HR: 1340h
AN: T23C-0575    [Abstracts]
TI: From Topography to Tectonics: Coupling geomorphic and geodynamic analyses to assess potential blind thrusts in the Marin County region
AU: * Johnson, C B
EM: cjohnson@geosc.psu.edu
AF: Pennsylvania State University, 542 Deike Building, University Park, PA 16802
AU: Furlong, K P
EM: kevin@geodyn.psu.edu
AF: Pennsylvania State University, 542 Deike Building, University Park, PA 16802
AU: Kirby, E
EM: ekirby@geosc.psu.edu
AF: Pennsylvania State University, 542 Deike Building, University Park, PA 16802
AB: To test the theory of an active blind thrust underlying the Marin County - Mt. Tamalpais region (Johnson, C., et al., AGU abstracts fall 2004), we have further examined different geomorphic, geodynamic, and geophysical aspects of the regional system. Previous work focused on channel steepness indices for channels along Bolinas Ridge, showing a systematic change with position along the ridge. Here we present new geomorphic analyses to extend this work. Distribution of hillslope gradients in watersheds along the ridge show distinct regions of near-threshold gradients (~35°) adjacent to channel floors. Relief within these `inner gorges' increases systematically from north to south, and may reflect a relatively recent change in incision rate. We use a range of empirical relationships between channel steepness index and erosion rate to estimate potential spatial variations in incision rate. We couple these analyses with elastic displacement modeling of potential fault geometries and slip rates. The resulting displacement curves, which now include erosional effects, places important constraints on the location and geometry of the underlying fault structure. The coupled blind thrust analysis using both deformational modeling and geomorphic constraints on erosion provides significant improvements over either approach alone. The sensitivity of surface displacement patterns produced by the deformational modeling has also been explored and shows that small differences in fault tip depth, fault length, and orientation result in relatively small changes in uplift pattern. In addition, we are also able to constrain the lateral extent of the blind fault by using a structure that has been imaged directly beneath Pt. San Pedro in the northern San Francisco Bay by the precise relocation of the local seismicity using double differencing techniques. We interpret this structure as marking the eastern limit for the blind thrust model.
DE: 8100 TECTONOPHYSICS
DE: 8175 Tectonics and landscape evolution
SC: Tectonophysics [T]
MN: Fall Meeting 2005