• Thesis title: Thermal and chemical imaging of the upper mantle anomalies: application to Western Mediterranean
  • Author: Ajay Kumar(GEO3BCN-CSIC)
  • Date: 24 November 2020, 11:00
  • Place: Online. By invitation.
Thesis supervisors
Thesis tutor
  • Dr. Juan José Ledo Fernández(UB)

The closure of the Ligurian-Tethys Ocean, opened during Jurassic and consisting of highly segmented margins in between Africa and Iberia, has produced the Alboran and Algerian basins in the Western Mediterranean through subduction and slab roll-back processes during the Cenozoic. Towards the end of the slab roll back,collision with the continental margins led to the formation of the Betic-Rif orogen in south Iberia and the Tell-Kabylies in north Algeria. Both, the Betics-Rif and Tell- Kabylies, shows the high-pressure and low-temperature (HP-LT) rocks exhumed from the subduction channel but with opposite tectonic vergence, to the NW in Betics and to the SE in Kabylies. While the Cenozoic evolution of the back-arc basins in the Central and Eastern Mediterranean (i.e., Liguro-Provenca, Tyrrehenian and Aegean) are well understood, the evolution of the Alboran and Algerian basins in the Western Mediterranean is under debate, leading to the proposal of different geodynamicevolution models. All the models agree on that the subduction and subsequent slabrollback was operating but argues for the direction of subduction trench and slabrollback.

At present, positive seismic velocity anomalies in the upper mantle are observed in the tomography models around the Alboran Basin and beneath the North-Algeria margin. These high velocity anomalies are qualitatively interpreted to be cold, hence, remnant of the subducted Ligurian-Tethys lithosphere in order to explain geodynamic evolution of the Alboran and Algerian basins. Subduction processes must have left its imprint on the crust and upper mantle structure, temperature and chemical composition, which dictate the present-day physical state. Physical state inside the Earth controls the physical properties (i.e., density, seismic velocities, and thermal conductivity) which in turn control the geophysical observables at the surface (i.e., elevation, gravity anomaly, geoid height, and surface heat flow). Integrated geophysical-petrological modelling of these surface observables allows exploring and reconciling observations from different datasets and methods. However, thermal and/or chemical nature of the imaged seismic velocity anomalies (e.g., subducted Ligurian-Tethys) needs to be incorporated in such models. In general, seismic tomography models reports relative positive or negative velocities with respect to a reference model which are further inferred qualitatively as cold or hot regions in the upper mantle, respectively. Quantitative interpretation of the seismic velocity anomalies in terms of temperature and/or chemical composition is challenging and is at the forefronts of the modern day geophysics.

Hence, the objectives of this thesis is twofold: 1) to develop a methodological framework to incorporate the sublithospheric anomalies observed in seismic tomography in the integrated geophysical-petrological modelling of the geophysical surface observables, and 2) its application to the Alboran and Algerian basins and their margins to model the present-day crust and upper mantle thermo-chemical structure yielding temperature, density (i.e., chemical composition) and seismic velocities to put constrains on their geodynamic evolution.

In the first part, an already existing tool, LitMod2D_1.0, is improved into a new LitMod2D_2.0 version which allows to model the sublithospheric anomalies and to be available for the scientific community. Various synthetic tests of the upper mantle anomalies have been performed to understand the sensitivity of temperature and chemical composition to the density and seismic velocities. Results show nonlinearity between the sign of thermal and seismic velocity anomalies, and that S-wave velocities are more sensitive to temperature whereas P-wave velocities are to composition. A synthetic example of subduction is made to understand the sensitivity of sublithospheric mantle anomalies associated with the slab and the corner flow, on surface observables (elevation, geoid height, and gravity anomalies). A new open-source graphic user interface is incorporated in the new version for ease of application. The output of the code is simplified by writing only the relevant physical parameters (temperature, pressure, material type, density, and seismic velocities) to allow the user to utilize predefined post-processing codes from a toolbox (flexure, mineral assemblages, synthetic passive seismological data, and tomography) or designing new ones. A post-processing example is demonstrated by calculating synthetic seismic tomography, Rayleigh-surface-wave dispersion curves, P-wave receiver functions and stable minerals distribution from the output file of LitMod2D_2.0.

In the second part of this thesis, I apply improved LitMod2D_2.0 to define the present day crustal and lithospheric structure along two 2D geo-transects beneath the Betics-Alboran and Greater Kabylies-Tell-Algerian orogenic systems to discuss the highly debated and contrasting existing models. Results 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. This sharp change in crustal thickness, from the orogenic wedge to the back-arc basins, contrasts with the gentler crustal thickening towards the respective opposed margins. Despite the similar LAB depth (~60 km) in both basins, the chemical composition of the lithospheric mantle beneath the Alboran Basin is slightly more fertile than beneath the Algerian Basin. At sublithospheric levels, results show that both the Alboran slab beneath the Betics and Algerian slab beneath the Kabylies, are about -400 oC colder than the ambient mantle but have different chemical composition. Alboran slab is slightly fertile compared to the typical oceanic lithospheric of the Algerian slab. Both slabs are detached from the respective continental lithospheric mantle of Iberia and Africa, since their weight is not transmitted isostatically to the surface. Results show that the uplift related to the slab break-off is ~700–1000 m in the Betics and is ~600–1200 m in north Algeria.

The Ligurian-Tethys slab beneath the SE Iberia shows an apparent dip to the SSE whereas the slab below Algeria dips to the NNW, matching the NW- and SE tectonic transport direction of the fold and thrust belts of the Betics and Greater Kabylies-Tell-Atlas subduction-related orogens, respectively. The large-scale configuration of present-day SE Iberia and Algerian margins as well as their mantle compositions in the Alboran and Algerian geo-transects is consistent with opposite dipping subduction of two segments of the Jurassic Ligurian-Tethys domain. Their present configurations agree with Neogene slab roll-back process triggering mantle delamination followed by slab break-off in both opposite subductions.

More information

Kumar, A., Fernàndez, M., Jiménez-Munt, I., Torne, M., Vergés, J., & Afonso, J. C. (2020). LitMod2D_2. 0: An improved integrated geophysical-petrological modeling tool for the physical interpretation of upper mantle anomalies. Geochemistry, Geophysics, Geosystems, 21(3), e2019GC008777.

Boonma, K., Kumar, A., García-Castellanos, D., Jiménez-Munt, I., & Fernández, M. (2019). Lithospheric mantle buoyancy: the role of tectonic convergence and mantle composition. Scientific reports, 9(1), 1-8.

Jiménez-Munt, I., Torné, M., Fernandez, M., Vergés, J., Kumar, A., Carballo, A., & García-Castellanos, D. (2019). Deep seated density anomalies across the Iberia-Africa plate boundary and its topographic response. Journal of Geophysical Research: Solid Earth, 124(12), 13310-13332.

In review

Kumar, A., Fernàndez, M., Vergés J., Torne, M., Jiménez-Munt, I. (2020). Opposite symmetry in the lithospheric structure of the Alboran and Algerian basins and their margins (Western Mediterranena): geodynamic implications. Journal of Geophysical Research: Solid Earth.

In preparation

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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