Shear zones are common strain localization structures in the middle and lower crust and play a major role during orogeny, transcurrent movements and rifting alike. Understanding seismic signatures of shear zones is critical to recognize and quantify strain in continental rifts and margins. The understanding of crustal deformation depends on the ability to recognize and map shear zones in the subsurface, yet the exact signatures of shear zones in seismic reflection data are not well constrained. To advance this knowledge, the authors of the paper Seismic expression of shear zones: Insights from 2-D point-spread-function-based convolution modelling simulated how three outcrop examples of shear zones would look in different types of seismic reflection data using a 2-D point-spread functions (PSF) based convolution modelling, with PSF being the elementary response of diffraction points in seismic imaging. The study, published on the Journal of Structural Geology, explored how geological properties (e.g. shear zone size and dip) and imaging effects (e.g. frequency, resolution, illumination) control the seismic signatures of shear zones.
Three shear zone networks were mapped in great detail (meter scale) over vast areas (several square kilometers): Holsnøy - Norway, Cap de Creus – Spain and Borborema – Brazil.
The models highlight that the seismic signature of shear zones depends on the complex interaction of a variety of geological and geophysical factors. This study focuses on the effects of two geological elements, aspect ratio and dip of the shear zone, as well as two geophysical aspects, seismic frequency and illumination. Nevertheless, the authors were able to consistently identify three characteristic features of shear zones on the resulting models: multiple, inclined reflections, converging reflections and cross-cutting reflections.
Geometric curvature of shear zones is a useful kinematic indicator, as seismic reflections forming an acute angle with the edge of the shear zone indicate movement towards the junction of reflections. These results will help identifying and mapping shear zones in 2-D and 3-D seismic reflection data, and helps understand the way strain is distributed in the ductile middle and lower crust in different tectonic settings.
To summarize, the authors suggest looking for combinations of subparallel, inclined reflection packages, cross-cutting reflections, and unidirectional reflection junctions to identify shear zones in seismic reflection data.
Read more about it in the paper: Wrona, T., Fossen, H., Lecomte, I., Eide, C.H., Gawthorpe, R.L., Seismic expression of shear zones: Insights from 2-D point-spread-function-based convolution modelling, Journal of Structural Geology (2020), doi: https://doi.org/10.1016/j.jsg.2020.104121
