STRUCTURE AND KINEMATICS OF A TRANSECT ACROSS THE KAHILTNA ASSEMBLAGE, SOUTH-CENTRAL ALASKA

A 50 km NW-SE transect across the Kahiltna assemblage in south-central Alaska reveals a polarity reversal from 1) south-vergent mesoscale fabrics in a melange that represents the remnants of the toe of a south-facing accretionary wedge to 2) north-vergent folds in flysch that comprises the collision zone between the Wrangellia composite terrane and North America. The melange, less than 5 km south of the McKinley strand of the Denali Fault in the Cantwell area, exposes an accretionary wedge and slope basin unconformity that provides evidence of subduction prior to collision. A several hundred meter package of intensely deformed argillite with a scaly fabric is overlain unconformably by a conglomerate with clasts ranging from 10 cm to tens of meters. Sandstone fish within the argillite matrix and the dominant fault set indicate top-to-the-south shear. A conspicuous unconformity separates the melange from the overlying conglomerate that is interpreted to represent the base of a perched slope basin or submarine channel near the toe of an accretionary wedge. Structural fabrics and sandstone injections indicate that top-to-the south shear occured when sediments were unlithified or partially lithified. The melange unit marks the northern edge of the Kahiltna assemblage south of the Denali Fault.

In the Kahiltna flysch south and north of the Denali Fault, the dominant structures are tight, north-vergent folds with wavelengths that range from 50 cm to 500 m. These folds have a steep, regionally extensive axial planar slaty cleavage that crosscuts the south-vergent fabrics of the melange. The presence of a gently plunging stretching lineation and locally, steep fold axes indicates the importance of strike slip deformation during the main phase of collision. The maximum finite stretch associated with slaty cleavage development ranges from 2.2 to 4.0. Incremental strain histories are consistent with a component of dextral shear in flysch south of the Denali Fault and sinistral shear north of the Denali Fault, an observation that suggests lateral variations in kinematics within the collision zone prior to Tertiary strike slip juxtaposition.