HR: 08:45h
AN: T31E-04 INVITED    [Abstracts]
TI: Stable Isotopic Constraints on the Cenozoic Topographic Evolution of the Sierra Nevada
AU: * Chamberlain, P
EM: chamb@stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, United States
AU: Mulch, A
EM: mulch@geowi.uni-hannover.de
AF: Institut fur Geologie, Universitat Hannover, Hannover, 30167, Germany
AB: The Sierra Nevada with mean elevations of 2500 in the north and 3000 m in the south produces a modern rain shadow as a result of the interception of moisture sourced in the Pacific. This rain shadow is reflected in a gradient in the oxygen and hydrogen isotopic composition of precipitation across the mountain range, with a west to east difference in δD of ~40 per mil and δ18O of ~5 per mil. The persistence of this isotopic gradient in the past allows us to place constraints on the surface elevation history of the Sierra Nevada. With the aim of reconstructing the rain shadow development over time, we undertook three different stable isotopic paleoaltimetry studies of the Sierra Nevada. These studies involved: 1) reconstructing the isotopic rain shadow on the east-side of the Sierra using the oxygen isotopic composition of smectite from weathered ashes and calcite from paleosols from Miocene to Recent sediments (Poage and Chamberlain, 2002; Tectonics); 2) determining the hydrogen isotopic composition of kaolinite from weathered Eocene stream deposits along an elevation gradient on the west-(windward-)side (Mulch et al., 2006; Science); and 3) examining the hydrogen isotopic compositions of hydrated volcanic glasses from Miocene to Recent ashes found on both the east- and west-side of the Sierra Nevada (Mulch et al., in review). All of these studies give consistent results and indicate that the Sierra Nevada has existed as an orographic barrier since the mid-Miocene and in the north may date back into the Eocene with no significant surface uplift (>1 km) since then. Taken together, these results suggest that the Sierra Nevada formed the edge of a pre-Eocene continental plateau and the current mountain range proper was formed by tilting and down-drop along its eastern edge by later Basin and Range faulting and maybe already earlier during the incipient demise of the plateau. Our results do not support models calling for greater than 1 km of surface uplift in the Pliocene as a result of removal of dense mantle lithosphere. The fact that different methods and models for reconstructing surface elevation histories (e.g. geomorphic analysis, thermochronology, cosmogenic radionuclides, stable isotope paleoaltimetry, geodynamic modeling, tilting and sedimentation analysis) give conflicting conclusions with regard to the elevation history of the Sierra Nevada is disconcerting, particularly since these methods have been applied in much more detail here rather than anywhere else on Earth. It is critical, therefore, for the scientific community to fully understand what these methods are telling us about surface elevation histories and perhaps the Sierra Nevada offers the ideal area to compare and reconcile these different approaches.
DE: 0473 Paleoclimatology and paleoceanography (3344, 4900)
DE: 8175 Tectonics and landscape evolution
DE: 8177 Tectonics and climatic interactions
DE: 9604 Cenozoic
SC: Tectonophysics [T]
MN: 2007 Fall Meeting