Dual petrological and strain-path controls on the seismic anisotropy of deformed continental crust

Recorded seismic anisotropy (SA) due to lattice preferred orientation (LPO) of elastically anisotropic minerals (especially micas and amphiboles) may be interpreted in terms of strain trajectories in deformed continental crust. Such usage assumes that LPO and SA indicate foliation and lineation. This assumption is tested using plausible rock compositions, microstructures and strain ellipsoids typical of different 3D strain paths to predict the in situ seismic response of deformed crust. For typical polymineralic lithologies the bulk SA depends upon individual mineral modal content and LPO modulated by the single crystal elastic properties. Here we assume single foliations that parallel components of the 3D strain trajectories. Content can be varied progressively and proportionately for constant LPO to investigate how changing composition impacts on SA. Such 'rock recipes' indicate large SA values for micaceous rocks (e.g. schists) and smaller SA values for mafic middle-to-lower crust compared to felsic middle crust. Although deformation impacts also on LPO, the relative strengths of planar and linear features may not be recognised, with concomitant implications for SA, as shown via modified Flinn Plots. Both micas and amphiboles appear sensitive to finite strain, rather than infinitesimal strain, strain magnitude and strain path. For micaceous rocks LPO and the resultant SA chart the orientation of the XY plane (foliation) but do not record the complete intensity or shape of finite strain ellipsoids. In contrast, amphibole-rich rocks generate SAs that give a complete (XYZ) description of 3D strain trajectories. The strain attributes are key for establishing the structural geometry of deformed terrains and underpin traditional kinematic studies at outcrop. The challenge now is to take these methods into the subsurface using SA as a proxy for different deformation states.