HR: 1340h
AN: T53A-0473    [Abstracts]
TI: Assemblage of Foreland Mineral Populations: Insights From Basin-Scale Mixing of U-Pb Zircon Ages
AU: Amidon, W H
EM: willyamidon@hotmail.com
AF: Univeristy of California Santa Barbara, Department of Geological Sciences, Santa Barbara, CA 93101 United States
AU: Burbank, D W
EM: burbank@crustal.ucsb.edu
AF: Univeristy of California Santa Barbara, Department of Geological Sciences, Santa Barbara, CA 93101 United States
AU: * Gehrels, G E
EM: ggehrels@geo.arizona.edu
AF: Univeristy of Arizona, Department of Geosciences, Tucson, AZ 85721 United States
AB: Fission-track and Ar-Ar dating of detrital minerals such as zircon and muscovite in foreland sediments provide estimates of exhumation rate during different periods of orogen growth. Interpretations of foreland cooling ages often assume that a population of foreland minerals provides a representative sample of the entire orogen. In reality, different mineral concentrations and erosion rates in source lithologies can have a profound effect on the proportion of any given mineral derived from each source region. Where the provenance of mineral cooling ages is often ambiguous, U-Pb ages can provide a lithology-specific fingerprint with which to trace sediment through the fluvial system because U-Pb ages are rarely "reset" in zircon. To better understand the construction of the foreland mineral population, we use mixing of U-Pb zircon ages in modern river sediment to evaluate transport and mixing of zircons in orogen-scale drainages. U-Pb ages were determined from zircons in modern river sands from the Narayani watershed in central Nepal. Tributary samples define the age contribution from each major lithology, whereas samples from main-stem rivers evaluate the relative age contribution from each lithology. The proportion of zircon ages derived from each lithology is determined by least-squares inversion as well as the size and position of component Guassian peaks in the downstream age distributions. The proportion of zircons derived from each lithology is then compared to proportions predicted by combining the exposed area, zircon concentration, and erosion rate of each lithology. Zircon concentrations were measured in bedrock and river sands by grain counting, and modern sediment flux is used as a proxy for erosion rate. Preliminary results suggest that foreland mineral populations are strongly controlled by spatial variations in the mineral concentration and erosion rate of the source region. Even if they are rapidly eroding, regions of low zircon concentration can be underrepresented in the foreland; thus, foreland cooling ages may not representatively record the exhumational history of the entire orogen.
DE: 8102 Continental contractional orogenic belts
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting