ONE VERSUS TWO LATE CENOZOIC UPLIFT EVENTS, SIERRA NEVADA, CALIFORNIA, RECORDED IN DRAINAGE GEOMORPHOLOGY

Long known for differences in topography and volcanic cover between northern and southern regions, the topography of the Sierra Nevada of California also appears to reflect a similar temporal and spatial variation of tectonic history. For example, the landscape of the San Joaquin River drainage forms topographic highs of basement exposures positioned above projected ~10 Ma paleothalwegs with latest Miocene-Pliocene volcanic remnants inset below the ~10 Ma paleo datum, whereas the northernmost Sierra topographic highs are predominantly volcanic draped peaks and interfluves capped by Mio-Pliocene andesites. Positioned at the transition between these disparate regions, the drainages of the Tuolumne and San Joaquin rivers display geomorphic characteristics of their respective regions to the north and south.

As stream incision outpaces erosion rates in a drainage system due to a lowering of base-level, incised stream-channel walls form a steeper gradient than channel walls above the pre-incision stream position. These gradient changes or ‘breaks-in-slope’ are preserved in the interfluve crests of tributary reaches oriented at high angles to main stream courses. The San Joaquin River drainage displays two breaks in slope. The higher one coincides with the 10 Ma paleothalweg of Huber (1981). New Ar/Ar ages, paleomagnetism and field-relationships show that ~3.6 Ma volcanics are inset on basement benches (straths) on the rim of the lower break in slope. Thus the first incision event took place between 10 and 3.6 Ma and the second after 3.6 Ma. The Kings River drainage and other river drainages to the south exhibit two breaks in slope in their canyon walls, and two knickpoints in some streams. In contrast, the Tuolumne River drainage has a single break in slope. The geomorphology suggests two periods of late Cenozoic incision and inferred uplift from the San Joaquin drainage southward, versus one for the Sierra Nevada from the Tuolumne River drainage northward. Thermochronologic data suggests that the first period of stream incision began at about 20 Ma in the Kings to Kern River drainage, much earlier than in the San Joaquin River drainage, whereas the second period of incision appears approximately coeval (ca. 3.5 Ma) from the San Joaquin River southward.