Development of a subhorizontal decoupling horizon in a transpressional system, Chugach metamorphic complex, Alaska: Evidence for rheological stratification of the crust

Field studies east of the Copper River provide an unusual view of the architecture of a ductile strike-slip system where the Chugach metamorphic complex plunges westward from high-grade gneiss to greenschist facies rocks of the Chugach terrane. In this down-plunge view of a transpressional system we observe a three-phase ductile deformational history similar to the Tana River region about 75 km to the east. In the Copper River area, however, spatial variations in structural development provide important insights into syntectonic flow in lithologically homogeneous rocks ranging from high-grade gneiss to greenschist facies phyllite. Early structures (D 1 ) formed prior to the Chugach metamorphic complex and produced a steeply dipping layering (So) and layer-parallel cleavage (S 1 ) following thrust imbrication and associated folding. Following D 1 , the earliest structures of the Chugach metamorphic complex (D 2 ) were associated with a vertical shortening and horizontal, margin-parallel extension. D 2 produced gently inclined to recumbent subisoclinal to isoclinal folds (F 2 ) in S 1 = S 0 layering with axes parallel to the ∼WNW-trending elongation lineation. These structures were in turn overprinted by a third set of structures (D 3 ) that produced upright, gently plunging folds (F 3 ) of S 2 that vary spatially. In the high-grade gneissic core of the complex, D 3 structures are homogeneously distributed with tight to subisoclinal mesoscopic folding throughout the core, and no significant spatial variation in fabric development. In contrast, structurally above a transition from schist to gneiss, D 3 fabric development is domainal with bands of intense S 3 foliation development separated by broad regions in which D 3 fabrics are indistinct or absent. Within these D 3 high-strain bands, numerous fabric asymmetries indicate that these bands are zones of distributed dextral shear. These observations suggest that during D 3 , upper crustal dextral strike-slip faults passed downward into discrete shear zones, but these shear zones did not penetrate to the base of the crustal column. Instead, the strike-slip motion was dispersed into a broad zone of distributed shear within the high-grade gneisses at depth, and the contact between these two crustal levels was a D 3 decoupling surface. We propose a tectonic model for the Chugach metamorphic complex that combines effects of highly oblique Kula-Farallon subduction with the coeval interactions of the Kula-Farallon-North American triple junction. Reanalysis of the plate velocities together with observations from the Border Ranges fault system immediately inboard of the Chugach metamorphic complex indicate that the Chugach metamorphic complex developed when the Chugach terrane was a forearc sliver. If slip partitioning were complete, this forearc sliver migrated northward faster than the triple junction. Thus, the triple junction moved slowly southward within the forearc sliver, which is consistent with regional geochronological trends. The proposed mid-crustal decoupling horizon formed within this evolving transform system and D 2 and D 3 probably overlapped in time. In this interpretation D 2 primarily records vertical displacement gradients related to mid-crustal decoupling that initiated during supracrustal strike slip along the Border Ranges fault. With prograde metamorphism of the Chugach metamorphic complex, the decoupling horizon rose to higher crustal levels, and strike slip stepped outboard to form D 3 dextral shear zones within the Chugach metamorphic complex. The inference of mid-crustal decoupling implies a grossly different rheologic behavior across the gneiss transition. We propose that the gneissic front represents a fundamental change in macroscopic flow behavior from homogeneous linear flow in the high-grade gneisses to strain-softening, nonlinear flow in the overlying rocks. This rheological change is interpreted as a consequence of a deformation mechanism switch from diffusion