Wim Spakman

Closing the gap between regional and global travel time tomography

Recent global travel time tomography studies by Zhou [1996] and van der Hilst et al. [1997] have been performed with cell parameterizations of the order of those frequently used in regional tomography studies (i.e., with cell sizes of 1°–2°). These new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations (5° cells). The inferred structures are, however, of larger scale than is usually obtained in regional models, and it is not clear where and if individual cells are actually resolved. This study aims at resolving lateral heterogeneity on scales as small as 0.6° in the upper mantle and 1.2°–3° in the lower mantle. This allows for the adequate mapping of expected small-scale structures induced by, for example, lithosphere subduction, deep mantle upwellings, and mid-ocean ridges. There are three major contributions that allow for this advancement. First, we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the Earths mantle by seismic waves [Spakman and Bijwaard, 1998]. Second, we exploit the global data set of Engdahl et al. [1998], which is a reprocessed version of the global data set of the International Seismological Centre. Their reprocessing included hypocenter redetermination and phase reidentification. Finally, we combine all data used (P, pP, and pwP phases) into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal-to-noise ratio is improved. In the approximate solution of the huge inverse problem we obtain a variance reduction of 57.1%. Synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells (0.6° or 1.2°) in the shallow parts of subduction zones decreasing to approximately 2°–3° resolution in well-sampled regions in the lower mantle. Vertical resolution can be worse (up to several hundreds of kilometers) in subduction zones with rays predominantly pointing along dip. Important features of the solution are as follows: 100–200 km thick high-velocity slabs beneath all major subduction zones, sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle; large high-velocity anomalies in the lower mantle that have been attributed to subduction of the Tethys ocean and the Farallon plate; and low-velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under Iceland, east Africa, the Canary Islands, Yellowstone, and the Society Islands. Our findings corroborate that the 660 km boundary may resist but not prevent (present day) large-scale mass transfer from upper to lower mantle or vice versa. This observation confirms the results of previous, global mantle studies that employed coarser parameterizations.

Geodynamics of flat subduction: Seismicity and tomographic constraints from the Andean margin

The cause and geodynamic impact of fiat subduction are investigated. First, the 1500 km long Peru fiat slab segment is examined. Earthquake hypocenter data image two morphologic highs in the subducting Nazca Plate which correlate with the posi- tions of subducted oceanic plateaus. Travel time tomo- graphic images confirm the three-dimensional slab ge- ometry and suggest a lithospheric tear may bound the NW edge of the fiat slab segment, with possible slab de- tachment occurring down dip as well. Other fiat slab re- gions worldwide are discussed: central Chile, Ecuador, NW Colombia, Costa Rica, Mexico, southern Alaska, SW Japan, and western New Guinea. Flat subduction is shown to be a widespread phenomenon, occuring in 10% of modern convergent margins. in nearly all these cases, as a spatial and temporal correlation is observed between subducting oceanic plateaus and fiat subduc- tion, we conclude that fiat subduction is caused pri- marily by (1) the buoyancy of thickened oceanic crust of moderate to young age and (2) a delay in the basalt to eclogite transition due to the cool thermal structure of two overlapping lithospheres. A statistical analysis of seismicity along the entire length of the Andes demon- strates that seismic energy release in the upper plate at a distance of 250-800 km from the trench is on aver- age 3-5 times greater above fiat slab segments than for adjacent steep slab segments. We propose this is due to higher interplate coupling and the cold, strong rhe- ology of the overriding lithosphere which thus enables stress and deformation to be transmitted hundreds of kilometers into the heart of the upper plate.

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.

Evidence for active subduction beneath Gibraltar

We report on a marine seismic survey that images an active accretionary wedge west of Gibraltar. Ramp thrusts offset the seafloor and sole out to an east-dipping decollement, indicating ongoing westward-vergent tectonic shortening. New traveltime tomographic re- sults image a slab of oceanic lithosphere descending from the Atlantic domain of the Gulf of Cadiz, passing through intermediate-depth (60-120 km) seismicity beneath the west- ernmost Alboran Sea, and merging with a region of deep-focus earthquakes 600-660 km below Granada, Spain. Together, these new data provide compelling evidence for an active east-dipping subduction zone.

Tethyan subducted slabs under India

Abstract Tomographic imaging of the mantle under Tibet, India and the adjacent Indian Ocean reveals several zones of relatively high P-wave velocities at various depths. Under the Hindu Kush region in northeastern Afghanistan and southern Tajikistan, a regional northward-dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the Indian plate. Under northern Pakistan this same slab shows a roll-over structure with the deeper portion overturned and dipping southward, as can also be seen in the distribution of earthquake hypocenters. Farther east-southeast (e.g., in the vicinity of Nepal), a well-resolved anomaly below 450 km depth is connected to the slab under the Hindu Kush, but seems to be separated from the lithosphere above 350 km. These upper-mantle anomalies are interpreted as the remnants of delaminated sub-continental lithosphere that went down when Greater India continued to converge northward with Asia after ∼45 Ma. The deeper high-velocity anomalies under the Indian sub-continent appear clearly separated from the shallower ones as well as from each other, and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the Indian plate. They occur at depths between 1000 and 2300 km and occasionally descend down to the core-mantle boundary. The anomalies form three parallel WNW-ESE striking zones. We interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the Neo-Tethys Ocean closed between India and Tibet in the Cretaceous and earliest Tertiary. The northern deep-mantle zone under northern Afghanistan, the Himalayas and the Lhasa block in southern Tibet may represent the last-subducted remnant of the Paleo-Tethys Ocean, which is thought to have closed before the Hauterivian stage of the Early Cretaceous. The middle zone continues southeastward as a rather straight high-velocity zone towards Sumatra, where it becomes convex southward and parallel to the subduction zone under the Sunda arc. Comparison of this straight middle zone near India with the shallower (upper 600–1000 km) northern zone, which displays a cusp-like shape near the Yunnan (SW China) Syntaxis of the eastern Himalayas, supports the notion that the shallow northern zone represents later subduction than the deeper middle zone. The suggestion of a counterclockwise rotation (>20°) of the Indian plate during Tertiary indentation of Asia is supported by these features. The present-day latitudes of 5°–35°N of the deep slabs under India and adjacent areas correspond to the approximate paleolatitudes of the Cretaceous subduction zones. The slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey, which implies that lateral movements in the deeper mantle were not large.

Zagros orogeny: a subduction-dominated process

This paper presents a synthetic view of the geodynamic evolution of the Zagros orogen within the frame of the Arabia–Eurasia collision. The Zagros orogen and the Iranian plateau preserve a record of the long-standing convergence history between Eurasia and Arabia across the Neo-Tethys, from subduction/obduction processes to present-day collision (from ~ 150 to 0 Ma). We herein combine the results obtained on several geodynamic issues, namely the location of the oceanic suture zone, the age of oceanic closure and collision, the magmatic and geochemical evolution of the Eurasian upper plate during convergence (as testified by the successive Sanandaj–Sirjan, Kermanshah and Urumieh–Dokhtar magmatic arcs), the P–T–t history of the few Zagros blueschists, the convergence characteristics across the Neo-Tethys (kinematic velocities, tomographic constraints, subduction zones and obduction processes), together with a survey of recent results gathered by others. We provide lithospheric-scale reconstructions of the Zagros orogen from ~ 150 to 0 Ma across two SW–NE transects. The evolution of the Zagros orogen is also compared to those of the nearby Turkish and Himalayan orogens. In our geotectonic scenario for the Zagros convergence, we outline three main periods/regimes: (1) the Mid to Late Cretaceous (115–85 Ma) corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper-plate fragmentation, (2) the Paleocene–Eocene (60–40 Ma) witnesses slab break-off, major shifts in arc magmatism and distributed extension within the upper plate, and (3) from the Oligocene onwards (~ 30–0 Ma), collision develops with a progressive SW migration of deformation and topographic build-up (Sanandaj–Sirjan Zone: 20–15 Ma, High Zagros: ~12–8 Ma; Simply Folded Belt: 5–0 Ma) and with partial slab tear at depths (~10 Ma to present). Our reconstructions underline the key role played by subduction throughout the whole convergence history. We finally stress that such a long-lasting subduction system with changing boundary conditions also makes the Zagros orogen an ideal natural laboratory for subduction processes.

The role of slab detachment processes in the opening of the western–central Mediterranean basins: some geological and geophysical evidence

Abstract A review of the geological and geophysical data from the central and western Mediterranean region and the present-day upper mantle structure derived from tomographic studies are utilized in order to define the Oligocene–Recent geodynamic evolution for the area. In line with previous work, we suggest that the Miocene–Quaternary opening of the western and central Mediterranean basins is the result of back-arc extension due to the roll back toward the southeast of a northwestward subducting African slab in a geodynamic setting pinned between the Alpine and Betic collisional zones. We find, however, that this general pattern is complicated by four different detachment events which occurred beneath the Alps (Early Oligocene), the Betic chain (Aquitanian), northern Africa (Langhian) and the Apennines (Late Miocene?–Pliocene). We show that each of these events determines a major tectonic reorganization within the European plate.

Greater India Basin hypothesis and a two-stage Cenozoic collision between India and Asia

Cenozoic convergence between the Indian and Asian plates produced the archetypical continental collision zone comprising the Himalaya mountain belt and the Tibetan Plateau. How and where India–Asia convergence was accommodated after collision at or before 52 Ma remains a long-standing controversy. Since 52 Ma, the two plates have converged up to 3,600 ± 35 km, yet the upper crustal shortening documented from the geological record of Asia and the Himalaya is up to approximately 2,350-km less. Here we show that the discrepancy between the convergence and the shortening can be explained by subduction of highly extended continental and oceanic Indian lithosphere within the Himalaya between approximately 50 and 25 Ma. Paleomagnetic data show that this extended continental and oceanic “Greater India” promontory resulted from 2,675 ± 700 km of North–South extension between 120 and 70 Ma, accommodated between the Tibetan Himalaya and cratonic India. We suggest that the approximately 50 Ma “India”–Asia collision was a collision of a Tibetan-Himalayan microcontinent with Asia, followed by subduction of the largely oceanic Greater India Basin along a subduction zone at the location of the Greater Himalaya. The “hard” India–Asia collision with thicker and contiguous Indian continental lithosphere occurred around 25–20 Ma. This hard collision is coincident with far-field deformation in central Asia and rapid exhumation of Greater Himalaya crystalline rocks, and may be linked to intensification of the Asian monsoon system. This two-stage collision between India and Asia is also reflected in the deep mantle remnants of subduction imaged with seismic tomography.

A Tomographic View on Western Mediterranean Geodynamics

During the Cenozoic, the Western Mediterranean region has experienced a complex subduction history which involved the destruction of the Late Triassic/Jurassic Ligurian ocean and the West Alpine-Tethys. Lithosphere remnants of this evolution have been detected in the upper mantle by seismic tomography imaging. However, no general consensus exists on the interpretation of these remnants/slabs in the context of Ligurian ocean and West Alpine-Tethys subduction. In this paper we search for subduction remnants of the entire Cenozoic evolution in the recent global tomography model of Bijwaard and Spakman (2000) and compare these tomography results and our interpretations with those obtained in previous studies. Next, we present an analysis of imaged mantle structure in the context of the tectonic evolution of the Western Mediterranean during the Cenozoic. Our analysis leads to the following main results:

The TRANSMED Atlas. The Mediterranean Region from Crust to Mantle

One - Printed Volume.- 1 The Mediterranean Area and the Surrounding Regions: Active Processes, Remnants of Former Tethyan Oceans and Related Thrustbelts.- Abstract.- 1.1 Introduction.- 1.2 Mediterranean Fold-and-thrust Belts.- 1.3 Mediterranean Marine Basins.- 1.4 Geological-geophysical Baseline.- 1.4.1 Heat Flow.- 1.4.2 Crustal and Lithospheric Structure.- 1.4.3 Gravity.- 1.4.4 Magnetic Field.- 1.4.5 Seismicity.- 1.4.6 Geodetic Data.- 1.4.7 Stress Field.- 1.5 Global Dynamics and Active Processes Exemplified in the Mediterranean.- 1.5.1 Subduction of the Eastern Mediterranean Lithosphere beneath the Calabrian and Aegean.- 1.5.2 Rifting and Passive Margin Development in Back-arc Regions and Other MediterranRelated to Tectonic Wedges, Tilted Blocks and Sedimentary Loadingean Domains.- 1.5.3 Mud and Salt Diapirism (Eastern Mediterranean Ridge, Alboran Sea, Nile Delta).- 1.5.4 Sea-level Changes, Salinity Crisis, Flooding (Messinian Mediterranean versus Pleistocene Black Sea).- 1.6 Record of Ancient Dynamics of the Tethyan Oceans, Ophiolitic Sutures, Mantle Tomography versus Paleogeography of the Mediterranean Realm.- 1.6.1 Collisional vs. Intracontinental Thrust Belts and Oceanic Sutures.- 1.6.2 Plate Dynamics and Palinspastic Restorations: Demise of the Concept of a Single Tethys.- 1.6.3 Cenozoic Magmatism in the Mediterranean Region.- 1.7 Conclusions.- Acknowledgements.- 2 A Tomographic View on Western Mediterranean Geodynamics.- Abstract.- 2.1 Introduction.- 2.2 The Global Tomography Model BS2000.- 2.3 Interpretation of Model BS2000 for the Western Mediterranean Mantle.- 2.3.1 Alps, Apennines, and the Western Mediterranean.- 2.3.2 The Betic-Rif and Alboran Region.- 2.4 Analysis: the Geodynamic Evolution of the Western Mediterranean.- 2.4.1 Tomographic Evidence for Slab Roll-back.- 2.4.2 Northern Apennines and Alpine-Tethys Subduction.- 2.4.3 Slab Detachment beneath the Central-southern Apennines.- 2.4.4 Calabria Subduction.- 2.4.5 The North African Margin.- 2.4.6 Betic-Rif and Alboran Region: I. Subduction and Roll-back of Predominantly Oceanic Lithosphere.- 2.4.7 Betic-Rif and Alboran Region: II. Development of Arc Geometry and Subduction Roll-back.- 2.4.8 Synthesis of Tomographic Constraints on the Geodynamic Evolution of the Western Mediterranean Region.- 2.5 Summary.- Acknowledgements.- Appendix 1 (CD-ROM).- Appendix 2 (CD-ROM).- 3 The TRANSMED Transects in Space and Time: Constraints on the Paleotectonic Evolution of the Mediterranean Domain.- Abstract.- 3.1 Introduction.- 3.2 The Western Tethys Main Plate Tectonic Constraints.- 3.2.1 The East Mediterranean-Neotethys Connection.- 3.2.2 The Apulia-Adria Problem.- 3.3 The Geodynamic Evolution of Greater Apulia and Surrounding Regions.- 3.3.1 Paleotethys Evolution (Figs. 3.2-3.6).- 3.3.2 Cimmerian Events and Triassic Marginal Oceans (Figs. 3.6-3.9).- 3.3.3 The Jurassic Oceans: Alpine Tethys, Central Atlantic and Vardar (Figs. 3.8-3.11).- 3.3.4 The Cretaceous Oceans: North Atlantic and the Pyrenean Domain (Figs. 3.10-3.14).- 3.4 The TRANSMED Transects in Space and Time.- 3.4.1 Transects I-II-III West.- 3.4.2 Transects IV-V-VI.- 3.4.3 Transects III East, VII and VIII.- 3.5 Conclusions.- Acknowledgements.- Appendix 3 (CD-ROM).- References: Preface, Chapters 1, 2 and 3.- References: CD-ROM.- Transect I: Iberian Meseta - Guadalquivir Basin - Betic Cordillera - Alboran Sea - Rif - Moroccan Meseta - High Atlas - Sahara Domain.- Transect II: Aquitaine Basin - Pyrenees - Ebro Basin - Catalan Coastal Ranges - Valencia Trough - Balearic Promontory - Algerian Basin - Kabylies - Atlas - Saharan Domain.- Transect III: Massif Central - Provence - Gulf of Lion - Provencal Basin - Sardinia - Tyrrhenian Basin - Southern Apennines - Apulia - Adriatic Sea - Albanian Dinarides - Balkans - Moesian Platform.- Transects IV, V and VI: The Alps and Their Forelands.- Transect VII: East European Craton - Scythian Platform - Dobrogea - Balkanides - Rhodope Massif - Hellenides - East Mediterranean - Cyrenaica.- Transect VIII: Eastern European Craton - Crimea - Black Sea - Anatolia C2014 Cyprus - Levant Sea - Sinai - Red Sea.