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Title: The lithospheric structure of Africa: Mapping crustal and lithospheric thickness using geoid and elevation constraints together with a thermal analysis
Author: Globig, Jan
Director: Fernàndez Ortiga, Manel
Torné Escasany, Montserrat
Keywords: Geodinàmica
Issue Date: 21-Sep-2016
Publisher: Universitat de Barcelona
Abstract: [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 años. En la actualidad, la mayoría del basamento de África consiste en cratones Archeanos flanqueados por cinturones móviles Proterozoicos y es considerado tectónicamente estable puesto que no ha sufrido deformación tectono-termal desde la orogenia Pan-Africana al final del Precámbrico. A pesar de todo, África es afectada por procesos activos como, volcanismo de punto caliente, rift continental y subsidencia intractratónica. Por lo tanto, África es el escenario ideal para estudiar el rol de la estructura litosférica y cortical en la deformación observada en el continente. El objetivo principal de esta tesis es presentar nuevos mapas de potencia litosférica y cortical del continente Africano basados en la integración de datos de elevación, geoide y térmicos en la modelización. El método asume isostasia local, régimen térmico estacionario e incremento lineal de la densidad en profundidad, así como interrelación entre la densidad del manto litosférico y temperatura. Los resultados obtenidos están constreñidos por una nueva compilación de la profundidad del Moho a partir de datos sísmicos, consistente en 551 puntos, y por modelos tomográficos publicados de la potencia litosférica. El mapa de potencia cortical presenta una distribución bimodal de Norte a Sur con valores mayores en los dominios cratónicos en el sur de África (38 – 44 km) relativos a los presentes en el norte (33 – 39 km). El resultado más 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 región de Afar, donde las asunciones del modelo no se cumplen, el ajuste a la información sísmica disponible es mejor (76.3%; RMSE=4.3 km). El mapa de potencia litosférica presenta grandes variabilidades (90 - 230 km), con una litosfera engrosada ligada a los dominios cratónicos y LAB somera relacionada con el Rift Mesozoico y Cenozoico, concordante con los modelos de tomografía sísmica. A pesar de que los mapas de la potencia litosférica y cortical presentan patrones regionales similares, existen grandes diferencias en el Atlas, el Sistema Oeste del Rift, y las cuencas intractratónicas. El efecto de las variaciones laterales de densidad así como la contribución no isostática a la elevación en la zona de Afar, se estima que es de ~1.8 km.
Appears in Collections:Tesis Doctorals - Departament - Geodinàmica i Geofísica

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