Nanga Parbat crustal anisotropy: Implications for interpretation of crustal velocity structure and shear‐wave splitting

Abstract

The Nanga Parbat‐Haramosh massif represents a unique exposure of mid‐lower continental crust from beneath the Himalayan orogen. Seismic velocity measurements on a suite of quartzofeldspathic gneisses show up to 12.5% velocity anisotropy for compressional waves and up to 21% for shear waves. The degree of anisotropy is a function of mica content and rock fabric strength. Over 30% of the samples have maximum compressional wave velocities of 6.4–6.5 km/s; velocities typically associated with more maficlithologies. These results have implications for the interpretation of crustal velocity structure obtained from wide‐angle seismic surveys where in situ velocity measurements are made from refracted or turning rays that potentially spend a substantial portion of their travel path propagating in the foliation plane. Velocities determined from these surveys may overestimate mean velocities of crustal rocks with well‐developed horizontal fabric. In addition, crustal anisotropy due to the development of pervasive rock fabric has the potential to be a significant contributing factor to shear‐wave splitting observations.