CONSTRAINING THE COLORADO PLATEAU CRUSTAL THICKNESS USING SR/Y AND LA/YB RATIOS TO SHOW CHANGES IN PEAK SUBDUCTION-RELATED VOLCANISM FROM THE OLIGOCENE TO THE HOLOCENE

Studies by Lieu and Stern (2019) show that the composition of magma varies with crustal thickness. Trace element ratios (Sr/Y and La/Yb) have provided good proxies for crustal thickness when compared to seismic profiles along modern volcanic arcs. The proven validity of this method begs the question: Can we calculate the thickness of the Colorado Plateau using trace element ratios from volcanic rocks of southern Utah? Volcanic activity in the region has spanned over 30 million years (Best et al, 2013; Johnsen et al., 2010; Smith et al., 2019). The majority of this volcanic activity is the direct result of the subduction of the Farallon Plate beneath the North American Plate similar to the tectonic setting of the Andes, or the Cascades today. These volcanic products are our best view of the magmatic activity of the region over that timeframe. Fortunately, many of the rocks are very well studied with published trace element data (Best et al., 2013; Johnsen et al., 2010), but the record focuses on the older end of the volcanic activity. We propose to calculate the thickness of the Colorado Plateau crust through time using the range of volcanic activity preserved on, and around the Colorado Plateau. To gain a complete record, we need to supplement the published data with new trace element analyses from younger eruptions represented by the Red Desert Lava Flows, Henrie Knolls Lava Flows, and Miller Knolls Lava Flows – all of which are exposed near Cedar Breaks National Monument. With these data, we hope to document the change in crustal thickness between the peak of subduction-related volcanic activity in the Oligocene through the Holocene.