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Farris, D.W., 2006

Magmatic and tectonic modification of convergent margins: An example from southern Alaska

Bibliographic Reference

Farris, D.W., 2006, Magmatic and tectonic modification of convergent margins: An example from southern Alaska: Los Angeles, California, University of Southern California, Los Angeles, Ph.D. dissertation, 358 p.


Southern Alaska is an excellent natural laboratory to study forearc/arc subduction zone magmatism and tectonics. Understanding these processes are important to understanding the evolution and modification of continental crust. This thesis focuses on the Kodiak Islands, but also examines larger-scale features throughout southern Alaska and cordilleran tectonics. Kodiak Island intrusive rocks differ in character across the Border Ranges fault system (BRF). North of the BRF is the tilted Triassic-Jurassic Talkeetna island arc. This arc section exposes ultramafic mantle through mid to upper crustal plutonic and volcanic rocks all of which are geochemically related, despite being fault-bounded blocks. South of the BRF lie Paleocene intrusive rocks related to eastward spreading ridge subduction migration. These rocks are distributed in two belts: the Kodiak batholith and the trenchward belt. The Kodiak batholith is composed of granitic plutons emplaced as a series of intermingled, 1-8 km wide, viscoelastic diapirs that ascended by downward transport of aureole rocks through and around the magmatic column, and range in age from 59.2-58.4 +/- 0.2 Ma (SW-NE). These plutons formed from equilibrium crystallization of an argillite/graywacke-derived magma, and contain fractally fragmented metasedimentary xenoliths. Trenchward belt rocks lie south of the Contact fault and are composed of small gabbroic to granitic plutons, dikes, and pillows that range in age from 62.6 +/- 0.6 - 60.15 +/- 0.86 Ma (SW-NE). They formed by assimilation fractional crystallization of MORB with an argillite assimilant. Trenchward belt rocks intruded when a spreading ridge first entered the accretionary prism, whereas the Kodiak batholith formed when a slab window opened at 15-20 km depth. Age differences between the two Paleocene magmatic belts are explained by oblique ridge subduction and Contact fault displacement. The Kodiak batholith is part of the 2,100 km Sanak-Baranof belt of forearc magmatism. Along-strike variations in pluton chemistry, spacing and thermal characteristics can be explained by oblique-ridge subduction along a curved continental margin. Inland, arc magmatism exhibits a flare-up in activity that tracks eastward migration of magmatism in the forearc, but precedes it by 1-3 My.

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