DEFORMATIONAL HISTORY OF THE LIBERTY CREEK AND ICEBERG LAKE BLUESCHIST BODIES, SOUTH CENTRAL ALASKA

The Iceberg Lake and Liberty Creek blueschist bodies of south central Alaska are located ~10 km south of and directly to the south of the Border Ranges Fault (BRF) respectively. These blueschists are being studied to determine the timing of initiation of this Jurassic megathrust system. New geologic mapping at nominal scales of 1:24,000 to 1:12,000 using digital mapping techniques within these bodies has revealed a more complex deformation history than previously noted (Nokleberg et al. 1989; Pavlis and Roeske, 2007). These two blueschist localities are distinctly different with the Liberty Creek schists forming a coherent terrane, where as the Iceberg Lake blueschists are kilometer scale blocks within a younger mélange. Despite these differences the blueschist bodies have a similar structural history. Both assemblages have a pronounced continuous cleavage (S1) defined by a crystallographic and dimensional preferred orientations. An S2 cleavage overprints previously described north dipping layering and S1 cleavages and has a shallow to moderate south dip. Intersection lineations created by both S0-S1 and S1-S2 overprints plunge shallowly to the ENE-WSW. These fabrics are overprinted by a ~N-S striking vertical cleavage, S3, which wraps earlier lineation around (f3) folds and crenulations. These fabrics complicate the already intricate history of the central Alaskan section of the BRF system. S2 strikes parallel to subduction-related deformation. It is not clear if S2 is associated with deformation within the accretionary prism or subduction erosion related uplift along the BRF. S3 is perpendicular to these earlier cleavages and strikes parallel to the Taral fault. S3 is interpreted as a younger fabric related to northeast-directed oblique thrust faulting along the Taral fault, an N-S striking fault that cuts older strike-slip structures. The current position of the blueschists show approximately 60-120 km of lateral east to west movement, and the similarity of fabrics within the blueschists indicates this movement occurred during or after S3 was recorded in the rocks. Microstructural and geochronological analyses are being carried out to further clarify the structural and temporal history of these blueschist bodies and their relationship to the surrounding rocks and the BRF.