Development of mountainous topography in the Basin Ranges, U.S.A.

We use the landcape evolution model Zscape to explore quantitatively the development of mountainous topography in the Basin and Range province (formerly the Basin Ranges), USA, as a function of faulting, surface processes and microclimate. Many of the classic morphologies of mountains in the Basin and Range were described in the late part of the 19th century. The varied topography coupled with differing experiences led to a similarly diverse set of explanations. We are able to demonstrate through a variety of numerical experiments that a diverse landscape is easily obtained by the simple, steady combination of tectonic and surface processes. Numerical landscapes reveal the same features observed in the field, including facets, spur benches, piedmonts, and relatively linear and regularly spaced drainages. In all cases, a steady-state landscape is generated on the order of 106 years, so that there remains little information in the landscape that can tell us about processes or conditions prior to 1 million years ago. The fundamental form of the steady-state landscape (including the facet and spur bench morphology) is governed by the spacing of rivers and bedrock strength and the resulting relief is for the most part strength-limited. Neither hillslope nor facet relief is dependent on the rate of fault slip or channel incision. Relief is incision-limited, however, nearer the headwaters of rivers where stream power is relatively low. Topographically asymmetric ranges may be generated over tectonically symmetric horsts by allowing precipitation to be driven by orographic processes, but the asymmetry is likely to be dependent on the ability to remove eroded material from the adjacent basins. The value of the experiments presented here is to demonstrate that the astute and impressive observations made by the likes of Gilbert, Davis and Dutton are reproducible using a relatively simple description of the relevant physics and that we can recognize and explain various landscape morphologies that have in the past been the subject of necessarily qualtitative reasoning