NORTHERN SIERRA NEVADA PALEOZOIC-MESOZOIC TECTONIC EVOLUTION RESPECULATED (Invited Presentation)

Metamorphic rocks of the northern Sierra Nevada lack continental basement slivers and comprise upper plate (formed above subduction zone) arc and related rocks and volumetrically subordinate subduction complexes tectonically accreted from subducting plate. Original low-angle primary tectonic contacts have been tightly folded, so that their local orientation is steep. Although this region did not experience continental collision, orogenic components have undergone severe structural attenuation from strike-slip excision, subduction erosion, and extension. This lithologic assemblage records initiation and termination of multiple intra-oceanic subduction zones. The earliest records of subduction are the imbricated upper plate rocks of the Sierra City mélange (SCM) of the Shoo Fly Complex (SF), whose coherent mafic components have an arc geochemical affinity. The SCM formed and assembled from ca. 420 to 372 Ma, then was tectonically stacked westward over lower allochthons of the SF, that lack immature igneous components, at about 370 Ma. Island arc rocks were deposited atop the eroded surface of the imbricated SCM from about 350-370 Ma and coeval plutons intruded the SF. Apparent metamorphic sole rocks structurally beneath the Feather River ultramafic belt, west of the SF, record at least two apparent high-temperature metamorphic subduction initiation events at ca. 350 and 255 Ma. The younger event may correlate to regional initiation of continental margin arc activity east of the Sierra whereas the older event lacks a corresponding arc. Structurally beneath the sole, the blueschist facies Red Ant schist may record continuation of the east-dipping subduction event that began at ca. 255 Ma. West in the foothills region, a younger metamorphic sole beneath ultramafic-mafic rock complexes yields dates of high-temperature metamorphism from ca. 220 Ma to 185 Ma attributed to west-dipping subduction. The earlier date may reflect subduction initiation whereas the latter may have resulted from ridge subduction and/or subduction termination. This is the youngest of the subduction interface zones preserved in the Sierra. After or possibly shortly before termination of this subduction episode, east-dipping Franciscan subduction initiated to the west; its accreted products preserved are preserved in the California Coast Ranges.