Ajay Kumar successfully defended his Ph. D. thesis online last 24 November. The thesis, entitled "Thermal and chemical imaging of the upper mantle anomalies: application to Western Mediterranean" has been directed by Manel Fernández and Jaume Vergés, both researchers at Geosciences Barcelona and has been tutored by Juan José Ledo Fernández from University of Barcelona. Kumar’s work was focused on characterizing the thermal and chemical structure of the upper mantle (upto 400 km depth) beneath Alboran and Algerian basins using gravity, elevation, mantle xenoliths and seismic tomography.

Previous studies have revealed, using seismic tomography, the presence of two positive seismic velocitiy anomalies indicating colder than the surrounding material beneath the Southern Spain and North Algerian. These "anomalies" had been interpreted as oceanic lithosphere of the ancient Tethys ocean that subducted into the mantle during the collision of Eurasian and African plates.

"Seismic tomography (like CT scan) is one of the most powerful tools in studying the internal structure and dynamics of the Earth. However, seismic tomography only tells us which regions are fast or which are slow. Accordingly, fast regions are interpreted to be cold and slow regions as hot regions. This interpretation is qualitative and leaves one other important factor controlling seismic velocities, the chemical composition", said Ajay Kumar.

Kumar's thesis, which has been developed in the framework of the SUBITOP project, has been unfolded into two well-defined phases. The first part of the thesis was to improve the already existing modeling software LitMod2D. Kumar modified the code to include the seismic velocity anomalies in his forthcoming models. The new LitMod version was key to determine the temperature and the chemical composition of the subducted lithosphere.

According to Kumar, "LitMod2D_2.0 was a pre-requisite for the thesis. The software numerically calculated the temperature, pressure, density, and seismic velocities inside the Earth. It then predicts the data dependent on these four parameters which can be measured at the surface of the Earth (elevation, gravity, SHF, earthquake waveforms). Model is changed in the process in order to minimize the difference between predicted and observed data".

Once the code was ready, Kumar could address the second part of his work. The efforts were focused on determining the temperature, composition, and architecture of the lithosphere along two transects beneath Alborán and Algerian basins and that also included the two main orogens of the region: the Betics and the Kabyles. "The main challenge was to infer temperature and/or composition from seismic velocities. This challenge comes from the fact that the physics relating them is highly non-linear", said Kumar.

The resulting model shows show “a thick crust (37 km and 30 km) and a relative deep LAB (130 km and 150 km) underneath the HP-LT metamorphic units of the Internal Betics and Greater Kabylies that contrast with the ~16 km thick magmatic crust of the Alboran Basin and the ~10 km thick oceanic crust of the Algerian Basin, respectively.”

The work also notes that, although the temperature of the Alboran and Argelian subducted slabs beneath the Betics and the Kabylians is about 400ºC colder than the surrounding material, they have a different composition.

Kumar integrated various data to model the lithospheric structure. "The data includes gravity field of the Earth, elevation, surface heat flow, waveforms from the earthquakes and the laboratory-based parameters for material inside the Earth", said Kumar.

In 2018, Kumar also carried out a month and a half research stay in the Laboratory of Experimental Tectonics at Roma 3 University directed by Claudio Faccena to learn how to carry out analog models.

"The main objective of the Rome visit was to study the influence of a continent lying laterally to a subducting ocean plate and to see the influence on the rate of subduction along the trench and its curvature.", explained Kumar.

The stay was an opportunity for Kumar to widen his experience in analog modeling."As most of the thesis work was done on a computer, Rome visit provided me with an opportunity to work with physical material. For instance, if I had to model an oceanic plate on my computer I would just give some numbers whereas in Rome I had to physically make an oceanic plate with silicon and iron."

"This thesis provides important and crucial constraints for the understanding of the geodynamic evolution of the complex Western Meditteranean, which is under debate. Apart from this, one outcome of this thesis is LitMod2D_2.0, a tool freely available to the scientific community to use in their region of interest.", said Kumar.

"Understaing the processes in the Earth helps to make better policies regarding the natural hazard these processes pose. For example, the structure of the deep Earth imaged in this thesis can be used and taken into the earthquake hazard assement and risk maps", concluded Kumar.

More information

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  • Kumar, A., Fernandez, M., Peral, M., Funiciello, F., Zlotnik, S., Faccenna, C., Vergés, J. Effects of kinematic boundary conditions on trench curvature in a retreating subduction zone: insights from analog modelling.

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