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Emplacement of the Kodiak batholith and slab-window migration

David W. Farris^,1, Peter Haeussler², Richard Friedman³, Scott R. Paterson⁴, R.W. Saltus⁵ and Robert Ayuso⁶

¹ Department of Earth Sciences, University of Southern California, 3651 Trousdale Avenue, Los Angeles, California 90089, USA
² U.S. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508, USA
³ Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada
⁴ Department of Earth Sciences, University of Southern California, 3651 Trousdale Avenue, Los Angeles, California 90089, USA
⁵ U.S. Geological Survey, P.O. Box 25046, MS 964, Denver, Colorado 80225, USA
⁶ U.S. Geological Survey, 954 National Center, Reston, Virginia 20192, USA

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 flow, collapse of wall rock, stoping, and diking. Stokes flow xenolith calculations suggest ascent rates of 5–100 m/yr and an effective magmatic viscosity of 10⁷–10⁸ 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.

Key Words: slab window • pluton displacement • Alaska • Kodiak • tectonics • ridge subduction