Thermal, density, seismological, and rheological structure of the lithospheric-sublithospheric mantle from combined petrological-geophysical modelling: Insights on lithospheric stability and the initiation of subduction

This thesis presents a first-of-its-kind combined geophysical-petrological methodology to study the thermal, compositional, density, rheological, and seismological structure of different lithospheric domains. The methodology is incorporated in a finite-element code (LitMod) that solves simultaneously the heat transfer, thermodynamical, geopotential, isostasy, and rheological equations for a particular lithospheric structure with any given composition.; LitMod has been applied to a number of synthetic and real transects in both oceanic and continental domains. It is found that the highly depleted composition typically assumed for Archean lithosphere cannot be representative of the whole lithospheric thickness. A model in which the cratonic keel is composed of at least two boundary layers (i.e. a thick thermal boundary layer including a chemical boundary layer in its upper part) is a necessary condition for reconciling petrological and geophysical data. The observed S-wave anomalies at depths of 350--400 km beneath cratons can be explained by allowing a slightly depleted composition down to the bottom of the thermal boundary layer, which is also consistent with other geophysical observables.; In continental compressional settings, the inclusion of compositional and compressibility effects suggest that mechanisms other than pure lithospheric thickening (e.g. eclogitization of injected melts and/or metasomatism) are necessary to destabilize the root of a thickened lithosphere.; In the oceanic domain, a plate model with an asymptotic thermal thickness (depth to the 1300°C isotherm) of 110 +/- 5 km is consistent with all the available geophysical and petrological data. Although the compositional (density) structure of mature oceanic lithosphere makes it gravitationally unstable with respect to the sublithospheric mantle after ≳ 30--80 Ma, the density contrast Deltarho never exceeds values of ∼40 kg m-3. Thus, the role of Deltarho in triggering/assisting subduction initiation is less critical than previously thought.; A new model for spontaneous initiation of subduction triggered by a Rayleigh-Taylor instability is presented. This model is consistent with density and lithospheric strength estimations in oceanic plates. It is shown that the finite growth of the instability provides the necessary forces to produce whole lithospheric failure, bend the elastic part of the plate, and initiate subduction.