Emplacement of the Kodiak batholith and slab-window migration

Abstract

The Kodiak batholith is one of the largest, most elongate intrusive bodies in the forearc Sanak-Baranof plutonic belt located in southern Alaska. This belt is interpreted to have formed during the subduction of an oceanic spreading center and the associated migration of a slab window. Individual plutons of the Kodiak batholith track the location and evolution of the underlying slab window. Six U/Pb zircon ages from the axis of the batholith exhibit a northeastward-decreasing age progression of 59.2 ± 0.2 Ma at the southwest end to 58.4 ± 0.2 Ma at the northeast tip. The trench-parallel rate of age progression is within error of the average slab-window migration rate for the entire Sanak-Baranof belt (~19 cm/yr).Structural relationships, U/Pb ages, and a model of new gravity data indicate that magma from the Kodiak batholith ascended 5-10 km as a northeastward-younging series of 1-8-km-diameter viscoelastic diapirs. Individual plutons ascended by multiple emplacement mechanisms including downward fl ow, collapse of wall rock, stoping, and diking. Stokes fl ow xenolith calculations suggest ascent rates of 5-100 m/yr and an effective magmatic viscosity of ≈10 7 -10 8 Pa s. Pre-existing structural or lithologic heterogeneities did not dominantly control the location of the main batholith. Instead, its location was determined by migration of the slab window at depth.

Textural record of the seismic cycle: strain-rate variation in an ancient subduction thrust

“…The Ghost Rocks Formation was deposited on a subducting ridge flank in the latest Cretaceous-earliest Paleocene (Byrne 1982(Byrne , 1984Haeussler et al 2003;Farris et al 2006). The structural base is a mélange zone along which Eocene marine sediments were thrust under the Ghost Rocks Formation (Moore 1969) (Fig.…”

Fluid‐rich damage zone of an ancient out‐of‐sequence thrust, Kodiak Islands, Alaska

“…This is demonstrated by the inclusion of clasts of dike material into two of the subsidiary faults in the Uganik Thrust footwall (Figure 3f) and the dike crosscutting the Uganik Thrust itself with only about 10 cm offset (Figure 2c). No thermal aureole was observed around dikes at Big Waterfall Bay, where the intrusions are smaller than those with greenschist facies metamorphic halos described elsewhere in the complex [ Farris et al , 2006; Paterson and Sample , 1988].…”

“…While giving excellent age control on the fault motion, these field relationships raise the question of local perturbation of thermal indicators collected in the vicinity of the dikes. Near the Kodiak Batholith, maximum metamorphic aureole temperatures were 650°C and contact metamorphism was associated with ductile deformation of the country rock, even outside the detectable thermal aureole [ Farris et al , 2006]. No structural or metamorphic effects could be found in the vicinity of the much thinner dikes at the Uganik Thrust.…”

Fluid-rock interaction recorded in black fault rocks in the Kodiak accretionary complex, Alaska

“…The Kodiak archipelago in Alaska (Figure 1a) consists of a Jurassic to Eocene accretionary complex composed of NW-dipping, thrust-bounded units showing progressively younger ages toward the southeast (Moore et al 1983;Plafker et al 1994) that are intruded by 58-to 59-Ma near-trench granitic rocks reflecting ridge subduction (Farris et al 2006;Ayuso et al 2009). The Ghost Rocks Formation, consisting of the latest Cretaceous-earliest Paleocene ridge-flank deposits, is distributed over the southeastern part of Kodiak Island (Fisher and Byrne 1987).…”