Deep Crustal Anisotropy and its Distortion Through the Seismological Lens

Seismic interpretations of crustal anisotropy often appear to be at odds with expectations based on structural geology. We provide a solution to the apparent discrepancy based on petrological data and synthetic seismograms and present results across the continental US. Seismic investigations of crustal anisotropy offer one of the best chances to observe lower crustal flow in situ, and receiver function (converted wave) studies have good horizontal and depth resolution and are less expensive than active source studies, and suffer from less tradeoff than tomographic studies. A puzzling observation in receiver function studies of the continental crust has been a prevalence of observed plunging axis anisotropy in subhorizontal layers interpreted to have accommodated a significant component of simple shear. In contrast, geological field observations and deformation experiments suggest that shear zones develop a significant boundary-parallel foliation (C-planes in S-C mylonite) after only modest amounts of strain accumulation (~gamma <3). The solution may lie in a preferential sensitivity of seismic waves. Receiver functions have ~5 times higher signal amplitudes for plunging compared to horizontal symmetry axis anisotropy. Even a small plunge (10-20 deg off horizontal) leads to a roughly twofold increase of signal amplitude compared to the horizontal symmetry axis case. For an S-C fabric with horizontal C and dipping S planes, the S fabric is well detected even for relatively small and realistic (3% - 6%) amounts of anisotropy. Horizontally aligned mica does not appear anisotropic to body waves, although it can be detected via radial anisotropy with surface waves. Amphibole alignment with a fast symmetry axis parallel to horizontal shear also produces a much weaker signal than in the plunging case. The behavior with backazimuth is also distinct. General S anisotropy and horizontal axis P anisotropy generate two pairs of relatively small amplitude peaks and troughs over the entire backazimuth range (hereinafter A2), while plunging P anisotropy shows a much higher amplitude single peak and trough (termed A1). Published crustal sample P versus S anisotropies range within a factor of 2 of each other, with the majority of samples showing comparable P and S anisotropy. While the A2 signal theoretically provides a robust detector for anisotropy, we suggest that a search for the larger A1 signal is more likely to be successful. We present seismic forward modeling results for petrological crustal deformation fabrics with aligned mica, amphibole, and quartz for different geometries. We also show results from the EarthScope Transportable Array across areas with presumed past or present lower crustal flow. When observed receiver function signal amplitudes are decomposed into A0 (isotropic, 1-D), A1, and A2 components, the A1 component dominates A2 by a factor of ~3 averaged across the entire network. The A1 component also contains information on isotropic dipping interfaces. However, isotropic dip and shallow structure such as sediment reverberations can theoretically be distinguished from deep anisotropic signals by matching the phase of early and late A1 arrivals. We present strategies to isolate the deep crustal fabric signal across the Transportable Array.