Gravitational instabilities in the lithosphere : numerical studies
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[EMBARGOED UNTIL 08/01/2026] Gravitational instability is one of the fundamental mechanisms controlling the evolution of planets. It drives convection in the Earth's mantle and core, as well as in the atmosphere. However, being cold and stiff, the Earth's lithosphere is supposed to resist the growth of internal gravitational instability. In this dissertation, I integrate numerical experiments with geophysical observations and geological data to explore three Earth's lithospheric deformation processes where gravitational instability plays a major role: salt diapirism in sedimentary basins, Archean dome-and-keel structures, and small-scale removal of mantle lithosphere beneath the central Andes. In the study of salt diapirism, high-resolution thermomechanical models reveal that an interplay of salt buoyancy, differential loading, and tectonic stresses drives salt diapirs. Findings challenge the conventional dichotomy of active versus passive diapirism and underscore the importance of plastic deformation in allowing the piercement of salt rocks into stiff overburdens. For Archean dome-and-keel structures, numerical experiments indicate that their formation through partial convective overturn is more difficult than previously assumed. The favorable conditions for forming dome-and-keel structures are synchronous voluminous mafic-ultramafic magma eruption and TTG (Tonalite--Trondhjemite--Granodiorite) magma intrusion, possibly related to mantle plume activity. Therefore, dome-and-keel structures are products of localized thermal anomalies rather than evidence of a globally dominant vertical tectonic regime suggested in previous studies. To explore the formation of discrete low-velocity zones beneath the central Andes, numerical experiments highlight that composition-induced convective dripping can cause the small-scale removal of the mantle lithosphere, triggered by rheological weakening and lithospheric heterogeneity. The small-scale removal of the mantle lithosphere may account for the seismic low-velocity zones, magmatism, and topographic evolution in the central Andes. This dissertation highlights that the lithosphere can be locally weakened to permit the growth of gravitational instability. Gravitational instability plays a major role in lithosphere tectonics across diverse spatial scales and geological timeframes. By integrating numerical methods with geophysical and geological studies, this dissertation enhances our understanding of lithosphere evolution from the perspective of gravitational instability.
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Ph. D