CRUSTAL SEISMIC ANISOTROPY: IMPLICATIONS FOR UNDERSTANDING CRUSTAL DYNAMICS

The Nanga Parbat - Haramosh massif, in the core of the western syntaxis of the Himalaya, represents an excellent exposure of mid-lower continental crust from beneath a collisional orogen. The exhumed core of the massif is a large scale antiform striking N10oE with lineations oriented north-south with near-vertical dips. Laboratory measurements of seismic velocity on a suite of quartzofeldspathic gneisses from the massif show a strong degree of anisotropy, up to 12.5% for compressional waves and up to 21% for shear waves. The degree of velocity anisotropy is a function of mica content and rock fabric strength. The strong anisotropy measured in these rocks is observable in seismic field data recorded by an IRIS/PASSCAL deployment across the massif and provides a means of mapping rock fabric at depth. While splitting delay normally increases with distance traveled through anisotropic material, the range of delay times can be due to heterogeneity in composition, lateral variation in % anisotropy, changes in orientation of the regional foliation within the massif, and velocity variance due to non-axial propagation through a wide range of event-station azimuths. Because the composition of the Nanga Parbat massif is basically homogeneous, structure is well constrained, raypaths are restricted to the crust, and source-receiver geometries sample a range of azimuths with respect to structure, this data set is ideal for studying and quantifying the affect of non-axial propagation through regional foliation. This type of analysis has important implications for understanding crustal dynamics. Vp, Vs, and Vp/Vs ratios are typically used to infer both lithology and rheology of subsurface materials providing constraints for thermo-mechanical models of deformation. Current tomography codes do not generally account for anisotropic effects and may potentially under or over estimate velocity structure in the crust. At Nanga Parbat, a prominent low–velocity zone is mapped beneath the core of the massif. The magnitude and extent of this zone constrains crustal flow paths focusing crustal strain, exhumation, and potential zones of partial melting in the crust. Accurate determination of velocity structure is necessary to understand crustal structure and modification during orogenesis.