Uplift History of the Colorado Plateau Since the Late Miocene: Analysis Using Vesicular Basalts as a Paleoaltimeter

Lehigh University, Bethlehem PA

Investigators

Abstract

The PI's have developed a method for deten-nining paleoelevations of highland areas on the basis of the vesicularity of lava flows. They demonstrated the validity of the approach by testing the method against recent lavas emplaced at known elevations, and recently applied the technique in an exploratory study on the Colorado Plateau to obtain a general constraint on the timing of uplift. This has been a subject of considerable concern and debate in the geological research community. The results of the exploratory study indicate that the Colorado Plateau experienced slow uplift from 25 to IO Ma, and rapid uplift after IO Ma. These results reconcile the longstanding controversy between interpretations of ancient vs. recent uplift by providing an uplift history curve for the Colorado Plateau. The PI's initial investigation did not resolve the nature of the slow-fast transition, nor the details of differential uplift between different parts of the plateau, but subsequent sampling of young flows (<10 Ma) should reveal these details and the extent (if any) of internal plateau deformation. They now propose to conduct a detailed analysis of the uplift of the Colorado Plateau, building on the results of our exploratory project, and focusing on the last 10 m.y. The importance of establishing the uplift history for the Colorado Plateau and Rocky Mountains lies in its pivotal role in paleogeographic reconstructions and paleoclimatic changes, as well as its reflection of lithospheric then-nal structure and mantle processes. Various approaches for deten-nining timing of uplift have been tried in the past including geomorphologic, paleofloristic, sedimentary, and isotopic investigations, but each has had sources of uncertainty stemming from using proxies that depend on factors other than elevation alone. Because the PI's recently developed technique measures paleoatmospheric pressure, it is not subject to these uncertainties (although like all measurements, has its own potential sources of error). Using the proposed analysis, the PI's will be able to reconstruct the detailed uplift history of the Colorado Plateau and adjacent Rocky Mountains. Vesicular lavas preserve a record of paleopressure at the time and place of lava emplacement because the difference in internal pressure in bubbles at the base and top of a lava flow depends on atmospheric pressure and lava flow thickness. At the top of the flow, the pressure is simply atmospheric pressure, while at the base, there is an additional contribution of rgH lava overburden. Thus the modal size of the vesicle (bubble) population is larger at the top than at the bottom. Because the thickness of the flow can easily be measured in the field, and the vesicle sizes can be measured in the lab, this leads directly to paleoatmospheric pressure. Given that sea level pressure has not changed significantly in Cenozoic time, the difference between the paleoelevation "preserved" in the lavas and their present elevation reflects the amount of uplift. Lava can be well dated, therefore a suite of samples of various ages will constrain the timing of epeirogenic uplift independent of climate, erosion rates or any other envirom-nental factors. In their recent work, the PI's developed new tools for analysis of vesicular volcanic rocks. These tools most notably include methods for identifying appropriate field sampling locations, and the computational software which allows tomographic data to be manipulated to produce vesicle population statistics necessary for deten-nining size distributions. The proposed method is based on the modal size of the vesicle population, so it is critical to be able to accurately measure the size distribution in collected samples from the tops and bottoms of flows. The new analytical techniques involve Computed X-Ray Tomography to measure vesicle sizes. Based on our ability to measure vesicle size to within 1.6% (by volume), a factor analysis of the sensitivity of the technique to atmospheric pressure provides an elevation within +-300 in. As done in their preliminary project in conjunction with local field experts, the PI's propose to collect samples from various identified flows that are now at high elevations. They will prepare samples for Xray Tomographic scanning to be done at the facility at U. Texas. The resulting data will be analyzed with software they have developed previously. The size distributions will be used to reconstruct paleopressure and thus paleoelevation. This should provide an unambiguous documentation of the details of uplift history of the region, as well as elucidate any differential uplift within the Colorado Plateau.