Abstract
The Central Belt of the Sierra Nevada in California consists of deformed ophiolitic, island arc, and subduction complex rocks of Triassic-Jurassic age. Near the town of Auburn, California, Central Belt rocks consisting of ultramafics, metavolcanics, metasediments, and metagabbros are exposed at the Auburn Damsite along the American River. These rocks have been termed the Auburn Mélange due to the map pattern of interleaved strips of different lithologies. Previous work has proposed that the Auburn Mélange was formed by deformation within the Bear Mountain Fault Zone (BMFZ), a major structure that bounds the western edge of the Central Belt. However, the mechanism of mélange formation, and the extent to which sedimentary and tectonic processes may have contributed, has not been fully investigated. This thesis uses outcrop-scale field mapping, thin-section petrography, and U-Pb geochronology to investigate timing and nature of mélange formation. Particular focus was on the nature of contacts between units to investigate whether the Auburn Mélange contains original depositional contacts or whether contacts indicate fault contacts between units, as well as the age of a deformed metagabbro. Field observations indicate that the map-scale mélange map pattern of rocks in the Auburn Mélange is primarily due to imbricate faulting of ocean-plate basalt-sedimentary packages that occurred between 168 and 161 Ma. A coarsening-upward sedimentary succession from cherts and black shales to sandstones and volcanogenic conglomerate can be recognized. Ultramafic slivers were incorporated between fault imbricates however it is not clear whether these are detached slices of ultramafic rock, or ultramafic oceanic basement linked to overlying basalts. The faults between imbricate slices are then cut by the 161 Ma Oregon Bar Pluton and several highly deformed gabbroic intrusions. A new ca. 150 Ma U-Pb zircon age from the metagabbro constrains the age of late ductile deformation and is synchronous with activity on the BMFZ to the south at the Guadalupe Igneous Complex at about 151 Ma.