Robert D. van der Hilst

The Geological Evolution of the Tibetan Plateau

The geological evolution of the Tibetan plateau is best viewed in a context broader than the India-Eurasia collision zone. After collision about 50 million years ago, crust was shortened in western and central Tibet, while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western Pacific and Indonesia. Cessation of rapid Pacific trench migration (∼15 to 20 million years ago) coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern Tibet. The latter appear to result from rapid eastward flow of the deep crust, probably within crustal channels imaged seismically beneath eastern Tibet. These events mark a transition to the modern structural system that currently accommodates deformation within Tibet.

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

Abstract 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 Earths 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 symmetric velocity 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 Fermats 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 800 km.

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 Earths 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 Earths 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.

The deep structure of the Australian continent from surface wave tomography

Abstract We present a new model of 3-D variations of shear wave speed in the Australian upper mantle, obtained from the dispersion of fundamental and higher-mode surface waves. We used nearly 1600 Rayleigh wave data from the portable arrays of the Skippy project and from permanent stations (from Agso , Iris and Geoscope ). Agso data have not been used before and provide better data coverage of the Archean cratons in western Australia. Compared to previous studies we improved the vertical parameterization, the weighting scheme that accounts for variations in data quality and reduced the influence of epicenter mislocation on velocity structure. The dense sampling by seismic waves provides for unprecedented resolution of continental structure, but the wave speed beneath westernmost Australia is not well constrained. Global compilations of geological and seismological data (using regionalizations based on tectonic behavior or crustal age) suggest a correlation between crustal age and the thickness and composition of the continental lithosphere. However, the age and the tectonic history of crustal elements vary on wavelengths much smaller than have been resolved with global seismological studies. Using our regional upper mantle model we investigate how the seismic signature of tectonic units changes with increasing depth. At large wavelengths, and to a depth of about 200 km, the inferred velocity anomalies corroborate the global pattern and display a progression of wave speed with crustal age: slow wave propagation prevails beneath the Paleozoic fold belts in eastern Australia and wave speeds increase westward across the Proterozoic and reach a maximum in the Archean cratons. The high wave speeds associated with Precambrian shields extend beyond the Tasman Line, which marks the eastern limit of Proterozoic outcrop. This suggests that parts of the Paleozoic fold belts are underlain by Proterozoic lithosphere. We also infer that the North Australia craton extends off-shore into Papua New Guinea and beneath the Indian Ocean. For depths in excess of 200 km a regionalization with smaller units reveals that some tectonic subregions of Proterozoic age are marked by pronounced velocity highs to depths exceeding 300 km, but others do not and, surprisingly, the Archean units do not seem to be marked by such a thick high wave speed structure either. The Precambrian cratons that lack a thick high wave speed “keel” are located near passive margins, suggesting that convective processes associated with continental break-up may have destroyed a once present tectosphere. Our study suggests that deep lithospheric structure varies as much within domains of similar crustal age as between units of different ages, which hampers attempts to find a unifying relationship between seismic signature and lithospheric age.

Structure of the upper mantle and transition zone beneath Southeast Asia from traveltime tomography

[1] Tomographic images of the mantle beneath East Asia were obtained from the inversion of traveltime data from global and regional seismograph networks and from temporary arrays on and around the Tibetan plateau. Our results are consistent with previous studies but the unprecedented resolution of mantle heterogeneity provides new insight into the large‐scale tectonic framework of the continental India‐Asia collision in the western part of the study region and subduction of the oceanic lithosphere in the east. In the realm of continental collision, west of ∼100°E, a relatively slow P‐wave speed characterizes the upper mantle beneath much of the Tibetan plateau but the wave speed is high beneath cratonic India, the southern and western part of the Tibetan plateau, Hindu‐Kush, and the Tian Shan. In the subduction realm, east of ∼110°E, the main structures are (i) pronounced low‐wave‐speed anomalies at a depth of between 100 and 400 km beneath Asia’s southeastern seaboard and the back‐arc regions of ongoing subduction; (ii) narrow, fast anomalies in the upper mantle beneath major subduction zones; and (iii) widespread fast anomalies at a depth of 500–700 km beneath the Sea of Japan, the northern part of the Philippine Sea plate, and southeastern China. If the latter anomalies represent stagnant slabs, their fragmented nature and large lateral extent suggest that they are produced by different episodes of subduction beneath western Pacific island arcs, along the old SE margin of Asia, or during the Mesozoic collision of cratonic units in Southeast Asia. Attribution to ancient subduction systems implies that slab fragments can reside in the transition zone for (at least) several tens of millions of years. Shallow, slow anomalies beneath the Red River fault region connect to deep anomalies beneath the South China fold belt and South China Sea, suggesting a causal relationship between the evolution of the continental lithosphere of SW China and deeper mantle processes. Between the collision and the subduction realms, tomography reveals high‐wave‐speed continental roots beneath the western part of the North China craton (Ordos block) and the South China, or Yangtze, craton (Sichuan Basin) to a depth of ∼300 km.

Tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases (PKP, Pdiff)

The mapping of variations in P wave speed in the deep mantle is restricted by the uneven sampling of P waves, in particular beneath the Southern Hemisphere. To enhance data coverage, we augmented the ∼1.6 million summary rays of P, pP, and pwP that we used in previous studies with differential travel times of diffracted and refracted core phases. For the core-refracted differential travel time residuals (PKPAB-PKPDF and PKPAB-PKPBC) we used 1383 cross-correlated digital waveforms as well as ∼27,000 routinely processed bulletin data. We used the waveform data to define quality criteria for the selection and reduction of the bulletin PKP data. For PKPDF-Pdiff we only considered 543 records derived from waveform cross correlation. No PcP data were used in this study. We used optical ray theory to calculate the ray paths associated with the P, pP, pwP, and PKP data, which are measured at 1 Hz. However, to account for the large Fresnel zones of the low-frequency (∼50 mHz) PKPDF-Pdiff data we estimated the three-dimensional shape of the Frechet sensitivity kernels from kernels calculated by normal mode summation. The use of these kernels allows us to properly distribute the sensitivity for a given seismic phase over a large mantle volume while allowing the high-frequency data to constrain small-scale structure. The differential times are relatively insensitive to source mislocation and to structure in the shallow mantle beneath source and receivers, and they have previously been interpreted exclusively in terms of lateral structure directly above the core mantle boundary (CMB). However, images thus obtained can be contaminated by effects of small scale structure elsewhere in the mantle. Here, we do not make a priori assumptions about the mantle source of anomalous time differentials. From test inversions we conclude that (both upper and lower) mantle structures that are poorly resolved by P data can be mapped into the core along PKP paths but that the effect of outer core structures, if any, on the mantle model is small. Compared to the inversion of the P, pP, and pwP alone, the inclusion of the PKPAB-PKPDF and PKPAB-PKPBC and PKPDF-Pdiff data improves the resolution of structure beneath 2200 km depth. In particular, the joint inversion puts better constraints on the long-wavelength variations in the very deep mantle and yields an increase in the amplitude of velocity perturbations near the CMB that is in agreement with but still smaller than inferences from shear wave studies. Resolution tests indicate that in some regions the enhanced definition of structure is significant, but in most regions the improvements are subtle and structure remains poorly resolved in large regions of the mantle.

A laboratory investigation of effects of trench migration on the descent of subducted slabs

A laboratory investigation of viscous slabs subducted from a migrating trench reveals a range of possible behaviours. The slab dip in a uniform mantle is found to be steady or oscillatory, depending on the rates of descent and trench migration. In addition, density and viscosity interfaces are used to model the increased resistance to sinking of stiff slabs through the seismic discontinuity at a depth of about 660 km in Earths mantle. If the slab is denser than the lower layer and its dip in the upper layer is steady, it can continue to descend through the lower layer in a tabular form and in either a steady or oscillatory manner, or it can be laid horizontally on the interface for a distance before descending in a chain of diapirs resulting from gravitational instability at the interface. If the slab dip in the upper layer is unstable, the slab sinks into the lower layer in a chain of large diapirs at a spacing determined by the frequency of oscillations set by instability of the slab within the upper layer. The style of penetration depends on the trench migration speed and the ratio of sinking velocities in the two layers. Estimates for mantle slabs indicate that they may range across the major regime transitions. The experimental system provides a gross simplification of mantle conditions but the results are a testimony to the possibility of a range of complex behaviour of subducted lithosphere. They also indicate that the relationship between the structural features in the lower mantle revealed by seismic imaging and present-day tectonic processes at the surface may not be obvious. Tomographic images of western Pacific subduction zones and data for the migration rates of associated trenches suggest a dependence of slab behaviour on migration rate similar to that seen in the experiments.

The Poisson 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. fl 2000 Elsevier Science B.V. All rights reserved.

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

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.

Tectonic implications of tomographic images of subducted lithosphere beneath northwestern South America

We used seismic tomography to investigate the complex structure of the upper mantle below northwestern South America. Images of slab structure not delineated by previous seismicity studies help us to refine existing tectonic models of subducted Caribbean-Pacific lithosphere beneath the study area. Beneath western Venezuela and Colombia we distinguish two slabs: a Maracaibo and a (redefined) Bucaramanga slab. The Maracaibo slab, coinciding with most of the Bucaramanga slab previously defined by W. D. Pennington, dips in a direction of 150° at an angle of 17° to a depth of 275 km and correlates to the subducted Late Cretaceous oceanic plateau of the Caribbean plate. Farther south, a second slab dips at an angle of 50° in a direction of 125° to a depth of at least 500 km and correlates to the subducted oceanic crust of the Nazca plate and the downdip extension of the Panama island arc. We refer to this slab as the redefined Bucaramanga slab, because it is different from the Bucaramanga slab segment defined by Pennington. The area of the South American plate overriding both slabs is characterized by the absence of an active volcanic arc, an anomalously wide topographically uplifted and tectonically active area, and the northward escape of the Maracaibo block along active strike-slip faults. In support of earlier studies, we attribute this to the underthrusting of the Caribbean oceanic plateau (our shallowly dipping Maracaibo slab) along the base of the South American lithosphere and to the recent collision of the Panama island arc rafted in on more steeply dipping crust of the Nazca plate (our redefined Bucaramanga slab).