Olivine orientation and seismic anisotropy at confining pressures of 0.1 to 10.0 kb have been studied for 14 samples collected from the Twin Sisters dunite. At 10 kb, the dunites show differences in compressional wave velocities with propagation direction ranging from 0.03 to 0.92 km/sec. All samples were geographically oriented in the field, and the measured velocities have been related to the geographic orientations. The results show a complex but definite pattern of anisotropy throughout the body in which the slow velocities tend to be horizontal and normal to the elongation of the body. Mean velocities in a vertical direction are slightly higher than mean north-south velocities.
The anisotropy is clearly related to preferred olivine orientation. Petrofabric studies of the field-oriented samples show concentrations of olivine b crystallographic axes parallel to slow velocities. High velocities have been measured parallel to concentrations of olivine a axes. The fabric studies further indicate that the olivine b axes throughout the body show a strong tendency to lie horizontal in a northeast-southwest to east-west direction.
The fabric of the Twin Sisters body is interpreted as having originated by recrystallization accompanying flow within the upper mantle. Locally this fabric has been modified by cataclasis and plastic flow. The dunite body appears to have been transported from the mantle as a solid along a major thrust fault.