New Mexico Geological Society Annual Spring Meeting — Abstracts

Fluvial incision data from the Great Plains of New Mexico and Colorado show differential incision across broad (50-100 km) convexities, suggesting that the region’s bedrock channel systems are in a period of adjustment to a regional-scale perturbation. Some incision data implicate hard bedrock knickpoints, but regional trends point to transient incision signals due to headwater uplift (or equivalent base level fall). Multiple datasets are currently available with which to address the controls on this phenomenon: (1) normalized channel steepness index (k_sn) analysis used to identify anomalously steep river slopes has revealed a spatial pattern of non-equilibrium stream profiles in the region, (2) ⁴⁰Ar/³⁹Ar ages on basalt-capped surfaces of the Raton-Clayton volcanic field show an onset of more rapid exhumation on the Great Plains starting at about 3.6 Ma, and (3) apatite fission-track data also indicate differential exhumation through time, with moderate rates beginning in the Oligocene followed by more rapid exhumation in the last 5 Ma. These datasets are suggestive of neotectonic uplift and we have used them in combination with new mantle tomographic images from the EarthScope experiment to identify possible dynamic topography above a well-imaged zone of low velocity in the upper mantle (the Jemez anomaly). The spatial patterns evident in the incision and exhumation data correspond to mantle velocity structure and therefore suggest mantle-driven controls on surface modification are at work on the Great Plains. This conclusion may be an important component to understanding the development of late Cenozoic relief across the western U.S. orogenic plateau and may also help constrain rates and processes of neotectonic mantle flow.