HR: 0800h
AN: H31A-1265    [Abstracts]
TI: Quantifying the Forcing Factors Responsible for the Tectono-Geomorphological Evolution of Neogene Rift Basins, Baja California
AU: * El-Sobky, H F
EM: elsobky@geo.tamu.edu
AF: Texas A&M University, MS 3115 TAMU, Dept of Geology & Geophysics, College Station, TX 77843-3115 United States
AU: Dorobek, S L
EM: dorobek@geo.tamu.edu
AF: Texas A&M University, MS 3115 TAMU, Dept of Geology & Geophysics, College Station, TX 77843-3115 United States
AB: The Gulf of California and its surrounding land areas provide a classic example of recently rifted continental lithosphere, where back-arc stretching of a continental volcanic arc has culminated in the ongoing seafloor spreading that characterizes the present-day axis of the gulf. The recent tectonic history of eastern Baja California, which includes most of the land area eastward of the main drainage divide that extends north-south along the length of the peninsula, has been dominated by oblique rifting that began at about 5 Ma. Thus, extensional tectonics, bedrock lithology, long-term climatic changes, and evolving surface processes have controlled the tectono-geomorphological evolution of the eastern part of the peninsula since 5 Ma. No previous studies, however, examined the effect of these combined factors on the current tectono-geomorphological characteristics of eastern Baja California. We assume that although long-term climate may have changed along the peninsula over the last several million years, precipitation amounts are likely to have changed in a similar way along the entire length of the peninsula, regardless of the long-term climatic trend. This suggests that climatic variation can be largely ruled out as an explanation for the geomorphologic variability between basins. In an attempt to quantify the factors that affected the geomorphologic development along the eastern side of Baja California, thirty-four drainage basins were extracted from a 15-m-resolution absolute digital elevation model (DEM). The stacked-vector method was applied to utilize the different terrain attributes (e.g., hillshaded relief, aspect, slope, etc.) for supervised classification of bedrock lithologies using object-oriented techniques. Stream-length gradient indices were then measured for the main stream in each of the basins. Bedrock lithologies and alluvium were plotted along the stream profiles to identify any relationship between lithology, structure, and stream gradient. Our results indicate that drainage basins in the eastern rift province of Baja California can be classified according to the dominant geomorphologic controlling factors (i.e., lithology-controlled, fault-controlled, or hybrid basins). Using slope-area analysis, steepness and concavity indices were extracted for bedrock channels within the thirty-four drainage basins. A reference concavity of 0.4 was used to allow a direct comparison of steepness indices for the thirty-four basins. The results are highly correlated with stream length-gradient indices for each basin. Nine basins exhibit steepness index values greater than 0.07, which indicates the strong tectonic signature and higher uplift rates in these basins.
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 1815 Erosion
DE: 1821 Floods
DE: 1824 Geomorphology: general (1625)
DE: 8122 Dynamics: gravity and tectonics
SC: Hydrology [H]
MN: Fall Meeting 2005