GROWTH AND DESTRUCTION OF THE WALLOWA TERRANE

Growth and Destruction of the Wallowa Terrane

Abstract. The Wallowa terrane in the Blue Mountains contains a constructional sequence composed of two groups of rocks: (1) a lower part that consists of Permian ophiolite and island arc rocks, and (2) an upper part represented by a Late Triassic arc whose detritus fed into an adjacent Late Triassic-Early Jurassic marine basin. The Wallowa ophiolitic and arc rocks and the adjacent Baker terrane are the remnants of Permian and possibly earlier oceanic lithosphere probably related to other Tethyan ocean floor in the Klamath Mountains (North Fork and Salmon Creek units of the Sawyers Bar terrane and the Stuart Fork or Fort Jones terrane) and in the northern Sierra Nevada (Tuolomne River belt and Calaveras Complex). Wallowa terrane Permian arc rocks are characterized by Na2O-enrichment, K2O-deficiency (e.g., spilites and keratophyres), a generally flat REE profile, and are associated with subgreenschist facies hydrothermal metamorphism. According to Vallier (1995), Triassic arc rocks show greater K2O and LREE-enrichment (LREE/HREE is ~2-2.5). The timing of Late Triassic D1 mylonitic shearing overlaps with arc volcanism and basinal development in the Wallowa terrane and subduction of the adjacent Baker terrane accretionary prism beneath the McCloud Olds Ferry arc terrane and its ophiolitic basement. The timing of Middle to early Late Jurassic folding (D2) and penetrative axial planar foliation (S2) probably reflects continued contraction of the Wallowa arc shortening the distance between the Triassic-Jurassic arc and basin and the Permian remnant arc and ophiolite. D3 structures refold D2 structures and truncate them during underthrusting of Late Triassic-Early Jurassic arc and sedimentary rocks beneath Permian arc and ophiolitic rocks. D2/D3 folding in the Wallowa terrane may reflect continued subduction of the remnant Tethyan ocean floor-arc and accretionary prism beneath the McCloud Olds Ferry arc in the Blue Mountains. According to Ferns and Brooks (1995), faults separating the Wallowa and Baker terranes may have developed subsequent to or during Baker terrane subduction beneath the McCloud arc and probably do not represent terrane suturing.