Strain variations in an ancient accretionary complex: Implications for forearc evolution

Abstract

Strain magnitudes and orientations and incremental strain histories are characterized throughout the Kodiak Formation, a portion of a Late Cretaceous‐early Tertiary accretionary prism. The results of these analyses are used to evaluate the history of underplating, penetrative strain, and unroofing in the forearc of the southwest Alaskan convergent margin. Five major deformation events are recognized in the Kodiak Formation: (1) layer‐parallel extension and layer‐parallel shear (D1) related to underthrusting, (2) seaward verging fold and thrust deformation (D2) with development of both a slaty cleavage (S2) and a downdip stretching lineation (L2) related to underplating and early penetrative strain within the wedge, (3) development of a regional scale antiform (D3), (4) development of a shear zone along the northwest limb of the D3 antiform (D4), and (5) right‐lateral strike‐slip faulting and penetrative right‐lateral simple shear in the seaward half of the Kodiak Formation (D5). Based on sharp differences in structural style and the magnitude and orientation of strain associated with D2, the Kodiak Formation has been subdivided into three structural units informally called the landward, central, and seaward belts. The structural variations observed in these three belts correspond to differences in structural relief of 1–5 km associated with the D3 antiform; the lowest D2 structural level is in the core of the antiform (central belt) and progressively higher structural levels are exposed on both limbs (landward and seaward belts). The regional distribution of incremental strain histories associated with D2, D3, D4, and D5 has been quantified by using syntectonic fibers in pressure shadows around spherical pyrite framboids. From the highest to the lowest structural level, D2 strain is characterized by (1) an increase in the magnitude of finite strain, (2) a change in the orientation of finite extension from nearly vertical to gently northwest plunging, (3) a change in the incremental extension direction from nearly vertical to moderately northwest plunging, and, finally, (4) a change in the strain history from coaxial to noncoaxial with a sense of rotation consistent with southeast directed thrusting. We interpret these D2 strain patterns to reflect a strain field that was fixed relative to the basal decollement, with coaxial subhorizontal shortening in the interior of the prism and noncoaxial strain (probably simple shear) near the base. Subsequent variations in the orientation of incremental extension associated with D3 are consistent with relatively rapid, vertical translation of the accreted sequences relative to the D2 strain field. We specifically propose two periods of wedge thickening in response to a combination of underplating and penetrative strain. These two wedge thickening events correspond with episodes of subaerial exposure and erosional unroofing in the Late Cretaceous and middle Tertiary.