Showing papers on "Slab published in 1996"

Dominant two‐dimensional solar wind turbulence with implications for cosmic ray transport

Abstract: Two new methods for distinguishing two-dimensional (2D) turbulence from slab turbulence are applied to Helios magnetometer data. Two-component models with varying slab and 2D ingredients are considered. Both methods indicate that solar wind magnetic turbulence possesses a dominant (∼85 % by energy) 2D component. The presence of such a large 2D component provides a natural solution to the long-standing problem of “too small” cosmic ray mean free paths derived from quasilinear scattering theory when using the slab model.

Petrogenesis of slab-derived trondhjemite–tonalite-dacite/adakite magmas

Abstract: The prospect of partial melting of the subducted oceanic crust to produce arc magmatism has been debated for over 30 years. Debate has centred on the physical conditions of slab melting and the lack of a definitive, unambiguous geochemical signature and petrogenetic process. Experimental partial melting data for basalt over a wide range of pressures (1–32 kbar) and temperatures (700–1150°C) have shown that melt compositions are primarily trondhjemite–tonalite–dacite (TTD). High-Al (> 15% Al 2 O 3 at the 70% SiO 2 level) TTD melts are produced by high-pressure (≥ 5 kbar) partial melting of basalt, leaving a restite assemblage of garnet + clinopyroxene ± hornblende. A specific Cenozoic high-Al TTD (adakite) contains lower Y, Yb and Sc and higher Sr, Sr/Y, La/Yb and.Zr/Sm relative to other TTD types and is interpreted to represent a slab melt under garnet amphibolite to eclogite conditions. High-Al TTD with an adakite-like geochemical character is prevalent in the Archean as the result of a higher geotherm that facilitated slab melting. Cenozoic adakite localities are commonly associated with the subduction of young ( −1 ) conducive for slab dehydration melting. Viable alternative or supporting tectonic effects that may enhance slab melting include highly oblique convergence and resultant high shear stresses and incipient subduction into a pristine hot mantle wedge. The minimum P–T conditions for slab melting are interpreted to be 22–26 kbar (75–85 km depth) and 750–800°C. This P–T regime is framed by the hornblende dehydration, 10°C/km, and wet basalt melting curves and coincides with numerous potential slab dehydration reactions, such as tremolite, biotite + quartz, serpentine, talc, Mg-chloritoid, paragonite, clinohumite and talc + phengite. Involvement of overthickened (>50 km) lower continental crust either via direct partial melting or as a contaminant in typical mantle wedge-derived arc magmas has been presented as an alternative to slab melting. However, the intermediate to felsic volcanic and plutonic rocks that involve the lower crust are more highly potassic, enriched in large ion lithophile elements and elevated in Sr isotopic values relative to Cenozoic adakites. Slab-derived adakites, on the other hand, ascend into and react with the mantle wedge and become progressively enriched in MgO, Cr and Ni while retaining their slab melt geochemical signature. Our studies in northern Kamchatka, Russia provide an excellent case example for adakite-mantle interaction and a rare glimpse of trapped slab melt veinlets in Na-metasomatised mantle xenoliths.

Metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere

Abstract: Earth's deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km. This depth interval closely brackets the mantle transition zone, characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher-pressure phases. Deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal, thermodynamic, and mechanical ferment inside slabs. Numerical simulations of reaction rates show that the olivine → spinel transformation should be kinetically hindered in old, cold slabs descending into the transition zone. Thus wedge-shaped zones of metastable peridotite probably persist to depths of more than 600 km. Laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting. This instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser, finer-grained phase. Hence in cold slabs, such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite. It is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs. This consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features. It explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone, where minerals in downgoing slabs should transform to spinel and ilmenite structures. The onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab. Even if a slab penetrates into the lower mantle, earthquakes should cease at depths near 700 km, because the seismogenic phase transformations in the slab are completed or can no longer occur. Substantial metastability is expected only in old, cold slabs, consistent with the observed restriction of deep earthquakes to those settings. Earthquakes should be restricted to the cold cores of slabs, as in any model in which the seismicity is temperature controlled, via the distribution of metastability. However, the geometries of recent large deep earthquakes pose a challenge for any such models. Transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics, including focal mechanisms indicating localized shear failure rather than implosive deformation, are so similar to those of shallow earthquakes. Finally, metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab. Such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and/or are detached from subducting slabs. Metastability in downgoing slabs could have considerable geodynamic significance. Metastable wedges would reduce the negative buoyancy of slabs, decrease the driving force for subduction, and influence the state of stress in slabs. Heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage, causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity. Because wedge formation should occur only for fast subducting slabs, it may act as a “parachute” and contribute to regulating plate speeds. Wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel, favoring the subsequent sinking of the slab into the lower mantle.

Extracting convergent surface energies from slab calculations

Abstract: The formation energy of a solid surface can be extracted from slab calculations if the bulk energy per atom is known. It has been pointed out previously that the resulting surface energy will diverge with slab thickness if the bulk energy is in error, in the context of calculations which used different methods to study the bulk and slab systems. We show here that this result is equally relevant for state-of-the-art computational methods which carefully treat bulk and slab systems in the same way. Here we compare different approaches, and present a solution to the problem that eliminates the divergence and leads to rapidly convergent and accurate surface energies.

Extracting convergent surface energies from slab calculations

Abstract: The formation energy of a solid surface can be extracted from slab calculations if the bulk energy per atom is known. It has been pointed out previously that the resulting surface energy will diverge with slab thickness if the bulk energy is in error, in the context of calculations which used different methods to study the bulk and slab systems. We show here that this result is equally relevant for state-of-the-art computational methods which carefully treat bulk and slab systems in the same way. Here we compare different approaches, and present a solution to the problem that eliminates the divergence and leads to rapidly convergent and accurate surface energies.

Structure and Evolution of Lithospheric Slab Beneath the Sunda Arc, Indonesia

Abstract: Tomographic imaging reveals seismic anomalies beneath the Sunda island arc, Indonesia, that suggest that the lithospheric slab penetrates to a depth of at least 1500 kilometers The Sunda slab forms the eastern end of a deep anomaly associated with the past subduction of the plate underlying the Mesozoic Tethys Ocean In accord with previous studies, the lithospheric slab was imaged as a continuous feature from the surface to the lower mantle below Java, with a local deflection where the slab continues into the lower mantle The deep slab seems to be detached from the upper mantle slab beneath Sumatra This complex slab structure is related to the Tertiary evolution of southeastern Asia and the Indian Ocean region

State of stress in the Southern Tyrrhenian subduction zone from fault-plane solutions

Abstract: SUMMARY In this paper we present revised locations and original focal mechanisms computed for intermediate and deep earthquakes that occurred within the Southern Tyrrhenian subduction zone between 1988 and 1994, in order to improve our knowledge of the state of stress for this compressional margin. In particular, we define the stress distribution within a large portion of the descending slab, between 40 and about 450 km depth. The seismicity distribution reveals a continuous 40-50 km thick slab that abruptly increases its dip from subhorizontal in the Ionian Sea to a constant 70" dip in the Tyrrhenian. We computed focal mechanisms for events with magnitudes ranging from 2.7 and 5.7, obtaining the distribution of P- and T-axes for many events for which centroid moment tensor (CMT) solutions are not available, thus enabling the sampling of a larger depth range compared to previous studies. We define three portions of the slab characterized by different distributions of P- and T-axes. A general down-dip compression is found between 165 and 370 km depth, whereas in the upper part of the slab (40-165 km depth) the fault-plane solutions are strongly heterogeneous. Below 370 km the P-axes of the few deep events located further to the north have a shallower dip and are not aligned with the 70" dipping slab, possibly suggesting that they belong to a separated piece of subducted lithosphere. There is a good correspondence between the depth range in which the P-axes plunge closer to the slab dip (-70") and the interval characterized by the highest seismic energy release (190-370 km).

Dynamic models of subduction: geophysical and geological evidence in the Tyrrhenian Sea

Abstract: SUMMARY Predictions based on a 2-D finite-element model for subduction underneath the Calabrian Arc in southern Italy are compared with a variety of geophysical and geological data, such as the present-day stress pattern within the slab, uplift from the elevation of marine terraces in Calabria and subsidence in the Tyrrhenian Marsili Basin from ODP Leg 107. We model the behaviour of the slab driven by slab pull, in agreement with the present tectonic style in this part of the Mediterranean as suggested by several investigators. The model accounts for the crustal, lithospheric and mantle structures in a vertical cross-section perpendicular to the Calabrian subduction zone. The shape of the slab is constrained on the basis of new tomographic images in the southern Tyrrhenian Sea, which were obtained from the regional seismic stations of the Istituto Nazionale di Geofisica, while the rheological properties of the mantle are taken from global dynamic models. Density contrasts between the subducted slab and the surrounding mantle, based on petrological models, drive the flow in our viscoelastic model; stress values, displacements and vertical velocities at the surface are sampled at different times after loading until dynamic equilibrium is reached. Our estimates are appropriate for a time window of 100 kyr; the validity of our comparison with the geological record is based on the assumption that the tectonic configuration in the past was not substantially different from that of the present day. Two families of models, with unlocked and locked subduction faults, are considered. The unlocked models allow for roll-back of the trench of about 20 mm yr-', in agreement with some geological estimates; the same family of models predicts uplift of the Calabrian Arc of about 1 mm yr-l and subsidence in the Marsili Basin of 1-2 mm yr-', in agreement with geological surveys. The deviatoric stress obtained from the unlocked model is consistent with the continuous distribution of deep seismicity in the southern Tyrrhenian Sea, with minor concentration within the lithospheric wedge. Locked models fail to reproduce these geophysical and geological observations. Predictions derived from a detached slab model are not consistent with the continuous hypocentral distribution of deep seismicity in the southern Tyrrhenian Sea. Deformation at the surface and the stress patterns at depth for a detached slab differ substantially from those of a continuous plate: dynamic topography and horizontal motions are reduced, when compared with the continuous plate, with deviatoric stresses concentrated within the relict slab. Our results indicate that subduction is a major tectonic process in the southern Tyrrhenian Sea.

Rheological control of oceanic crust separation in the transition zone

Abstract: Mineral physics observations suggest that distinct density and rheological differences exist between the crustal component of oceanic lithosphere and the underlying mantle. We have conducted numerical experiments to investigate the influence of both density and viscosity on the effectiveness of recycling of oceanic crust into the lower mantle. Confirming previous results, the density inversion at 670 km depth alone is not sufficient to prevent crustal recycling. However, a soft layer may exist between the strong garnet crust and cold slab interior. Models employing a simplified Newtonian sandwich model show that this thin, weak layer can effectively decouple the crust and slab. Once entrained into the lower mantle, the then lighter crust can rise sufficiently fast as a Rayleigh-Taylor instability to avoid further entrainment. These results suggest that the crustal component of slabs may be trapped at 670 km depth, leading to a garnet enriched transition zone.

Complex Structure of Mantle Discontinuities at the Tip of the Subducting Slab beneath Northeast China -A Preliminary Investigation of Broadband Receiver Functions-

Abstract: Broadband seismic waveform data recorded at stations in the western Pacific region are analyzed to investigate mantle discontinuities. By using teleseismic deep events, we observe unambiguous P-to-S conversion waves associated with the mid-mantle discontinuities in many of the individual seismograms. The commonly used method of stacking receiver functions is not so effective in this case due to the limited number of deep events. An inversion scheme is developed to determine a discontinuity response function at each station by fitting observed waveform data with superpositions of the P-to-S converted waves. Beneath the station in northeast China (MDJ) where the subducted Pacific plate appears to stagnate along the "660-km" discontinuity, the discontinuity response function has more complicated features than those of other stations. The preliminary results indicate no depression of the "660-km" discontinuity at the tip of the subducting slab beneath MDJ; instead a multiple-discontinuity structure down to a depth of 780 km is observed.

Lateral variation in slab orientation beneath Toba Caldera, northern Sumatra

Abstract: The Investigator Fracture Zone (IFZ) subducts beneath Toba Caldera, the Earth's largest Quaternary caldera, in northern Sumatra, suggesting a possible relationship between them. Locations of sub-crustal earthquakes based on arrival times of P and S waves at a seismograph network surrounding Toba reveal the geometry of the subducted slab and the IFZ beneath Toba. A vertical tear of less than 20 km in the slab across the IFZ, as previously suggested, cannot be ruled out but the large-scale geometry of the slab is dominated by a broad bend of slab contours parallel to the concaveseaward indentation of the trench. The slab shape is probably a response to the trench curvature, can explain the change in trend of the volcanic arc near Toba, and may cause shallowing of the forearc basin near Nias Island. The decrease in radius of curvature of the slab contours is not accompanied by an observable decrease in dip angle, possibly resulting in lateral compressive stress in the slab. The high rate of seismicity along the subducted Investigator Fracture Zone, that intersects the slab obliquely to its plunge direction, is uncommon at subducted fracture zones and is likely caused by such lateral stress in the slab.

Finite-Element Analysis of Temperature Effects on Plain-Jointed Concrete Pavements

Abstract: The finite-element study of the effect of temperature variation on plain-jointed concrete pavements is presented. Temperature variation causes curling and thermal-expansion stresses. Curling stresses result from temperature gradients through a slab depth. Thermal-expansion stresses are induced due to uniform changes in temperature that cause the slab to expand. The developed three-dimensional (3D) model consists of four slabs separated by longitudinal and transverse joints. The interaction between the ground and the concrete slab along with interaction at the joints were modeled using interface elements. These elements gave the model the capability to solve for partial contact between curled slabs and the ground to investigate the effect of compressive stresses that may develop at the joints during curling, and to study the influence of friction between slabs and the ground. The data obtained using the finite-element model has shown reasonable agreement with the results obtained from three computer models: KENSLABS, ILLI-SLAB, JSLAB, and the analytical solution proposed by Bradbury. The best correlation was obtained with JSLAB. The model was used to perform parametric studies on curling and thermal-expansion stresses to study the effect of superposition of both stresses and to address the effect of uniform temperature changes on joint opening. Another simpler model using nine layers across the depth of a pavement slab was used to introduce the effects of nonlinear temperature distribution. The results of parametric studies are presented and compared with other solutions. The arithmetic addition of positive curling stresses and thermal-expansion stresses were less than those stresses obtained by superposition. In some cases, the calculated joint openings were higher than the allowable joint opening. Nonlinear temperature distribution caused higher tensile stresses than the linear distribution of temperature. The difference in tensile stresses between the two distributions was approximately 3–13% of the modulus of rupture of concrete.

Cyclic stressing and seismicity at strongly coupled subduction zones

Abstract: We use the finite element method to analyze stress variations in and near a strongly coupled subduction zone during an earthquake cycle. Deformation is assumed to be uniform along strike (plane strain on a cross section normal to the trench axis), and periodic earthquake slip is imposed consistent with the long-term rate of plate convergence and degree of coupling. Simulations of stress and displacement rate fields represent periodic fluctuations in time superimposed on an average field. The oceanic plate, descending slab, and continental lithosphere are assumed here to respond elastically to these fluctuations, and the remaining mantle under and between plates is assumed to respond as Maxwell viscoelastic. In the first part of the analysis we find that computed stress fluctuations in space and time are generally consistent with observed earthquake mechanism variations with time since a great thrust event. In particular, trench-normal extensional earthquakes tend to occur early in the earthquake cycle toward the outer rise but occur more abundantly late in the cycle in the subducting slab downdip of the main thrust zone. Compressional earthquakes, when they occur at all, have the opposite pattern. Our results suggest also that the actual timing of extensional outer rise events is controlled by the rheology of the shallow aseismic portion of the thrust interface. The second part of the analysis shows the effects of mantle relaxation on the rate of ground surface deformation during the earthquake cycle. Models without relaxation predict a strong overall compressional strain rate in the continental plate above the main thrust zone, with the strain rate constant between mainshocks. However with significant relaxation present, a localized region of unusually low compressional, or even slightly extensional, strain rate develops along the surface of the continental plate above and somewhat inland from the downdip edge of the locked main thrust zone. The low strain rate starts in the middle or late part of the cycle, depending on position. This result suggests that the negligible or small contraction measured on the Shumagin Islands, Alaska, during 1980 to 1991, may not invalidate an interpretation of that region as being a moderately coupled subduction zone. In contrast, mantle relaxation causes only modest temporal nonuniformity of uplift rates in the overriding plate and of extensional stress rates in the subducting plate, even when the Maxwell time is an order of magnitude less than the recurrence interval.

Onset of mid-Cretaceous volcanism by elevation of the 670 km thermal boundary layer

Abstract: A pulse of ocean crustal production during the mid-Cretaceous begins at the same time that magnetic reversal frequency drops to zero. We suggest that slab penetration into the lower mantle caused a thermal boundary layer normally at 670 km to be rapidly advected upward as the slabs descended through the lower mantle. The onset of near-surface melting due to the rise of this boundary layer coincides with the slabs9 arrival at D″, which drove up the heat flux across the core-mantle boundary and stabilized the magnetic reversal process. The slab penetration event was coincident with the breakup of the Gondwana supercontinent and triggered the subsequent formation of oceanic plateaus in the central Pacific and Indian Ocean basins.

Phase transition buoyancy contributions to stresses in subducting lithosphere

Abstract: The sequence of phase transitions undergone by minerals with increasing depth in Earth's mantle is perturbed within subducting lithospheric slabs by their thermal structure. Such perturbation of equilibrium phase relations gives rise to relative buoyancy contrasts between slab and mantle that contribute to the state of stress within the slab. While other factors contribute to overall slab stresses, thermal and phase transition effects largely control the structure of the stress field within the slab. The resulting maximum in down-dip compressive stress within the slab corresponds to the observed peak in depth distribution of deep seismicity. Furthermore, metastable persistence of lower pressure phases within the cold slab should give rise to localized shear stresses whose distribution corresponds to observed features of subduction zone seismicity. These observations are independent of the variety of failure mechanisms proposed for deep seismogenesis.

Dispersion of regional body waves at 100–150 km depth beneath Alaska: In situ constraints on metamorphism of subducted crust

Abstract: Phase delays at high frequencies are observed in body waves that travel in the Alaska slab, along its strike at 100–150 km depth. The delays, between 2–6 Hz energy and the direct 0.5–1 Hz arrival, are 0.5–1.5 s for P waves and 1.5–4 s for S waves. Such dispersion suggests a waveguide structure that parallels the slab, perhaps near its top. A channel that is 2–6 km thick and 2.5–5% slower than surrounding mantle can explain the observations. The thickness of the layer is comparable to that of subducted oceanic crust or somewhat thinner. The layer may be crust that is slow at these depths. The required velocity anomaly is too small to be due to a continuous layer of metastable gabbro yet too large to represent an eclogite layer. It may indicate a mixture of the two, or persistence of hydrated mineral assemblages to depth.

The dynamic rupture process of the 1993 Kushiro-oki earthquake

Abstract: We have constructed a dynamic rapture model of the 1993 Kushiro-oki earthquake (Mw = 7.6) that is consistent with the observed seismograms. This earthquake occurred beneath the city of Kushiro, Japan, at a depth of about 100 km, and the aftershocks are limited to a horizontal plane between a double seismic zone in the subducting slab. Excellent near-field strong-motion records are available. In our analysis, a kinematic inversion is followed by a dynamic crack calculation, and this pair of operations is repeated until convergence to the data is obtained. Two different conditions concerning the dynamic stress drop were considered in the dynamic calculation, and a dynamic model was constructed for each condition. The difference in these conditions has little effect on the results. The final dynamic model shows that this earthquake occurred with a localized high stress drop of about 40 MPa in the subducting slab, which indicates that the region between the double seismic zone is strong enough to sustain such a high stress. One of the two high strength regions in the final model lies near the upper plane of the double seismic zone and represents a barrier due to a substantial change of stress state in the slab. In the region where the initial breaks occurred 5 s before the main rupture, both the strength and the stress drop are smaller than in the adjacent region. This implies that before the main rupture the accumulated stress had already been released in the area of the initial breaks. The aftershocks tend to occur near the high strength area; in some of them, the P wave first-motions are opposite to those of the main shock.

Insulated concrete slab assembly

Abstract: An insulated concrete slab assembly for use over expansive soils. A slab is cast in place by pouring concrete into preshaped insulating forms. The forms remain permanently in place and provide insulation to reduce heat transfer through the slab. The shape of the forms provides strengthening ribs on the slab. The ribs act as beams which allow the slab to bridge or cantilever over expansive soil between foundation walls. The forms are supported during construction by support members or legs which later prevent damage to the slab by eliminating or limiting the forces which can be applied to the slab by expanding soil.

Snow melt control system

Abstract: A snow and ice melt control system is provided to melt snow and ice from a slab, a temperature sensor means are provided at the input and output from the slab, and a differential temperature is generated reflecting the condition of the slab in respect to the ambient temperature. A controller is provided to adjust the amount of heat and heated fluid flowing to the slab to actively respond to environmental conditions to more efficiently melt snow and ice on the slab.

An optimization approach to time-domain electromagnetic inverse problem for a stratified dispersive and dissipative slab

Abstract: The one-dimensional electromagnetic inverse problem for a stratified dispersive and dissipative slab is considered in the time domain. An optimization approach is used to reconstruct the parameters using the measured reflected field and/or transmitted field. Wave-splitting is used, and an explicit expression for the gradient is derived by introducing dual functions. The reconstruction algorithm is tested with both synthetic and experimental data.