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CHAPTER 3: Tectonic and petrologic evolution of the Kodiak batholith and the
trenchward magmatic belt, Kodiak Island, AK: Juxtaposition along the Contact
fault*
ABSTRACT
The Paleocene intrusive rocks on Kodiak Island are part of the 2100 km long
Sanak-Baranof near trench plutonic belt. Within this belt plutonic rocks decrease in
age from 61-50 Ma from west to east. These plutons have been attributed to the
progressive subduction of a spreading ridge and resulting slab-window. The average
migration rate of the triple junction based on the above ages is 19 cm/yr. A similar
rate can also be calculated from internal Kodiak batholith ages, and the Kodiak
batholith ages align with the rest of the Sanak-Baranof belt plutons.
On Kodiak Island there are two Paleocene magmatic belts, which
traditionally have been explained as having formed from a single passage of a T-R-T
triple junction. In this hypothesis the basaltic and gabbroic rocks of the trenchward
belt formed as the spreading-ridge entered the toe of the accretionary prism, and the
larger granitic Kodiak batholith resulted from a slab-window at depth. However,
newly determined radiometric dates suggest that the two belts differ in age by 1-3
Ma. U/Pb zircon dates from the Kodiak batholith yield ages that range from 59.2-
58.4±0.2 Ma in the SW and NE, respectively. Ages in the trenchward belt range
from 62.6±0.6 (K-Ar) on the southwest side of the island to 60.15±0.86 (Ar-Ar
whole rock) in the northeast at Pasagshak Bay. Therefore, the Kodiak batholith and
the trenchward belt in the Ghost Rocks Formation cannot have formed from the
* Chapter 3 will be submitted to the Journal of Geophysical Research
65
Object Description
| Title | Magmatic and tectonic modification of convergent margins: an example from southern Alaska |
| Author | Farris, David W. |
| Author email | dfarris@usc.edu |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Geological Sciences |
| School | College of Letters, Arts and Sciences |
| Date defended/completed | 2006-04-12 |
| Date submitted | 2006 |
| Restricted until | Unrestricted |
| Date published | 2006-11-06 |
| Advisor (committee chair) | Paterson, Scott R. |
| Advisor (committee member) |
Anderson, Lawford Platt, John P. Davis, Gregory A. Thompson, Mark E. |
| Abstract | 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 meta-sedimentary 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 2100 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 Ma. |
| Keyword | Alaska; Kodiak; pluton; tectonic; magmatism |
| Language | English |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Type | texts |
| Legacy record ID | usctheses-m124 |
| Contributing entity | University of Southern California |
| Rights | Farris, David W. |
| Repository name | Libraries, University of Southern California |
| Repository address | Los Angeles, California |
| Repository email | cisadmin@lib.usc.edu |
| Filename | etd-Farris-20061106 |
| Archival file | uscthesesreloadpub_Volume35/etd-Farris-20061106.pdf |
Description
| Title | Page 82 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | CHAPTER 3: Tectonic and petrologic evolution of the Kodiak batholith and the trenchward magmatic belt, Kodiak Island, AK: Juxtaposition along the Contact fault* ABSTRACT The Paleocene intrusive rocks on Kodiak Island are part of the 2100 km long Sanak-Baranof near trench plutonic belt. Within this belt plutonic rocks decrease in age from 61-50 Ma from west to east. These plutons have been attributed to the progressive subduction of a spreading ridge and resulting slab-window. The average migration rate of the triple junction based on the above ages is 19 cm/yr. A similar rate can also be calculated from internal Kodiak batholith ages, and the Kodiak batholith ages align with the rest of the Sanak-Baranof belt plutons. On Kodiak Island there are two Paleocene magmatic belts, which traditionally have been explained as having formed from a single passage of a T-R-T triple junction. In this hypothesis the basaltic and gabbroic rocks of the trenchward belt formed as the spreading-ridge entered the toe of the accretionary prism, and the larger granitic Kodiak batholith resulted from a slab-window at depth. However, newly determined radiometric dates suggest that the two belts differ in age by 1-3 Ma. U/Pb zircon dates from the Kodiak batholith yield ages that range from 59.2- 58.4±0.2 Ma in the SW and NE, respectively. Ages in the trenchward belt range from 62.6±0.6 (K-Ar) on the southwest side of the island to 60.15±0.86 (Ar-Ar whole rock) in the northeast at Pasagshak Bay. Therefore, the Kodiak batholith and the trenchward belt in the Ghost Rocks Formation cannot have formed from the * Chapter 3 will be submitted to the Journal of Geophysical Research 65 |
