Selma Quadrangle, Josephine County, Oregon: Supplement 7 from "Geologic and tectonic evolution of jurassic marginal ocean basin lithosphere, Klamath Mountains, Oregon" (Thesis)

This study examines the geologic and tectonic history of the westernmost lithotectonic belt of the Klamath Mountains province, known as the western Jurassic belt (WJB). In Oregon the WJB exposes one of the world's best preserved examples of ancient marginal ocean basin lithosphere, including a complete ophiolite sequence, an arc volcano- plutonic complex, and older fragments of oceanic lithosphere that occur as screens within the ophiolite and which form the basement constructs of the arc. As such, the region provides an exceptional opportunity to directly examine the structural framework and geologic features of marginal ocean basins. Geologic mapping in the central Illinois River drainage area, Curry and Josephine Counties, Oregon has resulted in the following changes to previously reported work: 1) the discovery of a 5 x 45 km fragment of rocks correlative with the Rattlesnake Creek terrane; rocks previously known only to occur within the overthrust lithotectonic belt to the east, the western Paleozoic and Triassic belt , 2) the discovery of an intra-peridotite boundary separating pre-175 Ma serpentinized harzburgite and dunite from relatively unaltered peridotite associated with the Josephine ophiolite (166162 Ma), and 3) a re-interpretation of the region as a fold and fault belt rather than an imbricate thrust stack of separate subterrane units. Collectively, these relations confirm the prediction of a previous model for the Jurassic tectonic evolution of the region that invokes rifting of a Middle Jurassic arc and arc basement terrane to form a paired, Late Jurassic inter-arc basin ophiolite and volcanic arc. Radiometric ages determined from rocks in the map area and from arc plutons exposed to the east, in combination with a wealth of preexisting geochronometric data, indicate that the Klamath Mountains province experienced a continuum of arc-related magmatism from ~175-135 Ma, distinct episodes of thrusting at ~165 and ~155 Ma, and a prolonged episode of ophiolite genesis and extension from ~170- 160 Ma. These features characterize the province as a batholithic belt constructed in an entirely oceanic setting and represent an archetypal example of a long-lived oceanic arc system generated at a loosely coupled ocean/ocean convergent plate boundary