Seismic anisotropy beneath southwestern Norway

Project description
Motivation:
The southwestern Baltic Shield was assembled through a complex superposition of processes that span more than a billion years. These processes include episodes of terrane amalgamation, subduction, rifting, and mountain building. Insights into the protracted history of the region have been gleaned mainly from surface geology, but there are some key constraints that are only available at depth in the crust and lithospheric mantle. One such constraint is the fabric of the rocks that make up these domains, which can tell us about the scale of the various processes at play in the evolution of the continent. In southwestern Norway, rock fabrics can help us better understand the large shear zones that accommodated compressional deformation during the Caledonian orogeny and extensional deformation during subsequent orogenic collapse. Superimposed on these shear zones is a network of brittle faults that stem from the subsequent rifting of the North Sea. Though these structures are well imaged at the surface, only limited constraints exist at depth other than those from a set of offshore seismic reflection lines. The objective of this study is to probe subsurface rock fabrics beneath SW Norway through seismic methods to constrain the vertical extent of these structures. This will provide important new insight on the overall scale of the major tectonic episodes that have affected the region over the past 500 Myr. It will also give us insight into potential interconnections between the various consecutive episodes – i.e., how older episodes may have influenced more recent ones.

Hypothesis:
Several studies have investigated seismic anisotropy beneath the Baltic Shield to infer lateral/vertical variations in rock fabric across the region. Though the seismic station coverage has been rather sparse until now in SW Norway, a pattern has emerged whereby NW-SE oriented fast axes dominate where Proterozoic basement terranes are exposed, whereas NE-SW oriented fast axes dominate beneath the Caledonian nappes. These observations have been compared to the general trends of maximum stresses operating during Caledonian compression and post-orogenic extension. However, their potential links to large shear zones associated with the Caledonian orogenic cycle and to networks of brittle faults associated with the opening of the North Sea has not been established. The project will test the hypothesis that seismic anisotropy measured at a dense network of seismometers recently deployed across SW Norway may provide improved constraints on how these various tectonic processes have impacted the structure of the lithosphere at depth.

Proposed course plan during the master's degree (60 ECTS)
GEOV261 (10sp)
GEOV276 (10sp)
GEOV298 (10sp)
GEOV302 (10sp)
GEOV375 (10sp)
GEOV272 (10sp)

Field- lab- and analysis work
The work with start with an in-depth review of the literature concerning measurements of seismic anisotropy via shearwave splitting analysis and the implications of these measurement in terms of crust-mantle processes. The following step will involve training on software packages for seismic anisotropy analysis – e.g., SplitPy in Python, SplitLab in MATLAB. Seismic data from temporary and permanent stations in SW Norway will be then downloaded from the EIDA seismic archive of the University of Bergen. These data will first be analysed using fully automated tools, to get a general idea of anisotropic trends in the region. Manual analysis will then be done to refine the results such that they can be interpreted in the context of regional structure. If large azimuthal variations in anisotropic parameters are detected at some stations, further analysis will be carried out to constrain possible two-layer anisotropic models. Interpretation will be achieved by comparing the orientation of anisotropy with shear zones and networks of faults in the region, and by deriving implications based on documented models of tectonic evolution for the region.