Seismic expression of shear zones: Insights from 2-D point-spread-function based convolution modelling

T Wrona,H Fossen,I Lecomte,C.H Eide,R.L Gawthorpe

doi:10.1016/j.jsg.2020.104121

Abstract

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. Our understanding of crustal deformation depends on our 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 our understanding, we simulate how three outcrop examples of shear zones (Holsnøy - Norway, Cap de Creus - Spain, Borborema - Brazil) would look in different types of seismic reflection data using 2-D point-spread-function (PSF)-based convolution modelling, where PSF is the elementary response of diffraction points in seismic imaging. We explore 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. Our models show three consistent seismic characteristics of shear zones: (1) multiple, inclined reflections, (2) converging reflections, and (3) cross-cutting reflections that can help interpreters recognize these structures with confidence.

Highlights

Shear zones play an important role in accommodating tectonic deformation in the crust and mantle (e.g. Vauchez et al, 2012; Snyder and Kjarsgaard, 2013)
Our models show the seismic signatures of the Holsnøy, Cap de Creus, and Borborema shear zone networks for different frequencies (5–40 Hz) and maximum illumination angles (30◦, 45◦, 90◦) (Figs. 3, 4, 5)
Our models show that kilometre-scale shear zones mapped in the field can produce inclined seismic reflections similar to those observed in seismic reflection data from rifts and continental margins (Fig. 10a) (e.g. Fossen and Hurich, 2005; Fazlikhani et al, 2017)

Summary

Introduction

Shear zones (i.e. tabular volumes of rock with higher strain than the surrounding rocks) play an important role in accommodating tectonic deformation in the crust and mantle (e.g. Vauchez et al, 2012; Snyder and Kjarsgaard, 2013). As shear zones generally form in the ductile mid-to-lower crust, they are less often exposed at the surface than brittle faults. Geophysical techniques, such as 2-D and 3-D seismic reflection data can help us image and study shear zones in the subsurface Since strain variations often correlate with physical property changes (e.g. density and seismic velocity), we expect shear zones to appear on seismic reflection data, and in rare cases surface outcrops of exhumed shear zones can be directly correlated with reflection patterns in seismic im ages Understanding seismic signatures of shear zones is critical to recognize and quantify strain in continental rifts and margins

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Discussion

Conclusion