Seismic anisotropy has been observed to correlate with aligned cracks in the shallow crust and inferred to be related to lattice-preferred orientation (LPO) of olivine due to mantle flow. However, anisotropic behavior due to structural layering or inherent foliation of crustal material is not as well studied even though metamorphism and deformation are fundamental expressions of plate convergence and have the potential to pervasively structure the crust and influence wave propagation at regional scales. Regional foliations and compositional alignments are produced by deformation and metamorphism associated with tectonic processes and should result in "anisotropic terranes" detectable by seismic methods. Thus the identification and mapping of anisotropy within the crust can serve as a proxy for the presence and spatial extent of tectonic deformation.
We examine a three-dimensional exhumed regionally-foliated antiformal structure located at Nanga Parbat, Pakistan, to test our hypothesis that regional deformation and metamorphism result in crustal anisotropy and that this anisotropy can be observed in the recorded seismic wavefield and used as a proxy to map subsurface rock fabric. For our study we use an existing data set collected by the NSF-Continental Dynamics Nanga Parbat project (NSF EAR-9418849). We have identified 365 high-quality local earthquakes which were recorded within a temporary 56 short-period/broadband array. The combination of local earthquakes/stations provides internal illumination of the Nanga Parbat antiform. Using this data set, our research objectives are to: (1) quantify the observations of seismic anisotropy in the seismic data, (2) develop techniques to systematically analyze the anisotropic signatures recorded in the seismic wavefields (interpretive tomography), (3) calibrate these observations using petrophysical properties directly measured from Nanga Parbat rock samples, (4) use Christoffel anisotropy theory and synthetic seismograms computed by 3D anisotropy wave propagation code for verification of our analyses, (5) develop a set of tools that can be used to help design field experiments to measure the anisotropic wave field through complex 3-D structures, and (6) determine Vp, Vs, and Vp/Vs ratios to better constrain geodynamic modeling of crustal deformation. These objectives allow us to understand the relationship between seismic anisotropy and crustal tectonic deformation.
