New Mexico Geological Society Annual Spring Meeting — Abstracts

Geomorphology and surficial geology has undergone a recent dramatic transformation to increased numbers of studies regarding large-scale quantitative investigation of the fundamental linkage between crustal and surficial processes. The Earth's surface is now largely viewed as an important boundary condition in conceptualizing and solving continental-scale tectonic and climatic problems. To that end,
the study of earth surface processes plays the central role in any understanding of how earth systems are linked and what the potential driving mechanisms are for global change. For example, it has been proposed for several years now that major tectonic events, such as the uplift of the Himalayas, has resulted in the dramatic cooling of late Cenozoic climate as atmospheric CO₂ is absorbed during the weathering of silicates. Closer to home, changes in global climate and increased storminess have been linked to an increase in relief, and possible isostatic uplift of the Colorado Plateau and entire Rocky Mountain foreland. Both of these examples rely almost completely on an understanding of what controls large-scale landscape denudation -fundamentally a geomorphic process. The goal of this presentation is to examine the evidence for landscape denudation and illustrate how emerging computer technologies, specifically those associated with digital elevation models (DEM) and geographical information systems (GIS) will shape geomorphologic thought on this subject for the next decade.

Geomorphologists view climate and relief as primary end members that control large-scale landscape denudation. Classic studies of the effects of climate and relief on denudation rates can be revisited and reinterpreted as new fluvial sediment yield data and digital topography become increasingly available. Digital topography is easily rasterized by popular GIS software such as ARC/INFO. Once in ARC/INFO format, the raster data can be quickly analyzed to determine elevation, slope, and relief characteristics. Numerous recent studies of this kind have shown in a compelling way that mean elevation and mean relief of well-drained landscapes are highly correlated, and that both elevation and relief are highly correlated by a simple linear function to mean denudation. The slope of the regression line between mean elevation and mean denudation is K_d, the erosion constant. Use of DEM/GIS techniques has shown that K_d is a true constant over geologic time applicable to a region with a characteristic rock type and climate. The presence of an erosion constant allows for time-integrative calculations of the mass flux through an orogen and the subsequent reconstructions of paleotopography, paleohydrology, likely earth surface processes, and long-term landscape evolution.

To illustrate the concepts presented above, consider the case of the Sierra Nacimiento mountains of north-central New Mexico. This range has a strikingly linear range front which by all traditional geomorphic morphometric analyses, should be revealed as tectonically active; however, regional exhumation of the Laramide range-front fault could result in the same geomorphic expression of an essentially tectonically dead landscape. DEM/GIS analyses of the Sierra Nacimiento in comparison to other Laramide age structures (Colorado Front Range) and known tectonically active rift structures (Taos Range) are successful in distinguishing topographic characteristic unique to the tectonic development of those ranges. The respective rates of denudation of those ranges are commensurate with the mean relief measured from the DEMs. The conclusion is that the Sierra Nacimiento owes its geomorphic expression largely to exhumation rather than active tectonics. Further investigations of this kind are planned for the entire Rocky Mountains where they will be integrated with ongoing fission track thermochronologies to better understand the fundamental relationship between post-Laramide tectonics, denudation, and current topographic expression.