SPATIAL AND TEMPORAL PATTERNS OF MELT MIGRATION AND STRAIN LOCALIZATION IN THE TWIN SISTERS ULTRAMAFIC MASSIF, WASHINGTON STATE

We present results from integrated field, microstructural, and textural studies in the Twin Sisters ultramafic massif, Washington State, that document variations in spatial and temporal patterns of melt migration and strain localization in naturally deformed upper mantle. The Twin Sisters massif is composed primarily of olivine-rich harzburgite and bands of dunite (i.e. former channels of melt migration) that crosscut a steeply dipping ~N-S foliation and sub-horizontal lineation. Dunite bands have a characteristic conjugate geometry that varies systematically across the massif; in the east, bands form at high-angle to each other but progressively become subparallel to foliation in the west. This systematic variation correlates with an increase in the magnitude of coaxial strain toward the west of the massif, indicating the preservation of a non-localized, km-scale strain gradient. Moreover, the preservation of high-angle conjugate bands in low strain regions suggests that the geometry of the melt migration network contrasts patterns expected for the stress-driven segregation of partial melt, as inferred from experimental deformation studies. Rather, an inescapable observation is that melt channels initiated as tabular to lensoidal bodies oriented approximately 45 degrees to the trend of foliation and lineation, and therefore also to the maximum and minimum axes of the coaxial strain ellipsoid, followed by variable degrees of transposition during heterogeneous strain localization. Differential stresses were calculated for samples across the strain gradient using the grain size paleopiezometer for olivine. Values range between 30-90 MPa, with strain rates correspondingly varying by approximately two orders of magnitude (10^-14 to 10^-12 s^-1). Crystallographic preferred orientations, determined by electron backscatter diffraction, record complex [100](010), Axial-100, and Axial-010 textures in both harzburgite and dunite, likely related to modification during continued strain localization. Together with microstructural observations (e.g. subgrains, deformation bands), these data suggest that dislocation creep or dislocation accommodated grain boundary sliding was the dominant deformation mechanism during the localization of strain within the Twin Sisters mantle lithosphere.