Constrictional flow within the Eocene forearc of Southern Alaska: An effect of dextral shear during ridge subduction

Eocene deformation in metasedimentary rocks of the Valdez Group was examined along the Richardson Highway between Valdez and the Copper River basin. A topographic lineament along Stuart Creek and the lower Tiekel River separates two distinct structural domains: a southern domain (Domain 1) with two fabrics, and a northern domain (Domain 2) with three. Both domains possess an early, layer-parallel phyllitic cleavage (S 1 ) associated with south-directed thrust systems. In Domain 1, S 1 is steeply dipping and overprinted by a second, steeply south-dipping cleavage (S 2 ) with a subhorizontal elongation lineation, and F 2 folds are rare to absent. To the north in Domain 2, however, S 1 was first tightly folded during D 2 into a series of gently inclined folds (F 2 ) and then refolded by tight to close, steeply inclined F 3 folds that are asymmetric to the north. Metamorphic mineral assemblages indicate greenschist facies conditions accompanied the deformation: Biotite grew during D 2 in Domain 1 and during D 3 in Domain 2; and metatuffs in Domain 1 contain actinolite-chlorite-epidote-feldspar-quartz. Fluid inclusion analyses of multiple vein sets indicate a complex fluid history but indicate a metamorphic temperature of 350-400 °C, consistent with the observed mineral assemblages. Uniform metamorphic grade across the belt together with spatial variations in fabrics suggests that D 2 in Domain 2 is time-equivalent with D 3 in Domain 2. We evaluate the kinematics of linear and planar fabric development through a combination of finite strain analysis and geometry of syn-tectonic growth-fibers developed on sand grains. Finite strains are estimated by different methods including object shapes, object centers, and stretches estimated from growth fibers. The strain data show a predominance of constrictional strains, particularly in Domain 1, with subhorizontal stretching parallel to the mesoscopic lineation. Strain superposition models indicate that the main deformation (D 2 ) in Domain 1 was a constrictional flow indicating that D 2 was marked by subhorizontal orogen-parallel elongation and simultaneous vertical and horizontal shortening. Shear sense indicators generally indicate dextral shear on steeply dipping surfaces, but are not always well developed because the constrictional flow deviates strongly from simple shear. These characteristics are typical of the strain facies referred to as lengthening shear to lengthening-narrowing shear flow. We interpret this large-scale kinematic pattern as a combined effect of distributed dextral shear and vertical shortening related to a coeval ridge-subduction event. The vertical shortening could be associated with lateral flow driven by the migrating ridge, vertical displacement gradients related to a mid-crustal decoupling zone recognized in the Chugach metamorphic complex, or both. We infer that dextral shear is distributed through a wide zone but is concentrated along the major topographic lineament that is coincident with the structural domain boundary. We informally refer to this dextral structure as the Stuart Creek shear zone and conclude that the two structural domains were juxtaposed along this structure. D 3 structures recognized north of the Stuart Creek shear zone probably represent a back-thrusting event synchronous with the dextral shear system, which may record mid-crustal strain partitioning. Locating the westward continuation of the dextral shear system is difficult due to lack of detailed structural studies and the difficulty of recognizing strike slip within the homogeneous flysch of the Chugach terrane. The dextral shear system may transfer into complex out-of sequence thrust systems if the strike-slip systems were developed during or after the Alaskan orocline.