Late-orogenic backfolding and extension in the Brooks Range collisional orogen, northern Alaska

Abstract

New structural observations across a crustal section spanning relatively unmetamorphosed fold–thrust belt rocks and epidote–amphibolite grade rocks document the late-orogenic kinematic history of the Brooks Range, Alaska, and provide insight into the character, relative timing, and geodynamic setting of late-orogenic backfolding and extension in collisional orogens. Greenschist-grade rocks on the north flank of a metamorphic culmination display mesoscopic, open-to-close, S-vergent folds (and associated cleavages) and small-displacement normal faults, suggesting a sequence of D2 backfolding and D3 extension following the protracted phase of N-directed D1. At deeper levels, epidote–amphibolite facies rocks display a pervasive gently N-dipping S2 crenulation cleavage that is axial-planar to kilometer- and smaller-scale tight to isoclinal recumbent F2 folds. Porphyroblast–matrix relationships suggest that folds at the deeper levels initiated during synmetamorphic D2 backfolding. Much of the strain recorded by S2 and the tightening of folds, however, probably accumulated during post-metamorphic D3 extension.40Ar/39Ar data suggest an early- to mid-Albian age for D2 backfolding, which may be related to formation of a regional-scale backfold. Backfolding, together with apparently synchronous thrusting near the range front may represent the final stages of Cretaceous contractional deformation in the orogen. Contraction was immediately followed by extension, which, aided by erosion, produced a short-lived period of cooling and exhumation beginning at ∼103Ma. The available data suggest that extension is post-contractional, and thus is not viewed as a driving force of thrusting in the foreland. Extension was not limited to range-bounding normal faults, but rather was distributed over much of the southern half of the Brooks Range, with character of extensional fabrics being dependent on the geometry of D2 fabrics.Inasmuch as backfolding and extension are common features within collisional orogenic belts, the conclusions drawn here have implications regarding fundamental processes associated with the late-stages of collisional orogenesis. In particular, the study provides an example of the roles that backfolding and extension have played in the geodynamic development of a collisional orogen.