SEDIMENTARY RECORD OF TRIASSIC-JURASSIC TERRANE COLLISION AND ACCRETION IN THE BLUE MOUNTAINS PROVINCE, EASTERN OREGON

Sedimentary rocks in the Blue Mountains Province (BMP) of eastern Oregon contain a record of Triassic-Jurassic continental growth by collision and accretion of oceanic terranes. We are using analysis of stratigraphic architecture, trace-element geochemistry, detrital zircon ages, and Sm-Nd isotopes in the BMP to interpret the tectonic evolution of this and adjacent regions of the western U.S. Cordillera. Paleozoic and Mesozoic rocks include two Triassic magmatic arcs (Wallowa and Olds Ferry terranes), an intervening oceanic plate and subduction complex (Baker terrane), and a thick succession of Triassic-Jurassic sedimentary rocks (Izee terrane). We divide supracrustal rocks into two megasequences: (1) Late Triassic to Early Jurassic strata that change from (1a) volcanic and volcaniclastic rocks to (1b) marine argillite and turbidites with chert-bearing conglomerate derived from the Baker terrane; and (2) Jurassic deposits that record ~20-40 m.y. of deep subsidence in a large westward-migrating marine basin. Late Triassic chert-clast conglomerates of MS-1 record growth of a large thrust belt in the Baker terrane and syntectonic deposition in marine basins on opposite flanks of the thrust belt. Jurassic strata of MS-2 overlie older rocks along a major angular unconformity, and thus represent a regional overlap assemblage that post-dates amalgamation of the Wallowa, Baker, and Olds Ferry terranes.

Based on compilation of existing and new data, we propose a tectonic model for the BMP that includes: (1) Middle Triassic magmatism in the Wallowa and Olds Ferry arcs during closure of an ocean basin; (2) Late Triassic collision between facing accretionary wedges of the Wallowa and Olds Ferry arcs, and growth of flexural marine basins on both flanks of the Baker terrane thrust belt; and (3) Jurassic collision of the amalgamated BMP terranes with western North America, and growth of a large marine collisional basin that was produced by crustal thickening and loading in the Cordilleran thrust belt of Nevada. This analysis suggests that collisional tectonics played a major role in Triassic-Jurassic mountain building, crustal growth, and basin development in the western U.S. Cordillera. The Jurassic collisional margin in the BMP may have changed southward to a neutral or extensional convergent margin in the Klamath Mountains and southwestern U.S.