bluuebunny/crossref_metadata_2025 huggingface dataset:
A model for deformation in Long Valley, California, 1980–1983
Authors: Rundle John B. , Whitcomb James H.
Published: February 2008

Geodetic data collected in Long Valley, California, from 1975 through 1983 define a pattern of uplift and strain which is evidently associated with a sequence of earthquakes occurring in May 1980 and subsequent swarm activity continuing until the present. We have constructed a model to explain the deformation observed since May 1980 in terms of inflation of two subsurface magma chambers, faulting in the south moat region of the caldera, and slip on the Hilton Creek fault. The most significant new feature of the model is the shallow magma chamber at 5 km depth, located a few hundred meters to the east of the Casa Diablo hot spring area. Inflation of this chamber causes stresses which show consistency with various aspects of the seismicity in the south moat of the caldera. Calculations of stress across vertical planes over the magma chamber can be used together with failure criteria to estimate the inflation volume at which the rock layers intervening between the chamber and the surface will fail by extensional fracture.
An initial analysis of the Earth's crust under the Williston Basin: 979 Cocrust Experiment
Authors: Hajnal Z. , Fowler C. M. R. , Mereu R. F. , Kanasewich E. R. , Cumming G. L. , Green A. G. , Mair A.
Published: February 2008

Four 300‐km‐long refraction profiles from western Manitoba and eastern Saskatchewan were shot over the Canadian portion of the Williston basin. On the basis of three distinct seismic velocity intervals, 5.95–6.50, 6.60–6.75, and 7.00–7.61 km/s, the crust in the Nelson Front and the Churchill tectonic province can be vertically separated into three distinct zones. The crust thickens from east to west, reaching a maximum of more than 50 km in the central portion of the basin. The upper mantle velocity increases from 7.84 km/s in the east to 8.47 km/s in central Saskatchewan. Along the eastern flank of the basin, in the transition zone between the Churchill and Superior tectonic provinces, extensive structural variations are interpreted as major faults which appear to disturb the entire crust. This thick high‐velocity crust beneath the Williston basin indicates that it is not of extensional origin.
A hotspot model for Iceland and the Vøring Plateau
Authors: Vink Gregory E.
Published: February 2008

The proximity of a hotspot to a spreading center may result in the channeling of atmosphere to the section of rise crest closest to the hotspot. This produces more basalt and thicker crust at these locations, thus forming a plateau over time. The geometric constraints of such a model predict a unique orientation, location, and age progression for a plateau formed by this mechanism. The hotspot will channel material to the closest part of the rise; therefore the orientation of the plateau will differ from that of a hotspot track by the component of absolute motion perpendicular to the rise axis. The plateau will be symmetric with respect to the location of the rise axis at the time of formation. Also, the age progression of the plateau will be contemporaneous with the age of formation of the seafloor on either side because the plateau is seafloor, just with thicker crust. A set of reconstructions based upon magnetic isochrones and a fixed hotspot reference frame is presented for the Norwegian‐Greenland Sea as a means of evaluating the model's predictions. By locating the Iceland hotspot, reconstructing the relative positions of the Greenland and European plates, and then assuming material would be channeled from the hotspot to the closest section of the rise crest, we can trace the tectonic evolution of the Greenland‐Faeroe and Vøring plateaus. The model is able to locate the plateaus, explain their orientations, and predict an age progression that satisfies observed age determinations. The analysis demonstrates that both plateaus could have been formed by the Iceland hotspot with the Greenland‐Faeroe Plateau being in effect a continuation of the Vøring Plateau, which was cut off due to transform motion between the northern and southern spreading centers.
Evolution of oceanic lithosphere: A driving force study of the Nazca Plate
Authors: Richardson Randall M. , Cox B. Lea
Published: February 2008

The role of the evolution of oceanic lithosphere in driving oceanic plates is tested with elastic and viscous finite element modeling of the Nazca plate. Results indicate that ridge‐push forces, when distributed over that portion of the oceanic lithosphere giving rise to the force, predict a stress state that varies more spatially than does previous modeling with ridge forces concentrated at the ridge crest. In particular, the component of stress normal to the ridge vanishes near the ridge for the new modeling. Predicted stresses for distributed ridge force models are still in qualitative agreement with the limited data on Nazca intraplate stress. Stress states have also been calculated for a variety of slab‐pull models, which include possible dependence on age, rate, and angle of subduction. The predicted stresses vary substantially near the trench among these models, with less variation farther from the trench. Combinations of ridge‐push and slab‐pull models are consistent with the earlier result of ah upper limit of about a factor of 2 for net slab‐pull over ridge‐push. Simple viscous modeling with only distributed ridge forces predicts relative motion between the Nazca and South American plates that is in fairly poor agreement with the observed direction. The fit to the observed direction could probably be improved by either including slab forces or the effect of the largely transform boundary with the Antarctic plate.
Nucleation, crystal growth and the thermal regime of cooling magmas
Authors: Brandeis Geneviève , Jaupart Claude , Allègre Claude J.
Published: February 2008

Crystallization at the margin of a quiet cooling magma has been studied numerically, taking into account the kinetics of crystallization. The variables are the latent heat value, the growth and nucleation functions, the initial magma temperature, and the thermal contrast between magma and country rock. We have investigated a wide range of values for these parameters corresponding to natural conditions. We show that after a highly transient stage, crystallization tends toward an equilibrium between heat production (latent heat release) and heat loss. Given the small diffusivity of country rocks, latent heat release is the main factor controlling the temperature evolution. In order to minimize the latent heat release, crystallization occurs at a temperature where nucleation is small. This can be close to either the liquidus or the solidus, depending on the initial conditions. The main process controlling crystallization is nucleation and not crystal growth. Nucleation occurs as a series of sharp pulses followed by longer periods of crystal growth. The nucleation pulses give birth to thermal oscillations. These oscillations can be sustained if the interior magma temperature is above the liquidus independently of the heat loss mechanism. We show that the phenomenon occurs on the scale of a few centimeters which corresponds to the inch‐scale layering of many ultrabasic complexes. The model allows us to calculate crystal sizes which are in good agreement with geological observations. The crucial parameters which determine crystal size variations near the margins of igneous bodies are the initial thermal conditions as well as the nucleation and growth functions. In the main cooling regime close to the liquidus, significant size variations can be created by small thermal disturbances.
An inhomogeneous fault model for gaps, asperities, barriers, and seismicity migration
Authors: Rundle John B. , Kanamori Hiroo , McNally Karen C.
Published: February 2008

We develop a model for a fault in which various areas of the fault plane have different stress‐slip constitutive laws. The model is conceptually simple, involving nonlinear algebraic equations which can easily be solved by a graphical method of successive iterations. Application is made to the problem of explaining seismicity patterns associated with great earthquakes. The model quantitatively explains phenomena associated with seismic gaps, asperities, and barriers.
The volcanoseismic swarms of 1981–1983 in the Tahiti‐Mehetia Area, French Polynesia
Authors: Talandier Jacques , Okal Emile A.
Published: February 2008

During the years 1981–1983, three intense seismic swarms took place in the Tahiti‐Mehetia area of French Polynesia at the presumed location of the Society Island hot spot. The 1981 swarm featured 4000 earthquakes, with a maximum magnitude ML = 4.3, in the immediate vicinity of the island of Mehetia; the 1982 swarm occurred along the flank of the major Teahitia seamount, and involved more than 9000 events; a second swarm occurred at Teahitia in 1983 and involved 3000 events. Although no precise constraint can be placed on the depth of individual events from their travel times to Polynesian stations, features in the evolution of the Mehetia swarm are generally consistent with the probable ascent of a magma body toward the surface. In the case of Teahitia, the recording of abundant tremors of both high and low frequency, particularly intense during the 1983 swarm, is directly similar to cases of documented volcanic eruptions. The swarms are interpreted as episodes of active volcanism, part of the process of building the next island in the chain.

Appendix tables are available with the entire article on micro‐fiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document B84‐005; $2.50. Payment must accompany order.
Source mechanisms of earthquakes near mid‐ocean ridges from body waveform Inversion: Implications for the early evolution of oceanic lithosphere
Authors: Bergman Eric A. , Solomon Sean C.
Published: February 2008

To investigate the early tectonic evolution of oceanic lithosphere, we present a synthesis of the characteristics of near‐ridge earthquakes, i.e., earthquakes which occur off the axis of major mid‐ocean spreading centers but in lithosphere less than 35 m.y. old. Near‐ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems, with the exception of the central Indian Ocean and the eastern part of the Cocos plate, which have experienced a high rate of earthquake activity during that time. For 32 near‐ridge earthquakes we determined the double‐couple orientation, seismic moment, centroid depth, and source time function, using a formal inversion technique based on matching synthetic and observed P and SH waveforms. All types of faulting styles are observed. Seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m.y., and occurs almost entirely in the uppermost mantle, from just below the Moho to depths at which the temperature estimated from standard thermal models exceeds 800°C. Thrust faulting is not observed at depths greater than 10 km below the seafloor. The deepest near‐ridge earthquakes are generally characterized by normal‐faulting focal mechanisms, with the T axis oblique or perpendicular to the local spreading direction; most of these earthquakes occur in the central Indian Ocean. The transition from mechanisms characteristic of ridge axes to those characteristic of the near‐ridge environment occurs within the first few million years. For lithosphere less than 35 m.y. old, horizontal compressive stress associated with the cooling and subsidence of the lithosphere (“ridge push”) does not appear to be the dominant source of stress released in earthquakes; this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere. This bound, together with the observed depth of faulting, implies that the strain rate associated with seismic deformation is locally at least 10−15 s−1. We propose that thermoelastic stresses associated with plate cooling play an important role in near‐ridge earthquakes. This source of stress can account for the mechanisms and depths of many near‐ridge earthquakes and for the concentration of activity in very young lithosphere. The comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the Moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis. The higher level of near‐ridge seismicity in the Indian Ocean may be the result of modification or enhancement of the more typical near‐ridge stress state by such local processes as the continental collision between India and Asia or active secondary convection beneath the young lithosphere of the Indian plate.
Uplift across Long Valley Caldera, California
Authors: Castle Robert O. , Estrem John E. , Savage James C.
Published: February 2008

The results of levelings along U.S. Highway 395 between Lee Vining and Toms Place, California, in 1957, 1975, 1980, 1982, and 1983 define the development of a dome‐shaped uplift across the Long Valley caldera. These levelings indicate that Toms Place remained virtually invariant with respect to Lee Vining during the full interval 1957–1983, implying that the end points of the survey route lie beyond the range of the developing dome. No significant uplift occurred before 1975, but by October 1980 the uplift near Casa Diablo Hot Springs reached 0.25 m and the half width of the dome was of the order of 25 km. The uplift of Casa Diablo Hot Springs increased to 0.35 m by August 1982 and to about 0.40 m by August 1983 without any appreciable increase in the lateral extent of the dome. The August 1982 to August 1983 uplift apparently occurred before February 1983 with little or no uplift during the period February–August 1983. The 1975–1983 uplift is attributed to the injection of 0.19 km3 of magma into a spherical chamber at a depth of 10 km beneath the southern quadrant of the ancient resurgent dome within the caldera. Examination of the full vertical control record through the Long Valley area indicates that the 1975–1983 uplift is unique within the period 1905–1983. Although comparisons of the results of a 1932 leveling against a 1914 datum disclose a 1914–1932 down‐to‐the‐northwest tilt of about 10 µrad over the reach between Toms Place and Lee Vining, this tilt (if real) is more reasonably attributed to regional tectonic deformation than it is to the movement of magma at depth.
Petrology and chemistry of hyperferroan anorthosites and other clasts from lunar meteorite ALHA81005
Authors: Goodrich Cyrena Anne , Taylor G. Jeffrey , Keil Klaus , Boynton William V. , Hill Dolores H.
Published: February 2008

Thirteen previously undescribed clasts from lunar meteorite Allan Hills A81005 include one that is a cataclastic, “hyperferroan” noritic anorthosite. Mafic minerals in this clast (low‐Ca pyroxene with mg′ 52 and olivine with Fo 41) have very low mg compared to most lunar anorthosites described before the discovery of 81005. Siderophile element concentrations are low and indicate that the clast is compositionally pristine. Rare earth elements (REE) are 2.5–10 times more abundant and slightly more fractionated than in previously described ferroan anorthosites. Another probably pristine clast is a granulitic gabbroic norite that has low‐Ca pyroxene with mg′ 53. Hyperferroan anorthosites are a significant component of 81005. Extreme iron enrichment and high REE abundances indicate that these rocks formed late in the evolution of the magma from which the ferroan anorthosite suite formed. Plagioclase was not enriched in Na even at these late stages. Another clast is a cataclastic ferroan anorthosite (Fo 66) that might be pristine. Of the remaining 10 clasts, nine are granular to cataclastic rocks (primarily anorthositic troctolites or troctolitic anorthosites), and two are impact melt breccias, all of which probably represent mixtures of pristine rock types. The former have REE that are essentially unfractionated relative to chondrites and range in abundance from 1.5–3×Cl, and the latter are similar to bulk 81005 (REE 5–8×Cl). Siderophile element abundances are high and indicate the presence of a significant meteoritic component. Our data indicate that 81005 has only a small KREEP (potassium, rare earth elements, phosphorus) component. This is consistent with previous results.
Experimental methods for quenching structures in lunar‐analog silicate melts: Variations as a function of quench media and composition
Authors: Dyar M. Darby
Published: February 2008

Compositions analogous to lunar green, orange, and brown glasses were synthesized under consistent conditions, then quenched into a variety of different media when the samples were removed from the furnace. Iron valence and coordination are a direct function of quench media used, spanning the range from brine/ice (most effective quench), water, butyl phthalate, silicone oil, liquid nitrogen, highly reducing CO‐CO2 gas, to air (least efficient quench). In the green and brown glasses, Fe3+ in four‐ and six‐fold coordination is observed in the slowest‐quenched samples; Fe2+ coordination varies directly with quench efficiency. Less pronounced changes were observed in the Ti‐rich orange glass. Therefore the remote‐sensed spectrum of a glass‐bearing regolith on the moon may be influenced by the process by which the glass cooled, and extreme caution must be used when comparing spectra of synthetic glass analogs with real lunar glasses.
The polymict eucrites
Authors: Delaney J. S. , Prinz M. , Takeda H.
Published: February 2008

The polymict eucrites are a suite of about 14 polymict achondritic meteorites that contain at least 90% (vol) of eucritic material and are found both in Antarctica and elsewhere. They are part of a continuum of polymict breccias that includes howardites and polymict diogenites. Rare gas data suggest that some are gas‐rich regolith breccias. The clasts contained in polymict eucrites sample a variety of lithologies from their parent body and include eucritic clasts, alkali‐enriched eucrites, pyroxene‐cumulate eucrites, two‐pyroxene mafic rocks, possibly feldspar‐cumulate eucrites, diogenites, fayalitic lithologies, and a variety of other mineral clasts. The eucritic clasts are probably samples of near surface lithologies on the parent body, but the presence of two‐pyroxene mafic rocks is considered to be evidence of magmatic activity at pressures in excess of 1 kbar in the parent body. Partial melt models may explain some of the diversity among the clasts, but cumulus processes are believed to have been important. If pressures greater than 1 kbar are confirmed, then the basaltic achondrite parent body (BAP) must have a diameter in excess of 200 km. The variety of lithologies suggests that not all basaltic achondrites are samples of igneous rocks formed by single‐stage melting of a chondritic asteroid but are instead samples of an evolved and diverse crust produced by serial magmatism.
History of the Apollo 15 yellow impact glass and sample 15426 and 15427
Authors: Spangler R. R. , Delano J. W.
Published: February 2008

Individual pieces of the Apollo 15 yellow impact glass from 15426 and 15427 were studied using the laser microprobe to determine their 39Ar‐40Ar age and their 38Ar‐37Ar exposure age. Except for the extractions from one fragment, the age determinations required significant correction for trapped 40Ar. Data from multiple extractions on each fragment were used to make individual 40Ar/36Arsw versus 39ArK/36Arsw plots from which the apparent age and trapped component for each fragment was determined. An age of 3.35±0.05 AE was determined for the age of the impact event that formed these glasses. This age cannot be reconciled with the age prediction of Delano et al. [1982]. Since these glasses were probably produced by an impact into a target of moderate‐TiO2 basalts, and the only known moderate‐TiO2 basaltic flows are younger than the impact event [Boyce and Johnson, 1978], the target for these glasses must be covered up by later basaltic flows. The average 38Ar‐37Ar exposure age for these glasses was 274±74 m.y. Other glasses from clods 15426 and 15427 give similar exposure ages [Spangler et al. 1984]. Two possible reasons for this similarity is that (1) the clods formed at greater than 300 m.y. ago so that all glasses in 15426 and 15427 shared a common exposure history, and (2) the gardening at Station 7 produced a soil with an average exposure age of ∼300 m.y.
Petrologic comparisons of Cayley and Descartes on the basis of Apollo 16 soils from stations 4 and 11
Authors: Basu Abhijit , McKay David S.
Published: February 2008

Petrologic aspects of the Cayley and Descartes formations are reviewed in the light of new data on Apollo 16 soils. Specific comparison of the modal abundances of lithic fragments in drive tube sample 64001/2 from the slopes of Stone Mountain (station 4) and in soil 67941 from the North Ray Crater rim (station 11) shows that melt rocks, especially poikilitic rocks, are more abundant at station 4 than at station 11; the reverse is true for fragmental breccias. Such lithologic differences suggest that stations 4 and 11 do not belong to the same geologic formation. Metamorphosed breccias are pervasive in both the formations and may represent a local component that has been reworked and diluted as fresh materials were added. Lithologic compositions inferred from the study of soil samples are different from lithologic compositions inferred from the study of rake samples or breccia clasts. This difference may be related to a mixing of material of different grain size distributions. The petrology of soils at the Apollo 16 site may not accurately reflect original material associated with either the Descartes or the Cayley formation because of extensive mixing with local material.
On the production of cosmogenic nuclides in meteorites by primary galactic particles: Cross sections and model calculations
Authors: Michel Rolf , Stück Reinhard
Published: February 2008

Cross sections for the proton‐induced production of radionuclides (40≦A≦60) from natural titanium, iron, and nickel were measured for proton energies from 80 to 200 MeV. To critically survey the theoretical predictions of unknown excitation functions, the experimental data were compared with calculations according to Blann's hybrid model of nuclear reactions and to the semiempirical formulas of Rudstam and of Silberberg and Tsao. Then, based mainly on the new excitation functions, the production rates of cosmogenic radionuclides in meteorites by primary galactic protons were calculated. Since the interaction of primary solar and galactic particles with meteorites can be described with accuracy, the model calculations allow for a quantitative estimate of the action of secondary particles in meteorites. A method of completely describing the production of cosmogenic nuclides in meteorites that takes into account all sources of production is proposed.
Late Cenozoic tectonics of the Southern Chile Trench
Authors: Cande Steven C. , Leslie Rob B.
Published: February 2008

Marine geophysical data from off‐shore of southern Chile are used to define the interaction between the Chile Ridge and Chile Trench during late Cenozoic time. We identify three distinct ridge‐trench collision events. Between 14 and 10 Ma a 700‐km‐long, nearly continuous section of the Chile Ridge was sub‐ducted between 55°S and 48°S. Shorter sections of the ridge, offset by large transform faults, were subducted at 6 and 3 Ma between 48°S and 47°S. At the present‐day triple junction, the subduction of the ridge has a strong influence on the Chile Trench. In this region the landward trench slope has undergone a recent episode of subduction‐driven tectonic erosion: the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward. Evidence exists for late Neogene and Quaternary uplift and plutonism on the adjacent continental margin. South of 48°S, where collision took place between 10 and 14 Ma, the effects of collision are much less pronounced. In particular, the landward trench slope does not appear to have been subjected to extensive tectonic erosion. We conclude that the configuration of the spreading centers and transform faults on the Chile Rise is the chief factor controlling ridge‐trench tectonic interaction. Tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge, offset by large transform faults, are subducted. A major sub‐marine channel leads along the trench axis south‐ward from the triple junction. This channel cuts across the outer trench rise and carries sediment westward to the Mornington Abyssal Plain. The Paleogene tectonic history of the southern Chile Trench includes a southward migrating, ridge‐trench collision involving the Farallon‐Aluk spreading center in Eocene time.
Focal depths and mechanism of Mid‐Atlantic Ridge earthquakes from body waveform inversion
Authors: Huang Paul Y. , Solomon Sean C. , Bergman Eric A. , Nabelek John L.
Published: February 2008

We have determined the source mechanisms (double‐couple orientation, moment, centroid depth, source time function) of 14 earthquakes on the northern Mid‐Atlantic Ridge (0°–72°N) from an inversion of long‐period P and SH waveforms. The earthquakes are all characterized by nearly pure normal faulting on fault planes that dip at about 45° and strike parallel to the local trend of the ridge axis. Moments range from 3 to 15×1024 dyn cm, and the source time functions are all of simple form. The P and S waveforms for all earthquakes can be well matched using conventional values for t* (1 and 4 s, respectively). These earthquakes are all very shallow; centroid depths range between 1.2 and 3.1 km beneath the seafloor. The P waves from these earthquakes show strong water column reverberations, suggesting that fault rupture extended to the seafloor. The predominant period of these reverberations constrains the water depth in the epicentral region. On the basis of estimated water depth and epicentral location, all of these earthquakes can be shown to have occurred beneath the inner floor of the median valley. The centroid depths do not show a correlation with either spreading rate or seismic moment. Under the assumption that the centroid depth marks the mean depth of fault slip, earthquake faulting extended to depths of 2–6 km for these events.
A numerical model of earthquake‐induced piezomagnetic anomalies
Authors: Zlotnicki J. , Cornet F. H.
Published: February 2008

A numerical model for evaluating the order of magnitude of earthquake‐induced piezomagnetic anomalies is proposed. The stress field associated with normal or reverse faulting with a 10‐MPa mean stress drop is determined by mean of displacement discontinuity method. The induced piezomagnetic tensor is then used to predict the associated magnetic field variations observed at ground surface. The model shows that for earthquakes occurring in homogeneous magnetic masses with piezomagnetic properties of andesite rocks, the associated anomalies are of the order of 0.2 nT for faults with a 5‐km dip slip and 2 nT when the dip slip is 20 km long. The variation of magnetic field for the case of a piezomagnetic dyke intruding a nonmagnetic body is found to be much greater: for a 4‐km‐thick dyke parallel to the fault and located 4 km from it, the observed ground surface magnetic field variation is of the order of 10 nT. These results outline that induced piezomagnetic effects strongly depend on the stress drop and stress distribution along and outside the fault, the location of the earthquake source with respect to the piezomagnetic body, and the piezomagnetic properties of the rock.
Pore fluid and seismogenic characteristics of fault rock at depth on the Wasatch Fault, Utah
Authors: Parry W. T. , Bruhn R. L.
Published: February 2008

Fluid inclusions and structural fabric in fault rock in the exhumed footwall of the Wasatch fault allow measurement of fluid pressure, temperature, and composition and estimation of rheological characteristics of fault behavior. Hydrothermally altered and deformed Oligocene quartz monzonite of the Little Cottonwood stock forms a partially preserved footwall carapace at the southern end of the Salt Lake segment of the Wasatch normal fault zone. Cataclasite and phyllonite contain two syndeformational alteration mineral assemblages that formed during progressive faulting and displacement of the footwall. An early epidote‐chlorite‐sericite‐magnetite alteration assemblage developed in phyllonite and cataclasite, and a later laumontite‐prehnite‐hematite alteration assemblage developed during cataclasis. Fluid inclusions associated with the formation of alteration minerals are preserved in healed fractures in quartz grains. Fluid inclusions that record the transition temperature from quasi‐plastic flow to frictional deformation are associated with hydrothermal alteration to chlorite, epidote, and sericite and were trapped at minimum temperatures of 223°C–353°C and minimum fluid pressures of 1150–2800 bars. The fluids average 13 mol% CO2 and 8.2 wt % NaCl. A radiometric age of hydrothermal sericite suggests that the age of the alteration is 17.6±0.7 m.y. If maximum fluid pressure is near lithostatic, the minimum depth of formation of the alteration was 11 km at 337°C consistent with a thermal gradient of 30°C/km. The ratio of pore fluid pressure to lithostatic pressure varied from 0.52 to nearly 1.0 and indicates that effective normal stress on the fault at these depths may have approached zero. Structural fabrics and fluid inclusion characteristics are used to estimate seismogenic characteristics of the fault. Cleavage lineation and S‐C bands in the phyllonite indicate that deformation at temperatures above 350°C and depths greater than 11 km was dominated by aseismic creep. Unstable fracture propagation in cataclasite developed during seismic slip at shallower levels. The downdip, seismogenic length of the Salt Lake segment is estimated as 16 km using a fault zone dip of 45° and a minimum depth of 11 km for the quasi‐plastic/frictional transition. The segment strike length is 35 km, and the seismogenic area is about 600 km2, compatible with generation of M = 7.0 earthquakes.
Spectral properties (0.4 to 25 microns) of selected rocks associated with disseminated gold and silver deposits in Nevada and Idaho
Authors: Krohn M. Dennis
Published: February 2008

Visible, near‐infrared (VNIR) reflectance spectra and midinfrared (MIR) transmission spectra (2.5–25. µm) were measured for selected rock types from disseminated gold and silver deposits in northern Nevada and southern Idaho. Samples from the open‐pit mines at Pinson, Nevada, and Delmar, Idaho, were compared to weathered surface samples from a prospect at Preble, Nevada. Eight characteristics of the disseminated deposits were proposed that could be identified by remote‐sensing applications. Of these eight characteristics, jasperoids, the siliceous replacement of limestones were readily distinguishable from the carbonate host rock and other associated silicate rocks by their spectral characteristics in the MIR. Active or fossil hot‐spring sites, where present near the deposit, display similar VNIR spectral features at the surface as hydrothermally altered rocks found at depth in the open‐pit mines. However, most of the host rocks at the surface above a disseminated gold prospect show few VNIR absorption features, because of the presence of organic matter. VNIR spectral features of carbonate were detectable in a few cases immediately adjacent to a jasperoid. Ammonium spectral features are observed on some of the hydrothermally altered shale surrounding the prospect, also suggesting the presence of organic matter. Because organic matter exerts such a large influence on the spectral character of the host rocks, the combined MIR and VNIR wavelengths are needed for remote discrimination of disseminated gold prospects in the western U.S.
Heat flow in the Piedmont and Atlantic Coastal Plain of the southeastern United States
Authors: Costain J. K. , Speer J. A. , Glover L. , Perry L. , Dashevsky S. , McKinney M.
Published: February 2008

Seventy‐four new heat flow values have been determined at sites from New Jersey to Georgia, mostly in the Piedmont and Atlantic Coastal Plain provinces. Relatively high heat flows are characteristic of synmetamorphic and postmetamorphic granites exposed in the Piedmont and occurring in the basement beneath the sediments of the Atlantic Coastal Plain. The Piedmont heat flow and heat generation values, determined in granites, metagranites, and one Slate Belt site, lie in a belt approximately parallel to major structural trends in the Appalachians. These values fall on or near the regression line: q(mW/m2) = 29.8 ± 1.5(mW/m2) + 7.8(km) ± 0.475A(μW/m3) (R = 0.97477). It is proposed that the occurrence of a thrust fault that truncates granites at a depth, D, of 7.8 km is directly responsible for this linear relation in the eastern Piedmont. Basement core from beneath the sediments of the Atlantic Coastal Plain was recovered at ten sites. Results from five of these sites plus one site in the Baltimore Gabbro define a second linear relation with a slope (D = 8.0±0.380 km; R = 0.99550) approximately parallel to the Piedmont relation, but with an intercept of 48.2±0.8 mW/m2. These two relations may represent different heat flow provinces in the southeastern United States or the linear relations may simply be a consequence of differences in the thicknesses of the upper heat‐producing layer at least as far east as postmetamorphic granites sampled under the Coastal Plain.
Deep tow studies of the Vema Fracture Zone: 2. Evidence for tectonism and bottom currents in the sediments of the transform valley floor
Authors: Kastens Kim A. , Macdonald Ken C. , Miller Stephen P. , Fox P. J.
Published: February 2008

The Deep Tow geophysical instrument package has been used to survey a 65‐km segment along the principal transform displacement zone (PTDZ) of the Vema Fracture Zone, away from either ridge‐transform intersection. The surface trace of the PTDZ is a small groove, 5–80 m deep and 20–200 m wide, incised in the otherwise flat‐lying turbidites of the transform valley floor. The fault trace cuts across the transform valley diagonally from ridge crest to ridge crest, hugging the southern flank of the median ridge. The groove is straight or gently sinuous, relatively free of the splays and bifurcations which characterize the transform fault zone near the ridge‐transform intersection (Macdonald et al., this issue). A pull‐apart basin was observed at the location of a slight clockwise bend in the fault trace. Vertical offsets of turbidites suggest that the west end of the median ridge and the east end of the south wall are rising relative to the transform valley floor at rates of the order of millimeters per year. Sediment waves were observed buried under 10–20 m of sediment, suggesting that Antarctic Bottom Water may have flowed through the transform valley more vigorously several tens of thousands years ago than at present.
Evidence for small‐scale mantle convection from Seasat altimeter data
Authors: Haxby William F. , Weissel Jeffrey K.
Published: February 2008

Evidence for the early development of convective instability in the thermal boundary layer associated with cooling plates has been found from gravity anomalies and residual sea surface heights derived from Seasat altimeter data. Subtle lineated patterns trending in the direction of plate motion in the hot spot reference frame are observed over the younger portions of the fast‐moving Pacific and Indo‐Australian plates. In particular, for the east central Pacific Ocean, the Seasat‐derived data sets reveal lineations with (1) wavelengths in the range 150–500 km (perhaps increasing with plate age), and (2) peak‐to‐trough amplitudes of 5–20 mGal for gravity anomalies. The lineated pattern over the Pacific plate, which first becomes discernable over seafloor 5‐ to 10‐m.y.‐old west of the East Pacific Rise, is clearly oblique to the trends of the prominent Pacific‐Farallon fracture zones. Onset of convective instability at such early ages can be understood if young oceanic lithosphere is underlain by a layer having a viscosity of about 1018 Pa s (1019 P), which is about 3 orders of magnitude less than mantle viscosity inferred from postglacial rebound studies. Recent numerical studies of convection in fluids with a temperature‐ and pressuredependent viscosity support an early onset time for convective instability and low‐viscosity zones in the upper mantle beneath young lithosphere. Such low‐viscosity zones may correspond to similarly located regions of low shear wave velocities resolved through studies of surface waves.
Silicate volcanism on Io
Authors: Carr Michael H.
Published: February 2008

Io is currently emitting 1–1.5 W m−2 of tidal energy as a result of its volcanic activity. If the lithosphere is more than 20 km thick, as appears probable from the surface relief, only a fraction of the tidal energy can be dissipated within the lithosphere, otherwise it will become thinner. The rest of the tidal energy must be dissipated below the lithosphere. Io is likely to be highly differentiated as a result of the volcanic activity, with a low melting temperature fraction, such as basalt, near the surface and a high melting temperature fraction, such as peridotite, at depth. If solidus temperatures are reached at depths shallower than the thickness of the basalt layer, a partial melt zone will separate an all‐basalt lithosphere above from a peridotite mantle below, and part of the tidal energy will be dissipated by viscous deformation within the melt zone. Most of the energy dissipated below the lithosphere will be transported upward through the lithosphere as silicate magma, which is generated in quantities sufficient to resurface the satellite at a rate of a few tenths of a centimeter a year, depending on the lithosphere thickness. Many of the characteristics of the Ionian surface have been explained in terms of sulfur volcanism. However, most of the features observed can be as readily explained by silicate volcanism, and silicates are more consistent with the apparent strength of the surface as implied by the relief. Simulations of basaltic eruptions indicate that the surface temperatures that would result from basaltic eruptions are similar to those measured by the Voyager infrared interferometer spectrometer experiment. The high rates of emission by the Ionian hot spots imply eruption rates that are high compared with typical terrestrial eruptions. An eruption rate of 4000 m3 s−1 may be required to explain Loki which is currently emitting 1013 W. Although the near‐surface materials are mainly silicate, they may contain several percent of volatile components rich in S, Na, and K. Remobilization of these components by the ongoing silicate eruptions causes the plumes, provides material to the torus, and gives the satellite its characteristic reflectivity.
Finite strain calculations of continental deformation: 1. Method and general results for convergent zones
Authors: Houseman Gregory , England Philip
Published: February 2008

A finite element method is used to calculate stresses and strain rates in a thin viscous sheet, representing the continental lithosphere, that is bordered in one part by an indenting boundary. We calculate solutions for velocity fields, crustal thickness distributions, strain rates, rotation (time‐integrated vorticity), and finite strain ellipsoids to show the dependence of these quantities on the controlling parameters: the stress‐strain exponent n and the strength of the sheet relative to the gravitational forces (which may be expressed as a dimensionless number: the Argand number Ar). For n > 3 and Ar between 1 and 10 a plateau of thickened crust is formed in front of the indenter. The strain histories of individual elements within the viscous sheet may be quite complex, and the finite strain at the end of deformation may bear no obvious relation to the principal stress orientations during the deformation. The strain rate fields are interpreted in terms of the style of faulting that would be expected in the brittle upper crust if it were coupled to the ductile, but stronger, upper mantle. In general, the length scale of deformation increases with Ar and time since the start of the collision and decreases with n; for Ar ≃ 3, the increase in length scale is accompanied by a change in the style of deformation: from predominantly crustal thickening (with thrust faulting in the brittle layer) to predominantly transcurrent deformation (strike‐slip faulting in the brittle layer).
Deformation mechanism maps and micromechanics of rocks with distributed grain sizes
Authors: Freeman Brett , Ferguson Colin C.
Published: February 2008

Conventional deformation mechanism (DM) maps are constructed for single‐valued grain size. When diffusional creep is competitive with power law creep, such maps are a poor guide to rheology and microstructure development in distributed grain size materials because the strain rate dependence on grain size implies wide variations in local stress and/or strain rate. We present DM maps for quartzite with realistic grain size distributions using two bounding models: (1) homogeneous strain rate with local stress varying from grain to grain and (2) homogeneous stress with local strain rate varying. Distributed grain size systems are stronger at low stresses and have larger power law creep fields and larger regions where two or more mechanisms contribute significantly to the total strain rate. Our DM maps do not incorporate grain shape and packing, but we review theoretical work on the effects of these variables and compare rheological predictions with our own for the development of idealized core‐and‐mantle microstructure.
Can intrasolar diffusion contribute to isotope anomalies in the solar wind?
Authors: MacElroy J. M. D. , Manuel O. K.
Published: February 2008

The possible role of intrasolar diffusion is examined for the isotopes of He, Ne, and Xe to see if there is a theoretical basis for an empirical mass fractionation relationship between the isotopic compositions of planetary and solar‐wind‐implanted elements. It is shown that if diffusion is responsible, even in part, for the observed depletion of 4He in the solar wind, then significant fractionation effects across the isotopes of Ne and Xe as well as He can be predicted. It is also shown that isotopic fractionation need not necessarily infer extreme elemental abundance patterns in the solar interior.
Plate tectonic development of the south west Indian Ocean: A revised reconstruction of East Antarctica and Africa
Authors: Martin A. K. , Hartnady C. J. H.
Published: February 2008

A revised sequence of reconstructions for East Antarctica relative to Africa is generated from published seafloor spreading and fracture zone anomalies. These date from the Cretaceous/Tertiary boundary to the Late Jurassic and lead to a revised predrift reconstruction for east and west Gondwanaland: Euler pole 1.67°S, 35.99°E, rotation angle 53.43°. This reconstruction derived wholly from marine data places East Antarctica adjacent to the Lebombo Mountains and south of and subparallel to the Sabi acid volcanic suite, supporting the postulate that these rocks represent Mesozoic plate boundaries. This refit implies that part of the coastal plain of Mozambique is underlain by oceanic or very highly extended continental crust. The reconstruction is supported by the alignment of (1) the Cape Fold Belt with its Antarctic equivalents and (2) the western limit of Pan‐African reactivation of the Mozambique Mobile Belt with the western limit of the Sverdrupfjella metamorphic suite. Antarctica initially moved along a transform fault lying immediately east of and subparallel to the Lebombo Mountains, the eastern face of the Tugela Cone, the southeastern face of the Falkland Plateau in its reconstructed position, and the eastern flank of the Agulhas Plateau in its reconstructed position. At M10 time a triple junction formed near the tip of the Falkland Plateau implying that the oldest lineated magnetic anomalies in the eastern Weddell Sea associated with this triple junction cannot predate M10. At M2 time the Mozambique Ridge rifted from the Astrid Ridge in a ridge jump episode. Such an event may also explain the presence of magnetic anomalies in the southern Mozambique Basin. Gunnerus Ridge, Conrad Rise, and Del Cano Rise are all tectonically associated with the Madagascar Ridge.
Continuum calculations of continental deformation in transcurrent environments
Authors: Sonder Leslie J. , England Philip C. , Houseman Gregory A.
Published: February 2008

A thin viscous sheet approximation is used to investigate continental deformation near a strike‐slip boundary. The vertically averaged velocity field is calculated for a medium characterized by a power law rheology with stress exponent n. Driving stresses include those applied along boundaries of the sheet and those arising from buoyancy forces related to lateral differences in crustal thickness. Calculations are performed with either velocity or stress prescribed on the transcurrent boundary. Exact and approximate analytic solutions for a region with a sinusoidal strike‐slip boundary condition are compared with solutions for more geologically relevant boundary conditions obtained using a finite element technique. The across‐strike length scale of the deformation is approximately 1/[4π(n)1/2] times the dominant wavelength of the imposed strike‐slip boundary condition for both the analytic and the numerical solutions; this result is consistent with length scales observed in continental regions of large‐scale transcurrent faulting. An approximate, linear relationship between displacement and rotation is found that depends only on the deformation length scale and the rheology. Calculated displacements, finite rotations, and distribution of crustal thicknesses are consistent with those observed in the region of the Pacific‐North America plate boundary in California.
Magma mixing as a petrogenetic process in the development of the Taos Plateau volcanic field, New Mexico
Authors: McMillan Nancy J. , Dungan Michael A.
Published: February 2008

The package of lavas exposed in the Red River gorge of the Taos Plateau volcanic field, northern New Mexico, consists of hybrids produced by simple two‐component magma mixing. The sequence of flows, which is graded compositionally from basalt at the base of the section to dacite at the top, represents the outpouring of progressively more silicic hybrids during the mixing process. The end‐members were basalt (50% SiO2) and dacite only slightly more silicic than the uppermost dacite in the gorge (61–62% SiO2). Hybrids (52–57% SiO2) are characterized by large skeletal olivine phenocrysts and exhibit linear chemical variations between end‐member compositions for all major and trace elements. Major and trace element mass balance calculations concur on the proportions of end‐members necessary to form each hybrid; chemical models limit the amount of subsequent olivine fractionation to less than 5%. The presence of groundmass olivine in andesitic hybrids implies that eruption occurred shortly after, or during, mixing. Estimated density and viscosity contrasts between the end‐members would appear to inhibit mixing of basaltic and dacitic magmas, unless basalt were forcefully injected into a dacitic magma chamber. Chemical and petrographic similarities between other olivine andesites and two‐pyroxene dacites of the Taos Plateau volcanic field and the Red River gorge hybrids suggest that magma mixing was an important process in the Taos Plateau magmatic system.