X-RAY MICROTOMOGRAPHY AS A FAST NEW TECHNIQUE FOR MEASURING 3D STRAINS: AN APPLICATION TO THE DEFORMATIONAL HISTORY OF A LARGE EXPOSED SUBDUCTION COMPLEX IN THE OLYMPIC MOUNTAINS OF WASHINGTON STATE

As path-integrated records of a velocity field, finite strain measurements provide direct information on the ductile deformation and associated kinematics of orogenic wedges. Despite this link to orogenic behavior, difficulties associated with the actual strain measurements have limited the use of strain in interpreting ductile deformation and orogenic kinematics. Here, we develop a new technique to rapidly measure 3D strains directly from a sample's internal volume information, eliminating much of the labor and interpolation associated with traditional 3D strain measurements. Our technique uses an x-ray synchrotron source to image a sample's 3D internal x-ray absorption coefficient at micron resolution. The x-ray absorption coefficent is directly related to atomic number and density, allowing, in this application, quartz grains to be distinguished from overgrowths and selvages. We employ autocorrelation to rapidly and automatically measure the 3D strain from the x-ray absorption information. Here we present x-ray microtomography strain measurements (and provide comparisons with traditional thin-section measurements) for 52 pressure-solved sandstones from the Olympic subduction complex in Washington State. These strain measurements suggest overall vertical extension and sub-horizontal shortening in the direction of convergence for the Olympic subduction complex. This rapid new strain measurement technique will increase both the quality and quantity of strain measurements, leading to improved interpretations of ductile deformation and widespread field-based constraints on orogenic kinematics.