bluuebunny/crossref_metadata_2025 huggingface dataset:
Inversion of apatite fission track data for thermal history information
Authors: Corrigan Jeff
Published: February 2008

Apatite fission track length distributions and ages provide an integrated record of the low‐temperature (∼20°–150°C) thermal history of the host rock over geologic time scales. The information content of these data is quantified by applying a stochastic optimization technique (simulated annealing) to determine the range of thermal histories compatible with synthetic apatite fission track data given a mathematical description of fission track annealing. The goodness of fit between the observed and predicted data parameters and a slowly decreasing control parameter are treated as analogous to the free energy and temperature, respectively, of a thermodynamic system. On this basis a Boltzmann‐type probability distribution is defined and used to accept or reject random thermal history perturbations. A priori information used to constrain the model space may include the temperature range in which the solutions are sought, present‐day temperature, the complexity of the thermal history, and the maximum duration of the data‐sensitive thermal history. This approach allows determination of the range of thermal histories compatible with an observed confined track length distribution and associated fission track age, and it provides an estimate of the probability distribution of temperatures at a specific time. Results for synthetic data sets for diverse model thermal histories suggest that the resolution of thermal history is generally poor, parts of the thermal history to which the data are most sensitive can be resolved to no better than about ±10°C for ideal data. Bounds on acceptable time‐temperature space are sensitive to the assumed complexity of the thermal history but are relatively insensitive to typical uncertainties associated with the data. Application of this approach to real data is fundamentally limited by the uncertainty associated with extrapolating annealing models to geologic conditions.
Tertiary meteoric hydrothermal systems and their relation to ore deposition, northwestern United States and southern British Columbia
Authors: Criss Robert E. , Fleck Robert J. , Taylor Hugh P.
Published: February 2008

Tertiary meteoric hydrothermal systems have altered the rocks exposed over more than 5 % of the land surface of the northwestern United States and southern British Columbia, including at least 25,000 km2in Idaho. The systems typically involved convective circulation of fluid derived from ordinary meteoric groundwaters around crystallizing, calc‐alkaline, epizonal plutons emplaced into coeval volcanic cover rocks. These individual systems had widely ranging “lifetimes” of 103to 106years and operated locally throughout the Cenozoic, although the most profound development of such activity occurred during Eocene time. Individual systems varied in size from a few tens of square kilometers (Yankee Fork, Idaho) or less to several thousand square kilometers (Sawtooth and Castro ring zones, Idaho) Typically, regional propylitization aacompanied the fluid circulation, although the higher‐temperature alteration assemblages were developed locally, as were intense alteration effects (e.g., silicification, sericitization, etc.) near some veins and in mining districts. A significant amount, probably 25–50%, of the mineral production and potential in the region is closely related to Tertiary meteoric hydrothermal systems. Oxygen and hydrogen isotopic data clearly demonstrate the close geologic association of meteoric hydrothermal systems and mineralization in (1) the Paleocene, Cu‐Zn‐Pb‐Mn Main Stage mineralization at Butte, Montana; (2) numerous Eocene epithermal deposits principally valued for Au and Ag but also including significant deposits of Cu, Pb, Zn, F, Sb, etc., as at Republic, Washington, and in several mining districts in the Idaho batholith and the Challis volcanic field; (3) several Eocene skarn deposits valued for W (Ima, Idaho) and Cu (Mackay, Idaho); (4) important lead‐silver vein and replacement deposits of Tertiary (Bluebell, British Columbia) and of probable Cretaceous and early Tertiary age (Wood River, Idaho); (5) several potentially economic Mo‐, Be‐, and U‐bearing Eocene “porphyry” plutons; and (6) Miocene epithermal deposits, most prominently the Au and Ag bearing veins at Silver City and DeLamar, Idaho, the Hg deposits at the McDermitt caldera, Nevada and Oregon, and at Weiser, Idaho, and Au deposits in the Western Cascade Range and Lake County, Oregon. A close spatial association has been demonstrated between ore deposits and rocks having anomalous δ18O values and low δD values. The most important deposits are associated with relatively small (generally 5–300 km2) zones of low δ18O values, and they are particularly closely linked with zones of very steep18O/16O gradients in the altered rocks. These associations hold much promise for the use of δ18O and δD contour maps in future exploration efforts.
Paleomagnetism of the Moreton's Harbour Group, northeastern Newfoundland Appalachians: Evidence for an Early Ordovician Island Arc near the Laurentian Margin of Iapetus
Authors: Johnson Rex J. E. , van der Pluijm Ben A. , Van der Voo Rob
Published: February 2008

Paleomagnetic results have been obtained from mafic volcanic units in the upper part of the Moreton's Harbour Group, which is part of an accreted island arc terrane that is preserved in the Notre Dame Bay subzone of the Central Mobile Belt of the Newfoundland Appalachians. Detailed thermal and alternating field demagnetization reveals a stable characteristic component of magnetization, carried by magnetite, at a large number of sites in pillow basalts and coeval basaltic dikes. The intrusives contain both polarities, and a primary age for the characteristic magnetization is indicated by a positive contact test for one of the dikes, and by a positive structural test involving a correction for block rotations (strike‐correction). The overall mean direction for the characteristic component after tectonic (tilt and strike) correction (flows and intrusives:D= 171°,I= +22°,k= 22.6, α95 = 6.5°, pole 29°N, 135°E; flows:D= 166°,I= +22°,k= 33.2, α95 = 6.5°) corresponds to an Early Ordovician paleolatitude for the arc of 11°S, which is indistinguishable from the expected paleolatitude of the North American margin. This implies that the arc formed at or near the margin of the craton. In contrast, the Avalon block, which formed the other margin of Iapetus, was widely separated from the arc and the craton at this time. The Early Ordovician paleolatitude of the arc terrane supports a tectonic model in which coeval ophiolitic sequences inboard of the arc were formed in a narrow ocean basin between the arc and the craton. Subsequent convergence between the arc and Laurentia in Middle Ordovician time resulted in closure of this narrow ocean basin and obduction of the back‐arc basin oceanic crust onto the ancient margin of the craton, thus giving rise to the Taconic orogenic pulse in the Newfoundland Appalachians.
Low intensity of the geomagnetic field in Early Jurassic time
Authors: Perrin M. , Prevot M. , Mankinen E. A.
Published: February 2008

From a large collection of Jurassic continental tholeiites cropping out in Europe and Africa, we selected 90 samples for paleointensity determinations. The samples were carefully selected to avoid any secondary magnetizations, especially viscous magnetization. Use of the Thellier method reveals that magnetic property changes due to heating begin often at quite low temperatures but fortunately without modifying noticeably their natural remanent magnetization‐thermoremanent magnetization ratio. Twenty‐eight well‐clustered paleointensity estimates were obtained from two European dikes that were emplaced during Early Jurassic time: the Kerforne dike at Brenterc'h in Brittany (northwestern France) and the Messejana dike on the Iberian Peninsula (Spain and Portugal). Virtual dipole moments calculated from both magmatic units are similar and only about one‐third of present‐day values. These new data lend support to the recently postulated low dipole moment of the Mesozoic geomagnetic field.
The evolution of Mauna Kea Volcano, Hawaii: Petrogenesis of tholeiitic and alkalic basalts
Authors: Frey F. A. , Garcia M. O. , Wise W. S. , Kennedy A. , Gurriet P. , Albarede F.
Published: February 2008

Mauna Kea Volcano has three exposed rock units. Submarine shield‐building tholeiites form the oldest unit. Subaerial, interbedded tholeiitic and alkalic basalts form the intermediate age unit (70–240 Ka), and they are partially covered by evolved alkalic lavas, hawaiites and mugearites (4–66 Ka). In contrast to other Hawaiian volcanoes, such as Haleakala and Kauai, lavas from Mauna Kea do not define systematic temporal variations in Pb, Sr or Nd isotopic ratios. However with decreasing age the tholeiitic basalts are increasingly enriched in incompatible elements; therefore the shield and postshield tholeiites were derived from compositionally distinct parental magmas. Submarine shield lavas from the east rift contain forsterite‐rich olivine (up to Fo90.5) providing evidence for MgO‐rich (14.4 to 17%) magmas. Postshield tholeiitic and alkalic basalts with similar isotopic ratios may have been derived from the same source composition by different degrees of partial melting. If a compositionally and isotopically homogeneous source and a batch melting model are assumed, inversion of incompatible element abundance data for the postshield basalts requires low degrees (<2%) of melting of a garnet Iherzolite source which had near‐chondritic abundances of heavy rare‐earth elements (REE) but less than chondritic abundances of highly incompatible elements such as Ba, Nb and light REE. As the volcano migrated away from the hotspot, eruption rates decreased enabling high Fe‐Ti basalts to form by fractional crystallization in shallow crustal magma chambers. The associated phenocryst‐rich, high‐MgO postshield lavas (picrites and ankaramites) are products of phenocryst accumulation. Eventually basaltic eruptions ceased, and the youngest Mauna Kea lavas are exclusively hawaiites and mugearites which formed from alkalic basalt parental magmas by clinopyroxene‐dominated fractionation at lower crustal pressures.
Rifting in the northern Norwegian‐Greenland Sea: Thermal tests of asymmetric spreading
Authors: Crane Kathleen , Sundvor Eirik , Buck Roger , Martinez Fernando
Published: February 2008

The analysis of heat flow, seismic, and topographic data collected in the northern Norwegian‐Greenland Sea reveals an asymmetric evolution of the Eurasian and North American plates. These data are compared to predictions from three kinematic models of extension which produce asymmetry about the Knipovich Ridge: (1) uniform asymmetric pure shear, (2) lithospheric simple shear, and (3) rift/ridge jumping. The data are consistent with a range of deformation scenarios, from one ridge jump occurring at about 25 m.y. after the initiation of spreading to continuous asymmetric extension. The simple shear model can match the data only when a detachment fault dips more steeply than 45° under Svalbard. Tectonic and heat flow evidence suggests that asymmetry may have evolved from a combination of all three models. When the Mohns Ridge (propagating to the east) encountered the preexisting northward trending Spitsbergen Shear Zone, the direction of ridge propagation shifted to the north, being influenced by the abrupt change in the regional stress field at the shear zone‐ridge axis intersection. As a result, the nascent Knipovich Ridge entered into and propagated along the shear zone. Consequently, formerly active shear faults became the new detachment surfaces along which new crust is minted and asymmetrically extended. The high level of deviatoric stress about the Mohns and Knipovich Ridge intersection may cause a gradual eastward migration of the Knipovich Ridge, resulting in multiple zones of magma intrusion. “Off‐axial” bands of high heat flow and volcanism located along the Barents Sea and Svalbard margin, along the Yermak Plateau, and on continental Svalbard just southeast of the Yermak Plateau may be evidence for this migration. Propagation from the Nansen Ridge may have entered the same shear zone from the north, explaining the creation of the small Molloy Ridge and off‐axis volcanism on the Yermak Plateau.
Microcrack‐induced seismic anisotropy of sedimentary rocks
Authors: Sayers C. M. , van Munster J. G.
Published: February 2008

The seismic anisotropy often detected in the Earth's crust has been attributed in large part to the presence of open microcracks showing some degree of alignment. This alignment is believed to result from the anisotropic state of in situ stress, since any cracks remaining open at depth will tend to be oriented normal to the direction of the minimum in situ stress. Previous workers have calculated the change in elastic wave velocities due to either randomly oriented or perfectly aligned cracks. In this paper the P and S wave anisotropy in the long‐wavelength limit is calculated for an arbitrary orientation distribution of cracks. For the case of an orthotropic distribution of circular cracks it is found that a simple relation exists between the P and S wave anisotropy. To test this prediction, ultrasonic P and S wave velocities were measured for propagation in three orthogonal directions through three samples of Berea sandstone as a function of maximum compressive stress applied perpendicular to the bedding plane. The samples were 50‐mm cubes and were stressed to peak in a true triaxial loading frame, while the principal stress components parallel to the bedding plane were held constant at 4 MPa.
On postglacial geoid subsidence over the equatorial oceans
Authors: Mitrovica J. X. , Peltier W. R.
Published: February 2008

We develop two new spectral formalisms for the gravitationally self‐consistent solution of the “sea level equation” which governs the redistribution of glacial meltwater on a visco‐elastic Earth. The first is a purely spectral technique based on an extension of theory outlined by Dahlen (1976) for determining the equilibrium oceanic tide on an elastic planet. Using this technique, with a feasible spherical harmonic truncation level (ℓ ≤ 30), we can obtain convergence in the computed sea level variations within the degree range ℓ ≤ 10. We also develop a second, “pseudospectral” technique, however, which permits the construction of gravitationally self‐consistent solutions to much higher degree and order (ℓ ≫ 100). The pseudospectral formalism is employed here to develop a comprehensive physical explanation for the global pattern of present‐day sea level variations due to ongoing glacial isostatic adjustment. In particular, we focus upon a mechanism, which we term “equatorial ocean syphoning,” that acts to draw water toward the oceanic portion of the collapsing peripheral bulge that encircles previously glaciated regions. The collapse of the forebulge induces a flow of water which is required to maintain hydrostatic equilibrium. The syphoning mechanism dominates the relative sea level (RSL) variation in oceans in the far field of the ice sheets (that is, beyond the peripheral bulge) during periods, such as the interglacial of the past 4000 years, during which the volume of the ice sheets has not (apparently) changed appreciably, while, at the same time, isostatic adjustment persists. The RSL change in the near field is, in contrast, dominated by the (local) vertical displacement of the solid surface. Finally, comparison of gravitationally self‐consistent predictions of RSL change in the far field, with the observational data at small Pacific island sites, has provided an upper bound of 1 to 2 m for the eustatic sea level rise produced by any recent (last 3000 years) melting of the Antarctic ice sheet.
Heat flow from a scientific research well at Cajon Pass, California
Authors: Sass J. H. , Lachenbruch Arthur H. , Moses T. H. , Morgan Paul
Published: February 2008

The long‐standing “stress/heat flow paradox” was the primary scientific motivation for the Cajon Pass borehole. For nearly two decades, the absence of a fault‐centered heat flow anomaly from measurements to relatively shallow (∼200 m) depths had indicated low average shear stresses (≤20 MPa) on the San Andreas fault, while laboratory data on rock strength and in situ stress determinations to about a kilometer had indicated high stress (∼100 MPa). Initial results from an unsuccessful 1.7‐km‐deep oil well at the site gave a high heat flow (∼90 mW m−2) consistent with a strong San Andreas fault; however, the late Cenozoic geologic history of the Cajon Pass area suggested that the anomalous heat flow was the transient effect of rapid erosion. Theoretical studies predicted that the ∼30% surface anomaly would be substantially reduced at depths of 3–5 km. The research borehole reached a total depth of 3.5 km. Below a superficial covering of Tertiary sedimentary rocks, it penetrated gneissic rocks with composition ranging from gabbroic to granodioritic. Core recovery amounted to only about 3% of the total depth, necessitating the use of drill cuttings to characterize thermal conductivity. This, in turn, resulted in much higher uncertainties in average conductivity (±10–15%) than would have occurred with a continuously cored hole (±3–5%). From a time series of temperature logs, equilibrium temperature gradients were established over selected intervals of 250–500 m to within 95% confidence limits of 2%. These gradients were combined with harmonic mean thermal conductivities having larger uncertainties to give interval heat flows, which vary systematically from 100 ± 5 mW m−2 in the uppermost 400 m to 75 ± 3 mW m−2 in the lowermost 300 m. Thus, at the Cajon Pass site, heat flow is decreasing with depth at a mean rate of more than 7 mW m−2 per kilometer, consistent with a frictionless fault and with theoretical predictions based on local erosional history.
Geochemical constraints on formation fluid ages, hydrothermal heat flux, and crustal mass transport mechanisms at Cajon Pass
Authors: Torgersen Thomas , Clarke W. B.
Published: February 2008

He, Ne, and helium isotopic measurements have been made on borehole fluids collected during drill stem tests at the Cajon Pass scientific drill hole. These fluids represent a dominant component (greater than 90%) of the formation fluids as determined from fluoroscein tracer measurements. The results indicate a crustally derived radiogenic helium source with excess He concentrations up to 100x air‐saturated water conditions. The gas content and the chemical composition of the borehole waters reflect a variation in the formation waters entering the hole from several discrete, identified, and poorly interconnected fracture systems. These conditions indicate a local source for the excess 4He which allows 4He model ages of 33,000 to 5×106 years to be placed on the formation fluids at this depth. This constraint suggests an upper limit of <7% of the total heat flow can be removed by hydrothermal processes. These data thus provide geochemical support for a low stress fault. These model ages further indicate an effective vertical component of the fluid transport velocity of the order of 0.04–6 cm yr−1. Pressure head and in situ permeability measurements indicate flow velocities of 10−5 cm yr−1. To satisfy both of these extant conditions, fluid exchange must have occurred (as least once) in the last ∼33,000 to 5×106 years under flow conditions considerably faster than presently operable. Fluid flow in this region is therefore discontinuous with long periods of slow flow interrupted by periods of very rapid transport. Such a conclusion is consistent with both local and general observations of metamorphic geology and suggests a deep crustal fluid transport mechanism that is mechanically and/or tectonically controlled.
Tomography of the western United States from regional arrival times
Authors: Hearn Thomas , Beghoul Noureddine , Barazangi Muawia
Published: February 2008

First arrival times from regional distances (200–1200 km) in the western United States extracted from the International Seismological Centre (ISC) data set were inverted in a tomographic study to map the laterally varying Pn velocity structure of the uppermost mantle (i.e., the mantle lid) and to estimate the crustal static delay at each seismograph station. Synthetic data were used to evaluate resolution. Results correlate well with major tectonic features. We find low uppermost mantle velocities (V < 7.9 km/s) centered in the Basin and Range Province surrounded by uppermost mantle of higher, more normal Pn velocities (V > 7.9 km/s). These low apparent Pn velocities are primarily due to the effects of isostasy, hotspot volcanism, and crustal extension. We interpret the low apparent Pn velocities beneath the Sierra Nevada (7.6–7.7 km/s) to be the result of time delays introduced by ray paths tunneling through the deep crustal root. The Yellowstone hotspot, migrating along the Snake River Plain, has left low velocity (<7.6–7.8 km/s), hot upper mantle in its wake. Lower Pn velocities (∼7.8 km/s) beneath the extensional Basin and Range are also presumably due to hotter uppermost mantle. Based on station static delays for the Basin and Range, we infer that Moho depths there do not vary significantly from 30 km. Normal continental Pn velocities (∼8.0 km/s) as well as thicker crust (40–50 km) underlying the Rocky Mountains, Wyoming Basin Province, Colorado Plateau, and the northern Columbia Plateau indicate the presence of relatively cool uppermost mantle beneath these regions.
Shock wave equation of state of serpentine to 150 GPa: Implications for the occurrence of water in the Earth's lower mantle
Authors: Tyburczy James A. , Duffy Thomas S. , Ahrens Thomas J. , Lange Manfred A.
Published: February 2008

The shock equation of state of serpentine has been determined to 150 GPa. Four distinct regions occur along the Hugoniot: a low‐pressure phase, a mixed phase region, a high‐pressure phase, and a very high pressure phase. The low‐pressure phase (LPP) exists under shock pressures to about 40 GPa. This material exhibits shock properties that are partially consistent with those of low pressure serpentine, but steep release paths and a low value of K′ = 2.77 suggest transformation to another, possibly amorphous, assemblage. Thermodynamic calculations indicate that under equilibrium conditions, serpentine would decompose to oxides plus water at conditions below 10 GPa along the Hugoniot. A mixed phase region begins at 40 GPa with complete transition to a high‐pressure phase occurring by about 55 GPa. The high‐pressure phase (HPP) occurs at shock pressures between 55 GPa and 125 GPa. Model Hugoniots based on perovskite plus periclase plus water and brucite plus periclase plus stishovite reproduce the serpentine HPP Hugoniot within experimental error, so definitive identification of the HPP as a distinct hydrous mineral phase or as a free water containing mixture is not possible. Above 125 GPa a transition to a very compressible phase, possibly a hydrous partial melt, occurs. The serpentine HPP Hugoniot is about 15–20 % less dense than the Earth's lower mantle. Models of the lower mantle based on shock equations of state for olivine, pyroxene, and serpentine indicate that for an atomic Mg/(Mg+Fe) ratio of 0.80, the presence of 2 wt % H2O is consistent with seismically determined lower mantle density estimates. Greater amounts of H2O can be accommodated if accompanied by an increase in Fe content. Calculated Hugoniot sound speeds of the serpentine HPP, although poorly constrained, are broadly consistent with lower mantle sound speeds. Thus the high‐pressure density and sound speed of an H2O‐rich magnesium silicate determined from shock equation of state experiments indicate that the observed seismic properties of the lower mantle allow the existence of several weight percent of water in the lower mantle.
Seafloor compliance observed by long‐period pressure and displacement measurements
Authors: Crawford Wayne C. , Webb Spahr C. , Hildebrand John A.
Published: February 2008

Ocean surface waves with periods longer than 30 s create periodic, horizontally propagating pressure fields at the deep seafloor. Seafloor displacements resulting from these pressure fields depend on the density and elastic parameters of the oceanic crust. The displacement to pressure transfer function, the seafloor compliance, provides information about ocean crustal density and elasticity, and we outline a linearized inversion method to determine ocean crustal shear velocity from the compliance. By computing compliance partial differences with respect to changes in ocean crust shear velocity, we provide estimates of inversion stability and of the compliance sensitivity to crustal properties. Seafloor compliance, measured from pressure and acceleration spectra, is presented for two different sites: Axial Seamount on the Juan de Fuca Ridge and the West Cortez Basin in the California continental borderlands. The compliances and inverted structure for these two sites show significant differences; in particular, a zone of low shear strength is observed at depth within Axial Seamount, suggesting the presence of at least 3% partial melt within the upper 2500 meters of the ediface. These results suggest that the method provides a useful new geophysical prospecting tool.
Paleointensity of the geomagnetic field during the last 80,000 years
Authors: Tric Emmanuel , Valet Jean‐Pierre , Tucholka Piotr , Paterne Martine , Labeyrie Laurent , Guichard Francois , Tauxe Lisa , Fontugne Michel
Published: February 2008

High‐resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores. Three duplicate records were used to estimate the regional coherency of the data within a single area (Tyrrhrenian Sea) while the two others document the field variations in the eastern Mediterranean and the southern Indian Ocean. Careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes. The time‐depth control was provided by oxygen isotopes, and small‐scale variations in the deposition rates were constrained by means of tephrachronology. The synthetic curve calculated from the Mediterranean records provides a continuous record of the intensity variations during the last 80,000 years (80 kyr), which correlates well with the sparse volcanic data available for the period 0–40 kyr. The fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the Pacific Ocean establishes the truly dipolar character of these variations. The dipole field moment is characterized by large‐scale changes as shown by the existence of pronounced drops (at 39 and 60 kyr) alternating with periods of higher intensity. The record suggests a periodic nature for these intensity variations; however, the period studied is not sufficiently long to state this conclusively. These results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time.
A multiple‐crack model of brittle fracture: 1. Non‐time‐dependent simulations
Authors: Lockner David A. , Madden Theodore R.
Published: February 2008

A numerical multiple‐crack interaction model is developed to simulate the failure process in brittle solids containing significant populations of flaws. The model, which is two dimensional, allows for the growth of microcracks on a regular array of potential crack sites. Individual cracks may be oriented vertically, horizontally, or at 45° to the sample axes. Quasi‐static equilibrium equations are expressed in terms of finite difference approximations and are solved by applying a Renormalization Group theory approach. More than 5800 potential crack sites are included in the current version of the model. We have successfully duplicated a variety of brittle fracture phenomena observed in laboratory rock mechanics studies by employing a limited number of parameters and relations in the model. Included in the model are (1) Lamé constants λ and μ for intact material, (2) a coefficient of friction for friction on cracks, (3) a normal stress‐dependent crack closure algorithm and (4) an initial crack population. A fracture mechanics approach is used to determine crack growth. Approximate stress intensity factors are computed for all cracks, and when critical values are exceeded, cracks are allowed to grow in either mode I (tension) or mode II (in‐plane shear). Simulations are performed by specifying a combination of stress and strain boundary conditions. The model is capable of duplicating experimentally observed features such as elastic moduli, dilatancy, acoustic velocities, peak strength, Mohr‐Coulomb failure envelope and, to a limited degree, crack coalescence. A mode II critical stress intensity factor was required to produce a concave failure envelope, as is observed in laboratory experiments. This curvature in the failure envelope reflects a transition from mode I crack growth at low confining pressure to mode II growth at high confining pressure.
Viscous flow models of global geophysical observables: 1. Forward problems
Authors: Forte Alessandro M. , Peltier Richard
Published: February 2008

We present a gravitationally consistent method for calculating the “kernel” functions which relate global geophysical observables, such as the geoid, surface plate motions and core‐mantle boundary (CMB) topography, to internal density heterogeneities in a viscous, compressible mantle possessing an arbitrary radial variation of viscosity. We show that the influence of finite mantle compressibility is substantial in the case of the predicted nonhydrostatic geoid, with the largest effects occurring at spherical harmonic degree ℓ=2. On the basis of the geoid data we find that our best two‐layer viscosity model possesses a factor of 9 jump in viscosity at 1200 km depth. We argue, however, that the inferences of viscosity obtained from the geoid data are sensitive to the model of internal density heterogeneity which is employed and that this sensitivity probably explains the differences between our own viscosity inferences and those obtained by others. Our viscous flow models possess a spherically symmetric viscosity distribution and they cannot therefore “explain” the toroidal component of the flow of the tectonic plates. We therefore present a new scheme for predicting plate motions based on an explicit coupling of poloidal and toroidal flows which we derive on the basis of the assumption that the individual plates are perfectly rigid. We show how this new description of surface plate motions induced by buoyancy forces in the mantle may be used to constrain the absolute value of the long‐term (i.e., steady state) mantle viscosity.
Visible and near‐infrared (0.4–2.5 μm) reflectance spectra of Playa evaporite minerals
Authors: Crowley James K.
Published: February 2008

Visible and near‐infrared (VNIR; 0.4–2.4 μm) reflectance spectra were recorded for 35 saline minerals that represent the wide range of mineral and brine chemical compositions found in playa evaporite settings. The spectra show that many of the saline minerals exhibit diagnostic near‐infrared absorption bands, chiefly attributable to vibrations of hydrogen‐bonded structural water molecules. VNIR reflectance spectra can be used to detect minor hydrate phases present in mixtures dominated by anhydrous halite or thenardite, and therefore will be useful in combination with X ray diffraction data for characterizing natural saline mineral assemblages. In addition, VNIR reflectance spectra are sensitive to differences in sample hydration state and should facilitate in situ studies of minerals that occur as fragile, transitory dehydration products in natural salt crusts. The use of spectral reflectance measurements in playa studies should aid in mapping evaporite mineral distributions and may provide insight into the geochemical and hydrological controls on playa mineral and brine development.
P wave velocity anomalies between the Tonga subducting slab and the overlying mantle beneath the Lau Basin
Authors: Bock G. , Eguchi T. , Fujinawa Y.
Published: February 2008

During a 3‐week period in November‐December 1984, nine ocean bottom seismograph (OBS) stations were operating in the Lau basin concurrently with a land‐based station in the Tongan island of Vava'u (VAV). Data from two intermediate‐depth (200 km < h < 300 km) and 10 deep (.h > 400 km) earthquakes in the Tonga Benioff zone allowed the P wave velocity contrast between the subducting slab and the overlying mantle beneath the Lau basin to be determined. Based on relative residuals OBS‐VAV and on three‐dimensional ray tracing, a P wave velocity contrast of 7% is inferred. This is an average contrast for depths above 400 km. The standard error of the estimates is of the order of 2%. The data are consistent with a hot upper mantle beneath the Lau basin where temperatures are close to the solidus temperature, but they do not necessarily require the presence of partial melt in the asthenosphere beneath the Lau basin.
Seismological constraints on source properties of themb=4.0, 1985 Ardsley, New York, earthquake: A characteristic rupture?
Authors: Hough S. E. , Seeber L.
Published: February 2008

On October 19, 1985, a magnitude 4.0 earthquake occurred in southern Westchester County, New York, approximately 30 km north of central Manhattan. Barstow et al. (1988) concluded that this earthquake consisted of two sub events, with the larger sub event occurring 0.6 s after the first one. We present analysis of data for the main shock as well as aftershocks and foreshocks which were recorded on the New York State Seismic Network. Because this data set includes a range of source magnitudes from less than 2 to 4 recorded for virtually identical source‐receiver pairs, an empirical Green's function technique can be used to study the source characteristics of the larger events. The corner frequency of the main shock and an attenuation parametert* are resolved from the initial portion of the seismogram at station PAL in Palisades, New York, 8 km northwest of the epicenter. Spectra of small foreshocks and aftershocks can then be corrected for attenuation usingt*. Following an iterative procedure, it is then possible to make reliable determinations of seismic corner frequency for these events. Several conclusions are obtained: (1) The spectrum of the first subevent in the main shock can be well modelled by a Brune spectrum from 1 to 10 Hz, with a corner frequency of 7 ± 1 Hz. At higher frequencies, the source spectrum contains sharp peaks which do not fit an omega‐square model and may reflect complexities in the rupture process. (2) The average stress drop of the sequence is ≈ 205 bars. The stress drop of the main event is 540 bars, but no stress drop dependency on moment is resolved over the magnitude range 1.8–4.0. (3) Attenuation parameterst* are obtained for the 8 km northwestward path from Ardsley to Palisades and for a 52‐km westward path to a station in the Hudson Highlands, and their similarity suggests an increase ofQwith depth. (4) Assuming that the two main shock sub events did not rerupture the same area but were contiguous, a maximum rupture dimension is 440–680 m. The rupture diameter of the largest aftershock,mb=3, is determined to be approximately 320 meters. The combined rupture for the sequence inferred from spectral methods is consistent with the size of the rupture inferred from the area surrounded by the early aftershocks (700 m diameter). The rupture dimension of the Ardsley sequence may be characteristic of the Dobbs Ferry fault and other seismogenic faults in the Manhattan Prong, which are segmented with characteristic dimensions of 0.5–2 km (Seeber and Dawers, 1989). Assuming that the range of stress drops in the Ardsley sequence is also characteristic of the Manhattan Prong, this range of dimensions corresponds to a range of characteristic magnitudes betweenmbof 3.3 and 5.5. We note that this magnitude range encompasses the five largest known earthquakes in the New York City seismic zone: the 1985 Ardsley main shock and events in 1737, 1783, 1884, and 1895.
Stress observations and driving force models for the South American Plate
Authors: Stefanick Michael , Jurdy Donna M.
Published: February 2008

Stress observations are compared to predictions of driving force models for the South American plate. Stress observations have the potential to give detailed information about the forces driving the plates, in that sets of forces with the same net torque can produce quite different stress patterns. For a two‐dimensional (vertical section) model of a plate there are three conditions for dynamical equilibrium: the net forces and the net torque about any point must all be zero. These conditions are used to estimate the magnitudes of forces as a function of age, with ridge push and slab pull as active forces, and plate drag and slab resistance as passive forces. The results are consistent with earlier published models, and indicate that the primary balance for an oceanic plate is between slab pull and the resisting drag forces. We generalize the model for the possible forces driving the South American plate and compare with observed stress measurements and kinematics. The South American plate has a small slab pull component, amounting to about one‐fifth the ridge push force, with the other components reasonably isolated geographically, Thus, the plate balance is largely between ridge push and plate drag. This gives an opportunity of resolving the effects of trench suction or collision at the subduction zones. Most remarkable is the nearly uniform E‐W orientation of the regional stress field, SHmax directions, which is extensional in the high Andes; a second feature is the N‐S orientations in the Amazon basin. The two‐dimensional, horizontal stress patterns are compared for successive models of combinations of driving forces. Models with ridge push, slab pull at the Scotia and Caribbean arcs, and trench suction near the west coast, balanced by plate drag, produce similar stress patterns to those observed. The magnitude of a possible trench suction force is estimated using a model with an eddy between the Nazca slab and the overlying South American plate.
Validation of the ICE‐3G Model of Würm‐Wisconsin Deglaciation using a global data base of relative sea level histories
Authors: Tushingham A M. , Peltier W. R.
Published: February 2008

A global data base consisting of radiocarbon‐controlled relative sea level (RSL) histories from 392 different geographic locations has been compiled for use in analyses of the glacial isostatic adjustment process. These data may be employed to constrain both the spatiotemporal characteristics of the last deglaciation event of the current ice age and the rheological structure of the planetary interior. In a previous paper in this series (Tushingham and Peltier, 1991), we employed the RSL data in this set from sites that were once ice covered to refine the apriori model of deglaciation that had been delivered by past research in the area of Pleistocene geomorphology (Peltier and Andrews, 1976; Wu and Peltier, 1983) to produce a new global model of this event that we have called ICE‐3G. In the present paper, following a description of the full data base and of the methods used to compile it, we employ data from the sites that were not ice covered (200 time series) to verify the plausibility of this refined model of deglaciation and to test the extent to which the totality of the observations constrain the radial viscoelastic structure of the planet. The analysis of solutions to the forward problem presented here reinforce previous inferences of a preference for a thick lithosphere by RSL data from sites along the U.S. east coast and a preference by data from most near‐field sites for an increase in viscosity across the 670‐km discontinuity in the mantle by a factor of approximately 2, when the viscosity of the upper mantle is fixed to the value of 1021 Pa s originally deduced by Haskel (1935) on the basis of his analysis of the data from Fennoscandia.
Rift flank uplifts and Hinterland Basins: Comparison of the Transantarctic Mountains with the Great Escarpment of southern Africa
Authors: ten Brink Uri , Stern Tim
Published: February 2008

Uplifted rift margins are a common feature of continents and oceans. Two variants of rift flank morphologies have been recognized: One in which the topography warps down from an inland high toward the continental margin, and one where the tropographic peak lies close to the continental margin. The Great Escarpment of southern Africa and the Transantarctic Mountains are examples of the first and the second variants of rift flanks, respectively. Both rift flanks are bordered on their landward side by broad continental basins: the Kalahari and the Wilkes hinterland basins. If these basins are interpreted as flexural “outer lows” that deepen in unison with the uplift of the rift flanks, the lithosphere on the uplifted side is very rigid in both cases (elastic thickness Te of 100 ± 20 km for southern Africa and 110 ± 20 km for East Antarctica). We suggest that the variation in rift flank morphology is caused by the isostatic response to uplift forces of elastic plates sharing different boundary conditions. We model the uplift of the Transantarctic Mountains as an upward deflection of an elastic plate which is broken at the front of the Transantarctic Mountains, and we model the uplift of the Great Escarpment as an upward deflection of a continuous elastic plate that is modified by the downward load of sediments on the continental margin. Although the Transantarctic Mountain uplift is young (60–0 Ma) and the southern African uplift is old (<100 Ma), the different isostatic responses of the two margins are not a function of age, because most loading (sedimentation) and unloading (erosion) took place shortly after rifting. Detailed modeling of topography, gravity, geological markers, and the locations of depocenters suggests that lithospheric rigidity decreases under the Transantarctic Mountains, whereas in southern Africa the decrease occurs not under the Great Escarpment but far seaward under the continental shelf and slope. If the distribution of lithospheric rigidity is indicative of the thermal regime of the lithosphere, then uplifted rift flanks are not always underlain by a thermal anomaly. This and other geological evidence indicate that a single mechanism cannot explain the uplift of both the Antarctic and the African margins.
The effect of anisotropic surface roughness on flow and transport in fractures
Authors: Thompson Mollie E. , Brown Stephen R.
Published: February 2008

The topography of natural fracture surfaces has a strong influence on the flow and transport properties of fractures. In this paper we investigate the role of anisotropic surface roughness on fluid flow, solute transport, and electrical current flow in fractures. Our results show that the directional characteristics of the surfaces are more important in determining fracture transport properties than is the degree of roughness. Roughness oriented parallel to the primary flow direction substantially enhances fluid and solute transport rates through fractures, with the effect becoming more apparent as the amount of contact area between the surface increases. Roughness oriented transverse to the flow direction inhibits flow rates and delays the movement of solute through the fracture.
Reversed polarity characteristic magnetizations in the Albian Contessa Section, Umbrian Apennines, Italy: Implications for the existence of a Mid‐Cretaceous mixed polarity interval
Authors: Tarduno John A. , Lowrie William , Sliter William V. , Bralower Timothy J. , Heller Friedrich
Published: February 2008

Paleomagnetic and paleontologic data from the Valle della Contessa (Umbrian Apennines, Italy) span a 14 m.y. gap in previous magnetostratigraphic sections and reveal several intervals of reversed characteristic remanent magnetization (ChRM) in limestones and marls of the Albian Marne a Fucoidi. Rock magnetic data indicate that the reversed ChRM is carried by hematite having high unblocking temperatures (∼ 625–690° C). Hematite with these unblocking temperatures also carries a normal polarity ChRM in other intervals. A potential bias towards the identification of normal polarity is present since at every horizon measured a normal polarity component, carried by magnetite and hematite, is isolated at lower unblocking temperatures (∼300–600°C). Bulk magnetic properties vary throughout the measured section. In the portion containing most of the reversed ChRM intervals, hematite is the dominant carrier of natural remanent magnetization, while magnetite is the dominant carrier stratigraphically above and below. These changes correspond approximately with lithology. The zone of dominant hematite magnetization is marked by reddish bands (pigmentary hematite) which can be correlated throughout Umbria and are thought to record changes in the oxidation state of the seafloor during deposition. Two models may account for the observed magnetization directions. The reversed magnetizations may record a remagnetization which occurred at least 18 m.y. after deposition, while the beds were still flat‐lying, in reversed polarity chron 33R (83–79 Ma) or later. If the reversed ChRM represents a remagnetization, the process is potentially of great importance since it can produce magnetization patterns which resemble polarity intervals. Alternatively, the reversed magnetizations may have been acquired during intense seafloor oxidation episodes during the mid‐Albian (107–104 Ma) and may record unrecognized intervals of reversed geomagnetic field polarity. Milankovitch‐like bedding cyclicity can be used to tune the sedimentation rate and obtain estimates of the duration of the potential reversed intervals. Using these estimates, two of the potential reversed polarity intervals are of sufficient duration (> 100 kyr) to be recognizable in both detailed stratigraphic sections and marine magnetic anomaly surveys. A primary magnetization model predicts that several intervals of reversed magnetization should be found near the boundary of the Biticinella breggiensis and Ticinella primula foraminiferal zones and within the Prediscosphaera cretacea nannofossil zone on a worldwide basis.