My Ph.D. thesis, entitled Subduction of the Tethys Oceans reconstructed from plate kinematics and mantle tomography (2004), can be downloaded from the website of the Utrecht University Library. In addition, I have made an animation of the Tethyan subduction history reconstructed in my thesis for both the Arabian and Indian region.

I will give an introduction to my research here, but I also have a shorter abstract.
Ik heb ook een korte en lange Nederlandse samenvatting van mijn onderzoek...

[The Tethys Oceans]

The Tethys Oceans separated the African, Arabian and Indian continents from Eurasia in Mesozoic-Cenozoic times. When the Atlantic Ocean opened, the continents started to converge and the Tethys Oceans gradually began closing. The final continental collisions of Africa, Arabia and India with the southern margins of Eurasia resulted in the formation of the impressive Alpine-Himalayan mountain chain that extends from the Mediterranean region to the Indonesian archipelago. Whereas the subduction of the Tethys Oceans was dominated by the convergence of the continents, the process was complicated by the rifting of various intermediate fragments, back-arc spreading, and intra-oceanic subduction. The large orogenic belt of the Tethyan region is therefore an interesting but extremely complex domain of continental fragments, highly deformed suture zones and ophiolitic terranes. Although the large-scale evolution of the area is now relatively well constrained, the more detailed aspects are still poorly understood.

[Plate kinematics, mantle tomography, and subduction]

Tectonic reconstructions of past plate motions form a framework for other geological, oceanographic and climatology studies. Especially the plate kinematics of large-scale reconstructions, as those for the complete Tethyan region, can be useful in this respect. Tectonic reconstructions are generally based upon many integrated sources of information that can be found near the Earth's surface, like paleomagnetic and paleobiogeographic data, fracture zones and magnetic anomalies on the ocean floor, stratigraphy, and the occurrence of volcanism. For the Tethyan region, the currently available data are not always well constrained and can be interpreted in different ways. As a result, the numerous reconstructions that have been presented for the Mesozoic-Cenozoic evolution of the Tethyan region still show significant differences. The Tethys Oceans subducted into the mantle, but disappeared without leaving enough clues at the surface to arrive at one single geodynamic scenario so far.

Seismic tomography is a technique to image the velocity structure of the Earth's interior. The cold material of subducted oceanic lithosphere, re-heating slowly within the mantle, will generally be imaged as positive velocity anomalies in these models. Also beneath the Tethyan region, tomographic images show large anomalous volumes in the present mantle that seem to represent remnants of the Tethys Oceans. Although care should be taken when interpreting seismic velocity anomalies, the positive anomalies associated with subducted oceanic lithosphere are largely due to temperature perturbations. We can therefore use the temperature-derivatives of the seismic velocities to convert these anomalies to thermal perturbations.

To test the tectonic reconstructions of the Tethyan region, we connect the near-surface geological information to the deep mantle structure by using the tomographic anomalous volumes. Therefore, we have to determine the total amount of convergence, thus the subducted lithospheric surface, from the plate kinematic models. The prediction of the present thermal volumes of these plates comprises the effects of age-dependent lithospheric cooling and that of the subduction process itself. To use the present positions and geometries of the seismic anomalies in our comparison, we have to acknowledge the complex behaviour of slabs descending into the mantle. Processes of importance are, for example, slab sinking throughout the mantle, slab thickening when entering the lower mantle, ridge subduction, and possible slab break-off after continental collisions. When comparing the spatial distributions of the predicted and tomographic volumes, the absolute plate motion, i.e. the motion of the plate with respect to the underlying mantle, becomes especially relevant.

[Aim and approach]

The aim of this research is to reconstruct the Mesozoic-Cenozoic subduction of the Tethys Oceans, by integrating plate tectonic reconstructions, mantle tomography, and elements of subduction dynamics. We therefore:

1) Calculate the surface of subducted lithosphere from plate tectonic reconstructions

2) Estimate the initial thermal volumes of the subducted lithosphere calculated in 1

3) Predict the present thermal volumes of the slabs from the volumes estimated in 2

4) Estimate the size of the anomalous volumes in seismic tomography that may be related to subducted lithosphere

5) Compare the tomographic volumes estimated in 4 with the present slab volumes predicted in 3, and test the tectonic reconstructions and subduction scenarios

[Thesis outline]

An overview of the Mesozoic-Cenozoic evolution of the Tethyan region is presented in Chapter 2. Herein, we will primarily focus on the large-scale plate motions as proposed by various tectonic reconstructions. The chapter is concluded with a summary of the aspects of the evolution that will be of importance for our analysis of Tethyan subduction.

In Chapter 3, we discuss the most important methods used in this study, namely (1) the interpretation of the positive velocity anomalies from seismic tomographic models in terms of temperature, (2) the calculation of the amount of convergence from plate tectonic reconstructions, and (3) the prediction of the present thermal structure of the subducted oceanic lithosphere. Because most Tethyan lithosphere started to subduct in Mesozoic times already, and has almost completely disappeared today, we will develop a simplified method to approximate these slab volumes.

The large-scale history of subduction within the Tethyan region is explored in Chapter 4 by analysing the bulk volumes of oceanic lithosphere subducted in the past 200 Ma. The total lithospheric surface subducted between the converging continents is calculated from the tectonic reconstruction of Norton (1999). The present thermal volumes predicted from these subducted plates are compared to the total volume of the relevant positive velocity anomalies underneath the Tethyan region, as determined from the tomographic model of Bijwaard et al. (1998).

More detailed aspects of the Tethyan evolution are investigated in Chapter 5. We incorporate other reconstructions (Dercourt et al. (1993); Sengor and Natal'in (1996); Stampfli and Borel (2002,2004)) and analyse, among others, the effect of spreading ridge subduction, the Cenozoic continental collisions and possible subsequent slab break-off, and the role of oceanic back-arc basins. Each process will lead to a particular subdivision of the subducted lithosphere, of which the amounts, locations and timing of subduction can be compared to the volumes, positions and geometries of the separate tomographic anomalies. By evaluating the results for the different tectonic reconstructions, we will be able to present a preferred scenario for the subduction in the western and central Tethyan region.

Finally, in Chapter 6 we address the subduction in the easternmost Tethyan region, namely the Indonesian archipelago. Because this region is characterised by ongoing subduction, we will directly model the present thermal structure of the subduction zones, instead of approximating the thermal volumes as in the previous chapters. The subduction zones models, based on the regional tectonic reconstructions of Rangin et al. (1990a,b) and Lee and Lawver (1995), will be converted into seismic velocity anomalies which can be compared directly to the tomographic images of the mantle structure.