bluuebunny/crossref_metadata_2025 huggingface dataset:
Age, paleomagnetism, and tectonic significance of the Elberton Granite, northeast Georgia Piedmont
Authors: Ellwood Brooks B. , Whitney James A. , Wenner David B. , Mose Douglas , Amerigian Craig
Published: February 2008

The Rb/Sr whole rock age of 350±11 m.y., the remanent magnetism (RM), and petrology of samples from 21 sites within the granitic Elberton pluton (NE Georgia) have been determined. Two partially conflicting estimates of tectonic rotation are developed and evaluated. The first, based on the orientation of the magnetic foliation within the body and its reorientation, assuming an originally horizontal upper granite contact and coincident magnetic fabric, requires a post emplacement rotation of 30°–35° down to the SE about a N‐NE axis. The second, based on reorienting the body so that the pole determined for the granite before tilt correction (ϕ = 120.3; θ = 54.7; dp = 2.2; dm = 4.5) becomes coincident with an independently reported mean 350 m.y. B.P. paleopole (ϕ = 120; θ = 47), indicates a postemplacement rotation for the body of ∼15° down to the NW along a E‐NE axis. While these tilt estimates are different, they do have two very important points in common, namely, that the strike of rotation is similar and the magnitude of rotation is small. We conclude from these data that no major structural reorientation or relocation of the Elberton granite is necessary to produce a pole position consistent with the 350 m.y. Rb/Sr whole rock age. We suggest that the tilting of the granite (if any) that may have occurred resulted from uplift of the Inner Piedmont, with the Elberton pluton isolated within a rigid crustal block bounded by NE‐SW trending faults moving in response to that uplift. Our interpretation of these data is not consistent with a post 350 m.y. major continental collision event, with concurrent large‐scale thrust fault motion, involving the Southern Appalachians of North America. This conclusion is reinforced by tectonic analyses based on the regional gravity gradient and on oxygen isotopic data reported for late orogenic granites in the Southern Piedmont.
Thermodynamics of pyrope‐grossular garnets and their stabilities at high temperatures and high pressures
Authors: Haselton H. T. , Newton R. C.
Published: February 2008

Activity expressions for Mg3Al2Si3O12 and Ca3Al2Si3O12 in pyrope‐grossular garnets that include explicit enthalpy, entropy, and volume terms have been formulated from measured thermodynamic properties. Predicted Ca3Al2Si3O12 activities agree well with those given by Hensen et al. (1975) based on high‐pressure phase equilibrium experimentation, when the latter activities are corrected to 1 bar with the measured partial molar volumes. Calculations of the stabilities of garnets in the CaO‐MgO‐Al2O3‐SiO2 system reproduce fairly accurately the most reliable phase equilibrium data at 1000°–1200°C that do not heavily involve a clinopyroxene phase. Though the calculation of garnet‐clinopyroxene stability relations is in general agreement with available experimental observations, a critical comparison is hindered by the uncertainties in the high‐temperature heat capacities of the garnet and clinopyroxene components. The predicted solvus in the garnet join develops a ‘spike’ at high pressures which is displaced toward pyrope‐rich compositions.
A palaeomagnetic reconnaissance of northeastern Baluchistan, Pakistan
Authors: Klootwijk Chris T. , Nazirullah Russel , De Jong Kees A. , Ahmed Habib
Published: February 2008

A total of 560 samples from three areas in northeastern Baluchistan (the southern Sulaiman Range, the central Loralai Range, and the northern Kirthar Range) were analyzed using thermal demagnetization techniques. Thirteen formations of late Palaeozoic to early Tertiary age were studied palaeomagnetically. Inclinations of the obtained results show a general affinity with the Indian apparent polar wander path. Deviating declinations from the Loralai Range indicate a clockwise rotation over 50° with respect to the Indian shield. Secondary magnetization components probably of late Palaeocene to early Eocene age were consistently present in the samples from the Kirthar Range and the Sulaiman Range but were not observed in samples from the Loralai Range. Acquisition of these components is attributed to crustal upwarping during the Palaeocene, which is tentatively related to initial collision of continental Indo‐Pakistan with an island arc off south central Asia. The Baluchistan data support recent palaeomagnetic results from the Indus‐Tsangpo suture zone in Ladakh (NW Himalaya), which indicate that such an initial collision took place at very low northern palaeolatitudes.
Fragmentation of solids under impulsive stress loading
Authors: Grady D. E.
Published: February 2008

An analysis of fragmentation due to dynamic stress loading is presented which provides analytic functions for the distributions in fragment sizes. The analysis is restricted to one‐dimensional bodies under uniform tensile loading. Concepts of survival statistics are used to account for spatially random fracture nucleation. Fragment size distribution curves for both brittle and ductile fracture are derived, and the curve for the latter is compared with experimental data. Fragment distribution curves are shown to depend on both material deformation properties and loading conditions.
Lunar tidal acceleration from Earth satellite orbit analyses
Authors: Cazenave Anny , Daillet Sylviane
Published: February 2008

We present results for the M2 and O1 ocean tides based on the Starlette satellite orbit analyses from which the tidal acceleration of the moon
is derived. The solution for M2 is consistent with previous satellite solutions; for O1 we find a phase lag which is approximately zero. The uncertainty is still large, but there is an indication that the tidal dissipation at the O1 frequency is small. Using the weighted mean value of the best satellite solutions gives
= −21.32 ± 1.25 arc sec/(century)2 for the total dissipation in the earth at the M2 frequency. In principle, comparison between
derived from satellite solution and
derived from numerical models gives the amount of dissipation in the solid earth, but uncertainties are still too large to give a reliable estimate of the tidal mantle Q.
Rare‐earth element geochemistry of the Samail Ophiolite near Ibra, Oman
Authors: Pallister John S. , Knight Roy J.
Published: February 2008

Rare‐earth element (REE) analyses of 68 rock and mineral separate samples from the Samail ophiolite clearly differentiate the various units of the ophiolite suite and indicate that the crustal suite is cogenetic, produced by crystal fractionation of basaltic magma in a spreading ridge magma chamber. Mantle peridotites are residual in rare‐earth character, but cannot be clearly related to the overlying mafic rocks. Chromian spinel is probably insignificant in its effect on REE distribution during partial melting and crystal fractionation, as indicated by the low REE concentrations in chromitite from the Samail. Layered gabbro REE patterns are dominated by cumulus clinopyroxene and plagioclase. Large positive Eu anomalies demonstrate plagioclase accumulation. Modal mixing (mass balance) calculations reveal that most of these adcumulus gabbros have REE patterns that are the products of the REE concentrations of their constituent cumulus phases in the observed modal proportions; hence no appreciable REE‐rich mesostasis is present. Such calculations also allow the prediction of mineral REE concentrations not actually determined by mineral separate analyses. Several high‐level (noncumulus?) gabbros yield patterns with positive Eu anomalies suggesting relative plagioclase accumulation, probably due to liquid fractionation (filter pressing). Dike complex REE patterns show light rare‐earth element depletions and are similar to, but not necessarily diagnostic of, midocean ridge basalt. They cluster at similar abundance levels; however, the absolute variation in abundance is large, indicative of modification by crystal fractionation. Calculations using partition coefficient data indicate that many dikes represent liquids that could have existed in equilibrium with cumulus minerals of the plutonic suite. Mineral separate REE data from layered cumulus gabbro of the Khafifah stratigraphic section reveal cryptic variation trends that are correlated with major element variation, in support of a long‐lived (periodically replenished) magma chamber model (Pallister and Hopson, 1978, 1979, 1981). (Cryptic variation as used herein is defined as the change in mineral composition with respect to stratigraphic position.) The REE cryptic variation shows both direct and inverse correlation to major element variation, indicating that magmatic replenishment was complicated by changes in parent melt REE abundance. A crystal fractionation origin for the small plagiogranite bodies of the Ibra area is favored by REE modeling, although larger bodies (Dasir) may also be related to magmatic inclusion of roofrock with diabase level REE distribution (Gregory and Taylor, 1979).
An intermediate depth earthquake beneath Tibet: Source characteristics of the event of September 14, 1976
Authors: Chen Wang‐Ping , Nábělek John L. , Fitch Thomas J. , Molnar Peter
Published: February 2008

This paper presents the results of a detailed study of an intermediate depth earthquake beneath southern Tibet (September 14, 1976; 0643:51.6 UT; 29.8l°N, 89.57°E; focal depth 90±10 km; mb = 5.4). From the differences in arrival times of P, pP, and sP and from Rayleigh wave spectral densities the focus is estimated to be at depth of 90±10 km and therefore in the uppermost mantle. From the polarities of the first motions of P waves, inversion of the relative amplitudes of P, pP, and sP, and synthesis of their waveforms, the source mechanism is constrained to be predominantly normal faulting with the T axis approximately horizontal and trending east‐west. This mechanism is similar to those of shallow earthquakes in Tibet. The occurrence of an earthquake at depth of 90 km is consistent with rapid, deformation at a temperature of about 700° to 800°C in the mantle beneath the Tibetan crust. The seismic moment of an equivalent double couple point source is estimated to be about 9 × 1024 dyne*cm (with an uncertainty factor of 1.3) from the sP amplitudes in the period range near 10 s, and 5 × 1024 dyne*cm (also with an uncertainty factor of 1.3) from the spectral density of Rayleigh waves with periods of 50 s.
Spaceborne and airborne imaging radar observations of sand dunes
Authors: Blom R. , Elachi C.
Published: February 2008

Seasat and aircraft radar imagery of five areas of sand dunes in the southwestern United States and northwestern Mexico have been studied and compared to Landsat imagery and air photos. Radar imaging system parameters included two wavelengths (23.5 cm and 3.0 cm) and incidence angles ranging from 0° through 70°. Horizontal polarization was used on both transmit and receive for all radar imagery. Resolution ranged from 12 m for the best aircraft imagery to about 40 m for Seasat. Sand dunes studied behave as smooth surfaces for both 23.5 cm and 3.0 cm wavelength radars and the reflection of the radar energy from the dunes is specular. Measured surface irregularities of sand dunes are inadequate to cause either Bragg or incoherent scattering of the incident radar beam in the 3.0 to 23.5 cm wavelength region. This results in dunes that appear dark when imaged at incidence angles greater than the angle of repose because none of the dune faces are oriented properly (normal to the incident radar beam) to give a specular return to the antenna. A scattering model based on specular return from randomly oriented flat dune faces was used and experimentally verified using aircraft data. Imaging radar resolution must be adequate to define the changing distribution of dune faces that characterize the dune morphology. Linear dune features with interdune spacing of less than 3–4 resolution elements are not recognizable. Distributed dune types such as barchans or stars must be more than about 10 resolution elements in each direction to be defined on the imagery. Illumination direction of the radar beam is important; directional dune features must be oriented within about 60° of perpendicular to the radar illumination direction in order to be imaged. If such features have considerable topographic relief, the direction of radar illumination is less important. Because directional trends in unexplored regions cannot be predicted, availability of radar imagery from two directions greatly facilitates interpretation of dune morphology and hence conclusions about causative wind regimes.
Microearthquake activity on the Orozco Fracture Zone: Preliminary results from Project ROSE
Authors: Scientists Project Rose
Published: February 2008

We present preliminary hypocenter determinations for 52 earthquakes recorded by a large multi‐institutional network of ocean bottom seismometers and ocean bottom hydrophones in the Orozco Fracture Zone in the eastern Pacific during late February to mid‐March 1979. The network was deployed as part of the Rivera Ocean Seismic Experiment, also known as Project ROSE. The Orozco Fracture Zone is physiographically complex, and the pattern of microearthquake hypocenters at least partly reflects this complexity. All of the well‐located epicenters lie within the active transform fault segment of the fracture zone. About half of the recorded earthquakes were aligned along a narrow trough that extends eastward from the northern rise crest intersection in the approximate direction of the Cocos‐Pacific relative plate motion; these events appear to be characterized by strike‐slip faulting. The second major group of activity occurred in the central portion of the transform fault; the microearthquakes in this group do not display a preferred alignment parallel to the direction of spreading, and several are not obviously associated with distinct topographic features. Hypocentral depth was well resolved for many of the earthquakes reported here. Nominal depths range from 0 to 17 km below the seafloor.
Nonlinearity in rock: Evidence from Earth tides
Authors: Agnew Duncan Carr
Published: February 2008

The earth is sinusoidally stressed by tidal forces; if the stress‐strain relation for rock is nonlinear, energy should appear in an earth tide record at frequencies which are multiples of those of the larger tidal lines. An examination of the signals to be expected for different nonlinear deformation laws shows that for a nonlinear response without dissipation, the largest anomalous signal should occur at twice the forcing frequency, whereas for nonlinear laws involving dissipation (cusped hysteresis loops) the anomalous signal will be greatest at 3 times this frequency. The size of the signal in the dissipative case depends on the amount by which dissipation affects the particular response being measured. For measurements of strain tides this depends on whether dissipation is assumed to be present throughout the earth or localized around the point of measurement. An analysis of 5.7 years of strain tide records from Piñon Flat, California, shows a small signal at twice the frequency of the largest (M2) tide. Most of the observed signal can be explained by loading from nonlinear water tides in the Gulf of California and the Pacific Ocean; the residual nonlinear tide is 65 dB less than the M2 tide. The signal at 3 times the M2 frequency is compatible with a linear model or with nonlinear hysteresis loops provided that nonlinear dissipation occurs throughout the earth. Nonlinear dissipation in the rocks near the strainmeter would produce a larger signal than is seen.
Magnetic overprinting in southeastern Australia and the thermal history of its rifted margin
Authors: Schmidt P. W. , Embleton B. J. J.
Published: February 2008

The magnetizations of many rock units in southeastern Australia have been severely overprinted. Interpretation of early work was hindered by this, and one of the aims of the current research is to ascertain the nature of the overprint mechanism, its geological significance, and its regional extent. Results from three igneous bodies from the Sydney Basin, which have been studied previously in a reconnaissance fashion, have provided a new insight into the reason for such widespread magnetic overprinting. The Hornsby Breccia (HB) and Milton Monzonite (MM) yield similar magnetic signatures, both containing discrete high‐ and low‐temperature magnetizations with directions of dec = 72°, inc = +83° (α95 = 4°) and dec =13°, inc = −76° (α95 = 2°), respectively, for HB, and dec = 60°, inc = +78° (α95 = 4°) and dec = 348°, inc = −79° (α95 = 6°) respectively for MM. The corresponding pole positions are 29°S, 166°E (A95 = 7°) and 59°S, 139°E (A95 = 4°) for HB, and 22°S, 171°E (A95 = 7°) and 55°S, 158°E (A95 = 11°) for MM. The paleomagnetic direction and pole from the third body studied, the Mogo Hill Basalt (MH), are dec = 241°, inc = 81° (α95 = 5°) and 41°S, 130°E (A95 = 9°). This pole position and the two low‐temperature overprint pole positions from HB and MM are consistent with ages ranging from 100 m.y. to approximately 70 m.y. when compared to the Late Mesozoic apparent polar wander path. We relate the overprinting to uplift, erosion, and rapid supracrustal cooling of the southeastern margin of Australia associated with initial phases of marginal rift development prior to seafloor spreading in the Tasman Sea. The temperatures of rocks now exposed may have reached 200°C or higher, before the rapid cooling blocked and stabilized the overprint magnetizations.
An interpretation of episodic slip on the Calaveras Fault near Hollister, California
Authors: Langbein John O.
Published: February 2008

Data from a precision multi‐wavelength distance measuring (MWDM) instrument, located near Hollister, California, have been analyzed in terms of strike‐slip faulting in the region covered by the network. Four episodes of deformation that are readily identifiable in the MWDM data for the year following September 1975 can be modeled as slip on the Calaveras fault. Creepmeter data tend to support this interpretation, since the creep events and the observed episodes of deformation appear to be correlated in a temporal sense. Lower bounds on the moment for each episode of slip can be determined from the MWDM data by the technique of linear programing. The results indicate that aseismic slip is the dominant mechanism for strain release, since the combined moment of 1.16 × 1024 dyn cm for the four episodes far exceeds the moment of all of the earthquakes that occurred during the year. Creepmeter data and geologic evidence that are taken in conjunction with the lower bound of the moment indicate that the depth of aseismic slip for some of the episodes of fault slip could extend to below the greatest depth of earthquake hypocenters for this region of central California.
The November 29, 1978, Oaxaca, Mexico, earthquake: A large simple event
Authors: Stewart Gordon S. , Chael Eric P. , McNally Karen C.
Published: February 2008

The recent Oaxaca, Mexico, earthquake November 29, 1978 (MW= 7.6,Ms= 7.8, seismic moment,M0= 3.2 × 1027dyne cm), is of special interest because of its location within a predetermined seismic gap. The event excited long‐period (100–200 s) multiple Rayleigh and Love waves which were well‐recorded by the WWSSN. These data along withPwave first‐motion data andPwaveforms were used to constrain the source mechanism. The results indicate an oblique thrust mechanism consistent with subduction of the Cocos plate to the northeast beneath Mexico (dip = 14°, strike = N90°W, rake = +54°); hence this event is indeed of the type anticipated by Ohtake et al. (1977). A local network of stations, installed in a joint University of Mexico‐California Institute of Technology program, began operation 20 days prior to the mainshock (González, 1979; McNally et al., 1979; McNally, 1981; Ponce et al., 1980). The 43 foreshocks of magnitudeM≥2.8 were recorded by the network in a period of 20 days prior to the mainshock. These events show an interesting spatial and temporal pattern, which culminates in the last 1.8 days of the sequence with an apparent migration of activity toward the epicenter of the impending earthquake. This pattern can be interpreted as a buildup of stress or migration of stress toward a fault zone asperity. With supplemental stations, the network continued operation until December 12, 1978 (Singh et al., 1980) and provided good hypocentral control for the more than 169 aftershocks of magnitudeM≥ 2.8. The area of the aftershock zone determined from these events is 5525 km2(85 km by 65 km). In spite of the large size of the event,Pwaves for the Oaxaca event indicate an extremely simple source, at the period range of the WWSSN long‐period seismographs. This simplicity suggests that thePwaves were generated by a limited portion of the rupture surface, perhaps by the breaking of a fault zone asperity. This result may be further supported by the discrepancy between the larger surface wave moment compared with that determined from body waves. Such simplicity also appears to be the case for the 1965 and 1968 Mexican earthquakes on the adjacent eastern and western ends of the Oaxaca aftershock zone, respectively (Chael et al., 1980). This type of body wave simplicity for a large subduction zone earthquake is an important characteristic of the mode of strain release along some subducting plate boundaries.
Geometry and physical properties of the Socorro, New Mexico, magma bodies
Authors: Brocher Thomas M.
Published: February 2008

Analysis of multichannel seismic reflection profiles gathered by the Consortium for Continental Reflection Profiling (COCORP) constrains the geometry and material properties of a proposed magma body located within the Rio Grande rift. A new spectral ratio method is used to find the reflectivities of the proposed midcrustal intrusion; large and spatially variable reflectivities support the interpretation of the reflector as an intrusive molten magma complex. Modeling of the reflectivity suggests that in one location the intrusion took the form of several thin (30 to 40 m thick) layers. Seismic wave attenuation within the rift's midcrust is low.
Lunar floor‐fractured craters: Evidence for viscous relaxation of crater topography
Authors: Hall J. Lynn , Solomon Sean C. , Head James W.
Published: February 2008

In this paper we evaluate quantitatively the hypothesis that topographic modification of floor‐fractured craters on the moon was accomplished predominantly by viscous relaxation. Adopting the simple assumption that the moon may be modeled as having a uniform Newtonian viscosity, we compare the observed topographic profiles for a number of floor‐fractured craters with the profiles predicted from the viscous relaxation of topography of fresh craters of similar diameter. Despite the simplicity of the rheological model, the comparison is quite good. The floor uplift, the rim subsidence, and the apparent subsidence outside the rim for the several floor‐fractured craters considered are well matched by the viscous relaxation hypothesis. Floor fractures, while indicating that a purely viscous model is not strictly valid, can be explained by the effects of isostatic adjustment on a thin brittle lithosphere. The association of many floor‐fractured craters with impact basins and with the time of mare volcanism can be understood in terms of a pronounced acceleration of crater relaxation in local regions of anomalously high near‐surface temperatures and therefore of low effective viscosity and thin lithosphere. The quantitative extent of relaxation of floor‐fractured craters can be interpreted in terms of a limited time interval of substantial relaxation for each crater. That time interval ended for each crater after local cooling had been sufficient for the viscosity to rise, for the lithosphere to thicken, and for the present topographic relief to be ‘frozen in.’ Thus viscous relaxation is a viable hypothesis to explain the topographic profiles of a number of lunar floor‐fractured craters, and the extent of viscous relaxation of crater topography may serve as a tool to map lateral and temporal variations in the shallow thermal structure of the moon and of other planets and satellites.
Chemical evolution of magmas in the proterozoic terrane of the St. Francois Mountains, southeastern Missouri: 2. Trace element data
Authors: Cullers R. L. , Koch R. J. , Bickford M. E.
Published: February 2008

The St. Francois Mountains igneous complex of Proterozoic age consists of hypabyssal, granite plutons intruded into the overlying rhyolites of similar age. Melting and crystallization models of these rocks suggested by the field, petrographic, and major element contents are further refined using Rb, Ba, Sr, REE (rare‐earth element), Th, Sc, and Co contents. There are two groups of relatively undifferentiated plutons associated with the Butler Hill caldera and Taum Sauk area that could have formed by partial melting of lower crustal rocks. The plutons associated with the Butler Hill caldera (Knoblick and Silvermine granites) could have formed by about 30% aggregate melting of a quartz diorite, graywacke, or subgraywacke. The plutons associated with the Taum Sauk area could have formed by about 30% equilibrium melting of a subarkose or arkose. More silicic rocks of the Butler Hill caldera could have formed by fractional crystallization. The Silvermine granite is internally zoned. Crystallization of plagioclase/biotite/hornblende/magnetite/apatite/zircon/sphene in a ratio of 0.70/0.10/0.15/0.04/0.005/0.003/0.007 from the least differentiated portion of the Silvermine granite could have formed intermediate portions of the Silvermine. Further crystallization of the Silvermine requires more biotite (Ba begins to decrease) and less hornblende, zircon, and sphene (HREE increase) to precipitate and form the most differentiated portions. Also fractional crystallization of undifferentiated Silvermine‐type magmas could have produced the sequence of Butler Hill‐Breadtray‐Graniteville granites as residual liquids, respectively. The early crystallizing mineral assemblage is dominated by plagioclase and biotite, while the late crystallizing mineral assemblage is dominated by quartz, K‐feldspar, and lesser plagioclase. The Gneissoid granodiorite in the Hawn Park area has the mineral assemblage and composition of a cumulate derived from the Silvermine granite‐type differentiation sequence. The associated volcanic rocks could not have formed from melts of exactly the same composition of any of the associated intrusive rocks. Rather, the volcanic rocks formed from melts of slightly different chemistry.
Chemistry of rock‐forming minerals of the Cretaceous‐Paleocene batholith in southwestern Japan and implications for magma genesis
Authors: Czamanske Gerald K. , Ishihara Shunso , Atkin Steven A.
Published: February 2008

Petrographic descriptions and electron microprobe analyses of minerals are presented for 35 specimens from seven suites chosen to examine the transition from magnetite series to ilmenite series granitoids along two transects across the Cretaceous‐Paleocene Inner Zone batholith of southwestern Japan. Regularities in chemical compositions of amphiboles, biotites, and feldspars suggest that fundamentally similar processes produced the magmas that formed the two series. Constant or decreasing Fe/(Fe+Mg) for biotites and amphiboles with increasing host rock silica content, coupled with the absence of early formed magnetite and sphene, suggest that magnetite series rocks may have become oxidized during crystallization near the level of intrusion, through the processes of second boiling and differential loss of hydrogen. For the Daito‐Yokota, magnetite series suite, Fe/(Fe+Mg) for biotites decreased from 0.48 to 0.37 as SiO2 content of the host rock increased from 55.3 to 75.5 wt %; for an ilmenite series suite from the Takanawa Peninsula, Fe/(Fe+Mg) for biotites increased from 0.51 to 0.77 with an increase in host rock SiO2 from 53.4 to 75.5. Detailed consideration of amphibole chemistry shows predominance of edenitic and tschermakitic substitution schemes as well as coupling between substitutions of Ti in octahedral sites and AlIV. Interrelations between amphibole and biotite chemistry show that Fe/(Fe+Mg) and Mn contents can be interpreted in terms of equilibration, whereas Ti content cannot. The chemistry of chlorites correlates well with that of biotites; primary and secondary muscovites are distinct in composition. Plagioclase in all studied suites shows igneous zoning appropriate to host rock composition; perthitic alkali feldspars in all samples have lost albite component, and temperatures based on the two‐feldspar geothermometer are low. The biotite‐apatite geothermometer is also inoperative for this group of samples because fully fluorinated apatites typically occur in biotites of modest F content. Whereas magnetites have reequilibrated, analyses of ilmenites for the representative Daito‐Yokota and Takanawa suites corroborate biotite compositional data and suggest that fO2 probably differed by 2 to 3 orders of magnitude during crystallization of silica‐rich magnetite and ilmenite series granites. Whole‐rock chemistry supports mineral chemistry in suggesting that the studied granitoids have crystallized from magmas generated in a lower crustal environment in which mantle‐derived magmas partially melted source materials with igneous characteristics.
A new interpretive procedure for whole rock U‐Pb Systems applied to the Vredefort crustal profile
Authors: Welke H. , Nicolaysen L. O.
Published: February 2008

Granulite grade Precambrian gneisses have usually undergone at least one period of strong U depletion. Whole rock U‐Pb isotope studies can determine the time(s) of the severe depletion, and this work attempts to place such studies on a more rigorous footing. Two‐stage U‐Pb systems can be described in terms of one major, episodic differentiation into rocks with varying U/Pb ratios, while three‐stage systems can be described by two such distinct episodes. Most of the Precambrian granulites that have been isotopically analyzed have histories too complex to be described as two‐stage systems. However, it is demonstrated here that U‐Pb data on whole rock suites can yield the complete U‐Pb chemical history of a three‐stage system (in terms of U/Pb ratios). For a suite of granulites, present‐day 207Pb/204Pb and 206Pb/204Pb ratios and element concentration data allow these ratios to be calculated at a number of specific past times and plotted as an array. The degree of scatter in each of these ‘past arrays’ is graphed as a function of time. The point of least scatter denotes the age of the end of stage 2 in the history of the system. The array slope and the dating of the end of stage 2 also permit the beginning of stage 2 to be calculated. All other parameters in the system (U and Pb concentrations, Pb isotopic ratios) can now be determined for each individual rock throughout its history. The new interpretive method also distinguishes sensitively among various kinds of uranium fractionation which may have operated during the differentiation episodes. It is applied here to uranium‐depleted granulites in the deeper part of the Vredefort crustal profile. The times of the two fractionating episodes are calculated at ∼3860 and ∼2760 m.y., respectively. The Vredefort 3070 m.y. event, when geochemical systems in the upper half of the crystalline basement became permanently closed, evidently had little significance for the lower half of the crystalline basement. Some fundamental features of Archaean basement evolution, whose detailed significance is still poorly understood, were elucidated for the first time by this U‐Pb isotope study on whole rock samples. Rocks which suffered little uranium depletion at the end of stage 1 suffered least at the end of stage 2. Rocks which were most affected at the end of stage 1 suffered the strongest depletion at the end of stage 2. Evidently, repeated high‐grade metamorphism purges certain granulites of their U content but leaves others relatively unscathed.
Shock metamorphism in the Vredefort Collar: Evidence for internal shock sources
Authors: Lilly P. A.
Published: February 2008

Shock metamorphic microstructures in the Vredefort collar include planar features, crystallographically controlled cleavage, crystallographically controlled faults, and mosaic extinction. In addition, several recrystallization textures are developed in the quartzites of the collar, and quartz c axis distributions for both primary and recrystallized quartz grains are random. The degree of recrystallization decreases away from the core‐collar contact. Two events of shock deformation have been identified in the collar, and using planar feature orientations, shock pressures have been estimated using the technique of Robertson (1975). The first shock (D1) subjected the lowermost Witwatersrand rocks to shock pressures of about 150 kbar and the uppermost beds to pressures of about 60 kbar. Following a period of extensive recrystallization of the quartzites came the second shock event (D2), which was weaker than the first and subjected the lowermost strata in the collar to pressures of between 75 and 100 kbar. The D2 event has been shown to be separated in time from the D1 event. The results are used to show that the shock sources were probably within the earth and that the Vredefort ring structure has formed as a result of endogenous processes rather than hypervelocity meteorite impact.
Fracture mechanical models of dry slab avalanche release
Authors: McClung David M.
Published: February 2008

Experimental evidence shows that snow is a pressure sensitive, dilatant, strain‐softening material in slow, constant rate, shear deformation. When strain softening initiates in a weak layer underneath a snow slab, avalanche release is hypothesized to be possible with or without additional loading. Specifically, two cases are discussed: (1) a shear‐crack‐like disturbance can initiate after formation of a slip surface in the weak layer and traverse the layer by a self‐propagating progressive failure with or even without loading, and (2) a self‐propagating shear instability can develop when a region of the weak layer is driven past peak shear strength by loading. For these cases the implied fracture sequence, fracture geometry, time scale of release, and temperature effects are discussed and shown to be consistent with the known facts of dry slab avalanche release.
Compositional variation in normal MORB from 22°–25°N: Mid‐Atlantic Ridge and Kane Fracture Zone
Authors: Bryan W. B. , Thompson G. , Ludden J. N.
Published: February 2008

Basalts collected by dredging between 23°N and 25°N include samples from the active spreading ridges, median valley walls, and older walls of the Kane Fracture Zone transform fault at about 24°N. Chemical analyses of over 100 basalts and basalt glasses show the depletion in large ion lithophile elements characteristic of ‘normal’ ocean ridge basalts. Basalt suites recovered from the median valley north and south of the fracture zone are almost identical geochemically. There is no evidence of present or past volcanic activity within the transform zone. Petrogenesis of the basalts, as deduced from both petrographic and geochemical evidence, indicates that simple low‐pressure equilibrium crystallization during ascent in shallow vents or flow in seafloor lava tubes can account for a large part of the major element and trace element variation within discrete data subsets. However, more complex models are required to explain relations between samples showing large differences in large ion lithophile element enrichments. A subset of samples of intermediate composition can be explained by simple mixing of highly fractionated basalt with mafic parental basalt. A number of basalts enriched in Al2O3 may reflect accumulation and partial resorption of plagioclase. By comparison to existing phase equilibria experiments, it is shown that a basalt‐type common to the area is a logical choice as a mantle‐derived primary magma; it is close to the composition of average normal MORB with wt % TiO2 ≅ 1.50, Mg# ≅ 66, and normative olivine 10–15 wt %. This composition is inferred to have separated from the mantle at a pressure of about 8–9 kbar. Rare examples of more mafic ‘primary’ basalt are present and are inferred to represent melts which escaped from the mantle at higher pressures. We outline a comprehensive hypothesis for melting, fractionation, and mixing which we believe may be adequate to explain compositional variation in these and other suites of ‘normal MORB.’ An important consequence of this model is that MORB presently at a given position on a low‐pressure, multiply saturated cotectic may have reached that point by a variety of paths; some may be nearly direct mantle derivatives, while others may have experienced a complex history of polybaric and low‐pressure fractionation and/or mixing.
Petrology and petrogenesis of the Trinity peridotite, An upper mantle diapir in the eastern Klamath Mountains, northern California
Authors: Quick James E.
Published: February 2008

The Trinity peridotite is an enormous ultramafic massif situated in the eastern Klamath Mountains of northern California and composed of a diverse assemblage of ultramafic rocks including dunite, harzburgite, plagioclase lherzolite and clinopyroxene‐rich lithologies. These rocks preserve an excellent record of a complex mantle history involving plastic deformation, partial melting, recrystallization and reaction with transient silicate melts in a large peridotite mass as it ascended through the upper mantle. Structural data are utilized to determine the relative ages of events that affected the peridotite, and petrochemical data are utilized to place constraints on the pressures and temperatures at which the events occurred. The Trinity peridotite is inferred to have ascended through the upper mantle from an initial depth of not less than 30 km, based on evidence for replacement of spinel by plagioclase. Ariegite bands and dikes formed, and isoclinal folding occurred while the peridotite was in the spinel lherzolite stability field (P > 10 kbar). The peridotite passed its solidus and partially melted at a pressure of ≤10 kbar, producing a basaltic melt with tholeiitic affinities. Plagioclase lherzolite formed where partial melt was trapped and minerals reequilibrated during subsequent low pressure cooling of the peridotite. Clinopyroxene‐rich dikes were emplaced after the peridotite had passed into the plagioclase lherzolite field (P < 10 kbar). Plastic deformation, recorded by penetrative foliation and lineation, occurred continuously during the diapiric rise of the Trinity body. The emplacement of undeformed gabbro marks the arrival of the Trinity at the base of the crust. Large, tabular dunite bodies, clinopyroxene‐rich dikes and ariegite dikes mark channels through which melts passed and reacted with the peridotite en route to the surface. The interiors of the dunite bodies and the dikes are interpreted to have formed as crystal cumulates from the transient melts. The margins of the dunite bodies and depleted zones around some clinopyroxene‐rich dikes formed as a ‘restite’ by assimilation of pyroxene and plagioclase from the peridotite wall rocks by the transient melts. The melts are hypothesized to have formed by partial melting of peridotite at a great depth (e.g., 45–60 km) and to have penetrated the Trinity peridotite at shallower depths (<30 km) where they were not in equilibrium with pyroxene or plagioclase. Structural and stratigraphic data suggest that these events occurred near an oceanic volcanic highland in a setting analogous to a modern island arc or back‐arc basin.
Causes and consequences of the relation between area and age of the ocean floor
Authors: Parsons Barry
Published: February 2008

The distribution of area of the ocean floor with age, t, is approximately described by dA/dt = C0(1−(t/tm)) where C0 is the rate of crustal generation and tm the maximum age. A linear differential area versus age relation can be explained by a balance between crustal generation and consumption where consumption is uniformly distributed with age. The present distribution of consumption with age was estimated from the isochron map used to derive the area‐age relation and a recently published set of angular velocity vectors describing present plate motions. The rate of consumption per unit age shows considerable scatter about a mean value equal to C0/tm(∼1.7×10−8 km2/yr2), but consumption is sufficiently close to being uniformly distributed with age to explain the observed degree of approximation to a linear differential area‐age relation. The trenches appear to be distributed randomly with respect to age provinces in the oceans. It was not possible to discern any trend of mean subduction velocities with age apart from the smaller velocities observed for the two youngest age provinces (0–9 Ma). Changes in the rate of plate generation and the distribution of consumption with age result in shifts in the area‐age distribution. In turn, these shifts produce changes in the plate driving forces which act to restore the rate of plate generation and distribution of consumption to their initial states. The ridge pushing force, for example, decreases as the rate of crustal generation increases. This coupling between driving forces and the area‐age distribution provides a feedback mechanism limiting the extent of any changes. A measure of the magnitude of shifts in the area‐age distribution is given by global changes in sea level. If long‐term changes in sea level are less than 150 m, as suggested by recent estimates, then only small changes (∼12%) in the rate of plate generation and distribution of consumption with age are required. Sea level changes as large as 300 m require about a 50% change in the rate of crustal generation. The above area‐age relation can be combined with simple expressions for depth and heat flow versus age to obtain an empirical hypsometric distribution, parameterized in terms of age, and exact expressions for the heat loss from the ocean floor. The latter is estimated to be 2.9×1013 W, with 25% of this heat being lost by seafloor younger than 5 Ma. Of the total heat loss from the mantle 26% is estimated to be transported by small‐scale convection.
Recent large earthquakes along the Middle American Trench and their implications for the subduction process
Authors: Chael Eric P. , Stewart Gordon S.
Published: February 2008

Several large shallow earthquakes (Ms > 7.0) have occurred along the Middle American Trench since the installation of the WWSSN network. Included are the 1965, 1968, and 1978 Oaxaca events, the 1970 Chiapas event, the 1973 Colima event, and the 1979 Petatlan event. These earthquakes have been studied in an attempt to identify similarities and differences between them that may lead to a better understanding of fracture and subduction processes. The events have seismic moments ranging from 1.0×1027 dyne cm for the 1968 event to 3.2×1027 dyne cm for the 1978 event. All events are of predominantly thrust type, consistent with subduction to the northeast of the Cocos plate. Body waves for the 1965, 1968, 1978, and 1979 events along the trench indicate rather simple faulting processes. These events all had focal depths of 15 to 20 km and stress drops of the order of 10 bars. The 1970 and 1973 events, the easternmost and westernmost, respectively, of the events studied here, are located close to triple junctions for the Cocos–North America–Caribbean plates (1970) and the Cocos–North America–Rivera plates (1973). These two events generated more complex body waves than the Oaxaca and Petatlan quakes. This source complexity may have been related to the more complicated tectonics near the 1970 and 1973 epicenters. This may offer a new perspective to viewing source complexity in relation to subduction zones.
Compressional wave velocities of a lunar regolith sample in a simulated lunar environment
Authors: Johnson Douglas M. , Frisillo A. L. , Dorman James , Latham Gary V. , Strangway David
Published: February 2008

Ultrasonic compressional wave velocities have been measured in the laboratory for an Apollo 15 soil sample (15301, 38) under very low uniaxial stress and high vacuum conditions. The velocities measured range from 125 to 522 m/s. The velocities of the soil are stress dependent and are strongly affected by compaction history. Hertzian contact theory does not appear to fit the data adequately for the pressure range of the experiment. Moderate increases in temperature do not have a significiant effect on the compressional wave velocities.
Structure along the northwest edge of the Snake River Plain interpreted from seismic refraction
Authors: Pankratz Leroy W. , Ackermann Hans D.
Published: February 2008

The results of a seismic refraction survey at the Idaho National Engineering Laboratory along the northwest boundary of the Snake River Plain show that velocities of the volcanic rocks beneath the Plain increase from 1.5 km/s at the surface to 5.2–5.5 km/s at depths between approximately 2200 and 2500 m. An exploration well in the area (INEL‐1) encountered Cenozoic volcanic rock with some interbedded sediments to a depth of 2460 m underlain by 700 m of a rhyodacite porphyry. The refraction data demonstrate a nearly vertical faultlike discontinuity of about 1000‐m displacement beneath the plain, about 1.8 km from the southeast flank of the Arco Hills (Lost River Range). Whether this discontinuity represents the northwest flank of the Snake River Plain graben, the east flank of the Lost River Range fault, Or a caldera wall is not known. The results further indicate that Paleozoic rocks may extend beneath the plain as far as 5.6 km southeastward from the Arco Hills.
Anomalous shear wave delays and surface wave velocities at Yellowstone Caldera, Wyoming
Authors: Daniel Robert G. , Boore David M.
Published: February 2008

To investigate the effects of a geothermal area on the propagation of intermediate‐period (1–30 s) teleseismic body waves and surface waves, a specially designed portable seismograph system was operated in Yellowstone Caldera, Wyoming. Travel time residuals, relative to a station outside the caldera, of up to 2 s for compressional phases are in agreement with short‐period residuals for P phases measured by other investigators. Travel time delays for shear arrivals in the intermediate‐period band range from 2 to 9 s and decrease with increasing dT/dΔ. Measured Rayleigh wave phase velocities are extremely low, ranging from 3.2 km/s at 27‐s period to 2.0 km/s at 7‐s period; the estimated uncertainty associated with these values is 15%. We propose a model for compressional and shear velocities and Poisson's ratio beneath the Yellowstone caldera which fits the teleseismic body and surface wave data:it consists of a highly anomalous crust with an average shear velocity of 3.0 km/s overlying an upper mantle with average velocity of 4.1 km/s. The high average value of Poisson's ratio in the crust (0.34) suggests the presence of fluids there; Poisson's ratio in the mantle between 40 and approximately 200 km is more nearly normal (0.29) than in the crust. A discrepancy between normal values of Poisson's ratio in the crust calculated from short‐period data and high values calculated from teleseismic data can be resolved by postulating a viscoelastic crustal model with frequency‐dependent shear velocity and attenuation.