Faceted spurs at normal fault scarps: Insights from numerical modeling

Abstract

[1] We present a combined surface processes and tectonic model which allows us to determine the climatic and tectonic parameters that control the development of faceted spurs at normal fault scarps. Sensitivity tests to climatic parameter values are performed. For a given precipitation rate, when hillslope diffusion is high and channel bedrock is highly resistant to erosion, the scarp is smooth and undissected. When, instead, the bedrock is easily eroded and diffusion is limited, numerous channels develop and the scarp becomes deeply incised. Between these two end-member states, diffusion and incision compete to produce a range of scarp morphologies, including faceted spurs. The sensitivity tests allow us to determine a dimensionless ratio of erosion, f, for which faceted spurs can develop. This study evidences a strong dependence of facet slope angle on throw rate for throw rates between 0.4 and 0.7 mm/a. Facet height is also shown to be a linear function of fault throw rate. Model performance is tested on the Wasatch Fault, Utah, using topographic, geologic, and seismologic data. A Monte Carlo inversion on the topography of a portion of the Weber segment shows that the 5 Ma long development of this scarp has been dominated by a low effective precipitation rate ($1.1 m/a) and a moderate diffusion coefficient (0.13 m 2 /a). Results demonstrate the ability of our model to estimate normal fault throw rates from the height of triangular facets and to retrieve the average long-term diffusion and incision parameters that prevailed during scarp evolution using an accurate 2-D misfit criterion.

The unusual temporal and spatial slip history of the Wassuk Range normal fault, western Nevada (USA): Implications for seismic hazard and Walker Lane deformation

“…We also use the faceted-spur analysis method developed by Petit et al (2009a;2009b) to provide another estimate of the long-term (>1 Ma) vertical displacement rate for the range-front fault system. These two new slip rate estimates fill in important temporal gaps for the Wassuk Range fault system and permit us to better evaluate strain release over time.…”

“…To provide an estimate of vertical displacement rate along the Wassuk Range normal fault system at the >1 Ma time scale, we applied the results of numerical modeling by Petit et al (2009a;2009b) to faceted spurs along the Wassuk Range normal fault system. Petit et al (2009a) tested the sensitivity of climatic (diffusion coefficient, effective precipitation rate, and characteristic length scale) and tectonic (fault dip angle and slip rate) parameters upon the development of faceted spurs associated with major active normal faults.…”

“…Petit et al (2009a) tested the sensitivity of climatic (diffusion coefficient, effective precipitation rate, and characteristic length scale) and tectonic (fault dip angle and slip rate) parameters upon the development of faceted spurs associated with major active normal faults. Surprisingly, their sensitivity tests suggest that a fault scarp will only bear faceted spurs when diffusion and incision processes balance, with the formation of these features not strongly controlled by climatic conditions (Petit et al, 2009a).…”

“…Petit et al (2009a) tested the sensitivity of climatic (diffusion coefficient, effective precipitation rate, and characteristic length scale) and tectonic (fault dip angle and slip rate) parameters upon the development of faceted spurs associated with major active normal faults. Surprisingly, their sensitivity tests suggest that a fault scarp will only bear faceted spurs when diffusion and incision processes balance, with the formation of these features not strongly controlled by climatic conditions (Petit et al, 2009a). In addition, the fault dip angle does not significantly control facet height or slope angle; these values are instead primarily controlled by the vertical displacement rate (Petit et al, 2009a;2009b), an idea supported by the development of triangular facets on the spurs of the hanging walls of reverse faults (e.g., Bull, 2007).…”

“…In this study, we use new geologic data, terrestrial cosmogenic nuclide (TCN) age data, shallow seismic data, gravity data, and large-scale fault scarp analysis (using the numerical modeling results of Petit et al, 2009a;2009b) to construct a more complete record of vertical displacement rate over 6 temporal windows ranging from 10 6 yr to 10 1 yr.…”