bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of the Aira Caldera, southern Kyushu, ∼22,000 years ago
Authors: Aramaki Shigeo
Published: February 2008

About 22,000 years ago a series of large‐scale pyroclastic eruptions produced the Aira caldera 20 km×20 km wide at the northern end of Kagoshima Bay in southern Kyushu. It started with a Plinian pumice eruption (Osumi pumice fall, 98 km3) followed by oxidized, fine‐grained Tsumaya pyroclastic flow (13 km3), both erupted from a vent located at the present site of Sakurajima volano, 8 km south of the caldera center. After a very short pause, violent explosive ejection of the basement rock fragments and pumiceous materials occurred at the central vent, gradually changing itself to a huge eruption column rapidly collapsing to form the Ito pyroclastic flow about 300 km3 in volume. The earliest phase produced up to 30‐m‐thick Kamewarizaka breccia developed along the caldera rim and charged with basement (lithic) fragments up to 2 m across. The breccia is a near‐vent variety of the bottom concentration zone of lithics in the Ito deposit. Various textural features and monotonous petrologic character indicate that the main part of the Ito pyroclastic flow was emplaced by a simple, short‐lived eruptive mechanism. The Aira‐Tn ash, a fine‐grained counterpart of the Ito pyroclastic flow, covered a wide area more than 1000 km from the vent. Evacuation of more than 110 km3 of rhyolitic magma produced a funnel‐shaped collapse structure with the center of the magma chamber about 10 km deep. Like many other Japanese Quaternary calderas, the Aira caldera is considered to heave formed not by a piston cylinder‐type subsidence utilizing a ring fracture but by coring and high‐angle slumping of the wall rocks into a funnel‐shaped central vent. The outline of the caldera was strongly controlled by the faults bounding the volcano‐tectonic graben forming Kagoshima Bay.
Geochemical evolution of the Menengai Caldera Volcano, Kenya
Authors: Leat Philip T. , MacDonald Ray , Smith Robert L.
Published: February 2008

The Menengai volcano is composed almost entirely of strongly peralkaline, Si‐oversaturated trachytes. No mafic or intermediate products have yet been identified. The volcano has had a complex geochemical evolution, resulting from the interplay of magma mixing, crystal fractionation and liquid state differentiation. The controlling mechanism at any time was related to the growth stage of the complex, the presence of volatile gradients in the chamber, and the distribution of magma densities in the chamber. Prior to a major ash flow eruption, the magma reservoir was growing by the addition and mixing of two or more trachytic melts, only slightly different in composition. A volatile‐rich cap eventually separated from the lava‐forming zone and became compositionalIy zoned by liquid state processes. In late pfecaldera times, trachyte magma was able to penetrate into the cap zone, resulting in the eruption of mixed magma. The first Menengai ash flow tuff was erupted from a compositionally zoned magma chamber which showed strong roofward enrichment in Fe, Mn, Cs, Hf, Nb, Ni, Pb, Rb, Ta, Th, U, Y, Zn, Zr, and the REE (including Eu) and probably also Na, Cl, and F, and roofward depletion in Al, Mg, Ca, K, Ti, P, Ba, and Sc. Observed enrichment factors of up to 5 make this one of the most strongly zoned ash flow units yet recorded. Zonation was achieved by liquid state differentiation, probably involving volatile transfer and thermodiffusion, and minor crystal fractionation. After a period of rehomogenization of the magma remaining in the upper parts of the chamber, the second Menengai ash flow tuff was erupted, with formation of the present caldera. This unit is also compositionalIy zoned, although with lower observed enrichment factors than the first sheet. Caldera collapse was followed by convective overturn within the magma chamber and the rise to the roof zone of a Ba‐rich magma from a level not tapped by the as flow. Enrichment of volatiles in this zone resulted in the establishment of a stable density interface between an upper, tuff‐producing zone and the lower, lava‐forming zone. In the latter, 25% crystallization of syenite took place against the side walls. Crystallization in the tuff‐forming zone was more extensive (75%) and produced a series of chemically evolved tuffs. At some critical stage, liquid state processes became dominant in both zones and compositional variations comparable to those in the ash flow sheets were established. Some mixing of the upper and lower zone magmas may have occurred relatively recently. The Menengai volcano provides evidence of the development by liquid state mechanisms of extensive compositional zonations through thicknesses in excess of 102 m in times of 102–103 years. It also gives unique information on the senses and amounts of elemental enrichments and depletions during liquid state differentiation of trachytic magma.
Eruptive history of the rhyolitic Kane Springs Wash Volcanic Center, Nevada
Authors: Novak Steven W.
Published: February 2008

The 19×13 km Kane Springs Wash caldera formed 14 Ma ago following eruption of the three youngest members of the Kane Wash Tuff. Two older cooling units of the Kane Wash Tuff are dated 15.7±0.2 Ma and 14.7±0.2 Ma and apparently erupted from source areas west of the exposed caldera. Volcanic eruptions from the Kane Springs Wash center began about 14 Ma ago with extrusion of trachyte lavas now preserved north of the caldera. Member V1 of the Kane Wash Tuff overlies the trachytes and is zoned from rhyolite to trachyte. In contrast, overlying members V2 and V3 are slightly peralkaline rhyolites (comendite). Eruption of these tuffs 14.1±0.2 Ma ago initiated caldera collapse. Following collapse, trachyte lavas indistinguishable in age from the ash‐flow tuffs erupted within the caldera, forming a 5‐m‐diameter cumulodome whose interior portions crystallized to syenite. This magmatic resurgence formed an extrusive complex within the caldera rather than doming the cauldron block as in classic resurgent calderas. Metaluminous ferroedenite‐rhyolite lavas and associated ash‐flow tuffs then erupted in the “moat” surrounding the central trachyte/syenite complex. Pyroclastic deposits from the domes were covered by trachyandesite and basalt lava flows about 13.4 Ma ago. Biotiterhyolite domes dated 13.4±0.2 Ma continued to erupt in the northeastern moat and appear to be the last silicic eruptions from the Kane Springs Wash magma system. A 13.4 Ma metaluminous biotite‐rhyolite dome containing vapor‐phase topaz overlies the trachyandesite lavas and appears to be the youngest biotite‐rhyolite dome. These intracaldera units filled the moat, resulting in a caldera filled with its own eruptive products. Local basalt flows covered both intracaldera units and outflow sheets between 12.7 and 11.5 Ma ago, prior to Basin and Range faulting. The “dominant volume” of the system was trachytic, as trachyte lavas erupted both prior to and following caldera collapse, and trachyte inclusions occur as scoriaceous bombs at the top of member V2. Fractionation of this trachyte magma produced the slightly peralkaline rhyolitic magma that erupted as the Kane Wash Tuff. The shift from an anhydrous mineral assemblage in the tuffs to hydrous assemblages in the later intracaldera rhyolites shows that the system was cooling and was possibly more hydrous in its late stages. Petrographic characteristics of trachyandesite lavas erupted late in the history of the center suggest that they represent basaltic magmas contaminated with silicic material
Peralkaline ash flow tuffs and calderas of the McDermitt Volcanic Field, southeast Oregon and north central Nevada
Authors: Rytuba James J. , McKee Edwin H.
Published: February 2008

The McDermitt volcanic field covers an area of 20,000 km2 in southeastern Oregon and northwestern Nevada and consists of seven large‐volume ash flow sheets that vented from 16.1 to 15 Ma ago. The volcanic field is characterized by peralkaline, high‐silica rhyolite, and all but one of the sheets are comendites. The ash flow field overlies a 1.5‐km‐thick section of Miocene Steens volcanic series and Steens Basalt, indicating a minimum age of 16.1 Ma for the Steens Basalt. The Orevada rift, a 200‐km‐long NNW‐striking structure, developed just prior to eruption of the mafic lavas. Each ash flow sheet resulted in the formation of a large collapse caldera along the Orevada rift. The center of the volcanic field is dominated by four overlapping and nested calderas; satellitic to these are three additional calderas. The areas adjacent to the incipient calderas subsided during ash flow eruption. Thickening of the ash flow sheets, monoclinal warping outside the caldera ring fault, and tilting in toward the caldera of blocks bounded by curvilinear faults all indicate regional subsidence prior to caldera collapse. The McDermitt caldera complex is highly mineralized; it contains ore deposits of Hg, Sb, Cs, Li, and U. The peralkaline tuffs have high contents of these elements and were the source rocks from which metals were leached by hydrothermal systems developed during the last stage of caldera‐related volcanism.
Evolution of the Early Oligocene Bonanza Caldera, northeast San Juan Volcanic Field, Colorado
Authors: Varga Robert J. , Smith Brian M.
Published: February 2008

The 36‐Ma‐old Bonanza caldera developed within a slightly older sequence of intermediate composition volcanics (Rawley Andesite), equivalent to the Conejos Formation of the greater San Juan volcanic field. Vent facies lava flows, monolithic breccias, and numerous intrusions dominate the early sequence within and near the caldera. Distal lava flows and abundant lahars of Rawley Andesite dip radially outward, suggesting that the Bonanza caldera formed near the summit of a large composite volcano. The caldera collapsed in a trapdoor fashion when less than 50 km3 of dacite and rhyolite ignimbrite of the Bonanza Tuff erupted from a major vent area near the north/northwest caldera margin. Listric normal faulting along a western ring fault system dropped basal ignimbrites down to the east with a maximum displacement of nearly 1 km. Northwest trending basement structures may have influenced the orientation of the ring fault system. Andesite and dacite lava flows overlie the Bonanza Tuff, marking a return to passive volcanism following explosive eruptions. Slightly younger silicic stocks and exogenous rhyolite domes were emplaced along the caldera margins and were cut by ring fracture faults, suggesting a protracted history of caldera collapse. Rawley Andesite, lower Bonanza Tuff, and late lava flows are chemically similar, high‐K andesites and dacites. Initial 87Sr/86Sr ratios (0.70529–0.70688) of the intermediate suite preclude major contamination with 87Sr‐rich Precambrian upper crust. The compositional homogeneity of these rocks and the absence of Plinian airfall components to the ignimbrites suggest that subvolcanic magma chambers at Bonanza did not become strongly compositionally zoned. Rhyolite ignimbrite of the upper Bonanza Tuff (initial 87Sr/86Sr = 0.70850) may be consanguineous with late silicic stocks, rather than with the volcano‐building andesitic suite. The Bonanza caldera appears to be closely related to two Oligocene calderas to the north (Mount Aetna and Grizzly Peak), which share a common position along the western edge of the northern Rio Grande rift system. Early Oligocene silicic magmatism and caldera formation along this trend may mark the initial phases of rift evolution, perhaps heralding a change to a tensile stress regime prior to physiographic development of the rift valley.
Variations in caldera development in the Tertiary volcanic field of Trans‐Pecos Texas
Authors: Henry Christopher D. , Price Jonathan G.
Published: February 2008

Calderas, 4–30 km in diameter, are the major igneous centers of Trans‐Pecos Texas. Twelve calderas occur in two north‐northwest‐trending belts. Eight, including two in adjacent Chihuahua, occur in a western alkali‐calcic belt. Four are in an eastern alkalic belt. Additional calderas are probably associated with two major ash‐flow sheets in the eastern belt. Sources exist for all major tuffs of Trans‐Pecos, other than these two, so additional calderas are unlikely. All calderas were formed between 38 and 28 Ma ago. The two calderas in Chihuahua were formed 30 and 28 Ma ago and show significant differences in composition and eruptive style from the earlier calderas. Calderas are underlain by individual magma chambers that were active for no more than about 1 Ma. The caldera cycle in Texas is highly variable. Minor volcanism commonly occurred before ash‐flow tuff eruption and caldera collapse. Tumescence occurred in at most two calderas. Ash‐flow eruption and collapse were simultaneous in all calderas, as shown by thick, intracaldera tuffs that are interbedded with breccias shed from caldera walls. Collapse occurred along discrete fracture systems in most of the western calderas but along hinge zones in the eastern calderas and at least part of the two young calderas in Chihuahua. Sedimentation within calderas was minor. Preresurgent volcanism ranged from minor to extensive and was best developed in the western calderas, all of which show some form of resurgence. No eastern calderas were resurgent. Resurgence is expressed in many ways in western calderas, but only a few fit the classic model. Ring‐fracture volcanism was irregularly developed. Hydrothermal alteration and ore deposition were abundant and spatially and genetically related to calderas in the western alkali‐calcic belt. Alteration and ore deposition were minor to nonexistent in the eastern alkalic belt.
The recovery of true particle motion from three‐component ocean bottom seismometer data
Authors: Garmany Jan D.
Published: February 2008

The principal motivation for the development of multicomponent ocean bottom seismometers (OBS's) was the hope that measurement of vector particle velocities would permit sharper interpretation of seismic data, since wave types (P and S) could be better indentified with this knowledge. The practical results of OBS use have not entirely borne out this expectation. Instead, the OBS‐seafloor coupling system characteristically shows strong resonances, resulting in a ringy appearance of almost all OBS data. Previous attempts to deal with this problem have used seriously oversimplified one‐dimensional analyses. In the present work we first consider an ideal model of an OBS as a Hamiltonian system with three spatial degrees of freedom. This is generalized to a linear system having n degrees of freedom with three inputs and three outputs, and we find that the OBS site system is an all‐pole minimum delay multichannel filter whose properties may be estimated from in situ seismic data. We apply the inverse of this filter to noise series obtained with a three‐component OBS used in the Rivera Ocean Seismic Experiment. Resonance peaks in the noise power spectra are almost completely removed but at the cost of the gain obtained through resonant detection. The most significant implication of this work is that accurate ground motion can be recovered from almost any three‐component OBS. Nevertheless, poor performance of OBS's will lead to degraded recovery of ground motion, so that design considerations which reduce the resonances in OBS‐seafloor coupling remain important.
The effect of pressure on porosity and the transport properties of rock
Authors: Walsh J. B. , Brace W. F.
Published: February 2008

We reanalyze the flow model proposed by Wyllie and Rose (1950) in which the complicated flow network through the pore phase of rock is replaced by a single representative conduit. Although the model is a very simple representation of the complicated pore phase in rock, we find that it provides an adequate simulation of how the transport properties vary with external pressure. Expressions derived for fluid permeability k and formation factor F are combined to give an expression for the mean hydraulic radius of the pore phase. Using this expression, we show that the exponent r in the empirical relationship k ∝ F−r must fall in the range 1≤r≤3. Also, we use the expression for hydraulic radius to estimate the crack area per unit volume and the standard deviation of the height of the asperities on the rhicrocrack surfaces for two granites. The values are in reasonable agreement with other estimates.
Paleomagnetic study of thrust sheet rotation during foreland impingement in the Wyoming‐Idaho overthrust belt
Authors: Schwartz Susan Y. , Van der Voo Rob
Published: February 2008

Upper Jurassic and Lower Cretaceous strata in the Darby and Absaroka thrust sheets were sampled in order to paleomagnetically evaluate thrust sheet rotation in the Wyoming‐Idaho overthrust belt. No significant rotations were found. Lack of rotation in these thrust sheets suggests that previously reported rotation of the Prospect thrust sheet was not transmitted to the older Darby and Absaroka thrust sheets, indicating that the Prospect thrust sheet only experienced local rotations in response to collision with the Teton‐Gros Ventre uplift. The Upper Jurassic Stump Formation possesses a magnetization that passes the fold test and gives a paleomagnetic pole at 64°N, 170°E ( K = 18, A95 = 15°), in good agreement with other published Upper Jurassic poles for cratonic North America. Samples from all Lower Cretaceous sites are believed to possess a synfolding remagnetization of chemical origin.
Noncohesive critical Coulomb wedges: An exact solution
Authors: Dahlen F. A.
Published: February 2008

Active fold‐and‐thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of Coulomb failure everywhere, overlying a basal decollement where frictional sliding is occurring. Ignoring cohesion, the four strength parameters needed to describe a critical Coulomb wedge are its internal and basal coefficients of friction μ and μb and its internal and basal Hubbert‐Rubey fluid pressure ratios λ and λb. An exact relation between surface slope α and basal dip β of a noncohesive critical wedge with uniform properties is derived. The state of stress within such a wedge has the same orientation everywhere, and α is constant if β is, and vice versa. A coefficient of internal friction μ = 1.1 is consistent with the known surface slope, basal dip, and pore fluid pressures in the active fold‐and‐thrust belt of western Taiwan, assuming that Byerlee's law, μb = 0.85, is valid on the base. The wide variety of tectonic styles observed to occur along convergent margins, including subduction erosion, active accretion, subduction without accretion, and even extension and normal faulting, may be controlled by relatively small spatial or temporal variations in either μb or λb.
Origin of Mesozoic and Tertiary granite in the western United States and implications for Pre‐Mesozoic crustal structure: 2. Nd and Sr isotopic studies of unmineralized and Cu‐ and Mo‐mineralized granite in the Precambrian Craton
Authors: Farmer G. Lang , Depaolo Donald J.
Published: February 2008

In the Cordilleran region of the western United States, Mesozoic and Tertiary peraluminous granitic rocks display regional variations in initial 143Nd/144Nd (εNd); εNd = −10 to −12 in southern Arizona, − 17 to −19 in the northern Great Basin (NGB), and −30 in the northern Rocky Mountains. Initial 87Sr/86Sr values are between 0.710 and 0.721 and show no regional pattern. Metaluminous granitic rocks have a wider range of εNd values extending from values similar to those of the peraluminous granites to much higher values. The 87Sr/86Sr values are mostly fairly low, between 0.705 and 0.710 except in the NGB where values as high as 0.7157 are observed. No systematic differences between the εNd or 87Sr/86Sr values of Cu‐ or Mo‐mineralized and Unmineralized granite were discerned, except for Cu‐mineralized granite in eastern Nevada and Mo‐mineralized granite in Colorado, which have εNd values higher (∼0) and lower ( ∼−10.0), respectively, than Unmineralized granite in the same region. Comparison to εNd values of exposed Precambrian rock suggests that the peraluminous granite, and the Mo granite in Colorado, were derived exclusively from felsic Precambrian basement rocks and that the regional variations in the εNd values reflect the regional variation in the average crustal age. The Nd data confirm that the Precambrian basement underlying the NGB and eastern California is isotopically distinct from Precambrian crust in the remainder of the western United States. The similarity between the εNd values of peraluminous granite and Precambrian crust also suggests that the high 147Sm/144Nd (>0.13) and the low total light rare earth element (LREE) abundances characteristic of peraluminous granite in southern Arizona were imposed during the chemical evolution of the magmas. Metaluminous granite are interpreted to have formed via mixing of mantle‐derived magma and large proportions of low 87Sr/86Sr (granulite facies) lower crust, except in the eastern NGB where the mantle magmas mixed with a lower crustal source with a significantly higher 87Sr/86Sr ratio. REE abundance patterns for metaluminous granite in the NGB are characterized by extreme LREE enrichment, which supports the proposed origin for these rocks by mantle/crust mixing or by remelting of the lower crust alone. No systematic difference exists between the sources of Cu‐ or Mo‐mineralized and Unmineralized metaluminous granite, but the data suggest that the Cu sources are in the mantle and the Mo sources are in preexisting crust. Overall, the Nd data indicate that continental interior granite in the western United States was primarily derived from preexisting crust, and although changes in the thermal structure of the continental mantle may have triggered magma formation, the resulting granites do not represent significant juvenile additions to the continental crust.
Anomalously thin crust in oceanic fracture zones: New seismic constraints from the Kane Fracture Zone
Authors: Cormier Marie‐Helene , Detrick Robert S. , Purdy G. Michael
Published: February 2008

Three detailed and carefully positioned seismic refraction experiments have recently been carried out along the Kane fracture zone near 24°N, 44°W in order to place better geophysical constraints on the extent and origin of the anomalously thin crust reported from parts of this and other large Atlantic fracture zones. These three experiments, when combined with earlier studies, result in reversed structural control along about a 300‐km‐long segment of the Kane fracture zone extending from the ridge‐transform intersection out onto crust about 25 m.y. old. An analysis of refraction lines obtained along the fracture zone trough by using both travel time modeling and delay time function techniques reveals large variations in crustal thickness and/or velocity. However, along most of the eastern fracture zone trough, upper mantle‐type velocities are found at depths of only 2‐3 km below the seafloor, less than half the typical depth to Moho in the ocean basins. This anomalous fracture zone crust is generally characterized by unusually low compressional wave velocities, relatively high‐velocity gradients throughout most of the thinned crustal section (about 1–2 s−1) and a distinct crust‐mantle boundary. The very thin crust appears to be confined to the deepest parts of the Kane fracture zone trough (<10 km wide), although a more gradual crustal thinning may extend up to several tens of kilometers from the fracture zone. A specially designed delay time experiment at the intersection of the Kane fracture zone and the Mid‐Atlantic Ridge also revealed the presence of a thinned crustal section of a relatively uniform thickness (about 3 km) except beneath the nodal basin where the crust may be less than 1 km thick. The anomalous crust in the intersection area appears to extend at least part way down the median valley. We interpret this anomalous fracture zone crust as a primary accretionary feature that forms as a result of a restricted magma supply near ridge‐transform intersections, although tectonic processes may also play a part, at least locally, in thinning the crust. Our results support a “ribbon” model of the gross seismic structure of the ocean basins in which bands of relatively homogeneous, so‐called normal crust 50–100 km wide extend away from spreading centers, each formed at a discrete ridge crest segment and each separated by the thin crust underlying the fracture zone trough.
Intermediate‐wavelength magnetic anomalies over the central Pacific
Authors: LaBrecque J. L. , Cande S. C.
Published: February 2008

We have extracted the intermediate‐wavelength magnetic anomaly field from the marine survey data of the central Pacific in the band pass of 4000–400 km (spherical harmonic degree 10–100). Our technique minimizes the effects of external field sources, secular variation, and strike aliasing. The objective of this paper is to demonstrate this capability, compare the derived sea surface field to the equivalent MAGSAT data set, and demonstrate that anomalies observed in both fields are correctable to geologic features within the oceanic lithosphere. As expected, the sea surface field exhibits a strong spatial correlation to and a better resolution than the MAGSAT field. However, the MAGSAT field also displays a diminished resolution for wavelengths less than 1900 km when compared to the upward continued sea surface data. Two reasons are likely for this diminished resolution: (1) differences in the resolution of the spherical harmonic reference fields and (2) an increased noise level in the MAGSAT data at the shorter wavelengths. Other sources of error are also considered. We also demonstrate that the remanent magnetization of the central Pacific seamounts produces negative magnetic anomalies which are observed at satellite altitude. Other sources including the thickened crust of the oceanic plateaus and regional petrologic variations can be associated with intermediate‐wavelength anomalies observed in both the MAGSAT and the sea surface fields.
Petrology of Young Pacific Seamounts
Authors: Batiza Rodey , Vanko David
Published: February 2008

Twenty‐two young seamounts near the East Pacific Rise (EPR) consist predominantly of Ol‐Hy normative basalt chemically identical to depleted mid‐ocean ridge basalts (MORB). This basalt is systematically more primitive than the EPR lavas and is supplied directly to the seamounts from the mantle under the ridge, ridge‐transform intersections, and fracture zones. The seamounts also contain small volumes of differentiated basalt with low Mg # (Mg # = Mg/Mg + Fe2+) which fractionate from basalt parents of high Mg # mainly by processes of fractional crystallization. Seamounts also contain Ne normative alkali basalts, which are enriched in light rare earth, other incompatible trace elements, and volatiles, as well as a complete spectrum of transitional basalts of intermediate composition. We suggest that this spectrum of primitive basalt types forms owing to magma mixing during melt segregation in the region of melting of a chemically, isotopically, and perhaps mineralogically heterogeneous mantle. Melting probably takes place at about 100 MPa in the stability field of plagioclase or plagioclase‐spinel, and the extent of partial melting is highly variable: relatively low to produce alkalic basalts and relatively high to produce the tholeiitic MORB. All basalt types, however, are probably hybrids produced by mixing in the region of melt segregation. The thermal and mechanical regime which results in the magma diversity observed on young seamounts is evidently also present at very slow spreading ridges but not below fast‐spreading ridges. Normal ridge crest segments and fracture zones (seamount volcanism) thus exhibit a range of thermochemical characteristics which differ markedly from plume/hotspot volcanism.
Geophysical investigation of a Suture Zone: The Border Ranges Fault of southern Alaska
Authors: Fisher Michael A. , von Huene Roland
Published: February 2008

The Border Ranges fault separates structurally complex accreted Cretaceous rocks from less deformed middle or late Paleozoic and younger rocks in the Cook‐Shelikof basin. Of the five types of geophysical data used to investigate this fault, gravity data give the clearest indication of its presence and crustal structure. For at least 400 km along the fault, gravity anomalies include a +20 to +30 mGal peak along the fault's upper plate and a −40 mGal trough along the lower plate. The paired anomaly can be modeled satisfactorily by a simple step, in a deep dense layer, that lies within 3 km of the projected offshore location of the fault. Relatively low‐density rocks lie along the fault's lower plate to a depth of about 10 km, and the upper part of the fault dips within 20° of vertical. Satellite altimetry data show that two circular geoid lows lie along the Border Ranges fault and coincide with lows in free air gravity data. Seismic refraction and seismic reflection data suggest that the large‐scale density anomalies that cause both types of lows must lie at depths greater than about 1 km within the margin. Three regional magnetic anomalies (Knik Arm, Seldovia, and Shelikof) terminate at the Border Ranges fault, suggesting that the fault truncates obliquely rocks that lie along its northwest side. Six seismic reflection lines cross the Border Ranges fault, but none of them shows reflections from it. The absence of such reflections probably results from the fault's steep dip and from the presence of strong water bottom multiples in the data. From the Late Jurassic until the early Late Cretaceous, the magmatic arc near the Cook‐Shelikof basin was inactive, and we infer that the predominant motion along the Border Ranges fault was strike slip. Resurgent Late Cretaceous magmatism was contemporaneous with uplift of rocks along the northwest side of the Border Ranges fault and with deformation of turbidite sequences in the fault's lower plate. We propose that during the Late Cretaceous, motion along the Border Ranges changed from strike slip to reverse. Cenozoic rocks near the fault show no evidence for post‐Cretaceous fault movement.
Agglutinates as recorders of regolith evolution: Application to the Apollo 17 drill core
Authors: Laul J. C. , Smith M. R. , Papike J. J. , Simon S. B.
Published: February 2008

We report chemical data (27 major, minor, and trace elements by INAA) for agglutinates from 26 depth intervals of the Apollo 17 deep drill core (sections 70009–70001) and compare their compositions with those of the soils in which they occur. The results show that seven stratigraphic units contain agglutinate populations in which the “average agglutinate” is in equilibrium (E) with the soil in which it resides and thus has a chemical composition that falls within the compositional range for the various size fractions. Four units contain average agglutinate populations with compositions out of equilibrium (O) with the soils in which they reside, and two units contain agglutinate populations that are close to the compositional limits of the soil size fractions and are designated (M) for mixed. The sequence is: (0–22 cm) E; (28–47 cm) O; (58–70 cm) E; (81 cm) O; (90–98 cm) E; (108–118 cm) M; (113–178 cm) E; (189 cm) O; (198–210 cm) E; (220–242 cm) O; (253 cm) E; (258–269 cm) M; (278–286 cm) E. This agglutinate sequence suggests a scenario in which several closely‐spaced depositional events were involved in the formation of the A‐17 drill core rather than a continuous accumulation process. Chemical mixing models indicate that agglutinates from section 70008 (22–48 cm) are enriched in mare components (81%), containing 55% orange glass and 26% ilmenite basalt. Agglutinates from core section 70003 (220–242 cm) did not form from the KREEPy soils that occur in this section but were probably mixed in from stratigraphic horizons above and below this unit. The Apollo 17 drill core soils have complex histories and agglutinates occurring in this stratigraphic sequence are powerful recorders of soil maturation and mixing events.
The impact cratering rate in recent time
Authors: Grieve Richard A. F.
Published: February 2008

Data on the recent cratering rate in the earth‐moon system is somewhat ambiguous with respect to whether the rate has been constant over the last ∼3 b.y. Rate estimates based on the present population of earth‐crossing bodies are higher than estimates based on crater counts at the Apollo 12 and 15 sites, while some estimates based on terrestrial crater counts are intermediate in value. The terrestrial data base for structures with diameter (D)>20 km on the North American and European cratons has been investigated for variations in the rate with time. There appear to be two populations, with a 0–120 m.y. age group suggesting a higher cratering rate than an older 200–360 m.y. age group. Reexamination of the preservation levels of these structures indicates that the topography of a 20 km‐sized structure may be removed by erosion in 34 to 273 m.y. and the geologic anomaly resulting from structural uplift in 79 to 630 m.y. This suggests that the relatively lower rate for the 200–360 m.y. group may reflect problems in crater recognition associated with crater retention. If rate estimates are restricted to the possibly more complete sample of 0–120 m.y. craters on the cratons, the recalculated rate is 5.4±2.7× 0−15 km−2 y−1 for D<20 km. This is comparable to that based on earth‐crossers and reinforces previous suggestions that the recent cratering rate may be a factor of 2–3 higher than the average rate over the last ∼3.0 b.y. This rate estimate, when extrapolated back in time, is a better fit to lunar data from Tycho and Copernicus than a constant rate based on Apollo 12 and 15 data and may be interpreted to suggest that the relatively higher recent rate has been in effect for ∼1 b.y. Given the uncertainties, however, this must be regarded as speculative.
Cosmic ray exposure histories of Apollo 14, Apollo 15, and Apollo 16 rocks
Authors: Eugster O. , Eberhardt P. , Geiss J. , Grögler N. , Jungck M. , Meier F. , Mörgeli M. , Niederer F.
Published: February 2008

We investigate the exposure history of six lunar rocks using the abundances of cosmic‐ray produced noble gas isotopes and the 81Kr‐Kr ages. Single‐stage exposure histories are inferred for basalt 14053, breccia 14305, and anorthosite 65315. We propose that basalt 14053 with a 81Kr‐Kr age of 21.2±5.0 m.y. is a Cone crater ejecta and that anorthosite 65315 (1.3±0.7 m.y.) originates from South Ray crater. We present a comprehensive comparison of all cosmogenic noble gas ratios and abundances for rocks with single‐stage exposure and show that the agreement with values predicted by nuclear systematics and target element chemistry is good, in particular for isotope ratios. Basalt 14310 experienced a multistage exposure. The apparent 81Kr‐Kr age, the depth sensitive isotopic ratios, and fission 136Xe, mainly from 235U induced by secondary cosmic‐ray neutrons, lead to the conclusion that this sample was preexposed 1.75 AE ago to cosmic rays for a duration of 350 m.y. Basalt 15058 and anorthosite 15415 also reveal multistage exposures. The abundance of the radionuclide 81Kr in 15058 increases toward the surface of the rock, reflecting an enhanced production rate due to solar flare particles, whereas stable 83Kr concentrations are constant with increasing depth within the rock. This indicates that basalt 15058 received most of its irradiation in a more shielded location than that effective during the last million years.
Apollo 16: Impact melt sheets, contrasting nature of the Cayley Plains and Descartes Mountains, and geologic history
Authors: McKinley James P. , Taylor G. Jeffrey , Keil Klaus , Ma Maw‐S. , Schmitt Roman A.
Published: February 2008

Apollo 16 stations 4 and 5 rake samples were examined petrographically and by electron microprobe and INAA. Results were combined with published information (major and trace element compositions, petrologic information, and age determinations) for nonrake samples from stations 4 and 5 and some other Apollo 16 sample localities. Lithologic abundances support the idea (Korotev, 1981) that the variation of soil compositions at Apollo 16 is a result of mixing between a component represented by station 5 soil and components much like either the dimict breccias (station 4 soils) or feldspathic fragmental breccias (stations 1, 2, and 11) in composition. Dimict breccias, because of their restricted occurrence, are apparently not a constituent of either the Cayley or Descartes formations: they were contributed to the surface materials by South Ray crater and were probably excavated from beneath both formations by this event. Pyroxene, olivine, and coexisting plagioclase compositions from within the anorthosite portions of dimict breccias bridge the gap between the Mg‐rich and ferroan anorthosite fields. Analyses from associated cumulate and granulitic clasts indicate that they are the source of the intermediate material; dimict breccia anorthosites are thus polymict rocks, not pristine ferroan anorthosite. Compositions of Apollo 16 impact melt rocks suggest a number of compositional clusters. Discriminant and cluster analyses of compositional data confirm the existence and strength of these clusters, as do age determinations and Ir/Au ratios within each group. Each of three clusters represents an impact event in the Apollo 16 region. Petrologic information and age determinations indicate that dimict breccias formed approximately 3.92 b.y. ago. The Nectaris event occured between 3.84 and 3.92 b.y. ago, and the Cayley plains were deposited as a result of the Imbrium event sometime later than 3.84 b.y.
Degassing of meteorite parent bodies
Authors: Sugiura N. , Brar N. S. , Strangway D. W. , Matsui T.
Published: February 2008

Gas permeability of three chondrites was measured. A typical time for degassing of a 100‐km‐radius parent body was calculated based on these permeability values. It was found that an ordinary chondrite parent body would act as an open system during the metamorphic period (100 m.y.), while an enstatite chondrite parent body would act as a closed system. The possibility that volatile element concentrations in chondrites are controlled by permeability is discussed.
Thermal metamorphism of Si2O3 (a circumstellar dust analog)
Authors: Nuth Joseph A. , Donn Bertram
Published: February 2008

We have experimentally studied the thermal behavior of Si2O3, the metastable condensate from SiO vapor. Si2O3 and its successor, amorphous quartz, have previously been shown to have infrared spectral features similar to some that appear in the spectra of grains in oxygen‐rich circumstellar regions. Thermal annealing experiments show that only one Si2O3 decay process operates over the range 750K<T<1000K. This process is a unimolecular disproportionation. The rate of this transformation can be expressed as k(hr−1) = 109 exp (−40 kcal/mole/RT). Using this rate constant we find that a significant fraction of freshly nucleated circumstellar grains can survive passage through a typical circumstellar shell virtually unaltered in structure. We emphasize that this is only the first in a series of laboratory experiments intended to study the metamorphism of newly condensed circumstellar material ejected into the interstellar medium. Grains similar to these might have been incorporated into the primitive solar nebula provided that they could also survive passage through the general interstellar medium.
Erosion and modification of SO2 ice by ion bombardment of the surface of Io
Authors: Johnson R. E. , Garrett J. W. , Boring J. W. , Barton L. A. , Brown W. L.
Published: February 2008

We report measurements of total yields, energy spectra, and mass spectra of molecules sputtered from SO2 ice at low temperatures by keV Ar+ ions. These measurements are applicable to corotating sulfur and oxygen ion sputtering of SO2 on Io. We find that the yields are much larger than those estimated earlier for low energy ion sputtering of SO2 and the energy spectra are peaked at lower energies than expected. We observed the irradiation time dependence of the chemical alteration of fresh SO2, leading to the production of new molecular species, and identified masses 32 (O2 or S), 48 (SO), and 80 (SO3 or S2O) as ejected species in addition to SO2. These measurements are used to determine the rate of direct sputter ejection of sulfur into the Jovian plasma torus, the rate of erosion of patches of SO2 ice, and estimates of the surface column densities of SO3 produced in SO2 ice by the corotating and fast ions. We estimate that the direct sputter ejection rate of sulfur into the Jovian plasma is of the order of 1010 atoms/cm2/s, that the erosion rate of fresh SO2 deposits due to sputtering is of the order of 10−3 cm/year, and that only for penetrating ion bombardment can a significant, and possibly observable, column density of SO3 be produced in an SO2 frost.
A simple method for inverting the azimuthal anisotropy of surface waves
Authors: Montagner Jean‐Paul , Nataf Henri‐Claude
Published: February 2008

We investigate the problem of retrieving anisotropy as a function of depth in the mantle, from the observed azimuthal variations of Love and Rayleigh wave velocities. Following the approach of Smith and Dahlen, this azimuthal dependence is expressed in terms of a Fourier series of the azimuth θ. For the most general case of anisotropy (provided it is small enough), some simple linear combinations of the elastic tensor coefficients are shown to describe the total effect of anisotropy (both polarization anisotropy and azimuthal anisotropy) on the propagation of surface Waves. For the terms that do not depend on the azimuth the combinations are related to the elastic coefficients of a transversely isotropic mantle. For the azimuthal terms the relevant combinations are explicited. It is found that the partial derivatives of the azimuthal terms with respect to these combinations are easy to compute for they are proportional to the partial derivatives of a transversely isotropic model in the case of a plane‐layered model. In a first approximation the same property holds true for a spherical earth and we calculate from PREM all the partial derivatives needed for performing the inversion of the azimuthal anisotropy of surface waves in the period range 50–300 s. It is observed that very shallow anisotropy can be responsible for substantial azimuthal variations up to the longest periods. With this approach it is also easy to compute the azimuthal variations of surface wave velocities produced by any anisotropic model. When a Ci j elastic tensor is chosen for the upper mantle, azimuthal variations up to 2% are obtained for Rayleigh waves. The azimuthal variations of Love wave velocities are very small. The 2 θ term of the azimuthal variations of Rayleigh wave velocities is the dominant term. Its fast axis corresponds to the fast axis of P waves.
Peridotites from the Island of Zabargad (St. John), Red Sea: Petrology and geochemistry
Authors: Bonatti Enrico , Ottonello Giulio , Hamlyn Paul R.
Published: February 2008

Exceptionally fresh peridotite bodies outcrop on Zabargad Island, an uplifted fragment of sub‐Red Sea lithosphere. The peridotites are associated with basaltic dikes and are in tectonic contact with a metamorphic unit and with post‐Mesozoic sedimentary units. The peridotites can be divided into three main groups: (1) protogranular spinel Iherzolites (sp‐lherzolites), with average modal composition ol 65%, opx 16%, cpx 16%, spinel 3% (2) amphibole peridotites (amph‐peridotites), containing >2% magnesio‐hornblende (3) plagioclase peridotites (pl‐peridotites), containing >2% Ca‐plagioclase. Minor outcrops of dunite and wherlite were also observed. The pl‐peridotites and amph‐peridotites, which are found in localized zones or bands within the sp‐lherzolite, show textures ranging from por‐phyroclastic to cataclastic, indicating varying degrees of tectonic deformation. Olivine and opx have a rather constant composition in the three groups, Fo ranging between 87.3% and 90.5% and En between 88% and 89%, respectively. Clinopyroxene is chromian diopside but contains less Na in the pl‐peridotites than in the sp‐lherzolites. Both opx and cpx are moderately Al and Cr‐rich, as is typical of mantle‐equilibrated pyroxenes. Spinel has a very low Cr/Al ratio in the sp‐lherzolites, lower than in the pi‐ and amph‐peridotites. Plagioclase in the pl‐peridotites ranges between An 80% and 93%, while traces of it rimming spinel in some of the sp‐lherzolites are more sodic. The amph‐peridotites contain up to 28% magnesio‐hornblende and, in some cases, traces of phlogopite and apatite; opx, cpx, and spinel are scarce. The major element composition of the Zabargad sp‐lherzolites, their slight light rare earth element (LREE)‐depleted pattern, transition elements Sc, Ti, Cr, Mn, Fe, Co, and Ni data, together with modal and mineral chemistry data, are all consistent with the sp‐lherzolites having last equilibrated in the sp‐lherzolite stability field (>9 kbar, >30 km deep) and representing essentially undepleted parental mantle material, though some samples might have undergone minor partial melting. The pl‐peridotites may represent localized incor‐poration of a melt component by the ascending Iherzolite body. The amph‐peridotites are enriched in K, LREE, and occasionally in P relative to sp‐lherzolites; they were probably formed by localized contamination with H2O‐rich metasomatic fhids injected through the Iherzolite body during its ascent. The Zabargad peridotites were probably emplaced from the upper mantle into the crust during the development of the Red Sea rift, i.e., in post‐Mesozoic time. They show affinity with some mantle‐derived oceanic ultramafics, particularly with St. Paul Rocks in the Atlantic. They could be considered a sample of oceanic mantle before extraction of the basaltic oceanic crust.
Depth of seismicity in the Imperial Valley Region (1977–1983) and its relationship to heat flow, crustal structure and the October 15, 1979, earthquake
Authors: Doser Diane I. , Kanamori Hiroo
Published: February 2008

Focal depths from over 1000 earthquakes occuring between 1977 and 1983 in the Imperial Valley‐southern Peninsular Ranges are used to study relationships between the depth of seismicity, heat flow, and crustal structure. This study used relocated A and B quality events from the California Institute of Technology catalog that were carefully selected to insure focal depth precision of ±2 km. Regional variations in focal depth appear to be related to regional heat flow variation, whereas local variations in focal depth, especially in the central Imperial Valley, may be related to crustal structure. These variations are studied by rheologie modeling. A comparison of focal depths of earthquakes occurring before and after the October 15, 1979 (M=6.6), earthquake indicates that aftershocks during the first 2 months of the sequence were 2–3 km deeper than earthquakes occurring in other time periods. The deepest earthquakes in Imperial Valley are spatially associated with a subbasement dome near the northern end of the Imperial fault. This dome coincides with the region where the Imperial fault undergoes a transition from stick‐slip behavior to aseismic fault creep. Models of slip during the 1979 mainshock are also compared with premainshock and postmainshock seismicity. A relocation of the 1940 (M=7.1) mainshock suggests that this sequence began by rupturing the same portion of the fault that experienced maximum slip during the 1979 mainshock.
Permeability and electrical conductivity changes due to hydrostatic stress cycling of berea and muddy J sandstone
Authors: Dey Thomas N.
Published: February 2008

Cyclic loading affects the electrical conductivity and fluid permeability of kaolinite clay‐bearing sandstone. The effective confining pressure on two sandstones was repeatedly raised and lowered between 3.5 and 31.5 MPa. Permeabilities dropped by 30% for Berea sandstone and 90% for Muddy J sandstone after three cycles; however, total pore volume always returned to its initial value after each cycle. Water salinity had little effect on these results. Electrical conductivity showed no change when a very conductive pore fluid, 1 M KCl water, was used, however the conductivity showed the same decreases as permeability when demineralized water was the pore fluid. These results show that clay particle rearrangement in the pores is probably responsible for the permeability changes.