bluuebunny/crossref_metadata_2025 huggingface dataset:
Deep structure of the ocean‐continent transition in the southern Iberia Abyssal Plain from seismic refraction profiles: Ocean Drilling Program (Legs 149 and 173) transect
Authors: Chian Deping , Louden Keith E. , Minshull Tim A. , Whitmarsh Robert B.
Published: September 2002

We present a wide‐angle seismic refraction study of an 80×40 km region of the southern Iberia Abyssal Plain, south of Galicia Bank. An intersecting grid of two E‐W and four N‐S wide‐angle reflection/refraction profiles is used to define variations of the basement velocity structure within this unusually wide ocean‐continent transition (OCT). These structures can be systematically linked to variations in acoustic basement morphology and to results from Ocean Drilling Program (ODP) boreholes. Lateral changes in the velocity structure of the basement occur abruptly over distances of ∼20 km where complex variations may be found. Thinned upper continental crust, 2–5 km thick with velocities of 5.0–6.6 km/s, is limited to a series of N‐S fault blocks immediately south of Galicia Bank. This crust is underlain by a high‐velocity layer (7.3–7.9 km/s) of weakly serpentinized (i.e., 0–25%) peridotite, which exists throughout the eastern part of the survey area. Basement within the OCT appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally. In the southeast sector of our survey where basement topography deepens and becomes subdued, continental fault blocks are absent; instead, basement contains an upper layer of more pervasively serpentinized (i.e., 25–45%) peridotite that is ∼2 km thick. This layer is characterized by low velocity at the top of basement (4.2 km/s) that increases rapidly with depth, and it probably corresponds to a seismically unreflective layer, previously identified in reflection profiles to the south of our survey. In the western section of our survey, beneath a series of elevated basement ridges, velocities are reduced within both the upper basement layer (3.5–6.0 km/s) and lower layer (6.4–7.5 km/s). These changes suggest that both upper and lower layers have become more highly serpentinized (with values of 60–100% in the upper layer and 25–45% in the lower layer) probably during the last stages of rifting and immediately before formation of oceanic crust. A normal or slow spreading oceanic crustal structure is not found within the survey region. Thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at ODP Site 897 and east of the J magnetic anomaly.
How strong is an asperity?
Authors: Sammis Charles G. , Nadeau Robert M. , Johnson Lane R.
Published: September 2002

A recent study of repeating earthquakes on the San Andreas Fault in central California by Nadeau and Johnson [1998] found that the smallest events occurred on patches having a linear dimension of the order of 0.5 m, displacements of about 2 cm, and stress drops of the order of 2000 MPa, roughly 10 times larger than rock strengths measured in the laboratory. The stress drop for larger events was observed to decrease as a power law of the seismic moment reaching the commonly observed value of 10 MPa at about magnitude 6. These large strengths are shown here to be consistent with laboratory data if the preexisting microcracks are all healed. A hierarchical fractal asperity model is presented, which is based on recent laboratory observations of contact distributions in sliding friction experiments. This “Cantor dust” model is shown to be consistent with the observed power law decrease in stress drop and increase in displacement with increasing event size. The spatial distribution of hypocenters in the Parkfield area is shown to be consistent with this simple fractal model and with a hierarchical clustering of asperities having a fractal dimension of D=1 and discrete rescaling factor of about 20.
Wide‐angle seismic transect across the Torngat Orogen, northern Labrador: Evidence for a Proterozoic crustal root
Authors: Funck Thomas , Louden Keith E.
Published: September 2002

A refraction/wide‐angle reflection seismic transect across the Labrador peninsula covers the Core Zone of the SE Churchill Province, the Paleoproterozoic Torngat Orogen, and the Archean Nain Province including a portion of the Labrador continental margin. An airgun array was used as source, and 11 ocean‐bottom seismometers and 16 land stations recorded the shots. Forward modeling of travel times and amplitudes reveals a deep asymmetric crustal root beneath the Torngat Orogen, with a crustal thickness of >49 km and with P‐wave velocities of 6.9–7.0 km/s. The geometry of the velocity model and the root can be best explained by either westward dipping subduction or westward underthrusting of the Nain crust. Gravity modeling suggests a correlation of the crustal root with a gravity low that extends ∼100 km in an E‐W direction and ∼200 km from north to south. The preservation of the crustal root is attributed to the lack of postorogenic heating and ductile reworking consistent with the lack of abundant postcollisional magmatism in the SE Churchill Province. A discontinuity possibly cutting through the entire crust is interpreted as a zone of major transcurrent shearing associated with the main phase of deformation. West of the Torngat Orogen, the crustal thickness in the Core Zone of the Churchill Province varies between 35 and 38 km (P‐wave velocities of 5.8–7.0 km/s). East of the orogen, the crystalline crust in the Nain Province is ∼38 km thick (velocities from 5.8 to 6.9 km/s) but thins to 9 km in the shelf area of the Labrador margin, where it is covered with up to 8 km of sediments. No high‐velocity zone was found beneath the thinned continental crust at the margin.
Nature of radio echo layering in the Antarctic Ice Sheet detected by a two‐frequency experiment
Authors: Fujita Shuji , Maeno Hideo , Uratsuka Seiho , Furukawa Teruo , Mae Shinji , Fujii Yoshiyuki , Watanabe Okitsugu
Published: September 2002

A two‐frequency radio echo sounding experiment was carried out at Dome Fuji, the second highest dome in East Antarctica, and along a 1150‐km‐long traverse line from the dome to the coast. The goal was to determine the dominant causes of the radio echo internal reflections and to investigate their possible changes with depth ranges and regions. From the two‐frequency (60 MHz and 179 MHz) radio echo responses at various sites, we distinguished four zones. Each of the zones is characterized by a dominant cause of radio echo internal reflection as follows. In the “PD” zone, changes in dielectric permittivity are mainly due to density fluctuations; in the “PCOF” zone, changes in dielectric permittivity are mainly due to changes in crystal‐orientation fabrics; and in the “CA” zone, changes in electrical conductivity are mainly due to changes in acidity induced by past volcanic eruptions. In each of these three zones, the changes occur commonly along isochrones. In addition, a basal echo‐free zone, the fourth zone, was found to appear always below the PCOF zone. These four zones and their distribution suggested variations of the physical conditions within the ice sheet.
A detailed record of paleomagnetic field change from Searles Lake, California: 2. The Gauss/Matuyama polarity reversal
Authors: Glen Jonathan M. G. , Coe Robert S. , Liddicoat Joseph C.
Published: September 2002

This new study of the Gauss/Matuyama transition from Searles Lake, California conjoined with other records from the western United States, provides interesting insights into the structure of the reversing magnetic field. The present study employs improved measurement and data reduction techniques, multiple parallel strings of samples, and a finer sampling interval than was used in the original study by Liddicoat[1982]. Particularly crucial to this investigation was the use of overprint directions to reconstruct declinations, required because the core was rotary drilled. The results of this technique were corroborated by employing an independent method that uses anisotropy of magnetic susceptibility to resolve a sediment fabric: the fabric facilitated the alignment of core segments. The new record reveals that the main swing of the transition occurs over a significantly shorter time span than was found in the original study. In addition, it brings out several small scale variations that were absent in the old record, some of which take the form of relatively rapid jumps in direction that punctuate more steadily varying changes. This alternating steady and rapid field change is similar to behavior observed in volcanic records, which argues that such behavior is not merely an artifact of episodic volcanism. The Searles Lake record is strongly nonzonal and is defined in the Northern Hemisphere by a swath of virtual geomagnetic poles (VGPs) stretching from northern Eurasia to west Africa and to the northwest Pacific. Glen et al. [1994] showed that a collection (spanning >15 Myr) of western North American transition and excursion records displays this same pattern, indicating that the VGP swath is a persistent feature of the transitional field. In addition, the compilation reveals that the swath extends into the Southern Hemisphere, outlining a region marked by an absence of poles that is centered on the Indian Ocean. The fact that this pattern is offset from a similar one seen in global compilations suggests that the persistent fields have a significant nondipolar component. Seven additional records are now available, making the western North America data set perhaps the finest regional set of high‐resolution records consisting of both igneous and sedimentary records. The new records, which provide an important test of the existence of the VGP pattern, strongly support the findings that reveal the presence of persistent, long‐term (>15 Myr) nondipolar transitional fields.
Continuous near‐bottom gravity measurements made with a BGM‐3 gravimeter in DSV Alvin on the East Pacific Rise crest near 9°31′N and 9°50′N
Authors: Cochran James R. , Fornari Daniel J. , Coakley Bernard J. , Herr Randall , Tivey Maurice A.
Published: September 2002

A Bell BGM‐3 gravimeter has been used to collect continuous, underway, nearbottom (3‐ to 10‐m altitude) gravity measurements from the deep‐diving submersible DSV Alvin during surveys on the East Pacific Rise (EPR) crest near 9°31′N and 9°50′N. Closely spaced (20‐to 30‐m) gravity measurements were made along transects up to 8 km long in both regions. Repeatability of measurements made at the same location on different dives is ∼0.3 mGal. Along‐track spatial resolution of anomalies is ∼130–160 m, with the limiting factors being precision and sampling rate of the pressure gauge depth data used to calculate vertical accelerations of the submersible. The average upper crustal density of the ridge crest determined from the relationship between depth and free‐water gravity anomalies varies greatly between 9°31′N and 9°50′N. Average upper crustal densities of 2410 kg/m3 for the 9°50′N area and 2690 kg/m3 for the 9°31′N area were calculated. The different densities are not due to differing geometry of the Layer 2A‐2B boundary or a regional cross‐axis gravity gradient. Differences in porosity of the shallow crustal rocks, or a difference in the proportion of low density extrusives to higher‐density dikes and sills within Layer 2A in these two areas, are the likely causes of the different upper crustal densities. Bouguer gravity anomalies near the EPR axis are primarily small amplitude (0.5–2 mGal), are a few hundred meters across, and appear to be lineated parallel to the axis. Larger‐amplitude Bouguer anomalies of up to 4 mGal were found at a few locations across the crestal plateau and are associated with pillow ridges composed of lavas which are clearly younger than the surrounding seafloor. These ridges have distinct chemical compositions compared to lavas from the axial summit collapse trough (ASCT) at the same latitude. Probable sources of the 0.5‐ to 2‐mGal anomalies observed on the summit plateau include areas of collapsed and fissured terrain and dike swarms feeding melt through Layer 2A to the surface. A grid survey of the ridge axis near 9°50′N shows Bouguer anomalies lineated along the axis, suggesting that dike swarms do contribute to the observed Bouguer anomalies. The along‐axis continuity of the gravity anomalies is disrupted at a 75‐m offset of the ASCT, suggesting that shallow feeders of lava to the surface may be segmented on a finer scale than the deeper crustal magmatic system. This initial study confirms the ability to conduct high‐resolution, near‐bottom, continuous gravity measurements from Alvin. It also provides important information on how the shallow crustal structure of a fast spreading mid‐ocean ridge develops and how it varies with the surface morphology.
High‐frequency wave propagation in the uppermost mantle
Authors: Ryberg T. , Wenzel F.
Published: September 2002

Short‐period, three‐component recordings of the seismic wave field of peaceful nuclear explosions recorded on deep seismic sounding profiles (Quartz and Ruby‐I, collected in 1984 and 1988, respectively) in northern Russia are used to constrain the nature of the high‐frequency teleseismic Pn phase, which can be observed for receiver distances of >3000 km. We suggest that this phase is caused by velocity fluctuations in the upper mantle acting as scatterers. To test this hypothesis and to determine the properties of the upper mantle scatterers, the elastic reflectivity method (one‐dimensional isotropic models) was used to model the coda of the high‐frequency teleseismic Pn phase. Both the observed data and the synthetic record sections were analyzed by examining the coda decay rates of the teleseismic Pn phase at 5 Hz. Synthetic seismograms are computed for different models based on the global model IASP91 with an added zone of randomly distributed velocity fluctuations (lamellae) just below the crust‐mantle boundary. These models may be characterized by the thickness of the scattering layer L, the vertical heterogeneity correlation length a, and the heterogeneity standard deviation σ. The numerical simulation of the wave propagation in these models generated a high‐frequency teleseismic Pn phase with a well‐pronounced, long coda. A comparison of the modeling results with the observations shows clearly that a velocity model containing fluctuations in the upper mantle can easily explain the presence of a high‐frequency teleseismic Pn. Comparing the coda decay rates of synthetic seismograms with the observation, we tried to constrain the properties of the velocity fluctuations in the upper mantle, L, a, and σ. In the distance range of the uninterfered high‐frequency teleseismic Pn phase beyond 1300 km, the coda decay rates for our best fitting model are similar to the observed ones. This model has a 75 km thick zone of scatterers below the Moho, containing lamellae with an average thickness of 2 km and a RMS velocity perturbation of 5%. The modeling results show also that two other possible models, the whispering gallery phase along the crust‐mantle boundary and scattering in the lower crust, taken alone are not able to explain the coda properties of the high‐frequency teleseismic Pn phase.
Paleomagnetism of an east‐west transect across the Cascade arc in southern Washington: Implications for regional tectonism
Authors: Hagstrum Jonathan T. , Swanson Donald A. , Evarts Russell C.
Published: September 2002

Paleomagnetic data from a transect across the Cascade arc in southern Washington were collected to test models of vertical axis rotations for this part of the Pacific Northwest and to provide information on the tectonic history of the St. Helens seismic zone. The 75 site‐mean directions are divided into three groups based on isotopic, fission track, and stratigraphic ages. Group 1 consists of samples from rocks deposited between 39 and 30 Ma and shows a mean clockwise vertical axis rotation of 34° ± 13°, group 2 consists of samples from rocks deposited between 30 and 24 Ma and shows a mean clockwise rotation of 17° ± 11°, and group 3 consists of samples from rocks between 24 and 16 Ma and shows a mean clockwise rotation of 20° ± 12°. Although these three values of rotation are statistically indistinguishable at the 95% confidence level, we interpret them in combination with other data to indicate differential rotation across the St. Helens seismic zone (SHZ). The available paleomagnetic data for Eocene‐Oligocene rocks west of the seismic zone show clockwise vertical axis rotations of 30° ± 8° and 35° ± 9° consistent with the value for group 1. The rotational values for groups 2 and 3 and the intrusive suite of Kidd Creek (13 Ma), despite their different ages, have similar values east of the SHZ. Comparing these groups of data east and west of the seismic zone indicates a differential rotation of 10° ± 3° across it. In addition, the rates of rotation in southern Washington are similar to those for rocks in the western Cascades of Oregon and indicate that rotational deformation of Miocene age extends northward into southern Washington and eastward into the axis of the arc.
Groundwater flow and the 4He distribution in the Great Artesian Basin of Australia
Authors: Bethke Craig M. , Zhao Xiang , Torgersen Thomas
Published: September 2002

The relationship between the distribution of 4He and the pattern of groundwater flow within the Jurassic aquifer of the Great Artesian Basin has been a source of controversy. According to one explanation, a large and possibly heterogeneous flux of helium passes from the crystalline crust into basin strata, accumulating in groundwater flowing through the aquifer. A second explanation holds that contrary to current understanding of the basin's hydrology, groundwater in the aquifer is stagnant, allowing helium produced in situ to accumulate over geologic time. We use a numerical model to study the relationship between groundwater flow and helium transport in the basin. The calculations assume a moderate and homogenous flux of helium and uniform hydraulic conductivities of sandstone (aquifers) and fine‐grained sediments (aquitards). We show that the 4He concentration along the top of the Jurassic aquifer is controlled not only by the lateral flow of groundwater but by its descent and upwelling. Our simulations reproduce the pattern of 4He content across the basin, showing that the basin's isotope distribution and groundwater hydrology can be reconciled in a quantitative sense without calling on large or uneven helium fluxes or on stagnant flow conditions. These results further show that isotopic study can give important information about three‐dimensional patterns of groundwater flow within sedimentary basins.
Structure and evolution of the continental crust of northern east Greenland from integrated geophysical studies
Authors: Schlindwein Vera , Jokat Wilfried
Published: September 2002

East Greenland's continental crust is little explored compared to the geologically related Norway. As part of a comprehensive geophysical study of east Greenland, the Alfred Wegener Institute for Polar and Marine Research acquired eight seismic refraction profiles between 70°N and 76°N. We present the results of four profiles combined with magnetic and geological data. The data set revealed pronounced differences in the crustal architecture north and south of Kong Oscar Fjord (72°N). The crust north of Kong Oscar Fjord shows three structural units, a Caledonian, a Devonian, and a Mesozoic to Tertiary, whereas only a Caledonian and a Devonian to Tertiary unit can be distinguished in the southern area. A seismic high‐velocity layer was discovered in the lower crust north of Kong Oscar Fjord which is absent in the southern area. This high‐velocity layer coincides with a pronounced magnetic anomaly related to Tertiary magmatism and is interpreted as Tertiary magmatic underplate. By integrating geophysical and geological data, we showed that a different evolution of the areas north and south of Kong Oscar Fjord started after the Devonian extensional collapse of the Caledonides. Mesozoic extension shifted to the east and preserved Devonian structures in the northern area, whereas the Devonian crust was further thinned and weakened in the southern area. We propose that it allowed an easier ascent of Tertiary melts to the surface than the crust north of Kong Oscar Fjord, where most of the melts got trapped in the lower crust. The model explains the distribution of extrusives and intrusives in east Greenland's Tertiary igneous province.
Geomagnetic field intensity at Hawaii for the last 420 kyr from the Hawaii Scientific Drilling Project core, Big Island, Hawaii
Authors: Laj Carlo , Kissel Catherine
Published: September 2002

Four hundred twenty five new paleointensity (Thellier‐Thellier) determinations (out of 545 analyzed samples) have been obtained from core HSDP, which penetrates about 1000 meters (208 flows) of the Mauna Loa and Mauna Kea volcanic series encompassing the last 420 kyr. Rock magnetic investigations identify pseudo‐single‐domain magnetite as the main magnetic mineral. Inclinations are shallower than expected from a geocentric dipole field but are consistent with data from other geographical regions at the same latitude. The inclination record reveals three episodes of negative inclination whose interpolated age correlates well with that of known geomagnetic events. The paleointensity record from the Mauna Loa sequence is not very detailed and does not allow precise comparison with other data in the 0–50 kyr interval. The record from the Mauna Kea sequence, on the contrary, is very detailed and documents relatively short‐lived episodes of low and high field strength from 15 to 60 μT. The average virtual dipole moment (8.7±3.0 1022 A.m2) is not significantly different from the value reported by Kono and Tanaka [1995] for the last 2.5 Myr. A comparison with other data from Hawaii and other geographical regions is described in detail. There are no drastic changes in paleointensity with the inclination anomaly, in agreement with previous results from Hawaii but in contrast with most published results which, however, consider data from polarity transition. Spectral analysis of a particularly detailed portion of the record, between 420 and 326 kyr, documents significant periodicities at 36, 8, 5, and 4 ka in the inclination record but not in the intensity record, suggesting that changes in time of the inclination are to a certain extent independent from those of the intensity.
A global geoid model with imposed plate velocities and partial layering
Authors: Čadek Ondřej , Fleitout Luce
Published: September 2002

Most inversions of the long‐wavelength geoid in conjunction with the seismic tomographic information have so far been carried out under the assumption of either purely whole mantle or perfectly layered circulation. Moreover, modeling the lithosphere as a spherical shell with a uniform low viscosity was found to yield the best fit to the observed geoid. We have tested whether a good prediction of the geoid can also be achieved by including two constraints: a semipermeable behavior of the 660‐km discontinuity and surface plate velocities equal to the observed ones. The mass transfer between upper and lower mantle has been changed by imposing a surface density anomaly at a depth of 660 km, which is proportional to the mass anomaly needed to achieve perfectly layered circulation. On the top of the mantle we assume a stiff lithosphere that moves with a velocity corresponding to the observed plate motion. The viscosity only varies with depth. Considering a simple three‐layer viscosity structure and changing the permeability of the 660‐km interface, we have obtained a satisfactory variance reduction of the geoid data (∼75% for degrees 2–12). The best fit to the geoid is obtained if the mass transfer across the 660‐km boundary is reduced to one third in comparison with the purely whole mantle model. The best fitting viscosity profile is characterized by a clearly defined asthenosphere and a viscosity increase by at least 2 orders of magnitude in the lower mantle. The amplitudes of dynamic topography predicted by our model are remarkably small (∼100 m), thus fully compatible with the observation.
Paleoseismic studies of the San Francisco Peninsula segment of the San Andreas Fault zone near Woodside, California
Authors: Hall N. Timothy , Wright Robert H. , Clahan Kevin B.
Published: September 2002

A late Holocene slip rate is proposed for the San Francisco Peninsula segment of the San Andreas fault zone at the Filoli Center, located between the town of Woodside and Upper Crystal Springs Reservoir, California. At this site the primary trace of the San Andreas fault zone traverses N35°W across the head of a large alluvial fan, offsetting buried stream channel deposits. One deposit, with a minimum calibrated accelerator mass spectrometry (AMS) radiocarbon age of 2070±120 years B.P., is offset 30±2 m, yielding an average geologic slip rate of 17±4 mm yr−1for the late Holocene. A stratigraphically lower channel sequence, with a minimum calibrated AMS radiocarbon age of 330±200 years B.P., is offset 4.1±0.5 m. We interpret that this older sequence records both the penultimate earthquake on the peninsula segment, accompanied by 1.6±0.7 m of dextral slip, and the ∼2.5 m of slip that occurred in 1906. On the basis of radiocarbon analyses and historical descriptions we propose that the pre‐1906 earthquake occurred in June 1838. Our interpretations suggest that the San Francisco Peninsula segment of the San Andreas fault zone may rupture in earthquakes of at least two magnitudes: (1) a large‐magnitude (≈Mw8) event with a recurrence of ∼250–320 years that involves the North Coast, San Francisco Peninsula, and Santa Cruz Mountains segments, as occurred in 1906 and (2) aMw7.0–7.4, 1838‐type event that ruptures only the San Francisco Peninsula segment. The apparent difference in slip rate of 7 ± 7 mm ‐yr−1between the North Coast segment (24±3 mm yr−1) and the peninsula segment (17±4 mm yr−1) suggests a possible slip rate gradient on the northern California section of the San Andreas fault.
Constructing the upper crust of the Mid‐Atlantic Ridge: A reinterpretation based on the Puna Ridge, Kilauea Volcano
Authors: Smith Deborah K. , Cann Johnson R.
Published: September 2002

The volcanic morphology of a number of segments of the slow spreading Mid‐Atlantic Ridge (MAR) have been reinterpreted based on our understanding of dike emplacement, dike propagation, and eruption at the East Rift Zone of Kilauea Volcano, Hawaii and its submarine extension, the Puna Ridge. The styles of volcanic eruption at the submarine Puna Ridge are remarkably similar to those of the axial volcanic ridges (AVRs) constructed on the median valley floor of the MAR. We use this observation to relate volcanic processes occurring at Kilauea Volcano to the MAR. We now consider that volcanic features (e.g., seamounts and lava terraces) built on the flanks of the AVRs are secondary features that are fed from lava tubes or channels, not primary features fed directly from an underlying dike. We examine simple models of pipe flow and conclude that lava tubes can transport lava down the flanks of submarine rifts to build all of the volcanic features observed there. In addition, deep water lava tubes are strong enough to withstand the pressures of a few megapascals that the building of a volcanic structure 150 m high at the end of the tube would generate. The volumes of individual volcanic terraces and seamounts on the Puna Ridge and at the MAR are large (0.1–1 km3) and similar to the volumes of lava flows that are broadly distributed at the subaerial East Rift Zone of Kilauea. This striking difference in the volcanic morphology on a scale of 1–2 km (producing terraces and seamounts underwater and low‐relief flows on land) must be related to the enhanced cooling and to the greater mechanical stability of tubes in the submarine environment. We suggest that at the MAR a crustal magma reservoir, most likely located beneath shallow, flat sections of the segment, provides magma to the rift axis through dikes that propagate laterally tens of kilometers. The zone of dike intrusion, at least in the neighborhood of the magma body, is likely narrower than the width resurfaced by flows, yielding a crustal structure that has a rapid vertical transition from lavas to sheeted dikes. At segment ends the zone of dike intrusion is likely to be wider, giving a resulting structure with a more gradual transition from lavas to dikes.
Mafic dike swarms in the South Shetland Islands volcanic arc: Unravelling multiepisodic magmatism related to subduction and continental rifting
Authors: Willan Robert C. R. , Kelley Simon P.
Published: September 2002

Eight groups of mafic dikes and related high‐level stocks cut Triassic accretionary complex and Mesozoic magmatic arc formations on Livingston Island. Some are affected by silicic/sericitic alteration, related to Cretaceous hydrothermal activity, and propylitic/epidosite alteration, analogous to that in ocean floor sheeted dikes. Alteration was accompanied by major and trace element metasomatism. Ar‐Ar analysis of the freshest rocks indicates five intrusive events, some of which are unexpectedly young. Groups 1–3 were intruded in the mid to late Cretaceous (∼108–74 Ma) and were coeval with the calc‐alkaline arc. Between 70 and 50 Ma, relatively rapid and oblique plate convergence led to strike‐slip tectonism and a pause in magmatism. At ∼52 Ma, orthogonal, slow convergence resulted in extensional faulting and emplacement of calc‐alkaline (group 2) and primitive tholeiitic dikes (groups 4–6) between 51 and 45 Ma. Extension of Antarctic Peninsula‐southern South American crust culminated in emplacement of mafic to intermediate, medium‐grained plutons and group C porphyries between 44 and 36 Ma. Localized hydrothermal flow along fault zones resulted in partial to complete argon loss from nearby Cretaceous lavas and Ar‐Ar reset ages of ∼40 Ma in mid‐Cretaceous hydrothermal K‐feldspar. Primitive olivine basalts (group D) and epithermal carbonate veins (31–29 Ma) were emplaced during along‐arc extension accompanying the opening of Drake Passage and Powell Basin. Excess argon occurs in two forms: strongly held in melt? inclusions in the primitive tholeiites and weakly held in some secondary alteration. There is no radiometric evidence, in the area studied, for magmatism related to late Cenozoic subduction, nor to the Pleistocene‐Recent opening of the back arc Bransfield rift.
Bouvet Triple Junction in the South Atlantic: Geology and evolution
Authors: Ligi Marco , Bonatti Enrico , Bortoluzzi Giovanni , Carrara Gabriela , Fabretti Paola , Gilod Dolores , Peyve Alexander A. , Skolotnev Sergej , Turko Natalia
Published: September 2002

The South American, African, and Antarctic lithospheric plates meet in the Bouvet Triple Junction (BTJ) located in the South Atlantic near the island of Bouvet. Multibeam, magnetic, gravimetric, and seismic reflection data have been used to understand the evolution of the three accretionary/transform boundaries that converge in the BTJ. The easternmost segments of the American‐Antarctic Ridge (AAR) have a spreading full rate of 19.5 mm/yr for the last 8 m.y. They are oriented N‐S, except for some NE‐SW segments, probably created by magma‐poor extension. The southernmost portion of Mid‐Atlantic Ridge (MAR) (spreading full rate of 30.5 mm/yr for the last 9 m.y.) has elevated topographic anomalies; it is segmented by transform and overlapping discontinuities and shows evidence of axial propagation. The MAR axial valley bifurcates at its southern tip in two branches oriented 115° and 180° that are, or have been up to recently, loci of crustal accretion. The bifurcation represents a former ridge‐ridge‐ridge (RRR) triple junction. The westernmost segments of the Southwest Indian Ridge (SWIR) are anomalously high. The segment adjacent to the island of Bouvet (spreading rate 14.5 mm/yr) is shallower than normal by almost 1 km due to the influence of the Bouvet hot spot. The westernmost SWIR segment (Spiess Ridge) consists of a “swollen” volcanic ridge that reaches 320 m below sea level and has a deep caldera on its summit. Spiess Ridge narrows and deepens to the NW; V‐shaped topographic and magnetic lineations suggest that it propagates NW at a rate of 40 to 50 mm/yr. The Spiess magmatic event started at roughly 1 Ma, when it caused deactivation of the 115° spreading branch. Therefore the Antarctic, South American, and African plates meet presently not in a triple point but in a broad zone of diffuse deformation. An area of extensional deformation observed east of Spiess Ridge may be caused by excess crustal formation at Spiess Ridge that cannot be accommodated by motion of rigid plates. The evolution of the BTJ since 10 Ma involves stages of RRR, RFF and RRF configurations with highly variable geometry of the accretionary/transform boundaries. Topographic anomalies, anomalously thick crust and excess volcanism suggest that the upper mantle below this region is affected by widespread, strong thermal anomalies that have influenced the configuration of the BTJ, and determined indirectly intraplate deformation in wide areas of the BTJ region. The thermal anomaly that gave rise to the SWIR‐Spiess excess magmatism is the prime cause of the recent disruption of a former RRR configuration, and of the imminent establishment of a new RRR Triple Junction to the north.
Crustal deformation during 1994–1998 due to oblique continental collision in the central Southern Alps, New Zealand, and implications for seismic potential of the Alpine fault
Authors: Beavan John , Moore Margaret , Pearson Chris , Henderson Mark , Parsons Barry , Bourne Stephen , England Philip , Walcott Dick , Blick Graeme , Darby Desmond , Hodgkinson Kathleen
Published: September 2002

The positions of 115 ground marks in a 150 × 100 km area of oblique continental collision in the central Southern Alps, New Zealand, have been measured by Global Positioning System (GPS) two to four times between 1994 and 1998. Contemporary velocity and strain rate fields derived from these observations are largely invariant along the northeasterly strike of the mountains and Alpine fault. Across strike, more than 60% of the strain occurs within a band from 5 km NW to 20 km SE of the Alpine fault, but significant strain continues at least a further 60 km SE to near the edge of the Southern Alps foothills. Projections of the fault‐parallel and fault‐normal components of velocity onto an Alpine faultnormal profile show that about 85% of the NUVEL‐1A model relative plate motion is observed within the GPS network. The surface displacements in the high strain rate region are well fit by a model in which stable slip or shearing is occurring at 50–70% of the relative plate rate in a region deeper than about 5–8 km on the down‐dip extension of the SE dipping Alpine fault. Material shallower than this is behaving elastically and thus storing elastic strain in the region of the Alpine fault. The longer‐wavelength displacements can be modeled either as distributed deformation beneath the Southern Alps, or by localization of elastic strain around the upper end of a discrete NW dipping fault or shear zone that is slipping stably below about 30 km depth and would outcrop near the SE boundary of the mountains if extrapolated to the surface. Strain determined from a small‐scale survey network crossing the Alpine fault indicates no significant near‐surface aseismic fault slip on the central Alpine fault over the past 25 years. Our results are consistent with independent geological evidence that the central section of the Alpine fault is capable of producing large to great earthquakes.
Transitional field behavior during the Gilbert‐Gauss and Lower Mammoth reversals recorded in lavas from the Wai'anae volcano, O'ahu, Hawaii
Authors: Herrero‐Bervera Emilio , Coe Robert S.
Published: September 2002

Using a portable fluxgate magnetometer, we identified two polarity transitions, one a reversed to normal (R‐N) in a 600‐m section called Pu'u Heleakala and second a normal to reversed (N‐R) in a 300‐m section known as Pu'u Kamai'leunu of superposed basalt flows on the western flank of Wai'anae volcano, one of the two volcanoes that compose the Hawaiian island of O'ahu. Volcanic stratigraphic studies suggest that the R‐N reversal corresponds to the Gilbert‐Gauss polarity boundary and the N‐R reversal correlates with the Lower Mammoth polarity transition. We drilled an average of six to eight samples from each of the lava flows (36 flows from the Heleakala and 29 from the Kamai'leunu) spanning the two sections. Lightning‐induced secondary magnetization is common in some flows, requiring detailed alternating field demagnetization to remove it and to isolate the primary directions. Thermal demagnetization proved to be ineffective and, if employed, could lead to incorrect identification of lightning‐dominated remanence as primary. Our laboratory experiments indicate that we have successfully identified one transition zone on each section, composed of at least seven to eight transitional lava flows. The transitional VGPs of the Gilbert‐Gauss reversal are located in northwestern Africa, Sri Lanka, and Borneo, whereas for the Lower Mammoth reversal the transitional VGPs are located in northern Africa and in the southern Atlantic. These observations are not in agreement with the hypothesized antipodal longitudinal bands over the Americas and the Western Pacific Rim. The two transitions are characterized by periods of high negative inclinations corresponding to VGPs in Africa about 180° away from the site longitude. The obtained VGP paths could be explained by a transitional field affected by anomalous geomagnetic fields that exist beneath the Pacific hemisphere. These recurrent nonaxisymmetric components in the transitional fields suggest that lateral variations in lower mantle properties influence the geodynamo.
Micromechanical model for anisotropic rock joints
Authors: Misra Anil
Published: September 2002

Force‐deformation relationships of rock joints are important in the study of mechanical behavior of fractured or jointed rocks. Experimental studies have shown that rock joints often exhibit deformation hardening and anisotropic behavior under shearing loads. This paper focuses upon the mathematical modeling of force‐deformation behavior of anisotropic rock joints by explicitly considering interaction of asperities on a joint surface. Elastic deformations and inelastic frictional sliding are considered at inclined asperity contacts. A modified spherical harmonic expansion is introduced to model the orientation distribution of asperity contacts. The resultant joint force‐deformation model is used to obtain the normal and shear stiffness behavior of anisotropic joints. Results obtained from the model show that the asperity contact orientation distribution is essential for representing the anisotropic nature of interface geometry and for modeling deformation hardening of interfaces. The model results are compared with experimental data culled from the literature.
Geophysical contributions to satellite nodal residual variation
Authors: Chen J. L. , Wilson C. R. , Eanes R. J. , Tapley B. D.
Published: September 2002

Even‐degree zonal gravitational variations due to mass redistribution within the Earth's system, especially in the atmosphere, hydrosphere, and oceans will lead to variations in the nodal precession rate of satellite orbit. The accurately measured nodal variation for LAGEOS 1 provides a means to study planetary‐scale mass redistributions and gravitational variations from the space. In this paper, we investigate atmospheric and hydrologic contributions to the LAGEOS 1 nodal changes using barometric pressure, soil moisture, and snow accumulation values from data‐assimilating numerical models. Oceanic effects are estimated from nonsteric sea level change determined by TOPEX/Poseidon satellite radar altimeter observation and a simple model for steric sea level changes. The results are compared with the LAGEOS 1 nodal change time series observed by satellite laser ranging. At annual and semiannual time scales, the atmosphere and hydrosphere provide significant contributions. The atmosphere provides broadband excitation of nodal changes at intraseasonal timescales. Seasonal and intraseasonal nontidal oceanic effects are also significant. General agreement between predicted and observed nodal precession rate residuals is improved relative to earlier studies, in part because of the better estimation of hydrological effects and new assessment of nontidal oceanic effects.
Body wave inversion of the 1970 and 1963 South American large deep‐focus earthquakes
Authors: Estabrook Charles H.
Published: September 2002

A comprehensive set of teleseismic waveforms from two South American deep‐focus earthquakes of the predigital era, the 1970 Colombia (Mw = 8.1) and 1963 Peru‐Bolivia (Mw = 7.7) events, are inverted for source mechanism, seismic moment, rupture history, and centroid depth. The P and SH wave inversion of the Colombia event confirms previous work, indicating that rupture occurred on a plane that dips steeply west. Rupture direction paralleled the trend of the Wadati‐Benioff zone. We decompose the source into subevents, based on a source time function which shows two major moment release pulses separated by ∼20 s. The first subevent is located near the initiation point at a depth of ∼630 km. The main moment release was located ∼70 km to the southeast and ∼20 km shallower. Rupture subsequently propagated farther southeast. The source time function has an initial subevent accounting for ∼30% of the moment release of the entire event, whereas the long‐period centroid moment tensor (CMT) analysis [Russakoff et al., 1997] has the initial subevent yielding ∼50%. The high‐angle nodal plane rotated ∼15° clockwise during the rupture, explaining the large compensated linear vector dipole (CLVD) component inferred from CMT solutions. Individual subevents have large CLVD and compressive isotropic components. A full moment tensor inversion of the Colombia and 1994 Bolivia events suggests that the initial subevents might contain a large non‐double‐couple (NDC) component. For the 1963 Peru‐Bolivia event, using P waves, rupture propagated NNW for a distance of ∼70 km, parallel to the high‐angle nodal plane and the trend of the Wadati‐Benioff zone. The focal mechanism changed dramatically after the second subevent, causing a very large NDC component. Both events, together with the 1994 Bolivia earthquake, have a precursor separated in space and time from the main rupture and show rupture velocities varying between 3 and 4 km/s between subevents, with <2.0 km/s on average for the entire event. Low seismic efficiencies and rupture velocities support a highly dissipative, temperature‐dependent rupture mechanism for large deep‐focus South American earthquakes, compared with events in cold subducting slabs.
Formation age of Earth from 129I/127I and 244Pu/238U systematics and the missing Xe
Authors: Ozima M. , Podosek F. A.
Published: September 2002

The Earth's atmosphere contains excesses of 129Xe and 136Xe from decay of the short‐lived radionuclides 129I and 244Pu, respectively. The inferred abundances of these radionuclides are substantially less than those in meteorites, nominally indicating that Earth as a planet did not form until more than 100 Myr later than the solar system as a whole, an interval rather longer than that suggested by other lines of evidence. From the comparison of elemental abundance ratios between atmospheric (assumed to approximate the total terrestrial noble gas inventory) and solar noble gases, we conclude that ∼90% of Earth's primordial Xe is missing from the atmosphere. When account is taken of the radiogenic Xe that is likely missing as well, the 129I‐244Pu formation interval of Earth may be reduced significantly, down to about 50–70 Myr.
Influence of anelastic surface layers on postseismic thrust fault deformation
Authors: Lyzenga Gregory A. , Panero Wendy R. , Donnellan Andrea
Published: September 2002

We present the results of a systematic modeling study of postseismic deformation following blind thrust earthquakes. The results include qualitative and quantitative predictions of the surface movement caused by relaxation in viscoelastic near‐surface layers. Finite element forward models are used in conjunction with elastic dislocation inversions to characterize the post‐seismic deformation. A viscoelastic surface layer overlying a blind thrust fault in an elastic basement shows characteristic signatures of postseismic surface movement. Simple equivalent elastic dislocations located in the hanging wall wedge are found to provide an effective proxy for near‐surface postseismic relaxation in two‐dimensional numerical simulations. A model survey of a range of fault dip angles and layer geometries shows the time evolution and geometry of the proxy fault to be simply related to fault dip and sediment thickness. The results are of significance in the interpretation of postseismic Global Positioning System (GPS) strain data from the 1994 Northridge, California, earthquake and other similar events in regions characterized by poorly consolidated or otherwise anelastic layers overlying the brittle seismogenic zone.
Noble gases in the Cameroon line and the He, Ne, and Ar isotopic compositions of high μ (HIMU) mantle
Authors: Barfod Dan N. , Ballentine Chris J. , Halliday Alex N. , Fitton J. Godfrey
Published: September 2002

Ultramafic xenoliths, basaltic lavas, and CO2 gases from the Cameroon line volcanic chain provide the first characterization of combined He, Ne, and Ar isotopes in a high‐time‐integrated 238U/204Pb = μ (HIMU) magmatic system. Helium isotopic compositions typically range from 5.0 to 6.7 Ra, with an average of 6.3. These values are indistinguishable from the 3He/4He of other HIMU locales (Austral Islands, St. Helena). Neon isotopic compositions for xenoliths and CO2 gases are mid‐ocean ridge basalt (MORB)‐like, with a maximum 20Ne/22Ne of 11.87 and 21Ne/22Ne of 0.0508. Argon isotopic compositions in silicates range from atmospheric to 40Ar/36Ar = 4910±430 (crushing) and up to 16,300±1000 (single grain, laser step heating). The correlation between 20Ne/22Ne and 40Ar/36Ar in CO2 gases suggests a minimum 40Ar/36Ar = 1650±30 for the mantle‐derived component. Uniform 3He/4He in silicates and in CO2 fluids across both the continental and oceanic sectors of the Cameroon line argues strongly for a negligible lithospheric contribution to noble gas isotopic compositions. This inference is supported by high 238U/3He in lherzolites, indicating that noble gases in these samples must have been recently introduced (<50,000 years ago) to the sample, most likely from the host magma. Ocean crust recycling models of mixing between MORB source regions and highly radiogenic slabs cannot produce the observed He and Ne isotopic compositions. Isolation and aging of MORB source mantle can generate the isotopic compositions but require extreme 3He/22Ne fractionation. Involvement of plume‐derived gases, consistent with the lithophile element isotopic compositions, alleviates the need for strong 3He/22Ne fractionation. Closed‐system aging of plume‐derived heterogeneities can reproduce the data with minimum 3He/22Ne fractionation at reasonable 238U/3He ratios. However, diffusive exchange of He and to a lesser extent Ne between aged MORB source and aged plume veins could explain the occurrence of low 3He/4He compositions in all HIMU centers and the apparent low time‐integrated 3He/22Ne of the Cameroon line.