Development of mountainous topography in the Basin Ranges, USA

We use the landscape evolution model Zscape to explore quantitatively the d evelopment of mountainous topography in the Basin and Range province (forme rly the Basin Ranges), USA, as a function of faulting, surface processes an d 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 t opography 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, ste ady combination of tectonic and surface processes. Numerical landscapes rev eal the same features observed in the field, including facets, spur benches , piedmonts, and relatively linear and regularly spaced drainages. In all c ases, a steady-state landscape is generated on the order of 10(6) years, so that there remains little information in the landscape that can tell us ab out processes or conditions prior to 1 million years ago. The fundamental f orm of the steady-state landscape (including the facet and spur bench morph ology) is governed by the spacing of rivers and bedrock strength and the re sulting relief is for the most part strength-limited. Neither hillslope nor facet relief is dependent on the rate of fault slip or channel incision. R elief is incision-limited, however, nearer the headwaters of rivers where s tream power is relatively low. Topographically asymmetric ranges may be gen erated over tectonically symmetric horsts by allowing precipitation to be d riven by orographic processes, but the asymmetry is likely to be dependent on the ability to remove eroded material from the adjacent basins. The valu e of the experiments presented here is to demonstrate that the astute and i mpressive observations made by the likes of Gilbert, Davis and Dutton are r eproducible using a relatively simple description of the relevant physics a nd that we can recognize and explain various landscape morphologies that ha ve in the past been the subject of necessarily qualtitative reasoning.