Analysis of the Wallowa-Baker terrane boundary: Implications for tectonic accretion in the Blue Mountains province, northeastern Oregon

Joshua J. Schwartz, Arthur W. Snoke, Carol D. Frost, Calvin G. Barnes, L. Peter Gromet, Kenneth Johnson

Research output: Contribution to journalArticle

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Abstract

The Baker terrane, exposed in the Blue Mountains province of northeastern Oregon, is a long-lived, ancient (late Paleozoic-early Mesozoic) accretionary complex with an asso ciated forearc. This composite terrane lies between the partially coeval Wallowa and Olds Ferry island-arc terranes. The northern margin of the Baker terrane is a broad zone (>25 km wide) of fault-bounded, imbricated slabs and slices of metaigneous and metasedimentary rocks faulted into chert-argillite mélange of the Elkhorn Ridge Argillite. Metaplutonic rocks within tectonic units in this zone crystallized between 231 and 226 Ma and have low initial 87Sr/86Sr ratios (0.7033- 0.7034) and positive initial εNd values (+7.7 to +8.5). In contrast, siliceous argillites from the chert-argillite mélange have initial 87Sr/86Sr values ranging from 0.7073 to 0.7094 and initial εNd values between -4.7 and -7.8. We interpret this broad, imbricate fault zone as a fundamental tectonic boundary that separates the distal, Wallowa island-arc terrane from the Baker accretionary-complex terrane. We propose that this terrane boundary is an example of a broad zone of imbrication made up of slabs and slices of arc crust tectonically mixed within an accretionary complex, providing an on-land, ancient analog to the actualistic arc-arc collisional zone developed along the margins of the Molucca Sea of the central equatorial Indo-Pacific region.

Original languageEnglish
Pages (from-to)517-536
Number of pages20
JournalBulletin of the Geological Society of America
Volume122
Issue number3-4
DOIs
StatePublished - 2010

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