Deux aspects de l'influence des continents sur le manteau terrestre : initiation de la subduction : effet thermique des racines continentales

In this thesis which consists of two parts, we study some aspects of the influence of continents on the terrestrial mantle.In the first part, we present a model of subduction initiation. How subduction zones get generated remains largely an open question. We investigate whether the sinking of the lithosphere could be initiated at passive margins. The continental crust is less dense and thicker than its oceanic counterpart, and is therefore elevated with respect to the oceanic floor. Lateral stresses associated with this topography difference are large enough to induce an extension of the continental crust, which spreads over the adjacent oceanic lithosphere. The continental load deflects the oceanic plate. The latter may decouple and under certain conditions it may sink into the asthenosphere. We study this process with laboratory experiments and theoretical calculations. The oceanic lithosphere is modelled by an elastic plate, and the continent and asthenosphere are considered as viscous fluids. We find that three dimensionless numbers control subduction initiation. One such number involves the density contrast between the oceanic lithosphere and the underlying mantle. The system behaviour is more sensitive, however, to two other dimensionless numbers. One is the ratio between the horizontal distance over which continental crust extends and the flexural parameter for the oceanic plate. Initiating subduction is easier with low values of this number, i.e. if the continent thins over a short distance or if the oceanic plate is strong. The other important number is the ratio of the elastic thickness over the initial crustal thickness. A strong oceanic plate and a thin continental crust favour subduction. The transition from a passive to an active margin therefore depends on the properties of both the oceanic lithosphere and the adjacent continental crust. Subduction is not determined solely by the age of the oceanic plate. Our model also has implications for some characteristics of passive margins. In the second part, we investigate the thermal structure of continental roots and we study the constraints that the heat flux at their base brings on mantle convection. We focus our study on the Canadian Shield. We present new surface heat flux determinations in the Northern part of the craton, where one expects the lithosphere to be thickest. They define a large zone of very low heat flux (∼29mW.m−2). The surface heat flux records two contributions : the heat supplied by the mantle, and the one produced by radioelements in the crust. Combining the heat flux data set with travel-time delays from a tomographic model allows us to distinguish between these two contributions. We obtain that the heat flux at the base of the Canadian Shield lithosphere is not uniform. Its variations are of ±3mW.m−2 around a mean value of 14mW.m−2. They are associated with changes of lithospheric thickness as large as a hundred kilometres. Our study brings constraints on the mantle temperature and rheology. Existing methods to evaluate the mantle potential temperature only apply to oceanic domains. We show that combining heat flux measurements and a tomographic model, we can estimate the mantle temperature beneath the Canadian Shield. The range of values we obtain (1290-1450◦C) is consistent with the one derived from independent methods for oceanic mantle. We assume that heat at the base of the lithosphere is supplied by small-scale convection. We show that the observed basal heat fluxes can only be explained if the whole thickness of lithospheric mantle is depleted, i.e. if the chemical and mechanical lithospheres coincide. Moreover, they are only compatible with a sublithospheric mantle deforming by wet-dislocation creep. This mechanism is consistent with what suggest studies based on olivine deformation.

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