Structure of the uppermost mantle beneath North America : Regional surface wave tomography and thermo-chemical interpretation

Abstract

Seismology is the primary tool to probe the interior of the Earth. The main requirement to obtain a high quality image of the Earth's structure is the achievement of an extensive dataset of seismograms. The North American and Caribbean regions offer a good opportunity in that matter. The large deployment of broad-band seismological stations over the region, provides a dense regional dataset. Phase velocity measurements and waveforms derived from this data are used to image the seismic velocity structure of the uppermantle, before investigating its thermal and compositional structure. The phase velocity maps are first obtained by waveform inversion for periods between 40 and 150 s, using fundamental mode Rayleigh waves. The ray path coverage yields a high resolution over most of Canada, the United States and Central America. Comparison with a recent global scale tomographic model shows that regional phase velocity maps present higher resolution and better prediction of independent data. This last statement is an important issue for future moment tensor calculations. The resolution of small scale heterogeneities may, however, be limited, as the inversion method used relies on the great circle path approximation (ray theory), which states that a wave is sensitive to velocity heterogeneities only along the geometrical ray path that links source and receiver. Neglecting off-great circle path effects may affect regional tomographic results significantly. Therefore, in a second step, the effect of finite frequency of surface waves is incorporated into the inversion of phase velocities measurements (scattering theory). Comparison is made between phase velocity maps obtained with ray and scattering theories. The results show that small scale anomalies (less than $800 km) are not imaged with sufficient resolution. Due to the large uncertainties in the data, a regularization operator is required, leading to the creation of a significant model null space, and a poor constrain of small scale anomalies. The structure of the upper mantle, from 50 to 250 km, is afterwards imaged by ray theory inversion of the phase velocity dispersion curves obtained. The results show high velocities beneath Canada and the eastern United States from the Rocky Mountains to the Atlantic coast, associated with a thick archean lithosphere (up to 220 km). Low velocities are observed beneath the western Cordillera, along the Pacific ocean from Canada to Mexico, induced by the intense tectonic activity of the region. Mexico is defined as a low velocity zone associated with the subduction of the Cocos plate. Positive anomalies are imaged in the Gulf of Mexico. The Caribbean region displays two low velocity zones: one beneath eastern Caribbean, as a result of back arc magmatism, and the other beneath western Caribbean, separated by a positive velocity arm. Finally, the seismological velocity model obtained is interpreted in terms of temperature and compositional variations in the depth range 60 to 260 km. Seismic velocities are primarily sensitive to thermal perturbations. In order to simultaneously infer temperature and composition, independent data are needed. In this thesis, we use density anomalies as additional constrain on the compositional variations in the upper mantle. Estimates of these anomalies are obtained through the scaling factor between density and velocity, which is derived from gravity anomalies data. Our results show that tectonically active regions consist of a mantle close to the Earth's average in terms of temperature and composition. On the contrary, cratonic areas are underlain by a mantle cooler than average and depleted in iron, which supports the idea that negative buoyancy induced by thermal anomalies are balanced by positive buoyancy due to compositional anomalies. This may explain the stability of cratons over geological times