Landscape response in the California Coast Ranges to the stormy conditions of the Pleistocene to Holocene climatic transition

Introduction: temporal scales, spatial scales, climate, and tectonics

A long-standing problem in geomorphology is identifying the relative roles of climate and tectonism on landscape evolution. Here we examine the effect of the stormy climate of the time period between approximately 15,000 and 10,000 years ago ("15 to 10 ka"), known as the Pleistocene to Holocene climatic transition, on streams and hillslopes in Pancho Rico Valley. Pancho Rico Valley is an east-west trending stream valley within the area of modestly high topography known as the Gabilan Mesa (Figure 1). The Gabilan Mesa is a distinctive physiographic feature that is within the central Coast Ranges of California, and is adjacent to the San Andreas fault zone (Figure 1).

A useful framework for understanding the relative influences of climate and tectonics on landscape evolution is to consider the temporal and spatial scales over which they influence geomorphic processes (Harvey, 2006). In many localities, the predominant influence of tectonics on landscapes is the creation of fundamental topographic relief over relatively long timescales (100s of thousands to millions of years; Harvey, 2006). In the central California Coast Ranges, tectonism over the last 400,000 years ("400 ky"), has resulted in down to the southwest tilting, with little folding or faulting, of the 110 km long and 25 km wide structural block that forms the plateau-like Gabilan Mesa (Page et al, 1998). On a regional spatial scale (>1000 km²), the effect of tectonism over the last 400 ky is the formation of the distinctive topography of the Gabilan Mesa, which consists of relatively uniform, low altitude, accordant ridges that slope exclusively toward southwest, and west- as well as southwest-trending stream valleys (Dohrenwend, 1978; Page et al, 1998; Figure 1). The influence of tectonics within most of the west- and southwest-trending stream valleys of the Gabilan Mesa (Figures 1) is subtle or undetectable (Dohrenwend, 1978). In contrast, the effect of climate on the geomorphology of Pancho Rico Valley since approximately 15 ka is profound. Climate influences landscape development in places like Pancho Rico Valley because: (1) climatic factors affect hillslope processes (eg. Reneau et al, 1990); and (2) climate directly controls stream discharge and strongly influences the magnitudes of sediment loads delivered to stream networks (Bull, 1979). The ratio of stream discharge to sediment load is a factor that influences landform development because, for example, within a stream channel having a given slope, a decrease in discharge or an increase in sediment load will cause sediment deposition, which ultimately leads to the development of landforms such as stream terraces or alluvial-fan lobes (Bull, 1979; Figure 2).

The impact of the stormy climate of the Pleistocene to Holocene transition

The potent storms of the Pleistocene to Holocene climatic transition caused many debris flows in Pancho Rico Valley. Garcia and Mahan (2009) showed that the debris flows stripped Pancho Rico Valley hillslopes of sediment, and scoured the bottoms of the uppermost parts of stream drainages (referred to herein as "low-order drainages"). The debris flows produced relatively broad scour scars (Stock and Dietrich, 2003) at the bottoms of most low-order drainages in Pancho Rico Valley (Figure 3; a discussion of the morphometric signature of debris-flow erosion, which Stock and Dietrich (2006) identified, is presented in Figure 4 and its caption). The sediment scoured by debris flows from the hillslopes and bottoms of low-order drainages exceeded the carrying capacity of Pancho Rico Creek, and the Pancho Rico Creek channel was buried by debris flow deposits that are up to 20 m thick (Garcia and Mahan, 2009).

The responses of Pancho Rico Valley hillslopes and Pancho Rico Creek to the Pleistocene to Holocene climatic transition illustrate that influence of climate on landscape evolution and process can be both profound and complex. Firstly, the most conspicuous, local effect of the stormy Pleistocene to Holocene transition climate on the geomorphology of Pancho Rico Creek was that the convexity of interfluves were accentuated, and a low-relief valley floor was constructed (Figures 2 and 3). From a sedimentological perspective, the local effect of the Pleistocene to Holocene transition was significant sediment transport from Pancho Rico Creek hillslopes and low-order drainages to the bottom of Pancho Rico Valley (Figure 4). However, because the carrying capacity of Pancho Rico Creek was exceeded, sediment stripped from Pancho Rico Creek hillslopes and low order drainage bottoms remained within Pancho Rico Valley, and did not enter the larger Salinas River system during the Pleistocene to Holocene climatic transition (Garcia and Mahan, 2009). Incision by Pancho Rico Creek into the debris-flow deposits occurred some time after the Pleistocene to Holocene transition, likely as a consequence of stream capture (Figure 1) and the associated increase in stream discharge, but most of the sediment stripped from hillslopes remains stored in the prominent, constructional alluvial landform that dominates lower Pancho Rico Creek (Figure 2; Garcia and Mahan, 2009). Surprisingly, there are no landforms in the Salinas Valley that record the significant hillslope erosion that occurred in Pancho Rico Creek during the Pleistocene to Holocene transition climatic event (Garcia and Mahan, 2009). Lastly, although the Gabilan Mesa is adjacent to the San Andreas Fault zone, climate-driven processes more strongly influence landform evolution than tectonically-driven processes over thousand to 10 ky timescales in the west- and southwest-trending valleys of the Gabilan Mesa.

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