bluuebunny/crossref_metadata_2025 huggingface dataset:
Amphibole‐out phase boundary in partially melted metabasalt, its control over liquid fraction and composition, and source permeability
Authors: Rapp Robert P.
Published: February 2004

A number of experimental studies in recent years have demonstrated that partial melting of hydrated basalt at lower crustal to upper mantle pressures is capable of generating up to 40% high‐SiO2 liquids, closely comparable in many geochemical respects to tonalitic‐trondhjemitic granitoids (TTG). From these studies it is apparent that the initiation of melting coincides with the beginning of amphibole dehydration but that a broad reaction interval over which amphibole coexists with high‐SiO2 liquids is defined by the wet basalt solidus on one side, and the amphibole‐out phase boundary on the other. The phase relations of an alkali‐rich tholeiitic metabasalt have been examined in the outer half of this region, up to and beyond the amphibole‐out phase boundary. Results indicate that amphibole exerts a strong control over the amount and composition of coexisting liquid over this interval. Melt fraction increases slowly and gradually (from 0% up to 20–30%) between the wet basalt solidus and up to amphibole‐out but increases substantially (from 30% to >50%) across and immediately beyond it. Liquids are strongly to moderately peraluminous (molar Al2O3/(CaO + Na2O + K2O), A/CNK, > 1.0) up to amphibole‐out, neutral at the boundary (A/CNK ∼ 1.0), and increasingly metaluminous (A/CNK < 1.0) beyond it. Up to amphibole‐out, potassium is strongly partitioned into liquid relative to amphibole (i.e., mineral melt kd ∼ 0.2–0.3), whereas titanium is strongly partitioned into liquid (kd ∼ 2.0–6.0). Physical and chemical criteria suggest that efficient melt segregation leading to TTG plutonism requires more than 20–30% batch melting of a garnet‐bearing basaltic protolith, near or beyond the amphibole‐out phase boundary, leaving dry residues of eclogite or garnet granulite and producing metaluminous liquids that are strongly depleted in heavy rare earth elements (HREEs) and yttrium (Y). Lower degrees of melting may involve critical melting, where melt is squeezed out of the residue by deformation of the crystalline matrix, resulting in disequilibrium melt compositions.
Experimental melting of biotite + plagioclase + quartz ± muscovite assemblages and implications for crustal melting
Authors: Gardien Véronique , Thompson Alan Bruce , Grujic Djordje , Ulmer Peter
Published: February 2004

In order to understand the role of mica‐rich rocks as a source of granite magmas, a series of melting experiments was performed on two different starting materials. The first composition is a model biotite gneiss consisting of 30 wt % biotite, 30 wt % plagioclase, and 40 wt % quartz. The second composition is a model two‐mica pelites consisting of 15 wt % biotite, 15 wt % muscovite, 30 wt % plagioclase, and 40 wt % quartz. Experiments were performed under vapor‐absent conditions at 1.0 GPa and between 750° and 950°C. With only biotite in the starting material the volume of melt is always less than 15 vol % below 900°C and reaches 25 vol % at 950°C. In experiments that involve both biotite and muscovite in the starting material, the melt proportion increases up to 28 vol % at 825°C and reaches 60 vol % at 950°C. For the biotite‐plagioclase‐quartz (BPQ) assemblage, the solidus is located at 800°C at 1.0 GPa. The melting reaction produces a metaluminous granitic liquid and leaves a residuum consisting of garnet + biotite + orthopyroxene + plagioclase + quartz. In addition, the experiments show that at 1.0 GPa biotite can be stable above 950°C. With both micas in the starting material (BPQM), the solidus at 1.0 GPa is located at 750°C. The melting reactions produce a peraluminous granitic liquid and leave a residuum of garnet + sillimanite + biotite + quartz + plagioclase + Kfeldspar in experiments below 900°C. At 950°C the residuum consists of garnet + orthopyroxene + biotite + plagioclase. The melt fraction is determined by the proportions of the hydrous phases and of the amount of feldspar relative to quartz. Mineral modes of the source rocks, particularly the amount of quartz, are at least as important as the amount of available H2O in controlling the melt fraction generated during crustal anatexis.
Self‐similar cataclasis in the Saltville thrust zone, Knoxville, Tennessee
Authors: Babaie Hassan A. , Hadizadeh Jafar , Babaei Abdolali
Published: February 2004

Fault rocks from the Saltville thrust zone, Knoxville, Tennessee, display a fractal geometry of clast size over 3 orders of magnitude. The cataclastic fractal geometry occurs at each magnification in different clast size classes and at combined magnifications. The mean of the fractal dimension (D) measured at each of the optical photomicrographs is generally smaller than that of the scanning electron microscopy images because of the smaller clast density in the optical sections. The fractal dimensions measured on randomly selected areas of the sections cut parallel to the thrust and normal to the thrust along the dip and strike, show a normal distribution with its mean, median, and mode that correlate closely with the dimension of ideal, fractal cataclasis (DI) based on the Sierpinski carpet model. The cataclasis was a statistical random, isotropic, and homogeneous fractal process that deformed the carbonates similarly parallel and normal to the thrust plane and in different parts of the thrust zone.
An improved method for determining and characterizing alignments of pointlike features and its implications for the Pinacate volcanic field, Sonora, Mexico
Authors: Lutz Timothy M. , Gutmann James T.
Published: February 2004

We present an improved method for determining statistically significant alignments of pointlike features. One of the principal such methods now in use, the two‐point azimuth method, depends on a homogeneous distribution of points over the region of interest. Modification of this approach by use of the relatively new statistical technique of kernel density estimation permits treatment of heterogeneous point distributions without introducing substantial dependence on choice of the grid employed in the test for significance of apparent preferred orientations. The improved method can selectively reveal alignments on different spatial scales and can suggest the locations of alignments as well as their orientation. We use this method to analyze the spatial distribution of 416 vents, largely of Pleistocene age, in the Pinacate volcanic field, Sonora, Mexico, just east of the northern end of the Gulf of California. Apart from a few sets of aligned cinder cones, the distribution of Pinacate vents appears nearly random on aerial and space photography. However, when treated statistically, old Pinacate vents exhibit structural control trending approximately N10°E throughout the field and in all its subareas. In contrast, vents with ages estimated by comparison with dated cones to be younger than about 0.4 Ma show not only the N10°E control but also N20°W and N55°W alignments significant at the 95% confidence level. The N10° alignment probably reflects the current Basin and Range horizontal stress regime in this particular area, which is atop the mantle magma source of the Pinacate lavas. The N55°W direction is related to a major regional fracture of that orientation passing through the middle of the field and possibly related to normal faults associated with opening of the adjacent Gulf of California. The distribution of vents relative to the fracture trace is consistent with magma having been guided upward along a SW dipping fault plane. The origin of the N20° W alignment is unknown but of pre‐Pleistocene heritage. We found no evidence to support control of the Pinacate vent alignments parallel to rifting or transform directions in the adjacent Gulf. Intrusion along N20°W and N55°W fractures at or since about 0.4 m.y. ago could reflect either a shift in the crustal stress field or an increase in magma pressure in Pinacate conduits that allowed magma to ascend along structures that were not parallel to the maximum horizontal compressive stress.
Shock recovery experiments on dolomite and thermodynamical calculations of impact induced decarbonation
Authors: Martinez Isabelle , Deutsch Alexander , Schärer Urs , Ildefonse Philippe , Guyot François , Agrinier Pierre
Published: February 2004

We have studied experimentally shocked dolomites and calcites by scanning electron microscopy (SEM), analytical transmission electron microscopy (ATEM), X ray diffractometry (XRD) using Rietveld refinements, and mass spectrometry analysis of the abundance ratios of the stable isotopes of carbon and oxygen. The shock‐recovery experiments have been perfomed by the multiple reverberation technique on natural dolomite rocks at 60 GPa, using steel devices and high‐explosive driver‐flyer plates as plane shock wave generators. Modified assemblies with grooves and holes were built in order to facilitate the escape of CO2, in case the conditions of breakdown of the carbonates into oxides and CO2 would be reached during the shock or postshock history of the samples. In contrast with the results from previous studies, almost no evidence for outgassing, expressed, for example, by the identification of CaO or MgO, could be observed. Consequently, no isotope fractionation occured in the shocked samples. This result is consistent with the calculations of peak‐shock and postshock temperatures, as well as with the examination of outgassing conditions of carbonates, calculated in this study up to 80 GPa. We have shown that in a direct shock, outgassing in air of nonporous dolomites and calcites should occur in impacts at 55–65 GPa and 35–45 GPa, respectively. The effect of porosity, which strongly lowers these values, has been estimated. The experimental setup for shock‐recovery experiments is shown to be an important parameter: reverberated shocks lead to lower peak‐shock and postshock temperatures than direct shocks at the same pressure; differences among experimental setups might explain part of the discrepancies between previous studies. In this study, the only surviving shock‐induced phenomenon is pulverization leading to grain sizes smaller than in the starting material. The decrease in grain size has been quantified via a structure refinement by Rietveld analysis of X ray powder patterns, which also allows the estimation of the lattice strains. A future pressure scale of shock effects in carbonates could probably be based on such parameters.
Elastic and viscoelastic properties of α iron at high temperatures
Authors: Isaak Donald G. , Masuda Koji
Published: February 2004

Recent experiments done at low driving frequencies suggest that a large degree of dispersion exists in the measured value of the shear modulus, μ, of α iron at high temperature. Discrepancies between values for μ from ultrasonic measurements and those from low‐frequency torsional measurements have been interpreted in terms of viscoelastic relaxation. However, the ultrasonic data are not in agreement with one another, and the degree of dispersion is not accurately known. We present new high‐temperature data for the elastic moduli of single‐crystal iron (α phase). The elastic moduli were measured using the rectangular parallelepiped resonance method (0.27–0.59 MHz) from room temperature to 925 K. Our data show that the difference in μ at high temperature between ultrasonic‐based measurements and low‐frequency (1 Hz) torsional measurements is only 14 GPa, rather than 29 GPa, as inferred from previous analyses. Thus the possible effects of viscoelastic relaxation are reduced but not eliminated. We find no dispersion in measurements for μ of α iron when considering frequencies ranging from 0.27 to 70 MHz and discuss the possibility that significant viscoelastic effects on measurements of μ at high temperature are limited to frequencies below 3 Hz.
The OCEAN study area: Tectonic history from magnetic anomaly data and seismic reflectivity
Authors: Henstock Timothy J. , White Robert S. , McBride John H.
Published: February 2004

The OCEAN experiment is a detailed geophysical study of a region of the Cape Verde basin. A dense network of new magnetic and gravity profiles has enabled us to constrain the spreading rate history of the region and the location of fracture zones. The main features on the gravity profiles are lineated perpendicular to the seafloor spreading magnetic anomaly lineations. Significant along‐axis variability in spreading history suggests that the Mid‐Atlantic Ridge behaved as a series of loosely coupled segments within which spreading was fundamentally asymmetric. Such variability is associated with a minor jump in the ridge axis which changes the offset and expression of one of the fracture zones. Deep seismic reflection and refraction lines were oriented parallel and perpendicular to the magnetic lineations; seismic reflections occur at all levels within the crust, decreasing in amplitude and coherence below the level of the Moho. Analysis of the subbasement reflectivity provides compelling evidence that at least two major sets of dipping structure are present and are imaged separately on the two perpendicular sets of seismic profiles. Dipping reflections on flow line (“dip”) profiles, which are interpreted as faults due to their association with offsets in the basement surface, appear to strike parallel to the paleoridge axis. The majority of reflections that may be identified as faults dip toward the west, and although basement topography suggests that east dipping faults are also present, no reflections may be interpreted unambiguously as such. East dipping reflections observed only in the middle to lower crust have a more obscure origin. Dipping reflections seen on isochron (“strike”) profiles show clear contrasts in strength, lateral coherence, depth, and dip population; a number of these strike parallel to flow lines. Comparing reflection and refraction data shows that both the layer 2/layer 3 boundary and the Moho are marked by a change in the character of reflections and suggests that they may represent important structural, as well as seismological, boundaries within the oceanic lithosphere.
Self‐induced fracture generation in zircon
Authors: Lee James K. W. , Tromp Jeroen
Published: February 2004

The mineral zircon has particular geological significance because of its use in the age determination of rocks and as a structural analogue phase for radioactive waste forms. Despite its widespread utility, however, radiation damage of the crystal structure (metamictization) causes a volume expansion of the crystal lattice and the generation of internal stresses which can induce fractures in the crystal. A model has been developed which describes the state of stress in n concentric spherical shells, each of which may have different material properties; the case of a three‐shelled composite sphere has been examined explicitly. In combination with fracture mechanics theory, the model predicts that spatially distinct radial and/or concentric fracture sets can be produced, and such fracture patterns accurately describe the distribution and types of self‐induced fractures observed in natural zircons. In general, the formation of such microfractures in zircon is a function of the degree of metamictization, shell thickness, and the confining pressure. The model predicts that the maximum depths in the Earth's crust at which radial versus concentric fractures can form in a crystal are different, suggesting that the types of fractures found in a suite of zircons from a particular rock might be potential indicators of the depth of burial. Because metamictization‐induced fractures may serve as potential pathways for the rapid leaching of various elements from the zircon crystal, this may also have important implications in interpreting the U/Pb ages of fractured zircons or in evaluating the suitability of related crystalline phases as hosts for nuclear waste.
Gravity anomalies and crustal structure at the Mesozoic Blake Spur Fracture Zone
Authors: Minshull T. A. , Morris E. , Detrick R. S.
Published: February 2004

A number of recent studies have used gravity data from mid‐ocean ridges to infer along‐axis crustal thickness variations, and hence variations in melt supply. A problem with these studies is that it is difficult to distinguish the effects of crustal thickness variations from those of crustal and mantle density variations. We analyze gravity data from the 12–13 km offset Blake Spur Fracture Zone in 140 Ma crust in the western North Atlantic. The advantage of studying this area is that mantle density variations are likely to be much smaller than those at the ridge axis, and crustal thickness variations are well constrained by expanding spread profiles (ESPs). Thus we are able to separately quantify the contributions made to the gravity anomaly by crustal thickness and crustal density variations. After removal of the gravity effects of the seabed and the oceanic basement topography, as defined by a grid of multichannel seismic reflection profiles, the gravity data show a linear 5‐mGal high along the fracture zone. After removal of the effects of crustal thickness variations defined by the ESPs, and of a basal high‐velocity layer beneath the fracture zone interpreted to be partially serpentinized upper mantle, the data show a 5‐mGal gravity low at the fracture zone. An alternative interpretation of the basal layer, as a thickened Moho transition zone, also leads to a residual gravity low. These results, together with other recent results from fracture zones on the Mid‐Atlantic Ridge, suggest that crustal thickness variations at fracture zones may be consistently understimated by studies based on gravity data. The discrepancy may be explained by variations in crustal density due to enhanced fracturing in the vicinity of fracture zones, and to disproportionate thinning of oceanic layer 3 near segment boundaries.
The Mendocino triple junction: Active faults, episodic coastal emergence, and rapid uplift
Authors: Merritts Dorothy J.
Published: February 2004

A complex zone of rapid Holocene surface uplift and deformation occurs at the Mendocino triple junction, the juncture of three plate‐bounding faults: the Cascadia subduction zone, San Andreas fault, and Mendocino fault. Within this mountainous structural knot, up to 1.4 m of coastal emergence occurred during the 1992 Cape Mendocino MS 7.1 earthquake. Surveying and radiometric dating of ancient marine strandlines (<8000 years) along the trace of the southernmost Cascadia subduction zone indicate that the Holocene pattern of net surface uplift is very similar to the 1992 coseismic uplift pattern. Results of this investigation also indicate that episodic emergence occurred at least four times between about 600 and 7000 years ago and that some past events might have resulted in larger amounts of uplift (∼2.5 m) than the 1992 earthquake, perhaps during great earthquakes (M>7.5) along the Cascadia subduction zone megathrust. However, another plausible interpretation of the data is that multiple earthquakes resulting in smaller amounts of net surface uplift per event (similar to the 1992 earthquake) occurred closely spaced in time, giving the appearance in the geologic record of less frequent and larger events. Regardless of the number and timing of paleoearthquakes, the number of platforms is a minimum of the number of events that resulted in sudden, rapid uplift, because platform preservation is also a function of rising Holocene sea level. At present, rates and patterns of net surface uplift are better constrained than the timing and magnitude of paleoearthquakes. Periods of rapid Holocene emergence also are identified as far south of the area of 1992 uplift and the Mendocino fault as ∼30 km, along the unlocated San Andreas fault. These also might be associated with coseismic uplift. Based on new mapping of active faults in the region, it is proposed here that this uplift is the result of multiple discontinuous thrust and strike‐slip fault segments which distribute plate boundary deformation along a restraining bend at the termination of the San Andreas fault. This interpretation is consistent with the fact that a single, prominent, strike‐slip fault has not been identified in the Mendocino triple junction region, while several small reverse faults crossing Holocene deposits and geomorphic surfaces have been mapped and dated.
Paleoseismology in Canada: A dozen years of progress
Authors: Adams John
Published: February 2004

Canadian studies in paleoseismology have led to the publication of over 40 papers in the last 12 years, 80% of which have appeared since 1989. Considerable progress has been achieved by confirming that the last great earthquake on the Cascadia subduction zone happened about 300 years ago and by working toward a chronology of prior earthquakes from their tsunami and turbidite records. As‐yet‐sparse evidence suggests that the geological effects of large crustal earthquakes are also recorded in southwestern British Columbia, but there is as yet no chronology. In the stable craton of eastern Canada, studies of silt layers in lakes that are floored by organic sediments have shown considerable potential to provide local earthquake chronologies that can be tested against the more sporadically preserved evidence from liquefaction, landslides and other possible indicators of paleoshaking. By contrast, research using more traditional techniques, involving direct examination of faults through geomorphological studies or trenching, has been hampered by the difficulty of identifying active faults in the first place. On the basis of the past dozen years, an accelerating pace of discovery should be expected in the next decade.
Heat flux through an old (∼175 Ma) passive margin: Offshore southeastern United States
Authors: Ruppel C. , Von Herzen R. P. , Bonneville A.
Published: February 2004

New heat flow data on the southeastern United States passive margin show that measured, uncorrected flux averages 49±11.8 mW m−2 through old (∼175 Ma) oceanic crust. Nonuniform thermal gradients were measured at about half of the 114 penetrations that comprise the data set and over the full range of water depths (1900 m to 4250 m) at which data were collected. With the simplifying assumption that the nonuniform gradients were caused by a step function change in bottom water temperatures at some time before the heat flow cruises, concave down (decreasing dT/dz with depth) and concave up (increasing dT/dz) sediment thermal gradients can be explained by respective average temperature decreases and increases of 0.1–0.2 K amplitude occurring 28–35 days before the measurements. Thermal gradients throughout the entire region are strongly influenced by oceanographic phenomena and locally by the presence of subsurface diapiric structures, while sediment thickness variations and lateral differences in sedimentation rate and sediment lithology appear to exercise relatively little control over thermal regimes. Mean reduced heat flow in the study area is estimated at ∼49 mW m−2 by decompacting and back stripping the 5–8 km of sediment deposited on the margin since the formation of the underlying oceanic crust. This value agrees with previous measurements made on younger crust in the Blake Ridge area but is significantly higher than the reduced heat flow value in a similar passive margin setting at the Baltimore Canyon Trough. We use the calculated average thermal gradient value and an assumption of constant conductivity to estimate temperatures of 19.5°C to 24.5°C at the bottom‐simulating reflector (BSR) on the Blake Ridge slope.
Measurement of melting temperatures of some minerals under lower mantle pressures
Authors: Shen Guoyin , Lazor Peter
Published: February 2004

Melting temperature measurements of six minerals (stishovite (SiO2), corundum(Al2O3), diopside (CaMgSi2O6), and three perovskites (MgSiO3, CaSiO3, Mg3Al2Si3O12)) at high pressures were carried out in a YAG laser‐heated diamond anvil cell with rhenium metal as an absorber of the laser light. A polished or compressed disc of the sample was in contact with rhenium foil and heated by conduction. Melting was determined by plotting laser power/sample temperature function and looking for the thermal anomaly associated with the fusion of materials. All these solids were found to be highly refractory, requiring quite high temperatures for melting at the lower mantle pressures. The experimental melting results showed that for these minerals, melting temperatures increased with increasing pressure. Our results at low pressures are consistent with the data determined by other techniques (piston‐cylinder, multianvils). The high‐pressure melting of MgSiO3 perovskite agreed with the recent measurements by Zerr and Boehler (1993) within experimental uncertainties. Melting temperatures and melting slopes of CaSiO3 and Mg3Al2Si3O12 perovskites were found to be less than those of MgSiO3 perovskite, indicating that the presence of Ca and Al would decrease the melting temperatures of MgSiO3 perovskite in the Earth's lower mantle and that this effect will increase with increasing pressures. Melting temperature measurements on stishovite and corundum to pressures of 36 GPa and 25 GPa, respectively, are reported.
Gravity, heat flow, and seismic constraints on the processes of crustal extension: Northern margin of the South China Sea
Authors: Nissen Susan Spangler , Hayes Dennis E. , Bochu Yao , Zeng Weijun , Chen Yongqin , Nu Xiaupin
Published: February 2004

Multichannel seismic data and gravity data have been used to construct crustal thickness profiles for three transects (eastern, central, western) across the rifted northern margin of the South China Sea. The present‐day crustal configuration of the margin is then interpreted by modeling the effects of two end‐member classes of extension processes, pure shear and simple shear. The applicability of each of these processes to the extension of the south China margin has been evaluated by comparing model predictions of subsidence and heat flow with observations across the margin. Neither of these end‐member models satisfactorily fits the observed data on the eastern and central transects across the south China margin when typical values for standard input parameters are used; the resulting heat flow is significantly underestimated by both models. In the case of a pure shear model, heat flow observations may be matched either by assuming an uncommonly thin initial steady state lithospheric thickness (∼60 km) or by assuming an unusually large crustal radiogenic heat production within the original, unextended continental crust. A perhaps more reasonable alternative scenario presumes the existence of an initially slightly thinner than “normal” steady state lithosphere (thicknesses of ∼90–100 km) in conjunction with a significant amount of upper crustal radiogenic heat production. Such heat production could be accommodated by the presence of Cretaceous granitic bodies (hypothesized) within the basement beneath the south China margin. In the case of a simple shear model, however, the observed high heat flow on the rifted south China margin may only be matched if the steady state lithospheric thickness is assumed to be uncommonly thin (∼60 km). Because the observed geophysical data characterizing lithospheric extension may be matched using more realistic input parameters in the pure shear case, pure shear extension is preferred over simple shear extension as the dominant mechanism for explaining the large‐scale rifting of the south China margin. For extension within the crust, however, combinations of both processes are not only possible, but probable, given published seismic evidence for through‐going crustal faults on the south China margin.
Global patterns of radiated seismic energy and apparent stress
Authors: Choy George L. , Boatwright John L.
Published: February 2004

Radiated energies from shallow earthquakes with magnitudes ≥5.8 that occurred between 1986 and 1991 are used to examine global patterns of energy release and apparent stress. In contrast to traditional methods which have relied upon empirical formulas, these energies are computed through direct spectral analysis of broadband seismic waveforms. Velocity‐squared spectra of body waves are integrated after they have been corrected for effects arising from depth phases, frequency‐dependent attenuation, and focal mechanism. The least squares regression fit of energy Es to surface wave magnitude Ms for a global set of 397 earthquakes yields log Es = 4.4 + 1.5Ms, which implies that the Gutenberg‐Richter relationship overestimates the energies of earthquakes. The least squares fit between Es and seismic moment M0 is given by the relationship Es = 1.6 × 10−5 M0, which yields 0.47 MPa as the average global value of apparent stress. However, the regression lines of both Es–Ms and Es–M0 yield poor empirical predictors for the actual energy radiated by any given earthquake; the scatter of data is more than an order of magnitude about each of the regression lines. On the other hand, global variations between Es and Mo, while large, are not random. When subsets of Es–M0 are plotted as a function of seismic region and faulting type, the scatter in data is substantially reduced. The Es–M0 fits for many seismic regions and tectonic environments are very distinctive, and a characteristic apparent stress τc can be derived. The lowest apparent stresses (<1.5 MPa) are associated with thrust earthquakes at subduction zones. The highest apparent stresses (>3.0 MPa) are associated with strike‐slip earthquakes that occur at oceanic ridge‐ridge transforms and in intraplate environments seaward of island arcs. Intermediate values of apparent stress (1.5 < τa < 3.0 MPa) are associated with strike‐slip earthquakes at incipient or transitional plate boundaries. In general, the dominant mode of failure for a tectonic environment is associated with the faulting type that has the lowest apparent stress. An energy magnitude ME can complement moment magnitude Mw in describing the size of an earthquake. ME, being derived from velocity power spectra, is a measure of seismic potential for damage. Mw, being derived from the low‐frequency asymptote of displacement spectra, is more physically related to the final static displacement of an earthquake. When earthquake size is ranked by moment, a list of the largest events is dominated by earthquakes with thrust mechanisms. When earthquake size is ranked by energy, the list of the largest events is dominated by strike‐slip earthquakes.
Crustal structure of the Labrador Sea conjugate margin and implications for the formation of nonvolcanic continental margins
Authors: Chian Doping , Louden Keith E. , Reid Ian
Published: February 2004

Wide‐angle seismic studies have determined the detailed velocity structure along a 350‐km‐long profile across the Labrador margin. Combination of this model with a previously published cross section for the southwestern Greenland margin constitutes the first combined conjugate margin study based on seismic velocity structure. The results indicate three distinct zones across the Labrador margin, similar to the structure of the conjugate Greenland margin. Zone 1 represents 27 to 30‐km‐thick continental crust thinning gradually seaward over ∼100 km distance. Farther seaward, zone 2 is 70–80 km wide, characterized by a distinct lower crust, 4–5 km thick, in which velocity increases with depth from 6.4 to 7.7 km/s. Interpretation for this lower crustal block favors an origin by serpentinized peridotite rather than by magmatic under‐plating. Zone 3 represents two‐layered, normal oceanic crust. The cross sections from both margins are reconstructed to an early drift stage at Chron 27. This demonstrates that the serpentinites in zone 2 are symmetrically distributed between previous identifications of Chrons 31 and 33 on both margins. Zone 1 shows a marked asymmetry, with a gradual thinning of continental crust off Labrador contrasted with a rapid thinning off Greenland. The abundant serpentinization of upper mantle peridotite in zone 2 and the asymmetric shape of zone 1 are both probably related to a very slow rate of continental rifting which produced little if any melt.
Crustal velocity field near the big bend of California's San Andreas Fault
Authors: Snay Richard A. , Cline Michael W. , Philipp C. Randolph , Jackson David D. , Feng Yanjie , Shen Zheng‐Kang , Lisowski Michael
Published: February 2004

We use geodetic data spanning the 1920–1992 interval to estimate the horizontal velocity field near the big bend segment of California's San Andreas fault (SAF). More specifically, we estimate a horizontal velocity vector for each node of a two‐dimensional grid that has a 15‐min‐by‐15‐min mesh and that extends between latitudes 34.0°N and 36.0°N and longitudes 117.5°W and 120.5°W. For this estimation process, we apply bilinear interpolation to transfer crustal deformation information from geodetic sites to the grid nodes. The data include over a half century of triangulation measurements, over two decades of repeated electronic distance measurements, a decade of repeated very long baseline interferometry measurements, and several years of Global Positioning System measurements. Magnitudes for our estimated velocity vectors have formal standard errors ranging from 0.7 to 6.8 mm/yr. Our derived velocity field shows that (1) relative motion associated with the SAF exceeds 30 mm/yr and is distributed on the Earth's surface across a band (>100 km wide) that is roughly centered on this fault; (2) when velocities are expressed relative to a fixed North America plate, the motion within our primary study region has a mean orientation of N44°W ± 2° and the surface trace of the SAF is congruent in shape to nearby contours of constant speed yet this trace is oriented between 5° and 10° counterclockwise relative to these contours; and (3) large strain rates (shear rates > 150 nrad/yr and/or areal dilatation rates < −150 nstr/yr) exist near the Garlock fault, near the White Wolf fault, and in the Ventura basin.
Earthquake locations using single‐station deep borehole recordings: Implications for microseismicity on the San Andreas fault in southern California
Authors: Abercrombie Rachel E.
Published: February 2004

We have located and estimated source parameters of 109 earthquakes (0 to 5 M) using seismograms recorded at 2.5 km depth in the Cajon Pass borehole, southern California. The borehole is about 4 km from the San Andreas fault, at the boundary between the locked, almost aseismic Mojave and San Bernardino segments, where the San Jacinto fault approaches the San Andreas. This area is of interest both on account of its tectonic complexity and its high potential seismic hazard. The clear, relatively unattenuated downhole recordings are rotated to determine the incoming azimuth of the P wave, and the delay time between the P and S arrivals is used to estimate the hypocentral distance. The difference between the hypocenters located this way and the Southern California Seismic Network (SCSN) locations of the same earthquakes increases with hypocentral distance from the borehole. Of the earthquakes within 20 km of the borehole, 95% are within 1–2 km of the SCSN epicenters and all are within 5 km of the SCSN depths. All but three of the 58 earthquakes located here which the SCSN did not record were within 20 km of the borehole and so the location errors are similar to those of SCSN A and B quality events. Most of the small earthquakes occurred within the relatively active San Jacinto fault zone, but at least eight are located close to or within the San Andreas fault zone. The stress drops for these events range from 0.1 to 18 MPa. These earthquakes appear similar to the other events in this study suggesting that the San Andreas fault is similar to other faults at the scale of these small earthquakes. The variation in stress drop and slip orientation of these small earthquakes suggests that larger scale heterogeneity continues to these small scales. Also, the location of these small earthquakes in major established fault zones implies that earthquake source dimensions are not geometrically controlled by fault zone width.
SHELLS: A thin‐shell program for modeling neotectonics of regional or global lithosphere with faults
Authors: Kong Xianghong , Bird Peter
Published: February 2004

The thin‐plate method of modeling neotectonics uses isostasy and vertical integration of lithospheric strength to reduce three‐dimensional problems to two dimensions. We introduce new thin‐shell continuum elements and fault elements which satisfy the completeness and compatibility requirements, and permit extension of these methods to the lithospheres of spherical planets. Even a coarse grid of these elements with elastic rheology can reproduce low‐order toroidal free oscillations. In the program SHELLS, a realistic frictional/dislocation‐creep rheology is handled by iteration; this method converges to solutions which can be numerically tested to confirm that they satisfy both local equilibrium and the balance of torques on whole plates. Because this code can incorporate both natural plate shapes (with internal faults) and realistic rheologies, it yields models that are readily tested by their predictions of geodetic velocities, stresses, and fault slip rates.
Constraints on oceanic lithosphere structure from deep‐focus regional receiver function inversions
Authors: Zhang J. , Langston C. A.
Published: February 2004

We extend teleseismic P wave receiver function techniques to include regional P waveforms from deep‐focus earthquakes and document a new data source for the study of oceanic lithosphere structure. High microseismic noise in oceanic environments strongly contaminates waveform recordings for moderate teleseisms. The use of high‐amplitude deep‐focus regional body waves significantly improves signal‐to‐noise ratio. Deep sources can generate body waves that propagate as direct waves or turning waves at regional distances which have ray parameters similar to the teleseismic body waves. The amplitudes of these regional body waves may be 10 to 20 times that of the teleseismic body waves for constant magnitude and same radiation pattern effects simply due to large differences in the geometrical spreading and attenuation along the propagation path. To demonstrate the feasibility of utilizing deep‐focus regional waveform data in studying oceanic lithosphere structure, we examine body wave recordings collected at the seismic station GUMO (Guam, Mariana Islands). The oceanic lithosphere structure under the island is resolved using a jumping inversion approach that includes a singular‐value decomposition procedure along with the Tikhonov regularization. The thickness of the crust appears to be about 15.0 km. The shallow structure has large positive velocity gradients, and the Moho is a broad crust‐mantle transition zone. Velocity complexity occurs in the middle crust.
Dynamics of explosive degassing of magma: Observations of fragmenting two‐phase flows
Authors: Mader H. M. , Phillips J. C. , Sparks R. S. J. , Sturtevant B.
Published: February 2004

Liquid explosions, generated by rapid degassing of strongly supersaturated liquids, have been investigated in the laboratory with a view to understanding the basic physical processes operating during bubble nucleation and growth and the subsequent behavior of the expanding two‐phase flow. Experiments are carried out in a shock tube and are monitored by high‐speed photography and pressure transducers. Theoretical CO2 supersaturations up to 455 times the ambient saturation concentration are generated by a chemical reaction; K2CO3 solution is suddenly injected into an excess of HCl solution in such a way as to mix the two solutions rapidly. Immediately after the injection event, a bubble nucleation delay of a few milliseconds is followed by rapid nucleation and explosive expansion of CO2 bubbles forming a highly heterogeneous foam. Enhanced diffusion due to advection in the flow coupled with continuous mixing of the reactants, and hence on‐going bubble nucleation after injection, generates an increasingly accelerating flow until the reactants become depleted at peak accelerations of around 150 g and velocities of about 15 m s−1. Stretching of the accelerating two‐phase mixture enhances the mixing. Liberation of CO2 vapor is spatially inhomogeneous leading to ductile fragmentation occurring throughout the flow in regions of greatest gas release as the consequence of the collision and stretching of fluid streams. The violence of the eruptions is controlled by using different concentrations of the HCl and K2CO3 solutions, which alters the CO2 supersaturation and yield and also the efficiency of the mixing process. Peak acceleration is proportional to theoretical supersaturation. Pressure measurements at the base of the shock tube show an initial nucleation delay and a pressure pulse related to the onset of explosive bubble formation. These chemically induced explosions differ from liquid explosions created in other experiments. In explosions caused by sudden depressurization of CO2‐saturated water, the bubbles nucleate uniformly throughout the liquid in a single nucleation event. Subsequent bubble growth causes the two‐phase mixture to be accelerated upward at nearly constant accelerations. Explosively boiling liquids, in which heterogeneous nucleation is suppressed, experience an evaporation wave which propagates down into the liquid column at constant average velocity. Fragmentation occurs at the sharply defined leading edge of the wavefront. The chemical flows effectively simulate highly explosive volcanic eruptions as they are comparable in terms of flow densities, velocities, accelerations, and in the large range of scales present. The large accelerations cause strong extensional strain and longitudinal deformation. Comparable deformation rates in volcanic systems could be sufficient to approach conditions for brittle fragmentation. Tube pumice is a major component of plinian deposits and ignimbrites and preserves evidence of accelerating flow conditions.
Holocene activity of the Rose Canyon fault zone in San Diego, California
Authors: Lindvall Scott C. , Rockwell Thomas K.
Published: February 2004

The Rose Canyon fault zone in San Diego, California, has many well‐expressed geomorphic characteristics of an active strike‐slip fault, including scarps, offset and deflected drainages and channel walls, pressure ridges, a closed depression, and vegetation lineaments. Geomorphic expression of the fault zone from Mount Soledad south to Mission Bay indicates that the Mount Soledad strand is the most active. A network of trenches excavated across the Mount Soledad strand in Rose Creek demonstrate a minimum of 8.7 m of dextral slip in a distinctive early to middle Holocene gravel‐filled channel that crosses the fault zone. The gravel‐filled channel was preserved within and east of the fault but was removed west of the fault zone by erosion or possibly grading during development. Consequently, the actual displacement of the channel could be greater than 8.7 m. Radiocarbon dates on detrital charcoal recovered from the sediments beneath the channel yield a maximum calibrated age of about 8.1±0.2 kyr. The minimum amount of slip along with the maximum age yield a minimum slip rate of 1.07±0.03 mm/yr on this strand of the Rose Canyon fault zone for much of Holocene time. Other strands of the Rose Canyon fault zone, which are east and west of our site, may also have Holocene activity. Based on an analysis of the geomorphology of fault traces within the Rose Canyon fault zone, along with the results of our trenching study, we estimate the maximum likely slip rate at about 2 mm/yr and a best estimate of about 1.5 mm/yr. Stratigraphie evidence of at least three events is present during the past 8.1 kyr. The most recent surface rupture displaces the modern A horizon (topsoil), suggesting that this event probably occurred within the past 500 years. Stratigraphie and structural relationships also indicate the occurrence of a scarp‐forming event at about 8.1 kyr, prior to deposition of the gravel‐filled channel that was used as a piercing line. A third event is indicated by the presence of several fault strands that displace the channel but did not move during the most recent event. Other events may also have occurred, but these data suggest that the return time for surface‐rupturing earthquakes is no more than about 4 kyr.
Petrology of lavas from Sierra Negra volcano, Isabela Island, Galápagos archipelago
Authors: Reynolds Robert W. , Geist Dennis J.
Published: February 2004

Sierra Negra volcano is a voluminous, active basaltic shield situated in the western Galápagos archipelago, a hotspot‐related chain adjacent to the Galápagos spreading center. The volcano has erupted Fe‐rich tholeiitic basalts of very limited compositional range. Comparison of lavas from each of five stratigraphic units covering the entire volcano indicates that the lavas are mineralogically similar and compositionally restricted (4.4 to 6.9 wt % MgO). The lavas are Fe‐rich and hypersthene‐normative and plot in both the alkaline and tholeiite fields of the alkali‐silica diagram. Rare earth element (REE) patterns are steep and parallel. The major and trace element data indicate a comagmatic relationship by fractional crystallization of the observed phenocryst phases. Projections into pseudoternary phase diagrams suggest that the magmas cooled and crystallized at pressures between 1 and 3 kbar. Calculated melt densities range from 2.73 to 2.77 g/cm3, which are higher than those of typical mid‐ocean ridge basalt (MORB), Hawaiian tholeiite, and other Galápagos lavas. The high calculated melt densities may be caused by fractionation at greater depths than is ordinary for the other basaltic magmas. Suppression of plagioclase and olivine crystallization in favor of augite may eliminate the minimum density “window” that has been proposed for MORB fractionation. Sierra Negra lavas have the most radiogenic lead and strontium isotopic ratios in the western Galápagos, indicating that the magmas have a relatively large contribution of plume material and have been minimally contaminated by entrainment of MORB‐producing mantle. Magmatic 3He/4He isotopic ratios from Sierra Negra are approximately 15 times the atmospheric ratio; although these ratios clearly indicate plume helium, they are not the highest 3He/4He in the archipelago, suggesting that helium is decoupled from the heavier isotopes. High Sm/Yb ratios, light rare earth element (LREE) enrichment, and a steep REE slope are consistent with an origin by moderate extents (5–15 %) of partial melting of a garnet‐lherzolite source with REE characteristics that are between chondrite and depleted Earth mantle sources. Sierra Negra lavas are compositionally monotonous because homogeneous plume‐rich material has experienced long‐term exposure to a well‐regulated sublithospheric thermal environment with additional compositional restrictions imposed in the lithosphere by high magma flux.