COMPLEX CRUSTAL STRATIFICATION IN A DUCTILE ATTACHMENT ZONE WITHIN THE CHUGACH METAMORPHIC COMPLEX, SOUTHERN ALASKA

The western termination of the Chugach Metamorphic Complex in southern Alaska provides an excellent exposure of a low angle, oblique crustal section. Field mapping revealed a 10-15 km wide synformal fan-shaped structure within ductiley deformed accretionary prism material that was affected by high-T, low-P metamorphism. Finite strain analysis has shown that this “foliation fan” contains a high strain core with decreasing strain outwards across strike. Our observed foliation pattern and relative finite strain values have led to a modification of the crustal attachment zone model of Teyssier and Cruz (2004). This model allows for differential motion of crustal layers, fundamentally implying detachment within the crustal column. Our initial models simulated a transition from rigid upper crust to distributed shear at depth, following the model of Teyssier and Cruz (2004). Although this model reproduces our observed foliation pattern, it is inconsistent with observed finite strain data. Thus, we modified the boundary conditions. These boundary conditions include homogenous ductile shear (pressure solution?) in an upper layer, and deformed lower level blocks offset by a lower level shear zone. When combined to form an attachment zone, these two ductile layers produce a predictable foliation pattern in an intermediate layer that is compatible with our field mapping and finite strain analysis. This crustal stratification scheme implies greater variation in the vertical distribution of strain throughout the crustal column. These crustal layers bearing distinct structural form and deformation mechanisms lead to the characterization of the development of detachment zones. Alternatively, the lower level shear zone could have been caused by subduction of a transform fault adjacent to the spreading ridge responsible for the creation of the Chugach Metamorphic Complex.