HR: 0800h
AN: T51C-0695    [Abstracts]
TI: Role of Erosion on Closely-Spaced Fault Scarps: High-resolution Laser Scanning Data and Scarp Diffusion Modelling of the Rex Hills Flower Structure, Nevada
AU: * Baran, R
EM: ramona.baran@iaag.geo.uni-muenchen.de
AF: LMU Munich Department of Earth and Environmental Sciences, Luisenstr. 37, Munich, 80333, Germany
AU: Guest, B
EM: b.guest@iaag.geo.uni-muenchen.de
AF: LMU Munich Department of Earth and Environmental Sciences, Luisenstr. 37, Munich, 80333, Germany
AU: Friedrich, A M
EM: friedrich@lmu.de
AF: LMU Munich Department of Earth and Environmental Sciences, Luisenstr. 37, Munich, 80333, Germany
AB: Flower structures are often associated with strike-slip faults. Their subsurface geometry is well known but additional information about their geometry and kinematics is also contained in their surface expression, especially in surfaces exhibiting closely-spaced fault scarps. However, detailed surface descriptions of flower structures are rare. Here, we present for the first time topographic profiles extracted from high-resolution digital elevation model (DEM) data using a ground-based laser scanner (Riegl company) to examine the surface expression of a flower structure in great detail. Our study site, the Rex Hills flower structure (40 m high, ~1 km long), is located on the transpressional left-bend between the Pahrump and Amargosa segments of the dextral Stateline fault system (Guest et al., 2007, GSAB). The southern Rex Hills slope exhibits three reverse fault- scarp generations related to three reverse fault branches, respectively. The basal scarp is the youngest and most continuous one. It exhibits five fault segments with an approximately constant displacement along the scarp. The upper two fault-scarp generations are less continuous, and older. The topography of the southern Rex Hills slope also is characterized by alternating valleys and ridges (each ~100 m long) extending perpendicular to the main ridge crest. Our analysis shows that fault scarp morphology varies laterally along this slope. Scarps exposed on ridge crests are typically more numerous (up to 4-5 scarps) and smaller (on average 5 m high), whereas adjacent valleys often exhibit single large (>10 m high) scarps. The larger valley-scarps did not result from the offset across a single fault branch, but resulted from merging of the three scarps mentioned above by enhanced scarp degradation. The preservation potential of small, individual scarps is therefore better on ridge crests relative to adjacent valleys. Assuming a known age of 2 kyrs (Menges et al., 2003, AGU), we examined the spatial variability of the diffusivity parameter (κ in m²/ka) and the degradation coefficient (τ in m², τ = 2κt) by performing linear diffusion analysis. On a plot of slope-angle vs. scarp-height, κ ranges from 2 - 4.5 m²/ka and τ from 4 - 9 m² for the ridge crests, whereas for the valleys κ ranges from 4.5 - 9 m²/ka and τ from 9 - 18 m². This implies that for the past 2 kyrs erosion is twice as high in valleys than on ridges. This result is consistent with the better scarp preservation on ridge crests discussed above. In order to refine the age of the basal scarp, we selected two basal scarp profiles from a ridge and a valley location, respectively, with similar scarp height but different slope angles. The graphically determined τ of the steeper ridge-scarp is smaller than the τ of the lower angle valley- scarp. Combined with a reasonable κ (1.1 m²/ka for Nevada), we obtained scarp diffusion ages of ~4.3 kyrs for the valley scarp and ~1.4 kyrs for the ridge scarp. The ridge scarp age agrees with the independent estimate of 2 kyrs, whereas the valley scarp estimate is three times too old. Therefore, ridge crest profiles provide more accurate information about closely-spaced fault branches and hence should be used to assess the evolution of flower structures or, more generally, any fault system with closely-spaced scarps. Combining high-resolution surface data analysis with subsurface data should significantly improve the understanding of flower structures in the future.
DE: 8002 Continental neotectonics (8107)
DE: 8010 Fractures and faults
DE: 8040 Remote sensing
DE: 8175 Tectonics and landscape evolution
DE: 8194 Instruments and techniques
SC: Tectonophysics [T]
MN: 2007 Fall Meeting