The composition of the Earth

Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations

We relocate nearly 100,000 events that occurred during the period 1964 to 1995 and are well-constrained teleseismically by arrival-time data reported to the International Seismological Centre (ISC) and to the U.S. Geological Survey9s National Earthquake Information Center (NEIC). Hypocenter determination is significantly improved by using, in addition to regional and teleseismic P and S phases, the arrival times of PKiKP, PKPdf , and the teleseismic depth phases pP, pwP , and sP in the relocation procedure. A global probability model developed for later-arriving phases is used to independently identify the depth phases. The relocations are compared to hypocenters reported in the ISC and NEIC catalogs and by other sources. Differences in our epicenters with respect to ISC and NEIC estimates are generally small and regionally systematic due to the combined effects of the observing station network and plate geometry regionally, differences in upper mantle travel times between the reference earth models used, and the use of later-arriving phases. Focal depths are improved substantially over most other independent estimates, demonstrating (for example) how regional structures such as downgoing slabs can severely bias depth estimation when only regional and teleseismic P arrivals are used to determine the hypocenter. The new data base, which is complete to about Mw 5.2 and includes all events for which moment-tensor solutions are available, has immediate application to high-resolution definition of Wadati-Benioff Zones (WBZs) worldwide, regional and global tomographic imaging, and other studies of earth structure.

Global teleseismic earthquake relocation with improved travel times and procedures for depth determination

We present four companion digital models of the age, age uncertainty, spreading rates, and spreading asymmetries of the world's ocean basins as geographic and Mercator grids with 2 arc min resolution. The grids include data from all the major ocean basins as well as detailed reconstructions of back-arc basins. The age, spreading rate, and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments. The age uncertainties for grid cells coinciding with marine magnetic anomaly identifications, observed or rotated to their conjugate ridge flanks, are based on the difference between gridded age and observed age. The uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities. Asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges, resulting in ridge jumps toward hot spots. We also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age-depth relationships. The new set of grids helps to investigate prominent negative depth anomalies, which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow. A combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems.

/pdf/age-spreading-rates-and-spreading-asymmetry-of-the-world-s-5bj3otyvfc.pdf

Age, spreading rates, and spreading asymmetry of the world's ocean crust

Seismic tomography based on P-wave travel times and improved earthquake locations provides further evidence for mantle-wide convective flow. The use of body waves makes it possible to resolve long, narrow structures in the lower mantle some of which can be followed to sites of present-day plate convergence at the Earth's surface. The transition from subduction-related linear structures in the mid-mantle to long-wavelength heterogeneity near the core-mantle boundary remains enigmatic, but at least some slab segments seem to sink to the bottom of the mantle.

/pdf/evidence-for-deep-mantle-circulation-from-global-tomography-3unqcjsa05.pdf

Evidence for deep mantle circulation from global tomography

Two new global high-resolution models of the P-wave and S-wave seismic structure of the mantle were derived independently using different inversion techniques and different data sets, but they show excellent correlation for many large-scale as well as smaller scale structures throughout the lower mantle. The two models show that high-velocity anomalies in the lower mantle are dominated by long linear features that can be associated with the sites of ancient subduction. The images suggest that most subduction-related mantle flow continues well into the lower mantle and that slabs may ultimately reach the core-mantle boundary. The models are available from anonymous ftp at maestro.geo.utexas.edu in directory pub/grand and at brolga.mit. edu in directory pub/GSAtoday.

https://courses.washington.edu/ess502/GrandEtAl_GSA_1997.pdf

Global seismic tomography: A snapshot of convection in the earth

We made a region-by-region examination of subducted slab images along the circum-Pacific for some of the recent global mantle tomographic models, specifically for two high-resolution P velocity models and two long-wavelength S velocity models. We extracted the slab images that are most consistent among different models. We found that subducted slabs tend to be subhorizontally deflected or flattened in the upper and lower mantle transition region, the depth range of which corresponds roughly to the Bullen transition region (400–1000 km). The deflected or flattened slabs reside at different depths, either above or across the 660-km discontinuity as in Chile Andes, Aleutian, Southern Kurile, Japan, and Izu-Bonin; slightly below the discontinuity as in Northern Kurile, Mariana, and Philippine; or well below it as in Peru Andes, Java, and Tonga-Kermadec. There is little indication for most of these slabs to continue “directly” to greater depths well beyond the transition region. Mantle downflow associated with present slab subduction appears to be blocked strongly to turn into predominantly horizontal flow in the transition region. Recent global tomographic models show also a group of lithospheric slabs deeply sinking through the lower mantle, typically the presumed Farallon slab beneath North and Central America and the presumed Indian (Tethys) slab beneath Himalaya and the Bay of Bengal. These remnant slabs are not connected to the surface plates or to the presently subducting slabs and appear to sink independently from the latter. The presence of these deeply sinking slabs implies that the pre-Eocene subduction occurred in much the same way as in the present day to accumulate slab bodies in the transition region and that the consequent unstable downflow occurred extensively through the transition region in the Eocene epoch to detach many of the surface plates from the subducted slabs at depths and hence to cause the reorganization of global plate motion.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999RG000068

Stagnant slabs in the upper and lower mantle transition region

High-quality P and S travel times are now available from careful reprocessing of data reported to international agencies. A restricted data set has been extracted for which comparable ray coverage is achieved for P and S, and used for a joint inversion to produce a three-dimensional model for shear and bulk sound velocities represented in terms of 2° × 2° cells and 18 layers in depth through the mantle. About 106 times for each of P and S are combined to produce 312,549 summary rays for each wave type. Linearizing about the ak135 reference model, 583,200 coupled tomographic equations are solved using an iterative partitioned scheme. Clear high-resolution images are obtained for both bulk-sound speed and shear wavespeed. The bulk and shear moduli have differing sensitivity to temperature and mineral composition, and so the images of the two velocity distributions help to constrain the nature of the processes which produce the variations. Different heterogeneity regimes can be recognised in the upper mantle, the transition zone, most of the lower mantle, and the lowermost mantle. In the upper mantle, many features can be explained by thermal effects; but in some orogenic zones (e.g. western North America), the opposite sense of the bulk-sound and shear wave speed variation requires compositional effects or volatiles to outweigh any thermal effects. In the lower mantle, pronounced narrow structures which may represent remnant subduction are most marked in shear. The level of large-scale variations in bulk sound speed compared to shear diminishes with depth in the lower mantle reaching a minimum near 2000 km. Below this depth, the variability of both wave speeds increases. Near the core-mantle boundary the variations of the two wave speeds show little concordance, suggesting the presence of widespread chemical heterogeneity.

/pdf/joint-seismic-tomography-for-bulk-sound-and-shear-wave-speed-1cbg8y5z5p.pdf

Joint seismic tomography for bulk sound and shear wave speed in the Earth's mantle

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

Sri Widiyantoro

Papers

Evidence for deep mantle circulation from global tomography

Global seismic tomography: A snapshot of convection in the earth

Stagnant slabs in the upper and lower mantle transition region

Joint seismic tomography for bulk sound and shear wave speed in the Earth's mantle

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