EVOLUTION AND SUBDUCTION POLARITY OF THE MID-PALEOZOIC KLAMATH-SIERRA VOLCANIC ARC

Upper Silurian-Lower Mississippian arc related Klamath-Sierra magmatic rocks have been an integral part of Cordilleran models for the Early Mississippian Antler orogeny. Although debated for some thirty years, disagreements still persist over the polarity of the Klamath-Sierra arc. One set of models call upon a west-facing (east-dipping) subduction zone and collapse of a back arc basin. Other models call upon an east-facing (west-dipping) subduction zone that involved either arc collision or a Mediterranean-style slab rollback. Recent studies in Western Pacific subduction systems, in particular the Eocene-Recent Izu-Bonin-Marianas and the Quaternary New Britian systems, provide actualistic models for evaluating Klamath-Sierra arc polarity. In these modern systems, across arc variations in basalt compositions can be correlated with subduction polarity. New and previously published field, petrographic, geochronologic and geochemical data for Klamath-Sierra arc rocks are used to evaluate the evolution and polarity of the mid-Paleozoic subduction zone. The Eastern Klamath Trinity-Redding ophiolite records initial Late Silurian-Early Devonian (431-398 Ma) supra-subduction extension and magmatism. Trinity-Redding mafic rocks are consistent with partial melting of a depleted, residual NMORB mantle (Nb/Yb 0.08-0.5) enriched by subduction-derived fluids (Th/Yb 0.02-0.4) and compare with modern forearc rocks. A trench-proximal position is indicated by Devonian Yreka subterrane melange and turbidites deposited on Trinity basement. The Northern Sierra Shoo Fly complex correlates with Trinity-Yreka rocks and is overlain by Middle Devonian-Lower Mississippian (~378-368 Ma) subduction-related volcanic rocks. Northern Sierra basaltic rocks are consistent with partial melting of more enriched (~EMORB) mantle (Nb/Yb 1.5-3) enriched by subduction-derived fluids (Th/Yb 0.7-4) and compare with modern arc rocks ~200 km inboard of the trench. Thus petrologic and geochronologic constraints favor mid-Paleozoic development of a west-facing (east-dipping) Klamath-Sierra subduction zone. If the Klamath-Sierra arc continued to follow the development of the Cenozoic IBM system, back arc rifting would have commenced in Early Mississippian time and may have provided a trigger for the Antler orogeny.