Partitioned Waveform Inversion Applied to Eurasia and Northern Africa

This report summarizes the data analysis achieved during Heather Bedle's eleven-week Technical Scholar internship at Lawrence Livermore National Labs during the early summer 2006. The work completed during this internship resulted in constraints on the crustal and upper mantle S-velocity structure in Northern Africa, the Mediterranean, the Middle East, and Europe, through the fitting of regional waveform data. This data extends current raypath coverage and will be included in a joint inversion along with data from surface wave group velocity measurements, S and P teleseismic arrival time data, and receiver function data to create an improved velocity model of the upper mantle in this region. The tectonic structure of the North African/Mediterranean/Europe/Middle Eastern study region is extremely heterogeneous. This region consists of, among others, stable cratons and platforms such as the West Africa Craton, and Baltica in Northern Europe; oceanic subduction zones throughout the Mediterranean Sea where the African and Eurasian plate collide; regions of continental collision as the Arabian Plate moves northward into the Turkish Plate; and rifting in the Red Sea, separating the Arabian and Nubian shields. With such diverse tectonic structures, many of the waveforms were difficult to fit. This is not unexpected as the waveforms are fit using an averaged structure. In many cases the raypaths encounter several tectonic features, complicating the waveform, and making it hard for the software to converge on a 1D average structure. Overall, the quality of the waveform data was average, with roughly 30% of the waveforms being discarded due to excessive noise that interfered with the frequency ranges of interest. An inversion for the 3D S-velocity structure of this region was also performed following the methodology of Partitioned Waveform Inversion (Nolet, 1990; Van der Lee and Nolet, 1997). The addition of the newly fit waveforms drastically extends the range of the model. The model now extends as far east in Africa to cover Chad and Niger, and reaches south to cover Zambia. The model is also stretched eastward to cover the eastern half of India, and northward to cover the southern portion of Scandinavia

Lateral variations in thermochemical structure of the Eastern Canadian Shield

The origin of 3D seismic heterogeneity in Precambrian lithosphere has been enigmatic, because temperature variations in old stable shields are expected to be small and seismic sensitivity to major‐element compositional variations is limited. Previous studies indicate that metasomatic alteration may significantly affect average 1‐D structure below shields. Here, we perform a grid search for 3‐D thermo‐chemical structure, including variations in alteration, to model published Rayleigh‐wave phase velocities between 20 and 160 s for the eastern part of the Archean Superior and Canadian Proterozoic Grenville Provinces. We find that, consistent with constraints from surface heatflow and xenoliths, the lithosphere is coolest (Moho heatflow 12‐17 mW/m2) and the thermal boundary layer thickest (>250 km) in the northeastern Superior and warmest in the southeastern Grenville (Moho heatflow 20‐25 mW/m2, thermal boundary thickness 160‐200 km). Compositionally, the phase velocities for most of the Superior within our study region require little alteration, but in a few regions, fast velocities need to overlie slower velocities. These can be modelled with an eclogite layer in the mid lithosphere, consistent with active‐seismic and xenolith evidence for remnants of subducted Archean crust. The phase velocities from the Grenville Province require significant metasomatic modification to explain the relatively low velocities of the shallow lithosphere, and the required intensity of alteration is highest in parts of the Grenville associated with arc accretion. Thus, composition of the northeastern Canadian Shield appears to reflect different stages and styles of craton assembly