SEISMIC ANISOTROPY AND FINITE STRAIN IN AMPHIBOLITE FROM THE DEEP UNDERGROUND SCIENCE AND ENGINEERING LAB, LEAD, SOUTH DAKOTA, USA

Recent modeling of shear and pressure wave velocities indicates that the lower crust may be composed of amphibolite in certain areas. A primary constituent in amphibolite, hornblende, may have pronounced seismic anisotropy on the elongate c-axis. Thus, deformation fabrics defined by the alignment of hornblende crystals can affect the velocity and interpretations about the lower crust. Additionally, fabrics can provide information about the type of finite strain experienced by the rock. Using amphibolite samples collected from the 4850 level of the Deep Underground Science and Engineering Lab that underwent metamorphism and deformation at ~1750 Ma seismic velocity models were determined based on their fabric and composition. The samples that were selected exhibited the clearest fabric with the least amount of retrograde mineral alteration. The primary phases, not including secondary alteration, such as chlorite or quartz-calcite veins, where hornblende, plagioclase (An33), and quartz. The samples were then analyzed using the SEM-EBSD and hkl software to determine crystal preferred orientations (CPOs) for hornblende, plagioclase, and quartz. Seismic velocities for the rocks were calculated with MTEX code for the program MatLab using the Voigt, Ruess, Hill calculations. The differences in the CPO for hornblende in both samples may be explained by change in the type of strain. The CPO in sample D-F, the strong maximum associated with the pole to (100) and girdle and weak maximum with the c-axis is compatible with the flattening strain. The strong alignment of c-axis in D-A and girdle formed by the poles to the (100) plain is consistent with constrictional strain. Preliminary modeling indicates maximum velocity of pressure waves in sample D-A is 7.0 km/s and the minimum in 6.34 km/s with a difference of 9.4%. The maximum velocity of pressure waves in sample D-F is 6.8 km/s and the minimum is 6.21 km/s with a difference 8.64%. The samples that were analyzed are interpreted to represent two different types of strain, constrictional and flattening. Results for both samples are similar although, sample D-A, interpreted to be associated with flattening strain, shows slightly lower velocities and anisotropy.