A DOWN-PLUNGE SECTION OF A PALEOGENE STRIKE-SLIP SHEAR ZONE FROM LOW-GRADE PHYLLITE TO HIGH-GRADE GNEISS IN THE CHUGACH METAMORPHIC COMPLEX, SOUTHERN ALASKA

The Chugach Metamorphic Complex contains what may be the world’s most well-exposed down-plunge section of a strike slip fault system from greenschist facies shear zones downward into upper amphibolite facies gneiss. The downplunge section is developed in a monotonous interlayered metagreywacke-argillite assemblage that handicaps detailed descriptions of structural geometry, yet, at the same time, the homogeneous composition ultimately is an advantage because fabric variations reflect primarily changes in flow due to kinematic and rheologic changes within the crust. Early work suggested a two-phase overprinting (D2 to D3) observed within the complex was the product of changing kinematics during a ridge-subduction event with the latest event recording strike slip. We are now re-evaluating this model in light of the “attachment zone” model of Teyssier et al. (2003), and our observations to date indicate a modified version of this model could account for the observed deformation. A low-angle foliation is spatially associated with a decoupling horizon (attachment zone?) along the petrologic transition from schist to gneiss. Below this horizon the latest fabric (S3) is homogeneously developed as a steep foliation associated with upright folds, but above is limited to distinct bands of steeply dipping foliation and shallow plunging lineation that show clear evidence of dextral shear. One of these bands appears to pass through the entire down-plunge section as a 3-5km wide high-strain band, and we infer that this structure is the core of the main strike-slip shear zone. This high-strain band lies within a large synform in foliation with foliation rolling through broad antiforms on either side of the zone. The latter relationship is consistent with the predictions of the original attachment zone model, but the crustal penetrating shear zone is not. This discrepancy does not, however, eliminate the attachment model because it is almost certainly an artifact of modeling assumptions, and we are examining the problem through refinement of the model. New field work in summer 05 should clarify the structural geometry and shear-sense work in progress should provide further tests of the hypothesis.