The lithospheric structure of Africa: Mapping crustal and lithospheric thickness using geoid and elevation constraints together with a thermal analysis

[eng] The African continent preserves a >3.7 Ga long geological record, which comprises stabilization of oldest crust in the Archean, late Proterozoic joining of first cratonic units that lead to the formation of Gondwana and later the supercontinent Pangea, and post-breakup Mesozoic and Cenozoic continental rifting with transition to active oceanic spreading in the Afar and Red Sea region. Today, most of Africa’s basement consists of Archean cratons and blocks flanked by Proterozoic mobile belts and is considered to be tectonically stable, as it largely escaped tectonothermal deformation since the late-Precambrian Pan-African orogeny. Yet, Africa is affected by a number of active processes, many of them as young as the Cenozoic Era, including widespread hotspot volcanism, active rifting in East Africa, large-scale doming in eastern and sub-equatorial Africa and intracratonic subsidence in the Congo. The link between old Precambrian stable basement and recent tectonic activity makes Africa an ideal laboratory to study the role of the crustal and lithospheric mantle structure on the observed deformation within the continent. The main goal of this thesis is to provide new crustal and lithospheric thickness maps of the African mainland based on integrated modelling of elevation and geoid data and thermal analysis. The approach assumes local isostasy, thermal steady-state, and linear density increase with depth in the crust and temperature-dependent density in the lithospheric mantle. The obtained results are constrained by a new comprehensive compilation of seismic Moho-depth data consisting of 551 data points from active and passive source seismic experiments, and by published tomography models relative to lithosphere thickness. The calculated crustal thickness map shows a north-south bimodal distribution with higher thickness values in the cratonic domains of southern Africa (38 - 44 km) relative to those beneath northern Africa (33 - 39 km). The most striking result is the crustal thinning (28 - 30 km thickness) imaged along the Mesozoic West and Central African Rift Systems. The crustal model shows noticeable differences when compared to previous global and continent-scale models, especially for regions to the north of the equator. After excluding the Afar plume region, where the modeling assumptions are not fulfilled, the model shows the best fit with the available seismic data (76.3% fitting; RMSE=4.3 km). The new crustal thickness map correlates better with geological structures and tectonic provinces as well as gravity anomalies, and shows a higher spatial resolution. The resulting lithospheric thickness map shows large spatial variability (90 to 230 km), with thicker lithosphere related to cratonic domains and shallower LAB related to Mesozoic and Cenozoic rifting domains, which is in good agreement with seismic tomography models. Though the crustal and lithosphere thickness maps show similar regional patterns, major differences are found in the Atlas Mountains, the West African Rift System, and the intracratonic basins, i.e., the Taoudeni and Congo Basin, indicating strong strain partitioning most probably due to intra-lithospheric decoupling along the crust-mantle boundary. The effects of lateral variations in crustal density as well as the non-isostatic contribution to elevation in the Afar plume region, was estimated to be ~1.8 km, and are also discussed. [spa] La arquitectura del continente Africano es el resultado de una compleja historia >3.7 billones de anos. En la actualidad, la mayoria del basamento de Africa consiste en cratones Archeanos flanqueados por cinturones moviles Proterozoicos y es considerado tectonicamente estable puesto que no ha sufrido deformacion tectono-termal desde la orogenia Pan-Africana al final del Precambrico. A pesar de todo, Africa es afectada por procesos activos como, volcanismo de punto caliente, rift continental y subsidencia intractratonica. Por lo tanto, Africa es el escenario ideal para estudiar el rol de la estructura litosferica y cortical en la deformacion observada en el continente. El objetivo principal de esta tesis es presentar nuevos mapas de potencia litosferica y cortical del continente Africano basados en la integracion de datos de elevacion, geoide y termicos en la modelizacion. El metodo asume isostasia local, regimen termico estacionario e incremento lineal de la densidad en profundidad, asi como interrelacion entre la densidad del manto litosferico y temperatura. Los resultados obtenidos estan constrenidos por una nueva compilacion de la profundidad del Moho a partir de datos sismicos, consistente en 551 puntos, y por modelos tomograficos publicados de la potencia litosferica. El mapa de potencia cortical presenta una distribucion bimodal de Norte a Sur con valores mayores en los dominios cratonicos en el sur de Africa (38 – 44 km) relativos a los presentes en el norte (33 – 39 km). El resultado mas llamativo es el adelgazamiento de la corteza (28 – 30 km) calculado a lo largo del Sistema del Rift Mesozoico en el Oeste y Centro. Tras excluir la region de Afar, donde las asunciones del modelo no se cumplen, el ajuste a la informacion sismica disponible es mejor (76.3%; RMSE=4.3 km). El mapa de potencia litosferica presenta grandes variabilidades (90 - 230 km), con una litosfera engrosada ligada a los dominios cratonicos y LAB somera relacionada con el Rift Mesozoico y Cenozoico, concordante con los modelos de tomografia sismica. A pesar de que los mapas de la potencia litosferica y cortical presentan patrones regionales similares, existen grandes diferencias en el Atlas, el Sistema Oeste del Rift, y las cuencas intractratonicas. El efecto de las variaciones laterales de densidad asi como la contribucion no isostatica a la elevacion en la zona de Afar, se estima que es de ~1.8 km.