Structural evolution of coherent terranes in the Ghost Rocks Formation, Kodiak Island, Alaska

Abstract

Summary The Late Cretaceous to Palaeocene Ghost Rocks Formation, Kodiak Island, Alaska, comprises two structural styles: multiply deformed mélange and relatively coherent terranes of turbidite deposits which together constitute a probable ancient accretionary complex. The mélange consists almost exclusively of orientated, elongate sandstone boudins which lie in a foliated pelitic matrix. In the coherent terranes, structural histories, rock types and hornfels metamorphism delineate two NE trending units: Unit A, and a probably younger Unit B. Unit A can be subdivided into two gradational structural belts, a seaward conjugate fold belt and a landward spaced cleavage belt. The conjugate fold belt contains ‘S’ and ‘Z’ type kink folds that often intersect to form conjugate fold sets. The folds of this fold belt are hornfelsed by a sill which intruded partially lithified sediments; the fold belt therefore also formed when the sediments were partially lithified. Analyses of the fold axial surfaces indicate a shortening direction of 319°, when later folding is removed. Sill emplacement and hornfels metamorphism interrupted the conjugate fold deformation and resulted in a change in style of deformation in Unit B to broad, open to tight folds associated with a spaced cleavage. The cleavage strikes 046° and dips approximately vertically, fanning divergently about fold axial surfaces. The approximately coaxial conjugate folding and folding with cleavage in unit A constitute D 1 . Unit B generally contains sub-horizontal beds except for a large-scale, overturned, thrusted fold, which emplaced mélange to the NW and above Unit B. Slickenlines, fold axes and poles to bedding indicate this unit was shortened along a trend of 334°. This deformation is termed D 2 and is correlated elsewhere in the Ghost Rocks Formation to thrust faults which place mélange to the NW and under the coherent terranes. The structural histories of the coherent terranes in the Ghost Rocks Formations show that accretionary prism deformation can involve coaxial deformations and the tectonic deformation of partially lithified sediments. The structural histories also show that the Palaeocene deformation axes in the Ghost Rocks Formation have not rotated out of their original sub-horizontal orientations.