bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismicity of the Wrangell and Aleutian Wadati‐Benioff Zones and the North American Plate along the Trans‐Alaska Crustal Transect, Chugach Mountains and Copper River Basin, southern Alaska
Authors: Page Robert A. , Stephens Christopher D. , Lahr John C.
Published: February 2008

The configuration of the Aleutian and Wrangell Wadati‐Benioff zones in southern Alaska between longitudes 142°W and 148°W is determined from regionally recorded earthquakes from 1971 to 1986. The earthquakes are located with a simple velocity model calibrated with seismic refraction measurements conducted for the Trans‐Alaska Crustal Transect (TACT). West of the transect meridian (approximately 145°20′W) and beneath the Chugach Mountains, shocks in the depth range 25–60 km compose the shallow part of the northwest to north dipping Aleutian zone. This zone can be traced updip and seaward beneath Prince William Sound. East of the transect meridian and north of 61°N latitude, the weakly active, north‐northeast dipping Wrangell zone extends to a depth of at least 100 km beneath the Wrangell volcanoes and along strike for at least 150 km. The two zones appear to be continuous, at least in the depth range 20–45 km, and may define adjacent limbs of a buckle in the subducted oceanic lithosphere. North of 61°N, focal mechanisms in the shallow Aleutian zone are compatible with a uniform stress field characterized by in‐plane least compressive stress directed toward the steeply dipping, deeper part of the Aleutian zone north of Cook Inlet. Stress orientation in the Wrangell zone is unresolved because of the low level of seismicity. No focal mechanisms corresponding to slip on a gently northward dipping megathrust have been observed. Along the transect meridian, a change of focal mechanism with depth near the top of the dipping seismic zone is inferred to reflect the interplate boundary beneath the Chugach Mountains. This boundary coincides with a band of strong reflectors imaged by TACT vertical seismic profiling. The overriding plate appears to be aseismic beneath the northern Chugach Mountains, but minor shallow seismicity occurs beneath the Copper River Basin. First‐motion patterns for these shallow shocks suggest the most compressive stress within the overriding plate is oriented subhorizontally and approximately north‐south.
Tomographic inversion of local earthquake data from the Hengill‐Grensdalur Central Volcano Complex, Iceland
Authors: Toomey D. R. , Foulger G. R.
Published: February 2008

We have determined the three‐dimensional P wave velocity structure within the area of the Hengill‐Grensdalur central volcano complex, southwest Iceland, from the tomographic inversion of 2409 P wave arrival times recorded by a local earthquake experiment. The aperture of the 20‐element seismic network utilized in the inversion permitted imaging of a 5‐km‐thick crustal volume underlying a 15×14 km2 area. Within this localized volume are located the underpinnings of the active Hengill volcano and fissure swarm, the extinct Grensdalur volcano, and an active high‐temperature geothermal field. It was thus expected that the characteristic length scale of heterogeneity would be of the order of a kilometer. In order to image heterogeneous seismic velocity structure at this scale we paid particular attention to the fidelity of the assumed model parameterization, defined as the degree to which the parameterization can reproduce expected structural heterogeneity. We also discuss the trade‐off between the resolution of model parameters and image fidelity, compare results obtained from different parameterizations to illustrate this trade‐off, and present a synoptic means of assessing image resolution that utilizes the off‐diagonal information contained within the resolution matrix. The final tomographic image presented here was determined for a parameterization with fidelity that closely matches the geologic heterogeneity observed on the surface. For this parameterization, the resolution of individual parameters is generally low; however, a quantitative analysis of resolution provides an unambiguous assessment of well‐resolved volumes. Within the better resolved regions of the model the averaging volumes are 1–2 km and 2–4 km in vertical and horizontal extent, respectively. Results of tomographic inversion image three distinct bodies of anomalously high velocity, two of these extend from near the surface to a depth of about 3 km. These high‐velocity volumes are located directly beneath the surface expressions of the extinct Grensdalur volcano and the extinct Husmuli basalt shield. The third high‐velocity structure occurs in the depth range of 3–4 km but does not extend to the surface. These three high‐velocity bodies are interpreted to be solidified magmatic intrusions. Relatively low velocities underlay limited portions of the trace of the present accretionary axis and a low‐velocity body is imaged in the roots of the active Hengill volcano. The volume of lower velocities located beneath the surface expression of the Hengill volcano is interpreted to be a region of partial melt.
Extension during continental convergence, with application to the Tibetan Plateau
Authors: England Philip , Houseman Gregory
Published: February 2008

The Tibetan plateau is the product of crustal thickening caused by the collision between India and Asia and is the largest active example of extensional tectonics in a zone of continental collision. Throughout most of the Tertiary, the tectonics of the plateau were dominated by north‐south shortening, a significant proportion of which took place on east‐west striking thrust faults. For the last 5 m.y. or so the plateau has been thinning by the mechanism of extension on north‐south trending normal faults. Previous investigations of the collision have been able to account for the large‐scale features of the Tertiary deformation but have failed to explain the transition, in the late Tertiary to Quaternary strain rate field of the plateau, from north‐south compression to east‐west extension. The transition could, in principle, be effected by a reduction in the rate of convergence between India and Asia or by a uniform reduction in strength of the whole continental lithosphere of Asia. It could not, however, be effected by a reduction in strength of the elevated region alone; this produces increased compressional strain rates in the weakened zone. An alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere. The lower part of the lithosphere consists of a thermal boundary layer which, when thickened by horizontal shortening, is colder and denser than its surroundings. Convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere. The convective instability would happen in a time brief compared with the collision time scale (∼50 m.y. in the case of India and Asia) but would only occur after there had already been substantial thickening of the lithosphere. Numerical experiments show that for a range of lithospheric parameters, the increase in surface height (as much as 2 km) and of potential energy (5 to 10 × 1012 N m−1) resulting from convective instability of the lower lithosphere are sufficient for east‐west extension to replace north‐south compression as the dominant feature of the stress field of the Tibetan plateau.
Dilatancy during rock deformation at high temperatures and pressures
Authors: Fischer George J. , Paterson Mervyn S.
Published: February 2008

The dilatancy during deformation at high pressure and temperature and at various pore pressures has been measured in Carrara marble, Solnhofen limestone, and Gosford sandstone. The method consists of monitoring the volume of pore fluid (argon) that flows into or out of the specimen at constant pore pressure. The confining pressure was 300 MPa, temperatures were up to 873 K, and pore pressure ranged from 30 to 280 MPa. Positive dilatancy was observed in some cases in all three rocks in the ductile regime up to the highest temperature, provided the difference between confining pressure and pore pressure was relatively small, thus indicating the occurrence of microcracking even under these conditions. However, the behavior was complicated by the collapse of initial porosity in the limestone and sandstone and by microcracking due to intergranular thermal stresses upon heating, especially in the marble. The dilatancy observations have important implications for kinetics of fluid‐rock interactions under geological conditions.
Appalachian Stress Study: 3. Regional scale stress variations and their relation to structure and contemporary tectonics
Authors: Evans Keith F.
Published: February 2008

Using commercial hydrofracture data to supplement published stress data, we map regional variations in least horizontal stress magnitude (Sh) and orientation within the Paleozoic sediments of the Appalachian Plateau. Systematic trends revealed by the data are examined for correlations with structure (the study area includes a prominent salt detachment) and tectonic history. The dominant stress component present at all depths (including basement) throughout the region is ENE oriented compression and is most likely of tectonic origin. No deviations from this rule are found that are statistically significant. Near‐surface (<600 m) stresses are consistently greater than the overburden with “strike‐slip” regime conditions prevailing below. However, where the Salina salt beds are present, “thrust” regime conditions extend deeper, reaching the salt in most places where data are available (limited to where the salt is shallower than 1.5 km). At the salt, a discontinuity in the trend of Sh with depth marks the abrupt transition to the underlying strike‐slip regime characterized by least stress ratios (i.e., Rh = Sh/Sv) in the range 0.5–0.7 and extending to basement. The comparatively high stress found above the salt may reflect a difference between the strain histories of the overlying and underlying sections facilitated by mechanical decoupling by the salt and perhaps includes a component of stress remnant from NW oriented Alleghanian shortening of the upper section. The lower (subsalt) stress regime extends beyond the eastern salt margin and displays remarkable uniformity over distances of hundreds of kilometers. This is particularly evident across Ohio, where an Rh value of 0.6±0.05 appears valid down to basement. North of Ohio, values increase markedly, whereas to the south of the salt pinch‐out, in West Virginia and eastern Kentucky, values as low as 0.35 are found in Devonian shales at 1–1.5 km depth, although values deeper in the section may be higher. The northward increasing trend can be reasonably well modeled as the effect of persisting lithospheric flexure due to the Pleistocene ice sheet. However, flexure was much greater in the past, and as Devonian shales in south Ohio/West Virginia are currently only marginally stable, a modest increase over the present flexure would be expected to cause shear failure. This suggests that the very low stress in the shales of south Ohio/West Virginia is of different origin and more recent than the Pleistocene.
The effects of pressure and porosity on the micromechanics of the brittle‐ductile transition in quartzite
Authors: Hirth Greg , Tullis Jan
Published: February 2008

Room temperature experiments on a quartzite with 7% porosity show that with increasing pressure there is a transition from brittle faulting to ductile cataclastic flow at about 600 MPa. In contrast, experiments on nonporous quartzite show only faulting up to 1000 MPa. The transition to cataclastic flow in the porous quartzite is due to the changing influence of the pores. Microstructural observations and elastic theory suggest that the transition occurs at the pressure where the differential stress producing a compressive stress concentration sufficient to initiate pore collapse becomes less than that producing a tensile stress concentration sufficient to nucleate axial microcracks at the tops and bottoms of pores. Cataclastic flow of the porous quartzite is only transient. Deformation remains distributed and accompanied by work hardening only as long as pore collapse continues (to ∼16% strain); at higher strains, microcracking produces a net dilatancy, which causes strain weakening that leads to faulting (at ∼26% strain). At a confining pressure of 750 MPa the faults are stable, whereas at 1000 MPa they are unstable. The results indicate that porous rocks may undergo limited ductile deformation at shallow levels in the crust, but higher strains will cause deformation to localize.
Occurrence of small changes in electrical conduction of olivine arising from high‐temperature creep
Authors: Hirsch Lee M.
Published: February 2008

New measurements of the electrical conductance of single crystals of olivine during creep in the temperature range 1325–1725 K, under controlled oxygen fugacity and under uniaxial stresses of 60–180 MPa, are discussed. These measurements indicate a small but definite increase in the conductivity of olivine as a result of creep. This increase is weakly correlated with strain and stress. The change in conductivity of olivine that results from deformation is not diminished by annealing at high temperature. Similar measurements on corundum crystals lend support to the measured increase in olivine conductivity. These results suggest that dislocations do not directly act as channels for enhanced conduction in olivine; however, they may indirectly influence conduction through their interaction with other defects. Possible mechanisms to account for this influence include interactions between dislocations and point defect complexes and destruction of the preexisting cellular arrangement of dislocations with redistribution of impurities. The present study suggests that deformation is not a viable mechanism in producing high‐conductivity layers in the upper mantle.
Paleomagnetism of the Dewey Lake Formation (Late Permian), northwest Texas: end of the Kiaman superchron in North America
Authors: Molina‐Garza Roberto S. , Geissman John W. , Van der Voo Rob
Published: February 2008

The remanent magnetization of red beds of the Late Permian Dewey Lake Formation from northwest Texas was investigated using thermal, chemical, and alternating field demagnetization. After removal of a small present‐day viscous overprint, the Dewey Lake shows univectorial decay of the remanence revealing a dual‐polarity characteristic direction of D = 152.4°, I = −11.1°, k = 39.5, α95 = 5.9°, N = 16 sites. Three polarity transitions were observed defining at least four polarity zones. The observed polarity zones are roughly layer parallel and may correlate with the Illawarra magnetozone at the end of the Kiaman. If this is so, K‐Ar age determinations of an interbedded ash layer (Kolker and Fracasso, 1985) suggest that the Kiaman Superchron ended before 251±4 Ma. The paleopole, located at 51.4°N–126.2°E, confirms the reliability of previously reported Late Permian poles from southwest United States, which fall significantly off the Late Permian segment of the paleomagnetic Euler pole (PEP) track of Gordon et al. (1984). This implies that a more segmented set of small circles is required to model the North American apparent polar wander path, diminishing the usefulness of the PEP method.
Cosmic ray production rates of 10Be and 26Al in quartz from glacially polished rocks
Authors: Nishiizumi K. , Winterer E. L. , Kohl C. P. , Klein J. , Middleton R. , Lal D. , Arnold J. R.
Published: February 2008

We have studied the concentrations of 10Be and 26Al in quartz crystals extracted from glacially polished granitic surfaces from the Sierra Nevada range. These surfaces were identified with the glacial advance during the Tioga period ∼11,000 years ago. Our measurements yield the most accurate estimates to date for the absolute production rates of these nuclides in SiO2 due to cosmic ray nucleons and muons for geomagnetic latitudes 43.8°–44.6°N and altitudes 2.1–3.6 km. The estimates are relatively free from uncertainties in snow cover since we studied a suite of rock surfaces inclined 0°–75° with respect to the horizontal. The principal uncertainty arises due to the lack of a precise date for the glacial retreat event, about ±10%. The 26Al/10Be ratio at production (6.0) is determined more accurately since the exposure age of the samples is 2 orders of magnitude smaller than the mean lives of the two nuclides. Production rates of 10Be and 26Al in quartz at other latitudes and altitudes in the troposphere can be determined from the present measurements by scaling, using the known altitude and latitude dependence of cosmic ray fluxes of nucleons and negative muons. Knowledge of the production rates of these nuclides is a prerequisite for their application in erosion and geomorphological studies.
The 40Ar/39Ar thermochronometry for slowly cooled samples having a distribution of diffusion domain sizes
Authors: Lovera Oscar M. , Richter Frank M. , Harrison T. Mark
Published: February 2008

Many 40Ar/39Ar age spectra for alkali feldspars are significantly different from the model age spectra calculated for slowly cooled samples composed of diffusion domains of a single size, and the Arrhenius plots for these samples show departures from linearity that are inconsistent with diffusion from domains of equal size. The most plausible explanation for these discrepancies is the existence of a distribution of diffusion domain sizes. We have extended the single‐diffusion‐domain closure model of Dodson so that it applies to minerals with a distribution of domain sizes and have used it to explain many commonly observed features of 40Ar/39Ar age spectra and Arrhenius plots for 39Ar loss during step heating. For samples with a distribution of diffusion domain sizes, the form of the 39Ar Arrhenius curve is a function of the heating schedule (i.e., the temperature and duration of the steps used), and thus different heating schedules will result in different curves for the same sample. This effect can be used to confirm the existence of a distribution of diffusion domain sizes and to optimize the information contained in the Arrhenius plot. The multiple diffusion domain size model is used to reinterpret the age spectra, Arrhenius plots, and cooling history of three feldspars from the Chain of Ponds pluton, northwestern Maine, earlier interpreted assuming a single domain size. Interpreting the 40Ar/39Ar and 39Ar released during step heating in terms of a single domain size gives rise to a large discrepancy between the cooling rate determined from the age and closure temperature of the three samples compared to the cooling rate required to explain the shape of the individual age spectra. The single domain size model fails also to account for the observed departures from linearity of the Arrhenius plots. We show that a particular domain size distribution in each sample can explain in detail both the shape of the age spectra and the Arrhenius plots, and results in the three samples defining a common cooling history. There is thus good evidence for the three alkali feldspar samples studied here that the thermally activated diffusion measured by 39Ar release during step heating in the laboratory is also the mechanism responsible for argon loss or retention in the natural setting.
Structure of the Alula‐Fartak Fracture Zone, Gulf of Aden
Authors: Tamsett Duncan , Searle Roger
Published: February 2008

The Alula‐Fartak transform fault is associated with a broad, steep‐sided valley running NNE‐SSW across the mouth of the Gulf of Aden. A discussion of the tectonics of the fracture zone is based principally on GLORIA side scan sonar and gravity data. The transform valley is 180 km long and 25 km across, has 3.5 km of relief, and is bounded by transverse ridges. The northern two‐thirds is highly asymmetrical in cross section. The eastern wall dips 38° and appears to be a single scarp exposing a section of oceanic crust 3100 m thick. The western wall is much less steep and accommodates several normal faults oriented at about 15° to its general trend. A very fine, long sonar target running along part of the valley delineates the main active strike‐slip fault. The sonar data also reveal many fine faultlike targets between the main walls, running approximately at right angles to the trend of the valley, which may be anti‐Riedel shears. A 50‐km‐long, 7.5‐km‐wide, and 1.5‐km‐high median ridge runs along the southern third of the transform valley. Gravity data show that the valley is underlain by a significant mass deficiency thought to be a large serpentinite intrusion. The median ridge is suggested to be the surface manifestation of a large serpentinite diapir. The ridges flanking the transform valley are above a level compatible with isostatic equilibrium and are interpreted as flexural features raised in response to the upward loads imposed on the lithosphere by the fracture zone valley and its underlying mass deficiency. Very shallow crust is located close to the ridge‐transform intersections. This is considered to result from static flexure induced by the negative load of the transform valley superimposed on dynamic compensation of the spreading center valley. Evidence of a c. 3.5 m.y. B.P. change in spreading direction suggests the transform has experienced a recent component of opening, accounting for several aspects of its structure.
Convection‐driven tectonics on Venus
Authors: Phillips Roger J.
Published: February 2008

An analysis is presented for convective stress coupling to an elastic lithosphere as it applies to tectonic deformation on Venus. Theoretical solutions are introduced for the response of a mathematically thick elastic plate overlying a Newtonian viscous medium with an exponential depth dependence of viscosity. A Green's function solution is obtained for the viscous flow driven by a harmonic density distribution at a specified depth. It is shown that during the transient phase of the solution, the field quantities (stress, velocity, displacement) in both media depend upon the rheology of both media, whereas in steady state the solutions become uncoupled. Due to this uncoupling, stresses obtained from a purely viscous solution can be applied as boundary conditions at the base of an isolated elastic plate. I show for such a plate that (1) shear stress imparted at the base of the lower boundary can induce large in‐plane stretching mode forces and (2) these forces can be estimated directly from free‐air gravity anomalies without detailed knowledge of the density distribution in the underlying flow. Free‐air gravity anomalies for Venus imply in‐plane forces of about 2–3×1012 N/m. In‐plane forces of this magnitude are capable of extensive disruption of the Venusian lithosphere. To demonstrate this, an elastic‐plastic analysis is carried out for the deformation of a model Venus lithosphere; the calculation is based on a lithospheric strength envelope and uses gravity and topography as a guide to the magnitude of stresses imparted by convective flow. The results predict that dynamic uplift of Venusian topography must be accompanied by extensive brittle failure and viscous flow in the lithosphere. Geoid anomalies and geoid‐to‐topography ratios for Venusian topographic features are large compared to values for thermal swells in Earth's ocean basins; this suggests that dynamic support of topography must be an important process on Venus. Further, a low‐viscosity zone is probably not present beneath the Venusian lithosphere. A comparative analysis suggests that tectonic deformation of the lithosphere associated with dynamically supported topography on Earth is unimportant.
Introduction to Special Section on Polar Processes on Mars
Authors: Haberle Robert M.
Published: February 2008

The 13 papers contained in this issue are the product of a workshop on Mars Polar Processes held in Sunnyvale, California, in May 1988. The workshop was sponsored by NASA's Mars Data Analysis program in an effort to encourage continued interdisciplinary investigations into a variety of climate issues raised during the program's MECA project (Mars: Evolution of its Climate and Atmosphere). Even though the project ended before the workshop was held, funds were set aside for additional workshops because of MECA's resounding success. The Polar Processes workshop was one of these workshops, and I am grateful to Joseph Boyce of NASA headquarters for his support and to Bruce Jakosky, Dave Paige, and Steve Squyres for their help in organizing the workshop.
Sublimation and transport of water from the north residual polar cap on Mars
Authors: Haberle Robert M. , Jakosky Bruce M.
Published: February 2008

Several features of the Viking Mars atmospheric water detection observations have suggested that the residual north polar cap on Mars is a major source of atmospheric water during northern summer and that the water it supplies to the atmosphere is transported equatorward to low northern latitudes by summer's end. We have explored the role of the north residual cap in the current Martian water cycle by constructing models to assess the ability of the cap to supply water to the atmosphere and the ability of the atmosphere to transport it out of the polar regions. From our sublimation model we conclude that the residual cap can sublime the 7×1014 g of water that appears in the atmosphere after Ls = 80° provided surface winds are high (∼10 m/s), the cap emissivity is low (∼0.9), or the exposed soil in polar regions contains ice that is accessible to the atmosphere. In our view these requirements seem unlikely, and we are drawn to the conclusion that other sources of water exist. This conclusion is further supported by the results of two‐dimensional transport simulations which show that most of the water sublimed from the cap remains at high latitudes. The polar circulation simply lacks the intensity and scale to move water very far from the cap. A weak circulation places additional constraints on cap sublimation rates since abundant clouds and significant ice deposits would form just equatorward of the cap edge, according to the model, if the cap is allowed to sublime 7×1014 g of water. Such predictions are inconsistent with available data. We suggest that the nature of the additional source is water desorbing from the nonpolar regolith.
Simulation of the metamorphic and deformational history of the metamorphic sole of the Oman Ophiolite
Authors: Hacker Bradley R.
Published: February 2008

Emplacement of ophiolites onto less dense continental crust is an enigma. The rates of emplacement, the mechanisms of emplacement, and the stress and temperature histories of the rocks are all incompletely understood. The Oman ophiolite is well exposed and has been studied extensively by field geologists. An integrated thermal and kinematic model of the temperature, stress, rock type, and displacement fields during the early stages of the emplacement of the Oman ophiolite is developed here to explain field relations and provide insight into the development of the metamorphic rocks at the base of the ophiolite. The thermal evolution was calculated by a finite difference algorithm for heat conduction, considering heats of metamorphic reactions, deformational heating, and heat advection by rock. The stress and displacement fields were calculated by an analytical model using a velocity boundary condition, power law constitutive relations, and a brittle frictional sliding relationship. In simulations involving young, hot subducted lithosphere, a wider portion of the subduction zone deforms by power law creep, and deformation occurs at lower stresses and slower strain rates, minimizing the deformational heating compared to colder, older subducted lithosphere. Dehydration occurs at an accelerated rate in younger, hotter lithosphere, removing heat from the subducted plate at an earlier stage and causing the zone of brittle deformation to propagate downward more rapidly. Power law creep of subducted basaltic rocks is limited to temperatures greater than 500°C and occurs at differential stresses of the order of 100 MPa. The simulations predict that metamorphic field gradients and the spatial distribution of rock types in the metamorphic sole of Tethyan ophiolites might be used to infer the emplacement direction of the ophiolite.
Active submarine volcanism on the society hotspot swell (west Pacific): A geochemical study
Authors: Devey C. W. , Albarede F. , Cheminée J.‐L. , Michard A. , Mühe R. , Stoffers P.
Published: February 2008

The present work deals with the petrography and geochemistry of lavas dredged from five active submarine volcanoes (named Mehetia, Moua Pihaa, Rocard, Teahitia, and Cyana) from the southeast end of the Society Islands hotspot trace. Most samples are basic and alkaline, ranging from 16 to 5 wt % MgO, with about 5% normative nepheline. Fractionation modelling based on major and minor compatible element variations suggests that olivine and minor clinopyroxene were the major fractionating phases and implies a maximum range of fractionation of 30–35%. Rocard and Cyana have yielded more evolved, trachy‐phonolitic, glassy samples. These evolved samples are thought to be derived by removal of 70% cumulate from the basalts. Both basaltic and phonolitic samples are incompatible‐element enriched, with La/YbN ≈ 15 in most of the basalts. The trachy‐phonolite patterns show middle rare earth element (REE) depletion and negative Eu anomalies. The Moua Pihaa basalts have flatter patterns than the other basalts (La/YbN = 7.5–12.4). All samples, with the exception of a sample from Moua Pihaa which has elevated 206Pb/204Pb, fall on linear Sr‐Nd‐Pb isotopic arrays, suggesting two end‐member mixing. The most depleted end‐member is shown to be a pristine ocean island basalt magma with no detectable contribution from a depleted, mid‐ocean ridge basalt (MORB) upper mantle. The flatter REE patterns and higher 206Pb/204Pb of the Moua Pihaa sample are taken to indicate a more depleted, U‐enriched (high μ) component in its source. This component may be altered oceanic crust. The Sr isotopic variations in the samples excluding Moua Pihaa correlate positively with Rb/Nb, Pb/Ce, and SiO2 variations, indicating a component of mantle enriched by injection of material from a subducted oceanic slab. Correlation of 207Pb/204Pb with 87Sr/86Sr suggests that the subducted material is geochemically old. Mapping the geochemical variations shows that the contribution to the lavas from the subduction component is greater over the north of the hotspot than in the south. The absence of a MORB component in the Society magmatism, the small volumes of the Polynesian hotspot volcanoes, and the lack of more intense volcanic activity near the center of the Pacific Supers well, all lead us to conclude that the latter is unlikely to be caused by a large convective plume. The Superswell is more probably located above a region in the asthenospheric mantle which, due to its higher content of recycled continental debris, is anomalously hot.
On the building and classification of mountains
Authors: Wilson J. Tuzo
Published: February 2008

Physicists believe that known laws should suffice to explain the Earth's behavior, but the complexities of geology have defied simple explanation. Today three developments are helping to solve the problem. First, summary interpretations such as the centennial project of the Geological Society of America are being compiled of the upper 1% (60 km) of the Earth's radius. Second, new methods show the behavior at great depths in the Earth. Third, combining these studies suggests which physical laws apply at various depths. In the mantle the pattern of convection, by viscous fluid flow is becoming‐clear; Upwelling plumes start rifts which fragment the lithosphere. Subduction carries plates down. In the strong lithosphere the Coulomb‐Navier, laws of brittle failure apply and confine fractures to three types corresponding to plate boundaries. Other laws determine where island arcs form. The idea that some large areas of the surface act independently of the surrounding areas is becoming recognized and simplifies analysis. One large area which behaves independently lies in the southwestern United States, these developments and the observation that most plate boundaries are elevated provide a means of applying laws of physics to classifying ranges, including folded mountains, island arcs, rifts, and ridges, into three classes and many subclasses, illustrated by examples.
Isotopic structure and tectonics of the central Transantarctic mountains
Authors: Borg Scott G. , Depaolo Donald J. , Smith Brian M.
Published: February 2008

Regional patterns of Nd, Sr, and O isotopic ratios of ∼500 Ma granitic rocks are used to identify the ages and areal extents of three crustal provinces in the central Transantarctic Mountains. One of the provinces is the edge of the East Antarctic Craton, which isotopic analyses show is composed of Archean rocks thrust over Proterozoic rocks. The other two provinces compose the Beardmore microcontinent, which we deduce was allochthonous to East Antarctica and was emplaced in late Precambrian or early Paleozoic time. Evidence for a former ocean basin between the Beardmore microcontinent and East Antarctica is provided by basalt and gabbro of mid‐ocean ridge character, dated by Sm‐Nd at ∼760 Ma, associated with marine sediments now located at the suture. The granitic rocks formed over a westward‐dipping subduction zone that was active at ∼500 Ma. The East Antarctic Craton is exposed in the Miller Range, which is a tectonic composite of reworked Archean and early Proterozoic material containing ∼500 Ma peraluminous granites with model ages (TDM) of 2.0 Ga, high δ18O (+11 to +12‰) and high initial 87Sr/86Sr (0.7324 to 0.7417). East of the Marsh Glacier the granitic rocks are metaluminous to weakly peraluminous with model ages of 1.3 to 1.8 Ga, high δ18O (+9 to +13‰) and lower 87Sr/86Sr (0.7068 to 0.7191). East of the Shackleton Glacier, gabbro, tonalite, diorite, and granodiorite have low δ18O (+6 to +7‰), low initial 87Sr/86Sr (0.7045 to 0.7059) and high ϵNd (+0.4 to +1.7). These isotopic provinces correspond to differences in age and composition of the middle and lower crust at the time of formation of the granitic magmas. The boundaries of the isotopic provinces also correspond to discontinuities in provenance, lithology, structural style, and grade of metamorphism of prebatholithic metasedimentary rocks. The isotopic data indicate that the granitic magmas were formed mostly by crustal anatexis in the areas west of the Shackleton Glacier. This is typical of early Paleozoic granitic batholiths elsewhere in the world and has led to speculation that subduction was not involved in granitic magmatism at that time of earth history. However, the granitic rocks located west of the Shackleton Glacier, by virtue of their mantle‐like isotopic compositions and their association with metavolcanic rocks, appear to be subduction‐related. The tectonic history deduced for the Gondwana margin, as represented in the central Transantarctic Mountains, began with deposition of sediments on an Atlantic‐type rifted margin at ∼760 Ma. The Beardmore microcontinent was most likely accreted in association with folding of the clastic sedimentary rocks before middle Early Cambrian time (550 Ma). Carbonate sedimentation and volcanism along the eastern margin of the Beardmore microcontinent commenced in Cambrian time. Folding and metamorphism of all older units occurred in late Cambrian time followed by emplacement of granitic rocks at ∼500 Ma.
Thermal effects of compaction‐driven groundwater flow from overthrust belts
Authors: Deming David , Nunn Jeffrey A. , Evans David G.
Published: February 2008

The thermal consequences of compaction‐driven groundwater flow resulting from overthrusting are studied with a two‐dimensional numerical model. The model represents a foreland basin 5 km deep and 400 km wide and is used to estimate quantitatively the magnitude, direction, and thermal consequences of fluid expulsion. Model simulations in which the permeability structure is homogeneous lead to maximum Darcy velocities of the order of 1 cm/yr; temperature in the foreland increases by less than 5°C. A sensitivity analysis reveals that temperature in the foreland may be increased an additional 2°C by increasing porosity, speed of thrusting, or heat flow. However, if the area underneath the thrust sheet is not effectively sealed, fluid escapes upward and thermal perturbations in the foreland are negligible. Models with basal and midlevel aquifers produce maximum Darcy velocities of the order of 4 cm/yr, but temperature in the foreland again increases by less than 5°C. Models in which hot fluids at depth are channeled directly upward through a high‐permeability pathway can produce temperature perturbations as high as 50°C, over limited areas. Modeling results suggest that the expulsion of pore fluids from orogenic zones through the process of sediment compaction is likely to produce significant thermal perturbations in adjacent forelands only over areas that are spatially restricted, or spatially and temporally restricted.
Tectonic history and new isochron chart of the south Pacific
Authors: Mayes Catherine L. , Lawver Lawrence A. , Sandwell David T.
Published: February 2008

We have developed an internally consistent isochron chart and a tectonic history of the South Pacific using a combination of new satellite altimeter data and shipboard magnetic and bathymetric data. Highly accurate, vertical deflection profiles (1–2 μrad), derived from 22 repeat cycles of Geosat altimetry, reveal subtle lineations in the gravity field associated with the South Pacific fracture zones. These fracture zone lineations are correlated with sparse shipboard bathymetric identifications of fracture zones and thus can be used to determine paleospreading directions in uncharted areas. The high density of Geosat altimeter profiles reveals previously unknown details in paleospreading directions on all of the major plates. Magnetic anomaly identifications and magnetic lineation interpretations from published sources were combined with these fracture zone lineations to produce a tectonic fabric map. The tectonic fabric was then used to derive new poles of rotation for 12 selected times in the Late Cretaceous and Cenozoic. From our reconstructions, we estimated the former location of the spreading centers in order to derive a new set of isochrons (interpreted unes of equal age on the ocean floor). We believe that the use of new Geosat altimeter data in combination with a multi‐plate reconstruction has led to an improvement in our understanding of South Pacific tectonics.
Transient fluid flow through the toe of the Barbados Accretionary Complex: Constraints from ocean Drilling Program Leg 110 heat row studies and simple models
Authors: Fisher A. T. , Hounslow M. W.
Published: February 2008

Thirty‐four sediment and mud line temperatures were collected from six drill holes on Ocean Drilling Program (ODP) leg 110 near the toe of the Barbados accretionary complex. When combined with thermal conductivity measurements from sediment cores and results from earlier surveys, these data delineate the complicated thermal structure on the edge of this convergent margin. Heat flow values at the seafloor of 92–192 mW m−2 are 80–300% higher than those predicted by standard heat flow versus age models for oceanic crust but are compatible with earlier seafloor measurements made in this area at the same latitude. Heat flow tends to decrease downhole at four sites, suggesting the presence of heat sources within the sediments. These results are consistent with the flow of warm fluids through the complex along high permeability conduits, including thrust faults, the major décollement zone, and sandy intervals. Simple calculations suggest that this fluid flow is transient, occurring on time scales of tens to tens of thousands of years. Fluid flow velocities along the décollement zone, estimated with a simple thermal model, are about 10−7 m s−1, 100 times faster than predicted by numerical, steady state analyses. The estimated maximum sediment permeability within the décollement zone, which was based on the above fluid flow velocity, is about 10−12 m2, also 100 times higher than that calculated numerically. High heat flow in the vicinity of 15°20′N and not elsewhere along the deformation front suggests that the leg 110 drill sites may be situated over a prism‐water discharge zone, with dewatering more active here than elsewhere along the accretionary complex.
Mud volcano field seaward of the Barbados Accretionary Complex: A deep‐towed side scan sonar survey
Authors: Henry Pierre , Le Pichon Xavier , Lallemant Siegfried , Foucher Jean‐Paul , Westbrook Graham , Hobart Michael
Published: February 2008

A 30 km2 diapiric field has been identified near 13°50′N up to 12 km seaward of the deformation front of the Barbados accretionary complex. Using a deep‐towed side scan sonar with a 3.5‐kHz profiler, we identified 31 different diapiric structures. Based on seismic stratigraphy, we show that this field has been active for 200,000 years and that it is a transient feature triggered by the seaward propagation of the high pore fluid pressure associated with the décollement beneath the accretionary complex. Both basement and décollement in this area are anomalously shallow due to the presence of a N110° basement ridge. The height of the diapirs above seafloor does not exceed 40–50 m and can be related to the pressure head of the mud below the décollement. Two types of structures are identified: mud volcanoes and massively emplaced diapirs and ridges on the one hand, enigmatic circular very flat mud pies and conical mounds on the other hand. The second type of structures has steeper slopes and appears to be associated with very active venting, as confirmed by a submersible exploration reported in a companion paper (Le Pichon et al., this issue (b)). The venting results in the formation of a stiff carbonate crust and of large subsiding basins around the mud pies. Continuous active fluid expulsion through these structures indicates that an efficient piping system still connects them to the zone of anomalously high pore pressure below the protodécollement.
Mud volcano field seaward of the Barbados Accretionary Complex: A submersible survey
Authors: Le Pichon Xavier , Foucher Jean‐Paul , Boulègue Jacques , Henry Pierre , Lallemant Siegfried , Benedetti Marc , Avedik Felix , Mariotti André
Published: February 2008

We describe a 0.55 km2semicircular mud pie and three adjacent diapiric mounds explored using the submersibleNautilenear 13°50′N, 12 km seaward of the Barbados accretionary complex deformation front. This diapiric field had been previously explored with an acoustically navigated deep‐towed side scan sonar, thus providing an accurate base map. The mud pie is situated at a depth of 4938 m. In addition to visual observations, water, rock, sediment, and biological sampling, we measured temperature gradients and made geochemical analyses of the samples. Fluid venting associated with chemosynthetic animal communities is widespread on all four structures but is maximal in the central third of the mud pie. The chemosynthetic life distribution on the mud pie displays a concentric zonation which we relate to an increase in venting activity toward the center, as evidenced by thermal gradients. Recent mud flows are present in the centralmost part of the mud pie. The simplest interpretation of the structure is that it is a mud lake covered by a mechanically resistant carbonate crust. We determine that approximately 106m3of fluid are advected upward through the crust of the mud lake every year. This large amount of fluid implies large‐scale lateral transport.
Deformation bands from the toe of the Nankai Accretionary Prism
Authors: Karig D. E. , Lundberg Neil
Published: February 2008

Deformation in the toe of the Nankai accretionary prism of SW Japan occurs not only by displacement on seismically defined thrust faults but also by diffuse thickening, shortening, and porosity loss. Sediments recovered from this setting are only very slightly tilted, and the only structures noted were subtle deformation bands, which are semiplanar zones of brittle‐ductile shear associated with subhorizontal shortening. Most bands dip between 25° and 60°, both in and opposite to the direction of bedding dip, and show no obvious trends with depth or distance from the trench. There is no change of mineralogy within deformation bands and only a very slight reduction of grain size, but porosity is reduced by 5%. Bands with dips between 25° and 40° are often marked by fractures, the surfaces of which show reverse slip with slickenlines and downdip steps. These bands crosscut and are younger than the steeper bands. All deformation bands consist of subbands that dip 10°–15° less steeply than the main band and within which the clay fabric is rotated to near vertical. The subbands are probably Riedel shears, oriented at angles to 01 in the bands appropriate to a brittle Coulomb response. A few deformation bands show subsidary sets of many short bands, at angles of ∼85° to the main bands. These may be R′ shears. The steeper deformation bands, especially the conjugate sets that dip 50°–60°, are interpreted to have been passively rotated toward the maximum finite extension direction (ε1) by ductile flow. The strain necessary to rotate these bands would require an ellipticity, R = (1+ε1)/(1+ε3), of ∼2.5. Based on seismic profiles and porosity data, R is only 1.5, demonstrating that if ductile flow were responsible for band rotation, such flow is not representative of the bulk of the toe. Seismic profiles across the proto thrust zone show 45° dipping “seismic discontinuities,” which appear to be panels of slightly steeper bedding dips (10°–15°) and perhaps greater deformation. Ductile flow is to be expected in the compactively deforming sediments of the Nankai toe, but the coexistence with brittle to brittle‐ductile shears is not easily explained with experimental soil mechanics.
The interaction of fluid processes and progressive deformation during shallow level accretion: Examples From the Shimanto Belt of SW Japan
Authors: Agar Susan M.
Published: February 2008

The interaction of deformation and fluid flow is recorded in the mesostmcture and microstructure of the Shimanto Belt in the eastern Hata Peninsula of SW Japan. Large areas of disrupted strata, including the Okitsu mélange have been generated during the progressive deformation associated with shallow level accretion. The resulting assemblage of structures and their crosscutting relations reflect the complex interplay between pore fluid pressures, lithification states, and strain rates. Evidence for the relative timing of structures, diagenesis, and modes of fluid migration has been used to constrain a generalized model for the nature of fluid outflow during the accretion and burial of trenchslope lithologies. Early outflow was diffuse, but the flow direction was controlled mainly by a regionally pervasive structural anisotropy and the predominance of independent paniculate flow. As the sediments became more lithified, fluid flow was focused along faults in which transient microfractures controlled the permeability. Fractures released pore fluid pressure promoting further deformation by diffusive mass transfer. This in turn resulted in the buildup of pore fluid pressures and tectonic stresses and a recurrence of fracturing. In some fault zones and in the Okitsu mélange several such deformation mechanism cycles can be identified. The development of a complex array of faults during the late stages of accretionary complex evolution resulted in fluid flow paths becoming multidirectional.