bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of mantle heat source distribution on supercontinent stability
Authors: Lowman Julian P. , Jarvis Gary T.
Published: September 2002

A two‐dimensional Cartesian geometry numerical model is used to study the influence of internal heating on the ability of mantle convection to rift and disperse an assembled supercontinent. Our wide aspect ratio models incorporate migrating plate boundaries and finite thickness rigid mobile oceanic and continental plates (∼12,000 km across). We examine models characterized by both whole mantle and upper mantle convection parameters. Specifying different degrees of internal heating in the mantle, we compare the patterns of convection that develop below stationary continental plates that form as a result of the aggregation of smaller mobile continents. Following continental collisions, we find that a reorganization of mantle flow may result in the development of deviatoric stresses at the base of the continental lithosphere, which are capable of initiating continental breakup. Our results suggest that continental basal stresses are largely controlled by the wavelength of the subcontinental convection pattern and that the inclusion of internal heating in the mantle generally increases this wavelength. For models with 80% internal heating (in which active upwellings are intrinsically absent), basal stresses arise primarily from subduction at the continental margins and result in a geologically reasonable duration for our model supercontinent assemblages (∼200 Myr). For models with 40% or less internal heating, continental breakup is controlled by mantle upwellings and our supercontinents remain intact much longer (∼600–700 Myr).
Statistical self‐similarity of magmatism and volcanism
Authors: Pelletier Jon D.
Published: September 2002

Magmatism and volcanism exhibit spatial and temporal clustering on a wide range of scales. Using the spatial pair‐correlation function, the number of pairs of magmatic or volcanic events whose separation is between r − ½Δr and r + ½Δr per unit area, we quantify the spatial clustering of magmatism and volcanism in several data sets. Statistically self‐similar clustering characterized by power law spatial pair‐correlation functions is observed. The temporal pair‐correlation function is used to identify self‐similar temporal clustering of magmatism and volcanism in the Radiometric Age Data Bank of 11,986 dated intrusive and extrusive rocks in the North American Cordillera. The clustering of magmatism and volcanism in space and time in this data set is found to be statistically self‐similar and identical to those of distributed seismicity. The frequency‐size distributions of eruption volume and areal extent of basaltic flows are also found to be self‐similar with power laws analogous to the Gutenburg‐Richter distribution for earthquakes. In an attempt to understand the origin of statistical self‐similarity in magmatism and volcanism we present one end‐member model in which the ascent of magma through a disordered crust of variable macroscopic permeability is modeled with a cellular‐automaton model to create a distribution of magma supply regions which erupt with equal probability per unit time. The model exhibits statistical self‐similarity similar to that observed in the real data sets.
Heat transport in stagnant lid convection with temperature‐ and pressure‐dependent Newtonian or non‐Newtonian rheology
Authors: Dumoulin Caroline , Doin Marie‐Pierre , Fleitout Luce
Published: September 2002

A numerical model of two‐dimensional Rayleigh‐Bénard convection is used to study the relationship between the surface heat flow (or Nusselt number) and the viscosity at the base of the lithosphere. Newtonian or non‐Newtonian, temperature‐ and pressure‐dependent rheologies are considered. In the high Rayleigh number time‐dependent regime, calculations yield Nu ∝ RaBL1/3beff−4/3 where beff is the effective dependence of viscosity with temperature at the base of the upper thermal boundary layer and RaBL is the Rayleigh number calculated with the viscosity νBL (or the effective viscosity) at the base of the upper thermal boundary layer. The heat flow is the same for Newtonian and non‐Newtonian rheologies if the activation energy in the non‐Newtonian case is twice the activation energy in the Newtonian case. In this chaotic regime the heat transfer appears to be controlled by secondary instabilities developing in thermal boundary layers. These thermals are advected along the large‐scale flow. The above relationship is not valid at low heat flow where a stationary regime prevails and for simulations forced into steady state. In these cases the Nusselt number follows a trend Nu ∝ RaBL1/5beff−1 for a Newtonian rheology, as predicted by the boundary layer theory. We argue that the equilibrium lithospheric thickness beneath old oceans or continents is controlled by the development of thermals detaching from the thermal boundary layers. Assuming this, we can estimate the viscosity at the base of the stable oceanic lithosphere. If the contribution of secondary convection to the surface heat flux amounts to 40 to 50 mW m−2, the asthenospheric viscosity is predicted to be between 1018 and 2×l019 Pa s.
Lithospheric deformation beneath the San Gabriel Mountains in the southern California Transverse Ranges
Authors: Kohler Monica D.
Published: September 2002

High‐resolution tomographic images from Los Angeles Region Seismic Experiment (LARSE) array and southern California Seismic Network (SCSN) teleseismic data suggest that the entire lithosphere below the San Gabriel Mountains and San Andreas fault in the Transverse Ranges has thickened in a narrow, vertical sheet. P wave travel time inversions of the combined data support the presence of the well‐documented upper mantle high‐velocity anomaly that extends ∼200 km into the mantle under the northernmost Los Angeles basin and Transverse Ranges, and is associated with mantle downwelling due to oblique convergence. We find that the high‐velocity, high‐density upper mantle anomaly comprises a 60–80 km wide sheet of mantle material that lies directly below a substantial crustal root in the San Gabriel Mountains. The velocity perturbations are as large as 3% in the anomaly, corresponding to a ∼2% density increase. The tomographic images suggest that deformation in the ductile lower crust and mantle lithosphere may be partially coupled mechanically and thermally if the thickening is occurring together in response to convergence and that it may be a local compressional feature.
Divalent cation diffusion in Mg2SiO4 spinel (ringwoodite), β phase (wadsleyite), and olivine: Implications for the electrical conductivity of the mantle
Authors: Farber Daniel L. , Williams Quentin , Ryerson Frederick J.
Published: September 2002

Mg‐Fe and Mg‐Ni interdiffusion coefficients have been measured in single‐crystal olivine (α‐(Mg,Ni,Fe)2SiO4), polycrystalline β phase (β‐(Mg,Ni,Fe)2SiO4), and silicate spinel (γ‐(Mg,Ni,Fe)2SiO4) at 1473 K between 1 and 14 GPa under controlled thermodynamic conditions. In olivine, DMg‐Niα ranges from 10−17 m2 s−1 at 1 GPa to 4×10−18 m2 s−1 at 9 GPa, and DMg‐Feα ranges from 3×10−15 m2 s−1 at 1 GPa to 10−15 m2 s−1 at 4 GPa. At 9 GPa the cation diffusion rates in γ spinel are ∼3 orders of magnitude higher than those in olivine; between 9 and 14 GPa, DMg‐Niγ decreases from 8×10−15 to 9×10−16 m2 s−1, and DMg‐Feγ decreases from 2×10−13 to 5×10−14 m2 s−1. Between 10 and 14 GPa the diffusion coefficients for both Fe and Ni are similar in magnitude and pressure dependence between β phase and γ spinel. The activation volumes for Mg‐Ni diffusion are 3.2 and 6.7 cm3 mol−1 in the α and γ(β) phases, respectively, and for Mg‐Fe diffusion are 5.4 and 6.1 cm3 mol−1 in the α and γ(β) phases, respectively. Our results demonstrate that extreme differences in transport properties are expected to occur across the 400 km discontinuity. A model for electrical conductivity in the upper mantle involving two mechanisms for electrical conduction, with one mechanism being associated with a hydrogen or defect‐related conduction process and the other involving a straightforward application of the Nernst‐Einstein equation to our cationic diffusion data, reproduces geomagnetic sounding results for the upper mantle and shallow transition zone. This modeling yields an order of magnitude increase in electrical conductivity across the 400 km discontinuity, with little or no change in conductivity occurring near 520 km depth. Our modeling suggests that divalent cation diffusion is the dominant mechanism of charge transport between 400 and 670 km depth and that the high‐pressure olivine polymorphs dominate the electrical behavior of the deep upper mantle. Comparison between field‐based deep‐mantle conductivity profiles derived from magnetotelluric studies and extrapolation of our model suggests that a discontinuity in electrical transport properties exists at the base of the transition zone as well. Similarly, given the dramatic differences in chemical diffusion between different polymorphs of olivine, it is possible that other transport property‐mediated processes, such as viscous flow, could dramatically change across the 400 and 670 km discontinuities; such changes are less likely to occur at 520 km depth.
Vertical axis rotations across the Puna plateau (northwestern Argentina) from paleomagnetic analysis of Cretaceous and Cenozoic rocks
Authors: Coutand Isabelle , Chauvin Annick , Cobbold Peter Robert , Gautier Pierre , Roperch Pierrick
Published: September 2002

Between 10°S and 30°S, the central Andes are marked by both a major topographic anomaly, the Altiplano‐Puna plateau, and a westward concave geometry whose origin remains controversial. The arcuate shape is accompanied by a remarkable pattern of rotations about vertical axes. Indeed, in the central Andes paleomagnetic studies have demonstrated counterclockwise rotations on the northern limb of the arc (throughout Peru, northernmost Chile, and northern Bolivia) and clockwise rotations on the southern limb (throughout southern Bolivia, northwestern Argentina, and northern Chile). To fill a gap in data from northern Argentina and to contribute to the ongoing debate on the origin of rotations in the central Andes, we have undertaken a paleomagnetic study of 373 cores, taken at 29 sites (grouped into seven localities). The samples are from sediments and lava flows of Cretaceous to Tertiary age located in intermontane basins of the Puna plateau in northwestern Argentina. Vertical axis rotations, calculated from paleomagnetic declinations, are clockwise for all localities and confirm the pattern of clockwise rotations associated with the southern central Andes. However, significant variations in the amount of rotation occur from one locality to another, suggesting that they are, at least in part, influenced by local tectonics. As most faults in the Puna plateau have reverse dip‐slip components, we infer that the observed differential rotations between blocks are due to scissoring motions on thrust faults. Whether or not this mechanism has operated across the entire area of thickened crust in the central Andes remains to be demontrated. Even if such faulting has locally influenced rotations, Cenozoic oroclinal bending is a likely cause of the remarkable pattern of vertical axis rotations across the central Andes.
Detailed geological mapping of two contrasting second‐order segments of the Mid‐Atlantic Ridge between Oceanographer and Hayes fracture zones (33°30′N‐35°N)
Authors: Gràcia Eulàlia , Bideau Daniel , Hekinian Roger , Lagabrielle Yves
Published: September 2002

We present detailed geological mapping of the axial valley floor and ridge flanks of two neighboring but contrasting spreading segments (OH1 and OH3) of the Mid‐Atlantic Ridge between Oceanographer and Hayes fracture zones. New in situ observations from the submersible Nautile correlated with swath bathymetry and acoustic backscattering data from these segments reveal that the along‐and across‐axis volcanic‐tectonic variability within second‐order segments is larger than commonly acknowledged. Segment OH1 is a long and robust segment with a narrow and shallow axial valley. The most intense magmatic activity is found at the segment center where the thickest crust has been imaged, suggesting focused magma supply. Away from this part of the segment, faulting and sedimentary cover predominate. In contrast, the center of segment OH3 is dominated by extensive sedimentary cover, fissuring, and faulting. Furthermore, the most recent constructional volcanism is located away from the segment center in a region of greater depth and thinner crust. This relocation of volcanism suggests either a recent shift in the magmatic source or the last vestige of a centrally located source fed by lateral dike injection. Segment tip magmatic oscillations are suggested by the distribution of rock types at both segment ends. Serpentinized peridotites and associated dolerites are exposed at the massifs located at the intersection with nontransform offsets (NTOs), whereas only basaltic rocks crop out on the nodal basin floors. We suggest that the combination of low magmatic budget and extension taking place at the NTOs during a segment retreat favors the uplift and exposure of ultramafic massifs.
Large‐scale variations in inner core anisotropy
Authors: Creager Kenneth C.
Published: September 2002

I analyze nearly 2000 handpicked differential times of core‐penetrating compressional waves to image lateral variations in the anisotropic structure of the solid inner core. The inner core is strongly anisotropic (2–4% on average) throughout most of the western hemisphere from near the surface to its center and into the lowermost several hundred kilometers of the eastern hemisphere. In contrast, the outer half of the eastern hemisphere from 40° to 160°E (the quasi‐eastern hemisphere) exhibits very weak anisotropy with an average amplitude of only 0.5%. The symmetry direction is the fast direction and lies on or near the spin axis. Voigt's isotropic average of compressional wave speeds is the same in the eastern and western hemispheres, suggesting that there are no large‐scale lateral variations in the chemistry or temperature in the inner core. Instead, I suggest that the variations seen in anisotropy represent lateral variations in the degree of crystal alignment. The inner core appears to be organized in a very simple way with 60–90% of its volume containing well‐aligned crystals and the remaining part (uppermost 400–700 km of the quasi‐eastern hemisphere) containing less well aligned crystals. A mechanism consistent with axisymmetry and large‐scale variability of anisotropy is discussed. It incorporates the inference that the inner core is rotating faster than the mantle and that gravitational coupling to the mantle forces large‐scale inner core deformation, inducing flow with strain rates as high as 10−14 s−1. A positive feedback mechanism, related to anisotropic viscosity, may reinforce lateral variations in the strength of anisotropy.
High‐ and low‐resolution images of the Earth's mantle: Implications of different approaches to tomographic modeling
Authors: Boschi Lapo , Dziewonski Adam M.
Published: September 2002

Recently published images of the Earth's mantle are characterized by a nominal resolution much higher than that used in previous studies, where substantially different techniques were employed. The agreement between such “high‐resolution” and “low‐resolution” images often seems very poor. In an attempt to determine the reason for this discrepancy, we analyze how the choice of inversion algorithm (exact or iterative), regularization (norm or roughness minimization), and parameterization (spherical harmonics up to a variable degree, blocks) affects a tomographic model. We also investigate the effects of the varying density of the data coverage on the final solution. In our experiments we employ two seismic data sets: Rayleigh wave phase velocity at 75 s period and P wave travel times. We construct a new model of P velocity in the mantle (BDP98) based on the International Seismological Center bulletins 1964–1992. We use our findings in an evaluation of recent mantle models, including our own, focusing on similarities and discrepancies between models of different nominal resolution. In all the models the long‐wavelength component is the most stable. However, consistent high‐resolution details, probably corresponding to features of the real Earth, are also seen. In general, we conclude that most of the differences between existing tomographic models derive from the arbitrary choices made in the process of defining and solving the inverse problem, rather than from actual errors or approximations.
Growth folding and active thrusting in the Montello region, Veneto, northern Italy
Authors: Benedetti Lucilla , Tapponnier Paul , King Geoffrey C. P. , Meyer Bertrand , Manighetti Isabelle
Published: September 2002

The Montello is an elongated hill about 15 km long and 5 km wide located south of the Venetian Alps front and ∼100 km southwest of Gemona, site of the destructive Ms ∼6, 1976 earthquake sequence. Mio‐Pliocene strata in the core of the hill are folded. Seven Quaternary terraces across the western termination of the anticline have also been folded and uplifted. The terraces flank the abandoned Biadene valley, a former course of the Piave river which now flows eastwards along the north side of the hill. Topographic profiles along and transverse to the valley and terraces are used to measure the progressive development of the anticline. Fossil remains and archaeological sites dated with 14C suggest that the Biadene paleovalley was abandoned between 14 and 8 ka (11±3 ka). The successive terraces appear to have been emplaced at the onset of interglacials and interstadials, since about 350 ka. The best fitting terrace ages suggest vertical uplift rates of about 0.5 mm/yr before 172 ka and of about 1 mm/yr after 121 ka. The Montello thus appears to be a growing ramp anticline on top of an active, north dipping thrust that has migrated south of the mountain into the foreland. Modeling the deformation of the terraces as a result of motion on such a thrust ramp requires that it propagated both south and upwards with time but with a constant slip rate (1.8–2 mm/yr). For at least 300 kyr the lateral growth of the anticline kept pushing the course of the Piave river southwestwards, at a rate at first of 10 mm/yr, and then 20 mm/yr. Though the growth rate doubled more than 120 kyr ago, the anticline kept a constant height/length growth ratio (≃20) implying self‐similar depth/length growth of the thrust underneath. The clustering of historical earthquakes north of Treviso suggests that the thrust responsible for ongoing folding of the Montello slipped seismically three times (778, 1268, 1859 A.D.; intensity I ≥ VIII) in the last 2000 years, with events of maximum magnitude close to 6 and with average recurrence time between 500 and 1000 years. NW shortening on NE‐SW trending thrusts along the Venetian Alps front is compatible with the direction of convergence between Africa and Europe but does not suffice to absorb this convergence.
Geomagnetic paleointensity and directional secular variation at Ocean Drilling Program (ODP) Site 984 (Bjorn Drift) since 500 ka: Comparisons with ODP Site 983 (Gardar Drift)
Authors: Channell J. E. T.
Published: September 2002

Normalized remanence (relative geomagnetic paleointensity) records for the last 500 kyr can be matched between two Iceland Basin Ocean Drilling Program (ODP) sites (984 and 983) and correlated with other high‐resolution records. Directional secular variation is, however, not easily correlated between sites due to inadequate recording, at these mean sedimentation rates (12–15 cm/kyr), of the characteristic high frequency variability of the directional record. Both sites record the Iceland Basin event at ∼186–189 ka in which the characteristic magnetization component rotates through 180° and back, coincident with a paleointensity low that lasted about 3 kyr. Other Brunhes Chron geomagnetic excursions appear to be manifest as intervals of higher‐amplitude secular variation during lows in paleointensity. There is a tendency for paleointensity lows to correlate with peak interglacials in the oxygen isotope records; however, this does not translate into a correlation between paleointensity and percent carbonate or between paleointensity and magnetic concentration or grain size parameters.
Thermomechanical behavior of large ash flow calderas
Authors: Burov Evgenii B. , Guillou‐Frottier Laurent
Published: September 2002

The fundamental thermal and mechanical processes that occur within the “ash flow caldera‐magma chamber” systems remain largely enigmatic. To date, the only models of caldera collapse are simple, mostly elastic or viscoelastic mechanical models that can predict some of the conditions preceding the collapse. They cannot, however, predict the collapse itself because they are incapable either of reproducing the formation of faults or of accounting for the brittle‐ductile transitions and fault‐related thermal anomalies. We have constructed analytical and numerical thermomechanical models that account for both elastic‐plastic‐ductile rheology and physical properties of the caldera rocks. The overpressured magma is evacuated through a central vent and transforms into ash flow units deposited within the forming caldera. Brittle deformation, faulting, and subsequent collapse of the structure are reproduced. The results show that in the absence of a regional stress field the collapse on both sides will occur only for aspect ratios (i.e., caldera diameter to the depth of the magma chamber) exceeding 5 and that internal embedded faults may also appear when the aspect ratio exceeds 10. Thermal conductivity contrasts in ash flow calderas give rise to strong heat refraction that localizes deep seated thermal anomalies to the outer sides of the faults. In the presence of regional extension the border faulting can be attenuated or disappear, and the faults tend to localize around the central part of the chamber roof. Coupled thermomechanical modeling suggests that the outer sides of the border faults have high trapping capabilities for hydrothermal fluids. The geometry of the brittle‐ductile transition largely controls that of the fractured zones within and around the chamber roof, thus justifying a new “mechanical definition” of magma chamber geometry.
Oceanic angular momentum variability estimated from the Parallel Ocean Climate Model, 1988–1998
Authors: Johnson Thomas J. , Wilson Clark R. , Chao Benjamin F.
Published: September 2002

This paper describes the use of a global numerical model of the oceans to estimate variations in both the equatorial and axial components of angular momentum resulting from oceanic mass redistribution and circulation. The Parallel Ocean Climate Model, driven by daily wind fields and monthly heat fluxes from the European Centre for Medium‐Range Weather Forecasts for 1988–1998, provides insight into variations in the transport and exchange of angular momentum, a quantity which is essentially conserved within the Earth system. Exchange of angular momentum between the oceans and the solid Earth should be manifest as changes in the Earth's rotation (both polar motion and length of day), and it is possible to compare predicted Earth rotation changes with actual geodetic observations. Using an inverted barometer assumption for oceans, the numerical model predictions of rotation change agree in sign and magnitude and are significantly correlated with observed polar motion and length of day variations after subtracting the dominant atmospheric contributions. The correlation has a seasonal variation which suggests that the role of the oceans in the excitation polar motion is more important during the Northern Hemisphere summer. Our results indicate that the oceans, to a different extent, account for a significant part of the nonatmospheric angular momentum budget for the Earth, and its fluid envelopes on interannual to submonthly timescales.
Three‐dimensional Euler deconvolution and tectonic interpretation of marine magnetic anomaly data in the Puerto Rico Trench
Authors: Muszala S. P. , Grindlay N. R. , Bird R. T.
Published: September 2002

During the summer of 1996 single‐channel‐seismic, magnetic, gravity, Hydrosweep bathymetric and HMR1 sidescan data were collected north of the island of Puerto Rico to constrain the geologic and tectonic setting of the Puerto Rico trench. Magnetic data from this cruise are merged with other available data, then processed and interpreted with the aid of the Euler deconvolution method. The area north of the island of Puerto Rico is divided into three magnetic anomaly zones. Zone 1 is dominated by northwest to southeast trending magnetic anomalies. Zone 2 consists of an east‐west region of relatively low amplitude anomalies and occurs south of zone 1. Zone 3 is dominated by the highest magnetic values in the study area and the source region is roughly centered under the island of Puerto Rico. Respectively, these zones approximate three geologic provinces of the Puerto Rico trench composed of Early Cretaceous ocean crust with sedimentary cover, a blueschist belt and an Oligocene‐Cretaceous island arc with a limestone cap. Also mapped are the Main Ridge fracture zone and the Fourth of July fracture zone, both on the North American plate, which correlate with the Main Ridge and Fourth of July Ridge on the Caribbean plate. The 3‐D Euler deconvolution facilitates the identification of new faults as well as the mapping of known faults also evident in the seismic and bathymetric data. A model concerning the formation of the Puerto Rico trench is proposed that incorporates the existence of strike‐slip faults and the fracture zones associated with the subduction of the North America Plate. Evidence from this study support a tectonic interpretation of subduction followed by more recent strike‐slip faulting that is accompanied by only a minimal amount of subduction.
Properties of partial thermoremanent magnetization in pseudosingle domain and multidomain magnetite grains
Authors: Shcherbakova V. V. , Shcherbakov V. P. , Heider F.
Published: September 2002

An experimental study was carried out to investigate the thermal demagnetization properties of partial thermoremanent magnetization (pTRM). Fifteen different igneous rocks and five synthetic specimens containing crushed and hydrothermally grown magnetite of different grain sizes were studied. The domain structures (DS) of the samples cover the range from single domain (SD) to multidomain (MD) grains. Two different kinds of pTRM were considered: (1) pTRMa(T1, T2) acquired when the upper temperature T1 of acquisition of pTRM is reached by cooling from Tc and (2) pTRMb(T1, T2) acquired by heating to T1 from room temperature. As was shown previously, Thellier's laws of independence and additivity of pTRM(T1, T2) are violated in MD grains. The violations are as follows (1): There is a decrease of pTRM(T1, T2) on cooling below the lower temperature T2 of acquisition of pTRM; (2) the intensity of pTRM(T1, T2) depends on the thermal prehistory of the sample, e.g., pTRMb(T1, T2) ≠ pTRMa(T1, T2); and (3) a pTRM(T1, T2) has a rest (tail) that is not removed by thermal demagnetization at T1 < T ≤ Tc. Here is shown that the violations of the Thellier's laws occur simultaneously and are most pronounced in MD grains. Deviations from both laws are much weaker or almost absent for SD and pseudosingle‐domain (PSD) particles. The properties of pTRM(T1, T2) depend strongly on the temperature interval (T1, T2) where the pTRM was induced. Half of the natural samples under consideration have MD or PSD properties for the low‐temperature pTRMs combined with typical SD‐PSD behavior for the high‐temperature pTRMs. A linear relation was found for MD samples which states that pTRMa(T1, T2)‐pTRMb(T1, T2) is equal to intensity of a tail of pTRMa(T1, T2) after thermal demagnetization at T1. This relationship implies that the remanence carriers, which constitute the tail of pTRMa, do not participate in the acquisition of pTRMb. A modification of Néel's [1955] thermofluctuation model of MD TRM is proposed to explain this relation. The modification suggests that variations of DS during a cooling and heating cycle result in their stabilization accompanied by an irreversible increase of unblocking temperatures. The normalized value of the tail of pTRMa(300°C,Tr) after demagnetization is suggested as an independent means to evaluate the DS of a sample.
Paleomagnetic directions and K/Ar dating of 0 to 1 Ma lava flows from La Guadeloupe Island (French West Indies): Implications for time‐averaged field models
Authors: Carlut J. , Quidelleur X. , Courtillot V. , Boudon G.
Published: September 2002

Twenty‐six lava flows spanning the last million years were sampled in La Guadeloupe, French West Indies. Because of the lack of continuous volcano‐stratigraphic sections in La Guadeloupe, dating is necessary in order to describe the temporal evolution of the geomagnetic field in this time interval. New K/Ar ages ranging from 50 ka to 1 Ma have been obtained on andesites using the Cassignol‐Gillot technique at the Université Paris Sud‐Institut de Physique du Globe de Paris Orsay laboratory. Additional flows (with K/Ar ages obtained using the same dating technique) were also sampled for paleomagnetic investigations. More than 200 samples were analyzed using both alternating field AF and thermal stepwise demagnetization techniques. Duplicate samplings of two flows at three different sites demonstrate that within‐flow dispersion is negligible for the andesitic lava flows sampled in this study. Direct comparison with an earlier paleomagnetic study performed on the island indicates that, for the three investigated flows, modern demagnetization techniques yield much better defined paleomagnetic directions. The Matuyama‐Brunhes transition was recorded in a three‐flow section and is dated at 781±18 ka, in good agreement with other recent radiometric age determinations. The mean paleomagnetic pole calculated from the 23 normal polarity flows is indistinguishable from geographic north, which implies that no significant persistent axial quadrupole term can be identified at this site for the last million years. This result contradicts earlier results and has important implications for models of the time‐averaged field (TAF). An Occam algorithm was used to construct a TAF model from the global volcanic database from the last 5 Ma. Substitution of the mean direction calculated from the earlier study for the Lesser Antilles by our new mean value reduces the quadrupole term by more than 30%. This effect, which was produced by changing data from a single site, demonstrates that older paleomagnetic sites may need to be reinvestigated. Furthermore, it also highlights the limitations of TAF models that can be inferred from paleomagnetic databases in their present state.
Linear ridge groups: Evidence for tensional cracking in the Pacific Plate
Authors: Lynch Mary Ann
Published: September 2002

A new class of oceanic bathymetric feature, discovered in 1987, consists of en‐echelon groups of linear ridges. There are now at least three such sets of ridges known, and their remarkable similarity supports the notion that they result from a widespread tectonic process taking place in the interior of plates. In two of these major linear ridge groups, the Crossgrain and Puka Puka ridges, numerous morphologic features indicate that they originated as tension cracks. The form of the individual volcanic structures that make up the ridges correlates with the degree of tension that formed them, and the similar orientation of all ridges in a group indicates that the stress that formed them is relatively widespread geographically and temporally. The ridges show a characteristic sequence of development, beginning with a swath of small volcanoes followed by larger domical volcanoes where the ridge will eventually develop. This paper examines proposed mechanisms of formation and concludes that no single source of tension in the lithosphere could plausibly have caused both the Crossgrain and Puka Puka ridges. The similarity of the orientations of all the groups of en‐echelon linear ridges on the Pacific plate suggests that the individual ridges form normal to a least compressive stress direction that is geographically variable in the crust. Parallel ridges constrained to a narrow band then result in an en‐echelon arrangement, though some closely spaced, synchronously formed sets of two or three ridge segments may have influenced each other's form.
Evaluation of morphological indicators of magma supply and segmentation from a seismic reflection study of the East Pacific Rise 15°30′–17°N
Authors: Carbotte Suzanne M. , Solomon Adina , Ponce‐Correa Gustavo
Published: September 2002

Hydrosweep bathymetry and multichannel seismic reflection data from two contrasting segments of the East Pacific Rise 15°30′–17°N are used to assess the relationship between crustal structure, morphological indicators of magma supply, and ridge segmentation. From stacked and migrated seismic profiles we evaluate the width and depth of the axial magma lens and the geometry of the seismically inferred extrusive crust, layer 2A. In contrast to other studies, correlation between axial morphology and magma lens characteristics is found with a lens that, on average, is twice as wide and slightly shallower (100–200 m) beneath the unusually shallow and broad southern segment and that shoals and broadens beneath the shallowest part of the northern segment. However, large local‐scale variations in lens depth and width are also observed that are not related to morphology. We conclude that magma supply contributes to the regional characteristics of the magma lens beneath a ridge segment, with large variations within a segment due to local processes of magma eruption and delivery. A negative correlation between zero‐age extrusive layer thickness and morphology is found with a thinner extrusive layer along the southern segment. In this innermost axial region above the magma lens, factors such as magma pressure, not magma volume in the lens, may govern the thickness of extrusives that accumulate. The width of the zone over which the extrusive layer is built is positively correlated with morphology with a wide zone where extrusives approximately triple in thickness characterizing the southern segment (5–8 km), and a narrower zone (2–5 km) of minor thickening (less than double) along the northern segment and toward ridge segment ends. This relationship could reflect changes in lava flow characteristics in these areas of contrasting magma supply. We find a close correspondence between discontinuities in the narrow axial summit trough and changes in magma lens presence and geometry, indicating a genetic link between the finest‐scale tectonic segmentation of the ridge and segmentation of the magma lens. In several locations, discrete magma lenses at different levels within the crust (offset by several hundred meters) are imaged beneath adjacent fourth‐order ridge segments.
Flexural unloading and uplift along the Côte d'Ivoire‐Ghana Transform Margin, equatorial Atlantic
Authors: Clift Peter D. , Lorenzo Juan M.
Published: September 2002

Recent Ocean Drilling Program sampling of the Côte d'Ivoire‐Ghana margin of West Africa provides for the first time the opportunity to study the development of a marginal ridge that formed along a sheared passive margin adjacent to the continent‐ocean transition after the end of intracontinental wrenching. We model its evolution using a two dimensional flexural backstripping technique. The model is constrained by existing seismic refraction and reflection data on the crustal structure and stratigraphy and paleobathymetric evidence from the cores. Following rifting at 120 Ma (Aptian), intracontinental wrenching continued until ∼105 Ma (mid Albian), when South America and Africa separated along this transform. While the possible presence of thicker crust under the Marginal Ridge compared to the Deep Ivorian Basin may explain some of the ridge's topography, the entire uplift can be readily modeled as a flexural response to unloading along a shallow‐dipping (∼25°) fault at the time of continental separation. Forward subsidence modeling of the Marginal Ridge suggests that an effective elastic thickness of 2.5 km at 105 Ma is most appropriate to match the observed structure. Conduction of heat from the oceanic plate across the continent‐ocean transition drove temporary uplift of at least 1200 m peaking just before 89 Ma, when sedimentary data show deposition within the photic zone (0–50 m). This study shows that significant, transient thermal uplift can be found in sheared continental passive margin settings even where magmatism is insignificant.
Cause of continuous oscillations of the Earth
Authors: Tanimoto Toshiro , Um Junho
Published: September 2002

Spheroidal fundamental mode oscillations of the Earth for frequencies between 2 and 7 mHz (millihertz) are observed even on seismically quiet days. Two hypotheses of the cause of these oscillations are investigated: the cumulative effect of small earthquakes and atmospheric pressure variations. The cumulative effect of earthquakes, assuming that earthquakes follow the Gutenberg‐Richter law, is shown to be 1–2 orders of magnitude too small. The observed amplitudes of modes require an equivalent earthquake of magnitude 6.0 everyday, which cannot be achieved by summing up contributions from small earthquakes. The hypothesis of atmospheric excitation is favored because of the discovery of seasonal variations in stacked modal amplitudes for spheroidal modes between 0S20 and 0S40. It is also evaluated by comparing observed modal amplitudes with theoretical amplitudes, derived from a stochastic normal mode theory. The source of excitation is atmospheric pressure variations, which indicate turbulent motion of the atmosphere for the frequency range of interest and are estimated by barometer data. The observed modal amplitudes can be matched by the stochastic normal mode theory, indicating that atmospheric pressure variation is large enough to excite solid Earth normal modes up to the observed amplitudes. Therefore two lines of evidence, detection of seasonal variations and approximate match of overall modal amplitudes, support the hypothesis that the continuous background oscillations are excited by atmospheric pressure variations.
Slab temperature and thickness from seismic tomography: 1. Method and application to Tonga
Authors: Deal Michael M. , Nolet Guust , van der Hilst Rob D.
Published: September 2002

Delay times of compressional body wave phases from both teleseismic and local events are used to invert for a high‐resolution P wave velocity model in the Tonga subduction zone. The images obtained show a high‐velocity subducting slab with velocity deviations of the order of 3–4%. Assuming to first order that the positive velocity anomalies within the slab are caused by a temperature effect, a theoretical slab temperature model based on the diffusion equation is used to explain velocity anomalies within the tomographic slab. Temperature differences between the interior of the slab and the ambient mantle are converted to velocity perturbations using the scaling parameter dVp/dT ≈ 4.8 × 10−4 km s−1 °C−1 for lithosphere material. The optimal values for the parameters in the temperature model are found using a nonlinear optimization that compares the integrated velocity anomalies in the tomographic slab region to integral of high velocities in a synthetic slab derived from a temperature model. The parameters for slab thickness and mantle potential temperature are not uniquely determined; therefore a fixed value for the mantle potential temperature based on laboratory values for the temperature of the spinel‐to‐perovskite transition at 660 km is used. Using 1180°C as the potential temperature, the theoretical temperature model gives an optimal slab thickness of 82 km for a region near 29°S in Tonga. The uncertainty in the thickness is dominated by the uncertainty in the mantle temperature and would be 8 km for an uncertainty of 100° in mantle temperature, but nonsystematic errors are less. In order to enhance the tomographic result the velocity model is biased towards the theoretical slab model. However, a posteriori changes made to the tomogram will most likely violate the fit to the delay time data. To prevent this, the difference between the tomogram and the predicted slab model is projected onto the null‐space of the inversion to remove components which do not satisfy the seismic data. Using only null‐space components to modify the minimum‐norm solution, an enhanced model is obtained which has been biased toward the theoretical solution but has the same data misfit as the minimum‐norm solution. The final image shows a very narrow and continuous slab with maximum velocity anomalies of the order of 6–7%; many of the gaps within the slab, as well as artifacts around the slab which were present in the minimum‐norm solution, are absent in the biased image.
Faulting and fault scaling on the median valley floor of the trans‐Atlantic geotraverse (TAG) segment, ∼26°N on the Mid‐Atlantic Ridge
Authors: Bohnenstiehl DelWayne R. , Kleinrock Martin C.
Published: September 2002

A quantitative assessment of faulting on the median valley floor of a slow spreading ridge is accomplished through the analysis of high‐resolution DSL‐120 sidescan sonar and coregistered bathymetric data from the TAG segment near 26°N on the Mid‐Atlantic Ridge. At this location, faulting is exposed within a 3–5 km wide ridge‐parallel zone lying asymmetrically on the eastern half of the median valley floor. Mapped faults have a normal sense of displacement, are <2 km in length, and accommodate ∼1.5% brittle extension. Evidence of fault linkage within the fault population includes kinked and bent fault traces in map view, the development of overlapping fault segments or relay ramps, and the presence of multiple local maxima in the displacement‐distance profiles of some faults. Faults have a slight tendency to dip to the east, or outward relative to the valley axis, and exhibit little symmetry of fault dip about the axis of the faulted zone or any other ridge‐parallel line. Faults exhibit a roughly linear relationship between maximum fault throw and fault length, with a mean ratio of 0.030 for the population. Regression of length‐frequency data indicates a power‐law distribution, with an exponent of 1.64–1.96, demonstrating that fractal populations can exist in the mid‐ocean ridge environment. The fractal nature of this length‐frequency distribution and the ratio of maximum fault throw to fault length differ significantly from those described previously for populations of larger abyssal hill faults in the fast spreading environment, where the distribution is exponential and the throw‐to‐length ratio is ∼5 times lower. These results suggest that the scaling of fault populations in the midocean ridge setting may vary as a function of spreading rate and/or fault size.
A new numerical model of coupled inland ice sheet, ice stream, and ice shelf flow and its application to the West Antarctic Ice Sheet
Authors: Hulbe Christina L. , MacAyeal Douglas R.
Published: September 2002

We have developed a dynamic/thermodynamic finite element numerical model that couples inland ice sheet, ice stream, and ice shelf dynamics. This new model stands apart from other whole ice sheet models in its explicit treatment of ice stream flow. Additionally, the model accounts for both horizontal and vertical advection and diffusion of temperature in the flowing ice. In present day simulations of the West Antarctic Ice Sheet (WAIS), modeled ice velocity agrees well with observed ice flow. In particular, the model reproduces the pattern of speed variation across ice streams although the continuous downstream speed up of ice flow cannot be reproduced without concurrent downstream variation in basal friction. Model thermodynamics, evaluated qualitatively by model prediction of the spatial distribution of basal melting and quantitatively by comparison with ice temperature measured in boreholes at several locations, are sound. In particular, the model reproduces the broad pattern of frozen‐bed inter‐ice stream ridges and melted‐bed ice streams. Model initialization for long‐time simulations is somewhat limited by computation time and by a thermodynamic feedback at sites of large viscous heating that can be a problem in heat balance only model initializations. Methods for averting those initialization problems are discussed. The new model can accommodate a variety of boundary conditions (such as various bed rheologies) and is well‐suited to investigate the origin and evolution of WAIS ice streams within the context of the whole ice sheet system.
Activation volume of Si diffusion in San Carlos olivine: Implications for upper mantle rheology
Authors: Béjina Frédéric , Jaoul Olivier , Liebermann Robert C.
Published: September 2002

The effect of pressure on silicon diffusion in San Carlos olivine has been determined using a uniaxial split‐sphere apparatus (USSA‐2000) and the nuclear reaction analysis technique (NRA) on the 30Si isotope. Experiments were performed at high temperature, T = 1763 K, and pressures between 4 and 9 GPa. The specimens were inserted into a pure Fe capsule, which is very effective in maintaining the oxygen fugacity within the stability field of olivine, as well as providing a soft medium to mechanically protect the crystals. Diffusion profiles along the
crystallographic axis and of characteristic length of the order of 50 nm were obtained after annealing the olivine samples between 1 and 4 hours. We find the activation volume for silicon diffusion in San Carlos olivine to be VSi = +(0.7±2.3) × 10−6 m3/mol after a correction for oxygen fugacity which is pressure‐dependent. This result demonstrates that pressure has practically no effect upon silicon diffusion under our temperature and pressure conditions. Extrapolation of our high‐pressure measurements to 1 atm gives a Si diffusion coefficient, log(DSi) = −18.9±1.0 (with DSi in m2/s). Finally, our experiments show that according to the point‐defect model of Jaoul [1990], the activation volume for creep of olivine at high temperature is dominated by the effect of pressure on the Mg defect concentration and has an activation volume close to 6 × 10−6 m3/mol.
Friction of simulated fault gouge for a wide range of velocities and normal stresses
Authors: Mair Karen , Marone Chris
Published: September 2002

During earthquake rupture, faults slip at velocities of cm/s to m/s. Fault friction at these velocities strongly influences dynamic rupture but is at present poorly constrained. We study friction of simulated fault gouge as a function of normal stress (σn = 25 to 70 MPa) and load point velocity (V = 0.001 to 10 mm/s). Layers of granular quartz (3 mm thick) are sheared between rough surfaces in a direct shear apparatus at ambient conditions. For a constant σn, we impose regular step changes in V throughout 20 mm net slip and monitor the frictional response. A striking observation at high velocity is a dramatic reduction in the instantaneous change in frictional strength for a step change in velocity (friction direct effect) with accumulated slip. Gouge layers dilate for a step increase in velocity, and the amount of dilation decreases with slip and is systematically greater at higher velocity. The steady state friction velocity dependence (a‐b) evolves from strengthening to weakening with slip but is not significantly influenced by V or σn. Measurements of dilation imply that an additional mechanism, such as grain rolling, operates at high velocity and that the active shear zone narrows with slip. Data from slow (μm/s) and fast (mm/s) tests indicate a similar displacement dependent textural evolution and comparable comminution rates. Our experiments produce a distinct shear localization fabric and velocity weakening behavior despite limited net displacements and negligible shear heating. Under these conditions we find no evidence for the strong velocity weakening or low friction values predicted by some theoretical models of dynamic rupture. Thus certain mechanisms for strong frictional weakening, such as grain rolling, can likely be ruled out for the conditions of our study.