HR: 0800h
AN: H41D-0760    [Abstracts]
TI: Channel Morphology Response to Differential Rock Uplift Rates in the San Gabriel Mountains, CA
AU: * DiBiase, R A
EM: Roman.DiBiase@asu.edu
AF: Arizona State University, School of Earth and Space Exploration, Tempe, AZ 85287, United States
AU: Whipple, K X
EM: kxw@asu.edu
AF: Arizona State University, School of Earth and Space Exploration, Tempe, AZ 85287, United States
AU: Heimsath, A M
EM: Arjun.Heimsath@asu.edu
AF: Arizona State University, School of Earth and Space Exploration, Tempe, AZ 85287, United States
AB: The San Gabriel Mountains in southern California provide an excellent opportunity to study landscape response to both spatial and temporal rock uplift patterns. Both published low temperature thermochronometry data and our preliminary cosmogenic erosion rate data show a strong gradient in uplift rates from west to east, ranging from <0.1 mm/yr to >1.0 mm/yr respectively. Cosmogenic ¹⁰Be in stream sands from 13 basins have already been measured, and 50 more are currently in progress at Purdue University's PRIME Lab. These erosion rates will be a major focus of this study, revealing in detail the nature of the relationship between uplift rates and landscape morphology, including the channel steepness index. These erosion rates also allow for analysis of correlations between rock uplift rate and channel morphology (e.g. width, bed state). Detailed field surveys in summer 2007 conducted with a laser rangefinder provide georeferenced data on channel geometry, bedrock exposure, median and maximum grain size, and valley width for ~25 km of 7 channels, spanning millennial-scale erosion rates from 0.05 to 0.4 mm/yr. Preliminary observations focus mainly on channel response across knickpoints recording landscape response to a temporal increase in rock uplift rate, and detail the partitioning of bedrock and alluvial reaches in over-steepened zones. Much of the relief across knickpoints is expressed in waterfalls and steep bedrock reaches that interrupt alluvial reaches (likely transport-limited). In addition, though channel width appears to decrease with increasing channel steepness index, there is a tendency for over-narrowing in knickzones that is not matched in adjusted high uplift zones. Indeed, knickpoints, especially the tectonically induced knickpoints along the Big Tujunga, consistently produced steep, narrow, inner gorges that are uncommon elsewhere in the landscape, suggestive of a sediment-flux control on channel width. More field data is needed to answer the question of whether this over-narrowing is a transient condition or if it is simply the result of increasing rates of incision.
DE: 1824 Geomorphology: general (1625)
DE: 1825 Geomorphology: fluvial (1625)
DE: 8175 Tectonics and landscape evolution
DE: 9350 North America
SC: Hydrology [H]
MN: 2007 Fall Meeting