bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural features in granular flows
Authors: Drake Thomas G.
Published: February 2008

High‐speed motion pictures document a series of essentially two‐dimensional, free‐surface flows of 6‐mm diameter plastic spheres generated in an inclined glass‐walled chute 3.7 m long and 6.7 mm wide. Flows exhibit a diverse suite of structural features, depending on particle flux, chute inclination, and geometry of the fixed bed. Flows at sufficiently low inclination or high particle flux can generally be divided into factional and collisional regions. In the frictional region, intergranular momentum transfer occurs by enduring, frictional contacts; kinetic theories of granular flow do not apply. The frictional region typically consists of a quasi‐static zone extending up from the fixed bed and an overlying block‐gliding zone, in which coherent blocks of grains move parallel to the bed. The collisional region overlies the contact region in sufficiently deep flows; otherwise it extends to the bed. In it, momentum transfer is dominated by collisions. It typically consists of three zones: a lower grain‐layer‐gliding zone, in which grains appear to slide over one another; a middle chaotic zone, in which grain motions are highly random, as in a dense gas; and an overlying saltational zone, in which grains trace long, curved paths. Zones grade smoothly into one another, and some are missing in certain flows. In none of the flows does a frictional region overlie a collisional region. Zone thicknesses and block dimensions are previously unrecognized length scales in granular flows. The no‐slip velocity boundary condition applies only when a frictional region adjoins a geometrically rough fixed bed, otherwise significant slip occurs (up to 50% of the mean flow velocity). Binary collisions dominate only in the collisional region and there perhaps only in the chaotic and saltational zones. Because many collision‐dominated geophysical flows start from friction‐dominated ones and end by reverting to the frictional regime as the energy supply diminishes, kinetic theories of granular flow, which unanimously employ the binary‐collision assumption, can at best describe only a portion of the phenomena. Microstructural descriptions of the relatively slow, compact, and frictional regions evident in most geophysical flows are required to complement existing kinetic theories.
Seismic velocities from the Barbados Ridge Complex: Indicators of high pore fluid pressures in an accretionary complex
Authors: Bangs Nathan L. B. , Westbrook Graham K. , Ladd John W. , Buhl Peter
Published: February 2008

Three wide‐aperture (0–5 km offset) seismic profiles were shot perpendicular to the strike of the Barbados Ridge accretionary complex at 16°12′N (line 465), 14°23′N (line 480), and 13°20′N (line 484) to image the deep structure of the accretionary complex and obtain seismic velocity data. Velocity analyses on full‐fold common depth point (CDP) gathers were used to provide velocity/depth functions at 1 km separation along each line to map the variation in velocity with depth and distance across the accretionary complex. The sediments within the complex show consolidation increasing laterally with greater age of the complex as indicated by the increase in seismic velocity and the inferred decrease in porosity. The velocity at the base of the wedge along line 480 is ∼2.4 km/s at the toe. The velocity at the wedge base at ∼60 km from the toe, where the wedge thickness has increased from 1.8 km to ∼5.0 km, has increased to greater than 3.0 km/s. However, the velocities within the leading margins and within undeithrust sequences in each of the three sections are low, as much as 1.0 km/s less than velocities found in undeformed sedimentary sequences of equivalent thickness. The low‐velocity regions are interpreted as sediments with high porosity. Where porosity is inferred to be high, pore fluid pressures are also inferred to exceed hydrostatic and pore fluids actually support the increased lithostatic load produced by recent thrusting and thickening of the complex. The correlation between the most rapidly thickened regions and the velocity anomalies suggests that a rapid rate of loading relative to the rate of pore fluid expulsion produced these overpressured sediments. Velocities in the underthrust sequences, where the differences between loading histories of wedge sediments and the underthrust sequences are largest, are sufficiently low to produce velocity inversions. Low velocities also occur in regions adjacent to the complex where the pore fluid pressures may be elevated by the pressure produced in the adjacent wedge. West of the wedge, in the forearc basin along line 480, and near a seismically imaged mud diapir, velocities at a depth of 3.5 km below the seafloor are 0.5 km/s less than what is expected in a more normal sedimentary section. Immediately seaward of the complex, the deepest undeformed sequences have a velocity anomaly of −0.5 km/s. Velocities remain low for at least 12 km seaward of the complex, but they become closer to a more normal sediment section farther from the wedge.
Fluid flow and mass flux determinations at vent sites on the Cascadia Margin Accretionary Prism
Authors: Carson Bobb , Suess Erwin , Strasser Jeffrey C.
Published: February 2008

Fluid venting from the toe of the accretionary prism off Oregon was measured in situ during a series of dives with DSRV Alvin in 1987 and 1988. A benthic chamber was placed over active vent sites to sequentially collect samples of venting fluids and to make direct measurements of discharge rates. Calibrated flow meter measurements and flow rates determined from dissolved methane transfer indicate that discharge from two vent sites, Alvin 1428 and Alvin 1900, ranges roughly between 100 and 500 L m−2 d−1. with the most reliable estimates falling in the range of 125–150 L m−2d−1. These rates imply subsurface advective flow on the order of 100 m yr−1. Comparison of observed discharge rates with rates calculated for steady state expulsion supported by accretion‐related compaction indicates that the observed flow is greater than predicted flow by several orders of magnitude. The disparity dictates that fluids are not derived locally, but are transported laterally within the prism, or that flow is not steady state and that individual vents are short‐lived features in the ongoing accretion process.
Relationship between carbonate deposits and fluid venting: Oregon Accretionary Prism
Authors: Kulm LaVerne D. , Suess Erwin
Published: February 2008

Active fluid venting and its surface manifestations (unique animals and carbonates) occur over the accretionary prism in the Cascadia subduction zone located off central Oregon. A large variety of authigenic carbonate deposits and unique carbonate structures have been observed from submersibles and remotely operated vehicles and recovered with aid of submersibles and bottom trawls from the outermost continental shelf and lower continental slope. The carbonate deposits range from relatively thin crusts and slabs to irregular edifices and well‐formed circular chimneys that rise from 1 to 2 m above the seafloor. Mineralogically, the carbonate cement consists of aragonite, calcite, Mg‐calcite, or dolomite with varying amounts of detrital constituents. Stable carbon and oxygen isotope data identify four distinct subgroups of methane‐derived carbonates from several different vent sites and different fluid source zones. Subgroup I represents one vent site on the lower slope and is characterized by oxygen isotope values ranging from +6.8‰ to +4.7‰ PDB. Subgroup II represents another vent site about 1 km away and exhibits oxygen values of +3.4‰ to +4.9‰ PDB. Carbon isotopic values range from −40.96 to −30.23‰ versus −44.26 to −53.44‰ PDB, respectively, for the two vents. An irregular edifice from the outer shelf has the same isotopic composition as subgroup II. A companion study shows that the expelled fluids contain largely biogenic methane and methane‐derived dissolved carbonate; a shallow fluid source zone (<1 km) is indicated. The isotopic carbon values of the subgroup I and II carbonates are consistent with the carbon composition of the expelled fluids and apparently represent a historical record of the composition of these fluids. In subgroup III, strong 18O enrichment and heavier carbon values characterize the dolomitic chimneys from the outer continental shelf. Cemented sandstones from a “window” in the accretionary complex of the lower slope (subgroup IV) are characterized by extreme δ18O (−5.9 to −5.98 ‰) and moderate δ13C (−18.7 to −12.67‰)‐depleted carbonates. This “light” oxygen isotope composition most likely originated from the upward migration of warm hydrothermal fluids along the main décollement, which tapped the warm subducting basaltic slab, during the early stages of formation of the accreted complex. Well‐defined plumbing tubes within some carbonate chimneys on the shelf infer a single well‐defined subsurface conduit with a fairly energetic fluid flow. The majority of the chimneys probably formed above the seafloor as long as the rate of carbonate precipitation exceeded the rate of detrital input during their formation. We calculate a minimum of one conduit for each 35 m2 at one vent site on the shelf. A less energetic flow is suggested by the chaotic plumbing network of an irregular edifice and by the widespread occurrence of the carbonate slabs and crusts at numerous vent sites.
The nature and hydrogeologic significance of mud diapirs and diatremes for accretionary systems
Authors: Brown Kevin M.
Published: February 2008

In accretionary wedges, often morphologically similar sedimentary intrusions, when observed by remote geophysical means, may have one of two quite different driving mechanisms and a highly variable significance for the regional hydrogeologic picture. For example, mud diapirs are driven by buoyancy forces that arise from bulk density contrasts. In them, mud and pore fluid upwell en masse and fluid migration is a related (sometimes important) but generally subsidiary process. In contrast, diatremes contain sediments fluidized during rapid fluid advection and are forcibly and directly driven by the hydrogeologic system. The nature of fluid input from local and exotic source regions can, therefore, strongly affect sedimentary intrusive processes and vice versa. This complicates the process of defining the main features of the hydrogeological systems operating in accretionary wedges. Focused vertical advection through steep sided (piercement) mud diapirs requires conduit systems, otherwise flow will be diffuse and directed more horizontally out of the low‐permeability mud mass. However, where the permeability of the overburden is less than that of the diapir, the whole diapir may act as a conduit. Apart from this special case, conduits will be associated with highly anisotropic scaly fabrics that can sometimes develop in the marginal shear zone of diapirs. Scaly fabrics form during deformation and compaction of a mud matrix under conditions of constant or increasing effective stress. However, the effective stress path can be complex as it is both controlled by the relative rates of upward intrusion and burial (by sedimentation and/or structural thickening) and the hydrogeologic system. Due to this, it appears likely that even in a geographically related group of diapirs, effective stress histories will vary widely between intrusions so that some can form advective pathways for fluids and some cannot. Mixed systems of behavior may also be present with local diatremes developing within diapirs above the terminations of conduit systems and rapidly expanding methane gas pockets. The potentially heterogeneous near‐surface behavior may be why the surface manifestations of sedimentary intrusions are so variable when observed in the field. Diatremes can also form separately as large primary features above any structural or stratigraphic conduit that rapidly expels water or gas into the base of an unlithified sediment column. When active, large diatremes require enormous quantities of fluid (water or gas) to drive them, particularly if they are long lived features and hence are a direct indication of at least an episodically vigorous hydrogeologic system.
Hydrogeologic processes in the Oregon‐Washington Accretionary Complex
Authors: Wang Chi‐Yuen , Shi Yaolin , Hwang Win‐Tsuang , Chen Huiliang
Published: February 2008

Rapid, subduction‐induced venting of fluids was recently discovered at the lower slopes of several convergent margins. In this study we examine, by numerical modeling, the sources and pathways for the expelled fluids at the lower slope of the Oregon‐Washington margin. Models are constructed on the basis of existing multichannel seismic records, Deep Sea Drilling Project drilling data, and regional tectonic history. We show that the hydrogeologic processes at this margin may be cyclic and episodic. An important fluid source may be the underthrust turbidites beneath the frontal thrust, which dewater under the increasing overburden of the advancing thrust sheet The underlain hemipelagic unit, on the other hand, is a poor source of fluid, even though it is highly pressurized. Since the dewatering turbidites have not been subjected to temperatures greater than 100°C, the above conclusion is consistent with the low‐temperature signatures in the carbon isotope data for vented fluids and carbonate cementation at the lower slope. Cyclic but episodic seaward migration of the frontal thrust of the accretionary complex leads to corresponding seaward migration of the fluid sources. Between the distinct thrusting episodes, the fluid sources grow both in size and pore pressure in response to the increasing overburden of the advancing frontal thrust Other factors being equal, thrusting at the landward vergent margin is more effective in creating fluid sources than thrusting at the seaward vergent margin. Some aspects of these inferences may be common to other similar convergent margins and are testable with future Ocean Drilling Program drilling.
The character and distribution of mineralized fractures in the Kodiak Formation, Alaska: Implications for fluid flow in an underthrust sequence
Authors: Fisher Donald , Byrne Tim
Published: February 2008

The Kodiak Formation, an accreted slate belt in southwest Alaska, experienced two major episodes of fracturing and mineralization in the Late Cretaceous‐early Tertiary. The earlier of these episodes is related to underthrusting of sediments below a major décollement (D1), whereas a later episode is marked by development of fold and thrust structures and a regionally pervasive slaty cleavage within the accretionary wedge (D2). D1 veins are restricted to sand layers and are concentrated at the highest structural level of the Kodiak Formation. The most pervasive D1 fractures occur within a mélange zone, where the Kodiak Formation is thrust beneath the older Uyak Complex. Microstructures in the mélange suggest that these veins formed during the later stages of a deformation that involved layer parallel shear in conjunction with progressive lithification. D2 veins cut sand and shale layers and are concentrated at the lowest structural levels of the Kodiak Formation. Microstructures within these veins indicate that D2 veins are hydrofractures that were either (1) intermittentiy sealed along the length of the crack (e.g., in the smallest veins (<0.5 cm width)), (2) intermittently sealed at asperities along the fracture surface, or (3) open along the length of the crack through most of the vein history (e.g., in the largest veins (>0.5 cm width)). Fracturing coincided with development of a slaty cleavage in a regime of progressive simple shear. From highest to lowest structural levels, there is a gradual increase in measured finite strain, the abrupt appearance of pervasive D2 veins, and a greater noncoaxiality of strain related to southeast directed thrusting. These observations suggest that D2 veins formed above a décollement that was active at the base of a Late Cretaceous‐early Tertiary accreted package. In total, the D1 and D2 veins suggest substantial fluid flow within an ancient subduction complex, with development of hydrofractures first in the footwall and then in the hanging wall of a basal décollement that stepped from shallow to deeper structural levels.
Single plume model for asynchronous formation of the Lamont Seamounts and adjacent Eeast Pacific Rise terrains
Authors: Barone Angela M. , Ryan William B. F.
Published: February 2008

Sonar reflectance is used to quantify the relative ages of young volcanic terrains on a seamount chain and a nearby spreading center in the north equatorial Pacific. The reflectance is a measure both of the burial of lava by pelagic sedimentation and of the microrelief of the lava. The extent of sediment cover and the lava type distribution are calibrated with thousands of bottom photographs. The relative ages indicate that the locus of volcanic eruptions has migrated toward the East Pacific Rise axis. Zero‐age activity lies just to the west of the East Pacific Rise south of the Clipperton Transform and is probably the cause of an anomalously shallow neovolcanic zone. The evidence fits a scenario in which eruptive centers appear, evolve, and become extinct in succession, with little or no overlap in time. The eruptive centers are fed in sequence through conduits from buoyant, tightly focused magma plumes. The near identical heights of many of the centers at the time of their latest eruptions suggest that the seamounts tap the same mantle depth range. Broad summit plateaus with calderas are formed if the plume volume is such that the hydraulic head is reached and magma is trapped within the volcano in ephemeral magma chambers or conduits. If the plume volume is small and the hydraulic head is not reached, the seamount remains small with only a small crater. The plumes ascend asynchronously from a common region of nucleation in the asthenosphere. Plumes that follow in close succession build a composite seamount with a string of summit calderas. Plumes that rise less often result in a line of discrete circular volcanoes.
Teleseismic tomography of the compressional wave velocity structure beneath the Long Valley Region, California
Authors: Dawson P. B. , Evans J. R. , Iyer H. M.
Published: February 2008

In 1982 and 1984 the U.S. Geological Survey used several seismic networks, totaling over 90 stations, to record teleseismic P waves and measure travel time residuals in an area centered on the Long Valley caldera. We inverted the travel time residuals to obtain a three‐dimensional image of the velocity structure with resolution of 5–6 km to depths of 70 km beneath the array. Direct inversion of these data indicates that the 2‐ to 4‐km‐thick low‐velocity caldera fill contaminates the signal from any midcrustal velocity anomalies beneath the caldera. Thus two methods were used to strip the effects of the upper crust from the travel time residuals: (1) ray tracing through upper crustal velocity models provided by seismic refraction experiments and gravity surveys, and (2) an iterative stripping scheme using the inversion itself. The methods produce essentially identical results and adequately remove the effects of the shallowest crustal structures, including the caldera fill and hydrothermal alteration effects. The resulting “stripped” models show two well‐resolved midcrustal low‐velocity bodies in the Long Valley region. The first body is centered between 7 and 20 km depth beneath the resurgent dome of the Long Valley caldera and has a volume of 150–600 km3. The second, with a similar volume, is centered between 10 and 20 km depth beneath the Mono Craters, about 10 km north of Long Valley. Velocity contrasts in both of these bodies are about 6–10%, and the features are interpreted as silicic magma chambers. This experiment does not preclude the presence of additional pockets of magma smaller than 5 km across in the upper crust, particularly beneath the resurgent dome of the caldera (which would be removed with the stripping methods). It is likely that the midcrust anomaly beneath the resurgent dome is a remnant of the caldera forming magma chamber. Upper mantle velocities are lower than average beneath Mono Craters and higher than average beneath Long Valley. We tentatively interpret these anomalies as mantle partial melt beneath the Mono Craters and refractory upper mantle residuum beneath Long Valley. The high eruptive rate of the Mono Craters and these upper mantle structures suggest that the focus of volcanism is shifting north from Long Valley to the Mono Craters.
Geological link between fluid inclusions, dilatant microcracks, and paleostress field
Authors: Shepherd T. J.
Published: February 2008

Seismic investigations using three‐component instruments have demonstrated shear wave splitting in widely different geological and tectonic environments. Shear wave analysis suggests that the inferred anisotropy can best be explained if the rocks contain distributions of subparallel, subvertical, fluid‐filled microfractures which are aligned according to the current stress field (the Crampin “extensive dilatancy anisotropy” (EDA) model). Geological evidence for widespread fluid activity in the Earth's crust supports this hypothesis and suggests that the myriads of planes of microscopic secondary fluid inclusions which characterise most geological materials represent fossil EDA cracks. Field studies confirm a strong spatial relationship between the orientation of fluid inclusion arrays and the paleodirections of maximum compressive stress. At elevated temperatures and pressures, the healing of fluid‐filled cracks in the crust, of whatever origin, is considered to be a fairly rapid process. Moreover, the inclusions which form as a result of crack healing respond to changes in temperature, pressure, and stress by changing shape and/or volume. This indicates a high degree of fracture compliance; sufficient perhaps to react to rapid changes in the current stress field and associated shear wave splitting. Though individual inclusions (typically <20 μm) will have no effect on the propagation of seismic waves with wavelengths of tens to hundreds of meters, the rock volume sampled by the waves will be rendered anisotropic if the overall average of the inclusions shows a preferred orientation. Thus fluid inclusions may constitute one of the principal types of fluid‐filled microcrack described by the EDA hypothesis.
Mineral precipitation in the walls of black smoker chimneys: A quantitative model of transport and chemical reaction
Authors: Tivey Margaret K. , McDuff Russell E.
Published: February 2008

A quantitative model of transport and chemical reaction is developed to examine the interplay of physical and chemical processes controlling the evolution of black smoker chimneys. Diffusion and advection yield steady state profiles of temperature and concentration across a porous wall bound on one side by hydrothermal fluid and on the other by seawater. The pressure gradient is estimated from the fluid dynamics of high‐velocity flow inside the chimney. Parametric studies reveal the importance of diffusion driven by gradients in activity coefficients resulting from the strong thermal gradient. The model wall is a series of vertical layers, each with specified physical properties. Chemical reaction, calculated assuming chemical equilibria for the local temperature and concentration conditions, significantly modifies the physical properties of the wall as it evolves. Model calculations demonstrate the sensitivity of mineral zonation to the physical environment and can be used to re‐create the aging of chimneys inferred from petrologic studies.
Properties of chemical remanence in synthetic hematite: Testing theoretical predictions
Authors: Stokking Laura B. , Tauxe Lisa
Published: February 2008

We synthesized hematite under controlled conditions of the applied magnetic field in order to test the behavior of chemical remanent magnetization (CRM) predicted on the basis of theories developed to explain thermoremanent magnetization (TRM). We restrict this investigation to the remanence; acquired during mineral crystallization (grain‐growth CRM) and do not consider chemical remanence acquired during mineral alteration. The material produced by our technique is hematite, up to 10 μm in diameter. Thermal demagnetization to 200°C removes the viscous component acquired under identical field/temperature conditions to those prevalent during synthesis. As predicted by analogy to TRM, the CRM acquired parallels the orientation of the magnetic field applied during growth. The intensity of chemical remanence is a linear function of the field in which it grew, BCRM up to 7.5 mT, the maximum growth field investigated. Furthermore, the ratio of intensities of CRM to TRM acquired in the same field is independent of the strength of that field. The intensity of BCRM controls the growth and orientation of hematite crystals, a result not predicted from theory derived by analogy to TRM. The control on crystal growth by the field is reflected in the dependence on BCRM of the orientation of ellipsoids of anisotropy of isothermal remanence and the acquisition of isothermal and anhysteretic remanence.
Geochemistry of axial seamount lavas: Magmatic relationship between the Cobb Hotspot and the Juan de Fuca Ridge
Authors: Rhodes J. M. , Morgan C. , Liias R. A.
Published: February 2008

Axial Seamount, located along the central portion of the Juan de Fuca Ridge axis and at the eastern end of the Cobb‐Eickelberg Seamount Chain, is the current center of the Cobb Hotspot. The Axial Seamount lavas are transitional between N‐type and E‐type mid‐ocean ridge basalt (MORB), characteristics that they share with lavas along the rest of the Juan de Fuca Ridge. There are, however, subtle, but distinct, differences between the seamount lavas and those of the adjoining ridge segments. These include higher Na2O, CaO, and Sr at a given MgO content and lower silica saturation in the seamount lavas as compared with the ridge lavas. Lava chemistry and bathymetry indicate that Axial Seamount is a discrete volcanic unit, with a more productive shallow magmatic plumbing system separate from the adjacent ridge segments. These high magma supply rates have sustained a continuously replenished, steady state magma reservoir that has erupted remarkably homogeneous lavas over a long time period. Despite this classic association of spreading center and hotspot volcanic activity, there is no evidence in the lavas for geochemical or isotopic enrichment typical of hotspot or mantle plume activity. The differences in composition between the Axial Seamount lavas and the Juan de Fuca Ridge lavas are attributed to melting processes rather than to any fundamental differences in their mantle source compositions. The higher magma production rates, higher Sr, and lower silica saturation in the seamount lavas relative to the ridge lavas are thought to be a consequence of melt initiation at greater depths. The melting column producing the seamount lavas is thought to be initiated in the stability field of spinel peridotite, whereas the ridge lavas are produced from a melting column initiated at shallower levels, possibly within or close to the stability field of plagioclase peridotite. Implicit in this interpretation is the conclusion that the Juan de Fuca Ridge lavas, and by analogy most MORB, are generated at shallow mantle levels, mostly within the stability field of plagioclase peridotite. This interpretation also requires that for the upwelling mantle to intersect the solidus at different depths, the mantle supplying Axial Seamount must be hotter than the rest of the Juan de Fuca Ridge. Axial Seamount, therefore, reflects a thermal anomaly in the mantle, rather than a geochemically enriched ocean island basalt type mantle plume.
High‐resolution studies of the summit of Axial Volcano
Authors: Embley Robert W. , Murphy Kim M. , Fox Christopher G.
Published: February 2008

Sea Beam bathymetry, Sea MARC I side scan (30 kHz), towed camera, and submersible data from Axial Volcano, Juan de Fuca Ridge (JdFR) have been integrated to produce detailed maps of the volcano's summit region in order to elucidate its volcanic and tectonic evolution. The proximity of Axial Volcano, the youngest edifice of the Cobb‐Eickelberg Seamount Chain, to the JdFR superposes a hotspot‐derived radial stress pattern onto the linear stress field of a mid‐ocean ridge spreading center. The morphology of the summit of Axial Volcano reflects the differing eruptive styles produced from this superposition. Long rift zones begin at the edges of the caldera and extend as much as 75 km to the north and south. Axial Volcano has a distinct summit plateau similar to those found on many other submarine volcanos. The process by which summit plateaus are constructed is illustrated by the lava flows originating from the South Rift Zone, which have partially filled the southern half of the caldera and have smoothed the flanks. The summit plateau contains penetrative NE‐SW fracturing that is approximately parallel to the JdFR. The unusual rectangular caldera is 3×8 km in size, and its trend is about 160°. Three possible mechanisms are discussed for the caldera's formation: (1) Canary Island‐type “trapdoor” formation, (2) overlapping spreading center interaction, and (3) magma source migration. Although none of these models is fully compatible with the present data base, the overlapping spreading center model is preferred. Postcaldera volcanism probably began with the formation of the Central Caldera Eruptive Complex, which was characterized by relatively low‐volume, high‐viscosity flows. The most recent volcanism has apparently taken place along the northern and southern rift zones, which have erupted more fluid lavas resulting in areally extensive flows covering up to 60% of the caldera floor. Extensive low‐temperature venting and areas of high‐temperature venting near the northern and southern rift zones and caldera walls suggest a shallow magmatic source beneath the summit of Axial Volcano.
Hydrothermal venting from the summit of a ridge axis Seamount: Axial Volcano, Juan de Fuca Ridge
Authors: Baker Edward T. , McDuff Russell E. , Massoth Gary J.
Published: February 2008

We have mapped the distribution and intensity of submarine hydrothermal emissions from the summit caldera of Axial Volcano by means of hydrographic and chemical sampling of the water column during annual cruises in four consecutive years (1985–1988). These investigations were undertaken to ascertain the strength of hydrothermal venting relative to other vent fields on the Juan de Fuca Ridge and to gauge the importance of Axial Volcano as a source of hydrothermal emissions to the northeast Pacific Ocean. Measurable temperature anomalies are mostly restricted to a 200‐m‐thick water column within the caldera, where they range from a background level of ∼0.01°C to local maxima of 0.02–0.1°C above known high‐temperature vent fields. The temperature anomaly plume is significantly smaller in both extent and intensity than plumes emitted by vent fields on other segments of the Juan de Fuca Ridge. A maximum estimate of the total heat flux from the summit, based on the total excess heat and advective velocity of the plume, is 8×108 W. Hydrothermal venting seems surprisingly small for an edifice that testifies to a long‐term oversupply of magma at a single axial location. We speculate that additional heat may be lost from Axial Volcano by diffuse percolation of sea water over broad areas of its flanks and by magma eruption and venting along flank rift zones.
The sound field near hydrothermal vents on Axial Seamount, Juan de Fuca Ridge
Authors: Little Sarah A. , Stolzenbach Keith D. , Purdy G. Michael
Published: February 2008

High‐quality acoustic noise measurements were obtained by two hydrophones located 3 m and 40 m from an active hydrothermal vent on Axial Seamount, Juan de Fuca Ridge, in an effort to determine the feasibility of monitoring hydrothermal vent activity through flow noise generation. Most of the measured noise field could be attributed to ambient ocean noise sources of microseisms, distant shipping, and weather, punctuated by local ships and biological sources. Long‐period, low‐velocity, water/rock interface waves were detected with high amplitudes which rapidly decayed with distance from the seafloor. Detection of vent signals was hampered by unexpected spatial nonstationarity due to the shadowing effects of the caldera wall. No continuous vent signals were deemed significant based on a criterion of 90% probability of detection and 5% probability of false alarm. However, a small signal near 40 Hz, with a power level of 10−4Pa2/Hz was noticed on two records taken within 3 m of the Inferno black smoker. Hie frequency of this signal is consistent with predictions, and the power level suggests the occurrence of jet noise amplification due to convected density inhomogeneities.
Titanomaghemite in magnetic soils on Earth and Mars
Authors: Coey J. M. D. , Mørup S. , Madsen M. B. , Knudsen J. M.
Published: February 2008

Viking mission soil analyses and the constitution of the SNC group of meteorites have indicated that the Martian regolith is derived from basalt. On Earth such material often weathers to yield a highly magnetic soil (spontaneous magnetization σS ≥ 1 J T−1 kg−1), the magnetic constituent of which is fully oxidized titanomaghemite. We propose that titanomaghemite is likewise responsible for the magnetism of the soils on Mars.
Observational evidence of crystalline iron oxides on Mars
Authors: Bell James F. , McCord Thomas B. , Owensby Pamela D.
Published: February 2008

Visible to near‐IR (0.4–1.0 μm) spectral reflectance observations of Mars during the 1988 opposition were performed at Mauna Kea Observatory using a circular variable filter spectrometer at a spectral resolution R = λ/Δλ ≈ 80. On August 13 and 14 1988, UT, 41 regions 500–600 km in diameter were observed on Mars. The data have been reduced both to reflectance relative to solar analog (Mars/16 Cyg B) and to relative reflectance (spot/spot). The spectra show the strong near‐UV reflectance dropoff characteristic of Mars as well as absorptions at 0.62–0.72 μm and 0.81–0.94 μm both seen here clearly for the first time. These absorption features are interpreted as Fe3+ electronic transition bands that indicate the presence of crystalline ferric oxide or hydroxide minerals on the Martian surface. Comparison of these data with laboratory spectra obtained by other workers supports the conclusion that a single iron oxide phase, most likely hematite, could account for all of the observed spectral behavior of the Martian surface soils and airborne dust in the 0.4–1.0 μm region. This possibility must be reconciled with data from other possible spectral analogs and other wavelength regions as well as geochemical and mineral stability considerations to arrive at a more complete understanding of the role of ferric minerals in Martian surface mineralogy and weathering.
An assessment of the meteoritic contribution to the Martian soil
Authors: Flynn George J. , McKay David S.
Published: February 2008

The addition of meteoritic material to the Mars soils should perturb their chemical compositions, as has been detected for soils on the Moon [Anders et al., 1973] and sediments on Earth [Kyte and Wasson, 1986]. Using the measured mass influx at Earth and estimates of the Mars/Earth flux ratio, we estimate the continuous, planet‐wide meteoritic mass influx on Mars to be between 2700 and 59,000 t/yr. If distributed uniformly into a soil with a mean planetary production rate of 1 m/b.y., consistent with radar estimates of the soil depth overlaying a bouldered terrain in the Tharsis region [Christensen, 1986], our estimated mass influx would produce a meteoritic concentration in the Mars soil ranging from 2 to 29% by mass. Analysis of the Viking X ray fluorescence data indicates that the Mars soil composition is inconsistent with typical basaltic rock fragments but can be fit by a mixture of 60% basaltic rock fragments and 40% meteoritic material [Clark and Baird, 1979]. The meteoritic influx we calculate is sufficient to provide most or all of the material required by the Clark and Baird [1979] model. Particles in the mass range from 10−7 to 10−3 g, about 60–200 μm in diameter, contribute 80% of the total mass flux of meteoritic material in the 10−13 to 106 g mass range at Earth [Hughes, 1978]. On Earth atmospheric entry all but the smallest particles (generally ≤ 50 μm in diameter) in the 10−7 to 10−3 g mass range are heated sufficiently to melt or vaporize. Mars, because of its lower escape velocity and larger atmospheric scale height, is a much more favorable site for unmelted survival of micrometeorites on atmospheric deceleration. We calculate that a significant fraction of particles throughout the 60–1200 μm diameter range will survive Mars atmospheric entry unmelted. Thus returned Mars soils may offer a resource for sampling micrometeorites in a size range which is not collectable in unaltered form at Earth.
Numerical simulations of the decay of Martian global dust storms
Authors: Murphy James R. , Toon Owen B. , Haberle Robert M. , Pollack James B.
Published: February 2008

The decay of Martian global (great) dust storms is investigated. One‐dimensional (vertical, static atmosphere) and two‐dimensional (latitude‐height, steady state circulation) simulations carried out with an aerosol transport‐microphysical model indicate that atmospheric motions play a significant role in the observed decay of global dust storms. Spacecraft observations (Mariner 9, Viking) of the 1971 and the two 1977 planet‐encircling dust storms have provided suggestions about some characterisrics of storm decay. Specifically, the dust particle size distribution is inferred to have remained essentially unchanged for particles with radii between 1 and 10 μm during decay of the 1971 storm, and surface visible opacity declined quasi‐exponentially with time in northern mid‐latitudes during the decay of the two 1977 storms. The results from this investigation indicate that two‐ and three‐dimensional dynamical processes play a significant role in the observed decay features of Martian global dust storms. The most important processes are the lofting of dust by vertical motions in the dust source region of the southern hemisphere subtropics and a continuing advective resupply of atmospheric dust into the dust sink regions of the northern hemisphere. Our work has implications for Viking data analyses and future Mars observer observations and requires that the particle size distribution be treated as a time and latitude dependent quantity.
Amorphous material in high strain experimental fault gouges
Authors: Yund R. A. , Blanpied M. L. , Tullis T. E. , Weeks J. D.
Published: February 2008

The microstructures of gouges produced in room temperature, rotary shear sliding experiments were examined by transmission electron microscopy. Gouges were produced by sliding on ground surfaces of granite, quartzite, or marble except for one experiment in which a 1‐mm‐thick simulated gouge layer was used. Water was added to the sliding surfaces of all but one sample. Crystal plastic processes play no role in the granite and quartzite gouges and a minor role in the marbles. All of the gouges consist of mostly submicron crystalline fragments; in addition, the granite gouges contain 5–60% amorphous material, and the quartzite gouge contains −50% amorphous material. In the granite samples the composition of the amorphous material commonly lies between K‐rich and Na,Ca‐rich feldspars, although portions may be silica‐rich. The microstructural relations suggest that the amorphous material forms by comminution of fragments rather than by melting. The amount of amorphous material increases, and the size of the largest crystalline fragments decreases, with an increase in average shear strain, although the microstructure is nearly uniform throughout each granite gouge layer. These observations suggest that after slip becomes localized on “Y” shear surfaces and/or R1 Riedel shears the entire gouge layer must continue to undergo deformation. It is suggested that cyclic deformation in the gouge must occur to accommodate the passage of geometric irregularities on the active slip surfaces.
An experimental test of the spinel peridotite oxygen barometer
Authors: Wood Bernard J.
Published: February 2008

It has recently been proposed that the Fe3O4 content of spinel coexisting with olivine and orthopyroxene may be used as an oxygen barometer for spinel peridotites (O'Neill and Wall, 1987; Mattioli and Wood, 1988). This barometer relies on the equilibrium: 6 Fe2SiO4. (olivine) +O2 = 3 Fe2SiO6 (orthopyroxene)+2 Fe3O4 (spinel). The two calibrations, both based on a regular solution model for olivine (WFeMg = 8.36 kJ/mol), ideal two‐site model for orthopyroxene, and different activity data for spinel, generally give results within 0.7 log fo2 units of each other for mantle compositions. These calibrations have been tested, together with a recent spinel model of Nell and Wood (1990) by reequilibrating mixtures of olivine (Fo89) orthopyroxene (En90) and spinel of varying composition at known T and fo2 at 1 atm pressure. Experiments were performed at 1200°C and oxygen fugacities between fayalite‐Magnetite‐quartz buffer (FMQ) and 1.4 log units below FMQ. The equilibrium Fe3O4 content of spinel was reversed in experiments of 5–7 days duration at ratios of Cr/Cr+Al (spinel) of 0.13–0.53. Phases were analyzed by electron microprobe with Fe3+/total Fe of spinel being determined using spinel microprobe standards of known ferric iron content. The oxygen barometer was then tested by comparing oxygen fugacities calculated from the compositions of experimental products with the actual fo2 of the experiment. The Mattioli‐Wood version of the barometer gives fo2 values which average close (−0.15 log units) to the true values but which show a dependence on (Cr/Cr+Al) in the spinel. The O'Neill‐Wall version gives average values which are 0.65 log units low but which show no compositional bias. The Nell‐Wood version gives composition‐independent values which are closest to the true values, averaging 0.35 log units low. All three versions are sufficiently accurate to be petrologically useful and can be corrected, using these experimental data, to approximate the true values within a few tenths of a log unit in fo2.
The mechanics of the South Iceland Seismic Zone
Authors: Hackman M. Christine , King Geoffrey C. P. , Bilham Roger
Published: February 2008

The South Iceland Seismic Zone appears to act as a transform fault between offset sections of the Mid‐Atlantic Ridge. However, there is no morphological expression of a throughgoing transform between the ridges. The anticipated throughgoing east‐west transform is represented by a series of north‐south strike‐slip faults. We study the mechanics of the South Iceland Seismic Zone using boundary element methods. First, we examine the implications of the differences between the geometry of an orthogonal ridge‐transform‐ridge system and the geometry of the structures observed in Iceland. Then we consider the role of the north‐south faults in accommodating transform motion in an east‐west direction. We conclude that the north‐south faults can accommodate deformation if they are longer or more spatially frequent than mapped. The South Iceland Seismic Zone is subject to sequences of large earthquakes every 45–112 years. In comparing the seismic moment release derived from earthquake magnitudes with that predicted by our models, we confirm that the system of north‐south faults can act as a transform fault. The seismic moment release derived from the earthquake magnitudes is 1.4 times greater than that predicted by our models. If our application of the moment‐magnitude relation is correct, the above could imply that the brittle layer extends to 14 km (rather than the 10 km assumed in our models). Other possibilities are that the magnitudes of the earthquakes have been slightly overestimated or that the historical seismic record documents a period of seismicity greater than that expected from rates of plate motion inferred from geological and Very Long Baseline Interferometry (VLBI).
The 1967 Caracas Earthquake: Fault geometry, direction of rupture propagation and seismotectonic implications
Authors: Suárez Gerardo , Nábělek John
Published: February 2008

The fault plane orientation of the July 30, 1967, Caracas earthquake (Mw = 6.6) has been a source of controversy for several years. This strike‐slip event was originally thought to have occurred on an east‐west oriented fault plane, reflecting the relative motion between the Caribbean and South American plates. More recently, however, the complex seismic radiation from this event was interpreted as being indicative of a north‐south striking fault that ruptured along three en echelon segments. In this study we synthesize evidence based on the intensity and damage reports, the distribution of aftershocks, and the results of a joint formal inversion of the P and SH waves and show that these data clearly indicate that the rupture of the 1967 earthquake occurred on an east‐west trending fault system. Using a master event technique, the largest aftershock, which occurred 40 min after the main event, is shown to lie 50 km east of the epicenter of the mainshock. The epicentral distances of small aftershocks registered in Caracas, based on the S–P arrival time differences and the polarizations of the P waves, are also consistent with these events occurring on an east‐west oriented fault system north of Caracas. A joint inversion of the teleseismic P and SH waves, recorded on long‐period seismographs of the World‐Wide Standardized Seismographic Network, shows that in a time frame of 65 s, four distinct bursts of seismic moment release (subevents) occurred, with a total seismic moment of 8.6×1018 N m. The first three subevents triggered sequentially from west to east, in a direction that is almost identical to the east‐west trending nodal planes of the source mechanisms. The average depth of these three subevents is 14 km. The fourth, and last identifiable, subevent of the sequence shows a reverse faulting mechanism with the nodal planes oriented roughly east‐west. It occurred at a 21‐km depth, about 50 km to the north of the fault zone defined by the strike‐slip subevents. This fourth subevent appears to reflect compressional deformation of the southern Caribbean, possibly related to underthrusting along the proposed Curaçao trench. The complexity of the fault system causing the 1967 earthquake suggests that the relative motion along the Caribbean‐South America plate boundary in central Venezuela is taken up over a broad, highly faulted, and highly stressed zone of deformation and not by a simple, major throughgoing fault.