bluuebunny/crossref_metadata_2025 huggingface dataset:
Localized polymorphic phase transformations in high‐pressure faults and applications to the physical mechanism of deep earthquakes
Authors: Kirby Stephen H.
Published: February 2008

Earthquake mechanisms based on frictional instabilities are widely accepted for relatively shallow earthquakes. Such mechanisms, in unmodified form, are highly unlikely for deep earthquakes because of the expected high frictional shear stresses at depths of 300–700 km. Several alternative mechanisms have been proposed. Here, I discuss a new type of shear instability observed in experiments on ordinary polycrystalline ice and polycrystalline tremolite by my colleagues and me. These shear instabilities lead to rapid stress drops and sound emissions and have the following characteristics: (1) A single sharp shear zone forms in the maximum shear stress orientation. (2) Confining pressure does not suppress the instability and, under some conditions, raising confining pressure Pc lowers the differential stress σƒ at which it occurs. (3) Stress drops can occur repeatedly after reloading at about the same differential stress. (4) The mean stress vs. temperature conditions at which the instabilities occur in ice and tremolite place them in the fields of thermodynamic stability of ice II and diopside + talc, respectively. Although I have not as yet confirmed the existence of these high pressure phases in the shear zones, transformation in the shear zone is implicated because such shear instabilities do not occur in similar materials in which polymorphic phase changes do not occur. Reconstructive polymorphic phase changes localized along shear zones may be a general phenomenon in crystalline materials stressed non‐hydrostatically near transition boundaries. Moreover, by analogy, the reconstructive phase transformations that are thought to occur in deeply subducted lithosphere may lead to localized shear instabilities manifested by deep earthquakes and produce the observed double‐couple first‐motion distribution. Finally, if this is the earthquake source mechanism in deeply subducted lithosphere, then earthquakes should shut off when all of the polymorphic phase changes have occurred. This may explain the maximum depth of earthquakes at about 680 km.
Stress in the contorted Nazca Plate beneath southern Peru from local earthquakes
Authors: Schneider John F. , Sacks I. Selwyn
Published: February 2008

We study earthquake focal mechanisms in a region of highly contorted subducting lithosphere to identify dominant sources of stress in the subduction process. We observe a stress pattern in the contorted Nazca plate beneath southern Peru from an analysis of hypocentral trend and focal mechanisms of intermediate‐depth earthquakes. Expanding on previous studies, we examine the hypocentral trend using 1673 of 2178 well‐located local events from the nine‐station Arequipa network. The dip of the plate beneath southern Peru averages 25°–30° from 25‐ to 100‐km depth. Below this depth there is an 80‐ to 100‐km‐wide contortion between a zone of increasing dip (convex) to the southeast and a flat lying (concave) zone to the northwest. Using more than 6000 P wave first motions of events deeper than 50 km, we derive stress orientations from a moving average of composite focal mechanisms across a 200 by 350 km region including the contortion. The in‐plate distribution of tension (T) and compression (P) axes reveals a coherent stress pattern. The trend is most clear beneath south‐central Peru (NW section) and below 100‐ km depth in southernmost Peru (SE section). Both T and P axes tend to be dominantly in plate, especially below 100‐km depth. T axes orient toward the contortion in a fan‐shaped trend, which suggests that the deepest part of the seismic zone, within the convex SE section, is sinking and pulling the more buoyant NW section. We conclude that from 50‐ to 200‐km depth, slab‐pull forces are dominant in the observed stress. Our results suggest that a significant amount of plate extension occurs in this region of intermediate‐depth subduction.
Tomographic Inversion for Shear Velocity Beneath the North American Plate
Authors: Grand Stephen P.
Published: February 2008

A tomographic back projection scheme has been applied to S and SS travel times to invert for shear velocity below the North American plate. The data range in distance from 8° to 80°, and a total of 3923 arrival times were used. First arrivals were measured directly off the seismograms, while the arrival times of later arrivals were found by a waveform correlation technique using synthetic seismograms. The starting model was laterally heterogeneous in the upper 400 km to account for the first‐order differences in ray paths already known. The model was divided into blocks with horizontal dimensions of 500 km by 500 km and varying vertical thicknesses. Good resolution was obtained for structure from just below the crust to about 1700 km depth in the mantle. In the upper mantle a high‐velocity root was found directly beneath the Canadian shield to about 400 km depth with the Superior province having the highest velocity and deepest root. The east coast of the United States was found to have intermediate velocities from 100 to 350 km depth and the western United States the slowest velocities at these depths. Below 400 km depth the most significant structure found is a slab‐shaped high‐velocity anomaly from the eastern Carribean to the northern United States. Beneath the Carribean this anomaly is almost vertical and extends from about 700 km to 1700 km depth. Further to the north, the anomaly dips to the east with high velocities at 700 km depth in the central United States and high velocities below 1100 km depth beneath the east coast. The anomaly is about 1% in magnitude. This lower‐mantle anomaly may be associated with past subduction of the Farallon plate beneath North America.
A test of alternative Caribbean Plate relative motion models
Authors: Stein Seth , DeMets Charles , Gordon Richard G. , Brodholt John , Argus Don , Engeln Joseph F. , Lundgren Paul , Stein Carol , Wiens Douglas A. , Woods Dale F.
Published: February 2008

Two contrasting models have been proposed to describe the present‐day motions between the Caribbean Plate and neighboring plates. One model, based on both North America‐Caribbean and Cocos‐Caribbean data, assumes that North America‐Caribbean motion is reflected by the spreading rate (approximately 2 cm/yr) inferred from magnetic anomalies at the Cayman Spreading Center and the azimuths of nearby transforms (Jordan, 1975). This geometry was used by Minster and Jordan (1978) in deriving global plate motion model RM2. The other model, based only on North America‐Caribbean data, uses rates and azimuths inferred from the geometry of the Lesser Antilles Wadati‐Benioff zone (Sykes et al., 1982). The Cayman Spreading Center data were discounted assuming that they underestimate the full North America‐Caribbean motion owing to the complex tectonics of the Greater Antilles. The latter model assumes plate convergence at the Lesser Antilles at a rate about twice as fast as, and an azimuth differing by 25° from, the Jordan model. The two models also differ in their predictions of South America‐Caribbean motion; the Jordan model predicts oblique convergence whereas Sykes et al. predict oblique divergence. We use the NUVEL‐1 global relative motion data set, which incorporates recent data not used in earlier studies, to discriminate between the alternative models. We find that the Jordan geometry provides a better fit to the direction of North America‐Caribbean motion. Moreover, we find that models based on this geometry better fit the direction of Cocos‐Caribbean motion observed in earthquake slip vectors along the Middle America Trench. Since the prediction of this direction depends strongly on the rate of North America‐Caribbean motion, we conclude that the Jordan model better describes Caribbean Plate motions. The discrepancy between the two models results from the different methods of estimating plate motions. Sykes et al. (1982) report that they determined a Caribbean‐North America convergence rate of approximately 4 cm/yr from the length of the Lesser Antilles Wadati‐Benioff zone; we employ a similar procedure and obtain a rate closer to 2 cm/yr. We also find that the method they report using to determine the convergence direction is unable to distinguish between the alternative azimuths. We thus conclude that estimates of the rate and direction of plate motion from Wadati‐Benioff zone configurations are insufficiently robust and that such estimates are better derived using rates from magnetic anomalies and azimuths from transform faults and slip vectors. We further conclude that successful estimates of relative motion require internal consistency between the Euler vectors of all relevant plates, as demonstrated by the powerful Cocos‐Caribbean constraint not incorporated in the Sykes et al. model.
Heat flow in the Witwatersrand Basin and environs and its significance for the South African Shield Geotherm and lithosphere thickness
Authors: Jones M. Q. W.
Published: February 2008

The late Archaean‐early Proterozoic Witwatersrand basin is situated in the center of the Kaapvaal craton, South Africa. Two new heat flow determinations and 71 new heat flow estimates permit excellent definition of the heat flow pattern. Nonvertical heat conduction explains much of the observed local heat flow variability. Regionally, the heat flow in the basin is uniform, and the overall mean (51±6 mW m−2) is higher than the mean heat flow for Archaean cratons. Published data indicate that heat generated in the basin's strata (∼7.5 mW m−2) is largely responsible for the excess heat flow; the subsediment heat flux is ∼43.5 mW m−2. Seven new heat flow and heat production determinations in Archaean granite domes northwest of the basin average 33±2 mW m−2 and 2.3±0.8 μW m−3, respectively. Contrasted with the heat flux across the floor of the basin, the lower heat flow in the domes probably reflects a smaller radiogenic contribution from the crystalline crust below the domes. Published heat production data from upturned granitic basement exposed in the Vredefort dome in the center of the basin indicate that the radioactive contribution from the crust below the basin is ∼26.5 m W m−2. Subtracting this value and the contribution from the strata (∼7.5 mW m−2) from the basin's surface heat flow (51 mW m−2) indicates that the heat flux through the Moho is approximately 17 mW m−2. Calculated lithospheric temperatures agree closely with pressure‐temperature regimes inferred from studies of mantle inclusions in kimberlites in the Kaapvaal craton. Both data sets indicate that the cratonic Lithosphere is considerably cooler and thicker than that below an equilibrium ocean. Implications of the cold, thick cratonic Lithosphere for understanding the distribution of southern African diamondiferous kimberlites, and the density and buoyancy of the Lithosphere, are discussed.
Static compression and olivine flotation in ultrabasic silicate liquid
Authors: Agee Carl B. , Walker David
Published: February 2008

Piston‐cylinder techniques have been used to observe static compressions of ultrabasic silicate liquids to 6 GPa and 2000°C. Compressions are measured by bracketing sinking and floating of olivine crystals of known density at run conditions. Komatiite, peridotite, and their liquid mixtures with Fe2SiO4 all show constant density changes with pressure of ≅ 0.07 g/cm3/GPa along their liquidi. These results confirm shock wave studies and ultrasonic studies which show weak compositional effects on compressibility of basic to ultrabasic silicate liquid. The techniques converge in supporting the conjecture of Stolper et al. (1981) that ultrabasic silicate liquids may be denser than their equilibrium coexisting olivines and pyroxenes at high compressions. The pressure of this density inversion is 7–8 GPa for the compositions studied. Olivine flotation provides an important constraint upon Earth evolution models that include extensive melting during early differentiation. It also provides a mechanism for concentrating high Mg/Si olivine in the upper mantle without unduly perturbing other geochemical parameters. A full evaluation of the physical and geochemical consequences is required at an early date.
Temperature, pressure, and composition of hydrothermal fluids, with their bearing on the magnitude of tectonic uplift at mid‐ocean ridges, inferred from fluid inclusions in oceanic layer 3 rocks
Authors: Vanko David A.
Published: February 2008

Quartz‐bearing veins in metagabbroic rocks dredged from the Mathematician Ridge, east Pacific, contain abundant fluid inclusions. Heating and freezing data on nearly 400 inclusions from seven samples allow determination of the temperatures, pressures, and fluid compositions in the subseafloor hydrothermal system at the time of quartz growth. Coexisting dense halite‐saturated inclusions and low‐density, low‐salinity vapor‐rich inclusions (average 45 and 2 wt % NaCl equivalent, respectively) attest to an episode of phase separation in some samples. The phase separation occurred at temperatures of about 600°–700°C and pressures of 60–100 MPa (600–1000 bars). The fact that samples that formed at 60–100 MPa are now exposed on the seafloor, where ambient hydrostatic pressure is only 30–35 MPa, suggests that the samples have been tectonically uplifted of the order of 3 km. The fluids could originally have been part of a deep axial hydrothermal circulation cell, or alternatively, they could have been formed in a deep convection cell underlying the off‐axis edges of a magma chamber. Fluids are NaCl‐CaCl2 brines with molar Na: Ca of 4–8. This range of molar Na: Ca is very close to that of the inferred hydrothermal end‐member from various active black smokers, to the measured ratios from basalt‐seawater interaction experiments, and to the ratio calculated during numerical basalt‐seawater interaction calculations. Crushing experiments indicate little or no compressible gas within the fluids. Fluid inclusions in albite suggest trapping temperatures of around 410°–500°C. Those in epidote may have been trapped at around 500°C and 110 MPa (1.1 kbar) pressure, or around 3 km beneath the Mathematician Ridge seafloor.
Postshock temperatures in silica
Authors: Boslough M. B.
Published: February 2008

Experimental postshock temperatures for crystalline quartz released from shock states between 86 and 127 GPa and for fused silica released from 59 to 73 GPa have been determined using previously measured spectral radiance data. The temperatures range from 3660 to 4150 K and are consistent with the interpretation based on shock temperature measurements, that the Hugoniot of SiO2 crosses the phase boundary between stishovite and liquid. The new postshock temperatures were used to determine the Grüneisen parameter of liquid silica at high pressures and temperatures and the specific volume of the release state (γ/V = 3.5 Mg/m3 and Vr = 0.266 m3/Mg, respectively), although there is some ambiguity as to whether this volume corresponds to a partial or total release state. To show the validity of these postshock temperatures, a radiative transport model was extended to times after free surface arrival of the shock wave.
A geophysical study of the Earth's crust in central Virginia: Implications for Appalachian crustal structure
Authors: Pratt Thomas L. , Çoruh Cahit , Costain John K. , Glover Lynn
Published: February 2008

A regional seismic reflection line (I‐64) across the Virginia Piedmont has provided a stacked section suitable for an integrated interpretation of geophysical data in the region. A highly reflective upper crust, an allochthonous Blue Ridge Province, underlying thrust sheets including the Blue Ridge master decollement, and a basal decollement at a depth of about 9 km (3 s) are confirmed on the seismic data. Immediately east of the Blue Ridge Province, Appalachian structures plunge to as much as 12 km (4 s) depth. The Evington Group, Hardware terrane, and Chopawamsic metavolcanic rocks (Carolina terrane) crop out in the Piedmont Province, and numerous eastward dipping reflections originate from these rocks in the subsurface. These eastward dipping reflectors overlie a gently west dipping (10°–15°), highly reflective zone that varies in depth from 1.5 s (4.5 km) beneath the Goochland terrane to 4 s (12 km) beneath the rocks of the Evington Group. Some of the overlying eastward dipping reflections apparently root in this zone. The zone may include decollement surfaces along which the overlying rocks were transported. Relatively few reflections originate from within autochthonous Grenville basement at the western end of the profile. The Goochland granulite terrane is interpreted to be a westward thrust nappe structure that has overridden a portion of the Chopawamsic metavolcanic rocks. A broad zone of east dipping (20°–45°) reflections bounds the Goochland terrane on the east. These reflections may originate from deformation zones and continue to Moho depths. They appear to be correlative with similar events seen on other Appalachian lines. The pervasiveness of the zone of east dipping events on other seismic reflection lines and the continuity of the adjacent Piedmont gravity high suggest continuity of crustal features along the length of the Appalachians. A major conclusion of this study is that crustal thinning is responsible for the main components of the gravity field in Virginia, that is, the Appalachian gravity gradient and the Piedmont gravity high. The crust thins from about 52 km beneath the Appalachian mountains to about 35 km beneath Richmond, Virginia, and then rethickens by up to 10 km beneath the zone of east dipping reflections (mylonites?) east of Richmond. The I‐64 seismic data also contain a sequence of reflections at about 9–12 s, indicative of lower crustal layering; the base of this zone of reflections coincides almost exactly with the Mohorovicic discontinuity interpreted from earlier refraction work. The layering extends about 70 km west from Richmond, Virginia, and is interpreted as a lower crustal transition zone that is believed to persist across most of Virginia.
Relationships between mineralogical, chemical, and isotopic properties of some North American kimberlites
Authors: Alibert Chantal , Albarede Francis
Published: February 2008

Late Precambrian to Cretaceous kimberlites from North America have initial Sr and Nd isotopic compositions (εSr0 = −4 to 37, εNd0 = −4 to 6) which form a trend falling roughly within the mantle array. Two groups can be defined on the basis of mineralogical and geochemical characteristics: group A encompasses most of the northeast American kimberlites whose rare earth element (REE) concentrations are moderately fractionated, relative to chondrites. Nd isotopic ratios (εNd0 = 3 to 6) fall within the field of common ocean island basalts. They are also distinctive by the presence of phlogopite phenocrysts and high‐temperature garnet peridotite xenocrysts. Group B includes kimberlites from the Colorado‐Wyoming, Kansas and British Columbia areas and one sample from Pennsylvania. They lack the above quoted peridotite suite and abundant phlogopite. Their light REE concentrations show a strong enrichment, relative to chondrites, correlated with relatively unradiogenic Nd (εNd0 = −4 to 2). Kimberlites of both groups have radiogenic Pb (206Pb/204Pb = 18.17–19.41) and a wide range in 207Pb/204Pb. In both cases, melts were probably generated in the sublithospheric mantle, but group B melts interacted with an enriched component, resulting mainly from magmatic processes, and probably located within the deep lithosphere. By contrast, the Prairie Creek lamproite (Arkansas), as other ultrapotassic rocks, received its distinctive geochemical properties from a crustal component present in the mantle source.
Onset of mantle plumes in the presence of preexisting convection
Authors: Sleep Norman H. , Richards Mark A. , Hager Bradford H.
Published: February 2008

The onset of plumes where broad‐scale convection already exists is investigated along with the thermal history of the earth. Geological evidence relating to the onset time of mantle plumes is not definitive: no well‐defined hotspot tracks older than Mesozoic have been identified, but it is difficult to tell whether this absence is a reliable indication of a late onset of mantle plumes, since only continental crust of this age exists. Using the hotspot track distribution for the Pacific, the absence of Pre‐Mesozoic hotspot tracks on the Canadian Shield cannot easily be attributed to chance. However, young continental hotspot tracks are controversial, and it is not clear whether tracks such as the Monteregian Hills/White Mountain Magma Series would be recognized as such without considering the contributing evidence of the adjacent oceanic crust and its well‐defined seamount chains. Our simple models yield potentially testable predictions: the instability of plume formation in a system where broad‐scale convection already exists depends on horizontal conduit flow in the basal boundary layer toward the plume source. Geographically, plumes are most likely to form where the boundary layer is thickest and where broad‐scale ascending flow already exists. Parameterized convection indicates that the heat transport by conduit flow in the boundary layer toward plumes depends linearly on the ratio of effective viscosity for broad‐scale convection to effective boundary layer viscosity. In terms of material properties, this ratio is the ratio of viscosity across the boundary layer to about the 0.45 power for the case where the broad‐scale convection is heated from below. The viscosity contrast across the boundary layer depends on the temperature contrast across the layer. A late onset of mantle plumes is implied if the temperature contrast at the base of the mantle has increased with time, that is, if the core has cooled more slowly than the interior of the mantel. In one scenario, before about 3 b.y. B.P., the mantle temperature varied slowly because surface heat flow was nearly balanced by internal radioactivity. During this time the mantle was hotter than present, and its internal viscosity lower. Hie temperature contrast at the base of the mantel was thus small. After 3 b.y., internal radioactivity no longer balanced surface heat flow, and the mantle cooled about a few hundred degrees Celsius, as indicated by petrological observations. The core cooled slower than the mantle, and the temperature contrast across the bottom boundary layer increased with time, leading to a late onset of plumes. Phenomenological inferences are obtained, as Venus may have cooled more rapidly than the Earth. Its mantle may be too cool for rapid plate tectonics, and the gravity field and the convective pattern are dominated by hot upwelling regions.
Determination of in situ stress orientation from borehole guided waves
Authors: Barton Colleen A. , Zoback Mark D.
Published: February 2008

We have analyzed the horizontal particle motion of borehole guided waves from three‐component vertical seismic profile data recorded from three experiments. The polarization directions of the guided waves are consistent with the direction of horizontal principal stress determined independently. We suggest that stress‐induced velocity anisotropy in the rock adjacent to the borehole appears to cause antisymmetric flexural guided modes to be polarized in the direction of the maximum horizontal principal stress. Thus analysis of the horizontal particle motion of borehole guided waves appears to represent a new method for determining the orientation of horizontal principal stresses.
Modeling the occurrence of overpressures on the Scotian Shelf, offshore eastern Canada
Authors: Mudford Brett S.
Published: February 2008

Many wells on the Scotian Shelf, offshore from Nova Scotia, have drilled into overpressured zones. In particular, the wells comprising the Venture gas field have all encountered overpressures at depths below 4.5 km. The characteristics of the overpressure zones in the Venture field vary in some important details from overpressured zones on the U.S. Gulf Coast. In the Venture field the shales in the overpressured zones are observed to be normally compacted and interbedded with layers of overpressured sandstones. This is in contrast to the Gulf Coast, where the shales in the overpressure zone are undercompacted. Resistivity logs, sonic logs, and bulk density logs all provide some indications of overpressures in the Venture field but are not as unequivocal indicators of overpressure as they are on the Gulf Coast. Theoretical investigations of the causes of overpressures in an environment such as the Venture field show that overpressures are unlikely to be caused by loading of sediments. The most probable cause of overpressures is recent and continuing fluid generation, which is related to the transformation of kerogen to hydrocarbons. Limits are placed on the average permeabilities and fluid generation rates which are required if overpressures are to be produced by fluid generation.
Volcano monitoring using short wavelength infrared data from satellites
Authors: Rothery D. A. , Francis P. W. , Wood C. A.
Published: February 2008

Data from remote sensing satellites operating in the short‐wavelength infrared can be used to measure temperatures of about 150°C and above. The gain settings of the Landsat Thematic Mapper (TM) sensors are such that to record pixel‐integrated temperatures from 150°C to over 1000°C requires the use of several spectral bands. If the radiant source occupies less than a whole pixel, as is the case for fumaroles or a crusted lava surface with incandescent cracks, then the pixel‐integrated temperature is less than the actual surface temperature of the hot areas and appears to be different at each wavelength. By determining pixel‐integrated temperature in two spectral bands we can estimate both the temperature and size of hot areas which occupy less than a complete pixel. In some cases our observations provide the only available data on the state of activity of a particular volcano; in others they support and refine field observations. Our Landsat TM data reveal otherwise unobserved precursor and subsequent activity related to the September 1986 eruption of Lascar volcano (Chile) and supplement other data on the March 1986 eruption of Augustine volcano (Alaska). They also indicate the continued presence of lava lakes at Erta 'Ale (Ethiopia) in the absence of any ground‐based observations and document minor eruptive activity at Mount Erebus (Antarctica). Landsat Multispectral Scanner data have been used to measure the variation in temperature along a lava flow on Sierra Negra (Galapagos Islands) which occurred in 1979.
A model computation of moisture content in polythermal glaciers
Authors: Hutter Kolumban , Blatter Heinz , Funk Martin
Published: February 2008

Measurements in boreholes and internal radar horizons confirm the existence of polythermal structures in polar and subpolar glaciers and ice sheets. The occurrence of an intraglacial phase boundary makes it necessary to consider two layers and transition conditions between cold and temperate ice. The governing field equations and constitutive relations for a two‐layer model are stated with emphasis on boundary and transition conditions. A reduced plane flow model is introduced by using dimensionless variables and performing a scale analysis. According to the stated aim of this paper a parallel‐sided slab model is suitable for rough analyses of borehole measurements. Its governing equations are deduced, and their analytical and numerical treatment is discussed. Sensitivity tests reveal a strong dependence of moisture content profiles on the numerical value of the water diffusivity coefficient and especially on strain heating.
Compositional, Order/Disorder, and Stable Isotope Characteristics of Al‐Fe Epidote, State 2–14 Drill Hole, Salton Sea Geothermal System
Authors: Bird D. K. , Cho M. , Janik C. J. , Liou J. G. , Caruso L. J.
Published: February 2008

Epidote (Ca2Fe3Si3O12(OH)‐Ca2Al3Si3O12(OH)) is a common hydrothermal mineral in metasediments and veins at depths >900 m in the State 2–14 drill hole of the Salton Sea geothermal system. The mole fraction of Ca2Fe3Si3O12(OH) in epidotes (Xps) from this drill hole ranges from 0.11 to 0.42, and complex compositional zoning of octahedral Fe3+and Al is typical within single grains. With increasing depth there is an overall, but irregular, decrease in the Fe3+content of epidotes in metasandstones. In most samples, vein epidotes are more Fe3+‐rich and exhibit a wider compositional range than metasandstone epidotes from the same depth. Octahedral Fe3+in the M(1) sites in four epidotes, evaluated by57Fe Mössbauer spectroscopy, ranges from 7.5 ± 1.7% to 11.4 ± 1.5% of the total iron. The most ordered epidote formed in veins at 2618 m in the biotite zone. This epidote has an ordering parameter (σ = 1–2XFe3+,M(1)) of 0.85 ± 0.03 which corresponds to a calculated state of equilibrium order/disorder at ∼390° ± 60°C, in close agreement with the probable downhole temperature of ∼340°C, Two epidote samples from the chlorite + calcite zone (1420 m, ∼265°C; 1867 m, ∼300°C) are more disordered, corresponding to calculated states of equilibrium order/disorder of >450°C, These data suggest that the analyzed epidotes from the chlorite + calcite zone are in a metastable state of substitutional order/disorder. The measured value of δDepidotefrom 1420 m is −96‰, which is 5–6‰ lighter than epidotes from 1867 m (δDepidote= −90‰), 2227 m (δDepidote= −91‰), and 2618 m (δDepidote= −90 ‰). When compared to the reservoir fluid from the December 1985 flow test, the derived fractionation of hydrogen isotopes between epidote and the geothermal brine is −19‰ at 1867 m and ∼340°C. This value is in accord with published experimental hydrogen isotope fractionations between iron‐rich epidote and aqueous electrolyte solutions. Therefore, considering the complexity of the Salton Sea brine, the isotopic temperature is similar to the inferred downhole temperature.
THE KINETICS OF RADIOISOTOPE EXCHANGE BETWEEN BRINE AND ROCK IN A GEOTHERMAL SYSTEM
Authors: Hammond Douglas E. , Zukin Jeffrey G. , Ku Teh‐Lung
Published: February 2008

A wide range of isotopes in the 238U, 235U, and 232Th decay chains was measured in geothermal brines collected from two production zones at 1898 and 3220 m in the Salton Sea Scientific Drilling Project well. High concentrations of radium, radon, and lead isotopes are generated and maintained by the input of these isotopes from solid phases into brine by both recoil and leaching processes, by the high chloride content of the brine which complexes radium and lead, and by the apparent absence of suitable unoccupied adsorption sites. In contrast, uranium, thorium, actinium, bismuth, and polonium isotopes all have low concentrations due to their efficient sorption from brine to rock. Measurements of short‐lived isotopes in these decay series yield insights regarding the mechanisms controlling radioisotope exchange, and they permit estimation of rates of brine‐rock interaction. For example, the 228Ac/228Ra activity ratio of 0.2 in brines indicates that the mean residence time of actinium in solution before sorption onto solid surfaces is less than 2.5 hours. If molecular diffusion to fracture walls limits the rate of actinium sorption, the maximum width of the larger fractures in which sampled brine resides is 1–2 cm. However, the mean width of fractures must be only 1–2 µm, in order to account for the recoil input of 223Ra. The ratios of radium isotopes in the brine provide information about the mechanisms of recoil and leaching in transferring radium from rock to brine. Brine/rock concentration ratios [(dpm/g)b/(dpm/g)r] of radium isotopes increase with increasing half‐life, so that 223Ra (11 days) = 0.05–0.09, 228Ra (5.8 years) = 0.26–0.40, 226Ra (1600 years) = 0.45–0.97. Two mechanisms could explain this dependence on half‐life: (1) input of radium by the alpha recoil process occurs in microfractures and pore spaces, but the rate of diffusion down microfractures is so slow that it diminishes the effect of recoil input of the shorter‐lived isotopes to the larger fractures where the brine sampled resides, or (2) weathering and leaching of radium from solid phases occurs on time scales comparable to the half‐lives of 228Ra and 226Ra. The second mechanism is more likely because the 212Pb/224Ra activity ratios are within 20% of the value predicted by a simple recoil model with negligible adsorption, indicating that recoil input of 212Pb (10.6‐hour half‐life) is not limited by diffusion. This model permits the rates of chemical exchange between brine and rock to be estimated. These rates indicate the mean residence times of lead and radium in solution are about 30 years, and the residence time of radium in associated solid phases is about 2000 years. The estimated uncertainty in these residence times is about a factor of 2.
SEISMIC COUPLING AND OUTER RISE EARTHQUAKES
Authors: Christensen Douglas H. , Ruff Larry J.
Published: February 2008

Variation of interplate seismic coupling at subduction zones is a major factor controlling the size of the largest underthrusting events. This variation also has a profound effect on the regional intraplate stresses in the vicinity of the subduction zone. Outer rise seismicity is strongly correlated with variations in interplate coupling, reflecting the stress state of the interplate coupled zone. Over 200 outer rise earthquakes with known focal mechanisms are used to investigate the relationship between stresses in the outer rise and interplate seismic coupling. These events occur within the downgoing (i.e., oceanic) plate near the bathymetric trench axes and generally fall into the categories of tensional (normal) or compressional (thrust) with their tensional or compressional stress axes oriented approximately horizontal and perpendicular to the trench. In uncoupled subduction zones, only tensional outer rise earthquakes occur, which indicates that the outer rise is dominated by tensional stresses associated with plate bending and/or slab pull forces. In strongly coupled subduction zones, both tensional and compressional outer rise events are found. These events are related both spatially and temporally to the distribution of large underthrusting earthquakes and are thus an integral part of the earthquake cycle. In the strongly coupled regions, tensional outer rise events follow large underthrusting events as the outer rise is temporarily in tension due to the underthrusting motion. Compressional outer rise events take place as compressional stress slowly accumulates oceanward of locked sections of the interplate zone. In four instances, compressional outer rise earthquakes have been followed by large underthrusting events which have occurred 2, 4, 7, and 19 years after the associated outer rise event. The remaining compressional outer rise events are located in regions that are either known seismic gaps or in regions where the seismic potential is unknown. The occurrence of compressional outer rise earthquakes suggests that compressional stress is accumulating in the adjacent interplate region and that there is the potential for a future large underthrusting event in the region. Thirty compressional outer rise events have been located in trench segments of Middle and South America, the Kurile Islands, the Tonga and Kermadec islands, the New Hebrides Arc, and the Solomon Islands regions. In both the southern Kamchatka and northern New Hebrides regions the outer rise seismicity indicates that the stress regimes in the outer rise have changed with time from tensional, following a previous large underthrusting event, to compressional at present. Thus three stages of the cycle from underthrusting to tensional outer rise regime to compressional outer rise regime are present, requiring only the occurrence of the next underthrusting event to complete the cycle. The occurrence of compressional outer rise events is useful for assessing the seismic potential of a region on an intermediate time scale.
Stochastic Modeling of Seafloor Morphology: Inversion of Sea Beam Data for Second‐Order Statistics
Authors: Goff John A. , Jordan Thomas H.
Published: February 2008

At scale lengths less than 100 km or so, statistical descriptions of seafloor morphology can be usefully employed to characterize ridge crest processes, off‐ridge tectonics and vulcanism, sedimentation, and postdepositional transport. We seek to develop methods for the estimation of seafloor statistics that take into account the finite precision, resolution, and sampling obtained by actual echo sounding systems. In this initial paper we restrict our attention to the problem of recovering second‐order statistics from data sets collected by multibeam devices such as Sea Beam. The seafloor is modeled as a stationary, zero‐mean, Gaussian random field completely specified by its two‐point covariance function. We introduce an anisotropic two‐point covariance function that has five free parameters describing the amplitude, orientation, characteristic wave numbers, and Hausdorff (fractal) dimension of seafloor topography. We formulate the general forward problem relating this model to the statistics of an ideal multibeam echo sounder, in particular the along‐track autocovariance functions of individual beams and the cross‐covariance functions between beams of arbitrary separation. Using these second moments as data functionals, we then pose the inverse problem of estimating the seafloor parameters from realistic, noisy data sets with finite sampling and beamwidth, and we solve this inverse problem by an iterative, linearized, least squares method. The inversion method is applied to Sea Beam transit data from both the Pacific and Atlantic oceans. Sea Beam system noise stands out as a sharp spike on the along‐track autocovariance function and can be modeled as a white noise process whose amplitude generally increases with beam angle. The five parameters in our second‐order model can be estimated from the inversion of data sets comprising ∼100–200 km of track length. In general, the cross‐track wave number is the most poorly determined, although uncertainties in the assumed Sea Beam response may bias the values of the fractal dimension. Using the assumed beamwidth, the measured noise values, and the seafloor parameters recovered from the inversion, we generate Sea Beam “synthetics” whose statistical character can be directly compared with raw Sea Beam data. For most of the track segments we have processed thus far the synthetics are similar to the data. In the case of one Atlantic profile, however, the comparison clearly indicates the necessity of incorporating higher‐order statistics. The space domain procedures described in this paper can be extended for this purpose.
On the State of Lithospheric Stress in the Absence of Applied Tectonic Forces
Authors: McGarr A.
Published: February 2008

Numerous published analyses of the nontectonic state of stress are based on Hooke's law and the boundary condition of zero horizontal deformation. This approach has been used to determine the gravitational stress state as well as the effects of processes such as erosion and temperature changes on the state of lithospheric stress. The major disadvantage of these analyses involves the assumption of lateral constraint which seems unrealistic in view of the observational fact that the crust can deform horizontally in response to applied loads. If the same problems are addressed by assuming that the remote stress state is constant, instead of the condition of zero horizontal deformation, then the resulting stress states are entirely different and in good accord with observations. In the absence of applied tectonic forces the only likely gravitational stress states are those for which all three principal stresses are nearly equal. To the contrary, the gravitational stress states developed on the basis of the lateral constraint assumption can be ruled out. The processes of erosion and sedimentation have slight tendencies to increase and decrease, respectively, the state of deviatoric stress. In particular, for initial stress states in the range of slightly extensional to compressional, erosion has the effect of enhancing the ratio of average horizontal to vertical stress, which may explain, at least in part, the common observation of high near‐surface horizontal stresses. Temperature changes have only minor effects on the stress state, as averaged over the thickness of the lithosphere.
Heat Flow in the Utah‐Wyoming Thrust Belt From Analysis of Bottom‐Hole Temperature Data Measured in Oil and Gas Wells
Authors: Deming Dave , Chapman David S.
Published: February 2008

Bottom‐hole temperature (BHT) data from oil and gas wells represent a large, albeit low‐quality, data base for heat flow determinations. A method for analyzing BHT data sets that directly addresses the problem of noise in the data is presented and illustrated by application to 303 BHTs from the Anschutz Ranch (AR), Cave Creek (CC), and Anschutz Ranch East (ARE) oil/gas fields in the Utah‐Wyoming thrust belt, located in the Middle Rocky Mountain physiographic province of the western United States. Correction of raw BHT data by means of a Horner plot yields 81 corrected BHTs. An additional 90 BHTs, not suitable for a Horner correction, are corrected by means of a new BHT correction which may be applied to any single BHT for which a depth and time of measurement are known. The average geothermal gradient at both AR‐CC and ARE is found to be 23°C/km, although an average‐gradient analysis fails to explain scatter in the data. Analysis of corrected BHTs, through inversion of a layered‐earth model, yields estimates of formation thermal gradients and temperature fields at AR‐CC and ARE that are stable and unique, with an average precision of 2° to 3°C. Thermal conductivity measurements were made on 360 drill cutting samples representing 14 thrust belt formations; in situ conductivity is estimated using porosity from calibrated well logs. Heat flow is estimated by comparing temperature calculated by a two‐dimensional finite element model with temperature estimated from analysis of BHT data. Analysis of BHT data from the AR‐CC area yields an estimated regional heat flow of 60 mW/m2 (± 8 mW/m2); a similar analysis at ARE results in an estimate of 58 mW/m2 (± 8 mW/m2). These estimates are close to values determined for the Wyoming Basin and the central Colorado Plateau and imply that a thermal transition from the Middle Rocky Mountains to elevated heat flow characteristic of the Basin and Range province (80 to 90 mW/m2) must take place within 50 km.
A more precise chronology of earthquakes produced by the San Andreas Fault in southern California
Authors: Sieh Kerry , Stuiver Minze , Brillinger David
Published: February 2008

Improved methods of radiocarbon analysis have enabled us to date more precisely the earthquake ruptures of the San Andreas fault that are recorded in the sediments at Pallett Creek. Previous dates of these events had 95% confidence errors of 50–100 calendar years. New error limits are less than 23 calendar years for all but two of the dated events. This greater precision is due to larger sample size, longer counting time, lower background noise levels, more precise conversion of radiocarbon ages to calendric dates, and better stratigraphic constraints and statistical techniques. The new date ranges, with one exception, fall within the broader ranges estimated previously, but our estimate of the average interval between the latest 10 episodes of faulting is now about 132 years. Variability about the mean interval is much greater than was suspected previously. Five of the nine intervals are shorter than a century; three of the remaining four intervals are about two to three centuries long. Despite the wide range of these intervals, a pattern in the occurrence of large earthquakes at Pallett Creek is apparent in the new data. The past 10 earthquakes occur in four clusters, each of which consists of two or three events. Earthquakes within the clusters are separated by periods of several decades, but the clusters are separated by dormant periods of two to three centuries. This pattern may reflect important mechanical aspects of the fault's behavior. If this pattern continues into the future, the current period of dormancy will probably be greater than two centuries. This would mean that the section of the fault represented by the Pallett Creek site is currently in the middle of one of its longer periods of repose between clusters, and sections of the fault farther to the southeast are much more likely to produce the next great earthquake in California. The greater precision of dates now available for large earthquakes recorded at the Pallett Creek site enables speculative correlation of events between paleoseismic sites along the southern half of the San Andreas fault. A history of great earthquakes with overlapping rupture zones along the Mojave section of the fault remains one of the more attractive possibilities.
A large tsunamogenic landslide and debris flow along the Peru Trench
Authors: von Huene Roland , Bourgois Jacques , Miller John , Pautot Guy
Published: February 2008

Along the continental margin of northern Peru, Sea Beam bathymetry and seismic reflection records reveal features from mass wasting in the middle and lower slope areas. A curved scarp cuts the middle slope and marks a slip surface seaward of which a 20 by 33 km block was displaced 800 m and back rotated 5° as it moved downslope. The front of that block is itself marked by a 1‐km‐high curved scar where the block failed and created a ∼30‐km debris flow avalanche. The avalanche morphology of closed highs and lows without directional fabric covers the lower slope and trench axis seaward of the detached block. If the slip was catastrophic, a local ∼50‐m‐high tsunami was generated. This example documents the type of slope failure commonly inferred as a source of destructive tsunamis. Since the block was detached along faults that displace beds 3–5 km deep, it represents more than a superficial slide where sediment was locally oversteepened. The detached block includes rocks that were part of the continental margin since at least Eocene time in front of which a 15‐km‐wide accretionary complex subsequently developed. Apparently, only low levels of horizontal compression could be transmitted into the upper plate because of weak coupling across the Benioff zone. This may have permitted detachment and mass movement of the block despite the long history of plate convergence here. Tsunamogenic slides and mass wasting at trench depths are difficult to detect without modern high‐resolution techniques.
Role of crustal flexure in initiation of low‐angle normal faults and implications for structural evolution of the basin and range province
Authors: Spencer Jon E. , Chase Clement G.
Published: February 2008

The mechanics of low‐angle normal faulting has proved difficult to establish despite clear geological evidence for the existence of such faults. Gently dipping shear stress trajectories are necessary for nucleation and propagation of low‐angle faults in a structurally isotropic medium. Such trajectories are not produced by simple horizontal extension, but do arise in the presence of flexural stresses. We model stress conditions in a cross section through the strong upper crust using Airy stress functions for a two‐dimensional elastic medium. Numerical modeling indicates that high ratios of flexural stress to extensional stress produce conditions favorable for low‐angle normal fault initiation at intermediate depths in an elastic medium. We propose that the gentle dip of Cenozoic low‐angle normal faults in the Basin and Range Province resulted from flexural stresses that were produced by isostatically uncompensated surface and, especially, Moho relief. Resurgent mid‐Cenozoic magmatism and the consequent reduction of the thermally dependent flexural strength of the mantle lithosphere transferred flexural stresses to the upper crust, causing or promoting initiation of low‐angle normal faults. Temporal changes in structural style, from dominantly low‐angle to dominantly high‐angle normal faulting, may reflect reduction of the magnitude of flexural stresses in the upper crust due to flattening of the Moho and cooling of the mantle lithosphere.