bluuebunny/crossref_metadata_2025 huggingface dataset:
Upper mantle anisotropy beneath the Geoscope stations
Authors: Barruol Guilhem , Hoffmann Ruth
Published: September 2002

Seismic anisotropy has been widely studied this last decade, particularly by measuring splitting of vertically propagating core shear waves. The main interest in this technique is to characterize upper mantle flow beneath seismic stations. On the other hand, the major restriction in this method is that a single station gives a single anisotropy measurement. Alternative methods have been developed in order to avoid this restriction. An accurate determination of upper mantle seismic anisotropy beneath a seismic station may allow one, by doing anisotropy correction, to characterize remote or deeper anisotropy. The Geoscope network is ideal for this purpose because it is composed of a large set (about 26) of high‐quality, broadband seismometers globally distributed and because some of these stations have run for more than 10 years and most of them for more than 5 years. We selected about 100 events at each site, generally of magnitude (mb) > 6.0, and we performed systematic measurements of the splitting parameters (fast polarization direction ϕ and delay time δt) on SKS, SKKS, and PKS phases. Splitting on oceanic islands has been difficult to observe owing to the low quality of the signal but also perhaps owing to complex upper mantle structures beneath the stations. Station KIP (Kipapa, Hawaii) in the Pacific is the only oceanic Geoscope station with a clear anisotropy. We determined well‐constrained splitting parameters for 10 of the 17 continental stations that may be explained by a single anisotropic layer. The poor correlation between fast polarization directions and the absolute plate motion together with the apparent incoherence between the plate velocities and the observed delay times suggest that a simple drag‐induced asthenospheric flow alone fails to explain most of the observations. For some stations located on or near major lithospheric structures (TAM, Tamanrasset, Algeria, for instance), we observe a good correlation between fast polarization directions and regional structures. At station SCZ (Santa Cruz, California), we found clear variations of the splitting parameters as a function of the event backazimuth, compatible with two layers of anisotropy. Three stations (CAN (Canberra), HYB (Hyderabad, India) and SSB (Saint Sauveur Badole, France)) seem to be devoid of detectable anisotropy.
Spherical harmonic analysis of paleomagnetic data: The case of linear mapping
Authors: Kono Masaru , Tanaka Hidefumi , Tsunakawa Hideo
Published: September 2002

We consider the problem of describing the ancient magnetic field as completely as possible using paleomagnetic data. One of the best ways to do this is to represent the field by the use of Gauss coefficients. Unlike ordinary cases, however, spherical harmonic analysis of paleomagnetic data is a complicated problem because (1) the data cannot be considered as contemporaneous and (2) they are mostly available only in the form of directions. If the mapping between the model parameters (Gauss coefficients) and the data is linear (e.g., the three components of the field), there is one‐to‐one correspondence between the means of the parameters and the means of the data. Conventional least squares techniques can be applied to the means as the relations between the data and parameters are the same as in the instantaneous case. One‐to‐one correspondence also exists between the variances of the parameters and the variances of the data. However, the functions appearing in these relations are not orthogonal to each other, and the matrix to be inverted is quite ill conditioned. For magnetic field directions that are nonlinearly related to Gauss coefficients, one‐to‐one correspondence is completely absent between the means (variances) of the parameters and those of the data. This means that inversion becomes quite complicated because the mean of such data contains information on both the mean and fluctuations of the parameters. We applied inversion to the paleointensity data of the past 5 Myr and obtained the means and variances of Gauss coefficients for that period. The means are well determined and show that the time‐averaged field is dominated by the axial dipole component. It is more axisymmetric than the recent field, suggesting that equatorial dipole components are largely averaged out as assumed by paleomagnetic dipole hypothesis. Inversion of the variances gave less well determined results. Among the variances of Gauss coefficients that are significantly different from zero that of the axial dipole is much larger than those of other coefficients, indicating that the observed variation in paleointensity is largely due to the fluctuation of the magnitude of the dipole moment.
The relationship between open, dry microcracks and P wave velocity of crystalline rocks: Application to in situ stress estimates on KTB drill cores after stress relaxation
Authors: Zinke Jens C.
Published: September 2002

Experiments were performed to quantitatively confirm the relationship between open dry microcracks and P wave velocity on crystalline rocks. Equations derived by Hudson [1980] (first‐order corrections only) describe the experimentally obtained relationship. Microcrack analysis consists of chemical contact imagery to make the cracks visible, image analysis, and stereologic data processing to obtain three‐dimensional crack characteristics from data obtained in three orthogonal plane images. Pressure‐dependent measured P wave velocities are used to estimate intrinsic P wave velocities (velocities characteristic for the uncracked rock) by extrapolating the linear part of the pressure‐velocity curve from high pressure to low pressure. Therefore the impact of open microcracks on P wave velocity is given by normalized P wave velocities, which are the ratio between measured and intrinsic velocities at a hydrostatic pressure of 1 MPa. Three‐dimensional crack characteristics and normalized P wave velocities, are obtained in three orthogonal directions. Velocities in all other directions were estimated by interpolation assuming an orientation distribution that can be described by a triaxial ellipsoid. In this way it is possible to study the relationship between open dry cracks and P wave velocity experimentally and to compare the results with the theoretically predicted relationship using one single model. In a case study, pressure‐dependent measured P wave velocities have been used to infer the distribution of open microcracks in 84 drill cores from the Continental Deep Drilling Project of the Federal Republic of Germany (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland, or KTB) covering the depth range between 621–8080 m. The cores were studied after stress relaxation (1–7 years after drilling). It was not possible to obtain information about the in situ stress field owing to the anisotropic nature of the rock fabric.
The updip and downdip limits to great subduction earthquakes: Thermal and structural models of Cascadia, south Alaska, SW Japan, and Chile
Authors: Oleskevich D. A. , Hyndman R. D. , Wang K.
Published: September 2002

This study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes. Data on past great earthquake seismic limits have been compiled for four continental subduction zones, Cascadia, SW Japan, south Alaska, and Chile. These limits have then been compared to the predictions of several models for what constrains great earthquake rupture. Temperatures on the subduction thrusts have been estimated by finite element numerical models. The landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100°C, that is, depths of about 2 to 10 km for the subduction zones studied. This temperature agrees with the dehydration of stable sliding smectite clays to illite‐chlorite. The temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow. The downdip limits correspond to the depth on the thrust where either (1) the temperature reaches about 350°C, which corresponds to thermally activated stable‐sliding behavior for crustal rocks (with a transition to 450°C), or (2) about 40 km depth if 350°C is reached at greater depth. Depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc Moho, and this downdip limit may be a consequence of stable‐sliding serpentinite or talc and other hydrated forearc mantle rocks. The primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile. Secondary control comes from the thickness of incoming sediment, the convergence rate, and the radioactive heat generation in the overlying forearc. The 100°C updip limit occurs near the trench for young subducting plates with a thick sediment section such as Cascadia (6–8 Ma), and up to 80 km landward for older oceanic crust such as south Alaska (∼50 Ma). The 350°C downdip thermal limit is applicable for young oceanic plates (e.g., Cascadia and Nankai), whereas the forearc mantle limit applies for older plates (e.g., south Alaska and Chile except near the Chile Rise). For the margins studied that have experienced great earthquakes, there is generally good agreement between the postulated thermal and Moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform, tsunami and dislocation modeling. The downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits.
Plume‐related mafic volcanism and the deposition of banded iron formation
Authors: Isley Ann E. , Abbott Dallas H.
Published: September 2002

We have compiled a record of the geochronology of mantle plume activity between 3.8 and 1.6 Ga. Over this time period, the ages of komatiites, and those of global plumes, correlate strongly, at the 99% confidence level, with the ages of banded iron formations (BIFs). The ages of continental plumes correlate more weakly, at an overall 85% confidence level. Using the geochronological records of these events, we can define four periods characterized by mantle superplume activity. Three of these periods are also times of enhanced BIF deposition. The fourth mantle plume period may similarly be coeval with increased BIF accumulation, but the BIF chronostratigraphic resolution is not accurate enough to test this rigorously. Mantle superplume volcanism may promote BIF deposition by increasing the Fe flux to the global oceans through continental weathering and/or through submarine hydrothermal processes. It may also be enhanced by increasing the number of paleotectonic environments appropriate for BIF deposition (particularly plume‐induced ocean plateaus, seamounts, and intracratonic rifts) and by promoting global anoxic, Fe‐rich hydrothermal plumes in the shallow to intermediate marine water column.
Astronomically tuned geomagnetic polarity timescale for the Late Triassic
Authors: Kent Dennis V. , Olsen Paul E.
Published: September 2002

Cycle stratigraphic and magnetostratigraphic analyses of a ∼5000‐m‐thick composite section obtained by scientific coring in the Newark rift basin of eastern North America provide a high‐resolution astronomically calibrated geomagnetic polarity timescale (GPTS) spanning over 30 m.y. of the Late Triassic and earliest Jurassic. Only normal polarity is found in ∼1000 m of interbedded volcanics and continental sediments of earliest Jurassic age but a total of 59 normal and reverse polarity magnetozones are delineated in the underlying 4000+ m of Late Triassic continental sediments. Lithologic facies response to climatically induced lake level variation provides a full spectrum of Milankovitch cyclicity; the prominent 404 kyr orbital eccentricity climate cycle has a mean thickness of about 60 m and is the basis for scaling most of the stratigraphic section in time. When indexed to available radioisotopic dating, the resulting astronomically calibrated GPTS spans from the 202 Ma Triassic/Jurassic boundary to 233 Ma. Results of detailed sampling profiles across 42 magnetozone boundaries representing 35 different polarity reversals indicate transition durations that average 7.9 kyr, comparable to the estimated duration of recent polarity reversals. The polarity intervals have a mean duration of 0.53 m.y. with a corresponding reversal rate of 1.88 m.y.−1 and no significant polarity bias and are closely approximated by an exponential distribution with a gamma index k indistinguishable from 1. The longest polarity interval is about 2 m.y., and the shortest is about 0.02 m.y. The overall statistical properties indicate that the behavior of the geomagnetic field in the Late Triassic was not very different from that in the Cenozoic. This geomagnetic polarity record of the Late Triassic provides a well‐dated chronostratigraphic framework suitable for detailed global correlation.
Stress tensor inversion using detailed microearthquake information and stability constraints: Application to Ölfus in southwest Iceland
Authors: Lund Björn , Slunga Ragnar
Published: September 2002

Using detailed microearthquake data, we present a stress tensor inversion scheme with new methods for selecting the fault planes and allowing for errors in the focal mechanisms. The nonuniqueness of earthquake focal mechanisms is accounted for in our inversion scheme through the introduction into the inversion of a range of well‐fitting focal mechanisms for each event. The range of focal mechanisms significantly improves the quality of the estimated stress tensor. Relative localization of clusters of microearthquakes is used to obtain information about which nodal plane could be correct fault plane. The clusters frequently fall on a common fault plane, and if there are acceptable focal mechanisms where one nodal plane has orientation similar to the common plane, we assume this is the correct fault plane for the event. If there is no predefined fault plane, we utilize a simple Mohr‐Coloumb failure criterion to obtain a physical choice of fault plane between the two nodal planes in the focal mechanism. The nodal plane with highest relative instability is chosen as the fault plane. Differences between the instability and the standard slip angle criterion are investigated. The new inversion scheme has been applied to microearthquake data from the Ölfus area in the vicinity of the southwest Iceland triple junction. We estimate an oblique strike‐slip state of stress, maximum horizontal stress at N30°E, and minimum horizontal stress at N60°W, with significant normal faulting influence. The instability fault selection criterion predicts very well the orientation of faults mapped by relative localization.
P wave detection thresholds, Pn velocity estimates, and T wave location uncertainty from oceanic hydrophones
Authors: Slack Philip D. , Fox Christopher G. , Dziak Robert P.
Published: September 2002

P wave arrivals recorded by the U.S. Navy's SOund SUrveillance System (SOSUS) hydrophone arrays were used to estimate earthquake detection thresholds and Pn velocities in the northeast Pacific Ocean. The Navy hydrophones have been used successfully to detect and locate oceanic earthquakes using their waterborne acoustic tertiary (T) waves; however, use of these hydrophones for seismic body wave detection allows regional seismic analyses to be extended to the oceanic environment. The P wave detection threshold of the SOSUS hydrophones was quantified using the epicentral distance and magnitude of 250 northeast Pacific Ocean earthquakes. Earthquakes with body wave magnitudes as low as 2 have detectable P wave arrivals at epicentral distances of ≤500 km. Earthquakes with mb between 3.5 and 5 were detected ∼50% of the time at distances of 100–1500 km, while events with mb > 5 were all detected, even out to distances of 1000–1500 km. Both P and T wave hydrophone arrival times were used to estimate the epicenters of 100 earthquakes. The peak amplitude of the T wave coda and the onset of the P wave were used as the earthquake arrival times to estimate event locations. T wave arrival time residuals have a Gaussian distribution with zero mean, which implies that using T wave peak amplitude is consistent with using the P wave onset as the arrival time. There are typically ≤6 stations used to derive a T wave based location, hence location error ellipses are not well constrained. A Monte Carlo technique was employed to estimate T wave event location uncertainty. T wave locations have error bars of ∼1 km in latitude and longitude when >3 hydrophones are used for a location estimate. The detected P wave arrivals and earthquake locations were used to measure Pn velocities. Pn velocity values of 7.9 ± 0.1 and 8.0 ± 0.1 km/s were found for the Pacific and Juan de Fuca plates, respectively. A Pn velocity of 7.5 ± 0.1 km/s was measured for rays traveling northward from the Mendocino Triple Junction along the Cascadia subduction zone. A Pn velocity of 7.7 ± 0.3 km/s was estimated for ray paths originating onshore western North America and traveling to the offshore hydrophones.
Formation of natural gas hydrates in marine sediments: 2. Thermodynamic calculations of stability conditions in porous sediments
Authors: Henry Pierre , Thomas Michel , Clennell M. Ben
Published: September 2002

A thermodynamic model for hydrate formation is used to compute the solubility of methane in pore water in equilibrium with gaseous methane or methane hydrate or both. Free energy of water in the hydrate phase and of methane in gas bubbles are corrected to account for salt effects and capillary effects. Capillary effects increase the solubility of methane in fluid in equilibrium with either hydrate or gas. Natural sediments have a broad distribution of pore sizes, and the effective pore size for capillary effects is a function of the fraction of the pore space filled by hydrate or gas (phase fraction). The equilibrium conditions for hydrate + water + gas equilibrium thus depend on hydrate and gas phase fraction. Data acquired on Blake Ridge during Ocean Drilling Program Leg 164 show that the base of the hydrate stability there is shifted by −2°C or more with respect to the expected temperature and this shift has been attributed to capillary effects. We show that this explanation would require a very small effective pore radius (20 nm at 30 MPa). Mercury porosimetry indicates that the percolation threshold for Blake Ridge silty claystone is reached at 20–25% phase fraction and corresponds to a 100 nm pore radius. Hydrate and gas phase fraction determined with several independent methods are all lower than this percolation threshold, implying that gas and hydrate fill pores larger than 100 nm. We conclude that additional inhibition factors other than pore size effects must be involved to explain the −2°C bottom‐simulating reflector (BSR) shift as an equilibrium phenomenon. Capillary effects may, however, explain other observations such as large variations of the gas hydrate content in the sediment with lithology and porosity and the distribution of hydrate between interstitial hydrate and segregated masses. Capillary effects should also oppose the migration of gas bubbles when gas phase fraction is less than the percolation threshold and make unnecessary the assumption of a hydrate seal impermeable to fluids. Alternatively, we can go some way to explaining the offset position of the BSR by relaxing the assumption that the system is in thermodynamic equilibrium. Nucleation kinetics of hydrate and/or free gas bubbles may be inhibited by confinement of the methane‐bearing fluid in small pores. Equilibration may also be limited by possible rates of diffusional transport of gas, water, and salt components or be perturbed by significant flows of fluid or heat through the sediments.
Thermal modeling of convergent and extensional tectonic settings for the development of low‐grade metamorphism in the Welsh Basin
Authors: Robinson D. , Reverdatto V. V. , Bevins R. E. , Polyansky O. P. , Sheplev V. S.
Published: September 2002

The Lower Paleozoic Welsh Basin of the U.K. Caledonides shows a metamorphic transition from zeolite to low‐greenschist facies for which there are two radically opposing models of development. The traditional model is of a syntectonic metamorphism associated with crustal thickening during basin inversion and cleavage development. The alternative model links the metamorphism not to traditional ideas of crustal thickening but to thermal anomalies associated with the extensional development of a back arc basin. These two models are tested by applying quantitative thermal modeling to crustal thickening and crustal extension tectonic settings specific to the Welsh Basin. This basin has a unique and very well constrained record of its stratigraphic, sedimentological, and chronologic histories that is used here to provide the framework from which the dynamics of crustal evolution can be modeled. Crustal thickening models suggest that a higher‐pressure facies series with a pumpellyite‐actinolite to greenschist facies transition should be dominant. In contrast, extensional modeling suggests that a low‐pressure fades with a transition from prehnite‐pumpellyite to greenschist facies should be dominant. The thermal evolution of the extensional setting is more compatible with the lower‐pressure metamorphic series that is recorded in metabasites as well as pelitic rocks of the region. The metamorphic evolution is regarded as developing initially as a burial style, which is then over printed by fabric development while at or near peak P‐T conditions, thus giving an apparent syntectonic style of metamorphism.
Formation of natural gas hydrates in marine sediments: 1. Conceptual model of gas hydrate growth conditioned by host sediment properties
Authors: Clennell M. Ben , Hovland Martin , Booth James S. , Henry Pierre , Winters William J.
Published: September 2002

The stability of submarine gas hydrates is largely dictated by pressure and temperature, gas composition, and pore water salinity. However, the physical properties and surface chemistry of deep marine sediments may also affect the thermodynamic state, growth kinetics, spatial distributions, and growth forms of clathrates. Our conceptual model presumes that gas hydrate behaves in a way analogous to ice in a freezing soil. Hydrate growth is inhibited within fine‐grained sediments by a combination of reduced pore water activity in the vicinity of hydrophilic mineral surfaces, and the excess internal energy of small crystals confined in pores. The excess energy can be thought of as a “capillary pressure” in the hydrate crystal, related to the pore size distribution and the state of stress in the sediment framework. The base of gas hydrate stability in a sequence of fine sediments is predicted by our model to occur at a lower temperature (nearer to the seabed) than would be calculated from bulk thermodynamic equilibrium. Capillary effects or a build up of salt in the system can expand the phase boundary between hydrate and free gas into a divariant field extending over a finite depth range dictated by total methane content and pore‐size distribution. Hysteresis between the temperatures of crystallization and dissociation of the clathrate is also predicted. Growth forms commonly observed in hydrate samples recovered from marine sediments (nodules, and lenses in muds; cements in sands) can largely be explained by capillary effects, but kinetics of nucleation and growth are also important. The formation of concentrated gas hydrates in a partially closed system with respect to material transport, or where gas can flush through the system, may lead to water depletion in the host sediment. This “freeze‐drying” may be detectable through physical changes to the sediment (low water content and overconsolidation) and/or chemical anomalies in the pore waters and metastable presence of free gas within the normal zone of hydrate stability.
Crust generated above the Iceland mantle plume: From continental rift to oceanic spreading center
Authors: Smallwood John R. , Staples Robert K. , Richardson K. Ruth , White Robert S.
Published: September 2002

Since the North Atlantic continental breakup in the early Tertiary, the process of rifting above a mantle plume has produced large thicknesses of igneous crust. We report results of an integrated offshore‐onshore seismic study of the crust and upper mantle along a transect of the aseismic Faroe‐Iceland Ridge, between the continental fragment beneath the Faroe Islands and the present‐day spreading center in northeast Iceland. Normal‐incidence seismic data provide an image of the uppermost crust, which is complemented by a velocity model from streamer refraction analyses. These data together image four sedimentary basins, up to 400 m deep, along the crest of the ridge. The streamer refraction velocity model in turn forms the uppermost section of a full crustal velocity model derived from travel time modeling of air gun and explosive data. The compressional to shear wave velocity ratio in the crust of the Faroe‐Iceland Ridge is 1.77 ± 0.02, similar to that of eastern Iceland. The Moho beneath the Faroe‐Iceland Ridge lies at a depth of 25–30 km. Crust of 25–30 km thickness requires an upper mantle potential temperature elevated 200–250°C above normal if formed by passive adiabatic decompression melting of the mantle or a somewhat less elevated temperature if, as is likely, there is a component of active convection in the underlying mantle plume core.
Reconciling rapid strain accumulation with deep seismogenic fault planes in the Ventura Basin, California
Authors: Hager Bradford H. , Lyzenga Gregory A. , Donnellan Andrea , Dong Danan
Published: September 2002

Global Positioning System measurements across the east central Ventura basin, Transverse Ranges, southern California, before the nearby 1994 Northridge earthquake show high strain rates. Interpreting this rapid strain accumulation using the usual model of deep slip on a dislocation in a uniform elastic half‐space requires slip to extend to within 5 km of the surface. Such shallow slip is difficult to reconcile with the substantial coseismic displacement at depths from 7 to 19 km during the Northridge earthquake. Here we model the displacement and velocity fields throughout the earthquake cycle using a two‐dimensional finite element model with a viscoelastic rheology. Displacements are driven by far‐field and basal velocity boundary conditions and by imposed periodic earthquakes on the thrust faults bounding the basin. The thrust faults rupture through an elastic upper crust to a depth of 15 km. After a transient stage, during which stresses and strains build up to quasi‐equilibrium values, the behavior of the model becomes periodic. The sum of the coseismic displacement divided by the repeat interval, plus the average interseismic velocity, is equal to the geologic velocity. The temporal variation in surface velocity depends mainly on the Elsasser relaxation time (proportional to the product of the Maxwell time of the lower crust and the ratio of the thicknesses of the entire crust and viscoelastic lower crust). We are able to match the observed high strain rate only if we include the observed variations in elastic modulus associated with the deep basin sediments. The model reconciles geologic, geodetic, and seismological observations of deformation. There are trade‐offs among the far‐field convergence rate, the Elsasser time, the earthquake repeat time, and the time into the earthquake cycle. Acceptable convergence rates range from 8 mm/yr, for a relaxation time of the lower crust of 300 years, to 12 mm/yr, for a 30‐year relaxation time.
A relative geomagnetic paleointensity stack from Ontong‐Java Plateau sediments for the Matuyama
Authors: Kok Yvo S. , Tauxe Lisa
Published: September 2002

We present a stack of geomagnetic paleointensity estimates for the Matuyama Chron (0.778–2.582 Ma). Sedimentary magnetizations from 4 Ocean Drilling Program (ODP) cores from the Ontong‐Java Plateau (OJP) are analyzed with thermal techniques. Thellier‐Thellier experiments are used to determine magnetic properties and indicate that a thermal treatment to 250°C can be used for paleointensity estimates. The correlated records form a coherent OJP stack, which is dated at reversal boundaries. A coherent feature with two other data sets spanning the Matuyama Chron is the occurrence of a 150‐kyr cyclicity. However, the time control of the OJP stack is not accurate, except for the 10 tie points at the reversals. This could, at least in part, explain why the three Matuyama paleointensity records show no other coherent frequencies.
Formation of the axial relief at the very slow spreading Southwest Indian Ridge (49° to 69°E)
Authors: Cannat Mathilde , Rommevaux‐Jestin Céline , Sauter Daniel , Deplus Christine , Mendel Véronique
Published: September 2002

The comparison of segment lengths, relief, and gravity signature along the very slow spreading Southwest Indian Ridge (SWIR) between 49°E and 69°E suggests that the marked change in segmentation style that occurs across the Melville transform (60°45′E) reflects a change in the modes of formation of the axial topography. We propose that the axial relief east of Melville is largely due to volcanic constructions that load the axial lithosphere from above. By contrast, the axial relief in segments west of the Melville fracture zone appears to be primarily due, as proposed for segments of the faster spreading Mid‐Atlantic Ridge, to along‐axis changes in the depth of the axial valley, and to partial compensation of negative loads (thicker lower crust and/or lighter upper mantle) acting within the plate, or at the bottom of the plate. In terms of geology, this means that the contribution of the uppermost, effusive, part of the crust to along‐axis crustal thickness variations may be greater east of Melville than in other regions of the study area. Regional axial depths suggest that the ridge east of Melville is also characterized by a low melt supply and is underlain by cold mantle. A simple model of mantle melting and regional isostatic compensation suggests that differences in mantle temperature and in melt thickness between this deep eastern ridge region, and the shallower region west of the Gallieni transform (52°20′E), are of the order of 80°C and 4 km, respectively.
Some remarks on the relation between vertical motions of the lithosphere during extension and the necking depth parameter inferred from kinematic modeling studies
Authors: Govers R. , Wortel M. J. R.
Published: September 2002

A well‐established fact of continental stretching is that during the extension phase the lithosphere retains a finite (flexural) strength. Vertical loads caused by the redistribution of material with crustal and mantle densities during lithospheric thinning can therefore be supported by the lithosphere. Whether these loads cause flexural uplift or subsidence depends on the strength distribution within the continental lithosphere. In kinematic model studies, stratigraphic and other data acquired in the vicinity of sedimentary basins and passive margins are “inverted” for these loads. In these models, vertical rebound loads are parameterized in terms of a “necking depth” parameter. We use thermal‐mechanical finite element simulations to investigate whether rheological information about the continental lithosphere can be extracted from the necking depth parameter derived from kinematic models. We conclude that spatial and temporal variations in necking depths near basin edges and passive margins as derived from our dynamic simulations are not consistent with the uniform and stationary necking depth that is usually assumed in kinematic models. This is very important since, typically, the stratigraphic data used in kinematic model studies are acquired in these regions. We therefore conclude that there is no one‐to‐one relation between the necking depth parameter and the strength distribution in the lithosphere.
Seismic‐reflection and potential‐field studies of the Vulcan structure, western Canada: A Paleoproterozoic Pyrenees?
Authors: Eaton David W. , Ross Gerald M. , Clowes Ronald M.
Published: September 2002

The Vulcan structure is a major tectonic boundary between the Medicine Hat and Loverna Blocks, Archean crustal domains that are buried beneath the Western Canada Sedimentary Basin. It is characterized by prominent east‐trending gravity and magnetic anomalies, more than 350 km in length, which cut across the potential‐field fabric of southern Alberta at a high angle. Three decades ago, the Vulcan structure was the target of one of the first deep‐crustal seismic profiles, and it has since been variously interpreted as a failed Proterozoic rift, an intraplate collisional zone, and (or) a largely amagmatic Proterozoic suture. Several recent Lithoprobe seismic‐reflection profiles, coupled with new, coincident high‐resolution gravity data, regional gravity data, and updated aeromagnetic coverage, provide the basis for a revised model of the crustal architecture in this region. We interpret the Vulcan structure to be a relatively narrow (40–70 km) axial zone of a continental collisional belt. The seismic images imply crustal delamination and southdirected subduction of the lower crust of the Loverna Block. The axial gravity low and corresponding magnetic anomaly are interpreted to originate from a large granitic pluton, while prominent gravity highs on the flanks of the structure are modeled as thrust slices of lower‐crustal and/or mantle material brought up during the collision. Cross‐cutting relationships inferred from aeromagnetic data indicate that the timing of collision postdated formation of Archean fabrics in the Medicine Hat Block but predated terminal collision in the adjacent TransHudson Orogen. The geographic extent, inferred net shortening and tectonic setting of the Vulcan structure appear to resemble the modern Pyrenees belt, although the deep structure appears to be more akin to the Scandinavian Caledonides. Either scenario is consistent with an interpretation of the Vulcan structure as the Proterozoic collisional suture between the Wyoming and Hearne Provinces of the Laurentian craton.
Development of an Antarctic digital elevation model by integrating cartographic and remotely sensed data: A geographic information system based approach
Authors: Liu Hongxing , Jezek Kenneth C. , Li Biyan
Published: September 2002

We present a high‐resolution digital elevation model (DEM) of the Antarctic. It was created in a geographic information system (GIS) environment by integrating the best available topographic data from a variety of sources. Extensive GIS‐based error detection and correction operations ensured that our DEM is free of gross errors. The carefully designed interpolation algorithms for different types of source data and incorporation of surface morphologic information preserved and enhanced the fine surface structures present in the source data. The effective control of adverse edge effects and the use of the Hermite blending weight function in data merging minimized the discontinuities between different types of data, leading to a seamless and topographically consistent DEM throughout the Antarctic. This new DEM provides exceptional topographical details and represents a substantial improvement in horizontal resolution and vertical accuracy over the earlier, continental‐scale renditions, particularly in mountainous and coastal regions. It has a horizontal resolution of 200 m over the rugged mountains, 400 m in the coastal regions, and approximately 5 km in the interior. The vertical accuracy of the DEM is estimated at about 100–130 m over the rugged mountainous area, better than 2 m for the ice shelves, better than 15 m for the interior ice sheet, and about 35 m for the steeper ice sheet perimeter. The Antarctic DEM can be obtained from the authors.
Orogeny and orography: The effects of erosion on the structure of mountain belts
Authors: Willett Sean D.
Published: September 2002

A numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes. Crustal deformation is calculated by a two‐dimensional finite element model of deformation in response to subduction and accretion of continental crust. Erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power. The relative importance of the tectonic and erosion processes is given by a dimensionless “erosion number” relating convergence velocity, rock erodibility, and precipitation rate. This number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt. Fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction. These characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt. The two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity. Models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior. Models of the latter case predict a focused zone of exhumation at the margin of the orogen and, at high erosion number, a reversal in the topographic asymmetry. Natural examples of these two cases are presented. The Southern Alps of New Zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate. The asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry. The contrasting example of dominant wind in the direction of subduction motion is provided by the Olympic Mountains of Washington State. In this case, exhumation of deep levels of the Cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation.
Widespread extension in the core of the western Alps revealed by earthquake analysis
Authors: Sue Christian , Thouvenot François , Fréchet Julien , Tricart Pierre
Published: September 2002

The western Alps are an active collision belt whose current stress field is inhomogeneous [Müller et al., 1992]. We report new seismological data which significantly improve our knowledge of this stress field. About 1600 earthquakes which occurred in the western Alps during the last 10 years were precisely located, and 79 new focal solutions were computed. The analysis of this database shows that widespread extension affects all the internal zones of the belt. To better constrain the associated stress regime, six stress tensors were computed using the Gephart and Forsyth [1984] method. They show that the current tectonics of the western Alps are contrasted with close variation in the stress regime (transpression to the front of the belt contrasting with extension in the core of the belt). The extensional direction is radial to the arcuate geometry of the belt and bounded outboard by the former thrust of the internal zones onto the external zone, suggesting extensional reactivation of this inherited crustal discontinuity. Such widespread extension within the inner part of an actually ongoing collision belt cannot be explained by simple collision‐related tectonics. We propose that intrabelt buoyancy forces, such as those produced by a slab retreat or slab break‐off, interfere with the boundary forces driven by the ongoing Europe‐Africa convergence.
Relative geomagnetic field intensity across the Jaramillo subchron in sediments from the California margin: Ocean Drilling Program Leg 167
Authors: Leonhardt R. , Heider F. , Hayashida A.
Published: September 2002

Two relative paleointensity records of the Jaramillo subchron were obtained during Ocean Drilling Program Leg 167 at Sites 1010 and 1021 on the California margin. The two sites are ∼1300 km apart and differ in sediment composition and deposition rate. The demagnetization of the natural remanent magnetizations (NRM) of discrete samples revealed a vertical drilling overprint at both sites which could be removed by alternating field demagnetization at 20 mT for the sediments from Site 1010. This was confirmed by a positive reversal test on the characteristic remanent magnetization of the discrete samples across the Jaramillo subchron. Rock magnetic criteria for determining relative paleointensities are fulfilled for the samples from Site 1010. Because of variations in the rockmagnetic properties, two short intervals had to be excluded from relative paleointensity determinations at Site 1021. Normalization of the NRM at several demagnetization steps with various magnetic parameters yielded similar results only for Site 1010, and contributions from viscous remanent magnetization could not be detected in the sediments from this site using the pseudo‐Thellier method of Tauxe et al. [1995]. The relative paleointensity data of Site 1021 show variations of up to 50% between normalizations with different magnetic parameters and therefore had to be excluded from further analysis. Several fine‐scale intensity features are detectable in the record of Hole 1010C, displaying a maximum in relative paleointensity shortly above the lower Jaramillo transition, a prominent local minimum in the middle of the subchron, and a gradual decrease toward the upper Jaramillo reversal. A comparison of this record from the California margin with several published records of relative paleointensity across the Jaramillo subchron with different sedimentation rates and sediment composition shows reasonable agreement between the paleointensity patterns but differences in amplitudes. This suggests an influence of postdepositional magnetization processes which were not removed during normalization.
Penetration of Antarctic subglacial lakes by VHF electromagnetic pulses: Information on the depth and electrical conductivity of basal water bodies
Authors: Gorman Michael R. , Siegert Martin J.
Published: September 2002

Owing to the high level of absorption of very high frequency radio waves in water, previous investigators of airborne radio echo sounding (RES) data from Antarctica have assumed that the depth of subglacial lakes cannot be measured directly by this method. However, we have identified a number of RES returns from beneath the ice‐water interface at the surface of eight subglacial lakes that we have interpreted as being reflected from the lake floor. The returns allow us to measure the depth of subglacial lakes, since the velocity of radio waves in water (33.4 m μs−1) is relatively unaffected by electrical conductivity. Attenuation of radio waves within water is controlled largely by its electrical conductivity. Consequently, by examining the decay of the radio wave amplitude with depth we can gain information about the conductivity of subglacial water bodies. Our results indicate that the minimum water depths of eight subglacial lakes vary between 8 and 21 m. The lakes from which our depth measurements were taken are distributed widely around the ice sheet. Thus it may be concluded for the first time that Antarctic subglacial water bodies are generally at least several meters in depth. By examining the attenuation of radio waves through subglacial water, the electrical conductivity of the water is estimated to be extremely low (i.e., fresh pure water).
Three‐dimensional P wave velocity structure in the New Madrid seismic zone
Authors: Vlahovic Gordana , Powell Christine A. , Chiu Jer‐Ming
Published: September 2002

A three‐dimensional P wave velocity model for the New Madrid seismic zone (NMSZ) has been developed using a nonlinear travel time tomography method. The inversion involved 709 earthquakes recorded by digital, three‐component Portable Array for Numerical Data Acquisition stations deployed in the NMSZ over the time period 1989 through 1992. Analysis of ray coverage and inversion of a synthetic data set showed that the model has high resolution to a depth of 11 km. Low‐velocity anomalies correlate with the prominent northwest and northeast trending arms of seismicity. Lowest‐velocity anomalies (−8%) occur at the intersection of these arms and are associated with shallow seismicity and a high swarm rate. These low‐velocity regions are interpreted to be the result of increased fluid pressure associated with structurally disrupted rocks beneath the Blytheville‐Pascola arch complex. Small regions of high velocity, limited to the upper few km of the crust, correlate with gravity and magnetic anomalies and are interpreted to be igneous intrusions. High‐velocity anomalies also parallel the edges of the Reelfoot rift margin and probably are associated with crystalline Precambrian rocks that form the margin of the Reelfoot rift graben. Most earthquakes are associated with low‐velocity regions and avoid regions of high velocity.
Stresses in a subducting slab in the presence of a metastable olivine wedge
Authors: Philippe Devaux Jean , Fleitout Luce , Schubert Gerald , Anderson Charles
Published: September 2002

The transformation of olivine to spinel in a subducting slab gives rise to thermal strain, transformation strain, and buoyancy forces inside the slab. We use a finite element method to solve the mechanical equation, and we follow the evolution of the stresses inside the slab during its descent. We use two different rheologies: elastic and viscoelastic with a viscosity that depends on pressure, temperature, and grain size. The stresses due to buoyancy are found to be negligible compared to the internal stresses due to the phase transformation. When the viscoelastic rheology is used, a zone of very high differential stress coincides with the olivine‐spinel phase boundary. This zone is associated with large downdip compressive stresses inside the metastable olivine wedge, and large downdip tensile stresses in the spinel region, along the wedge. If we assume that the failure criterion can be related to the differential stress, our model is in agreement with the patterns of deep seismicity. However, patterns of deep seismicity are unlikely to reflect buoyancy stresses, which in the presence of a metastable olivine wedge, are negligible compared to the internal stresses.