A LAU BASIN MODEL FOR THE FORMATION OF THE JOSEPHINE OPHIOLITE AND RELATED RIFT FACIES, KLAMATH MOUNTAINS, OREGON-CALIFORNIA

The 162-164 Ma Josephine ophiolite (JO) and overlying Galice Fm. are part of an east-dipping thrust sheet extending beneath a 179-159 Ma arc (episodes 1-3 of Barnes & Snoke, this session) and its ~200 Ma disrupted ophiolitic Rattlesnake Creek Terrane basement (RCT). Locally the RCT is intruded by 164 Ma diabase dikes and overlain by diabase breccia and rare siliceous argillite interpreted by Saleeby et al. (1982, JGR) as a rift facies related to the JO. The main body of the JO grades northward into a similar rift facies that is overlain by the volcanic Rogue Fm. (Yule, 1996, Caltech Ph.D.), a predominantly volcaniclastic sequence which, like the JO, is overlain by the Galice Fm. The southernmost occurrence of the JO is also a 164 Ma rift facies with RCT-like basement (Wyld and Wright, 1988, GSA Bull). Structurally beneath the JO and the Rogue Fm. are the plutonic roots of the Rogue arc, the ~161-157 Ma Chetco Complex (episode 4 of Barnes & Snoke, this session). Originally, the JO was interpreted to have formed by back-arc spreading following rifting of the 179-159 Ma west-facing arc, but current high-resolution ages show that formation of the JO overlapped the 179-159 Ma “remnant arc” and is older than Rouge-Chetco “coeval arc.” It appears instead that Josephine rifting and at least initial seafloor spreading occurred in the forearc. Alternatively, rifting and spreading may have taken place in pull-apart basins along an intra-arc transform as suggested in previous models. Geochemistry of lavas and sheeted dikes of the JO shows a remarkably wide range of magma types including high-Ca boninite, transitional island-arc tholeiite/mid-ocean ridge basalt (IAT/MORB), and late MORB. The latter includes unusual highly fractionated Fe- and Ti-rich basalts, such as those that occur at modern propagating spreading centers. The Lau Basin is a possible modern analog for the JO and related rift facies. It originated by forearc rifting but after ~4 m.y. became a back-arc basin when arc volcanism jumped to its current position; this was followed by the onset of seafloor-spreading in the basin. The Lau Basin has two propagating spreading centers, one of which is erupting Fe-Ti basalts and the other--located only 40 km from the arc--erupts highly fractionated transitional IAT-MORB lavas and is flanked on either side by seamounts with boninites.