Deep Slabs Research Articles

Analyzing Gravity and Magnetic Data for the Detection of Deep-Seated Faults and Faults within the Sedimentary Cover Near Habbanieyah and Razzaza Lakes in the Middle of Iraq

This research deals with the processing and analyzing of magnetic and gravitational data for an area covering the region of Habbanieyah - Razzaza Lakes and its adjacent areas. The study includes data processing and mapping of the total gravity and magnetic anomalies for only the concerned region, then separating the residual anomalies by adopting the polynomial regression graphical method. The residual gravity anomaly reflects the variations of rock densities within the sedimentary cover. The horizontal gradient filter has been applied to the residual gravity anomaly in order to conduct the locations of fault planes within the sedimentary cover where sudden variations of gravity field take place. The quantitative interpretation for both gravity and magnetic anomalies yielded a preliminary determination for the depth to the center of major faults within the sedimentary cover. By constructing a gravity model along a profile which directed NE-SW and passing through the middle part of the study region, depth to the center of the effective faults found. This depth variation is due to the effect of tectonic activity which produced a set of faults, such faults caused the upward and downward structural motions and were responsible for positioning the deep high density causative slabs of bedrock. The residual magnetic field quantitative interpretation along two profiles crosses over anomalies at the NE and SW parts of the region yielded the depth to the top of magnetized basement rocks. The difference in depth of the basement rocks and the shifted anomaly locations reflects the effect of tectonic activity which may relate to a strike slip faulting in the higher depths.

Effect of trabeculectomy on optic nerve head and macular vessel density: an optical coherence tomography angiography study.

To investigate the effect of trabeculectomy on peripapillary and macular vessel density (VD) using optical coherence tomography angiography (OCT-A) in patients with primary open-angle glaucoma. This is a prospective, interventional study on 32 glaucoma eyes (21 patients) who underwent trabeculectomy. Optic nerve head (ONH) and macular structural and OCT-A scans were performed before surgery and at one and six months postoperatively with the Avanti device (AngioVue System, Optovue Inc., Fremont, CA). The primary outcomes of interest were changes in global and regional peripapillary and macular VD. The average (standard deviation) age and visual field mean deviation of the sample were 58.9(9.9) years and -14.5(9.0) dB, respectively. The average (standard deviation) baseline intraocular pressure significantly decreased from 24.5(10.4) mmHg to 9.6(2.1) mmHg, 6months after the surgery (P< 0.001). The mean (standard deviation) whole image VD of ONH had increased from 37.6(8.3)% to 40.5(9.1)% at 6months after the surgery (P = 0.01) while its changes were not significant at month 1 (38.1(9.5)%, P = 0.64). The average of radial peripapillary circle (RPC) VD increased from 36.5(10.3)% to 39.3(10.8)% at 6months (P = 0.04) compared to 1-month post-trabeculectomy (37.0(11.4)%, P = 0.71). Similar patterns at postoperative months 1 and 6 were also observed in RPC VD at superior and inferior hemifields as well as nasal and temporal quadrants. The observed changes in VD of macular, foveal, peri-, and parafoveal were not significant at superficial or deep slabs at 1 and 6months after trabeculectomy (all P values > 0.05). Our study demonstrated significant improvements in peripapillary vessel density at 6-month following trabeculectomy.

Seismic wavefield imaging of Earth’s interior across scales

Seismic full-waveform inversion (FWI) for imaging Earth’s interior was introduced in the late 1970s. Its ultimate goal is to use all of the information in a seismogram to understand the structure and dynamics of Earth, such as hydrocarbon reservoirs, the nature of hotspots and the forces behind plate motions and earthquakes. Thanks to developments in high-performance computing and advances in modern numerical methods in the past 10 years, 3D FWI has become feasible for a wide range of applications and is currently used across nine orders of magnitude in frequency and wavelength. A typical FWI workflow includes selecting seismic sources and a starting model, conducting forward simulations, calculating and evaluating the misfit, and optimizing the simulated model until the observed and modelled seismograms converge on a single model. This method has revealed Pleistocene ice scrapes beneath a gas cloud in the Valhall oil field, overthrusted Iberian crust in the western Pyrenees mountains, deep slabs in subduction zones throughout the world and the shape of the African superplume. The increased use of multi-parameter inversions, improved computational and algorithmic efficiency, and the inclusion of Bayesian statistics in the optimization process all stand to substantially improve FWI, overcoming current computational or data-quality constraints. In this Technical Review, FWI methods and applications in controlled-source and earthquake seismology are discussed, followed by a perspective on the future of FWI, which will ultimately result in increased insight into the physics and chemistry of Earth’s interior. Advances in numerical methods and high-performance computing have enabled imaging of Earth’s interior across scales using the physics of seismic-wave propagation. This Technical Review summarizes the use of full-waveform inversion in controlled-source and earthquake seismology.

Separation and Thickness Measurements of Superficial and Deep Slabs of the Retinal Nerve Fiber Layer in Healthy and Glaucomatous Eyes

Describe a new method to analyze retinal nerve fiber layer (RNFL) thickness maps. Cross-sectional study. RNFL thickness maps of healthy and glaucomatous eyes. Optical coherence tomography (OCT) RNFL raster scans from 98 healthy and 50 glaucomatous eyes were analyzed. The RNFL thickness maps were separated into superficial (SNFL) and deep (DNFL) slabs through a partial thickness plane set at the modal thickness (mode). Association between mode and OCT signal strength (SS), age, axial length, and visual field mean deviation (VFMD) was tested (Pearson coefficient, r). Thicknesses of inferior and superior SNFL regions (i-,s-SNFL), and inferior, superior, nasal, and temporal DNFL regions (i-,s-,n-,t-DNFL) were calculated. The regions thicknesses were compared between healthy and glaucomatous eyes (t-test) and between glaucomatous eyes with early, moderate, and severe disease (ANOVA and linear regressions of thickness on VFMD). Diagnostic accuracy and correlation with VFMD of RNFL regions thicknesses were calculated as the area under the receiver operating characteristic curve (AUC) and Pearson r, respectively. P<0.05 was considered significant. Thickness of regions in SNFL and DNFL slabs. The mode was not associated with SS, age, axial length, or VFMD, it circumscribed the thicker RNFL around the optic disc of healthy and glaucomatous eyes, and it was used to separate the SNFL and DNFL slabs of RNFL thickness maps. The thickness of the SNFL slab was less in glaucomatous eyes than in healthy eyes (P<0.001). S-SNFL and i-SNFL thicknesses (respectively) were 86.0±8.2μm and 87.3±9.6μm in healthy eyes vs. 66.1±9.1μm and 63.4±8.2μm in glaucomatous eyes (P<0.001 for both). The thickness of the DNFL slab was similar between groups (P=0.19). T-DNFL thickness was 37.0±5.3μm in healthy eyes vs. 33.9±5.0μm in glaucomatous eyes (P<0.001); thicknesses of all other DNFL regions were similar. The SNFL regions only thinned with progressively worse glaucoma severity, had excellent AUCs (AUC≥0.95, P<0.001), and correlated strongly with VFMD (r≥0.60, P<0.001). Glaucomatous RNFL thinning is predominantly detected within a slab with thickness greater than the mode. SNFL thickness has great AUC and correlation with VFMD in glaucomatous eyes. The usefulness for diagnosis and monitoring of glaucoma needs further study.

Seismic Structure of the Upper Mantle Beneath Eastern Asia From Full Waveform Seismic Tomography

AbstractTo better understand the subsurface behavior of subducting slabs and their relation to the tectonic evolution of the overriding plate, we conduct a full waveform inversion on a large data set to determine a high‐resolution seismic model, FWEA18 (Full Waveform inversion of East Asia in 2018), of the upper mantle beneath eastern Asia. FWEA18 reveals sharper, more intense high‐velocity slabs in the upper mantle under the southern Kuril, Japan, and Ryukyu arcs, than previous studies have found. The subducting Pacific plate is imaged as a roughly 100 km thick high‐velocity slab to near 550 km depth indicating relatively little deformation. Stagnation near 600 km depth is observed over horizontal distances of 600 km or less. The Pacific plate we image accounts for roughly 25 Myr of subduction with older slab likely located in the lower mantle. The Philippine plate, subducting beneath the Ryukyu Islands, has a clear termination at about 450 km depth. This may indicate a tearing event in the past or that less Philippine Sea plate has subducted than previously thought. We found a double‐layer high‐velocity anomaly above and below 660 km under the Yellow Sea and eastern coast of North China. This may correspond to parts of the Philippine Sea plate that detached in the past and Pacific plate that have intersected at depth or a complicated behavior of the Pacific plate at that depth. Slow cylindrical anomalies cross the entire upper mantle are imaged beneath major Holocene volcanoes, which are likely upwellings associated with the edges of deep slabs that are entering the lower mantle.

Compositional mantle layering revealed by slab stagnation at ~1000-km depth

Improved constraints on lower-mantle composition are fundamental to understand the accretion, differentiation, and thermochemical evolution of our planet. Cosmochemical arguments indicate that lower-mantle rocks may be enriched in Si relative to upper-mantle pyrolite, whereas seismic tomography images suggest whole-mantle convection and hence appear to imply efficient mantle mixing. This study reconciles cosmochemical and geophysical constraints using the stagnation of some slab segments at ~1000-km depth as the key observation. Through numerical modeling of subduction, we show that lower-mantle enrichment in intrinsically dense basaltic lithologies can render slabs neutrally buoyant in the uppermost lower mantle. Slab stagnation (at depths of ~660 and ~1000 km) and unimpeded slab sinking to great depths can coexist if the basalt fraction is ~8% higher in the lower mantle than in the upper mantle, equivalent to a lower-mantle Mg/Si of ~1.18. Global-scale geodynamic models demonstrate that such a moderate compositional gradient across the mantle can persist can in the presence of whole-mantle convection.

Meteorological variables to aid forecasting deep slab avalanches on persistent weak layers

Deep slab avalanches are particularly challenging to forecast. These avalanches are difficult to trigger, yet when they release they tend to propagate far and can result in large and destructive avalanches. We utilized a 44-year record of avalanche control and meteorological data from Bridger Bowl ski area in southwest Montana to test the usefulness of meteorological variables for predicting seasons and days with deep slab avalanches. We defined deep slab avalanches as those that failed on persistent weak layers deeper than 0.9m, and that occurred after February 1st. Previous studies often used meteorological variables from days prior to avalanches, but we also considered meteorological variables over the early months of the season. We used classification trees and random forests for our analyses. Our results showed seasons with either dry or wet deep slabs on persistent weak layers typically had less precipitation from November through January than seasons without deep slabs on persistent weak layers. Days with deep slab avalanches on persistent weak layers often had warmer minimum 24-hour air temperatures, and more precipitation over the prior seven days, than days without deep slabs on persistent weak layers. Days with deep wet slab avalanches on persistent weak layers were typically preceded by three days of above freezing air temperatures. Seasonal and daily meteorological variables were found useful to aid forecasting dry and wet deep slab avalanches on persistent weak layers, and should be used in combination with continuous observation of the snowpack and avalanche activity.

Generation of tectonic over-pressure inside subducting oceanic lithosphere involving phase-loop of olivine–wadsleyite transition

We conducted a two-dimensional numerical model to analyze the generation of tectonic over-pressure, which is a positive deviation from lithostatic pressure, for deep slabs which are anchored at the 660 km phase boundary. The formation of the ductile shear zone under a compressional tectonic setting induces tectonic over-pressure. We first propose that an apparent shear zone originated from an elastic heterogeneity in the phase loop, which is the two-phase (i.e., olivine and wadsleyite) coexistence interval around the 410 km boundary within subducting oceanic lithospheres, can cause tectonic over-pressure with a range from 0.3 to 1.5 GPa. This over-pressure significantly impacts the formation of the olivine–wadsleyite phase transition. The flattening of the olivine–wadsleyite interface by over-pressure is well-resolved. Therefore, we argue that the over-pressure should be applied when analyzing the phase boundary within the subducting lithosphere. Our results provide a new insight on the interplay among the phase transition, shear zone formation and tectonic over-pressure.

Contact Problems for a Plate as an Inclusion in an Elastic Half-Space

The contact problem for a flexible plate as an inclusion in an elastic half-space is solved by the method of orthogonal polynomials. The singularity of the contact stresses at the plate edge is separated exactly. The problem of the rotation of the plate is solved as well. The results can be used to design deep anchor slabs plates and inserts for ferroconcrete structures.

The relationship of intermediate- and deep-focus seismicity to the hydration and dehydration of subducting slabs

Previous studies suggest that intermediate- and deep-focus earthquakes in subducting slabs may result from dehydration reactions. We investigate the importance of dehydration in facilitating intermediate- and deep-focus earthquakes by comparing the seismicity rate with the calculated slab dehydration flux (van Keken et al., 2011) as a function of depth in 56 subduction zones worldwide. For each region, the seismicity rate per km of trench length per year as a function of depth between 1990 and 2009 was determined from the Preliminary Determination of Epicenters (PDE) catalog. The number of earthquakes between 75 and 240km depth was compared to the calculated slab dehydration flux in 25km increments. Deeper than 240km, dehydration rates were not calculated, so seismicity is instead compared to the calculated flux of water that remains mineralogically bound in the slab at that depth. No strong correlation between the dehydration flux and the seismicity rate was found at any depth. This suggests that some factor besides the presence of water, such as the stress state of the slab, controls intermediate-focus seismicity. For depths greater than 240km, there is a correlation between the amount of mineralogically-bound H2O and the seismicity rate. In particular, although deep slabs with small amounts of bound water show a wide range of seismicity rates (and many have no seismicity whatsoever), all deep slabs with appreciable water transported below 240km show significant seismicity. We conclude that the presence of water may be a necessary condition for deep seismicity in conjunction with other factors. We also note that slab temperature and dehydration are linked. Traditionally the limitations on deep earthquake occurrence have been linked to temperature, but this study suggests that similar arguments can be made linking deep earthquakes with the presence of water.

Co‐seismic deformation of deep slabs based on summed CMT data

We assess the co‐seismic deformation inferred from earthquake moment tensor solutions for subducting slabs at depths greater than 50 km globally. We rotate each moment tensor into a local slab reference frame, then sum tensors within 50 km depth bins to approximate long term deformation characteristics. This builds upon previous analyses by using the up‐to‐date global Centroid Moment Tensor catalog, incorporating a more complete slab geometry, and focusing on the 3‐D aspects of slab deformation. Results show a general consistency with Isacks and Molnar (1969), who found that most slabs can be divided into intermediate‐extensional, intermediate‐extensional‐deep‐compressional, and intermediate to deep‐compressional categories. Exceptions to these three categories can be related to slab bending in the top 100 km, plate convergence that is oblique to the trench normal direction, and regions of higher focal mechanism heterogeneity. The regions of higher focal mechanism heterogeneity appear where there are along‐strike changes in slab geometry and/or evidence of double‐seismic zones. We find that the sense of deformation in the intermediate strain axis direction is opposite to that of the down‐dip direction, in agreement with Kuge and Kawakatsu (1993). By quantitative comparison to numerical models of global mantle flow, we show that these observations are consistent with deformation of viscous slabs responding to their own negative buoyancy and an upper to lower mantle viscosity increase.

Numerical Modelling of Shear Connection in Steel-Concrete Composite Beams with Trapezoidal Slabs

This paper presents a review of the numerical analysis of composite steel-concrete beams, with particular attention on the modelling of composite beams containing deep trapezoidal slabs cast onto profiled steel sheeting. It is concluded that while robust algorithms are available, there is considerable scope for improvement of these models as an alternative to undertaking expensive testing programs in order to formulate practical design procedures. This is of particular relevance to Australian practice, as timely revisions of its composite structures standard are needed.

Mechanism for generating stagnant slabs in 3-D spherical mantle convection models at Earth-like conditions

Seismic tomography reveals the natural mode of convection in the Earth is whole mantle with subducted slabs clearly seen as continuous features into the lower mantle. However, simultaneously existing alongside these deep slabs are stagnant slabs which are, if only temporarily, trapped in the upper mantle. Previous numerical models of mantle convection have observed a range of behavior for slabs in the transition zone depending on viscosity stratification and mineral phase transitions, but typically only exhibit flat-lying slabs when mantle convection is layered or trench migration is imposed. We use 3-D spherical models of mantle convection which range up to Earth-like conditions in Rayleigh number to systematically investigate three effects on mantle dynamics: (1) the mineral phase transitions, (2) a strongly temperature-dependent viscosity with plastic yielding at shallow depth, and (3) a viscosity increase in the lower mantle. First a regime diagram is constructed for isoviscous models over a wide range of Rayleigh number and Clapeyron slope for which the convective mode is determined. It agrees very well with previous results from 2-D simulations by Christensen and Yuen (1985), suggesting present-day Earth is in the intermittent convection mode rather than layered or strictly whole mantle. Two calculations at Earth-like conditions (Ra and RaH=2í107 and 5í108, respectively) which include effects (2) and (3) are produced with and without the effect of the mineral phase transitions. The first calculation (without the phase transition) successfully produces plate-like behavior with a long wavelength structure and surface heat flow similar to Earth's value. While the observed convective flow pattern in the lower mantle is broader compared to isoviscous models, it basically shows the behavior of whole mantle convection, and does not exhibit any slab flattening at the viscosity increase at 660km depth. The second calculation which includes the phase transitions successfully exhibits the coexisting state of stagnant and penetrating slabs within the transition zone. These results indicate the importance of both a viscosity increase and mineral phase transitions for generating the structure of stagnant slabs observed by seismic tomography.

Survival times of subducted slab remnants in numerical models of mantle flow

We examine the rate of sinking of truncated subducted slabs in a series of idealized three-dimensional and two-dimensional simulations using a 3D Cartesian numerical convection model. Our goal is to account for the inference from seismic tomographic studies that some slab remnants have persisted at mid-mantle depths for 150 Myr or more while others have descended to the base of the mantle, where they have spread laterally for great distances along the core–mantle boundary. We consider the influence of initial slab dimensions, mantle viscosity stratification, slab dip angle and three-dimensionality on the ‘slab survival time’ — the time required to clear the upper half of the mantle of all measurable traces of a previous subduction zone. We find slab survival time is determined by two principal factors: the initial slab length and the viscosity stratification of the mantle. Survival time decreases with increasing slab length and increases with increasing viscosity stratification. For a viscosity jump by a factor of 100 at a depth of 660 km, shallow slab remnants remain in the upper 1500 km of the mantle for times well in excess of 150 Myr while deep slabs draw their upper portions out of the upper half of the mantle in about 150 Myr. The dependence of the age of slab remnants on both the total depth extent of the original slab and the degree of viscosity stratification has not been studied previously; it can account for the different rates of descent of slabs lying in close geographic proximity.

Effects of source radiation patterns on the phase S670P beneath the Tonga subduction zone

SUMMARY Observations of short-period S-P converted phases from the 670 km discontinuity (S670P) provide unique constraints on the nature of the discontinuity and the seismic structure of deep subducted slabs. Because this is a subtle phase in the P codas from deep earthquakes, great care is necessary in its identification. In past work we identified S670P phases from a large number of deep events beneath Tonga primarily by showing that the arrival times for S670P had the correct dependence upon hypocentral depth. Here we offer further confirmation of our interpretation of this S-P converted phase by showing that the occurrence of the phase is a predictable consequence of the earthquake source mechanism. We show this in two ways: (1) independently determined moment tensor solutions for the earthquakes generally predict relatively high-amplitude S-P conversions where they are observed; and (2) the observed amplitudes of S670P, ScP, PP, and P arrivals vary systematically in a way which confirms that S670P, is generated with large amplitude when the S-wave energy pattern is optimal. Comparison of ScP observations with moment tensor solutions also suggests that much of the S-wave energy results from the non-double-couple or, for CMT solutions, linear vector dipole (LVD) part of the source mechanism for the northern part of the Tonga subduction zone, where we observe the strongest S670P, phases. Because these LVD components tend to be directed along fault planes inferred from deep seismicity patterns, we suggest that they may be caused by rupture propagation directivity. We conclude that the occurrence of S670P, is primarily controlled by the source radiation pattern in a way that is consistent with our previous interpretation, thus confirming that S670P, is an S-P conversion from an almost undeformed phase change boundary at about 670 km depth.

Using shear wave amplitude patterns to detect metastable olivine in subducted slabs

Olivine within the lowest temperature regions of subducting oceanic lithosphere may persist metastably to depths well below the equilibrium phase boundary for transformation to β phase. This may produce a tapering low seismic velocity wedge within the slab in the transition zone; transformational faulting of such metastable olivine has been proposed as a possible explanation of deep earthquakes. Detecting seismic velocity structure associated with metastable olivine within a slab using seismic wave arrival time information alone has proven problematic. Motivated by previous two‐dimensional numerical calculations for upgoing and downgoing seismic waves, a three‐dimensional Gaussian beam method is used to assess whether the azimuthally varying teleseismic amplitude signature produced by a slab with an internal wedge of metastable olivine is distinguishable from that for a slab with equilibrium transitions. Five slab velocity structures are considered: a thermal model with horizontal phase transitions at the same depths as in ambient mantle, two models incorporating equilibrium transitions within the slab, and two models with tongues of metastable olivine. A depth‐ and temperature‐dependent thermal conductivity term and the effect of the latent heat of transformation of olivine to β phase are included in the thermal modeling. Three‐dimensional dynamic ray tracing and Gaussian beam summation are performed for events located at different depths and lateral positions for each of the slab models. For sources near 400 km depth the presence of low‐velocity metastable olivine produces relatively complex azimuthal shear wave amplitude patterns compared to thermal and equilibrium models. Comparison with observed amplitudes for four events in the Kurile slab indicates that allowing for internal slab velocity complexity in the transition zone provides better agreement with observations than simple thermal and equilibrium slab models. This indicates that careful modeling of teleseismic shear wave amplitude patterns may have the resolution to detect and characterize the presence of metastable olivine in deep slabs.

Seismological constraints on the mechanism of deep earthquakes: temperature dependence of deep earthquake source properties

Seismological observations are of vital importance for understanding the mechanism of deep earthquakes. Most of the seismological observables for earthquakes deeper than 300 km are similar to shallow earthquakes. Deep earthquakes clearly represent shear failure on a planar surface as shown by their double couple mechanisms. Deep earthquake aftershock sequences show temporal decay rates and magnitude–frequency relations ( b-values) that are similar to shallow earthquakes, with the aftershocks occurring preferentially along the mainshock fault planes. In addition, deep earthquake rupture velocities are similar to those of shallow earthquakes. However, there are also some observations that are clearly distinctive relative to shallow earthquakes. Stress drops of deep earthquakes show a large variation but are larger, on average, than shallow earthquakes. Different deep seismic zones show very different b-values, in contrast to shallow earthquakes, which show similar b-values worldwide. In addition, deep earthquakes show fewer aftershocks than shallow earthquakes. A variety of observations suggest that deep earthquakes are highly sensitive to the temperature of the slab. Both deep earthquake b-values and the rate of deep earthquake aftershock occurrence are inversely correlated with the temperature of the deep slabs, suggesting that these factors are temperature-controlled to an extent much greater than with shallow earthquakes. Large deep earthquakes in warm slabs show slower rupture velocities, larger stress drops, and lower seismic efficiencies than similar earthquakes in cold slabs. The width of deep seismic zones is also probably temperature controlled, but deep earthquake rupture can propagate outside the normal limits of Benioff zone seismicity. Simple thermal models for the Tonga slab near the 1994 deep earthquake suggest that the temperature at the rupture termination point was at least 200°C warmer than the temperature that limits smaller earthquakes in the slab. These observations can be used to evaluate physical models for deep earthquakes, including brittle slip along fluid-weakened faults, transformational faulting, and thermal (and perhaps melt lubricated) shear instabilities. It is difficult to explain the large fault widths of some deep earthquakes using a fluid weakening model, since hydrated materials are expected in only a narrow depth zone at the top of the slab, and the fault planes do not have the expected orientations for reactivated faults. The lateral extent of the largest deep earthquakes cast doubt on the transformational faulting model, in which events should be confined within a narrow metastable wedge. Seismological studies have also failed to find evidence for the existence of metastable olivine in slabs. The temperature dependence of deep earthquakes argues in favor of a temperature-activated phenomenon, such as thermal shear instabilities and perhaps fault zone melting.

Subduction zones: observations and geodynamic models

This review of subduction and geodynamic models is organized around three central questions: (1) Why is subduction asymmetric? (2) Are subducted slabs strong or weak? (3) How do subducted slabs interact with phase transformations, changes in mantle rheology, and possibly chemical boundaries in the mantle? Based on laboratory measurements of the temperature dependence of olivine, one would conclude that the core of a subducting slab is at least 10,000 times more viscous than ambient mantle; however, there are a number of complementary but independent observations that suggest that slabs are much weaker than this. Slabs undergo significant deformation in the upper mantle and may thicken to twice their original width by the time they reach the base of the transition zone. The lack of a clear correlation between the observed dip angle of deep slabs and plate velocity, rate of trench migration, and slab age in modern subduction zones is consistent with hypothesis that subduction is a time-dependent phenomenon. Both tank and numerical convection experiments with plates conclude that subduction is not a steady phenomenon, but that slabs bend, thicken, stretch, and change dip through time. This is at odds with the assumptions used in steady-state slab thermal models, where slab deformation is not considered.

Boron isotope systematics of slab fluids as inferred from a serpentine seamount, Mariana forearc

Serpentinite clasts and muds erupted from Conical Seamount, Mariana forearc, show substantial enrichment in boron (B) and 11B (δ 11B up to +15‰) relative to mantle values. These elevated B isotope signatures result from chemical exchange with B-rich pore fluids that are upwelling through the seamount. If the trends of decreasing δ 11B with slab depth shown by cross-arc magmatic suites in the Izu and Kurile arcs of the western Pacific are extended to shallow depths (∼25 km), they intersect the inferred δ 11B of the slab-derived fluids (+13‰) at Conical Seamount. Simple mixtures of a B-rich fluid with a high δ 11B and B-poor mantle with a low δ 11B are insufficient to explain the combined forearc and arc data sets. The B isotope systematics of subduction-related rocks thus indicate that the fluids evolved from downgoing slabs are more enriched in 11B than the slab materials from which they originate. Progressively lower δ 11B in arc lavas erupted above deep slabs reflects both the progressive depletion of 11B from the slab and progressively greater inputs of mantle-derived B. This suggests that the slab releases 11B-enriched fluids from the shallowest levels to depths greater than 200 km.

Strain pattern of the Southern Tyrrhenian slab from moment tensors of deep earthquakes: implications on the down-dip velocity

Seismic strain is analysed along the slab face of the Southern Tyrrhenian subduction zone using focal mechanisms of deep earthquakes which occurred in the period 1960-1998. Results show that the slab is mostly affected by down-dip shortening which strongly increases below 250 km depth. Extensional strain is mainly confined to the direction perpendicular to the slab face. The dominance of down-dip shortening and the minor along strike inplane extension implies thickening of the slab below 250 km depth. Assuming constant seismic efficiency along the slab, this strain pattern also implies a decrease of the down-dip velocity below 250 km depth. We also locate lower magnitude intermediate-depth and deep earthquakes using arrival times since 1985 available from the Italian seismic national network. These data show that the slab reaches the deeper part of the upper mantle, as suggested by the occurrence of a few ??600 km depth earthquakes, and that a large portion of the Tyrrhenian slab, between 100 and 250 km depth beneath the offshore of the Calabrian arc, is aseismic. Only a short part of the Tyrrhenian slab is seismically continuous from the top to the bottom. The lack of seismicity may indicate either that aseismic subduction is occurring or that the slab is detached from its upper part. Although the data are still inconclusive, they suggest that an aseismic subduction is the most appropriate interpretation, considering recent tomographic images of the slab and the results of this study, which agree well with the presence of a neutral down-dip stress zone, as also observed worldwide in deep slabs.