THE LATE MIOCENE DIXIE MOUNTAIN VOLCANIC CENTER, NORTHERN SIERRA NEVADA: ERODED STRATOVOLCANO OR LACCOLITHIC INTRUSION?

Based on previous mapping at a scale of 1:62,500, Grose (2000) interpreted the Dixie Mountain area of the northern Sierra Nevada as the eroded remnants of a stratovolcano that was originally the size of Mount Shasta, using the presence of interstratified volcaniclastic and lava flows that dip radially away from a core of 10 Ma hypabyssal intrusions. Our new mapping at 1:12,000 confirms most of these observations, but also documents other features that are not consistent with the eroded stratvolcano interpretation. We have subdivided the ~400-800 m thick stratigraphic section into mappable units. Our detailed mapping confirms that bedding dips very systematically away from the intrusive core; it also shows that dikes that intrude the stratigraphic section are radial. Facies mapping and analysis of the stratigraphic section shows that it is composed of >85% polylithic volcanic debris flow deposits, with <15% primary block-and-ash-flow tuffs of restricted lateral extent that were produced by localized lava dome/flow collapse. All the volcanic debris flow deposits and block-and-ash-flow tuffs are coarse-grained, with abundant angular blocks, and massive to very thick-bedded. Features that are typical of a stratovolcano but absent from the Dixie Mountain center include a nearly total lack of lava flows, except for a ~200 m thick, locally columnar-jointed sill complex with minor flows, which is restricted to the southern sector at the top of the section. There are also no products of explosive eruptions, such as pumiceous pyroclastic flow, surge or fall deposits.

On the basis of these observations, we present an alternative hypothesis for the origin of the Dixie Mountain Center. We infer that the volcanic debris flows and block-and-ash-flow tuffs were ponded in a basin, which we speculate was produced by Late Miocene faulting in the northern Walker Lane. The section shows an upward increase in primary volcanic rocks (block-and-ash-flow tuffs and the sill/ lava flow complex), which we interpret to record the first stages of magma emplacement. In the later stages of magmatism, the section was lifted by the force of the intrusions, in the manner of a laccolithic intrusion, although the erosional level is not deep enough to determine whether or not the intrusive complex is flat-floored.