Showing papers on "Slab published in 2004"

Slab detachment control on mafic volcanic pulse and mantle heterogeneity in central Mexico

Abstract: Seismic tomography studies and plate reconstructions suggest that the Farallon slab broke off shortly before subduction ended off southern Baja California. However, the progress of detachment in time and space and its consequences on the volcanism of central Mexico have not so far been considered. Here I use the Neogene geologic record of central Mexico to propose a lateral propagation of slab detachment beneath the Trans-Mexican volcanic belt during the late Miocene. I suggest that the trace of the detachment is expressed by a short (2–3 m.y.), eastward-migrating pulse of mafic volcanism that took place from ca. 11.5 to ca. 6 Ma to the north of the Pliocene–Quaternary volcanic arc, as hot, subslab material flowing into the slab gap produced a transitory thermal anomaly in the mantle wedge. Slab detachment of the deeper and denser part of the plate was initiated in the southern Gulf of California area by the incoming of progressively younger oceanic lithosphere at the paleotrench that produced an increasing coupling between the Magdalena microplate and the overriding North American plate. The tear in the slab propagated eastward from the Gulf of California to the Gulf of Mexico, paralleling the southern Mexico trench system. The decrease in the Rivera–North America convergence rate between ca. 9 and 7 Ma appears to be related to the loss of slab pull after the detachment. Sparse oceanic-island–type basalts emplaced since the end of the Miocene in the Trans-Mexican volcanic belt are located above a trench-parallel slab window between the inferred detachment trace and the leading edge of the present slab, which has been detected seismically. In this context, the occurrence of these unusual intraplate magmas is easily explained by the infiltration of enriched asthenosphere into the subarc mantle.

Kinematics of subduction and subduction-induced flow in the upper mantle

Abstract: [1] Results of fluid dynamical experiments are presented to model the kinematics of lithospheric subduction in the upper mantle. The experiments model a dense high-viscosity plate (subducting lithosphere) overlying a less dense low-viscosity layer (upper mantle). The overriding lithosphere is not incorporated. Several important features of slab behavior were investigated including the temporal variability of hinge line migration, the kinematic behavior of the slab and the subduction-induced upper mantle flow. Both fixed and free trailing edge boundary conditions of the subducting plate were investigated. Results show that hinge line retreat is a natural consequence of subduction of a negatively buoyant slab. The migration rate increases until the slab approaches the upper-lower mantle discontinuity, resulting in a decrease in migration rate followed by a renewed increase and finally approaching a steady state. Slab retreat results in mantle flow, with material initially located underneath the slab flowing around the lateral slab edges toward the mantle wedge. Experimental results indicate that all rollback-induced flow occurs around the lateral slab edges, forcing the hinge line to attain a convex shape toward the direction of retreat. No signs for poloidal flow underneath the slab tip have been detected. Only a small component of toroidal-type flow was observed underneath slanting slab tips. For a fixed trailing edge, the slab does not sink vertically downward, but sinks at an angle in a regressive manner. For a free trailing edge, slab sinking is oriented more vertically while the surface part of the subducting plate is pulled into the subduction zone.

Systematic variation in the depths of slabs beneath arc volcanoes

Abstract: SUMMARY The depths to the tops of the zones of intermediate-depth seismicity beneath arc volcanoes are determined using the hypocentral locations of Engdahl et al. These depths are constant, to within a few kilometres, within individual arc segments, but differ by tens of kilometres from one arc segment to another. The range in depths is from 65 km to 130 km, inconsistent with the common belief that the volcanoes directly overlie the places where the slabs reach a critical depth that is roughly constant for all arcs. The depth to the top of the intermediate-depth seismicity beneath volcanoes correlates neither with age of the descending ocean floor nor with the thermal parameter of the slab. This depth does, however, exhibit an inverse correlation with the descent speed of the subducting plate, which is the controlling factor both for the thermal structure of the wedge of mantle above the slab and for the temperature at the top of the slab. We interpret this result as indicating that the location of arc volcanoes is controlled by a process that depends critically upon the temperature at the top of the slab, or in the wedge of mantle, immediately below the volcanic arc.

Variable‐rate selective excitation for rapid MRI sequences

Abstract: Balanced steady-state free precession (SSFP) imaging sequences require short repetition times (TRs) to avoid off-resonance artifacts. The use of slab-selective excitations is common, as this can improve imaging speed by limiting the field of view (FOV). However, the necessarily short-duration excitations have poor slab profiles. This results in unusable slices at the slab edge due to significant flip-angle variations or aliasing in the slab direction. Variable-rate selective excitation (VERSE) is a technique by which a time-varying gradient waveform is combined with a modified RF waveform to provide the same excitation profile with different RF power and duration characteristics. With the use of VERSE, it is possible to design short-duration pulses with dramatically improved slab profiles. These pulses achieve high flip angles with only minor off-resonance sensitivity, while meeting SAR limits at 1.5 T. The improved slab profiles will enable more rapid 3D imaging of limited volumes, with more consistent image contrast across the excited slab.

Quantifying the net slab pull force as a driving mechanism for plate tectonics

Abstract: [1] It has remained unclear how much of the negative buoyancy force of the slab (FB) is used to pull the trailing plate at the surface into the mantle. Here I present three-dimensional laboratory experiments to quantify the net slab pull force (FNSP) with respect to FB during subduction. Results show that FNSP increases with increasing slab length and dip up to ∼8–12% of FB, making FNSP up to twice as large as the ridge push force. The remainder of FB is primarily used to drive rollback-induced mantle flow (∼70%), to bend the subducting plate at the trench (∼15–30%) and to overcome shear resistance between slab and mantle (0–8%).

One-dimensional models of shear wave velocity for the eastern Mediterranean obtained from the inversion of Rayleigh wave phase velocities and tectonic implications

Abstract: SUMMARY On a SW‐NE profile from the Libyan coast towards central Turkey phase velocity curves of the fundamental Rayleigh mode were measured using a two-station method. The inversion of phase velocity curves yields 1-D models of shear wave velocity down to approximately 200 km depths that may be interpreted as estimates of average models between neighbouring stations on the profile. Strong lateral variations in the shear wave velocity structure are imaged along the profile. The subducted oceanic African mantle lithosphere is indicated in 1-D models for the region around Crete by significantly enlarged shear wave velocities. It is also imaged by an average model of the structure between stations on Crete and Santorini. On a path crossing the Libyan Sea south of Crete the resulting model is slower than a model expected for 110 Myr old oceanic lithosphere. The passive African margin is thus assumed to extend northwards beneath the Libyan Sea. Anomalous low shear wave velocities are found for the uppermost mantle beneath central Turkey down to a depth of approximately 130 km. Using two stations on Crete the average depth of the oceanic Moho within the subducting slab is estimated to be at approximately 50 km beneath Crete. For this arc-parallel path, an enlarged standard deviation of the measured phase velocities of approximately 0.2 km s −1 between 10 and 30 mHz is observed that is probably caused by strong lateral heterogeneity related to the subducting slab. In addition, in this frequency range an anomalous propagation of the fundamental Rayleigh mode is detected that is indicated by measured phase velocities that are approximately one standard deviation faster than phase velocities expected from a great-circle approximation. An average shear wave velocity of approximately 3.5 km s −1 is observed above the oceanic Moho. In order to explain the recent lithospheric structure of the Hellenic subduction zone a tectonic model is assumed for the NE‐SW striking profile considered. It is based on the calculated 1-D models, tectonic reconstructions and on a model derived from the metamorphic history of rocks exposed on Crete. The suggested model summarizes the tectonic development at a lithospheric scale starting in the Late Cretaceous. Accretion of crustal material of two microcontinents to Eurasia is assumed, while continuous subduction of the oceanic lithosphere of different ocean basins and possibly of the mantle lithosphere of the microcontinents resulted in a single slab. The length of the oceanic lithosphere that was subducted south of Crete is estimated to be not greater than approximately 550 km.

Depth to the Juan De Fuca slab beneath the Cascadia subduction margin - a 3-D model for sorting earthquakes

Abstract: We present an updated model of the Juan de Fuca slab beneath southern British Columbia, Washington, Oregon, and northern California, and use this model to separate earthquakes occurring above and below the slab surface. The model is based on depth contours previously published by Fluck and others (1997). Our model attempts to rectify a number of shortcomings in the original model and update it with new work. The most significant improvements include (1) a gridded slab surface in geo-referenced (ArcGIS) format, (2) continuation of the slab surface to its full northern and southern edges, (3) extension of the slab surface from 50-km depth down to 110-km beneath the Cascade arc volcanoes, and (4) revision of the slab shape based on new seismic-reflection and seismic-refraction studies. We have used this surface to sort earthquakes and present some general observations and interpretations of seismicity patterns revealed by our analysis. For example, deep earthquakes within the Juan de Fuca Plate beneath western Washington define a linear trend that may mark a tear within the subducting plate Also earthquakes associated with the northern stands of the San Andreas Fault abruptly terminate at the inferred southern boundary of the Juan de Fuca slab. In addition, we provide files of earthquakes above and below the slab surface and a 3-D animation or fly-through showing a shadedrelief map with plate boundaries, the slab surface, and hypocenters for use as a visualization tool.

Role of lateral mantle flow in the evolution of subduction systems: insights from laboratory experiments

Abstract: SUMMARY We present 3-D laboratory experiments constructed to investigate the pattern of mantle flow around a subducting slab under different boundary conditions. In particular we present a set of experiments, characterized by different conditions imposed at the trailing edge of the subducting plate (that is, plate fixed in the far field, plate detached in the far field, imposed plate motion). Experiments have been performed using a silicone slab floating inside a honey tank to simulate a thin viscous lithosphere subducting in a viscous mantle. For each set, we show differences between models that do or do not include the possibility of out-of-plane lateral flow in the mantle by varying the lateral boundary conditions. Our results illustrate how a subducting slab vertically confined over a 660-km equivalent depth can be influenced in its geometry and in its kinematics by the presence or absence of possible lateral pathways. On the basis of these results we show implications for natural subduction systems and we highlight the importance of suitable simulations of lateral viscosity variations to obtain a realistic simulation of the history of subduction.

Finite-size effects of a left-handed material slab on the image quality.

Abstract: The characteristics of an imaging system formed by a left-handed material (LHM) slab of finite length are studied, and the influence of the finite length of the slab on the image quality is analyzed. Unusual phenomena such as surface bright spots and negative energy stream at the image side are observed and explained as the cavity effects of surface plasmons excited by the evanescent components of the incident field. For a thin LHM slab, the cavity effects are found rather sensitive to the length of the slab; the bright spots on the bottom surface of the slab may stretch to the image plane and degrade the image quality.

Low electrical resistivity associated with plunging of the Nazca flat slab beneath Argentina.

Abstract: Beneath much of the Andes, oceanic lithosphere descends eastward into the mantle at an angle of about 30 degrees (ref. 1). A partially molten region is thought to form in a wedge between this descending slab and the overlying continental lithosphere as volatiles given off by the slab lower the melting temperature of mantle material. This wedge is the ultimate source for magma erupted at the active volcanoes that characterize the Andean margin. But between 28 degrees and 33 degrees S the subducted Nazca plate appears to be anomalously buoyant, as it levels out at about 100 km depth and extends nearly horizontally under the continent. Above this 'flat slab', volcanic activity in the main Andean Cordillera terminated about 9 million years ago as the flattening slab presumably squeezed out the mantle wedge. But it is unknown where slab volatiles go once this happens, and why the flat slab finally rolls over to descend steeply into the mantle 600 km further eastward. Here we present results from a magnetotelluric profile in central Argentina, from which we infer enhanced electrical conductivity along the eastern side of the plunging slab, indicative of the presence of partial melt. This conductivity structure may imply that partial melting occurs to at least 250 km and perhaps to more than 400 km depth, or that melt is supplied from the 410 km discontinuity, consistent with the transition-zone 'water-filter' model of Bercovici and Karato.

Adsorption of Water on the TiO2 (Rutile) (110) Surface: A Comparison of Periodic and Embedded Cluster Calculations

Abstract: Periodic density functional theory (DFT) based on plane waves (PW) and Hartree−Fock (HF) based on the linear combination of atomic orbitals (LCAO) calculations using slabs separated by vacuum gaps were carried out to model the H2O−TiO2 (rutile) (110) interface. Positions of all atoms were allowed to relax except atoms in the central layer of the slab. Both associative and dissociative adsorption mechanisms were considered for half-monolayer and monolayer coverages. Five different orientations of H2O molecules on the TiO2 surface were studied to determine the most energetically favorable water positions for the associative mechanism. Two slab thicknesses (three Ti layers and five Ti layers) were chosen to test the effect of slab depth on calculated surface structures and adsorption energies. Results indicate that associative adsorption is favorable by −8 to −20 kJ/mol/H2O depending on the slab thickness for full-monolayer coverage. Embedded cluster HF calculations were also performed for comparison. Adsorp...

Toroidal mantle flow around the Calabrian slab (Italy) from SKS splitting

Abstract: [1] SKS-wave birefringence is likely caused by ongoing and/or fossil mantle flow. Splitting parameters in the southern Tyrrhenian subduction zone define a quite complex pattern of fast directions with large delay times up to 2.7 s. Fast directions are prevalently trench parallel below the slab and rotate to trench normal at the western edge of the slab depicting a ring around it. We interpret these anisotropic patterns, also observed in laboratory experiment, as toroidal mantle flow induced by rollback subduction.

The temporal evolution of plate driving forces: Importance of “slab suction” versus “slab pull” during the Cenozoic

Abstract: Although mantle slabs ultimately drive plate motions, the mechanism by which they do so remains unclear. A detached slab descending through the mantle will excite mantle flow that exerts shear tractions on the base of the surface plates. This "slab suction'' force drives subducting and overriding plates symmetrically toward subduction zones. Alternatively, cold, strong slabs may effectively transmit stresses to subducting surface plates, exerting a direct "slab pull'' force on these plates, drawing them rapidly toward subduction zones. This motion induces mantle flow that pushes overriding plates away from subduction zones. We constrain the relative importance of slab suction and slab pull by comparing Cenozoic plate motions to model predictions that include viscous mantle flow and a proxy for slab strength. We find that slab pull from upper mantle slabs combined with slab suction from lower mantle slabs explains the observation that subducting plates currently move similar to4 times faster than nonsubducting plates. This implies that upper mantle slabs are strong enough to support their own weight. Slab suction and slab pull presently account for about 40 and 60% of the forces on plates, but slab suction only similar to30% if a low-viscosity asthenosphere decouples plates from mantle flow. The importance slab pull has been increasing steadily through the Cenozoic because the mass and length of upper mantle slabs has been increasing. This causes subducting plates to double their speed relative to nonsubducting plates during this time period. Our model explains this temporal evolution of plate motions for the first time.

All-pass transmission or flattop reflection filters using a single photonic crystal slab

Abstract: We show that a single photonic crystal slab can function either as optical all-pass transmission or flattop reflection filter for normally incident light. Both filter functions are synthesized by designing the spectral properties of guided resonance in the slab. The structure is extremely compact along the vertical direction. We expect this device to be useful for optical communication systems.

High-resolution imaging of lowermost mantle structure under the Cocos plate

Abstract: [1] Broadband seismic shear waves are analyzed to investigate the fine-velocity structure in the lowermost mantle beneath the Cocos plate, a region where previous studies have indicated the presence of a shear velocity increase about 200–300 km above the core-mantle boundary. Data from 14 South American earthquakes recorded at California broadband networks provide dense ray path sampling of the lowermost mantle in an approximately 700 km long north-south corridor, roughly 150 km wide. Application of a simplified seismic migration method that uses a homogeneous background velocity model suggests topography of the previously imaged positive impedance jump, varying in depth from north to south by as much as 150 km, with a weakly reflecting transition zone in between. The migration approach enables examination of small-scale spatial variations and out-of-plane scattering effects. Topography of the discontinuity may account for observed variations in the amplitude of reflected arrivals or there may be lateral variations in the velocity contrast across the boundary. Lateral variations of the shear velocity structure within the D″ layer may produce some apparent topography in the discontinuity image, but any such volumetric structure is not yet well enough determined to incorporate in the migration. A localized negative impedance contrast reflector or scatterer is imaged at depths about 100 km below the positive reflector in the northern portion of the study area. Several scenarios can explain these results, including (1) a slab that reaches the lowermost mantle, (2) the birth of an upwelling beneath a recumbent slab, or (3) chemical layering in this region.

Moment–Rotation Parameters for Composite Shear Tab Connections

Abstract: Shear tab connections, acting compositely with the floor slab, are capable of providing some lateral resistance in steel frame buildings. This resistance was suspected from the events of the 1994 Northridge Earthquake and verified in a series of tests. For performance evaluation of steel structures, it is necessary to quantify the non-negligible rotational strength and stiffness of these composite connections, a task that cannot be accomplished through the use of simple rules of thumb. Therefore, trends in the moment–rotation behavior of the connections are identified from comparison of the cyclic behavior against parameters such as number of bolts, type of concrete, use of slab reinforcement, and the presence of a column web cavity. Analysis of these trends leads to guidelines for determining parameters for a basic backbone moment–rotation curve. These guidelines depend heavily on applicable limit states for elements of the connections and connection geometry. Predicted values of moment and rotation capa...

Anisotropy and mantle flow in the Chile-Argentina subduction zone from shear wave splitting analysis

Abstract: [1] We examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the South American subduction zone. Data is from the CHARGE network, which traversed Chile and western Argentina across two transects between 30� S and 36� S. Beneath the southern and northwestern parts of the network, fast polarization direction (j) is consistently trench-parallel, while in the northeast j is trench-normal; the transition between these two zones is gradual. We infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab. We explain our observations with a model in which eastward, Nazca-entrained asthenospheric flow is deflected by retrograde motion of the subducting Nazca plate. Resulting southward flow through this area produces N-S j observed in the south and northwest; E-W j result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab. INDEX TERMS: 7203 Seismology: Body wave propagation; 7218 Seismology: Lithosphere and upper mantle; 8123 Tectonophysics: Dynamics, seismotectonics; 8150 Tectonophysics: Plate boundary—general (3040); 9360 Information Related to Geographic Region: South America. Citation: Anderson, M. L., G. Zandt, E. Triep, M. Fouch, and S. Beck (2004), Anisotropy and mantle flow in the Chile-Argentina subduction zone from shear wave splitting analysis, Geophys. Res. Lett., 31, L23608, doi:10.1029/ 2004GL020906.

Fiber-Reinforced Polymer Strengthening of Two-Way Slabs

Abstract: This paper experimentally evaluates the strengthening of 2-way slabs using fiber-reinforced polymers (FRPs). Two different types of FRP materials were evaluated: carbon FRP strips and glass FRP laminates. The dominating failure mode for 2-way slab, flexural, or punching shear is based on the slab steel reinforcement ratio. The reinforcement ratios were chosen to serve the purpose of demarcating the 2 possible modes of failure. The tested specimens were classified according to the purpose of strengthening into specimens strengthened in flexure and specimens strengthened in punching shear. Specimens strengthened in flexure had 2 steel reinforcement ratios: 0.35 and 0.5%. Results show that the flexural capacity of 2-way slabs can increase to an average of 35.5% over that of the reference (unstrengthened) specimen. An increase of the initial stiffness was achieved for flexural specimens; however, an apparent decrease in the overall ductility was evident. FRP materials can be used to increase the flexural capacity of 2-way slabs. However, an average decrease in the values of the energy absorption of approximately 30% for flexural strengthening specimens was observed. Specimens strengthened for punching shear have an original slab reinforcement ratio of 1.0%. A strengthening technique that combines the use of carbon FRP strips and steel bolts increases the strength of the slab by 9.0%. An analytical model for the analysis of FRP strengthening of 2-way slabs under flexure or punching shear is introduced.