NEOGENE UPLIFT HISTORY OF THE COLORADO PLATEAU FROM VESICULAR BASALTS

We have analyzed vesicular basalts to determine the timing of uplift of the Colorado Plateau during the Neogene. By examining the vesicularity at the top and bottom of a lava flow, we can measure paleo-atmospheric pressure. The size distributions provide paleo-atmospheric pressure, which leads directly to paleoelevation. We collected basalts around the plateau perimeter from lava flows in the areas of Marysvale, Springerville, Grand Mesa, and the San Juan Mts. Flow ages ranged from 0-23 Ma, and present elevations ranged from 1.7-3.3 km. Vesicle size distributions were derived from Computed X-ray Tomographic (CXT) data, providing rapid measurement of tens of thousands of vesicle sizes in each sample. The results of our analysis indicate uplift amounts throughout the Colorado Plateau of 340 m to 2170 m, depending on age. There is no clear relationship between lava age and elevation of emplacement across the suite of sampled flows. This means that eruption sites were determined by the physics of magma generation and delivery to the vent rather than by surface elevation. There is, however, a clear relationship between age and amount of uplift (original elevation subtracted from present elevation). The results indicate slow uplift of roughly 40 m/m.y. between 25 Ma and 5 Ma (only 800 m uplift during the Miocene), and rapid uplift of 220 m/m.y. since 5 Ma (1100 m during the Pliocene). These results reconcile the long-standing controversy between interpretations of early vs. recent uplift by providing an uplift history curve for the Colorado Plateau. In general, all available proxies and lines of evidence should be used to determine paleoelevation. While vesicular lavas may provide a single, powerful tool, as many approaches as possible should be employed to address specific cases of paleoelevation and epeirogeny.