Field Investigations of Strain Partitioning and Multi-Phase Deformation in the Chugach Metamorphic Complex, Southern Alaska

Spatial and temporal differences in crustal flow are identified in the Chugach metamorphic complex (CMC) in south-central Alaska, as a result of dynamic changes during Eocene ridge subduction with potential complications from mid-crustal detachment during strike-slip. Previous work in this area showed a two-phase deformational history during prograde metamorphism associated with the ridge subduction, but new work suggests a spatial variation in this deformation. In the northern part of the CMC, adjacent to the dextral-strike-slip backstop to the accretionary complex (Border Ranges Fault), early sub-horizontal fabrics are overprinted by steeply dipping fabrics with clear evidence of dextral shear. A subhorizontal extension lineation on both fabrics has been used as evidence that the fabric overprints are coeval in a single progressive deformation associated with dextral strike slip, implying detachment, or attachment, within the crustal section at the petrologic transition from schist to gneiss. However, cross-cutting observations of Tertiary intrusions suggest that deformational phases are not coeval and fabrics developed at different periods. Tertiary intrusions will ultimately provide absolute age constraints on deformational phases when our U-Pb geochronology is completed. In contrast, observations along the southern edge of the complex show a two-phase fabric development but the oldest deformation fabric shows a prominent down-dip lineation with evidence of top to the south shear sense. This spatial variation in fabric develop is suggestive of strain partitioning during northward transport of the Chugach accretionary prism, with down dip lineations forming in a compressional regime at the toe of the prism and horizontal lineations forming in a strike-slip regime further inboard, adjacent to the backstop.