Abstract

We have developed a technique for measuring paleoelevation on the basis of the vesicularity of basaltic lava flows. We are currently applying this technique to determine the timing and extent of Cenozoic uplift of the Colorado Plateau. Because the technique measures paleoatmospheric pressure, it is not subject to uncertainties stemming from the use of proxies that depend on environmental factors other than elevation alone. 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. The modal size of the vesicle (bubble) population is larger at the top than at the bottom. This leads directly to paleoatmospheric pressure and thus elevation because the thickness of the flow can easily be measured in the field, and the vesicle sizes can be measured in the laboratory. On the Colorado Plateau, volcanic fields are generally found around the margins of the plateau. Basaltic lavas range in age from 0 to 23 Ma; present elevations range from 1.7 km to 3.3 km. Samples were collected from lava flows in the areas of Marysvale, Springerville, Grand Mesa, and the San Juan Mountains and analyzed to determine the timing and extent of uplift. The analysis of the samples using X-ray computed tomography imaging and our numerical techniques for determining vesicle population statistics indicate uplift of 350–2200 m, depending on age. There is a clear relationship between age and amount of uplift (original elevation subtracted from present elevation). The results indicate slow uplift of ∼40 m/m.y. between 25 Ma and 5 Ma (only 800 m of uplift during that time), and rapid uplift of 220 m/m.y. since 5 Ma (1100 m during that time). These results reconcile the long-standing controversy between interpretations of ancient versus recent uplift by providing an uplift history curve for the Colorado Plateau.

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