bluuebunny/crossref_metadata_2025 huggingface dataset:
Mid‐Cretaceous to early Tertiary apparent polar wander path of the Pacific Plate
Authors: Sager William W. , Pringle Malcolm S.
Published: February 2008

Pacific apparent polar wander was delineated for mid‐Cretaceous to early Tertiary time using 22 seamount paleomagnetic poles, 11 paleocolatitudes from azimuthally unoriented cores, five effective inclinations from two‐dimensional magnetic anomaly inversions, and one magnetic lineation relative amplitude datum. Three of the seamount paleomagnetic poles were new data calculated in this study. The apparent polar wander path was defined with six mean poles having ages of 39, 66, 76, 82, 85, and 94 Ma. An alternate path was constructed combining data from two pairs of mean poles (66 and 76 Ma, 85 and 94 Ma) that were statistically indistinguishable. The apparent polar wander path is hook‐shaped with a nearly right angle bend at 82 Ma. Its post‐82 Ma section trends north–south, the pre‐82 Ma part, east–west. The path indicates uneven polar wander. Unresolvable polar motion occurred between 66–76 Ma and 85–94 Ma, suggesting Stillstands. However, the large distances between the 85 and 82 Ma poles (15.2°±3.8°) and 82 and 76 Ma poles (11.3°±2.8°) imply rapid polar wander at rates of 5.1°±1.3°/m.y. and 1.9°±0.5°/m.y., respectively. Both the sharp bend and rapid polar wander occurred at the end of the Cretaceous Normal Polarity Superchron. The mid‐Cretaceous paleomagnetic data show a trend suggestive of southward motion of the Pacific plate that may be the end of a period of southward drift postulated by others to have begun in the Jurassic. The evolution of various features of the apparent polar wander path are discussed in terms of plate motion, true polar wander, and time‐varying non‐dipole geomagnetic fields. We argue that the overall shape of the polar wander path resulted from the change of Pacific plate motion from southward to northward, perhaps brought about by the beginning of the subduction of the plate beneath Asia.
Shallow Drilling in the Salton Sea Region: The Thermal Anomaly
Authors: Newmark Robin L. , Kasameyer Paul W. , Younker Leland W.
Published: February 2008

During two shallow thermal drilling programs, thermal measurements were obtained in 56 shallow (76.2 m) and one intermediate (457.3 m) depth holes located both onshore and offshore along the southern margin of the Salton Sea in the Imperial Valley, California. These data complete the surficial coverage of the thermal anomaly, revealing the shape and lateral extent of the hydrothermal system. The thermal data show the region of high thermal gradients to extend only a short distance offshore to the north of the Quaternary volcanic domes which are exposed along the southern shore of the Salton Sea. The central thermal anomaly has an arcuate shape, about 4 km wide and 12 km long. Across the center of the anomaly, the transition zone between locations exhibiting high thermal gradients and those exhibiting regional thermal gradients is quite narrow. Thermal gradients rise from near regional (0.09°C/m) to extreme (0.83°C/m) in only 2.4 km. The heat flow in the central part of the anomaly is greater than 600 mW/m2 and in the two local anomalies exceeds 1200 mW/m2. The shape of the thermal anomaly is asymmetric with respect to the line of volcanoes previously thought to represent the center of the field, with its center line offset south of the volcanic buttes. There is no broad thermal anomaly associated with the magnetic high that extends offshore to the northeast from the volcanic domes. The Salton Sea thermal anomaly can be described as three nested thermal features superimposed on the regional Salton Trough heat flux: a broad region of elevated conductive heat flow with constant temperature gradients of about 0.1° C/m, a central anomaly of high (0.4°C/m) near‐surface gradients observed through a conductive thermal cap underlain by a nearly isothermal zone of hydrothermal circulation, and two local anomalies with shallow temperature gradients as high as 0.8°C/m superimposed on the central geothermal system. These observations of the thermal anomaly provide important constraints for models of the circulation of the hydrothermal system. Thermal budgets based on a simple model for this hydrothermal system indicate that the heat influx rate for local “hot spots” in the region may be large enough to account for the rate of heat flux from the entire Salton Trough.
Epidote‐Bearing Veins in the State 2–14 Drill Hole: Implications for Hydrothermal Fluid Composition
Authors: Caruso L. J. , Bird D. K. , Cho M. , Liou J. G.
Published: February 2008

Epidote‐bearing veins in State 2–14 drill core from 900 to 2960 m depth were examined using backscattered electron microscopy and electron probe microanalysis to characterize the mineralogy, parageneses, texture, and composition of vein minerals. In order of decreasing abundance, minerals in epidote‐bearing veins are pyrite, calcite, K‐feldspar, quartz, anhydrite, hematite, chlorite, Fe‐Cu‐Zn sulfides, actinolite, titanite, and allanite. The downhole distribution of minerals in epidote‐bearing veins (+ pyrite and quartz) varies as a function of depth and includes: (1) calcite above ∼2000 m, (2) K‐feldspar between 1700 and 2745 m, (3) anhydrite between 2195 and 2745 m, (4) hematite ± sulfides above 2773 m, and (5) actinolite below ∼2890 m. Where present, K‐feldspar was the first mineral to precipitate in veins followed by epidote. In all other veins, epidote was the earliest vein mineral to form. Calcite, quartz, anhydrite, hematite, and sulfides were paragenetically later. Compositional zoning, common in most vein epidotes, is typically symmetric with Al‐rich cores and Fe3+‐rich rims. The minimum mole fraction of Ca2Fe3Si3O12(OH) (XPs) in vein epidotes decreases systematically with increasing depth from ∼0.33 at 906 m to ∼0.21 at 2900 m, and the maximumXPsat any given depth is greater than 0.33. Thermodynamic analyses of phase relations among vein‐filling minerals and aqueous solutions at depths near 1867 m and 300°C indicate that the modern reservoir fluid in the Salton Sea geothermal system is in equilibrium with calcite + hematite + quartz + epidote (XPs= 0.33) ± anhydrite. The predicted fugacity of CO2(∼14 bars) for the modern Salton Sea brine is in close agreement with the calculated value offCO2for the 1867 m production fluid. Theoretical phase diagrams in the system CaO‐K2O‐Fe2O3‐Al2O3‐SiO2‐H2O‐O2‐S2‐CO2demonstrate that the mineralogies and mineral parageneses recorded hi epidote‐bearing veins and the observed variations in Al‐Fe3+content of vein epidotes may result from only minor changes in the fugacity of CO2, O2, and S2of the geothermal fluid.
Surface Deformation and Shallow Dike Intrusion Processes at Inyo Craters, Long Valley, California
Authors: Mastin Larry G. , Pollard David D.
Published: February 2008

The Inyo craters are the two largest of four phreatic craters that lie within a 2.5‐km‐long, 500‐ to 700‐m‐wide N‐S trend of faults and fissures at the south end of the Inyo volcanic chain in eastern California. The alignment of these features with dike‐fed silicic volcanic centers of the same age a few kilometers to the north suggests that they were produced during intrusion of a dike at about 650–550 yr B.P. E‐W extension south of south Inyo crater is ten to several tens of meters, suggesting that the dike is at least that thick. To understand how the faults and fissures developed, we mapped and studied the fault and fissure pattern; used a theoretical boundary element model to determine the surface strain profile above a shallow dike in a purely elastic medium; and conducted physical model experiments of fault and fissure growth. Results of the field studies and experiments indicate that deformation develops according to the following sequence: 1) extension fractures and perhaps other inelastic deformation develop immediately above the dike top and on the limbs of a shallow syncline which forms above the dike; 2) fissures form along two parallel trends at the surface on opposite sides of the dike plane, leaving a relatively unfractured region in between; 3) dip‐slip movement on subsurface fractures, and linkage of these fractures with surface fissures produces inward‐facing normal fault scarps that bound a nested graben above the dike. Experimental graben widths are up to several times narrower than predicted from the locations of theoretical strain maxima above a dilating crack in a linearly elastic medium. This fact is apparently due to the growth of a zone of inelastic deformation above the experimental dike top in the subsurface. If the ratio between the depth to the dike top and the distance between the outermost surface fissures at Inyo craters is within the range of ratios measured experimentally, then the depth to the dike top is between 250 m and several times that. A slant hole drilled in the summer of 1987 by the Department of Energy, Office of Basic Energy Sciences, intersected three breccia bodies of possibly intrusive origin directly below the center of south Inyo crater between 550 m and 610 m below the crater rim. No intact juvenile igneous rock was intersected, suggesting that the hole passed above the (magma‐filled) dike top, perhaps passing through the zone of inelastic deformation which is inferred to exist from our model results.
A SYSTEMATIC APPROACH TO THE INTERPRETATION OF MAGNETOTELLURIC DATA IN VOLCANIC ENVIRONMENTS WITH APPLICATIONS TO THE QUEST FOR MAGMA IN LONG VALLEY, CALIFORNIA
Authors: Park Stephen K. , Torres‐Verdin Carlos
Published: February 2008

Magnetotelluric (MT) data in Long Valley, California, are principally sensitive to the complex structure in the rocks filling the caldera. While the overall shapes of the sounding curves are set by the deep crustal structure and mantle, the lateral variations of that response can be entirely explained using structure of the upper 1600 m. A conductive body at depths from 1350 to 1600 m is present in the northern, southern, and western moats and beneath the resurgent dome. This conductor effectively masks structures at intermediate depths. We believe that this conductor represents fractured Bishop Tuff within the caldera and possibly graphitic metasediments beneath it. Electrical logs from wells penetrating these formations are used to corroborate our interpretation. A large, conductive (5 ohm m) body of magma is precluded by the MT data. The response to a spherical chamber of 4‐km radius at a depth of 10 km is several times larger than the mismatch between our modeled and observed sounding curves. Small (2‐km radius), isolated pockets of magma may be present, however. We have developed a systematic approach to interpreting MT data in a complex volcanic environment. We clustered sites on the basis of similar responses to perceive patterns in spite of severe distortions by local heterogeneity. Our model was built from the surface downward, incorporating other published geophysical surveys and a time domain electromagnetic survey released by Unocal. Three‐dimensional modeling was employed from the start, and our interpretation was based primarily upon the phases. This systematic approach of stripping off the known, shallow structure in order to study the intermediate and deeper structure was quite successful. An important finding of our study is that the phase is sensitive to shallow (1350 m), three‐dimensional structure even at frequencies of 0.01 Hz. Three‐dimensional modeling simply cannot be avoided in complex geological environments.
The Growth of Geological Structures by Repeated Earthquakes 2. Field Examples of Continental Dip‐Slip Faults
Authors: Stein Ross S. , King Geoffrey C. P. , Rundle John B.
Published: February 2008

A strong test of our understanding of the earthquake cycle is the ability to reproduce extant fault‐bounded geological structures, such as basins and ranges, which are built by repeated cycles of deformation. Along strike‐slip faults, the coseismic and interseismic deformation can be nearly equal in magnitude and opposite in sign, resulting in little permanent deformation except for the fault offset. For dip‐slip faults, portions of the crust are lifted and dropped, and so buoyancy forces are exerted. The seismic and interseismic deformations do not balance, and structures grow and become subject to erosion and deposition. We consider three examples for which the structure and fault geometry are well known: the White Wolf reverse fault in California, site of the 1952 Kern County M=7.3 earthquake, the Lost River normal fault in Idaho, site of the 1983 Borah Peak M=7.0 earthquake, and the Cricket Mountain normal fault in Utah, site of Quaternary slip events. Basin stratigraphy and seismic reflection records are used to profile the structure, and coseismic deformation measured by leveling surveys is used to estimate the fault geometry. To reproduce these structures, we add the deformation associated with the earthquake cycle (the coseismic slip and postseismic relaxation) to the flexure caused by the observed sediment load, treating the crust as a thin elastic plate overlying a fluid substrate. The cumulative deformation is principally dependent on the elastic plate thickness, modestly sensitive to the sediment‐substrate density difference, and insensitive to the fluid viscosity for the 4‐ to 8‐Ma structures. We deduce a longterm flexural rigidity of 2–15 × 1019 Nm; this is equivalent to an elastic plate thickness of 2–4 km for a Young's modulus of 2.5 × 1010 Nm−2. This value is found where independent estimates of the elastic thickness from the coherence between surface topography and gravity yield values of about 4 km, but where coseismic fault slip extends to a depth of 10–15 km. Thus much of the seismogenic crust must weaken substantially during the life of active faults, causing the fault‐bounded basins to narrow over time.
ACTIVE DEFORMATION IN THE NEW GUINEA FOLD‐AND‐THRUST BELT: SEISMOLOGICAL EVIDENCE FOR STRIKE‐SLIP FAULTING AND BASEMENT‐INVOLVED THRUSTING
Authors: Abers Geoffrey , McCaffrey Robert
Published: February 2008

The New Guinea fold‐and‐thrust belt forms a 1000‐km‐long mountain chain running the length of New Guinea, but its trend is oblique to the predicted convergence direction. A large component of left‐lateral shear across the mountain belt is expected, but geological evidence for such motion is ambiguous. Surface structures in the fold‐and‐thrust belt show mountain building by folding and thrusting, supposedly thin skinned, of pre‐Pliocene continental shelf sediments, although some evidence indicates that active faulting penetrates the basement. We examine the active deformation of this mountain belt by using teleseismic long‐period P and SH waveforms, short‐period P waveforms, and P wave first motions to determine reliable focal mechanisms and source depths for the 18 largest earthquakes in the New Guinea fold‐and‐thrust belt since 1964. Eight thrust events, with seismic moments (M0) between 1017 and 2 × 1018 N m, are distributed throughout the thrust belt at depths ranging from 0.5 to 45 km and have steeply dipping fault planes that strike parallel to surface structural trends. Six of the eight events are between 11 and 25 km deep. The steep dips of nodal planes, the depths of these earthquakes, and the known thicknesses of the sedimentary section show that thrust faulting is not exclusively thin skinned (i.e., confined above a shallow detachment surface) but penetrates the crystalline basement at high angles. Most earthquakes in the western half of the thrust belt, including the three largest earthquakes examined (M0 > 5 × 1018 N m), show strike‐slip faulting with the probable fault plane oriented east‐west, so that displacement is left lateral. The nine strike‐slip earthquakes account for almost 90% of the observed seismic moment in the period 1964–1985 and indicate a seismic slip rate of 5–25 mm/a. All but one of these strike‐slip earthquakes are shallower than the thrust earthquakes, suggesting that east‐northeast convergence across the fold‐and‐thrust belt is accommodated by a decoupled system of strike‐slip and dip‐slip faults. While the great height of the mountain belt is an obvious indication that crustal thickening is an important consequence of plate motion in New Guinea, seismological evidence suggests that translation by strike‐slip faulting may play a much greater role than previously recognized. By analogy, in older collisional orogens, whose motions can be inferred only from remnant structures and not from seismicity, strike‐slip faulting may have been equally important in accommodating convergence. We estimate from seismic moments of earthquakes and from the volume of the mountains that deformation of the New Guinea Highlands may account for 5–20% of the total convergence between the Pacific plate and Australia in New Guinea.
Dipping Seismic Reflectors, Electrically Conductive Zones, and Trapped Water in the Crust Over a Subducting Plate
Authors: Hyndman R. D.
Published: February 2008

The LITHOPROBE program across the northern Cascadia subduction zone at Vancouver Island has obtained geophysical data that suggest a dipping zone of trapped free pore water in the overlying continental crust. Land and marine multichannel seismic reflection profiles show several bands of reflectors. The primary one at a depth of about 30 km is about 5 km thick, dips inland, and appears to coincide with an electrically conductive layer defined by magnetotelluric data. The downgoing oceanic crust is imaged as a thin reflector dipping at about 15° and 10 km deeper than the primary reflector, a location that is consistent with seismicity and seismic refraction data. Both the reflectors and the conductor have been modeled by near‐horizontal lamellae with 1–4% saline water porosity. Detailed geothermal measurements defining heat flow that decreases inland indicate that the dipping bands are nearly isothermal at a temperature of 400°–500°C. It is suggested that free water generated by dehydration of the downgoing oceanic plate migrates upward until reduced temperature results in hydration reactions and mineral precipitation that forms an impermeable barrier. Low vertical permeability is required to keep the water from separating to the top of the layer. This can be reconciled with the low electrical resistivity if pore interconnection is through thin grain boundary tubes as predicted by theoretical equilibrium pore geometry, although very small effective grain size is necessary. An abrupt transition from high permeability to very low permeability is predicted at a several percent porosity, in agreement with the modeled porosity. Such pore geometry with low permeability may be restricted to temperatures above about 400°C, i.e., in greenschist to amphibolite facies conditions, where grains will deform to equilibrium geometry and cracks are annealed. The region above the reflective and conductive band is estimated to be within blueschist conditions, consistent with the inferred high seismic velocity from refraction data and high density from gravity data.
PLATE KINEMATICS OF THE SOUTH ATLANTIC: CHRON C34 TO PRESENT
Authors: Cande S. C. , LaBrecque J. L. , Haxby W. F.
Published: February 2008

A high‐resolution seafloor spreading history of the South Atlantic since chron C34 is constrained by a combination of Seasat altimeter data and underway marine geophysical data. A set of 45 finite rotation poles defines the relative position of Africa and South America at roughly 2‐m.y. intervals. A set of 12 stage poles constrain the relative motion of these two plates at 5‐ to 10‐m.y. intervals. The position of the stage poles continuously migrates, reflecting the continuously changing azimuths of the fracture zones. Major changes in spreading direction are observed in the Late Cretaceous and early Cenozoic as the fracture zones sweep out broad S‐shaped curves similar to the pattern seen on the Kane fracture zone in the central Atlantic. Small offset fracture zones were found to be the most accurate recorders of changes in plate motion; large offset fracture zones, such as the Agulhas‐Falkland fracture zone, were the least reliable recorders. At 30°S, spreading rates decrease throughout the Late Cretaceous from a high of 75 mm/yr at the end of chron C34 to a low of 30 mm/yr around chron C27. A period of slow spreading between chron C30 and chron C20 corresponds to a zone of fracture zone proliferation, an increase in the amplitude of geoid anomalies over fracture zones, greater relief on topographic profiles, and locally, evidence of intraplate crustal deformation. Spreading rates increase at chron C20 to about 50 mm/yr and then gradually decrease during the late Paleogene and Neogene. A comparison of synthetic fracture zones based on the South Atlantic stage poles to the observed trends of fracture zones in the equatorial Atlantic indicates that the Vema and Marathon fracture zones were generated by South Atlantic spreading, as opposed to central Atlantic spreading, at least as far back as 35 m.y. ago and possibly 50 m.y. ago.
STRAIN, MAGNETIC FABRIC, AND PALEOMAGNETISM OF THE DEFORMED RED BEDS OF THE PONT‐REAN FORMATION, BRITTANY, FRANCE
Authors: Cogné Jean‐Pascal
Published: February 2008

A joint study of strain, anisotropy of magnetic susceptibility (AMS), and paleomagnetism has been conducted in the deformed red beds of the Ordovician Pont‐Réan formation, Brittany, France. Analysis of results from 13 sampling sites shows that homogeneous strain, estimated from the shape of elliptical reduction spots in each site, controls the development of a magnetic fabric during the Hercynian folding of this formation. The good correlation between strain and AMS tensors is interpreted as mainly arising from a strain‐induced progressive rotation of planar hematite that tends to align within cleavage plane. This rotation of magnetic carriers in turn induces a rotation of net measurable pretectonic remanent magnetization toward cleavage. Because of the high angle between the initially subvertical Ordovician rémanent magnetization and the regionally subhorizontal shortening direction, this deviation of the pretectonic magnetization is achieved without any scattering but with a probable clustering effect upon within‐site initial dispersions. For the same reason the overall distribution of mean‐site pretectonic magnetization vectors is clustered at the in situ deformed state. This results in an anomalous behavior of the classical fold test, that could erroneously lead to assignment of a syntectonic to post tectonic age to the magnetizations. Finally the strain removal technique based upon the passive behavior hypothesis is applied to the deviated vectors. It allows recovery of the initial direction of these pretectonic Ordovician magnetizations (D = 235°, I = 75°, α95 = 6.5°).
Plume formation and lithosphere erosion: A comparison of laboratory and numerical experiments
Authors: Olson Peter , Schubert Gerald , Anderson Charles , Goldman Peggy
Published: February 2008

The mechanics of thermal plume formation and intrusion into the lithosphere are investigated using a combination of laboratory experiments and numerical simulations. In the experiments a deep layer of strongly temperature‐dependent viscous sucrose solution is heated from below and cooled from above to produce high‐viscosity surface and low‐viscosity basal thermal boundary layers. Plumes develop from instabilities in the hot boundary layer, ascend, and partially intrude into the cold layer (lid). A set of two‐dimensional finite element calculations are made using the experimental geometry, fluid properties, and boundary and initial conditions. Satisfactory agreement is obtained for the number and shape of plumes and the time variation of surface heat flux, basal temperature, and, to a lesser extent, lid thickness. The major discrepancy is the onset time for instability, which is typically 30% late in the calculations. Numerical simulations of mantle plume‐lithosphere interaction with subsolidus creep rheology show that thermal plumes do not effectively intrude and erode the lithosphere if upper mantle viscosity is a function of temperature only, unless the activation energy is anomalously low, 50 kJ mol−1 or less. However, plumes can significantly erode the lithosphere on time scales of 10–20 m.y. if a low‐viscosity asthenospheric channel, due to pressure dependence of viscosity, is present.
Terrestrial heat flow above the Andean Subduction Zone in Bolivia and Peru
Authors: Henry Steven G. , Pollack Henry N.
Published: February 2008

We report 35 new and 9 revised terrestrial heat flow measurements overlying the Andean subduction zone in Bolivia and Peru. The measurement sites are distributed in the Andean Cordillera, the Altiplano, the sub‐Andean ranges, and the sedimentary platform and basins to the east of the Andes. They fall in the distance range 75–900 km from the Peru‐Chile trench. Sites in the Peruvian Cordillera have a mean heat flow of 41 mW m−2, whereas those in the Bolivian Cordillera and Altiplano average 84 mW m−2. The sub‐Andean ranges and the adjacent sedimentary platform have a mean heat flow of 50 mW m−2. The higher heat flow of the Bolivian Cordillera and Altiplano lies to the east of Quaternary volcanoes along the Bolivia‐Chile border and in southernmost Peru and thus can be recognized as a “back arc” heat flow high. Neither Quaternary volcanism nor high heat flow are present in central and northern Peru. This contrasting along‐strike pattern correlates with the variable angle of subduction of the Nazca plate beneath the region. Beneath Bolivia the subduction is at about 30°–35°, whereas beneath Peru it is subhorizontal and may be providing a cold underplate to the overlying Peruvian lithosphere. Extensive Miocene volcanism in Peru suggests, however, that heat flow in Peru 10 Ma ago was likely similar to that in Bolivia in the present day and implies a change in subduction and a rapid diminution of heat flow in Peru over the past 10 Ma. Imbrication of cold oceanic lithosphere beneath Peru, analogous to that beneath western British Columbia, may provide a mechanism for rapid reduction of the heat flow.
Roughness and wear during brittle faulting
Authors: Power William L. , Tullis Terry E. , Weeks John D.
Published: February 2008

In many natural fault systems, the thickness of gouge and breccia increases approximately linearly with displacement. In contrast, many experimental faults show non linear thickness/displacement relationships. The linear relationship for natural faults has been explained in the past using engineering models for adhesive or abrasive wear. Non linear relationships for experimental faults have not been explained. A model for wear during brittle faulting which considers the scaling of surface roughness can successfully describe the difference between wear on experimental faults and wear on natural faults. We suggest the linear relationship for natural faults results from the approximately self‐similar roughness of the fault surfaces. Experimental faults do not generally follow linear relationships because the roughness of ground surfaces normally used in experimental studies scales differently than the roughness of natural rock surfaces. A simple model which assumes that the volume of wear material created is proportional to the volume of mismatch between the surfaces can explain the differences between wear on experimental faults and wear on natural faults. For ground surfaces of experimental samples, the volume of mismatch is independent of the total slip because at the largest scales these surfaces are flat. In contrast, for natural, self‐similar surfaces the volume of mismatch increases with slip, because slip isolates larger and larger asperities from their original positions in the opposite surface. Natural and experimental faults evolve differently because of the difference in scaling of their respective surface roughnesses.
The 1960 Chilean earthquakes
Authors: Cifuentes Inès L.
Published: February 2008

An unusual foreshock sequence that began with an earthquake of moment 2.0 × 1021 N m and rupture length of about 150 km preceded the great 1960 Chilean earthquake. Earthquake relocations of the sequence show a progression of seismic activity toward the initiation of the main shock. The rupture length of the great 1960 Chilean earthquake is estimated from the distribution of aftershocks and crustal deformation to be 920 ± 100 km. The source mechanisms of a foreshock and two aftershocks are determined and used to constrain the source mechanism of the main shock. The first event in the sequence ruptured the segment of the Peru‐Chile Trench between 37.03°S and 38.74°S, whereas the great 1960 Chilean main shock ruptured the adjoining segment to the south. The great 1960 Chilean main shock initiated within the rupture area of the first event and terminated near the intersection of the Chile Ridge with the Peru‐Chile Trench (46.5°S).
Phase equilibria constraints on liquid lines of descent generated by paired assimilation and fractional crystallization: Trace elements and Sr and Nd isotopes
Authors: Nielsen Roger L.
Published: February 2008

Utilizing a phase equilibria based trace element isotope model for low pressure igneous differentiation, it is now possible to update the methods presently being used to evaluate the effect of assimilation and fractional crystallization (AFC) processes. The results of the calculations presented here indicate that bulk partition coefficients (D) for Sr and Nd are strongly dependent on the chemistry of the assimilant and the rate of mixing. Virtually all silicic assimilants will increase bulk D for network‐modifying cations because of the positive correlation of most trace element partition coefficients with reciprocal temperature, and the amount of network forming components in the melt. In addition to the effect of temperature and melt composition, the addition of a magma of different composition from the magma chamber will generally cause a change in the fractionating mineral proportions. For example, a peraluminous assimilant will increase the proportion of plagioclase, and a peralkaline assimilant will increase the proportion of augite. This will in turn have an effect on bulk D. For most multiply saturated mafic to intermediate systems differentiating at low pressure, bulk DSr is >1, and bulk DNd is <1. Thus the effect of a peraluminous assimilant is to drive bulk DSr away from 1 and a peralkaline assimilant drives bulk DNd toward 1. Therefore a peralkaline assimilant will generally drive the liquid path away from a simple mixing curve (or D = 1 AFC curve) and a peralkaline assimilant will drive the liquid path toward a simple mixing curve. Compared to the results derived in this study, existing models using fixed bulk Ds calculated from closed system conditions will generate results that underestimate the assimilation rate, 87/86Sr and bulk Sr content for a peraluminous assimilant, and over‐estimate the assimilation rate, 143/144Nd and Nd content of the assimilant for a peralkaline assimilant.
Platinum group elements in a 3.5 Ga nickel‐iron occurrence: Possible evidence of a deep mantle origin
Authors: Tredoux Marian , de Wit Maarten J. , Hart Rodger J. , Armstrong Richard A. , Lindsay Nicholas M. , Sellschop Jacques P. F.
Published: February 2008

The Bon Accord (BA) Ni‐Fe deposit occurs in chemically depleted ultramafic rocks of the circa 3.5 Ga Jamestown ophiolite complex in the Barberton greenstone belt of the Kaapvaal craton, South Africa. The host rocks of the BA body are high‐temperature ultramafic tectonites which probably represent the upper mantle residue from which the overlying magmatic rocks were separated during partial melting. BA is unusual both mineralogically and chemically. It consists of a rare Ni‐rich assemblage: Ni‐oxide (bunsenite), ‐spinels (trevorite, nichromite), and ‐silicates (e.g., liebenbergite, the Ni end‐member olivine) and their altered equivalents. NiO (average = 38%) and FeO + Fe2O3 (average = 34%) are the major chemical constituents. The Cu and S contents are very low (both <100 ppm), and no evidence for replacement of preexisting sulphides has been detected. BA is enriched in platinum group elements (PGE) and shows bimodal patterns on the standard PGE diagrams. Differences between the patterns are much greater for Os and Ir than for the other PGE. This observation is interpreted as being indicative of a high‐temperature process (>2000°C; lower mantle). High Ni/Fe and Ni/Co ratios (relative to C‐1 chondrite) suggest that BA might have been derived from siderophile‐rich material that remained in the lower mantle after inefficient core formation. A model is presented wherein such a metal‐silicate heterogeneity is fractionated and oxidized during ascent through the mantle in a thermal plume which originates in the lowermost mantle (the D″ layer). Some of its residue is finally incorporated as pods into the lithospheric mantle during formation of the Archean oceanic crust. Inclusion of such fractionated pods in the old subcontinental “keel” of the Kaapvaal craton might constitute a potential PGE source, which could have been tapped by subsequent magmatic activity.
The timing of uplift, volcanism, and rifting peripheral to the Red Sea: A case for passive rifting?
Authors: Bohannon Robert G. , Naeser Charles W. , Schmidt Dwight L. , Zimmermann Robert A.
Published: February 2008

Prior to the formation of the Red Sea the northeastern Afro/Arabian continent had low relief and was largely below sea level from the Late Cretaceous to the early Oligocene. The events leading to the formation of the Red Sea followed the sequence (1) alkaline volcanism and rifting beginning about 30–32 Ma affecting a narrow linear zone in the continent, (2) rotational block faulting and detachment faulting, well underway by 25 Ma, (3) gabbro and diorite magmatism, andesite to rhyolite volcanism, and fine‐grained nonmarine sedimentation in the rift between 20 and 25 Ma, (4) fine‐grained marine sedimentation in the rift as the early shelves started to subside in the middle Miocene, and (5) uplift of the adjacent continents (about 3 km) and subsidence of the shelves (about 4 km) between 13.8 and 5 Ma. The youth of the uplift is suggested by 44 fission track dates on apatites from rocks of the Proterozoic Arabian Shield that range in age from 13.8 to 568 Ma. The youngest of these ages, coupled with the present high relief along the Arabian escarpment and published heat flow measurements, indicate that 2.5–4 km uplift has occurred in the last 13.8 m.y. The sequence volcanism/rifting followed by uplift leads to our adoption of a passive mantle model for rift origin. Models that require uplift to create the rift are rejected, because of the late uplift. We advocate a model of lithospheric extension caused by two‐dimensional plate stress over those requiring tractional drag at the base of the lithosphere caused by vigorous flow in the asthenosphere. It is acknowledged that traction models could explain the observed data, but they imply a rigid, static lithosphere and seem to require a link between the direction of flow in the asthenosphere and plate motions. Neither requirement is necessary in the extension model. The rift starts with mechanical extension in a narrow zone of lithosphere between 25–32 Ma in our model. The thinned lithosphere is replaced by upwelling asthenosphere and by rocks from the adjacent deep continental lithosphere which flow into the rift. Ductile flow of the deep continental lithosphere is accelerated by partial melting as rocks flow upward toward the rift axis. Once partially melted, rocks formerly part of the continental lithosphere join the upwelling asthenosphere, resulting in a rapid erosion of the lithospheric mantle beneath the continent near the rift edge. The resulting density decrease explains the uplift. We think that the Red Sea began as a consequence of changing plate geometries resulting from the collision of India and Eurasia. After the collision, the segment of the Owens fracture zone north of the Carlsberg Ridge became locked, forcing the northeast corner of Afro/Arabia to rotate with the Indian plate away from the rest of Africa.
Paleomagnetism of the Upper Ordovician Juniata Formation of the central Appalachians revisited again
Authors: Miller John D. , Kent Dennis V.
Published: February 2008

Two components of magnetization were isolated in the Upper Ordovician Juniata Formation sampled in the area of the Pennsylvania salient. The thermally distributed, reversed polarity B component was most likely acquired during Alleghenian deformation, and although it is poorly grouped, it is similar to other Appalachian synfolding magnetizations. The pre‐Alleghenian age C magnetization is entirely of normal polarity and shows a difference in declinations between the mean magnetizations isolated on the northern and southern limbs of the salient of 24° ± 23°. This anomaly is consistent with the sense and magnitude of declination anomalies observed in pre‐Alleghenian magnetizations isolated in other throughgoing Appalachian red beds of Silurian, Devonian, and early Carboniferous age. The mean inclination of −44.7° suggests a paleolatitude of about 26°S for the central Appalachians in the Late Ordovician. This paleolatitude fits a trend of southward motion of North America from the Ordovician to the Early Devonian, followed by northward drift through the remainder of the Paleozoic.
Numerical models of Plinian eruption columns and pyroclastic flows
Authors: Valentine Greg A. , Wohletz Kenneth H.
Published: February 2008

Numerical simulations of physical processes governing the large‐scale dynamics of Plinian eruption columns reveal conditions contributing to column collapse and emplacement of pyroclastic flows. The simulations are based on numerical solution of the time‐dependent, two‐phase, compressible Navier‐Stokes equations for jets in a gravitational field. This modeling effort is directed toward studying the steady discharge phase of eruptions in contrast to our previous models of the initial, unsteady blast phase. Analysis of 51 eruption models covers a wide range of vent exit pressures, inertial and buoyancy driving forces, and coupling of energy and momentum between gas and pyroclasts. Consideration of three dimensionless groups (Richardson and Rouse numbers and thermogravitational parameter) facilitates this analysis and defines conditions leading to column collapse. For eruptions with similar particle size characteristics, exit pressure ratios are also very important in determining column behavior; column behavior is much more sensitive to exit pressure ratio than to the density ratio between the column and the atmosphere. Model eruption columns with exit pressures exceeding atmospheric pressure have diamond‐shaped patterns at their bases with internal dynamics that correspond closely to observations of overpressured jets in laboratory experiments. Collapsing fountains form pyroclastic flows that consist of low‐concentration fronts, relatively thick heads, vortex development along the top surfaces, and rising clouds of buoyant ash. The presence of coarse‐grained proximal deposits primarily reflects tephra size sorting within the eruption column before collapse, as opposed to that which occurs during lateral transport of the material in pyroclastic flows. The dynamics and particle behavior in the proximal zone around collapsing eruption columns is examined; the modeling indicates that flow within a few kilometers of a vent will be at its highest particle concentration relative to other parts of the flow field.
Late Neogene motion of the Pacific Plate
Authors: Harbert William , Cox Allan
Published: February 2008

Oceanic crust between 38°S and 65°S along the Pacific‐Antarctic ridge records a complete history of Neogene Pacific‐Antarctic motion. We have modeled magnetic stages corresponding to portions of isochrons 1, 2, 2A, 3, 3A, 4, and 5 from 17 high‐quality marine magnetic traverses of the ridge to determine the Neogene finite rotation poles for this plate pair. We chose to model velocities over time intervals of about 0.5 m.y. because this greatly increased the number of estimates of Pacific‐Antarctic plate velocity. Our analysis shows that a change in Pacific‐Antarctic relative motion occurred between chron 2A, 3.40 Ma, and the beginning of chron 3 time, 3.86 Ma. We used 203 estimates of Pacific‐Antarctic spreading velocity to calculate a 0–3.4 Ma and 3.86–10.3 Ma Euler pole. Combined with tracks of seamounts that record the passage of the Pacific plate over mantle plumes our data show that this change in relative motion corresponds to a change in the absolute motion of the Pacific plate with respect to the hotspot frame of reference. Using these Euler poles to calculate Pacific‐Antarctic‐Africa‐North America circuit Euler poles show that this recent change in Pacific plate absolute motion corresponds to a change from strike‐slip to transpressive motion along the coastal California Pacific‐North America plate boundary. Interestingly, this age of increased tectonism agrees in time with deformation in New Zealand and northern Japan.
Gravity anomaly across the Peruvian Andes
Authors: Fukao Yoshio , Yamamoto Akihiko , Kono Masaru
Published: February 2008

We carried out gravity surveys along four routes across the Peruvian Andes, one in northern Peru, one in central Peru, and two in southern Peru. Two of the routes extend from the coast to the flat lands of the Amazon Basin across the high Andes, while the other two routes extend only from the coast to the central high plateau or the Altiplano. One route through Nazca contains 250 data points over a length of 800 km and thus offers one of the best gravity profiles across the Andes. This profile shows quite asymmetric gravity anomalies associated with the Western Cordillera and the Eastern Cordillera, in marked contrast with the symmetric pattern of topographic profile. Using these profiles, we constructed crustal models varying layer thicknesses rather than layer densities. The crustal thickness has a maximum beneath the Western Cordillera, although the maximum thickness varies from 45 km in northern Peru to 55 km in central Peru and 65 km in southern Peru in contrast to the assumed thickness of 35 km beneath the stable Brazilian shield. Possible effects of lateral variations of crustal and mantle densities tend to reduce the above estimates of maximum thickness. The very shallow Moho beneath the Peruvian coast steeply deepens eastward in correspondence to the steep western slope of the Andes. The crust is thicker in the Western Cordillera than in the Eastern Cordillera. The Moho beneath the Eastern Cordillera tends to be relatively flat and shallows abruptly eastward across the sub‐Andes. The Wadati‐Benioff zone lies, in general, well below the model South American continental crust, leaving a mantle wedge in between. A notable exception is the Nazca profile, where the base of the model South American continental crust is in direct contact with the top of the Wadati‐Benioff zone. This provides a direct evidence for uplifting of the buoyant slab with an aseismic ridge (the Nazca ridge) at its top, which tends to attach itself to the bottom of the continental crust. We examined whether or not the load due to mountain topography is balanced with the buoyancy load due to excess mass of the crust in the Andes. The Western Cordillera is found to be approximately in isostatic equilibrium, but the Eastern Cordillera is not. This contrast in mechanical state and the difference in recent geology suggest that the Western Cordillera and the Eastern Cordillera have not uplifted through entirely the same tectonic process.
Global Positioning System Network analysis with phase ambiguity resolution applied to crustal deformation studies in California
Authors: Dong Da‐Nan , Bock Yehuda
Published: February 2008

A 5‐year campaign is in progress in central and southern California to monitor tectonic motions using observations of the Global Positioning System (GPS) satellites. Evidence from geological and space‐geodetic data indicates that significant horizontal deformations of order 10 mm/yr are occurring west of the San Andreas fault along the California margin. We demonstrate, using data from initial epoch measurements in January 1987, that the goal of measuring horizontal deformations with an accuracy of 5 mm/yr over the duration of the campaign should be readily achievable. The GPS network was designed to span a wide range of baseline lengths within California and across North America to aid in phase ambiguity resolution, an important factor in achieving highest‐accuracy baseline determinations. We present an efficient algorithm for multisession network analysis of GPS data with simultaneous orbit determination and ambiguity resolution. Using this algorithm with the California data, we demonstrate the improvement obtained in station position and satellite orbit estimation when the phase ambiguities are resolved. We demonstrate several millimeters +1 part in 108short‐term repeatability over five single‐session (daily) analyses for horizontal baseline components and length and several tens of millimeters for the vertical. Comparison with the independently determined very long baseline interferometry baseline between Mojave and Owens Valley suggests that the accuracy is at the same level as the repeatability. We show that simultaneous multisession analyses with ambiguity resolution improve the baseline and satellite orbit estimates compared to the single‐session analyses. With the single‐session analyses we were able to resolve all phase ambiguities on baselines within California. With the multisession analyses we were able to resolve the phase ambiguities for most of the network, over regional and continental scales.
Micromechanics of the brittle to plastic transition in Carrara marble
Authors: Fredrich Joanne T. , Evans Brian , Wong Teng‐Fong
Published: February 2008

Samples of Carrara marble were deformed at room temperature to varying strains at confining pressures spanning the range in mechanical behavior from brittle to plastic. Volumetric strain was measured during the experiments, and the stress‐induced microstructure was characterized quantitatively using optical and transmission electron microscopy. The range of confining pressure over which transitional (or semibrittle) deformation occurs is 30–300 MPa. The macroscopic initial yield stress is constant for confining pressures greater than 85 MPa, whereas the differential stress at the onset of dilatancy increases with pressure up to 300 MPa. The dilatancy coefficient decreases rapidly with increasing pressure up to 100 MPa, and then asymptotically approaches zero for pressures up to 300 MPa. The work hardening coefficient increases with pressure up to 450 MPa; the pressure sensitivity is greatest for pressures up to 100 MPa. Active deformation mechanisms include microcracking, twinning, and dislocation glide. Transmission electron microscopy observations indicate that dislocation glide occurs, at least on a local scale, in samples deformed in the semibrittle field at pressures as low as 50 MPa and applied differential stress well below the critical resolved shear stress for glide on the easiest slip system. Cracks and voids frequently nucleate at sites of stress concentration at twin boundaries, at twin terminations, and at the intersection of twin lamallae. Geometries suggestive of crack tip shielding by dislocations are also observed. Stereological measurements indicate that at constant strain in the semibrittle field, the stress‐induced crack density and anisotropy decrease with increasing pressure. Crack density and anisotropy in samples deformed to strains of 3–5% in the semibrittle field at pressures up to 120 MPa are comparable to those in the prefailure brittle sample, although an analysis of the energetics of deformation suggests that the ratio of brittle energy dissipation to total energy dissipation is at least 60% lower. We also detect a qualitative difference in the characteristic length of the cracks in the brittle and semibrittle fields. The mean dislocation density at constant differential stress increases significantly for samples deformed at pressures of 230 MPa and greater. Our results suggest that although semibrittle flow occurs over a wide range of pressure, the most marked changes in strain partitioning, and hence the style of deformation occur over a small range in pressure.