bluuebunny/crossref_metadata_2025 huggingface dataset:
Nucleogenic production of Ne isotopes in Earth's crust and upper mantle induced by alpha particles from the decay of U and Th
Authors: Leya Ingo , Wieler Rainer
Published: September 2002

The production of nucleogenic Ne in terrestrial crust and upper mantle by alpha particles from the decay of U and Th was calculated. The calculations are based on stopping powers for the chemical compounds and thin‐target cross sections. This approach is more rigorous than earlier studies using thick‐target yields for pure elements, since our results are independent of limiting assumptions about stopping‐power ratios. Alpha induced reactions account for >99% of the Ne production in the crust and for most of the 20,21Ne in the upper mantle. On the other hand, our 22Ne value for the upper mantle is a lower limit because the reaction 25Mg(n,α)22Ne is significant in mantle material. Production rates calculated here for hypothetical crustal and upper mantle material with average major element composition and homogeneously distributed F, U, and Th are up to 100 times higher than data presented by Kyser and Rison [1982] but agree within error limits with the results by Yatsevich and Honda [1997]. Production of nucleogenic Ne in “mean” crust and mantle is also given as a function of the weight fractions of O and F. The alpha dose is calculated by radiogenic 4He as well as by the more retentive fissiogenic 136Xe. U and Th is concentrated in certain accessory minerals. Since the ranges of alpha particles from the three decay chains are comparable to mineral dimensions, most nucleogenic Ne is produced in U‐ and Th‐rich minerals. Therefore nucleogenic Ne production in such accessories was also calculated. The calculated correlation between nucleogenic 21Ne and radiogenic 4He agrees well with experimental data for Earth's crust and accessories. Also, the calculated 22Ne/4He ratios as function of the F concentration and the dependence of 21Ne/22Ne from O/F for zircon and apatite agree with measurements.
Fluid flow in discrete joint sets: Field observations and numerical simulations
Authors: Taylor W. Lansing , Pollard David D. , Aydin Atilla
Published: September 2002

The distribution of flow within conductive joint sets is influenced by the geometric arrangement of joints and the hydraulic properties of both joints and matrix. We use finite element simulations with an equivalent porous media joint representation to understand the distribution of flow through joints and porous matrix. Isolated joints in a porous media create characteristic flow perturbations in the matrix with reduced fluid potentials near the upstream joint tip, elevated potentials near the downstream tip, and flow shadows adjacent to the joint. In more complex joint systems, flow in any given joint is influenced by its proximity to other joints, resulting in characteristic enhancement or reduction of flow velocities. The permeability ratio (equivalent joint permeability divided by matrix permeability) plays a major role in determining the distribution of flow within complex joint systems. When the permeability ratio is <3.0 orders of magnitude, all joints are indirectly connected to the flow system through the matrix. As joint conductivity increases, flow becomes increasingly localized into directly connected joints. When the permeability ratio exceeds 6.5 orders of magnitude, significant flow occurs only in the directly connected joints. We compare these numerical results with field observations from an ancient reactive flow system now exposed at the Earth's surface. In the field, 32% of joints are associated with chemically altered halos. By explicitly representing mapped joint distributions in numerical simulations, we estimate that 32% of the joints would conduct significant volumes of fluid if joint permeability is 5 orders of magnitude greater than the matrix permeability. This corresponds to an in situ joint aperture of 2.3 mm, closely resembling the 1.8‐mm average joint aperture measured on the present‐day outcrop.
Active stress map of Italy
Authors: Montone Paola , Amato Alessandro , Pondrelli Silvia
Published: September 2002

We present a new map of the present‐day stress field in Italy obtained from all the available data. The map reports 200 horizontal stress directions inferred from 109 borehole breakout data, 44 centroid moment tensor solutions, 34 other focal mechanisms, most of which are from polarity distributions, seven stress inversions of microearthquake data, two averages of T and P axes of earthquake focal mechanisms in zones of diffuse seismic activity, and four fault slip data. The integration of breakout data, which yield horizontal stress directions, with fault plane solutions, which reflect the stress regime, allows us to obtain an improved map of the present‐day stress in Italy. This stress field map can be used for a better comprehension of active tectonic processes, for seismic hazard assessment, and to foresee the behavior of faults recognized with other methods. Stress directions obtained from different data, although relative to different depth intervals (e.g., 0–7 km for breakouts and 0–20 km for most of the earthquakes) and to different tectonic units, are consistent. Since many regions in Italy are characterized by an extensional stress regime, we report the minimum horizontal stress (Shmin) orientations. The map shows that an extensional regime affects most of the Apenninic belt. Conversely, a compressional (or transpressional) regime characterizes the eastern Alps, the eastern side of the northern Apennines, and the southern Tyrrhenian to northern Sicily zone. An abrupt change in stress directions marks the transition between northern and southern Apennines, suggesting that the two arcs are characterized by a different tectonic setting and recent evolution. In this paper we report all the data analyzed to date, with their geographic coordinates and average stress directions, and we describe the main stress provinces in Italy in the framework of the tectonic evolution of the region.
Numerical simulations of depth‐dependent anisotropy and frequency‐dependent wave propagation effects
Authors: Rümpker Georg , Tommasi Andréa , Kendall J.‐Michael
Published: September 2002

A numerical investigation of the effects of shear wave splitting for vertical propagation in a smoothly varying anisotropic medium is presented. Through forward modeling, we predict the olivine lattice preferred orientation (LPO) developed in the oceanic upper mantle in response to the absolute plate motion (APM). We consider the effect of a change in APM similar to the one that presumably caused the kink in the Emperor‐Hawaii seamount island chain in the north Pacific. This results in an oblique orientation between lithospheric and asthenospheric anisotropy. Numerical simulations of shear wave propagation are used to estimate the characteristics of shear‐wave splitting. Ray theory does not account for coupling between shear waves in the depth‐dependent anisotropic medium due to the implicit assumption of high frequency. A forward propagator technique for calculating waveforms and splitting parameters is used to assess frequency‐dependent effects. The results show that ray theory is valid for estimating the splitting only for frequencies above 1 Hz. At frequencies more realistic for SKS propagation, apparent splitting parameters exhibit a π/2 dependence on the incoming shear wave polarization (back azimuth). For certain back azimuth ranges, shear wave splitting is very frequency dependent with apparent delay times ranging from 1 to 4 s and apparent fast polarization directions changing rapidly by up to 80°. Thus stacking of shear wave splitting measurements for largely different initial polarizations and frequencies should be avoided. Depth‐dependent anisotropy implies that shear wave splitting analyses will be sensitive to filtering. Anisotropic depth variations cannot be resolved unambiguously from splitting observations at relatively long periods (>5 s). It is not possible, for instance, to discriminate between smooth and abrupt transitions separating the anisotropic regions. Shorter‐period waveforms provide further information on the fine structure of anisotropic depth variations. A comparison between splitting calculations and observations from Hawaii suggests a divergent past APM direction or may indicate an alternative mechanism responsible for the lithospheric anisotropy.
Solid friction at high sliding velocities: An explicit three‐dimensional dynamical smoothed particle hydrodynamics approach
Authors: Maveyraud Claude , Benz Willy , Sornette Anne , Sornette Didier
Published: September 2002

We present realistic three‐dimensional numerical simulations of elastic bodies sliding on top of each other in a regime of velocities ranging from 0.1 to 10 m/s using the so‐called smoothed particle hydrodynamics method. This allows us to probe in detail the response of the bodies and the nature of the friction between them. Our investigations are restricted to regimes of pressure and roughness where only elastic deformations occur between asperities at the contact surface between the slider block and the substrate. In these regimes, solid friction is due to the generation of vibrational radiations which then escape to infinity or are damped out; in which case, energy is dissipated. We study periodic commensurate and incommensurate asperities and various types of disordered surfaces. In the elastic regime studied in this paper, we report evidence of a transition from zero (or nonmeasurable μ<0.001) friction to a finite friction as the normal pressure increases above 106 Pa. For larger normal pressures (up to 109 Pa) we find a remarkably universal value for the friction coefficient of μ ≈ 0.06, which is independent of the internal dissipation strength over 3 orders of magnitudes and independent of the detailed nature of the slider block‐substrate interactions. We find that disorder may either decrease or increase μ due to the competition between two effects: (1) disorder detunes the coherent vibrations of the asperties that occur in the periodic case, leading to weaker acoustic radiation and thus weaker damping, and (2) a large disorder leads to stronger vibration amplitudes at local asperities and thus to stronger damping. Our simulations have confirmed the existence of jumps over steps or asperities of the slider blocks occurring at the largest studied velocities (10 m/s). These jumps lead to chaotic motions similar to the bouncing‐ball problem. We find a velocity strengthening with a doubling of the friction coefficient as the velocity increases from 1 to 10 m/s. This reflects the increasing strength of vibrational damping.
New constraints on the Upper Permian and Lower Triassic geomagnetic polarity timescale from the Abadeh section (central Iran)
Authors: Gallet Yves , Krystyn Leopold , Besse Jean , Saidi Abdolah , Ricou Luc‐Emmanuel
Published: September 2002

We present the magnetostratigraphy of the Abadeh section (central Iran). Thermal demagnetization shows two magnetic components: one at low unblocking temperatures, which corresponds to a present day‐field direction, and another at high temperatures. This second component, which contains both polarities, is interpreted as a primary magnetization acquired during sediment deposition at a paleolatitude of about 7.7°S. A sequence of 14 magnetic polarity zones is obtained in the intervals during the Upper Dzhulfian‐Lower Changhsingian (Upper Permian) and during the Griesbachian (Lower Triassic). Combining data from south China, Canada, Pakistan, and Iran, we propose a composite (but still uncomplete) Upper Permian and Lower Triassic geomagnetic polarity timescale.
On the shape of paleomagnetic data sets
Authors: Beck Myrl E.
Published: September 2002

A quick and easy method to analyze the shape of paleomagnetic data sets is described and illustrated. The elongation E of a set of directions of virtual geomagnetic poles VGP is defined as E ≡ k2/k3, where k2 and k3 are the second and third eigenvalues of the Bingham distribution. It is shown that E<3 or so indicates that the distribution is significantly elongate, although the size of the sample and the paleolatitude both affect this conclusion. Because the azimuth of k2 gives the direction of elongation, it can be used to help determine why a particular distribution is elongate. It is shown that for most units from the craton both directions and VGP are nearly circular, except for units magnetized at very low latitudes. In general, poles are more circular than directions. Some units formed in plate margin locations also are circular, but many are highly elongate. Many units from the western Cordillera of North America have highly elongate distributions that are compatible with origin within a shear zone; others are obviously underformed internally. Units formed during extension are less commonly elongate, but some show the probably effects of normal faulting. It is hoped that shape analysis will be part of any complete paleomagnetic study in the future.
Antarctic crustal modeling from the spectral correlation of free‐air gravity anomalies with the terrain
Authors: von Frese Ralph R. B. , Tan Li , Woo Kim Jeong , Bentley Charles R.
Published: September 2002

We investigated the use of enhanced spectral correlation theory for modeling the crustal features of the Antarctic from regional observations of gravity and terrain. The analysis considered 1°‐gridded free‐air gravity anomalies and topographic rock, ice, and water components for the region south of 60°S. We modeled terrain gravity effects at 150‐km altitude by Gauss‐Legendre quadrature (GLQ) integration assuming densities of 2800 kg/m3 for rock, 900 kg/m3 for ice, and 1030 kg/m3 for seawater. These effects are substantial relative to the free‐air anomalies and must be compensated by the effects of subsurface density variations. Significant terrain‐correlated free‐air anomalies were revealed by the wavenumber correlation spectrum between the free‐air anomalies and the modeled terrain gravity effects, which we interpreted mostly to reflect possible isostatic imbalances of the crust. Subtracting the terrain‐correlated free‐air anomalies from the total free‐air anomalies and topographic gravity effects yielded terrain‐decorrelated free‐air anomalies and the gravity effects of isostatically compensated terrain features, respectively, which are uncorrelated with each other. The compensating effects that annihilate the latter were attributed to undulations of the Moho, which we estimated by inversion using GLQ integration and a mantle‐to‐crust density contrast of 400 kg/m3. The inversion produced a Moho map with nearly 40 km of total relief that agrees very well with deep seismic refraction soundings. For East Antarctica, a bimodal variation in crustal thickness was found: the crustal wedge between the eastern Weddell Sea (≃330°E) and the eastern flank of the Gamburtsev Subglacial Mountains (≃90°E) has a mean thickness of about 37 km, whereas the mean crustal thickness is near 32 km for the northern half of the rest of East Antarctica up to the western flank of the Transantarctic Mountains (≃150°E). For West Antarctica and the oceanic regions, mean crustal thicknesses of about 30 km and 14 km, respectively, are inferred. The terrain‐decorrelated free‐air anomalies may be related to long‐wavelength, large‐amplitude subcrustal density variations and to much shorter‐wavelength, smaller‐amplitude intracrustal density variations.
Paleoseismology of the 1912 Acambay earthquake and the Acambay‐Tixmadejé fault, Trans‐Mexican Volcanic Belt
Authors: Langridge Robert Max , Weldon Ray J. , Moya Juan Carlos , Suárez Gerardo
Published: September 2002

The Acambay Graben is a major intra‐arc basin of the Trans‐Mexican Volcanic Belt. Trenching studies at four sites along the Acambay‐Tixmadejé fault reveal evidence for the 1912 Acambay earthquake and at least four late Pleistocene and Holocene ground‐rupturing earthquakes with similar displacements to the 1912 event. Evidence for multiple events was recognized by a combination of upward terminating faults, fissure fills, erosionally truncated fissures, colluvial wedge fills, and downward increasing separation of stratigraphic units. Sites along the central part of the ESE‐WNW trending rangefront fault indicates large dip slip separations across a multi event scarp in Holocene colluvium with a penultimate event that occurred since ∼5230 calibrated years before present (cal. yr B.P.). On the Huapango Plain, near the eastern end of the 1912 rupture where slip was oblique, the penultimate earthquake occurred after ∼4700 cal. yr B.P., and there is evidence for at least four more earthquakes in the last ∼34,000 cal. yr B.P. Three‐dimensional excavation of lacustrine and channel deposits at Las Lomas yields a left‐normal slip vector of S40±5°E with 105±10 cm of slip for the last three events there. This slip vector is consistent with NNE‐oriented extension across the graben. The amount and style of slip documented for prehistoric ruptures are also consistent with the 1912 rupture, with a mean of ∼60 cm/event of dip slip on the rangefront and a mean of ∼35 cm/event of oblique slip on the N48°W trending Huapango Plain. Assuming these values represent a characteristic behavior yields an average recurrence interval of ∼3600 years for the last four large earthquakes (since ∼11,570 cal. yr B.P.), and a slip rate of ∼0.17 mm/yr across the rangefront. Empirical earthquake parameters calculated from our paleoseismic data support instrumental measures indicating that the 1912 Acambay earthquake was ofM6.8–7.0. The 1912 event is representative of ground‐rupturing earthquakes on the Acambay‐Tixmadejé fault and demonstrates that large seismic events (M∼7) can be generated by faults in the Trans‐Mexican Volcanic Belt and in other intra‐arc settings.
Recurrent large‐scale landsliding on the west flank of La Palma, Canary Islands
Authors: Urgeles Roger , Masson Douglas G. , Canals Miquel , Watts Anthony B. , Le Bas Tim
Published: September 2002

A large area of debris avalanche deposits has been discovered on the western submarine flanks of the island of La Palma. Multibeam bathymetry and its derivative backscatter data, Towed Ocean Bottom Instrument (TOBI) sidescan sonar images, and 3.5 kHz and airgun seismic reflection data have been used to identify at least two, and possibly as many as four, major landslide events. The youngest of the events, the Cumbre Nueva Debris Avalanche, extends onshore into the valleys bounded by the Caldera de Taburiente and Cumbre Nueva Ridge, which mark the degraded collapse scars. Radiometric dating of the volcanic flows in the headwall indicate an age of between 536 and 125 ka for this landslide. The debris avalanche covers an area of 780 km2, has a maximum thickness of 500 m, and has an estimated volume of 95 km3. Older deposits, collectively referred to as the Playa de la Veta Debris Avalanche Complex, are probably, as the name indicates, an amalgamation of at least two or three events rather than the result of a single catastrophic failure. The Playa de la Veta Debris Avalanche Complex is associated onshore with an unconformity dated as late Matuyama (1 Ma to 800 ka). It covers an area of 1200 km2, has a maximum thickness of 1300 m, and may represent a total volume of up to 650 km3. The greater thicknesses and limited areas occupied by debris avalanches on the western flank of La Palma, compared to other landslides in the Canary Archipelago, suggest that the La Palma landslide masses have relatively low mobility. The different debris avalanche lobes formed by each landslide event are separated by channels 2–2.5 km wide. The clear relationship between channel position and the boundaries of each debris avalanche lobe indicates that debris avalanches control later channel formation and pathways. The relief of the submarine flanks of the La Palma volcanoes, in the areas of island slope unaffected by landslides, is mainly the result of constructional volcanic processes. However, the older submarine slopes, such as in the northern Taburiente volcano, may also have been modified by smaller‐scale submarine mass wasting and sediment flows.
Modeling dynamic recrystallization of olivine aggregates deformed in simple shear
Authors: Wenk H.‐R. , Tomé C. N.
Published: September 2002

Experiments by Zhang and Karato [1995] have shown that in simple shear dislocation creep of olivine at low strains, an asymmetric texture develops with a [100] maximum rotated away from the shear direction against the sense of shear. At large strain where recrystallization is pervasive, the texture pattern is symmetrical, and [100] is parallel to the shear direction. The deformation texture can be adequately modeled with a viscoplastic self‐consistent polycrystal plasticity theory. This model can be expanded to include recrystallization, treating the process as a balance of boundary migration (growth of relatively underformed grains at the expense of highly deformed grains) and nucleation (strain‐free nuclei replacing highly deformed grains). If nucleation dominates over growth, the model predicts a change from the asymmetric to the symmetric texture as recrystallization proceeds and stabilization in the “easy slip” orientation for the dominant (010)[100] slip system. This result is in accordance with the experiments and suggests that the most highly deformed orientation components dominate the recrystallization texture. The empirical model will be useful to simulate more adequately the development of anisotropy in the mantle where olivine is largely recrystallized.
Seismotectonics of the Norwegian continental margin
Authors: Byrkjeland Unni , Bungum Hilmar , Eldholm Olav
Published: September 2002

The Norwegian continental margin and surrounding areas are seismically less active than some other passive margins worldwide, indicating a potential earthquake deficit. The adjacent oceanic crust is mostly aseismic except for parts of the Lofoten and Norway Basins which have experienced rapid deposition of glacial sediments. The excess load has enhanced the local stress field and, in turn, the seismic activity. The marginal highs along the continent‐ocean transition and the part of the Møre and Vøring Basins which experienced crustal extension prior to the early Tertiary breakup are practically aseismic. This region is also underlain by high‐velocity lower crust emplaced during breakup, suggesting crustal strengthening, which also may increase the return periods for the largest events. Farther landward, a spatial correlation of seismic activity and the glacial sediment wedge suggests a causal relationship, expressed through preferential rejuvenation of Late Jurassic‐Early Cretaceous faults. The locally high seismic activity in the coastal region occurs in areas where the continental crust is relatively thick (25–30 km) but still thinner than in the shield area farther east (45–50 km). Although stress relations are complex, we suggest that increased stress due to appreciable postglacial rebound gradients in the coastal region may be a contributing factor. While the stress field along the margin complies in general with the ridge push force, we infer that regional and local stress enhancement factors are not only present in this region but also necessary for explaining the earthquake activity. The relative importance of local stress sources is supported by several cases of 90° stress rotations relative to the ridge push direction. The superposition of regional and local stress, together with the existence of weakness zones and faults, yields potentials for earthquakes, primarily through structural reactivation.
Influence of clay fraction on pore‐scale properties and hydraulic conductivity of experimentally compacted mudstones
Authors: Dewhurst David N. , Aplin Andrew C. , Sarda Jean‐Paul
Published: September 2002

We report the results of a series of hydraulic conductivity tests carried out on seven natural, well‐characterised specimens of London Clay mudstone. The clay fractions of the samples range from 27% to 66% and enabled a test of the influence of clay fraction on the hydraulic conductivity, pore size distribution, compressibility and specific surface area of natural mudstones. Hydraulic conductivities were determined at effective stresses between 1.5 and 33 MPa. Hydraulic conductivities of clay‐rich samples (49–66% clay fraction) decreased from ∼10−11m s−1to ∼10−14m s−1over a porosity range of 48% to 25%. At a given porosity the hydraulic conductivities of two silt‐rich samples (27 and 33% clay fraction) were 40–250 times greater than those of the five clay‐rich samples. Variations in hydraulic conductivity are directly related to pore size distributions and are accurately predicted by a model which uses pore size distribution as its primary input. Clay‐rich samples have unimodal pore size distributions with modal throat radii around 60–120 nm. Silt‐rich samples have bimodal pore throat size distributions. One modal size is similar to that observed in clay‐rich samples with a second modal value at 3–6 μm. Compaction under effective stresses up to 10 MPa results in the preferential collapse of larger pores, so that the rate of loss of hydraulic conductivity is greater in the silt‐rich samples. Differences in hydraulic conductivity between silt‐rich and clay‐rich mudstones therefore decline with decreasing porosity. The range of porosity‐hydraulic conductivity relationships means that hydraulic conductivity is not easily predicted from porosity alone; additional constraining parameters such as grain and pore size distributions are required.
Determination of rock properties by low‐frequency AC electrokinetics
Authors: Pengra David B. , Xi Li Sidney , Wong Po‐zen
Published: September 2002

In brine‐saturated rock the existence of a mobile space charge at the fluid/solid interface leads to the electrokinetic phenomena of electroosmotic pressure and streaming potential. The coupling coefficients of these electrokinetic effects, when combined with the conductivity of the brine‐saturated rock, determine the brine permeability of rock exactly. A sensitive low‐frequency AC technique has been used to measure electrokinetic response of a collection of eight rock and four glass bead samples saturated with NaCl brine as a function of salt concentration (fluid conductivity of 0.5 to 6.38 S/m); the response of four of the original 12 samples has also been measured as a function of temperature from 0° to 50°C. All data verify the predicted permeability relationship. Additionally, the frequency response of the electroosmotic pressure signal alone can also be used to determine the permeability, given knowledge of experimental parameters. The concentration and temperature dependence of electroosmosis and streaming potential is found to mostly conform to the predictions of a simple model based on the Helmholtz‐Smoluchowski equation, the Stern model of the electrochemical double layer, and an elementary theory of ionic conduction.
Lithospheric Structure Beneath the Alboran Basin: Results from 3D Gravity Modeling and Tectonic Relevance
Authors: Torne M. , Fernàndez M. , Comas M. C. , Soto J. I.
Published: September 2002

A three‐dimensional gravity modeling combined with integrated heat flow and elevation modeling is conducted to map out the crustal and lithospheric mantle thickness in the Alboran Basin, in the westernmost Mediterranean. A “sediment”‐corrected Bouguer anomaly has been derived using a depth‐to‐the‐basement map and densities determined from well logs and seismic data. The gravity effect of the base of the lithosphere has been removed from the sediment‐corrected Bouguer anomaly to obtain a “crustal” Bouguer anomaly, which has been inverted for crustal thickness. The resulting lithospheric structure is further constrained by elevation data under the assumption of local isostasy. The low residual elevation anomalies obtained (±100 m in average) suggest that the area is in local isostasy, particularly the medium‐ and long‐wavelength topography features. Variations in crustal thickness range from 36 km underneath the Betic and Rif Chains to <12 km beneath the easternmost part of the Alboran Sea Basin, in the transition to the South Balearic Basin. In western Alboran the Moho lies at a rather constant depth of ∼18 km, deepening sharply toward the Gibraltar Strait down to 30–32 km. The base of the lithosphere shallows from 140 km depth in the Gibraltar Strait to <45 km depth in the easternmost Alboran Sea. Lithospheric thinning penetrates to the southeastern side of the Iberian margin crosscutting topographic highs of the central and eastern Betic Chain. Our results favor mantle delamination produced by detachment and subsequent peeling away of the lithospheric mantle rather than convective removal of the lithospheric mantle either by orogenic collapse or detachment and sinking of a lithospheric slab.
Gravitational collapse of orogenic crust: A preliminary three‐dimensional finite element study
Authors: Liu Mian , Shen Yunqing , Yang Youqing
Published: September 2002

Although gravitational collapse has been widely accepted as a viable explanation for both synorogenic and postorogenic extension, its mechanisms have not been well understood. Previous models of continental deformation that assumed a viscous or viscoplastic rheology are not well suited for simulating stress states and brittle deformation within the upper crust, where most extensional collapse occurs. Here we present preliminary results of a three‐dimensional (3‐D) finite element modeling of stress and faulting patterns within the orogenic crust using a viscoelastic rheology. Major parameters controlling orogenic extension, including topographic loading, tectonic compression, basal shear, thermal structure, and 3‐D tectonic boundary conditions are systematically explored in the model. For typical tectonic compression (60–100 MPa) and >2–3 km elevation the model predicts synorogenic extension in high plateaus and concomitant thrusting in the lowlands near the foothills. Viscous relaxation within the ductile crust may amplify deviatoric stresses within the brittle crust by a few times, and stress amplification is greater when the crust is hotter, and therefore the brittle crust is thinner. Most synorogenic extension occurs in the direction orthogonal to regional compression, while postorogenic extension is more likely to occur normal to the trend of mountain belts. We apply the model results to the Tibetan plateau and suggest that the age of the north trending grabens is not a reliable proxy for the time when the plateau had reached it present mean high elevation. Conversely, development of the east trending South Tibetan Detachment system in the Miocene by gravitational collapse was more difficult and may require a higher elevation of the Himalayas than present or much of the Tibetan plateau to have remained low till late Miocene.
Experimental study of caldera formation
Authors: Roche O. , Druitt T. H. , Merle O.
Published: September 2002

Scaled experiments have been carried out on caldera collapse mechanisms, using silicone as analogue magma and dry sand as analogue rock. Experiments were carried out in two and three dimensions using a range of roof aspect ratios (thickness/width 0.2 to 4.5) appropriate for caldera collapse. They reveal a general mechanism of collapse, only weakly dependent on the shape of the reservoir. For low roof aspect ratios (≤1), subsidence starts by flexure of the roof and the formation of outward dipping, reverse ring faults, which in turn trigger formation of peripheral inward dipping, normal ring faults. The subsidence always occurs asymmetrically. In cross section the reverse faults delimit a coherent piston, bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets. The surface depression consists of a nondeformed area (piston) surrounded by an annular extensional zone (tilted strata). For high aspect ratios (>1), multiple reverse faults break up the roof into large pieces, and subsidence occurred as a series of nested wedges (2‐D) or cones (3‐D). The extensional zone dominates the surface depression. In the case where preexisting regional faults do not play a major role, the collapse mechanics of calderas probably depends strongly on the roof aspect ratio. Calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults. The annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias. Collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant.
Deep structure of the ocean‐continent transition in the southern Iberia Abyssal Plain from seismic refraction profiles: The IAM‐9 transect at 40°20′N
Authors: Dean S. M. , Minshull T. A. , Whitmarsh R. B. , Louden K. E.
Published: September 2002

We present a crust and mantle velocity structure for the West Iberia passive continental margin derived from a 320‐km‐long wide‐angle seismic profile acquired in the southern Iberia Abyssal Plain. We observe a 170‐km‐wide ocean‐continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault‐bounded blocks of continental crust. The profile lies ∼40 km south of the transect sampled by Ocean Drilling Program (ODP) Legs 149 and 173. The transition zone structure can be divided into an upper layer, 2–4 km thick with velocities of between 4.5 and 7.0 km s−1 and generally a high‐velocity‐gradient (1 s−1), and a lower layer up to 4 km thick with a velocity of ∼7.6 km s−1 and a low‐velocity‐gradient. A weak Moho reflection in this zone was seen only on wide‐angle profiles at an offset of ∼30 km. The upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope. Conversely, the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust. On the coincident seismic reflection profile, fault‐bounded crustal blocks, identified in unequivocal extended continental crust, are not observed in the transition zone. The upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges, but no oceanic layer 3 velocity structure is present. While magnetic anomalies have been identified within the transition zone, they have not been modeled successfully as seafloor spreading magnetic anomalies, nor do they generally form long linear margin‐parallel features. Finally, ODP boreholes, ∼40 km north of our profile and within the interpreted transition zone, have recovered up to 140‐m‐thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material. We conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust. Although current melting models predict a considerably thicker crust of decompression melt products, we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite.
Tectonic rotations within the Bolivian Altiplano: Implications for the geodynamic evolution of the central Andes during the late Tertiary
Authors: Roperch Pierrick , Fornari Michel , Hérail Gérard , Parraguez Georgina V.
Published: September 2002

Paleomagnetic results from 61 sites in Tertiary red beds and volcanic rocks from the Bolivian Andes allow quantification of tectonic rotations within the Altiplano domain. A total of 16 sites were also obtained in lower Miocene ignimbrites that cover the forearc of the north Chilean Andes near Arica. In the southern Altiplano (Lipez region) a local clockwise rotation of up to 38° is recorded in lower Miocene volcanic rocks. Farther north, near the Salar de Uyuni, there is no evidence of significant rotations. Counterclockwise rotations are observed in the Northern Altiplano. The largest counterclockwise rotation (28°) is found in Eocene‐Oligocene red beds (Tiwanaku Formation) along the eastern border of the Corque syncline. Middle Miocene sediments located within the center of the Corque basin record a counterclockwise rotation of only 10°. About 200 km north of the Corque basin, Eocene‐Oligocene sedimentary rocks near the locality of Viacha record only 10° counterclockwise rotation. Paleomagnetic results in lower Miocene ignimbrites from the forearc near the Arica bend do not show evidence for late Cenozoic rotation of the forearc. These new results suggest that the 20° to 25° counterclockwise rotation of the southern Peruvian forearc occurred prior to the Miocene. Anisotropy of magnetic susceptibility (AMS) data show a magnetic fabric mostly controlled by bedding, but slight AMS lineations are found almost parallel to the major regional structural trends such as fold axes. The apparent relationship between tectonic rotations, AMS lineations, and structural trends suggests that rotations occured after or during the final stage of folding. Paleomagnetic data obtained in Paleozoic rocks show evidence of clockwise rotation of the southern sub‐Andean ranges. Tectonic rotations during the Neogene are mostly localized on the eastern side of the central Andes. The curvature of the occidental margin near Arica was likely acquired prior to the last stage of Andean deformation. Although the magnitude of the rotations may vary from one locality to the other, there is a consistent pattern showing counterclockwise rotations to the north and clockwise to the south. A large‐scale regional mechanism is needed to explain this pattern of tectonic rotations within the central Andes. We propose a model in which along‐strike segmentation of the Andean foreland and indentation of the Altiplano by a curved forearc are the major factors controlling tectonic rotations within the Altiplano, Eastern Cordillera, and sub‐Andean ranges.
Effect of a large outer rise earthquake on seismic cycles of interplate earthquakes: A model study
Authors: Kato Naoyuki , Hirasawa Tomowo
Published: September 2002

A numerical simulation of the sliding process of a plate interface is performed to investigate the effect of stress perturbation due to a large outer rise earthquake on seismic cycles of large interplate earthquakes in a subduction zone. A two‐dimensional model of uniform elastic half‐space is set up for repeatedly occurring large interplate earthquakes by assuming that the frictional stress on the plate interface obeys a rate‐ and state‐dependent friction law derived from laboratory experiments. An increase in shear stress on a shallow part of the plate interface due to a tensional outer rise earthquake promotes aseismic sliding on the shallower aseismic zone of the plate interface. This aseismic sliding advances the occurrence time of the next interplate earthquake. On the other hand, a compressional outer rise earthquake tends to delay the occurrence time of the next interplate earthquake. The magnitude of time advance or delay of the next interplate earthquake depends on the time of the outer rise earthquake in a seismic cycle due mainly to the complex behavior in propagation of aseismic sliding governed by the rate‐ and state‐dependent friction law.
CO2 emissions related to the Yellowstone volcanic system: 1. Developing a stratified adaptive cluster sampling plan
Authors: Boomer K. , Werner C. , Brantley S.
Published: September 2002

Statistical sampling theory offers several sampling designs that are well‐suited to geophysical research. Simple random sampling is the most commonly used formal methodology, in which the probability that any unit is sampled is known. Generally, units to be sampled are selected using a random number generator so that each unit has an equal chance of selection. In stratified sampling, the sampling frame is partitioned so that similar observed variable measurements are in each stratum. Systematic sampling attempts to capture the full variability within an area by obtaining samples at some predetermined interval. Adaptive cluster sampling is a relatively new methodology that allows additional units to be sampled, based on preceding sampling observations. The systematic adaptive sampling method applies adaptive cluster sampling to a systematic design. Selecting an appropriate design requires an understanding of the variability in the parameter of interest as well as the spatial covariance structure. This paper addresses such critical issues as sampling density, number of samples to collect, and assessment of the dimensions of a subregion to be sampled. While many researchers use the sampling designs presented herein, it is possible that inappropriate estimating equations were used. Formulae required to calculate unbiased, precise estimators for each of the designs are presented. The collection of CO2 flux degassing measurements from the Mud Volcano area, Yellowstone during the 1997 field season is used as a case study to outline the logical steps in selecting a sampling design. Although the case study used a stratified adaptive cluster sampling design, simpler designs can be developed based on the theory presented herein.
Coswarm and preswarm crustal deformation in the eastern Izu Peninsula, central Japan
Authors: Okada Yoshimitsu , Yamamoto Eiji , Ohkubo Tadashi
Published: September 2002

Remarkable crustal deformation has repeatedly been observed in the eastern Izu Peninsula, central Japan, synchronized to the occurrence of extensive earthquake swarms in the past 20 years. Among the instrumentation, a borehole tiltmeter installed at station KWN, which is located within a few kilometers of the swarm region, has faithfully recorded large signals of ground tilt associated with more than 10 swarm sequences in the past 10 years. In addition to coswarm signals it became clear that precursory tilt signals preceded major swarm activities. The amplitudes of the precursory tilt signals are of order of 0.1 μrad, and they precede the onset of major swarms by several hours to half a day. This signal is convincing because the same phenomena appeared at least 7 times and were also detected by other independent observations. We speculate that a dike intrusion at depth causes crustal deformation and only as it reaches depths shallower than ∼10 km does seismic activity begin. The precursory tilt signal can be utilized for practical prediction of the start of swarm activity in the eastern Izu Peninsula.
Style and age of late Oligocene‐early Miocene deformation in the southern Stillwater Range, west central Nevada: Paleomagnetism, geochronology, and field relations
Authors: Hudson Mark R. , John David A. , Conrad James E. , McKee Edwin H.
Published: September 2002

Paleomagnetic and geochronologic data combined with geologic mapping tightly restrict the timing and character of a late Oligocene to early Miocene episode of large magnitude extension in the southern Stillwater Range and adjacent regions of west central Nevada. The southern Stillwater Range was the site of an Oligocene to early Miocene volcanic center comprising (1) 28.3 to 24.3 Ma intracaldera ash flow tuffs, lava flows, and subjacent plutons associated with three calderas, (2) 24.8 to 20.7 Ma postcaldera silicic dikes and domes, and (3) unconformably overlying 15.3 to 13.0 Ma dacite to basalt lava flows, plugs, and dikes. The caldera‐related tuffs, lava flows, and plutons were tilted 60°–70° either west or east during the initial period of Cenozoic deformation that accommodated over 100% extension. Directions of remanent magnetization obtained from these extrusive and intrusive, caldera‐related rocks are strongly deflected from an expected Miocene direction in senses appropriate for their tilt. A mean direction for these rocks after tilt correction, however, suggests that they were also affected by a moderate (33.4° ±11.8°) component of counterclockwise vertical axis rotation. Paleomagnetic data indicate that the episode of large tilting occurred during emplacement of 24.8 to 20.7 Ma postcaldera dikes and domes. In detail, an apparent decrease in rotation with decreasing age of individual, isotopically dated bodies of the postcaldera group indicates that most tilting occurred between 24.4 and 24.2 Ma. The onset of tilting immediately following after the final caldera eruptions suggests that the magmatism and deformation were linked. Deformation was not driven by magma buoyancy, however, because tilting equally affected the caldera systems of different ages, including their plutonic roots. It is more likely that regional extension was focused in the southern Stillwater Range due to magmatic warming and reduction of tensile strength of the brittle crust. Faults that accommodated deformation in the southern Stillwater Range initially dipped steeply and cut deeply to expose more than 9 km of crustal section. The exposed crustal sections are probably rotated blocks above an unexposed basal detachment that lay near the early Miocene brittle‐ductile transition.
Crustal rheology and faulting at strike‐slip plate boundaries: 2. Effects of lower crustal flow
Authors: Roy Mousumi , Royden Leigh H.
Published: September 2002

We present a numerical model of deformation at a strike‐slip plate boundary within a linear viscoelastic crust, which is driven by far‐field plate motions and basal mantle velocities. The crust is assumed to have uniform elastic properties but continuously varying viscosity as a function of depth. Brittle faulting is represented by static elastic dislocations that are imposed when stresses exceed a critical threshold for fracture or frictional sliding. The locations and depth extents of faults in this model are not prespecified but, instead, are governed by stress evolution within the crust. We find that when a primarily elastic upper crust is underlain by a low‐viscosity lower crustal layer, the deformation zone broadens in time to encompass many parallel strike‐slip faults in an interacting network. In contrast, when the entire crust behaves elastically, the deformation zone remains narrow and focused on a single plate‐bounding fault, reflecting imposed mantle motions. Surface strain rate patterns within the interacting fault network are complex and reflect significant faulting‐related strain rate perturbations that decay over timescales of postseismic relaxation in the lower crust (10–100 years). The fault network has a characteristic spacing, with complex fault interactions and with the depth extents of faults increasing with time to a maximum depth governed by crustal rheology. The maximum depth of faults is limited by stress relaxation and large‐scale viscous flow in the lower crust, which confines brittle failure to shallow and midcrustal levels.
Particle recycling and oscillations of volcanic eruption columns
Authors: Veitch Graham , Woods Andrew W.
Published: September 2002

We study the dynamics of sedimentation and reentrainment of particles from the umbrella cloud above an axisymmetric, turbulent, particle‐laden buoyant plume. We develop a model to show that the reentrainment of particulate material into the uprising plume will cause the particle flux to increase by a factor of e between the plume source and the umbrella cloud. A buoyant plume rising in an environment of uniform density may thereby become negatively buoyant if its particle loading becomes sufficiently high. We compare the predictions of the model to a series of laboratory experiments and show that at high particle loadings the plume undergoes an oscillatory collapse. This periodic collapse generates dense gravity flows down the flanks of the plume. In the context of volcanic eruptions, such an instability, associated with particle recycling may lead to the formation of interleaved fall and flow deposits, such as have been observed near the collapse horizon in a number of pyroclastic deposits.