ANALYSIS OF AN INTRACANYON LAVA FLOW IN THE CENTRAL SIERRA NEVADA AS EVIDENCE FOR NEOGENE TILTING AND UPLIFT

The Table Mountain Latite, part of the Stanislaus Group, erupted near the Sierran crest around Sonora Peak and the Little Walker Caldera in a high-K pulse of volcanism from about 10.4-9.3 Ma that interrupted ordinary andesitic arc volcanism. The Table Mountain lava filled Miocene drainage channels that with subsequent erosion produced extensive inverted topography. One portion of the Table Mountain Latite near Columbia, CA has not suffered this inversion and thus demonstrably represents an “instantaneous” snapshot of the Stanislaus River still within its paleochannel. Here we present a study that reexamines this site and demonstrate that the current paleodownstream direction of this portion of the Table Mountain Latite is inconsistent with the slope of its upper surface in some places. The present Table Mountain Latite surface slopes westwards and/or southwards, but some reaches in this section flowed eastwards and/or northwards. Either the Sierra Nevada have not tilted since emplacement and lava flowed uphill, or the lava flowed downhill and subsequent tilt of the Sierra Nevada has modified the slope. Ad hoc geomorphic explanations of this feature have been proposed, but a simpler explanation is that the Sierra Nevada tilted southward in the Neogene. To test this, using recent LiDAR-derived DEMs in a GIS environment we extracted the data needed to plot at high precision the top surface slope against the azimuth. This shows a sinusoidal relationship with the largest slopes pointing southwards and negative slopes pointing north and east. When we backtilt the data for Sierran tilt, the sinusoid disappears yielding a stream gradient that is similar to pre-tilt gradients proposed elsewhere. Post-emplacement erosion has modified the surface somewhat, increasing data dispersion, but the conclusion remains robust.