bluuebunny/crossref_metadata_2025_split huggingface dataset:
Seismic velocity gradients in the upper mantle
Authors: Kennett B. L. N. 
Published: February 2008

The radial velocity gradients for P‐ and S‐waves in the upper mantle are of considerable interest for petrological studies of the upper mantle but are difficult to constrain directly. Comparison of three different styles of velocity model, constructed within a common parametrisation to give a good fit to the travel times of P‐ and S‐waves to 30°, provide a useful framework for constructing bounds on the radial gradients. For depths from 250–1000km a somewhat subjective set of bounds are suggested for the velocity gradients and velocity contrasts at the major discontinuities in an ‘average’ mantle.One of the most significant factors affecting the estimation of radial gradients is the presence of small‐scale heterogeneity in the mantle. Modest heterogeneity, around 1 percent perturbations on scale lengths of about 300km, can give rise to horizontal velocity gradients which can overwhelm the radial gradients in the shallower parts of the mantle and still be a quarter of the size of the radial gradients at greater depths. However such scales of heterogeneity are unlikely to simulate the presence of additional upper mantle discontinuities for ray paths travelling near vertical.
The spreading rate dependence of three‐dimensional mid‐ocean ridge gravity structure
Authors: Lin Jian , Morgan Jason Phipps 
Published: February 2008

We analyze over 1300 km of high resolution along‐axis gravity profiles at ridges with half‐spreading rates ranging from 1.2 to 5.5 cm/yr. The results show consistently higher along‐axis gradients of mantle Bouguer anomaly at the slow‐spreading Mid‐Atlantic Ridge (MAR) (0.3–1.2 mgal/km) than at the intermediate‐ to fast‐spreading Cocos‐Nazca Ridge and East Pacific Rise (EPR) (0.1–0.2 mgal/km). The regional peak‐to‐trough amplitude of mantle Bouguer anomaly is also greater along the MAR (30–60 mgal) than the Cocos‐Nazca Ridge and the EPR (10–20 mgal). With increasing spreading rate, the regional peak‐to‐trough amplitude of axial seafloor depth decreases from 1000–1700 m to 200–700 m. 3‐D numerical experiments suggest that mantle contributions to the gravity can be significant only near large‐offset transforms. At the more commonly observed non‐transform offsets, gravity anomalies will reflect crustal thickness variations.The along‐axis gravity data thus indicate that the amplitude of along‐axis crustal thickness variation decreases with increasing spreading rate. We propose that this spreading rate dependent crustal accretion style may originate in the mantle: finite‐amplitude mantle upwelling is intrinsically plume‐like (3‐D) beneath a slow‐spreading ridge but more sheet‐like (2‐D) beneath a fast‐spreading ridge. Such a transition in mantle upwelling may occur if the relative importance of passive upwelling over buoyant upwelling increases with increasing spreading rate. Small amplitude 3‐D upwellings may occur at a fast‐spreading ridge, but their effects on crustal thickness variations will be significantly reduced by along‐axis melt flows along a persistent low‐viscosity crustal magma chamber. In contrast, the large crustal thickness variations due to 3‐D mantle upwellings will be maintained at a slow‐spreading ridge because less along‐axis melt flows can occur in the colder and more rigid crust there.
Basic magmatism associated with Late Cenozoic extension in the western United States: Compositional variations in space and time
Authors: Fitton J. Godfrey , James Dodie , Leeman William P. 
Published: February 2008

Widespread basic magmatism across much of the western United States in the late Cenozoic followed the cessation of subduction along the Pacific coast. This volcanism accompanied lithospheric extension, block faulting and regional uplift In an attempt to assess the relative contribution of asthenosphere and mantle lithosphere to magmas across the western United States we have analyzed, for major and trace elements, a suite of 750 basic (MgO>4%) lava samples from all the major volcanic fields in the region. The data were divided into seven sets representing the main tectonomagmatic provinces: Basin and Range (BR), Western Great Basin, Transition Zone (TZ), Colorado Plateau, Snake River Plain, Southern Rocky Mountains, and Great Plains. It was further divided into relatively recent (<5 Ma) and older (>5 Ma) subsets on the basis of field relations and K‐Ar data. The <5 Ma subset shows striking chemical differences between the provinces. AU the BR lavas and some of the TZ lavas are indistinguishable from ocean island basalt (OIB) and therefore appear to have a source within the asthenosphere. In contrast, lavas from the other provinces generally show enrichments in Ba and depletions in Nb and Ti compared to OIB. This is accompanied by higher 87Sr/86Sr and lower 143Nd/144Nd than in the BR lavas. The older (>5 Ma) subset shows no great differences between the BR and the other tectonomagmatic provinces; all have high La/Nb and Ba/Nb. Crustal contamination alone cannot be responsible for these variations. We conclude that many of the magmas have inherited their chemical and isotopic characteristics from a lithospheric mantle source enriched by fluids expelled from a subducted slab. Pelagic sediment, returned to the mantle by subduction, is a possible agent for fluids rich in Ba, radiogenic Sr and unradiogenic Nd, but very poor in Nb. At least some of this enrichment must have accompanied the formation of the Proterozoic crust. It appears that subduction‐enriched lithospheric mantle was involved in the generation of all extension‐related basic magmas across the western United States until relatively recently. Only in the younger BR and parts of the TZ have asthenosphere‐derived magmas, uncontaminated by lithosphere, reached the surface. These observations conflict with models in which uplift and extension are caused by the replacement of mantle lithosphere by asthenosphere. Triey are best explained by the progressive erosion of the lithospheric manue over a plume currently located beneath the Southern Rocky Mountains.Supplemental data are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B91‐001; $2.50. Payment must accompany order.
Oceanic crust in the Japan Basin of the Japan Sea by the 1990 Japan‐USSR Expedition
Authors: Hirata Naoshi , Karp Boris Ya. , Yamaguchi Takashi , Kanazawa Toshihiko , Suyehiro Kiyoshi , Kasahara Junzo , Shiobara Hajime , Shinohara Masanao , Kinoshita Hajimu 
Published: February 2008

In September of 1990, a seismic refraction and reflection survey was conducted in the Japan Basin, in the northeastern part of the Japan Sea, as a part of the Japan‐USSR joint expedition. Twenty‐six ocean bottom seismometers (OBSs) were deployed on two 200‐km long lines. Explosives and an airgun were fired as controlled seismic sources on the two mutually perpendicular lines. Airguns were also used as the source for the multi‐channel reflection profiles.The crustal structure deduced is that of a typical oceanic basin: the crustal thickness is about 8.5 km including 2 km of sediment. We obtained a more detailed crustal structure than that obtained previously. From the dense airgun shooting data, the crustal structure is well resolved to show layer 1A, layer 1B, layer 2A, 2B, 2C, layer 3, and the mantle. The crust basically consists of laterally homogeneous layers but the Moho deepens slightly westward.
Downhole Logging as a Paeoceanographic Tool on Ocean Drilling Program Leg 138: Interface Between High‐Resolution Stratigraphy and Regional Syntheses
Authors: Lyle Mitchell , Mayer Larry , Pisias Nicklas , Hagelburg Terri , Dadey Kathy , Bloomer Stephen 
Published: February 2008

On Ocean Drilling Program (ODP) Leg 138, standard shipboard procedures were modified to allow for the real‐time monitoring of several laboratory core‐scanning systems that provide centimeter‐scale measurements of saturated bulk density, magnetic susceptibility and digital color reflectance. These continuous, high‐resolution data sets were used to ensure the proper offset of multiple holes and to splice together complete sedimentary sections. Typically, the spliced, continuous sediment sections were found to be about 10% longer than the section drilled, as measured by the length of the drill string. While the source of this elongation is not yet fully understood, it must be compensated for in order to property determine sediment fluxes and mass accumulation rates. Downhole logging, in conjunction with inverse correlation techniques provided a means to determine where the distortion occurred and to correct back to true in situ depths.Downhole logging also provides a means, through the generation of synthetic seismograms, of precisely relating the paleoceanographic events found in the core record to the high‐resolution seismic record. Once correlated to the seismic record, the spatial and temporal extent of paleoceanographic events can be traced well beyond the borehole. Most seismic events in the equatorial Pacific are related to rapid changes in carbonate content that, in turn, are related to both productivity events (often expressed as monospecific laminated diatom oozes) and times of enhanced dissolution. While many of these events may have oceanwide extent, others, like the absence of carbonate in the late‐Miocene to Recent in the Guatemala Basin have been shown to be regional and confined to only the deeper portions of the Guatemala Basin. As we identify and trace specific paleoceanographic events in the seismic record, we can begin to explore the response of the ocean through gradients of latitude, productivity, and depth.
On the Structure and Origin of Major Glaciation Cycles 1. Linear Responses to Milankovitch Forcing
Authors: Imbrie J. , Boyle E. A. , Clemens S. C. , Duffy A. , Howard W. R. , Kukla G. , Kutzbach J. , Martinson D. G. , McIntyre A. , Mix A. C. , Molfino B. , Morley J. J. , Peterson L. C. , Pisias N. G. , Prell W. L. , Raymo M. E. , Shackleton N. J. , Toggweiler J. R. 
Published: February 2008

Time series of ocean properties provide a measure of global ice volume and monitor key features of the wind‐driven and density‐driven circulations over the past 400,000 years. Cycles with periods near 23,000, 41,000, and 100,000 years dominate this climatic narrative. When the narrative is examined in a geographic array of time series, the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles. We argue that the 23,000‐ and 41,000‐year cycles of glaciation are continuous, linear responses to orbitally driven changes in the Arctic radiation budget; and we use the phase progression in each climatic cycle to identify the main pathways along which the initial, local responses to radiation are propagated by the atmosphere and ocean. Early in this progression, deep waters of the Southern Ocean appear to act as a carbon trap. To stimulate new observations and modeling efforts, we offer a process model that gives a synoptic view of climate at the four end‐member states needed to describe the system's evolution, and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems.“Solutions to problems involving systems of such complexity are not born full grown like Athena from the head of Zeus. Rather they evolve slowly, in stages, each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step.”—Victor P. Starr
Age and evolution of Western Brooks Range ophiolites, Alaska: Results from 40Ar/39Ar thermochronometry
Authors: Wirth Karl R. , Bird John M. , Blythe Ann E. , Harding David J. , Heizler Matthew T. 
Published: February 2008

Basalt, gabbro, and peridotite in the western Brooks Range structurally overlie Mississippian through Late Jurassic platform sediments that were deformed during Late Jurassic ‐ Cretaceous time. 40Ar/39Ar step‐heating experiments on hornblende, biotite, plagioclase, and potassium feldspar from gabbro, diorite, plagiogranite, and granite yielded complex release spectra. On isotope correlation diagrams, the incremental‐heating data indicate that gabbro, diorite, and plagiogranite cooled below the closure‐temperature for argon in amphibole between 187 and 184 Ma. Because the closure temperature of argon retention in amphibole is relatively high, these ages are interpreted to be the time since crystallization. Biotite and hornblende from metamorphic rock exposed along faults separating peridotite and gabbro from underlying basalt yielded plateau and isochron dates of 169 – 163 Ma; these dates are interpreted to record postmetamorphic cooling following intraoceanic detachment and transport of “hot” peridotite and gabbro over “cold” basalt and sediment. Detachment and faulting of the gabbros and peridotites is interpreted to have occurred approximately 20 Ma after crystallization, and is longer than the 5–10 Ma interval commonly observed in studies of other ophiolites. Fossils in flysch underlying the ophiolites indicate that the ophiolites were emplaced onto the continental margin during the Tithonian, approximately 15–20 Ma after initial detachment. The ages of feldspars (149 – 137 Ma) in the ophiolites and associated volcanic rocks are similar to the fossil ages of the underlying flysch and might record uplift during emplacement of the ophiolites onto the continental margin. A fission‐track age of apatite implies that the ophiolitic rocks were overlain by >4 km of rock until ∼59 Ma ago, the time of regional uplift in the Brooks Range.Supplement Appendix 1 is available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. document T92‐005; $2.50. Payment must accompany order.
The cosmic ray produced ³He/21Ne ratio in ultramafic rocks
Authors: Staudacher Thomas , Allègre Claude J. 
Published: February 2008

Although production of noble gases in meteorites are well studied since many years, [e.g. Bogard and Cressy, 1973] the first analysis of cosmic ray produced helium in terrestrial rocks were performed only in 1986 by Kurz and by Craig and Poreda. Later, Marti and Craig (1987) reported cosmic‐ray produced neon in summit lavas from Maui. More data on cosmogenic helium and neon were reported by Staudacher and Allègre (1991) on ultramafic nodules from Asia. The absolute production rate of cosmic‐ray produced helium has been determined by Kurz (1986, 1987) in olivine rich basalts from Hawaii with known K‐Ar or 14C ages. The resulting ³Hec production rate is 3.72×10−18cm³STP/g/a. This allows cosmogenic ³He to be used to determine cosmic ray exposure ages of surface samples. For neon, no such absolute production rate has yet been determined. In order to develop a complementary dating method using cosmogenic 21Ne, Staudacher and Allègre (1991) measured the cosmogenic ³He/21Ne ratio in ultramafic nodules. Their value was 1.4, which is significantly smaller than the value of 3.5, obtained in this work.Here we discuss the cosmogenic ³He/21Ne ratio in view of new noble gas data on olivine separates of oceanites from Réunion Island and on ultramafic nodules from Mongolia.
A field demonstration of variable density diffraction tomography
Authors: Witten Alan 
Published: February 2008

Geophysical diffraction tomography (GDT) was first suggested in 1984 as a quantitative method for high‐resolution subsurface imaging. In this early form, the imaged quantity is the so called ”object function” which is linearly related to the square of the complex refractive index. More recently, two algorithms have been proposed for separately imaging the spatial variations of the more fundamental mechanical properties of density and compressibility based on the GDT approach, acoustic measurements, and the exploitation of multiple frequencies. It was concluded in these works that these algorithms would not be sufficiently robust for the inherently noisy field applications. The postulated problems result from the existence of the density gradient appearing in the governing equation.In this study, these variable density algorithms were applied to acoustic data acquired at a site known to contain the buried skeletal remains of a dinosaur. For one of the algorithms tested, images of object function and normalized density and compressibility of a buried bone were reconstructed that exhibited image sharpness consistent with the frequencies employed. In addition, the images of this bone, that was subsequently excavated, properly render its size and location and yielded qualitatively correct density and compressibility.
Rotation of stress and blocks in the Lake Mead, Nevada, Fault System
Authors: Ron Hagai , Nur Amos , Aydin Atilla 
Published: February 2008

The Coulomb criterion, has been widely used to infer paleostresses from fault slip data, assuming that faults are optimally oriented relative to the tectonic stress direction. Consequently if the stress direction is fixed during deformation so must be the faults. Freund [1974] has shown however that faults, when arranged in sets, must generally rotate as they slip. Nur et al. [1986] showed that large rotations require the development of new faults more favorably oriented to the principal direction of stress. This leads to older faults sets offset by younger ones, both having the same sense of slip. Because stress rotates as well, paleostress analysis must include the possible effect of stress field rotation.The combined effects of stress field rotation and material rotation were found in the Lake Mead, Nevada Fault System (LMFS). Fault inversion results imply an apparent 60° clockwise (CW) rotation of the stress field since mid‐Miocene time. In contrast, structural data from the Great Basin suggest only a 30° CW stress field rotation. By incorporating paleomagnetic declination anomalies it is inferred that slip on faults caused a local 30° counterclockwise (CCW) rotation of blocks and faults in the Lake Mead area, so that the inferred 60° CW rotation of the stress field in the LMFS is actually only 30°.
On the structure and origin of major glaciation cycles 2. The 100,000‐year cycle
Authors: Imbrie J. , Berger A. , Boyle E. A. , Clemens S. C. , Duffy A. , Howard W. R. , Kukla G. , Kutzbach J. , Martinson D. G. , McIntyre A. , Mix A. C. , Molfino B. , Morley J. J. , Peterson L. C. , Pisias N. G. , Prell W. L. , Raymo M. E. , Shackleton N. J. , Toggweiler J. R. 
Published: February 2008

Climate over the past million years has been dominated by glaciation cycles with periods near 23,000, 41,000, and 100,000 years. In a linear version of the Milankovitch theory, the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity. But the 100,000‐year radiation cycle (arising from eccentricity variation) is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing. We present phase observations showing that the geographic progression of local responses over the 100,000‐year cycle is similar to the progression in the other two cycles, implying that a similar set of internal climatic mechanisms operates in all three. But the phase sequence in the 100,000‐year cycle requires a source of climatic inertia having a time constant (∼15,000 years) much larger than the other cycles (∼5,000 years). Our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source. When these ice sheets, forced by precession and obliquity, exceed a critical size, they cease responding as linear Milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses. In our model, the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity‐driven modulations in the 23,000‐year sea level cycle. During an interval when sea level is forced upward from a major low stand by a Milankovitch response acting either alone or in combination with an internally driven, higher‐frequency process, ice sheets grounded on continental shelves become unstable, mass wasting accelerates, and the resulting deglaciation sets the phase of one wave in the train of 100,000‐year oscillations.Whether a glacier or ice sheet influences the climate depends very much on the scale…. The interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger, thereby gradually increasing its radius of influence.Johannes Oerlemans [1991, p. 155]
Energetic neutral atom precipitation as a possible source of midlatitude F region winds
Authors: Tinsley Brian A. 
Published: February 2008

Midlatitude F region winds velocities observed during main phases of magnetic storms have been shown by Hernandez and Roble to be correlated with the rate of energy input into the ring current as measured by d (Dst)/dt.The attempt by Hernandez and Roble to relate the winds to the ring current decay mechanism of Cummings and Dessler fails because a correlation of wind velocity with positive values of d (Dst)/dt is necessary to support this mechanism rather than the observed correlation with negative values of d (Dst)/dt.Thus an alternative mechanism for deposition of ring current associated energy into the midlatitude thermosphere is needed. The first event discussed by Hernandez and Roble shows a wind divergence at midlatitudes which is taken as evidence of direct energy input there, and there are several suitable mechanisms, some or all of which would act in addition to high latitude sources (clearly present in the absence of changes in Dst).The mechanism of the precipitation of energetic neutral atoms arising from the charge exchange neutralization of ring current ions is suitable in all respects for a direct midlatitude energy input, except that the energy flux is several times too small, but there are reasons for the flux being greater than hitherto inferred.
Energy budget of the Volcano Stromboli, Italy
Authors: McGetchin T. R. , Chouet B. A. 
Published: February 2008

Field data, including eruption movies, are used to construct a budget of the various modes of energy transport for the volcano Stromboli, Italy. At least eight distinct energy transport mechanisms can be identified. In its "normal" steady state eruption mode, Stromboli delivers approximately 1.1 Mw of power to the surface by energy transport mechanisms other than heat conduction; conducted heat probably is of the order of 6 Mw (for the part of the volcano above sea level). Convection is certainly an important mode as well and may exceed all the others, but we have no data to address the problem.The energy budget for Stromboli, in its normal mode of activity appears to be dominated by heat conduction (and probably convection) through the surface of the ground (about 84% of the total). Heat content of the gas accounts for about 10% while the heat carried by condensed ejecta (lava fragments) and radiated heat from the vent, each contributes about 3%; the sum of all other modes is less than 1% (seismic, kinetic energy of ejecta, kinetic energy of gas, acoustic noise, heat carried in rare large flows).Because there are at least 8 different energy transport mechanisms available and operating, we stress the importance of defining terms when comparisons are made between the energy release associated with different volcanic eruptions. The relative importance of different energy transport modes probably varies greatly both with eruption mechanism and site.