Absent anisotropy: The paradox of the Southern Alps orogen

Abstract

A high resolution active source dataset recorded across the Southern Alps of New Zealand displays unusually strong S-wave phases, which show minimal shear wave splitting. A suite of methods was used to analyse the data to minimize the uncertainty in the anisotropy measurements. We use the Silver and Chan [1991] method, cross correlation of stacked data, and deconvolution in combination with the other methods. The results were consistent at each station, but differed between stations. Fault-parallel fast directions with delay times of 0-0.08 s were calculated from the most impulsive S-wave arrivals. The crustal rocks from the Southern Alps are strongly anisotropic in the laboratory, contrasting with results shown here. The minimal delay times suggest that the shear waves are affected by multiple phases of deformation in the crust. © 2003 by the American Geophysical Union.