EVALUATING ROCK UPLIFT-EROSION FEEDBACK: A CASE STUDY OF THE WESTERN GREAT PLAINS, USA

There are compelling reasons to believe that feedback relationships exist between rock uplift and erosional processes in orogenic belts, primarily through the medium of isostatic response to erosional unloading. It has proven difficult, however, to document such feedback in mountainous areas, due to the paucity of preserved pre-deformation datum surfaces. If a datum of known initial geometry can be identified, it can be used to assess erosional unloading, to evaluate patterns and magnitudes of rock uplift, and to partition uplift into tectonic and isostatic components. Such surfaces are rarely preserved in mountainous areas, but may be preserved in lower-relief areas where geomorphic processes are less efficient.

In this study, relationships between tectonic rock uplift, fluvial erosion, and erosionally driven isostatic rock uplift are examined in an area of low uplift rates (0.2 mm/yr or less) on the western Great Plains of New Mexico, Colorado, and Wyoming. The base of the Miocene Ogallala Group provides a widespread datum of fairly simple initial geometry, allowing an assessment of post-Miocene patterns of rock uplift, fluvial incision, and regional erosion, and indirectly a separation of tectonic and isostatic components of rock uplift.

Feedback involving tectonic rock uplift, erosion, and isostatic rock uplift is evident throughout the study area. However, the degree to which fluvial systems have adjusted to tectonic uplift, and to which the feedback system has become established, varies markedly. In general, upstream areas close to the Rocky Mountain front have relatively high tectonic rock uplift rates and have responded most fully to tectonic uplift. In these areas the feedback system is best developed and a large proportion of total rock uplift is attributable to erosional isostasy. In lower tectonic uplift rate areas, typically further from the mountain front, fluvial system response is less well developed, the erosion/isostatic uplift feedback is limited, and only a small proportion of the total rock uplift is isostatic. This relationship between tectonic uplift rate, fluvial system adjustments, and development of an uplift/erosion feedback system is complicated in some areas by effects of climatically controlled difference in stream discharge and/or by differences in bedrock resistance.