TECTONIC EVOLUTION OF THE WALLOWA COMPOSITE TERRANE

The Wallowa composite terrane in the Blue Mountains consists of two parts: (1) a Paleozoic volcanoplutonic island arc built on oceanic lithosphere, and (2) a Mesozoic supracrustal arc whose detritus fed into an adjacent Late Triassic-Early Jurassic marine basin. The arcs are juxtaposed along a Mesozoic thrust-fault. We suggest that the structure is a feature of Late Jurassic regional deformation. The structures and metamorphism of the two arcs suggest that they evolved separately prior to their juxtaposition along the Late Jurassic thrust. Layering (~S0) in both arcs is at high angles along the thrust. At least three deformational episodes involving contraction affected all the terranes in the Blue Mountains (Avé Lallemant ,1995) . The episodes began in the Late Triassic and ended in the Late Jurassic after terrane amalgamation. Late Triassic D1 deformation overlaps Wallowa terrane D1 transpressional mylonitic shearing, has essentially the same timing as transtensional arc volcanism and basinal development, and has about the same timing as subduction of the adjacent Baker terrane deeper ocean floor prior to its juxtaposition adjacent to the McCloud-Olds Ferry arc terrane. Middle to early Late Jurassic folding (D2) and formation of penetrative axial planar foliation (S2) in the Wallowa arc apparently affected only the Mesozoic arc and basin. D3 structures refold D2 structures in the Mesozoic arc of the Wallowa terrane and truncate them during underthrusting beneath the Permian arc. Growth and deformation of Tethyan arcs and related ocean floor in the Blue Mountains may yield important clues to tectonic events in other parts of the Cordillera. For example, the Wallowa and Baker terranes, respectively, may have counterparts in the Klamath Mountains (North Fork and Stuart Fork terranes) and in the northern Sierra Nevada (base of Don Pedro terrane plus Tuolumne River belt and Calaveras Complex). The paired terranes have similar age–ranges, stratigraphic characteristics, petrology and structure. Their presence in these provinces suggests that they are plate remnants of Tethyan-Panthalassan ocean floor.