Graduate Thesis Or Dissertation
 

Spatial and Temporal Landscape Responses to Progressive Tectonic Forcing along California Fault Systems

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  • Understanding the degree to which topography of erosional landscapes in active mountain belts encode the rates and patterns of active deformation in the upper crust is a primary goal in the field of tectonic geomorphology. In particular, the convolved influence of variations in rock mass quality and the erodibility of landscapes, climate and the efficiency of transport processes, and differential rock uplift in space and time makes extracting quantitative histories of deformation from topography challenging. In this dissertation, I examine the association among channel and hillslope morphology, erosion rate, and sediment delivery in two mountain ranges developed along active fault systems. Each field site experiences relatively uniform climate, and the channel systems in each range are developed within relatively monolithologic substrate. These conditions help isolate the influence of tectonic forcing on landscape topography, and I combine quantitative analysis of channel and hillslope morphology with estimates of erosion rate to explore how each landscape adjusts to spatial and temporal differences in erosion rates driven by differential uplift of rock. In the first field site, a series of first-order watersheds draining the western flank of Bolinas Ridge in the Coast Range of central California exhibit systematic spatial differences in topographic relief, channel steepness, and ridgecrest curvature atop interfluves. Grain size distributions of channel bed sediment co-vary with channel steepness; median grain size fractions are coarser and distributions exhibit wider variance in steeper watersheds. These observations imply that transport thresholds likely co-vary with rock uplift and erosion rate. I employ a stochastic threshold incision stream-power model driven with runoff variability calibrated against discharge distributions in nearby watersheds to explore the implications of variations in grain size for channel steepness. Analysis of decadal records of daily discharge along nearby rivers suggests that frequency-magnitude relationships are best fit by a stretched exponential distribution. When combined with the stochastic threshold river incision model, these discharge distributions predict a non-linear relationship between steady-state channel steepness and erosion rate, similar to findings in other tectonically active mountain ranges. However, the co-variance of grain size with erosion rate leads to systematic differences in the transport threshold, which in turn implies that the effective scaling between erosion rate and channel steepness is close to linear. Thus, I conclude that the spatial pattern of channel steepness along Bolinas Ridge is a reasonable approximation of the spatial pattern of differential rock uplift. In the second field site, I explore the response of channels to a temporal increase in relative rock uplift rate. Along the Inyo Mountains, in eastern California, steep watersheds are developed in the footwall of the Saline Valley fault system. Dextral, oblique normal slip along this fault leads to uplift of the footwall, and the presence of knickpoints at relatively uniform elevations implies that channel profiles record temporal acceleration in transient channel incision and rock uplift driven by fault throw. Recent studies to invert profile shape for the history of fault throw suggest an acceleration in the past 1-3 Ma. Determinations of erosion rate above and below knickpoints using cosmogenic 10Be in modern sediment document a non-linear scaling relationship between channel adjustment and erosion rate. This is interpreted to reflect the influence of a threshold for incision. Analysis of response timescale that includes measured erosion rate suggests a relatively recent increase in fault throw rate, in the past 0.5-1 Ma. These results suggest caution when using a linear model to invert channel profile shape for uplift history. Overall, my integrated results highlight the use of topographic adjustment in actively channel-incising landscapes as an efficient reconnaissance tool to describe the dynamics of active deformation over space and time.
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