Tomographic evidence for compositional heterogeneity deep in earth’s mantle

In the past decade, tomographic imaging has revealed that trajectories of mantle convection are more complex than expected from end-member models of unhindered whole mantle circulation or layered convection with an interface at 660 km depth. In the context of recently proposed mantle flow models, we discuss evidence for compositional heterogeneity in the deepest 1000 km of the mantle, and describe how this could survive in a system of thermochemical convection

Effects of relative plate motion on the deep structure and penetration depth of slabs below the Izu-Bonin and Mariana island arcs

An increasing number of seismological studies indicate that slabs of subducted lithosphere penetrate the Earth's lower mantle below some island arcs but are deflected, or, rather, laid down, in the transition zone below others. Recent numerical simulations of mantle flow also advocate a hybrid form of mantle convection, with intermittent layering. We present a multi-disciplinary analysis of slab morphology and mantle dynamics in which we account explicitly for the history of subduction below specific island arcs in an attempt to understand what controls lateral variations in slab morphology and penetration depth. Central in our discussion are the Izu-Bonin and Mariana subduction zones. We argue that the differences in the tectonic evolution of these subduction zones - in particular the amount and rate of trench migration - can explain why the slab of subducted oceanic lithosphere seems to be (at least temporarily) stagnant in the Earth's transition zone below the Izu-Bonin arc but penetrates into the lower mantle below the Mariana arc. We briefly speculate on the applicability of our model of the temporal and spatial evolution of slab morphology to other subduction zones. Although further investigation is necessary, our tentative model shows the potential for interpreting seismic images of slab structure by accounting for the plate-tectonic history of the subduction zones involved. We therefore hope that the ideas outlined here will stimulate and direct new research initiatives

New mantle convection model may reconcile conflicting evidence

Recently, a new model for mantle convection was proposed that may be more realistic than previous standard models. Exciting questions remain, of course, but we believe it can be used to reconcile otherwise conflicting evidence from different research fields and thus provide a new framework for further studies of convection

Zoned mantle convection

We review the present state of our understanding of mantle convection with respect to geochemical and geophysical evidence and we suggest a model for mantle convection and its evolution over the Earth’s history that can reconcile this evidence. Wholemantle convection, even with material segregated within the D00 region just above the core{mantle boundary, is incompatible with the budget of argon and helium and with the inventory of heat sources required by the thermal evolution of the Earth. We show that the deep-mantle composition in lithophilic incompatible elements is inconsistent with the storage of old plates of ordinary oceanic lithosphere, i.e. with the concept of a plate graveyard. Isotopic inventories indicate that the deep-mantle composition is not correctly accounted for by continental debris, primitive material or subducted slabs containing normal oceanic crust. Seismological observations have begun to hint at compositional heterogeneity in the bottom 1000 km or so of the mantle, but there is no compelling evidence in support of an interface between deep and shallow mantle at mid-depth. We suggest that in a system of thermochemical convection, lithospheric plates subduct to a depth that depends|in a complicated fashion|on their composition and thermal structure. The thermal structure of the sinking plates is primarily determined by the direction and rate of convergence, the age of the lithosphere at the trench, the sinking rate and the variation of these parameters over time (i.e. platetectonic history) and is not the same for all subduction systems. The sinking rate in the mantle is determined by a combination of thermal (negative) and compositional buoyancy and as regards the latter we consider in particular the e¬ect of the loading of plates with basaltic plateaux produced by plume heads. Barren oceanic plates are relatively buoyant and may be recycled preferentially in the shallow mantle. Oceanic plateau-laden plates have a more pronounced negative buoyancy and can more easily founder to the very base of the mantle. Plateau segregation remains statistical and no sharp compositional interface is expected from the multiple fate of the plates. We show that the variable depth subduction of heavily laden plates can prevent full vertical mixing and preserve a vertical concentration gradient in the mantle. In addition, it can account for the preservation of scattered remnants of primitive material in the deep mantle and therefore for the Ar and 3He observations in oceanisland basalts

The Poisson’s ratio of the Australian crust : geological and geophysical implications

The Poisson ratio, which depends on the VP/VS ratio, provides much tighter constraints on the crustal composition than either the compressional or the shear velocity alone. The crustal Poisson ratio can be determined from the joint analysis of the travel times of waves converted at the Moho and of crustal multiples reflected at the top of the Moho. We have analyzed the records of the permanent stations installed on the Australian continent, complemented by the data of the SKIPPY experiment. The results reveal substantial variations in the Poisson ratio in the different tectonic units. For the Proterozoic crust, an increase of the Poisson ratio with increasing crustal thickness is systematically observed while for the Phanerozoic crust, the Poisson ratio tends to decrease for increasing crustal thicknesses. These observations are in remarkable agreement with the results of the deep seismic soundings that were performed in the former Soviet Union. The variations observed in the Proterozoic provinces can perhaps be explained by underplating of mafic materials at the base of the crust

Compositional heterogeneity in the bottom 1000 kilometers of earth's mantle : Toward a hybrid convection model

Tomographic imaging indicates that slabs of subducted lithosphere can sink deep into Earth's lower mantle. The view that convective flow is stratified at 660-kilometer depth and preserves a relatively pristine lower mantle is therefore not tenable. However, a range of geophysical evidence indicates that compositionally distinct, hence convectively isolated, mantle domains may exist in the bottom 1000 kilometers of the mantle. Survival of these domains, which are perhaps related to local iron enrichment and silicate-to-oxide transformations, implies that mantle convection is more complex than envisaged by conventional end-member flow models

Travel-time tomography of the European-Mediterranean mantle down to 1400 km

The 3-D P-wave velocity structure of the mantle below Europe, the Mediterranean region and a part of Asia Minor is investigated. This study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper-mantle structure only. Here, the Earth’s volume under study encompasses the mantle to a depth of 1400 km, and we increase the number of International Seismological Centre (ISC) data for inversion by a factor of four by taking more years of observation, and by including data from teleseismic events. The most important departure from the earlier study is that we do not use the Jeffreys—Bullen model as a reference model, but an improved radially symmetricvelocity model, the PM2 model, which is appropriate for the European—Mediterranean mantle. Our inversion procedure consists of two steps. First, the radial model PM2 is determined from the ISC delay times by a nonlinear trial-and-error inversion of the data. As opposed to the Jeffreys—Bullen model, the new reference model has a high-velocity lithosphere, a low-velocity zone, and seismic discontinuities at depths of 400 and 670 km. Next, the ISC data are corrected for effects related to the change in reference model and inverted for 3-D heterogeneity relative to the PM2 model. We follow this two-step approach to attain a better linearizable tomographic problem in which ray paths computed in the PM2 model provide a better approximation of the actual ray paths than those computed from the Jeffreys—Bullen model. Hence, the two-step scheme leads to a more credible application of Fermat’s Principle in linearizing the tomographic equations. Inversion results for the 3-D heterogeneity are computed for both the uncorrected ISC data and for the PM2 data. The data fit obtained in the two-step approach is slightly better than in the inversion of ISC data (using the Jeffreys—Bullen reference model). A comparison of the tomographic results demonstrates that the PM2 data inversion is to be preferred. To assess the spatial resolution an analysis is given of hit count patterns (sampling of the mantle by ray paths) and results of sensitivity tests with 3-D synthetic velocity models. The spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross-section. In the well-sampled regions of the mantle the spatial resolution for larger-scale structure can (qualitatively) be denoted as reasonable to good, and at least sufficient to allow interpretation of larger-scale anomalies. A comparison is made of the results of this study with independent models of S-velocity heterogeneity obtained in a number of investigations, and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the Mediterranean region. In the context of this comparison, interpretations of large-scale positive anomalies found in the Mediterranean upper mantle in terms of subducted lithosphere are given. Specifically addressed are subduction below southern Spain, below the Western Mediterranean and Italy, and below the Aegean. In the last region a slab anomaly is mapped down to depths of 80

Seismic evidence for olivine phase changes at the 410- and 660-kilometer discontinuities

The view that the seismic discontinuities bounding the mantle transition zone at 410- and 660-kilometer depths are caused by isochemical phase transformations of the olivine structure is debated. Combining converted-wave measurements in East Asia and Australia with seismic velocities from regional tomography studies, we observe a correlation of the thickness of, and wavespeed variations within, the transition zone that is consistent with olivine structural transformations. Moreover, the seismologically inferred Clapeyron slopes are in agreement with the mineralogical Clapeyron slopes of the (Mg,Fe)2SiO4spinel and postspinel transformations

Compositional stratification in the deep mantle

A boundary between compositionally distinct regions at a depth of about 1600 kilometers may explain the seismological observations pertaining to Earth's lower mantle, produce the isotopic signatures of mid-ocean ridge basalts and oceanic island basalts, and reconcile the discrepancy between the observed heat flux and the heat production of the mid-ocean ridge basalt source region. Numerical models of thermochemical convection imply that a layer of material that is intrinsically about 4 percent more dense than the overlying mantle is dynamically stable. Because the deep layer is hot, its net density is only slightly greater than adiabatic and its surface develops substantial topography

A geological and geophysical context for the Wenchuan earthquake of 12 May 2008, Sichuan, People's Republic of China

On 12 May 2008, a magnitude 7.9 earthquake ruptured the Longmen Shan margin of the eastern Tibetan plateau. This event occurred within the context of long-term uplift and eastward enlargement of the plateau. The area has numerous geological features not typical of active convergent mountain belts, including the presence of a steep mountain front (>4 km relief) but an absence of large-magnitude low-angle thrust faults; young high topography (post ca. 15 Ma) and thickened crust but low global positioning system (GPS) shortening rates (<3 mm/yr); and no coeval foreland subsidence. In our interpretation, crustal thickening beneath the eastern Tibetan plateau occurred without large-scale shortening of the upper crust but instead is caused by ductile thickening of the deep crust in a weak (low-viscosity) layer. Late Cenozoic shortening across the Longmen Shan could be as little as 10-20 km, with folding and faulting mainly accommodating differential surface uplift between the plateau and the Sichuan Basin. The earthquake of 12 May probably reflects long-term uplift with slow convergence and right-slip, of the eastern plateau relative to the Sichuan Basin. GPS-determined rates in the vicinity of the 12 May event suggest an average recurrence interval of ∼2,000-10,000 yr