Surface Tectonics-Matched SKS Splitting: Vertically Coherent Mantle Deformation or Crustal Anisotropy?

SKS splitting measurements are generally assumed to reflect mineral alignment in the upper mantle, which in turn is caused by mantle deformation or flow. A crustal contribution to SKS splitting is usually considered negligible. More recently, ultrasonic laboratory measurements have suggested that crustal materials can possess anisotropy strong enough for a layer of order 10 km thickness to produce splitting comparable to a mantle signature. A counterargument, against the relevance of crustal SKS splitting, is that the crust is unlikely to have homogeneous anisotropy over 10 km thick layers. We perform numerical modelling of splitting in realistic crustal models with anisotropy that varies on a < 1 km scale in depth. The crustal models are built by taking elastic tensors from ultrasonics and varying their orientations according to geological mapping of foliation and lineation changes on a macroscopic scale, with examples from the Californian Sierra Nevada and other regions. We find significant splitting in these models. SKS splitting is often found to be related to surface tectonics; usually, this observation is interpreted as deformation of the upper mantle coherent with the crust. Our modelling offers an alternative explanation, namely that a significant part of the splitting signal may stem from the crust itself.