bluuebunny/crossref_metadata_2025 huggingface dataset:
Modes of continental lithospheric extension
Authors: Buck W. Roger
Published: February 2008

A simplified model of continental extension including lower crustal flow is developed. The model employs the thin sheet approximation in estimating lithospheric yield strength and gravitational buoyancy forces arising from lateral variations in crustal thickness and temperature. The effect of advection and diffusion of heat on the temperature structure and yield strength of the extending region is calculated. The viscosity of the lower crust, which controls its rate of flow, is estimated in a manner consistent with the yield strength calculation. The change in the force required to extend the lithosphere is calculated after a finite amount of extension. When this force increases, the zone of extension is assumed to widen; when it decreases, the extension remains localized. Material parameters affecting the model results are the density difference between the crust and mantle, the gradient of brittle yield stress with depth, the abundance of heat‐producing elements within the crust, and the ductile strengths of the crust and mantle as a functions of temperature and strain rate. The model predicts three distinct modes of extension depending on the model crustal thickness, heat flow, and strain rate. The modes are (1) core complex mode (concentrated upper crustal extension with lower crustal thinning over a broad area and any mantle lithosphere extending in a local area), (2) wide rift mode (uniform crustal and mantle lithospheric thinning over a width greater than the lithospheric thickness), and (3) narrow rift mode (concentrated crustal and mantle lithospheric extension). Models with a dry olivine mantle rheology and a dry quartz or dry anorthosite crustal rheology are consistent with geologic observations of crustal thickness and heat flow for all three extensional modes. An analysis of the force changes due to extension, ignoring conduction of heat, is carried out to give greater insight into how variations in crust and mantle rheologic parameters affect model results. Crustal rheologies several orders of magnitude stronger or weaker than dry quartz give results inconsistent with observations. The model implies a temporal progression from core complex mode to wide rift mode to narrow rift mode. Data from the southern Basin and Range Province are consistent with this progression.
Mantle layering from ScS reverberations: 2. The transition zone
Authors: Revenaugh Justin , Jordan Thomas H.
Published: February 2008

This is the second paper in a four‐part sequence that examines the nature of mantle layering using the ScSn phases and internal reflections observed within the reverberative interval of SH‐polarized seismograms. Mantle reflectivity profiles for 18 seismic corridors sampling portions of Indonesia, Australia, and the western and central Pacific are constructed from low‐frequency (25‐mHz) ScS reverberations; i.e, ScSn‐type phases reflected once from internal mantle discontinuities. Modeling the reflectivity profiles using synthetic seismograms yields precise, path‐specific estimates of the travel times to the 410‐km and 660‐km discontinuities and their reflection coefficients. The data are consistent with an olivine‐dominated model of the transition zone, marked by two major phase transitions having average apparent depths of 414±2 km and 660±2 km and impedance contrasts of 0.046±0.010 and 0.072±0.010, respectively. The interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km (peak to peak) and is negatively correlated, consistent with an endothermic transition at 660 km whose Clapeyron slope magnitude is comparable to that of the 410‐km (exothermic) transition. In the case of the 660‐km discontinuity, we have been able to supplement these long‐wavelength (1500–5000 km) observations with an estimate of intrapath topography, inferred from the frequency dependence of the 660‐km reflection coefficient, which gives peak‐to‐peak variations less than 40 km on scale lengths of 500–1500 km. For the 660‐km discontinuity, the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction. However, we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660‐km discontinuity which may involve small compositional variations. Plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition, loss of reflected energy due to local curvature (or roughness) of the reflector, and/or extreme topography occurring on a small compositional component to the discontinuity (≤40% of the total impedance contrast). By stripping the 410‐km and 660‐km peaks from the composite path reflectivity profile, we identify three minor reflectors at mean depths of 520, 710, and 900 km. The shallowest may mark the β‐phase → γ‐spinel phase transition. The 710‐km discontinuity can be attributed to the ilmenite (gamet) → perovskite phase transition, although changes in perovskite symmetry can potentially provide explanations for both the 710‐km and 900‐km features.
Crustal velocity and Moho topography in central New Hampshire
Authors: Shalev Eylon , Park Jeffrey , Lerner‐Lam Arthur
Published: February 2008

The White Mountain Batholith in central New Hampshire is a major plutonic, topographic and gravimetric feature, superimposed on the NE‐SW trending regional pattern of accreted terranes in northeastern North America. Compressional and shear‐wave arrival times from a two‐dimensional seismic array experiment are used to constrain a one‐dimensional crustal velocity model and Moho topography for central New Hampshire. Our deployment, a “piggyback” on the Ontario‐New York‐New England refraction experiment (ONYNEX), recorded 10 explosive sources using 20 three‐component digital seismometers. The crust can be modeled as 34.5 km thick, and with three layers having linear gradients without midcrust discontinuities. For this model, P‐velocities are 6.1–6.2 km/s in the upper crust, and 6.2–6.8 km/s in the lower crust. S‐velocities are 3.5–3.7 km/s in the upper crust, and 3.7–3.9 km/s in the lower crust. Because Moho reflections comprise our principal constraints on lower‐crustal velocities, there is a tradeoff between lower‐crustal velocity and crustal thickness. In particular, an average crustal thickness of 39–40 km, reported elsewhere in New England, is not precluded by our data. The Poisson's ratio for the crust varies between 0.23–0.26, in the lower range of values reported for crustal minerals. Our dataset is consistent with ≲2% lateral variation in the upper crust underlying the New Hampshire and White Mountain Plutonic Series, but larger variations deeper than 15 km. If modeled in terms of Moho topography, PmP traveltime residuals and PmP–P residuals suggest thicker crust (36–37 km) under the White Mountain batholith, with thinner crust (33–34 km) to the east and southwest of the White Mountains. Such topography of the Moho correlates well with both the surface elevation and the Bouguer gravity anomaly in the region. Gravity data do not favor the simplest alternative model: a thick low‐velocity granitic lower crust beneath the batholith. Using a simplified density model, roughly 30–35% of the gravity anomaly in the White Mountain region can be modeled as the result of the inferred Moho depression, requiring only a 5 km low‐density root for the White Mountain batholith. The short wavelength of the Moho topography is consistent with a very low flexural rigidity for the lithosphere (≈ 1021 Nt‐m), suggesting that the plutons were intruded into a weak lithosphere. A rapid transition from positive to negative PmP residuals falls along a line that parallels the onshore extension of the Kelvin Seamounts, and may correspond to the “line of weakness” hypothesized by Sykes (1978) to have been tectonically active during the opening of the North Atlantic Ocean at 100–200 Ma.
Inversion of borehole breakout orientation data
Authors: Qian Wei , Pedersen Laust Børsting
Published: February 2008

Well bore breakouts are zones of spalling and fracture that form on opposite sides of a well bore and tend to change the cross‐sectional shape of the borehole from circular to roughly elliptical. In vertical holes drilled in areas where one principal stress is vertical, breakout orientations tend to parallel the direction of the regional minimum principal stress (compressive). Such breakout orientations are often valuable indicators of the direction of the principal stresses. We will show in this paper that the variation of the breakout direction as the borehole deviates from vertical gives further information about the relative magnitudes of all the three principal stresses. Based upon the variation of breakout directions as a function of deviations and azimuths of the borehole along its depth, an inverse problem is formulated to estimate both the direction of the minimum horizontal stress and the relative magnitudes of the horizontal stresses to the vertical stress. Tests with synthetic data sets show that the power of the method is dependent on the stress regime (strike‐slip, normal and thrust). Generally the inverse problem in the case of thrust faulting is less constrained, whereas, in the case of normal faulting, it allows for a determination of all three parameters (SH/SV, SK/SV and ν) with a reasonable certainty. Interpretation of the data from the Siljan Deep Drilling Project in Sweden shows that the azimuth of the minimum principal horizontal stress is very well constrained to N(18.4°±0.3°)E, while the ratio of the maximum principal horizontal stress to the vertical stress is constrained to 1.1 ±0.1. The ratio between minimum principal horizontal stress and principal vertical stress is poorly constrained to the interval (0.0–0.95). Information of drilling mud leakage and borehole stability analysis provides a value of 0.61 [Stephansson et al., 1990]. These results are in fair agreement with the results from earthquake fault plane solutions in South Central Fennoscandia with dominant strike‐slip faulting and an inferred minimum horizontal stress direction of N45°E [Slunga, 1981].
Interaction of mantle plume heads with the Earth's surface and onset of small‐scale convection
Authors: Griffiths R. W. , Campbell I. H.
Published: February 2008

We investigate the behavior of a spherical blob of buoyant fluid as gravity forces it toward either a rigid horizontal boundary or a free surface. The diapir fluid is assumed much less viscous than the ambient fluid. This fundamental problem is the simplest unsteady model relevant to the ascent of hot plumes of buoyant material toward Earth's surface or the base of the lithosphere and closely models the heads of starting plumes. As the diapir approaches the boundary, it collapses in the vertical and spreads horizontally while a layer of the surrounding mantle is slowly squeezed out from betweeen the diapir and the surface. Experimental results for the thinning and lateral spreading of the bouyant fluid, and for the thinning of the squeeze layer, are given for both the case of a rigid, nonslip boundary and that of a free surface. These are compared with similarity scaling laws based on a balance between the bouyancy of the diapir and viscous stresses in the diapir's surroundings. We also observe that the squeeze layer can become gravitationally unstable, leading to a bifurcation from convection on the scale of the original plume to convection on scales much smaller than the diapir. The vertical exchange on smaller horizontal scales enables the plume to more rapidly approach the boundary. At the time instability occurs the diapir has spread to roughly twice its initial diameter. Application of these results, and previous results from surface uplift, to the plumes responsible for continental flood basalts is subject to knowledge of the local value of upper mantle viscosity. If this is taken to be 3×1020 Pa s, most uplift takes place ovcer approximately 5 m.y. Eruption of voluminous basalts will not take place until at least 5–10 m.y. after the surface has reached its maximum elevation. If small‐scale instabilities do develop within mantle plume heads, they may be an essential mechanism allowing the top of the plume to ascend to the shallow depths required for extensive melting. It may also explain the observation of Hooper (1990) that volcanism in the Deccan and Columbian Plateau begins before the onset of crustal extension.
Structural development of Sumisu Rift, Izu‐Bonin Arc
Authors: Taylor Brian , Klaus Adam , Brown Glenn R. , Moore Gregory F. , Okamura Yukinobu , Murakami Fumitoshi
Published: February 2008

Geophysical swath mapping, multichannel seismic profiling, and ocean drilling data are used to document the structural evolution of Sumisu Rift and to analyze the pattern of strain resulting from extension of an intraoceanic island arc. The ∼120‐km‐long, 30–50‐km‐wide Sumisu Rift is bounded to the north and south by structural and volcanic highs west of the Sumisu and Torishima calderas and longitudinally by curvilinear border fault zones with both convex and concave dip slopes. The zig‐zag pattern of normal faults (average strikes 337° and 355°) indicates extension oriented 076°±10°, orthogonal to the volcanic arc. Three oblique transfer zones divide the rift along strike into four segments with different fault trends and uplift/subsidence patterns. Differential strain across the transfer zones is accommodated by interdigitating, rift‐parallel faults and sometimes by cross‐rift volcanism, rather than by strike‐ or oblique‐slip faults. From estimates of extension (2–5 km), the age of the rift (∼2 Ma), and the accelerating subsidence, we infer that Sumisu Rift is in the early synrift stage of back arc basin formation. Following an early sag phase, half graben formed with synthetically faulted, structural rollovers facing large‐offset (2–2.5 km throw) border fault zones. In the three northern rift segments the largest faults are on the arc side and dip 60°–75°W, whereas in the southern segment they are on the west side and dip 25°–50°E. The present “full graben” stage is dominated by hanging wall antithetic faulting, basin widening by footwall collapse, and a concentration of subsidence in an inner rift. The hanging wall collapses, but not necessarily as a result of border fault propagation from adjacent rift segments. Whereas the border faults may penetrate the Theologically weak lithosphere (Te ≈ 3 km), many of the hanging wall and footwall collapse structures are detached only a few kilometers below the seafloor. Back arc volcanism, usually erupted along faults, occurs in the rift and along the protoremnant arc during both stages. Where drilled, the arc margin has been uplifted 1.1±0.5 km concurrently with ∼1.1 km of rift basin subsidence. Extremely high sedimentation rates, up to 6 m/kyr in the inner rift, have kept pace with synrift faulting, created a smooth basin floor, and resulted in sediment thicknesses that mimic the differential basin subsidence. A linear zone of weakness caused by the greater temperatures and crustal thickness along the arc volcanic line controls the initial locus of rifting. Rifts are better developed between the arc edifices; intrusions may be accommodating extensional strain adjacent to the arc volcanoes. No obvious correlations are observed between the rift structures and preexisting cross‐arc trends.
Lava flow cooling estimated from Landsat Thematic Mapper infrared data: The Lonquimay Eruption (Chile, 1989)
Authors: Oppenheimer Clive
Published: February 2008

Surface temperatures of a 1989 andesite lava flow at Volcán Lonquimay, Chile, derived from an analysis of Landsat Thematic Mapper (TM) infrared data, show a progressive cooling of crusted lava from 250°C to 170°C down a 1.5 km segment of the flow. These results are obtained by solving “dual‐band” equations for the two short‐wavelength infrared TM bands, assuming that the lava surface can be represented by two thermal components, exposed core and chilled crust, and unlike previous investigations, by predetermining the temperature of the hot component rather than the cool component. This approach is consistent with both observational data and theoretical models, and shows that any exposed core of the flow occupies tiny fractions, possibly less than 0.03%, of the surface area. As a result, the crust made by far the greatest contribution to radiative heat losses, estimated at between 1 and 3 MW per 30×30 m pixel, along most of the flow's length. Convective heat transfer into the air above the flow is about two thirds the corresponding radiative flux. These estimated surface heat losses suggest thicknesses of less than 1 m for a conductive thermal boundary layer component of the crust. Radiance values in the thermal infrared band 6 of the TM image are consistent with temperatures derived from the short‐wavelength infrared data and reveal the thermal manifestation of a topographic step beneath the flow.
Modeling of fluid flow and pore pressure at the toe of Oregon and Barbados Accretionary Wedges
Authors: Henry Pierre , Wang Chi‐Yuen
Published: February 2008

Fluid venting was recently found on the central Oregon accretionary wedge and evidence of fluid flow along the decollement were obtained in the Barbados wedge. The chemistry of hydrocarbons contained in the migrating fluids is different in these two areas and indicates that the main source is deep for the Barbados decollement and shallow for the Oregon vents. We apply a finite element model, with imposed thrusting kinematics and nonlinearities in the compaction equation, to the toe of both wedges. No deep fluid source is included; thus, the main source is the footwall just below the frontal thrust. The model indicates the following: (1) Pore pressure build up would be essentially controlled by kN, the equivalent permeability in the direction normal to the strata. In spite of very different sediment lithologies, kN is of the order of 10−17 m2 for both Oregon turbidites (Horath, 1989) and Barbados muds (Taylor and Leonard, 1990), and no high pore pressures can be obtained inside the offscraped sediments. (2) Permeability anisotropy greatly modifies fluid flow paths. A fault with a permeability anisotropy of 103 would channel fluids efficiently but leave the footwall pressure almost unchanged. A stratigraphic permeability anisotropy of 102 (as documented for the Oregon turbidites) would redirect the flow of pressurized fluids in the footwall along strata into the trench. If clogging of the frontal thrust occured, fluids expelled from the footwall would move across the frontal thrust, migrate along permeable strata in the hanging wall and then be vented near the top of the eroded anticline (as observed in Oregon). (3) Comparison of published estimates from data of fluid explusion fluxes with the modeling results suggests that a deeper and larger fluid source is needed at the Barbados site but is not required for the Oregon site.
Effects of ridge crest‐trench interaction on Antarctic‐Phoenix Spreading: Forces on a young subducting plate
Authors: Larter Robert D. , Barker Peter F.
Published: February 2008

Precise measurements of spreading rates on marine magnetic profiles collected to the west of the Antarctic Peninsula have enabled some consideration of the forces governing plate motion, since Antarctic—Phoenix motion has been controlled by the local rather than global force balance over the last 35 m.y. The total effective driving force per unit length of trench is calculated to have ranged between 2.6 and 3.6×1012 N/m, which is much less than is commonly thought necessary to support subduction. Conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench. The low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed. Driving forces show a strong correlation with observed spreading rates, which indicates that resistive forces were largely velocity dependent. Fluid migration up the subduction zone may elevate temperatures in and around the shear zone, reducing resistive forces below the levels required by purely conductive models. Changes in convergence rate may affect the depths of both the brittle/ductile deformation boundary and the basalt/eclogite phase change, causing a negative feedback which would appear as a velocity‐dependent resistive force. The different driving forces acting on the NE and SW parts of the Phoenix plate, as a consequence of older oceanic lithosphere at the trench in the NE, caused Antarctic–Phoenix spreading to take place about a near pole to the SW since 21 m.y. ago, and ultimately resulted in disruption of the Phoenix plate about 9 m.y. ago. Spreading rates decreased abruptly about 6 m.y. ago, probably because of E‐W compression across the long transform faults bounding the Phoenix plate. However, spreading on the last three segments of the Antarctic–Phoenix Ridge continued at least until 4 m.y. ago. Either spreading stopped progressively from SW to NE, or the final stage took place about a very near pole to the SW. A magnetic quiet zone extends up to 95 km from the margin between the Tula Fracture Zone and the North Anvers Fracture Zone, and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision. The absence of a magnetic quiet zone associated with the most recent ridge crest–trench collisions suggests a change in sedimentary regime during the late Miocene. Anomalously fast apparent spreading rates between 23 and 21 m.y. ago are thought to indicate an error in this part of the magnetic reversal time scale.
Structure of the crust and upper mantle beneath the Great Valley and Allegheny Plateau of eastern Pennsylvania 1. Comparison of linear inversion methods for sparse wide‐angle reflection data
Authors: Hawman Robert B. , Phinney Robert A.
Published: February 2008

We use τ(p) data extracted from a small number of wide‐angle recordings of quarry blasts to construct averaged, one‐dimensional velocity models of the crust beneath portions of the Great Valley, Newark Basin, Valley and Ridge, and Allegheny Plateau of the central Appalachians of Pennsylvania. Using the linear form of the equations for τ(p) in terms of slowness‐depth structure, we compare inversion results for the extremal, Backus‐Gilbert, and generalized least squares methods. Although the uncertainties are large, the models do show a well‐constrained increase in midcrustal velocities and crustal thickness beneath the Allegheny Plateau. Inversion of precritical reflections from the Moho suggests bounds of 7.1–7.6 km/s on P wave velocity and 3.9–4.2 km/s on S wave velocity near the base of the crust. P wave velocity models for the Great Valley show a crustal thickness between 40 and 45 km, with an average velocity between 6.4 and 6.6 km/s. Models for the Allegheny Plateau show a larger crustal thickness (47–52 km) and a much higher average velocity (6.8–6.9 km/s). Estimates of average shear wave velocities for the Great Valley range from 3.6 to 3.8 km/s, with crustal thickness estimates between 37 and 44 km. For the relatively small number of singular values retained, standard errors in depth for models derived by generalized least squares range from ±1 to 2 km for the P wave slowness models and from ±2 to 4 km for the S wave models; extremal depth bounds in general are 2 to 3 times as wide. Corresponding uncertainties in interval velocity, estimated from resolving kernels in slowness, range from 0.15 to 0.45 km/s for P wave models and from 0.15 to 0.40 km/s for S wave models. T2–X2 inversion of PmP data gives similar estimates for total crustal thickness and average velocity after correction for refraction effects. T2–X2 inversion of PmP data for a fourth profile suggests the possibility of a slight thickening of the crust (47–48 km) directly beneath the axis of the Great Valley gravity low. Estimates of average VP/VS for the crust based on average velocities for models derived by generalized least squares inversion range from 1.73 to 1.77. Estimates based on travel time ratios for events interpreted as P wave and S wave reflections from the Moho lie between 1.75 and 1.79. For a crust in which the effects of velocity anisotropy can be neglected, these estimates correspond to crustal averages between 0.25 and 0.27 for Poisson's ratio. The one‐dimensional velocity models derived here provide estimates of the long‐wavelength component of velocity structure. Besides demonstrating the maximum resolving power inherent in a limited τ(p) data set, these averaged models can be used for migrating reflection data and as starting models for determining two‐dimensional velocity structure.
Diminished pore pressure in low‐porosity crystalline rock under tensional failure: Apparent strengthening by dilatancy
Authors: Schmitt Douglas R. , Zoback Mark D.
Published: February 2008

Rupture tests on internally pressurized, thin‐walled hollow cylinders of Westerly granite with impermeable inner membranes suggest that the conventional, or Terzaghi, effective stress law does not describe tensile failure at high internal pressurization rates near 6 MPa/s. Unjacketed and saturated samples, with an initial pore pressure and for which the inner cavity pressure was increased rapidly with respect to the diffusivity, display substantially increased apparent tensile strengths and deformational moduli much higher than similarly configured but more slowly pressurized tests. Alternatively, the properties of completely dry test pieces with no pore pressure show little, if any, dependence on pressurization rate. Further, the behavior of the rapid unjacketed tests was similar to that for completely dry samples. These observations cannot be explained by the predicted undrained response, but they provide indirect evidence for diminished pore pressure effects reminiscent of dilatant hardening observed in compressive failure experiments. Calculated pore pressure diffusion rates support this suggestion as pore pressure perturbations cannot be damped out on the time scale of the rapidly pressurized tests. It is not clear if these effects are produced by elastic microcrack dilatancy, of which the nonlinear stress‐strain curve of granites is symptomatic, or the irreversible production of new porosity as in compressive shear failure tests.
Heat sources for Tertiary metamorphism and anatexis in the Annapurna‐Manaslu Region central Nepal
Authors: England Philip , Le Fort Patrick , Molnar Peter , Pêcher Arnaud
Published: February 2008

Following the collision of India with southern Tibet, crustal rocks of the leading edge of India (1) underwent regional metamorphism to upper amphibolite grade, (2) melted locally to produce anatectic granitoids, and (3) were sheared and thrust onto lower grade rock along the Main Central Thrust, yielding an inverted metamorphic sequence. This sequence is exemplified in the Annapurna‐Manaslu region. We use simple physical calculations to examine the heat sources involved in the different phases of metamorphism. The regional metamorphism apparently is due to the burial of the northern edge of India beneath the accretionary prism along the southern edge of Tibet. The observed temperatures and pressures of this first phase of metamorphism are consistent with the thermal relaxation, during the 10–30 m.y. before slip on the Main Central Thrust began, of thickened continental lithosphere whose original surface heat flux was between 50 and 70 mW m−2. The release of water from the footwall of the Main Central Thrust apparently facilitated melting of the overlying crust in the second phase. Such melting could have occurred in the first million years or so of thrusting, if warm (550–650°C) crust in the footwall contained the necessary water. If melting did not occur in the earliest stages of slip on the Main Central Thrust, dissipative heating, with shear stresses of 10 to 100 MPa, is required for temperatures near the Main Central Thrust to have remained high enough to generate melting above the fault during the underthrusting of cold material. The thickness (6–8 km) of the zone of inverted isograds associated with the fault, if undisrupted and due solely to thermal diffusion, implies that the time required to carry the rocks preserving the inverted metamorphism from the surface to depths of 30–40 km was 4–8 m.y. The apparent inverted temperature gradients (about 10–25°C/km) in this zone can be understood as the combined result of two processes. Diffusion of heat from hot rock thrust over cold rock expunged the original temperature gradient near the fault and could have created an inverse gradient of 10–20°C/km. The peak temperatures in such a zone, however, would not have exceeded about 350°C without an additional source of heat. Dissipative heating at shear stresses of about 100 MPa can account for peak temperatures in excess of 600°C during slip on the fault and would have contributed as much as 13°C/km to the inverse gradient. Although inversion of metamorphic isograds could have occurred as a result of deformation within the Main Central Thrust zone, the high temperatures during slip on this zone still require dissipative heating, unless the duration of slip exceeded 25 m.y.
Scattering of monochromatic acoustic and electromagnetic plane waves by two quarter spaces
Authors: Sampaio Edson E. S. , Fokkema Jacob T.
Published: February 2008

The plane wave scattering in a two‐dimensional medium consisting of two quarter spaces below a half‐space, also known as the vertical fault model, is the simplest two‐dimensional model applicable to geophysical exploration. The wave field obeys the two‐dimensional Helmholtz equation. Its formulation is equivalent to the electromagnetic case when the fundamental unknown is the horizontally polarized electric field vector and to the acoustic case when the unknown is the pressure. In the electromagnetic case the medium parameters are the electrical conductivity and the magnetic permeability, and in the acoustic case they are the compressibility and the density. The formulation of the scattered field in all space is defined by an integral representation composed of two one‐sided Fourier transforms in each medium. It represents an extension of Weaver's formulation. The spectral constituents of the Fourier integrals are determined by means of the Neumann's series, as a solution to the set of integral equations obtained by application of the boundary conditions. The numerical results show that the series converge and that the field values have a precision always better than 5%. The same formulation can be extended to solve the problem of scattering in more complex two‐dimensional or in three‐dimensional medium constituted of perpendicular blocks. Fields and spectral constituents may be used in the inversion of geophysical data.
Depth distribution of moment release in underthrusting earthquakes at subduction zones
Authors: Zhang Zhi , Schwartz Susan Y.
Published: February 2008

We present a global analysis of the depth distribution of moment release along the thrust contact at subduction zones using data from Harvard's centroid moment tensor catalogue. Substantial regional variations in the depth distribution of moment release are observed. A significant correlation exists between the depth of maximum moment release and the convergence rate of subducting lithosphere and the sediment thickness at the trench. Subduction zones with fast convergence rates and thick sediment accumulations concentrate moment release at greater depths than subduction zones with slow convergence rates and little sediment accumulation. The correlation between the convergence rate of oceanic lithosphere and the depth distribution of moment release may be best explained by differences in the thermal structure of the plate interface. Subduction zones with fast convergence velocities transport cold material to greater depths than subduction zones with slow convergence velocities. The colder temperature of the deep portions of the thrust contact at subduction zones with fast convergence velocities allows seismic failure to occur at greater depth than possible in subduction zones with slow convergence velocities. As has been demonstrated in intraplate environments, our observations in interplate thrust settings support the idea that temperature is the most important factor controlling the maximum depth of seismic failure. In subduction zones with thick sediment accumulations, the stable sliding properties of unconsolidated sediments may be responsible for the low levels of shallow moment release.
An Eulerian technique for thermomechanical modeling of lithospheric extension
Authors: Christensen U. R.
Published: February 2008

A two‐dimensional Eulerian spline function finite element method for modeling the coupled thermomechanical processes which control lithospheric extension is presented. The model domain comprises a cross section through the lithosphere and part of the asthenosphere. Material may enter or leave through the bottom and side boundaries. On the side boundary a fixed velocity or a fixed force condition can be applied. Highly nonlinear and thermally activated creep laws are used for the brittle and ductile rheologies of the lithosphere. Compositional differences between crust and mantle are described by a continuous function which is advected and which affects density, rheology, and heat production. An approximate topography is calculated from the normal stress at a free‐slip surface boundary. A set of sample case studies is presented which highlights the capabilities and possible applications of the method. It also serves to study the influence of numerical parameters and of conceptual assumptions in various nontrivial applications. The examples comprise narrow rifting continuing into the stage of seafloor spreading, formation of a wide basin, formation of a metamorphic core complex, and impact of a mantle plume onto the bottom of the lithosphere. In the last case the entire upper mantle is modeled, and the instability of a thermal bottom boundary layer gives rise to a diapiric mantle plume. The main weakness of the Eulerian approach as compared to the Lagrangian technique was found to be the effective blurring of material discontinuities like the Moho. This is of little consequence in pure shear deformation, but when strong simple shear occurs, it impairs the resolution. The main advantage of the Eulerian technique is that very large deformation poses no problem as it does for the Lagrangian method. The new approach is especially suited to study processes where strong differential flow occurs in part of the modeled system. A further possible advantage of the method is its efficiency in terms of computer time.
A seismotectonic model for western Hawaii based on stress tensor inversion from fault plane solutions
Authors: Gillard D. , Wyss M. , Nakata J. S.
Published: February 2008

For 57 fault plane solutions of earthquakes with magnitude ML≥3.0 located west of the southwest rift zone of Mauna Loa volcano in Hawaii, which occurred between 1972 and 1988, the dominant focal mechanisms (44 events) are decollement type with one nodal plane nearly horizontal (dip ≤ 30°). The average slip vector of the upper crust on the decollement plane points in an azimuth of 260° toward the ocean, away from Mauna Loa's southwest rift zone at an angle of about 150° with respect to the NE‐SW oriented rift. Two other types of focal mechanisms are present: normal faults (four events) and strike‐slip faults with normal component (nine events). The orientation of the principal stresses was derived by minimizing the sum of the misfits between the theoretical and observed fault geometry for each focal mechanism. After subdividing the data set into three different regions we found that the stress tensor is not homogeneous in west Hawaii. The orientation for the stress tensor is best resolved in the area located between latitudes 19.27°N and 19.4°N and longitudes 155.7° and 155.9° W where the greatest principal stress directions within the 95% confidence limits are nearly vertical with some spreading to the west. Their plunge varies between 53° and 85°. The intermediate and least principal stresses are mostly horizontal and have similar magnitudes. Their plunge varies between 1° and 36°. The area between latitudes 19.4° and 19.6°N and longitudes 155.7° and 156°W is characterized partly by near‐vertical and partly by east‐west oriented greatest principal stresses. The least principal stresses are approximately horizontal and have magnitudes similar to the intermediate principal stresses. In the two areas where the stress tensor was well resolved, the principal strain directions differed from the principal stress directions by 30° in the plunge, suggesting that the faulting in western Hawaii takes place on a weak plane, but this result could not be established at the 95% confidence level. The tectonic model proposed for west Hawaii is similar to the model for the Kalapana region. The strain is accumulated through magmatic intrusions in the southwest rift zone of Mauna Loa, and the earthquakes occur along a zone of weakness composed of oceanic sediment at about 10 km depth. The west flank of Mauna Loa slips in the direction away from the rift.
Influence of the lower crustal structure on the early coda of regional seismograms
Authors: Campillo Michel , Paul Anne
Published: February 2008

We use synthetic seismograms for a flat‐layered Earth model to study the influence of the widely observed strong reflectivity of the lower crust on the propagation of seismic regional phases. The lower crustal layering induces a clear increase in the duration of the Pg and Lg phases on both the vertical and radial components beyond the group velocity of their terminations in a model with a homogeneous lower crust. The transverse component is weakly affected. The amplitude and the spectral content of the early Lg coda are strongly dependent on the distribution of layer thicknesses. The predominance of the low‐frequency components (below 2 Hz) observed in the Lg coda of real data is correctly accounted for with 1‐km‐thick layers in the lower crust. From particle motions at various depths and the analysis of the dispersion curves of normal modes, we hypothesize that the late arrivals result from the interaction of plate‐type disturbances of the high‐impedance layers. These waves leak energy in the mantle at a low rate. This mode of generation of the coda waves explains the puzzling observation that the spreading coefficient of Lg seems to be applicable to the early coda of regional seismograms. The study of the influence of the lamellae on the average Q factor of the continental crust shows that the lower crust layering does not account for the observed low values of the apparent Qs. Another origin is needed for this strong attenuation, for example scattering by randomly distributed heterogeneities.
Geodetic analysis of dike intrusion and motion of the magma reservoir beneath the summit of Kilauea Volcano, Hawaii: 1970–1985
Authors: Yang Xuemin , Davis Paul M. , Delaney Paul T. , Okamura Arnold T.
Published: February 2008

We use leveling and trilateration data collected on Kilauea volcano to constrain the location of deformation sources caused by magma accumulation, intrusion, and eruption. For the 13 inflationary epochs examined, combinations of an expanding point source and one or two opening rectangular dislocations mimic inflation of the summit reservoir and formation of dike(s), respectively. The combined model adequately accounts for the deformation data and is consistent with the seismicity observed during each epoch. For 10 deflationary epochs, however, the data require only a contracting point source. Confidence in these results is gained by noting that locations of the sources of both inflation and deflation are coincident, within the observed uncertainties of the data, the function of network geometry, and the inversion procedure. It appears, therefore, that magma accumulation at Kilauea volcano may be characterized by the growth of dikes during inflation of the summit reservoir. Drainage of the reservoir, on the other hand, is not accompanied by significant closure of dikes. In contrast to previous studies (e.g., Fiske and Kinoshita, 1969; Dvorak et al., 1983) that do not include the dislocation (or dike growth) component of summit magma accumulation and concluded that the source of inflation migrates over a 5 km2 area, we find that a single magmatic reservoir source accounts for data collected during all inflationary and deflationary epochs, results, which compare favorably with those obtained from the point ellipsoid model, can be used to estimate the distribution of stresses within the volcano in the near field of the source.
nonhydrostatic and nonequilibrium thermodynamics of deformable materials
Authors: Shimizu Ichiko
Published: February 2008

The nonequilibrium thermodynamics of polycrystalline solids under nonhydrostatic stress are formulated, based on the conservation laws of classical dynamics and the second law of thermodynamics. For a closed system, the thermodynamic potential function corresponding to the Gibbs free energy, in classical thermodynamics, is defined as a Legendre transformation of the internal energy of the system, whereas it cannot be defined in an open system under stress. Chemical potential is defined at source points where addition of matter is possible. Chemical potential for grain growth (μ+) under nonhydrostatic stress varies with the orientation of grain surfaces and with direction of grain growth. Pressure solution, diffusion creep, and anisotropic phase equilibrium under stress are explained by the orientation dependence of μ+.
How deep do common basaltic magmas form and differentiate?
Authors: Albarede Francis
Published: February 2008

This work investigates the potential of experiments on peridotite melting to provide quantitative estimates of the temperature and pressure at which basaltic melts were formed. As the primary chemical signal is systematically obscured by crystal fractionation, the first part of this study is devoted to the analysis of the main differentiation trends in the basaltic series from the major geodynamic sites: mid‐ocean ridges, hot spots (both intraplate and near the ridge axis), continental floods, and orogenic boundaries. Two main sorts of primary magmas can be identified: (1) the parent magmas of mid‐ocean ridge, continental flood, and orogenic tholeiites which are poor in Ti and Fe, rich in Al and, to a lesser extent, Na and (2) the parent magmas of within‐plate ocean island basalts (OIB) which have the opposite characteristics. The importance of plagioclase during postmelting fractionation increases in the following order: OIB, island arc tholeiites (IAT), low‐Ti continental flood basalts (CFB), high‐Ti CFB, and mid‐ocean ridge basalts (MORB) corresponding to differentiation pressure decreasing from >10 to ∼5 kbar. Ocean islands that formed near ridge axis have parent magmas and differentiation conditions intermediate between those of MORB and within‐plate OIB. Along‐ridge variations in TiO2 and Na2O contents in MORB are ascribed to mantle heterogeneities and suggest pervasive infiltration of an OIB component. The chemical compositions of synthetic melts in equilibrium with peridotite have been fitted not as a function of the degree of melting, which is neither available from the sandwich experiments nor independent of the fertility of the source, but as a function of temperature (mean error, 40°C) and pressure (mean error, 2.7 kbar). Although fractionation of multimineral assemblages cannot be accounted for with precision, the effect of individual minerals, plagioclase, clinopyroxene, and olivine can be calculated. Assuming that primary melts are in equilibrium with a mantle olvine Fo90–92, it is suggested that MORB segregated from their mantle source at ∼15 ± 5 kbar. Shield‐stage ocean island basalts (Hawaii, Gough, Réunion) and Solomon Island picrites segregated at 20–30 kbar, whereas even greater depths are suggested for Tahiti lavas. The estimated thickness of the molten layer from which the lavas evolve depends on the degree of melting assumed. It is ∼50 km under the ridges, and 20 km or even less at the base of the lithosphere under intraplate volcanoes.
Experimental determination of elastic constants of Oshima granite, Barre granite, and Chelmsford granite
Authors: Sano Osam , Kudo Yozo , Mizuta Yoshiaki
Published: February 2008

Employing polyhedral specimens, longitudinal and shear wave velocities were measured in various directions of propagation and polarization. Sound velocities showed orthorhombic elasticity in all of the rocks. With the assumption of orthorhombic elasticity the nine stiffness constants of all samples were determined by the sound velocities under atmospheric pressure and Kelvin‐Christoffel's equation. Twenty‐one stiffness constants of Oshima granite, determined without assuming any symmetry, also showed orthorhombic features. Directions of the symmetry axes agreed well with the orientation of the preexisting cracks. Akaike's Information Criterion showed that the orthorhombic model with nine nonzero elastic stiffnesses was better than the model having 21 nonzero elastic stiffnesses for Oshima granite. The polyhedrons of two granitic rocks were loaded under hydrostatic pressure. All components of the stiffness constants increased with pressure. Under pressure of more than 120 MPa, two granitic rocks were approximately isotropic. The results show that oriented microcracks are mainly responsible for the orthorhombic elasticity of the granitic rocks and also indicate that dry oriented cracks can not be a cause for the anisotropic elasticity of granites at depths of more than 6–8 km.
Depth and degree of melting of komatiites
Authors: Herzberg Claude
Published: February 2008

High pressure melting experiments have permitted new constraints to be placed on the depth and degree of partial melting of komatiites. Komatiites from Gorgona Island were formed by relatively low degrees of pseudoinvariant melting (< 30%) involving L + Ol + Opx + Cpx + Gt on the solidus at 40 kbar, about 130 km depth. Munro‐type komatiites were separated from a harzburgite residue (L + Ol + Opx) at pressures that are poorly constrained, but were probably around 50 kbar, about 165 km depth; the degree of partial melting was <40%. Komatiites from the Barberton Mountain Land were formed by high degrees (∼50%) of pseudoinvariant melting (L + Ol + Gt + Cpx) of fertile mantle peridotite in the 80‐ to 100‐kbar range, about 260‐ to 330‐ km depth. Secular variations in the geochemistry of komatiites could have formed in response to a reduction in the temperature and pressure of melting with time. The 3.5 Ga Barberton komatiites and the 2.7 Ga Munro‐type komatiites could have formed in plumes that were hotter than the present‐day mantle by 500° and 300°, respectively. When excess temperatures are this size, melting is deeper and volcanism changes from basaltic to komatiitic. The komatiites from Gorgona Island, which are Mesozoic in age, may be representative of komatiites that are predicted to occur in oceanic plateaus of Cretaceous age throughout the Pacific [Storey et al., 1991].