Regional Full-Waveform Tomography using Spectral Element Method of North America: Application of Box Tomography
IPGP - Îlot Cuvier
Soutenances de thèses
The EarthScope TA deployment provides dense array coverage throughout the continental United States and with it, the opportunity for high resolution 3D seismic velocity imaging of the stable part of the north American upper mantle. Building upon our previous long-period waveform tomographic mod- elling, we present a higher resolution 3D isotropic and radially anisotropic shear wave velocity model of the North American lithosphere and astheno- sphere. The model is constructed using a combination of teleseismic and regional waveforms down to 40 s period and wavefield computations are per- formed using the spectral element method both for regional and teleseismic data. Our study is the first tomographic application of "Box Tomography", which allows us to include teleseismic events in our inversion, while comput- ing the teleseismic wavefield only once, thus significantly reducing the com- putational cost when computing several iterations of the regional inversion. We confirm the presence of high velocity roots beneath the Archean part of the continent, reaching 200-250 km in some areas, however the thickness of these roots is not everywhere correlated to the crustal age of the correspond- ing cratonic province. In particular, the lithosphere is thick (?250 km) in the western part of the Superior craton, while it is much thinner (?150 km) in its eastern part. This may be related to a thermo-mechanical erosion of the cratonic root due to the passage of the north American plate over the Great Meteor hotspot during the opening of the Atlantic ocean 200-110 Ma ago. Below the lithosphere, an upper mantle low velocity zone is present everywhere under the north American continent, even under the thickest parts of the craton, although it is less developed there. The depth of the minimum in shear velocity has strong lateral variations, whereas the bot- tom of the low velocity zone is everywhere relatively flat around 270-300 km depth, with minor undulations of max 30 km that show upwarping under the thickest lithosphere and downwarping under tectonic regions, likely re- flecting residual temperature anomalies. The radial anisotropy structure is less well resolved, but shows negative signatures in highly deformed regions.