Subduction of the Tethys Oceans reconstructed from plate kinematics and mantle tomography

Abstract

This thesis is concerned with the large-scale history of subduction within the Tethyan region, the Alpine-Himalayan mountain chain that stretches from the Mediterranean to the Indonesian archipelago. We investigate whether we can contribute to a better understanding of the Tethyan evolution by combining plate tectonic reconstructions of the area with independent seismic tomography results. From tomographic images of the present mantle structure, the volumes and locations of the positive seismic velocity anomalies are determined. The large tomographic volumes, and the large depths at which they are found, indicate that they must be associated with long periods of subduction. We therefore examine the large-scale surface motions within the region since 200 Ma, the timespan in which all Mesozoic-Cenozoic movements of the African-Arabian and Indian plates relative to Eurasia took place, and that is thought to be necessary to explain the inferred tomographic anomalies. The amount of convergence is calculated from plate tectonic reconstructions and gives us an estimate of the surface of the subducted oceanic lithosphere. From this surface, we predict the present thermal volumes of subducted material and test these against the volumes inferred from the seismic tomographic models. Plate tectonic reconstructions of the Tethyan region generally agree on the first-order motions. In this study, we find that the volumes, locations and depths of the tomographic anomalies can be readily explained by the predicted thermal volumes: For the Tethyan region as a whole, as well as for the Aegean/Arabian and Indian regions separately, the predicted volumes are found to be significantly larger than the tomographic volumes. Taking into account slab thickening, the predicted volumes are somewhat smaller than the tomographic volumes, which allows for additional material subducted as a result of oceanic spreading during subduction. More detailed aspects of the tomographic anomaly distribution in the mantle are discussed in the context of the differences among the current tectonic reconstructions. We 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 material, 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 are able to present a preferred scenario for the subduction in the western and central Tethyan regions. The easternmost Tethyan region is characterised by active subduction of various oceanic basins. Therefore, we directly model the present thermal structure of the Indonesian subduction zones, instead of approximating the thermal volumes of the subducted material as in the previous. The subduction zones models, based on regional tectonic reconstructions, are converted into seismic velocity anomalies which can be compared directly to the tomographic images of the mantle structure