bluuebunny/crossref_metadata_2025 huggingface dataset:
Laboratory studies of the condensation and properties of amorphous silicate smokes
Authors: Nuth Joseph A. , Donn Bertram
Published: February 2008

As part of an investigation of cosmic grains, we have measured the critical partial pressure of SiO necessary to initiate nucleation from SiO‐H2 and Mg‐SiO‐H2 vapors as a function of the temperature of the ambient gas (750°K<T<1010°K). Although the presence of magnesium vapor lowers the critical partial pressure of SiO at ambient temperatures less than ∼925°K when compared to the pure SiO‐H2 system, it does not effect the onset of nucleation at higher temperatures. The infrared spectra of particles collected from our system, and of those particles which have subsequently been annealed, are quite similar to spectra of the circumstellar shells of oxygen rich stars. To a lesser extent, the spectra of these particles also resemble the spectra of some individual chondritic stratospheric dust particles and the matrix material of some carbonaceous chondrites. Determination of the temperature dependent rate of recrystallization in our synthetic materials could elucidate the temperature histories of similar ‘cosmic’ grains.
A two‐pyroxene thermometer
Authors: Lindsley Donald H. , Andersen David J.
Published: February 2008

Experimentally determined pyroxene phase relations at 800°–1200°C are combined with calculated phase equilibria for the Di‐En and Hd‐Fs joins to produce a graphical two‐pyroxene thermometer that should be applicable to a wide variety of rocks from the earth, moon, and meteorites. The thermometer can be used directly with natural pyroxenes that have low contents of Al and other minor components. Samples with higher contents of ‘others’ components require special projection onto the Ca‐Mg‐Fe pyroxene quadrilateral; Wo, En, and Fs as normally calculated will not yield correct temperatures! The special projection is necessary to approximate the activities of those components in natural pyroxenes. The effects of pressure are nonnegligible but can be corrected for. Use of the thermometer for slowly‐cooled rocks may pose special problems if the pyroxenes have undergone granule exsolution (coalescence of exsolved material to form separate grains). For example, published analyses of Mg‐rich augites from the Skaergaard intrusion give temperatures that are 50°–100°C below expected magmatic values. Textural and experimental evidence confirms that these augites have undergone granule exsolution. The primary pyroxene compositions must be reconstructed from textural evidence before correct igneous or peak‐metamorphic temperatures can be obtained from this or any two‐pyroxene thermometer. Of the numerous two‐pyroxene thermometers proposed in the literature, those of Ross and Huebner [1975] and of Kretz [1982] Ca‐reaction] yield temperatures most similar to our own, although the former tends to give temperatures about 50°C lower for some igneous pyroxenes. Other thermometers generally overestimate temperatures for metamorphic rocks (hornblende granulite facies) by 50°–100°C.
Active faulting and tectonics of Burma and surrounding regions
Authors: Le Dain Anne Yvonne , Tapponnier Paul , Molnar Peter
Published: February 2008

This study of the active tectonics of Burma and surrounding regions is based mostly on an interpretation of Landsat imagery and on fault plane solutions for shallow and intermediate earthquakes. Fold axes in the Indoburman ranges and an east dipping inclined zone of intermediate depth earthquakes suggest that a slab of oceanic lithosphere was recently subducted to the east under the Indoburman ranges. Fault plane solutions of shallow earthquakes, however, do not show underthrusting at the present time. Instead, P axes are oriented roughly north‐south, parallel, not perpendicular, to the fold axes in the ranges. P axes for fault plane solutions of intermediate shocks also trend north‐south, parallel to the strike of the subducted slab, and may indicate that the hanging slab is being dragged north by India through the surrounding asthenosphere. The Sagaing fault probably accommodates most of the right‐lateral slip of India past Indochina, but India's northward movement also causes internal deformation within Burma, Thailand, and Yunnan. Differences between the tectonic fabric seen on the Landsat images and the tectonic regime compatible with the fault plane solutions throughout the region imply major changes in the directions of relative motion, in the orientations of strain, and in the style of deformation in the late Cenozoic. Presumably, these changes reflect the transition from subduction to collision and strike‐slip motion as India penetrated into Eurasia past Burma. Large parts of Burma and Thailand may have rotated several tens of degrees clockwise in this process.
Eruption age of a ∼100,000‐year‐old basalt from 40Ar‐39Ar analysis of partially degassed xenoliths
Authors: Gillespie A. R. , Huneke J. C. , Wasserburg G. J.
Published: February 2008

We have applied 40Ar‐39Ar dating, using stepwise thermal extraction of Ar, to five potassium‐rich granitic xenoliths and their host basalt from Sawmill Canyon in the Sierra Nevada, California. Previous K/Ar analyses showed the age of the basalt to be roughly 100,000 years or less. The xenoliths, which had accumulated large amounts of radiogenic 40Ar since their crystallization ∼100 m.y. ago, were partially degassed upon their inclusion in the basaltic magma. Ar released from the xenoliths in the laboratory at temperatures substantially below the melting temperature of the basalt, was created since the host magma cooled. Isotopic compositions of Ar released from the xenoliths in several extraction steps at temperatures below ∼900°C were colinear in 36Ar/40Ar versus 39Ar/40Ar diagrams and defined isochrons giving a mean age of degassing of 119,000±7000 (2σ) years. 40Ar extracted at higher temperatures included ancient radiogenic 40Ar that was never diffused from the xenoliths during immersion in the magma. This 40Ar caused an increase in the apparent age for the high‐temperature extractions. The high precision of the eruption age determined by this method is comparable to that obtained elsewhere by conventional K/Ar dating of sanidine. 40Ar‐39Ar analysis of granitic xenoliths to date young basaltic lava flows may prove to yield results superior to those found from analysis of the lava itself. Establishing the age of eruption of the basalt flow in Sawmill Canyon establishes age limits for two Sierran glaciations which left moraines stratigraphically above and below the lava. Thus the younger glaciation must be Wisconsin; the older must be pre‐Wisconsin in age.
Seismotectonics of the Himalayan Collision Zone: Geometry of the underthrusting Indian Plate beneath the Himalaya
Authors: Ni James , Barazangi Muawia
Published: February 2008

Fault plane solutions and well‐determined focal depths of medium‐sized earthquakes, topography, and Landsat imagery in conjunction with seismicity maps, cross sections, and available geological information are used to investigate the present tectonics of the Himalayan continental collision zone. Most of the accurately located epicenters of events along the Himalayan arc (78°E–95°E) that occurred between 1961 and 1981 are concentrated in a narrow zone, about 50 km wide, lying between the northerly dipping Main Boundary (MBT) and Main Central (MCT) thrusts. Most of these events are located just south of the MCT. Though the epicenters of the events are, in general, well located, their depths as determined by teleseismic travel time data are very unreliable. Events with accurately determined depths obtained from identification of surface‐reflected phases define a simple, planar zone from about 10‐km and 20‐km depth, with an apparent dip of about 15°. This result is all the more remarkable considering that the events used were located along about an 1800‐km length of the Himalyan arc. Except for one, all available focal mechanisms of events within this zone indicate shallow ( ≲30°), north dipping thrusts. This shallow, north dipping zone apparently defines a part of the detachment that separates the underthrusting Indian plate from the Lesser Himalayan crustal block. The spatial extent and the geometry of this interplate thrust zone strongly indicate that the MBT and nearby subsidiary surface and blind thrusts, rather than the MCT, are currently the most active structures of the Himalayan arc. We suggest that the great Himalayan earthquakes (M>8) occur along the same detachment surface as defined by the thrust‐type, medium‐sized events. Events located to the south of the MBT and beneath the Ganges foredeep show normal faulting withTaxes perpendicular to the Himalayan trend. The above results suggest that the Indian continental plate is underthrusting the Himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones. The November 19, 1980, earthquake that occurred near the MCT (near 88.5°E) shows a predominantly strike‐slip focal mechanism. One of the nodal planes of this mechanism is transverse to the Himalayan structural grain, and moreover, this plane has a trend similar to that of the recently mapped Yadong‐Gulu rift in the Tethyan Himalaya and in southern Tibet just northeast of the earthquake. We interpret this predominantly left‐lateral, strike‐slip mechanism to indicate a possible genetic relationship between transverse structural features in the Underthrusting Indian plate (the Kishangang basement fault) and the upper Himalayan blocks and Tibet.
Magnetostratigraphy, paleomagnetism, and remanence acquisition in the Triassic chugwater formation of Wyoming
Authors: Shive Peter N. , Steiner Maureen B. , Huycke David T.
Published: February 2008

The magnetic reversal sequence determined at five sections of the Chugwater Formation can be correlated over a distance of more than 200 km. A detailed study of 203 cores constituting a grid of samples taken from a normal to reverse transition zone at the Red Grade section shows that the transition is characterized by a fairly smooth 180° direction change accompanied by an intensity decrease in about 80 cm of section. The transition occurs in the same way in all vertical columns within the grid and in other columns at the same stratigraphic level up to 80 m from the grid and appears identical to the transition reported by Herrero‐Bervera and Helsley at the same lithologic horizon about 1 km from the grid. Thermal and chemical demagnetization of grid samples removes a secondary remanence in the general direction of the earth's present magnetic field and does not reveal the presence of normal and reverse components in the same sample. The transition appears to have been magnetized during a reversal of the earth's field. Samples probably became magnetized in a few hundred years or less while they were close to the deposition surface. Paleomagnetic pole positions for all five sites are very closely clustered, and there appears to be no consistent long‐term polar wander from bottom, to top in the sections. The mean pole position (46.6°N, 113.5°E; α95 = 1.88°) is significantly different from pole positions for the Early Triassic Moenkopi Formation from the Colorado Plateau, suggesting that post‐Triassic rotation has occurred between the plateau and the basement uplifts of Wyoming.
The morphology of propagating spreading centers: New and old
Authors: Mammerickx J.
Published: February 2008

Recent Seabeam investigations over the East Pacific Rise in the northeast Pacific have shown the existence of a propagating spreading center at 18°N. The propagating and dying spreading centers are joined by a zone of orthogonal lineations which appear to be the traces of a continuously migrating transform fault. Similar orthogonal lineations occur at 16°N, 107°30′W on the western side of the trace of a 5‐m.y.‐old propagating spreading center. The amplitude and the complexity of the topographic signature over the earlier propagating spreading center are higher and greater than over the presently active center, suggesting that the relief of the earlier center may owe its origin to differential subsidence of thermally contrasting segments of lithosphere, analogous to fracture zone topography of similar origin. Traces of former propagating spreading centers are thus a combination of two sets of landforms: those resulting from propagation and those resulting from the juxtaposition of two blocks of lithosphere of different thickness.
Sm‐Nd and Rb‐Sr systematics in volcanics and ultramafic xenoliths from Malaita, Solomon Islands, and the nature of the Ontong Java Plateau
Authors: Bielski‐Zyskind Miriam , Wasserburg G. J. , Nixon P. H.
Published: February 2008

A Rb‐Sr and Sm‐Nd study was carried out on five ultrabasic inclusions, three alnoite host rocks, and one sample of basalt from Malaita in an effort to determine the isotopic stratigraphy of the mantle in this area. The results of this study show the presence of relatively undepleted mantle segments underlying relatively older depleted upper mantle. The alnoite samples, three megacrysts and one Iherzolite inclusion, and the basalt are relatively uniform isotopically and yielded εNd (T) ≈ +3.5 and εSr (T) ≈ −6 to +17 (T = 100 m.y. for the basalt and Iherzolite and 34 m.y. for the rest of the samples). It is suggested that a mantle segment of that isotopic composition exists at a depth of more than 100 km. This mantle segment is relatively undepleted. A Iherzolite inclusion with εNd (T) = 6.6 is included in the alnoite and indicates the location of a light rare earth element depleted layer overlying a less depleted mantle at a depth of approximately 100 km. The mantle source for the alnoite and basalt cannot be old, depleted oceanic mantle or subducted old continental crust but may come from a relatively young, slightly depleted mantle segment. The isotopic results are compatible with a mixture of about 99% depleted oceanic mantle homogenized with 1% continental crust possibly by metasomatic processes. Such a mixture is only slightly fractionated and has ƒSm/Nd ≈ 0.13 and ƒRb/Sr ≈ 0. Assuming that the basalt is characteristic of the Ontong Java Plateau, a volume greater than 108 km3 of mantle source with εNd ∼ 3.5, εSr ∼ 0 was involved. This corresponds to a block of continental crust of ∼ 106 km3 which had to reach a depth greater than 100 km to produce the proposed nearly homogeneous isotopic mixture. Alternatively, the source of the Ontong Java Plateau may be a segment of a type of young continental crust with the appropriate isotopic signature which was subducted below 100 km. This would not require homogenization with a large volume of depleted mantle.
Deformation of the Mitaka Rhombus: Strain buildup following the 1923 Kanto earthquake, central Honshu, Japan
Authors: Thatcher Wayne , Fujita Naomi
Published: February 2008

Frequently repeated surveys of a local network near Tokyo provide considerable detail on the horizontal strain changes immediately adjacent to the rupture zone of the M = 8.1 Kanto earthquake. Changes in the three independent tensor strain components are determined for 35 epochs during 1916–1980 to a precision of about ±1.5 μstrain. No significant shear or elongational strain changes preceded the earthquake; a change in dilational strain is marginally significant. Postseismic movements are large, accumulating to 40% or more of the coseismic strain drop. They have at least two time scales. A distinct short‐term transient lasting less than a year is explained well by slip or localized deformation that occurs downdip of the coseismic rupture plane and has a time constant of about 3 months. More complex, longer‐period postseismic movements continue for at least another 10 years. Their precise duration, as well as the transition to presumably steadier interseismic deformation, is obscured by notable irregularities in the strain buildup at Mitaka. As a result, neither the interseismic strain rate nor the occurrence time of the next Kanto earthquake can be reliably estimated from these observations.
Deformation of clinopyroxenite: Evidence for a transition in flow mechanisms and semibrittle behavior
Authors: Kirby Stephen H. , Kronenberg Andreas K.
Published: February 2008

A systematic suite of constant strain rate experiments was performed on a vacuum‐dried, high‐purity, fine‐grained clinopyroxenite using NaCl and NaF as confining media in a Griggs‐type piston‐cylinder apparatus. The experiments were carried out over a range of temperatures from 400° to 1100°C, strain rates from 10−3 to 10−7 s−1, and confining pressures from 170 to 1990 MPa. At T = 600°C and
= 1.1 × 10−5 s−1, three modes of deformation occur with increasing confining pressure: (1) Macroscopic faulting associated with low strength and stress drops, (2) stable microfracturing and plastic deformation associated with pressure‐dependent strength, and (3) plastic deformation (mechanical twinning and 〈001〉 slip) with high strengths which are insensitive to pressure variations. In experiments at P = 1500 MPa, within this high‐pressure plastic mode, two regimes of flow are clearly defined. At low to intermediate temperatures and high strain rates, flow strengths are insensitive to changes in strain rate and temperature. Optical and transmission electron microscope observations indicate that plastic strain is accomplished by mechanical twinning on (100) and (001) and by {100}〈001〉 slip. In contrast, at high temperatures and low strain rates the flow stress is strongly dependent on temperature and strain rate. Specimens deformed in this regime show evidence of recovery, multiple slip, and recrystallization; and plastic strain is much more homogeneous. The flow data within each regime can be satisfactorily fit to thermally activated power laws. In the low‐temperature regime n (the stress exponent) = 83 ± 16 and E* (the activation energy for flow) = 220 ± 40 kJ/mol. We believe that these parameters reflect flow dominated by the kinetics of dislocation glide associated with mechanical twinning and (100)〈001〉 slip. In the high‐temperature regime, n = 5.3 ± 1.1 and E* = 380 ± 30 kJ/mol. These parameters describe creep by multiple slip accompanied by increased rates of diffusion and recovery.
Water in minerals? A peak in the infrared
Authors: Aines Roger D. , Rossman George R.
Published: February 2008

The study of water in minerals with infrared spectroscopy is reviewed with emphasis on natural and synthetic quartz. Water can be recognized in minerals as fluid inclusions and as isolated molecules and can be distinguished from hydroxide ion. The distinction between very small inclusions and aggregates of structurally bound molecules is difficult. New studies of synthetic quartz using near‐infrared spectroscopy are reported. These demonstrate that water molecules are the dominant hydrogen containing species in synthetic quartz but that this water is not in aggregates large enough to form ice when cooled.
The effect of varying water contents on the creep behavior of Heavitree quartzite
Authors: Jaoul Olivier , Tullis Jan , Kronenberg Andreas
Published: February 2008

Creep and constant strain rate experiments have been performed on Heavitree quartzite samples with different amounts of available water, at 15 kbar confining pressure, 800°–1100°C, 10−4 to 10−7/s strain rate, and 1–10 kbar deviatoric stress. Some samples were dried by vacuum heating, others were left as is, and others had 0.1–0.5 wt % water added to them before being mechanically sealed in a Pt tube and deformed. When the creep data are fit to a power law form of flow law, they show, with increasing water available, a decrease in the activation energy from 44 to 41 to 35 kcal/mol, and a decrease in the stress exponent from 3.3 to 2.3 to 1.8. Samples deformed at 900°C and 10−6/s showed corresponding changes in preferred orientations from a diffuse maximum parallel to to a small circle girdle about σ1 and a change in deformation lamellae orientations from basal to basal plus prismatic. The samples also showed corresponding textural changes, from little recovery and no recrystallization, to greater recovery with moderate amounts of fine grain boundary recrystallization and small isolated melt pockets (<1 vol %), to even greater recovery with less continuous but coarser recrystallization and somewhat more extensive grain boundary melt (<3 vol %). The low stress exponent of the water added samples may be due to a component of grain boundary sliding, allowed by the grain boundary recrystallization and melt in these samples, but there is no microstructural evidence for such a mechanism. The change in slip systems of the original grains in the water added samples might be due in part to some relaxation of grain boundary constraints. The ease of recovery in the as is and water‐added samples suggests that deformation of these samples was glide controlled and the chief mechanical effect of the added water may be to enhance dislocation glide.
Basin formation along the Bocono‐Moron‐El Pilar Fault System, Venezuela
Authors: Schubert Carlos
Published: February 2008

Within the boundary zone between the Caribbean‐South America plates, which includes the Boconó‐Morón‐El Pilar right‐lateral fault system that has been active since the late Tertiary, there are several sedimentary basins in the Venezuelan Andes and the Caribbean Mountains. These were formed in zones of crustal depression, where faults diverge, and as pull‐apart basins in zones of en echelon stepovers (or along releasing bends) in the faults. In addition, there are pull‐apart basins along the Avila and La Victoria faultzones in the Caribbean Mountains, which are associated with the plate boundary. The basins are filled with alluvial, lacustrine, fluvial, and marine sediments, mainly of Quaternary age. On the basis of model studies, dimensions of the basins, and age of their sedimentary fill, it is estimated that right‐lateral offset necessary to form them along the Boconó‐Morón‐El Pilar fault system varies from 2 to 125 km. Along the Avila and La Victoria fault zones, this right‐lateral offset is estimated to range from 2 to 30 km. On this basis, the maximum right‐lateral offset along the southern Caribbean plate boundary that can be documented along the Boconó‐Morón‐El Pilar fault system has not exceeded 100–125 km. If there has been larger offset, it has to be documented by other means, or it has taken place along other fault zones within the system.
Roughness at the base of the seismogenic zone: Contributing factors
Authors: Sibson Richard H.
Published: February 2008

The cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300°C. It can generally be modeled as the transition from frictional to quasi‐plastic behavior in quartzofeldspathic crust. Shear resistance increases with depth through the frictional regime to peak at the transition, beneath which it falls off exponentially with increasing temperature. Larger earthquake ruptures (ML>5.5) nucleate around this transition depth where the highest concentrations of strain energy may accumulate. Varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone. Factors affecting the depth of the transition include crustal composition, geometry and mode of faulting, fluid pressure levels in the frictional regime, and water content in the quasi‐plastic regime, quasi‐plastic strain rate, and geothermal gradient. Evaluation of their relative importance is complicated because several are interdependent. However, compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance, while regional variations in heat flow look to be particularly effective in creating long‐wavelength heterogeneities in strain energy concentration affecting faulting style.
On the constitution of the San Andreas Fault Zone in central California
Authors: Wang Chi‐Yuen
Published: February 2008

The constitution of the San Andreas fault zone in central California is modeled by interpreting the recently available geophysical data and by using some broad geochemical and mineralogical constraints. The available geophysical data in this region are consistent with a model that the fault zone throughout the seismogenic depths is composed of saturated, clay‐rich fault gouge, with composition similar to the materials exposed along its surficial trace and those recovered from shallow depths. This model appears compatible with the present understanding of mineral stability under the probable PT conditions and the geochemical environment along the fault zone. It implies that materials in the fault zone are the product of extensive hydrothermal alteration of cataclastic rocks with an aqueous fluid rich in CO2 and that large‐scale transfer of material between the surrounding crust and the fault zone has taken place. Gravity anomalies and physiographic characteristics of the fault zone provide additional support for this model and suggest that the observed weltlike pressure ridges and uplifted sag ponds along the San Andreas may result from upwelling of low‐density fault zone material under the lithostatic pressure of the surrounding crust and that such processes may still be continuing at the present time. Experimentally determined seismic wave velocities and electrical resistivity of serpentinites in the California Coast Range and elsewhere are mostly significantly higher than the properties of the San Andreas fault zone, suggesting that serpentinite might not be a voluminous fault zone component, although isolated slivers of serpentinite in the fault zone are likely sources for the observed magnetic anomalies.
Recent faulting and microearthquakes at the intersection of the Vema Fracture Zone and the Mid‐Atlantic Ridge
Authors: Rowlett Hugh , Forsyth Donald W.
Published: February 2008

New air gun reflection profiles, 3.5‐kHz reflection profiles, and microearthquake data recorded by an array of ocean bottom seismographs form the basis for this study of the transition from a spreading center to a major transform fault. Disturbances of the thick, normally flat‐lying, turbidite deposits provide indications of recent vertical motions. At the western intersection of the fracture zone with the median valley there is a depression in the sediments that represents the southerly extension of the median valley into the fracture zone valley. The depression is terminated abruptly on the south by the active transform fault, which acts as a locus for vertical as well as horizontal displacement. Flat‐lying, undisturbed sediments terminate abruptly at the fault. The western boundary of the depression is much broader and is characterized by a series of slumplike steps. To the west, there is little or no evidence for uplift or tilting of sediments which might indicate vertical recovery of the crust as it spreads away from the depression. This suggests that uplift and recovery out of the depression is episodic in nature and has been inactive over the last million years along the western boundary. To the east there is clear evidence of uplift and tilting of sedimentary layers. A basement ridge emerging from the sediments is currently being uplifted and rotated in a manner analogous to processes responsible for the creation and cancellation of median valley relief. The transition between the spreading center and the transform fault appears to take place within 1–2 km. The width of the transform fault just east of the depression is less than a kilometer. Microearthquakes were located and displayed by new methods that directly account for nonlinearities associated with small arrays. Microearthquakes located by three or more ocean bottom seismometers show that the greatest seismic activity occurs along the eastern walls of the median valley, at the basement ridge, in the eastern portion of the depression and in the crestal mountains. Very little activity is associated with the western edge of the transform depression and the trace of the transform fault.
Marine seismic refraction study between Cape Simpson and Prudhoe Bay, Alaska
Authors: Bee M. , Johnson S. H. , Chiburis E. F.
Published: February 2008

A marine seismic refraction study conducted between Cape Simpson and Prudhoe Bay, Alaska, provides data for subsurface structural and geological cross sections of the western Beaufort Shelf. The results suggest that the western Beaufort Shelf is underlain by a sedimentary prism with beds dipping and prograding to the northeast. Correlation of the refraction data with wells drilled on land and offshore reflection profiles permits tentative identification of geologic sequences on the basis of their seismic velocity. This study associates near‐surface velocities of 1.60–1.65 km/s with Quaternary sediments and 1.82–2.35 km/s velocities with Tertiary strata. Velocities of 1.60–3.40 km/s are correlated to Upper Cretaceous rocks, 2.10–4.14 km/s to Lower Cretaceous beds, 5.66–5.68 km/s to Mississippian‐Pennsylvanian carbonates, and 4.24–6.08 km/s to the Franklinian basement composed of argillites and phyllites. West of Cape Halkett, the sedimentary section is mainly Lower Cretaceous, whereas east of Cape Halkett, Upper Cretaceous and Tertiary strata dominate. Higher velocities (6.40–7.07 km/s) are thought to represent the crystalline basement, probably silicic in composition. Although no seismic velocities typical of the upper mantle are present on the record sections, a minimum depth calculation places the Mohorovicic discontinuity no shallower than 20 km. The interpretation indicates that the stratigraphic sequence present onshore at Prudhoe continues to the offshore and implies that the western Beaufort Shelf may be a good prospect for oil. The structures derived from seismic data indicate that the Beaufort continental margin is not different from Atlantic‐type margins and add further evidence to the concept of a rifted and rotated Beaufort continental margin.
Magnetostratigraphy and geochronology of northwest Iceland
Authors: McDougall Ian , Kristjansson Leo , Saemundsson Kristjan
Published: February 2008

The NW peninsula of Iceland is built mainly by tholeiitic, low K flood basalts that show a regional southeasterly dip, generally of less than 10°. From the lavas mapped and drilled for paleomagnetic measurements (a total of 1261 flows), two continuous composite sections were constructed, one on the western side of the peninsula 4055 m thick comprising 456 lavas and the other adjacent to the east coast, 3165 m thick comprising 396 lavas. K‐Ar age measurements were made on more than 70 lavas distributed throughout the two sections. These two lava sequences are about 90 km apart along strike, and although direct stratigraphic correlation is not possible between them, the evidence indicates that the eastern section is stratigraphically younger than that in the west, with some overlap. The results show that the lava sequences were erupted over an interval extending from about 14 to 8 Ma ago in the middle to late Miocene. Remarkably linear relations are found between the measured age and cumulative stratigraphic thickness for both sections, indicating rather uniform local rates of growth of the lava piles. For the western composite section a growth rate of 1820 m/Ma is found, contrasting with a much lower growth rate of 670 m/Ma for the eastern sequence. The average thickness of individual lavas in both sections is close to 8 m. The average time between eruptions of lavas in the western section is 5000 years, compared with a value of 12,000 years for the eastern section. The regional geology together with our measurements are interpreted in terms of crustal accretion processes by which the NW peninsula of Iceland was constructed by eruption of lavas from a spreading center of northeasterly strike. It is not clear whether the differences in rates of growth of the lava pile in the western and eastern sections are because of variation in eruptive activity with time or because eruptive activity has varied along the strike of the spreading center. Paleomagnetic measurements on the lavas provide a detailed history of polarity of the geomagnetic field during the interval 14–8 Ma ago. For the eastern composite section there is excellent and unambiguous correlation of the observed polarity pattern with the marine magnetic anomaly record. From these data we obtain an age of 9.6 Ma for the younger boundary of marine magnetic anomaly 5

Table 1 is available with, entire article on microfiche. Order from American Geophysical Union, 2000 Florida Ave., N.W., Washington, D.C. 20009.
Florida: A Jurassic transform plate boundary
Authors: Klitgord Kim D. , Popenoe Peter , Schouten Hans
Published: February 2008

Magnetic, gravity, seismic, and deep drill hole data integrated with plate tectonic reconstructions substantiate the existence of a transform plate boundary across southern Florida during the Jurassic. On the basis of this integrated suite of data the pre‐Cretaceous Florida‐Bahamas region can be divided into the pre‐Jurassic North American plate, Jurassic marginal rift basins, and a broad Jurassic transform zone including stranded blocks of pre‐Mesozoic continental crust. Major tectonic units include the Suwannee basin in northern Florida containing Paleozoic sedimentary rocks, a central Florida basement complex of Paleozoic age crystalline rock, the west Florida platform composed of stranded blocks of continental crust, the south Georgia rift containing Triassic sedimentary rocks which overlie block‐faulted Suwannee basin sedimentary rocks, the Late Triassic‐Jurassic age Apalachicola rift basin, and the Jurassic age south Florida, Bahamas, and Blake Plateau marginal rift basins. The major tectonic units are bounded by basement hinge zones and fracture zones (FZ). The basement hinge zone represents the block‐faulted edge of the North American plate, separating Paleozoic and older crustal rocks from Jurassic rifted crust beneath the marginal basins. Fracture zones separate Mesozoic marginal sedimentary basins and include the Blake Spur FZ, Jacksonville FZ, Bahamas FZ, and Cuba FZ, bounding the Blake Plateau, Bahamas, south Florida, and southeastern Gulf of Mexico basins. The Bahamas FZ is the most important of all these features because its northwest extension coincides with the Gulf basin marginal fault zone, forming the southern edge of the North American plate during the Jurassic. The limited space between the North American and the South American/African plates requires that the Jurassic transform zone, connecting the Central Atlantic and the Gulf of Mexico spreading systems, was located between the Bahamas and Cuba FZ's in the region of southern Florida. Our plate reconstructions combined with chronostratigraphic and lithostratigraphic information for the Gulf of Mexico, southern Florida, and the Bahamas indicate that the gulf was sealed off from the Atlantic waters until Callovian time by an elevated Florida‐Bahamas region. Restricted influx of waters started in Callovian as a plate reorganization, and increased plate separation between North America and South America/Africa produced waterways into the Gulf of Mexico from the Pacific and possibly from the Atlantic.
The role of water in the deformation of dunite
Authors: Chopra P. N. , Paterson M. S.
Published: February 2008

Specimens of two natural dunites (Anita Bay, of 100 μm grain size, and Åheirn, of 900 μm grain size) have been deformed in experiments at temperatures between 1200° and 1400°C and constant strain rates between 10−4 and 10−6 s−1 in a gas‐medium deformation apparatus at 300 MPa confining pressure. Prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200°C for >60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water. The predried specimens are much stronger than specimens tested under wet conditions in an earlier study. The predried specimens also do not show the apparent grain size dependence seen under wet conditions. Infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock, flow stress can be correlated with the intensity of a residual “water related” broad absorption. The addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component. The weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water‐related species in the grain boundaries, but intragranular weakening may also be contributing. It is suggested that the concentration of the water‐related species in the grain boundaries is governed by its concentration in the small melt fraction present. The amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0.01 wt %, amounts much less than most estimates of the water content of the earth's undepleted upper mantle. Such water may have an important role in determining upper mantle flow behavior.

Stress‐strain curves for all experiments are available with entire manuscript on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009‐ Document B84‐008, $2.50. Payment must accompany order.
Craters, calderas, and hyaloclastites on young Pacific seamounts
Authors: Batiza Rodey , Fornari Daniel J. , Vanko David A. , Lonsdale Peter
Published: February 2008

Craters, calderas, and bedded hyaloclastites are commonly associated with seamounts. New Sea MARC 1 side‐looking sonar data for the summit of MOK seamount located near the East Pacific Rise (EPR) at 10°N show that MOK has a large caldera consisting of three coalesced circular depressions. These data also reveal many features such as stepped crater walls, talus deposits, ring faults, intracaldera flows, small cones, lava tubes and channels, and other features. Results of an ALVIN submersible dive in the crater of seamount “F” near the EPR at 21°22′N, 108°37′W revealed the presence of bedded hyaloclastite deposits; bedded hyaloclastites were also recovered by dredging of six other volcanoes all with summit craters. The bedded hyaloclastites are thin blankets of several types of basalt glass shards in a matrix of bottom sediment, clay, and ferromanganese minerals. The glass shards are chemically homogeneous and similar to midocean ridge basalts and transitional basalt lavas typical of young seamounts. Hyaloclastite deposits are crudely inverse graded with tabular, platy shards parallel to bedding. The characteristics of bedded hyaloclastites on seamount suggest an origin by rapid eruption rate, explosive mixing of magma and seawater, followed by rapid transport and deposition, such as may occur during submarine lava fountaining.
Tertiary Ishizuchi Cauldron, southwestern Japan Arc: Formation by ring fracture subsidence
Authors: Yoshida Takeyoshi
Published: February 2008

The Tertiary Ishizuchi Cauldron, in the Setouchi volcanic belt of Miocene age in northwestern Shikoku, is 7–8 km in diameter and includes outer and inner ring fractures, inward dipping welded tuffs, central plutons, and ring fault complexes. Andesitic to dacitic welded tuffs within the central subsided block demonstrate repeated explosive activity during evolution of the cauldron. Inward dips of ring fractures surrounding the cauldron indicate that the bounding faults have the form of an upward opening cone. The cone shape is substantiated by peripheral upturning of intracaldera welded tuffs downdropped against the encircling ring faults. The ring fault complexes consist of rhyolitic to andesitic composite dikes, as well as intrusive tuffs and breccias from which some of the adjacent pyroclastic volcanic deposits were probably erupted. Central plutons of porphyritic granodiorite and fine‐grained adamellite were intruded about 14 Ma ago, have walls dipping outward and flat roofs, and may have formed concave‐upward tabular bodies which pass downward into ring dikes and upward into resurgent domes. Many Valles‐type calderas including Ishizuchi Cauldron are located in the area where the direction of the maximum horizontal compression of regional origin is parallel to arc structure in the middle Miocene southwestern Japan arc.
Late Quaternary caldera volcanoes of the Kenya Rift Valley
Authors: Williams L. A. J. , MacDonald Ray , Chapman G. R.
Published: February 2008

There are seven major, late Quaternary (≤1 m.y.), trachytic caldera volcanoes in the Kenya Rift Valley; the Barrier, Emuruangogolak, Silali, Paka, Menengai, Longonot, and Suswa. These are situated in the inner troughs of the rift. Their positions are not apparently related to transverse lineaments or basement structures. The structural development was complex, involving combinations of the formation of large calderas and small calderas/pit craters, the growth of lava and tuff cones and the eruption of fissure basalts. Two groups of volcanoes are broadly distinguished. A southern group, comprising Suswa, Longonot, and Menengai, are overwhelmingly of trachytic composition. Krakatau‐style formation of calderas approaching 100 km2 in area was accompanied by the eruption of voluminous ash flow and airfall tuffs (20–50 km3). In the remaining, more northerly volcanoes, basalts and mugearites form bimodal associations with trachytes. Caldera formation was accompanied by only sparse pyroclastic activity and the collapse may have been initiated by withdrawal of magma from depth. Knowledge of the geochemical evolution of the complexes is limited by the scarcity of published analytical data. The only detailed study is that of Menengai, where chemical variations were caused, at different structural stages, by magma mixing, crystal fractionation and various liquid‐state differentiation processes
Internal geology and evolution of the Redondo Dome, Valles Caldera, New Mexico
Authors: Nielson Dennis L. , Hulen Jeffrey B.
Published: February 2008

Deep geothermal drilling in the resurgent Redondo dome of the Valles caldera has allowed us to define a consistent intracaldera stratigraphic sequence that differs in a number of respects from the temporally equivalent sequence outside the caldera. Above the deeply eroded, Pliocene Paliza Canyon Formation, felsic ash flows and sediments in the dome area form a complex sequence of undetermined age that we call the Lower Tuffs. An erosional interval separates these rocks from the overlying Otowi Member (1.4 m.y.) of the Bandelier Tuff. Another period of erosion, during which a tuffaceous sandstone was deposited, separates the Otowi from the overlying Tshirege Member (1.1 m.y.) of the Bandelier. Both the Otowi and Tshirege members, with maximum thicknesses of 833 and 1155 m, respectively, are substantially thicker within the caldera than outside, indicating simultaneous deposition and cauldron subsidence. Both are predominantly densely welded with distinctive interior zones of granophyric crystallization. Resurgent doming was initiated after emplacement of the Tshirege Member as evidenced by erosion of the upper portions of the cooling unit prior to deposition of overlying units. An isopach map of the sandstone deposited during this erosional period shows that streams draining the uplifting dome were localized along the present Redondo Creek trend. Subsequent volcanic activity resulted in the formation of at least three additional ash flow tuff cooling units prior to deposition of caldera fill and eruption of the Redondo Creek Member of the Valles Rhyolite. A numerical model applied to formation of the Redondo dome suggests that the top of the causative magma body is located at a depth of about 4700 m, 1458 m beneath the bottom of the deepest geothermal well in the dome. No wells have penetrated intrusives that could be related to this magma. We suggest that the locations of faults bounding the apical graben of the dome were influenced by older faults associated with the northeast trending Jemez lineament. These faults were active early in the uplift history of the dome and account for many of the structural differences between hypothetical dome development and reality.
Calderas and ash flow tuffs of the Mogollon Mountains, southwestern New Mexico
Authors: Ratté J. C. , Marvin R. F. , Naeser C. W. , Bikerman M.
Published: February 2008

The Mogollon Mountains are a major volcanic source area in the southwestern Mogollon‐Datil volcanic field, where about 2000 m of middle‐Tertiary rocks are exposed. The volcanic sequence includes eight major (>100 km3) ash flow units, believed to represent four caldera‐related, compositionally zoned ash flow sequences. Each caldera‐related sequence shows normal compositional zonation from high‐silica rhyolite to rhyolite and dacite. The Mogollon caldera (34 Ma) is preserved only as a fragment in the wall of the Bursum caldera. The Emory caldera (34 Ma) is about 50 km east of the Mogollon Mountains, and only the high‐silica facies of its presumed outflow sheet (Fall Canyon Tuff) reaches the Mogollons. The Gila Cliff Dwellings caldera (30 Ma), earlier interpreted as the source of the Bloodgood Canyon Tuff, probably is a large remnant of a caldera or pair of calderas that were the source of the Davis Canyon and Shelley Peak Tuffs. The Bloodgood Canyon Tuff was most likely erupted from the Bursum caldera (29–28 Ma), but ponded in the Gila Cliff Dwellings caldera. The resurgent dome of the approximately 40‐km‐diameter Bursum caldera is the predominant topographic and structural feature of the Mogollon Mountains. The caldera probably collapsed initially in response to eruption of the Bloodgood Canyon Tuff, then was largely filled with Apache Spring Tuff prior to resurgent doming. The cyclical eruption, beginning about 34 Ma, of similar compositionally zoned ash flows from separate shallow magma chambers, suggests derivation from a common parent magma of batholithic proportions. A regional Bouguer gravity low of about 40 mGals has been interpreted to reflect the presence of such a batholith. Major ash flow eruptions in the Mogollon‐Datil field were followed 26–25 Ma by fundamentally basaltic volcanism, characterized mainly by andesite. About 22–21 Ma, high‐silica rhyolite, tholeiitic basalt, and alkali basalt were erupted as a bimodal suite roughly coincident with the beginning of basin‐range extensional faulting. Alkali olivine basalt as young as 5.5 Ma is interlayered with basin fill (Gila Conglomerate) adjacent to the Mogollon Mountains.
Calderas of the Marysvale Volcanic Field, west central Utah
Authors: Steven T. A. , Rowley P. D. , Cunningham C. G.
Published: February 2008

Five calderas (Three Creeks, Big John, Monroe Peak, Mount Belknap, and Red Hills) have been identified and a sixth (Lion Flat) tentatively identified in the middle Tertiary Marysvale volcanic field of western Utah. The calderas are manifestations of separate magma bodies that worked their way to shallow depths between 27 and 19 Ma. No coherent ash flow field exists in the Marysvale volcanic field; instead, the ash flow units are interleaved at different places in the general volcanic sequence. The magma chambers that vented ash flows and collapsed as calderas formed discrete high‐level plutons; they are probably underlain by a deep composite batholith consisting of many individual plutons from two distinct petrogenetic types, calc‐alkalic and bimodal (mafic‐silicic). The four largest calderas can be separated into two groups by contrasting structural behavior. The Three Creeks and Big John calderas are broad faulted downwarps that sagged into relatively deep magma chambers whose roofs were sufficiently strong to inhibit development of ring faults. In contrast, the Monroe Peak and Mount Belknap calderas are subcircular calderas that collapsed into shallow magma chambers with thin roofs, so easily breached that large volumes of lava were erupted within the calderas. Neither type shows strong postcollapse resurgence; roofs above the deeper magma chambers were too strong to be uplifted, and those above the shallow chambers were too weak to contain the magmatic pressures.