FACTORS FOR THE USE OF SEISMIC ANISOTROPY TO EXAMINE CRUSTAL DEFORMATION AND METAMORPHISM WITHIN CRATONS

Recent seismological studies within cratonal provinces have used seismic anisotropy to study sub-Moho lithospheric structure and aesthenospheric mantle flow fields. Teleseismic split shear waves and long-wavelength surface waves identify relict lithospheric fabrics, current directions of plate motion or underlying flow, and coarse-scale seismic velocity structure and associated bulk anisotropy. Application of seismic anisotropy methods to cratonal crust offers opportunities to examine regions that were subject to tectonic and metamorphic processes. Besides cracks in the shallow crust, seismic waves can respond to fabrics associated with low-to-high grade metamorphic rocks; observed anisotropy signals may be diagnostic of subsurface structure and internal texture.

Cratonal regions have characteristics that complicate crustal seismic anisotropy studies. First, they are far from rich zones of local-to-regional seismicity as is found at plate boundaries. Thus, seismic waves for cratonal crustal anisotropy studies include (a) near-vertical teleseismic P-to-S converted at Moho, (b) long period surface waves and shorter period ambient noise for horizontal anisotropy, (c) controlled sources, and (d) any induced seismicity and quarry blasts. Predominant wave directions and wavelengths differ markedly. Second, cratonal provinces contain complex patterns of structure and composition due to long histories of tectonic accretion and deformation. Spatially dense seismic arrays are needed to resolve crustal patterns - often not included in studies that target Moho and LAB depths. Third, petrophysical lab and EBSD studies show that high grade gneisses and amphibolites (amphibole-rich) have lower %anisotropy compared to low grade metasediments and schists (phyllosilicate-rich). In general, both grades exhibit velocity slow axis symmetry. Fourth, large scale structure can map local rock anisotropy into bulk fast-axis symmetry at the seismic scale. Fifth, elastic tensor analysis of seismic-scale effective media is a way to combine microscale fabrics and 3D structure to examine seismic wave response. Averaging methods such as asymptotic expansion homogenization (AEH) are needed. We examine these factors for the applicability of seismic anisotropy to study craton crustal provinces.