Testing Models for Incision and Neotectonics of the Grand Canyon Region: Field Studies, Ar-Ar Dating of Neogene Basalts, and Detrital Zircon and Monazite

University Of New Mexico, Albuquerque NM

Investigators

Abstract

Defining an accurate incision history for Grand Canyon can provide the critical missing element for understanding the magnitude of neotectonic uplift of the Colorado Plateau. This research project is testing the hypothesis that the eastern Grand Canyon block has been uplifted approximately 700-900 meters relative to the lower Colorado River block in the last 6 million years through movement on neotectonic normal faults. The century-old controversy about carving of Grand Canyon is addressed through a two-fold approach. The first part of the project aims to verify the fault-dampened model for Quaternary incision in which the observed differential incision in discrete river reaches of Grand Canyon is explained by relative vertical movement of regional blocks, separated by normal fault systems. A strategic campaign of dating of basalt flows, combined with geochemical correlations between dated flows and undated flows will verify the hypothesized coherent block behavior of eastern, western, and lower Colorado River blocks as well as the slip rates and displacement history of the faults that separate them. Restoration of faulting history, combined with accurate incision rates, will allow paleo river profiles to be constructed to evaluate the extent of surface uplift, using present and 5.5 million year old paleo-sea level as reference horizons. The second part of the project uses detrital zircon and monazite dating of quartzite cobbles to understand provenance. Quartzite cobbles are ubiquitous components of river gravels in this region; they will be fingerprinted and linked to their source terrain using: 1) detrital zircon age spectra, 2) compositions of detrital monazite, 3) detrital monazite age spectra, 4) metamorphic assemblages, and 5) trace phase geochemical characteristics. Because of the increasing knowledge of Proterozoic quartzite domains in the western U.S., and the limited number of quartzite outcrop areas in the Rocky Mountain provenance terrains, it should be possible to tightly constrain the source regions of many. The project will establish whether the gravels beneath dated lava flows represent paleo-Colorado River deposits. The incision of the Grand Canyon and the uplift of the Colorado Plateau have been of interest to earth scientists and the public ever since John Wesley Powell's first expedition of the Colorado River almost 140 years ago. Despite nearly a century of research, these are still poorly understood and remain controversial. This project poses a new model to explain the incision of the Grand Canyon and, if successful, has the potential to resolve long-standing debates about this very popular tourist destination. The research team will continue efforts to educate the public about the geological history of the Grand Canyon through the Trail of Time outreach program at the Grand Canyon National Park.