Imaging and interpreting seismic heterogeneity in the North American lithosphere

Abstract

Interpretation of seismic wavespeed anomalies inferred from global and continental-scale tomographic models is complicated by the competing effects of temperature and chemical composition. Understanding the origin of a seismic anomaly requires constraining multiple seismic parameters and quantifying how they are influenced by thermal and geochemical variations. In this Thesis I jointly interpret inverse modeling of seismic data and forward modeling of chemical and thermodynamic data in order to investigate the origins of seismic heterogeneity within the cratonic lithosphere. I present a joint inversion of body and surface wave travel-times to determine independent but mutually-constrained variations in V[subscript p] and V[subscript s] within the continental United States. From this, the V[subscript p] /V[subscript s] ratio may be determined, which is sensitive to compositional changes such as Fe - Mg substitution and Si enrichment.

The seismically-inferred wavespeeds are compared to predictions of V[subscript p] and [subscript p] /V[subscript s] made from forward-modeling of mantle rock compositions at a range of temperature and pressure conditions. By combining both forms of modeling it is possible to identify where thermal or compositional factors dominate the seismic and density structure. The first-order seismic structure within the North American lithosphere may be attributed to variations in temperature, but in certain regions compositional anomalies must be invoked. Subsequently, this framework is applied to questions related to the cratonic lithosphere. Compared to younger orogenic belts, Archean cratons have a relatively Fedepleted composition and low temperatures, the latter of which is sustained by a thick lithosphere. Finally, I investigate two anomalies within the North American craton that have been affected by mantle plumes.

Plumes influence the lithosphere in several ways, including thermal perturbations as well as the emplacement of compositionally distinct plume material into the lithosphere. The structure of the lithosphere at the time of plume passage influences how these interactions are manifested. The ways that the continental lithosphere can be altered therefore depend on its initial properties as well as on its history, and both must be considered to make a full geologic interpretation.