DETAILED STRUCTURAL MAPPING AND MAGNETIC ANALYSIS OF THE TWIN SISTERS DUNITE, WASHINGTON STATE

The Twin Sisters dunite is located in the NW part of Washington state, approximately 40 km east of Bellingham, Washington. It is elongate in a NW direction, with a length of 16 km, width of 5.5 km, and an estimated depth of 2 km. This dunite lies within a large melange zone that was accreted to North America during the mid-Cretaceous. The Twin Sisters massif consists entirely of dunite and serpentinization is virtually absent in the NW part of the body, allowing us to study mantle flow fabrics.

The lattice preferred orientation (LPO) of olivine is relatively consistent across the entire massif, as determined by previous studies. [100] axes tend to have point distribution which are oriented NS with shallow plunges, although the [010] and [001] can have either girdle or point distributions. Our fieldwork indicates more deformation than reported by previous workers. Isoclinally-folded and boudinaged enstatite-rich dikes occur throughout the massif. Fold hinges have a steeply NW-plunging orientation and axial planes are NW-oriented and vertical, where studied. These structures are similar in attitude to previously-reported, isoclinally-folded chromite layers. Given the high-temperature microstructures preserved at these sites, these structures formed in the mantle. Detailed maps of key areas were produced to highlight the compositional and deformation heterogeneity within this massif.

Despite the low degree of serpentinization, the low-field magnetic susceptibility is dominated by secondary magnetite that forms during serpentinization. We have used high-field AMS analysis - at fields of 0.9 T, which is considerably larger than the saturation magnetization, thereby eliminating the ferromagnetic component - to determine fabric associated with mantle flow. The non-ferromagnetic high-field AMS is dominated by the paramagnetic magnetocrystalline anisotropy of olivine. The high-field AMS fabrics correlate well with the LPO measurements on the same samples, which are not parallel to the low-field AMS fabrics. Our results suggest that if over 10% of the primary silicates are present and have a bulk fabric anisotropy larger than 1.05, the high-field AMS method can reliably measure the bulk silicate fabric.