997, at 1002 UT in a remote region of northern Tibet. We used four distinct methods to investigate the broadband source process and thereby better understand the tectonic implications of this event. We relocated aftershocks using a master event technique and found that the distribution of aftershocks covers a region of 200 km in lateral extent. We also employed a surface wave spectral inversion technique to estimate the mainshock moment, depth, centroid location, and centroid time and utilized an empirical Green's function technique to extract rupture directivity information and a detailed source time function from observed seismograms. We also inverted body waves to estimate the moment release along the fault and the source time function. The 1997 earthquake ruptured a strike‐slip fault that appears to be an extension or splay of the Kun Lun fault system. This fault is one of the most seismically active strike‐slip faults within the Tibetan plateau and has had events with surface wave magnitudes of 6.1, 7.4, and 7.9 in this region since 1973. The rupture released most of the energy within the first 20 s and propagated bilaterally initially, with the later rupture propagating westward for 20–30 s. The absence of large aftershocks suggests that the earthquake efficiently released the stored strain. Comparing mainshock to the largest aftershock energy ratios for this event and other large strike‐slip events shows that faulting within the plateau has the characteristics of weak faults (e.g., fracture zone faulting).
New constraints on Antarctic plate motion and deformation from GPS data
Authors: Bouin Marie‐Noëlle , Vigny Christophe
Published: February 2004

Four years (1995–1998) of continuous Global Positioning System (GPS) data recorded by 11 permanent International GPS Service (IGS) stations in Antarctica and surrounding regions have been processed using an optimized method for regional geodetic networks. The 4‐year time series has allowed us to extract nonlinear variations and constant horizontal velocities for all the stations. In December 1997 we have set up a permanent GPS station at Dumont d'Urville in Terre Adélie. The resulting data, along with those of the Scientific Committee on Antarctic Research (SCAR) campaigns (1995 and 1996) are included in our processing. A coseismic displacement (1 to 2 cm) is detected in the horizontal components of this station. We relate this to the March 25, 1998, Mw = 8.1 Balleny Islands earthquake for which a dislocation model yields prediction in the right direction. For the six Antarctic stations the horizontal rates are very consistent with the rigid plate rotation, all the residual velocities are negligible (less than 2 mm/yr), except at O'Higgins in the head of the peninsula (7 mm/yr toward the continent). The position of the rotation pole (62.0°N, 146.7°W) and its rate (0.264°/Ma) are significantly different from the NNR‐Nuvel‐lA predictions for the Antarctic tectonic plate but are consistent with the Australian relative motion. The high horizontal residual in the peninsula area, where iceberg calving rates and ice shelf disintegration have recently increased, can be interpreted as the horizontal component of the elastic crustal response to the load variations.
Geomorphology and seismicity of the Roum Fault as one of the active branches of the Dead Sea Fault System in Lebanon
Authors: Khair Kamal
Published: February 2004

To the south of Lebanon the Dead Sea Transform fault has a well‐documented left‐lateral displacement of about 105 km. In Lebanon, most of the movement is taken up by several major and minor faults. These are the NNE trending Yammouneh, and its NE branch Mid‐Beqa'a, Serghaya, Rachaya, and Hasbaya faults, and the NNW trending Roum fault. Geophysical, geomorphological, and geological study of the Roum fault zone and its northern extension in the Chouf area has led to the following conclusions: (1) The sinistral displacement along the Roum fault, as reflected by the offset of drainage patterns, decreases northward from 8.5 km across the Litani river to ∼1 km across the Awali river. (2) In the Chouf area the lateral displacement is dispersed into a complex network of faults and fractures reflected by clearly pronounced lineaments and therefore is not entirely transmitted northward. (3) The seismicity of the Roum fault zone and the Chouf area is characterized by earthquakes of moderate magnitude with a release interval of 40–50 years. (4) The seismicity of Lebanon, in general, is characterized by strong earthquakes with very long recurrence interval. (5) The bifurcation of the Dead Sea transform fault into several faults in Lebanon has probably occurred in response to the clockwise rotation of the upper part of its crust. (6) In the future “geologic time” the Roum fault will, probably, take up more of the offset, while the significance of other fault branches will decline.
Constraints on the energy and chemical balances of the modern TAG and ancient Cyprus seafloor sulfide deposits
Authors: Humphris Susan E. , Cann Johnson R.
Published: February 2004

The size, chemical composition, energy flux, and fluid composition of the TAG hydrothermal sulfide deposit at the Mid‐Atlantic Ridge and the size, chemical composition and reaction zone characteristics of the Skouriotissa volcanogenic massive sulfide deposit of the Troodos ophiolite in Cyprus are used to examine the energy requirements and chemical balances associated with the generation of a large volcanogenic massive sulfide deposit. We conclude that formation of large sulfide deposits from oceanic hydrothermal systems is a geologically rapid process and occurs on timescales of hundreds of years, with episodes of activity as short as a few tens of years separated by thousands of years of inactivity. About 2×1019 J of energy supplied at high temperature is required to form a deposit the size of the TAG mound. Metals (up to 4 times the mass of any element present in the sulfide deposit) are leached out of relatively small reaction zones most likely at the base of the sheeted dikes. Chemical balance can be struck for all elements except sulfur with a reaction zone 1–2 km3 in volume from which a small proportion of iron and large proportions of copper, zinc, and manganese are removed. A sulfur balance requires that a significant fraction of sulfur be derived from reduction of seawater sulfate, as suggested by stable isotope analyses. We argue that the principal source of energy that drives hydrothermal circulation is latent heat of crystallization of magma close to the top of the plutonic section. Furthermore, we speculate that activity of the TAG hydrothermal system is related to periods of more rapid magma supply from the mantle at magma supply rates similar to those observed in volcanoes in Hawaii and Iceland.
Geometrical heterogeneities and permeability anisotropy of rough fractures
Authors: Méheust Y. , Schmittbuhl J.
Published: February 2004

We develop a numerical model for describing a single phase viscous flow in a rough fracture and compare it successfully to previously‐published experimental results. The aperture fluctuations are introduced as an isotropic self‐affine fractal which includes geometrical heterogeneities, especially at the fracture scale. The model is based on the lubrication approximation and solutions are obtained from two independent numerical schemes. The curves describing the evolution of the hydraulic aperture as a function of the mean fracture separation are shown to be well fitted by power law functions. A grid rotation technique is developed to explore the influence of the pressure drop orientation over a continuous range of orientations. A single fracture is shown to be either flow enhancing or flow inhibiting by comparison to a parallel plate model of identical mean separation, depending on the pressure drop orientation. This anisotropy of the fluid flow results from the geometrical heterogeneities at the fracture scale. Statistical analyses of the results when exploring different apertures distributions with the same measured scale invariance property show a large variability of the permeability, which is due to the same phenomenon. A prediction of the hydraulic transmittivity that takes account of the pressure drop orientation is proposed.
Global seismicity characteristics of subduction‐to‐strike‐slip transitions
Authors: Bilich Andria , Frohlich Cliff , Mann Paul
Published: February 2004

There are at least 30 major plate boundary segments worldwide where the plate boundary changes from subduction to strike‐slip; these include six triple junctions and 24 two‐plate boundaries. This study investigates earthquake seismicity in the 24 two‐plate subduction‐to‐strike‐slip transition (SSST) regions by utilizing recently published earthquake relocations, ternary diagrams of focal mechanisms, and moment rate calculations. To facilitate cross‐regional comparisons, we categorize the geometry of SSST plate boundaries in terms of (1) their radius of curvature, (2) their sense of curvature, that is, whether they are convex or concave as viewed from the downgoing plate, and (3) their tectonic complexity, that is, the variability of crustal thickness and the segmentation of the plate boundary trace. We observe three main trends in SSST regions: (1) there is a conspicuous scarcity of strike‐slip earthquakes along plate boundary segments that plate motion models indicate are strike‐slip boundaries; (2) in these apparent strike‐slip segments, both the rate of occurrence of earthquakes of any kind and the moment release rate are low compared to adjacent subduction segments; and (3) there were few observable differences in seismicity between convex and concave boundaries. The observation that transform zones exhibit moment rate deficiencies, that is, have few large‐magnitude earthquakes in the historical record, may have important implications for seismic hazard assessment in SSST regions. In particular, is motion along these boundaries aseismic with little seismic hazard, or is motion expressed in very large magnitude, infrequent, but potentially devastating earthquakes? In at least three such regions, New Zealand, the Philippines, and the Dominican Republic, paleoseismic evidence and the historical record of seismicity suggest that very large, infrequent earthquakes do occur.
Do intermediate‐ and deep‐focus earthquakes occur on preexisting weak zones? An examination of the Tonga subduction zone
Authors: Jiao Wenjie , Silver Paul G. , Fei Yingwei , Prewitt Charles T.
Published: February 2004

It has been proposed that intermediate‐ and deep‐focus earthquakes occur on preexisting planes of weakness that were created at shallow depth. The purpose of this study is to test this hypothesis. We examined fault plane solutions for 360 events that occurred in the Tonga subduction zone from January 1976 to February 1999. For events in the slab, each fault plane solution is rotated by the slab dip angle so that the slab is in its original horizontal position. We then compare fault plane solutions for events in the slab with those for events at the outer rise. We find that an asymmetric fault system, corresponding to that found for outer rise events, persists down to about 450 km depth. This suggests that earthquakes down to this depth are caused by the reactivation of preexisting faults created in the oceanic plate before subduction. A similar pattern, though less well constrained, is found in the Kurile subduction zone down to 200 km depth. For earthquakes deeper than 450 km the fault plane solutions are much more scattered. In some places the fault plane pattern suggests the creation of new fault planes along orientations of maximum shear with zero internal friction. The pattern of fault plane solutions in particular localized volumes cannot be easily explained by the creation of new faults in the direction of maximum shear under a coherent regional tectonic stress field. Thus, either a highly heterogeneous stress field exists or preexisting planes of weakness also account for some deep earthquakes.
Source parameters for the 1952 Kern County earthquake, California: A joint inversion of leveling and triangulation observations
Authors: Bawden Gerald W.
Published: February 2004

Coseismic leveling and triangulation observations are used to determine the faulting geometry and slip distribution of the July 21, 1952, Mw 7.3 Kern County earthquake on the White Wolf fault. A singular value decomposition inversion is used to assess the ability of the geodetic network to resolve slip along a multisegment fault and shows that the network is sufficient to resolve slip along the surface rupture to a depth of 10 km. Below 10 km, the network can only resolve dip slip near the fault ends. The preferred source model is a two‐segment right‐stepping fault with a strike of 51° and a dip of 75° SW. The epicentral patch has deep (6–27 km) left‐lateral oblique slip, while the northeastern patch has shallow (1–12.5 km) reverse slip. There is nearly uniform reverse slip (epicentral, 1.6 m; northeast, 1.9 m), with 3.6 m of left‐lateral strike slip limited to the epicentral patch. The seismic moment is M0 = 9.2 ± 0.5 × 1019 N m (Mw = 7.2). The signal‐to‐noise ratio of the leveling and triangulation data is reduced by 96% and 49%, respectively. The slip distribution from the preferred model matches regional geomorphic features and may provide a driving mechanism for regional shortening across the Comanche thrust and structural continuity with the Scodie seismic lineament to the northeast.
Comparison of Mount Etna, Kilauea, and Piton de la Fournaise by a quantitative modeling of their eruption histories
Authors: Aki Keiiti , Ferrazzini Valérie
Published: February 2004

From published data we found characteristic relations between the amount V of erupted lava and the duration d of eruption for Mount Etna, Kilauea, and Piton de la Fournaise. The relation is similar between Mount Etna and Kilauea, where the increase of V with increasing d is slow, showing a trend of a lower flow rate for a larger eruption. For Piton de la Fournaise, however, the trend is distinctly different, showing a higher flow rate for a larger eruption. We constructed quantitative models of a magma system with reservoirs at various levels and tested hypotheses about the existence of large reservoirs under these volcanoes using the observed V‐d relations. We found that the observed V‐d relation is consistent with the presence of a large reservoir at a shallow depth under Kilauea, and with the presence of a large reservoir near the bottom of the volcanic edifice under Piton de la Fournaise. The above models for Kilauea and Piton de la Fournaise are characterized by a simple path from the mantle reservoir which leads to the shallowest reservoir connected to a hierarchy of channels with varying resistance to eruption sites. We obtained less satisfactory agreement between the observed V‐d relation and the predicted using our model for Mount Etna. Time histories of pressures in the reservoirs at various levels obtained by the modeling explain why inflation‐deflation cycles observed at Kilauea have not been reported for Piton de la Fournaise. The absence of volcano‐tectonic earthquakes with magnitude greater than ∼2.5 under Piton de la Fournaise is attributed to the simplicity of the magma path from the mantle to the shallowest reservoir and the underdeveloped rift zone, which result in a stress concentration localized only beneath the summit area.
Fracture networks in frozen ground
Authors: Plug Lawrence J. , Werner B. T.
Published: February 2004

Fractures in frozen ground self‐organize into networks through interactions between sequentially emplaced fractures, tensile stress and the developing fracture pattern. From this viewpoint we model the development of networks on a lattice representing the ground surface on which fractures initiate, propagate and arrest under a combination of uniform thermally induced tensile stress, stress reduction near existing fractures and stochastic parameterization of heterogeneity in frozen ground and in insulating snow. Tensile stress from cooling, tensile strength, propagation threshold, fracture depth and elastic properties are chosen to approximate properties of frozen ground. Using these parameters, model networks assemble with properties similar to natural icewedge networks, including (1) individual fracture paths have lengths ranging from tens to hundreds of meters; (2) fractures intersect to enclose regions with characteristic spacing between fractures of approximately 22 m; (3) intersections between fractures are predominantly orthogonal, with less common three‐way approximately equiangular intersections. Joint distributions of relative orientation and spacing between fractures from modeled networks and ice‐wedge networks at Espenberg, Alaska, are comparable at the level of variability in natural examples. This similarity is consistent with the hypotheses that networks self‐organize by stress‐interactions between sequentially placed fractures in frozen ground and that networks are insensitive to the many details of fracture dynamics omitted from the model. Spacing between fractures in modeled networks is influenced by suboptimal placement of fractures during network development and increases nonlinearly with the length scale of stress reduction around a fracture. Three‐way approximately equiangular intersections form where modeled fractures arrest on the outside of bends in earlier fractures.
Magma flow regimes in sills deduced from Ar isotope systematics of host rocks
Authors: Wartho Jo‐Anne , Kelley Simon P. , Blake Stephen
Published: February 2004

The best place to seek evidence of the style of past magma flow through a conduit is in the country rock. Heat flow has been studied in country rock adjacent to two Tertiary dolerite sills intruding the Caledonian schists and quartzites, on the Isle of Mull, Scotland. Radiogenic 40Ar loss within mica grains in the thermal aureoles of the intrusions has been measured at high spatial resolution using the Ultra‐Violet Laser Ablation Micro‐Probe, to discriminate between a history of prolonged magma flow, a history of conductive cooling following laminar flow, and instantaneous emplacement of the intrusions. The 40Ar/39Ar mica data and thermal modeling suggest that a prolonged period of magma flow of 3–5 months resulted in extensive argon loss from the micas, country rock melting, and mineral breakdown adjacent to a 6‐m sill. These features were absent from the wall rocks of a smaller 2.7‐m‐thick sill, which exhibited even less argon loss than might have been predicted for an instantaneous intrusion. If the heat loss from the 6‐m sill observed in one locality had been repeated along its length, it would have formed an important magma conduit to the Mull volcano, but dolerite is not a common flow composition on Mull. If on the other hand, the heat loss from the sill varies along strike, it constitutes strong evidence for channelling and heterogeneous flow within the sill.
Flow of variably fluidized granular masses across three‐dimensional terrain: 2. Numerical predictions and experimental tests
Authors: Denlinger Roger P. , Iverson Richard M.
Published: February 2004

Numerical solutions of the equations describing flow of variably fluidized Coulomb mixtures predict key features of dry granular avalanches and water‐saturated debris flows measured in physical experiments. These features include time‐dependent speeds, depths, and widths of flows as well as the geometry of resulting deposits. Three‐dimensional (3‐D) boundary surfaces strongly influence flow dynamics because transverse shearing and cross‐stream momentum transport occur where topography obstructs or redirects motion. Consequent energy dissipation can cause local deceleration and deposition, even on steep slopes. Velocities of surge fronts and other discontinuities that develop as flows cross 3‐D terrain are predicted accurately by using a Riemann solution algorithm. The algorithm employs a gravity wave speed that accounts for different intensities of lateral stress transfer in regions of extending and compressing flow and in regions with different degrees of fluidization. Field observations and experiments indicate that flows in which fluid plays a significant role typically have high‐friction margins with weaker interiors partly fluidized by pore pressure. Interaction of the strong perimeter and weak interior produces relatively steep‐sided, flat‐topped deposits. To simulate these effects, we compute pore pressure distributions using an advection‐diffusion model with enhanced diffusivity near flow margins. Although challenges remain in evaluating pore pressure distributions in diverse geophysical flows, Riemann solutions of the depth‐averaged 3‐D Coulomb mixture equations provide a powerful tool for interpreting and predicting flow behavior. They provide a means of modeling debris flows, rock avalanches, pyroclastic flows, and related phenomena without invoking and calibrating rheological parameters that have questionable physical significance.
Magnetic anomaly interpretation across the southern central Andes (32°–34°S): The role of the Juan Fernández Ridge in the late Tertiary evolution of the margin
Authors: Yáñez Gonzalo A. , Ranero César R. , von Huene Roland , Díaz Juan
Published: February 2004

Marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of Chile between 32°–34°S. Three magnetic domains have been identified: oceanic, continental margin, and subaerial. The oceanic domain has seafloor spreading anomalies (16 (∼37 Ma) to 18 (∼39.5 Ma)) disturbed by anomalies of the Juan Fernández hot spot chain. In the continental margin, the most prominent fabric are E‐W anomalies in the upper slope corresponding to onshore E‐W anomalies of large intrusive bodies. Onshore, a N‐S lineament of short‐wavelength anomalies defines the roots of a Cretaceous volcanic arc. A resembling lineament offshore indicates a submerged older volcanic arc and that continental basement extends to ∼50 km landward of the trench. Absolute Cenozoic plate motion for Nazca and South American plates and dating of the Juan Fernández chain provide a kinematic model of ridge‐continent collision. The reconstruction indicates rapid southward migration of the collision point along ∼1400 km of the margin from 20 to 11 Ma (∼20 cm yr−1). From 11 Ma to present the collison point has migrated at a slower rate along ∼375 km of the margin (3.5 cm yr−1). The predicted location of the subducted portion of the Juan Fernández chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship. In the last ∼10 Myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates. These observations indicate that subduction of the Juan Fernández chain plays a major role in arc‐forearc tectonics.
Seismic polarization anisotropy beneath the central Tibetan Plateau
Authors: Huang Wei‐Chuang , Ni James F. , Tilmann Frederik , Nelson Doug , Guo Jingru , Zhao Wenjin , Mechie James , Kind Rainer , Saul Joachim , Rapine Richard , Hearn Thomas M.
Published: February 2004

SKS and SKKS shear waves recorded on the INDEPTH III seismic array deployed in central Tibet during 1998–1999 have been analyzed for the direction and extent of seismic polarization anisotropy. The 400‐km‐long NNW trending array extended south to north, from the central Lhasa terrane, across the Karakoram‐Jiali fault system and Banggong‐Nujiang suture to the central Qiangtang terrane. Substantial splitting with delay times from 1 to 2 s, and fast directions varying from E‐W to NE‐SW, was observed for stations in the Qiangtang terrane and northernmost Lhasa terrane. No detectable splitting was observed for stations located farther south in the central Lhasa terrane. The change in shear wave splitting characteristics occurs at 32°N, approximately coincident with the transcurrent Karakoram‐Jiali fault system but ∼40 km south of the surface trace of the Banggong‐Nujiang suture. This location is also near the southernmost edge of a region of high Sn attenuation and low upper mantle velocities found in previous studies. The transition between no measured splitting and strong anisotropy (2.2 s delay time) is exceptionally sharp (≤15 km), suggesting a large crustal contribution to the measured splitting. The E‐W to NE‐SW fast directions are broadly similar to the fast directions observed farther east along the Yadong‐Golmud highway, suggesting that no large‐scale change in anisotropic properties occurs in the east‐west direction. However, in detail, fast directions and delay times vary over lateral distances of ∼100 km in both the N‐S and E‐W direction by as much as 40° and 0.5–1 s, respectively. The onset of measurable splitting at 32°N most likely marks the northern limit of the underthrusting Indian lithosphere, which is characterized by negligible polarization anisotropy. Taken in conjunction with decades of geophysical and geological observations in Tibet, the new anisotropy measurements are consistent with a model where hot and weak upper mantle beneath northern Tibet is being squeezed and sheared between the advancing Indian lithosphere to the south and the Tsaidam and Tarim lithospheres to the north and west, resulting in eastward flow and possibly thickening and subsequent detachment due to gravitational instability. In northern Tibet, crustal deformation clearly follows this large‐scale deformation pattern.
Particle aggregation in volcanic eruption columns
Authors: Veitch Graham , Woods Andrew W.
Published: February 2004

We develop a model to describe the formation of aggregates in a volcanic eruption column. The model combines a description of the rate of collision and sticking of particles with a model of the vertical transport in the eruption column. We thereby determine the evolution of the grain size distribution as a function of height in the eruption column. We consider aggregates in which liquid water provides the binding agent. For sufficiently large eruption columns we find that this limits the vertical extent of the zone where aggregates may form since near the source, all water is in the vapor form, while in the upper part of the column the mixture becomes very cold and freezes. However, in many cases, we find that the particles spend sufficient time in the central region, where the water is in the liquid form, that a substantial amount of aggregation occurs. Furthermore, we predict that owing to the reduction in the binding efficiency of water as particle size increases [Gilbert and Lane, 1994], there is a relatively narrow size distribution of aggregates at the plume top. We also model the airfall deposits associated with this aggregate‐rich distribution of particles which is injected to the top of the eruption column. We show that the relatively narrow size distribution of particles at the top of the column, coupled with the gravitational settling and transport by ambient winds, may lead to enhanced deposition close to the source and in some cases secondary thickening of the deposit. The relatively near‐source deposition of fine ash in these deposits is associated with the fallout of aggregates. As a simple application, we show that the present dynamical aggregation model is consistent with the secondary thickening of the deposit from the May 18, 1980, eruption of Mount St. Helens.
A detailed comparison of repeated bathymetric surveys along a 300‐km‐long section of the southern East Pacific Rise
Authors: Dunn Robert A. , Scheirer Daniel S. , Forsyth Donald W.
Published: February 2004

Little is known about the frequency‐magnitude relationship for volcano‐tectonic events over long sections of fast spreading ridges. A technique that can both detect and characterize the size, shape, and approximate time of a volcanic or tectonic event is to survey an area of seafloor on multiple occasions using multibeam sonar systems. We have performed a detailed comparison of repeated bathymetric surveys of a 300‐km‐long section of the southern East Pacific Rise with the objective of detecting and characterizing any changes in seafloor morphology associated with eruptions and perhaps tectonic deformation. Repeat bathymetry data were collected as part of three cruises that surveyed the axial high between 16°S and 18°40′S latitude in late 1995, in late 1996, and in mid‐1997. During the latter two surveys the way points of the 1995 survey were repeated with the same survey vessel, thus closely approximating the acquisition geometry of the first survey. We have developed new methods for removing systematic errors and suppressing random noise in the bathymetry data (which reduce errors between surveys to a root‐mean‐square value of <2 m) and for selecting regions of the seafloor that surpass threshold values in depth difference and lateral area from survey to survey. As the ability to resolve a depth anomaly is a function of both its thickness and lateral area, anomalies of small lateral area require a larger depth anomaly than larger area features to be statistically significant. For this 1.5‐year survey period, no significant seafloor changes in the bathymetric data were detected with areas ≤0.2 km2 and depth differences >8 m or with areas >0.2 km2 and depth differences >4 m. We estimate that any seafloor depth changes that occurred between the survey times must have dimensions less than these values or they would have been detected. For comparison, lava flows detected along the Juan de Fuca and Gorda ridges using similar techniques are ∼10–75 m in thickness and have estimated total areas of 0.52–2.8 km2.
Annual vertical crustal motions predicted from surface mass redistribution and observed by space geodesy
Authors: Mangiarotti S. , Cazenave A. , Soudarin L. , Crétaux J. F.
Published: February 2004

Temporal variations of surface mass redistribution among atmosphere, oceans, and continental water reservoirs deform the Earth's crust, in particular in the vertical direction. These displacements can now be measured by space geodesy and predicted from climatic loading data. In this study we first compute globally theoretical vertical displacements of the Earth's crust caused by the main annual surface mass redistributions (atmosphere and ocean mass, soil moisture, and snow load). For that purpose we consider atmospheric pressure data from the National Centers for Environment Prediction (NCEP), soil moisture data from Huang et al. [1996] and from the Global Soil Wetness Project (GSWP), snow data from the International Satellite and Surface Climatology Project (ISLSCP) and GSWP, and ocean mass data from the Parallel Ocean Climate Model (POCM) and from TOPEX‐Poseidon satellite altimetry after correcting for steric effects. Annual vertical displacements are computed for each load individually as well as for the total climatic contribution on global 2.5°×2.5° grids. We then present space geodesy‐derived annual variations of vertical coordinates of 16 Doppler orbitography and radiopositioning integrated by satellite (DORIS) stations globally distributed around the globe. A comparison is then performed for each station between observed (by DORIS) and predicted (from climatology) results.
Rapid fluid disruption: A source for self‐potential anomalies on volcanoes
Authors: Johnston M. J. S. , Byerlee J. D. , Lockner D.
Published: February 2004

Self‐potential (SP) anomalies observed above suspected magma reservoirs, dikes, etc., on various volcanoes (Kilauea, Hawaii; Mount Unzen, Japan; Piton de la Fournaise, Reunion Island; Miyake Jima, Japan) result from transient surface electric fields of tens of millivolts per kilometer and generally have a positive polarity. These SP anomalies are usually attributed to electrokinetic effects where properties controlling this process are poorly constrained. We propose an alternate explanation that contributions to electric fields of correct polarity should be expected from charge generation by fluid vaporization/disruption. As liquids are vaporized or removed as droplets by gas transport away from hot dike intrusions, both charge generation and local increase in electrical resistivity by removal of fluids should occur. We report laboratory observations of electric fields in hot rock samples generated by pulses of fluid (water) through the rock at atmospheric pressure. These indicate the relative amplitudes of rapid fluid disruption (RFD) potentials and electrokinetic potentials to be dramatically different and the signals are opposite in sign. Above vaporization temperatures, RFD effects of positive sign in the direction of gas flow dominate, whereas below these temperatures, effects of negative sign dominate. This suggests that the primary contribution to observed self‐potential anomalies arises from gas‐related charge transport processes at temperatures high enough to produce vigorous boiling and vapor transport. At lower temperatures, the primary contribution is from electrokinetic effects modulated perhaps by changing electrical resistivity and RFD effects from high‐pressure but low‐temperature CO2 and SO2 gas flow ripping water molecules from saturated crustal rocks. If charge generation is continuous, as could well occur above a newly emplaced dike, positive static potentials will be set up that could be sustained for many years, and the simplest method for identifying these hot, active regions would be to identify the SP anomalies they generate.