Denudation rate chronologies and the topographic development of the San Bernardino Mountains, California

How does the topography of mountains develop? What roles do crustal processes such as faulting and surface processes such as erosion play? These questions have long been a focus of geomorphic research. In the past decade, improved techniques for measuring rates of denudation (removal of material leading to a reduction in the relief and elevation of a landscape) have sparked new insights into these topics. Here we present a case study for a landscape astride the San Andreas fault zone that uses denudation rates measured over different time scales to track topographic development. We construct a denudation rate chronology that compares rates averaged over millions of years with rates averaged over thousands to hundreds of thousands to years.

The San Bernardino Mountains

The San Bernardino Mountains in southern California (Fig. 1) is an ideal location for examining how topography develops. This orogen retains remnants of topography formed prior to the beginning of mountain building in the late Miocene about 6 million years ago (Oberlander 1972; Meisling and Weldon, 1989). The relict topography consists of a gently undulating plateau in the north and central portions of the mountains. Much of this low-relief landscape is mantled by deeply weathered granite originally contiguous with a similar granitic weathering mantle found in the lower lying Mojave Desert to the north. The weathering of the granite formed under a more humid climate prior to uplift of the San Bernardino Mountains. The southern third of the range comprises a series of east-west–trending ridges of high, rugged topography separated by major faults located in the intervening valleys. Here the relic landscape has been erased by vigorous erosion. These characteristics make it possible to track topographic development from early block uplift to later slicing up of the landscape by faulting.

Denudation Rates

Denudation rates (reported in units of millimeters per thousand years or mm/ka) were measured in three different ways and over three different timescales in the San Bernardino Mountains. First, by dating and reconstructing the pre-uplift topography, rates of denudation since uplift began can be determined (Spotila et al., 2002). These rates are computed by dividing the amount of incision of the relic topography by the age of uplift initiation. Second, in the southern part of the mountains, thermochronology was used (Spotila et al., 2001). Thermochronology uses radiometric dating of minerals in rocks together with information about the thermal structure of the earth's crust. Each type of mineral has a closure temperature corresponding to the temperature at which the mineral begins to retain the products of radioactive decay. The depth below the earth's surface at which closure begins can be determined from geothermal gradients (rate at which temperature increases with depth). Dividing the closure depth by the radiometric age gives a denudation rate (some geologists term this rate "exhumation" rather than denudation). Thermochronologic denudation rates in the San Bernardino Mountains are also averaged over millions of years. The third method uses terrestrial cosmogenic nuclides. Terrestrial cosmogenic nuclides are generated by cosmic rays penetrating the top few meters of the Earth's surface. The concentration of these nuclides reflects a balance between the rate of production and the rate at which they are lost due to denudation and radioactive decay (Lal, 1991). By measuring the concentration of nuclides in surface rocks, calculating a value for the nuclide production rate at the sampling site, and using known values for radioactive decay, a denudation rate can be determined. As this denudation rate is averaged over the length of time the nuclides have resided in the near-surface, the period over which the rate is applicable is itself a function of the denudation rate, but will typically be on the order of thousands to hundreds of thousands of years. These principles also apply to spatially averaged denudation rates for whole river basins determined by sampling alluvial (stream) sediments (Bierman and Steig, 1996; Granger et al., 1996); this is the procedure used in our study.

Results

Our results (Fig. 2) show that denudation rates measured with all techniques are much lower in the northern part of the San Bernardino ranges where relic topography is preserved than in the southern part. When incision and thermochronologic rates averaged over million year timescales are compared with cosmogenic rates averaged over thousands to hundreds of thousands of years, we see that rates in the south are similar despite the very different averaging periods. This suggests that denudation rates of the narrow, rapidly uplifting ridges in the south have been relatively consistent over the last million years or so. Denudation rates in the north and central parts, on and around the edges of the plateau, are more rapid over shorter timescales, with the increase being more pronounced around the southern margin of the plateau than on the top (Fig. 3). One explanation for this faster, more recent denudation is that the topography of the plateau is evolving, and doing so by the headward retreat of the steep drainage basins around the southern plateau periphery cutting back and gradually removing the plateau and the pre-uplift topography (Binnie et al., 2008).

Evolving topography and threshold topography

Comparing the cosmogenic nuclide-derived denudation rates with average hillslope gradients (measured in angles of the steepness of slopes) of the San Bernardino Mountains illustrates another important result (Fig. 4). A broadly linear relationship exists between denudation rates and hillslope gradients; that is, denudation increases as hillslopes steepen. This relationship decouples when hillslopes are steeper than around 30°, whereupon denudation rates no longer correspond to hillslope angles. This approximately 30° value delineates a change in how slopes erode. When hillslopes are at their threshold angle of stability (or angle of repose), incision at the base of the slopes causes steepening beyond the mechanical strength of the substrate, resulting in landsliding. Sudden removal of material by landslides lowers the hillslope gradient, stabilising the slope until basal down-cutting again causes the slope to exceed its threshold angle. This mechanism has been proposed to explain the decoupling of relationships between hillslope gradients and denudation rates in tectonically active regions such as the margins of the Tibetan plateau and Olympic Mountains, USA (Burbank et al., 1996, Montgomery and Brandon et al., 2002, Ouimet et al., 2009). In the San Bernardino Mountains, the threshold topography is typically found in the south, whereas sub-threshold topography is most common on the plateau (Binnie et al., 2007).

Sequence of landscape evolution

The survival of pre-uplift topography on the plateau, coupled with the denudation rate chronologies, suggests a sequence for the landscape evolution of the San Bernardino Mountains. The left hand side of the denudation rate-hillslope gradient plot (Fig. 4) shows data mostly from the top of the plateau, where pre-uplift surfaces indicate that topography has been relatively unaffected by orogenesis. The right hand side of the plot contains data mostly from the southern parts of the mountains, where hillslopes are at threshold. Different parts of the San Bernardino Mountains are, therefore, experiencing different stages of the topographic response to orogenesis. As mountains begin to develop, their slopes steepen, denudation rates increase and, if crustal uplift is rapid enough, topography achieves threshold hillslopes. This notion of how threshold topography develops may be applicable to other ranges similar to the San Bernardino Mountains.

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