FIELD CHARACTERIZATION OF TECTONIC AND SEDIMENTARY RELATIONSHIPS BETWEEN UNITS IN THE AUBURN MÉLANGE HIGH-STRAIN ZONE

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. Using outcrop-scale field mapping, thin-section petrography, and U-Pb geochronology, several conclusions can be made about the Auburn Mélange. Field observations indicate that the map-scale 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. 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. The Auburn Mélange reflects imbrication of ocean plate stratigraphy followed by intrusion of Oregon Bar Pluton and gabbroic bodies. These lithologies were then deformed by deformation associated with the BMFZ.