Dependence of the Stress Field on Plate-Mantle Coupling and Lithospheric Structure.

Abstract

Deformation in the Earth’s mobile tectonic plates (lithosphere) is governed by the forces acting on and within the lithosphere and the lithosphere’s response to the resulting state of stress (rheology). Determining the origins of variations in the lithospheric stress field is a fundamental aspect of understanding processes at multiple scales, including global tectonic patterns, intra-plate deformation, regional faulting behavior and grain-scale seismic anisotropy. The forces acting on and within the lithosphere that give rise to variations in stress can be broadly broken down into plate boundary interactions, mantle flow acting on the base of the lithosphere and changes in lithostatic pressure resulting from gradients in density and surface topography. To date, numerous studies have explored the role these different forces play in generating observed lithospheric stress patterns, although calculations of global stress patterns in a mechanically homogenous lithosphere still fail to match observed stress patterns in many regions. The discrepancy between calculated and observed stress patterns reflects either uncertainty in the forces applied to the lithosphere or the assumption of a mechanically homogenous lithosphere. While maintaining the assumption of a mechanically homogenous elastic lithosphere, this thesis re-examines the forces acting on and within the lithosphere in the light of recent advances in our knowledge of lithospheric structure, mantle rheology and mantle flow modeling. The modeling results from Chapters I and II reveal that lithospheric stress patterns are highly sensitive to the assumed isostatic state, density structure and thickness of the lithosphere, which vary significantly across tectonic provinces that exhibit large changes in lithospheric rheology. In contrast, taking into account enhanced mantle flow-induced basal shear beneath thick continental roots does not generate an equivalent increase in local stress magnitudes within the overlying lithosphere, which reflects the integration of basal shear over plate-scale wavelengths and effective transmission of stresses within the homogenous elastic lithosphere. The dominant theme that emerges from these results is the critical importance of moving from mechanically homogenous models to models with both vertical and lateral variations in lithospheric rheology. This transition is necessary to calibrate the contribution of different sources of stress to the total lithospheric stress field.