The character and distribution of mineralized fractures in the Kodiak Formation, Alaska: Implications for fluid flow in an underthrust sequence

Abstract

The Kodiak Formation, an accreted slate belt in southwest Alaska, experienced two major episodes of fracturing and mineralization in the Late Cretaceous‐early Tertiary. The earlier of these episodes is related to underthrusting of sediments below a major décollement (D 1 ), whereas a later episode is marked by development of fold and thrust structures and a regionally pervasive slaty cleavage within the accretionary wedge (D 2 ). D 1 veins are restricted to sand layers and are concentrated at the highest structural level of the Kodiak Formation. The most pervasive D 1 fractures occur within a mélange zone, where the Kodiak Formation is thrust beneath the older Uyak Complex. Microstructures in the mélange suggest that these veins formed during the later stages of a deformation that involved layer parallel shear in conjunction with progressive lithification. D 2 veins cut sand and shale layers and are concentrated at the lowest structural levels of the Kodiak Formation. Microstructures within these veins indicate that D 2 veins are hydrofractures that were either (1) intermittentiy sealed along the length of the crack (e.g., in the smallest veins (<0.5 cm width)), (2) intermittently sealed at asperities along the fracture surface, or (3) open along the length of the crack through most of the vein history (e.g., in the largest veins (>0.5 cm width)). Fracturing coincided with development of a slaty cleavage in a regime of progressive simple shear. From highest to lowest structural levels, there is a gradual increase in measured finite strain, the abrupt appearance of pervasive D 2 veins, and a greater noncoaxiality of strain related to southeast directed thrusting. These observations suggest that D 2 veins formed above a décollement that was active at the base of a Late Cretaceous‐early Tertiary accreted package. In total, the D 1 and D 2 veins suggest substantial fluid flow within an ancient subduction complex, with development of hydrofractures first in the footwall and then in the hanging wall of a basal décollement that stepped from shallow to deeper structural levels.