TIME-EVOLVING SURFACE AND SUBSURFACE SIGNATURES OF QUATERNARY VOLCANISM IN THE CASCADES ARC

Increased resolution of data constraining topography and crustal structures provides new quantitative ways to assess province-scale surface-subsurface connections beneath volcanoes. Using a database of ~3000 mapped Quaternary vents, we extract topographic volumes of edifices with known epoch ages (Holocene and Late/Middle/Early Pleistocene) from digital elevation models in the U.S. Cascades arc (western North America). We determine total Cascades edifice volumes of ~1930 km3, which likely represents ~50% of the total eruptive output over the last 2 My. We then analyze the correlation between edifice volumes, spatial vent density, and an array of diverse geophysical data (isostatic residual gravity, surface-wave phase velocities, heat flow, and GPS-derived rotation rates) to understand the relationships between upper-crustal magmatism and volcanic edifices.

We find increases in both magnitude and number of correlations between datasets at vent locations compared to non-vent locations within the arc, with many relationships expected from magmatism. For example, we find arc-wide positive correlations between vent density, topography, and heat flux; negative correlations between vent density, seismic velocities, and gravity. Furthermore, correlations are more strongly associated with younger vents, suggesting migration of the upper-crustal magma structure throughout the Quaternary that is coincident with surface vent migration. Spatially, geophysical magmatic signatures increase in the central and southern Cascades, where eruptive output is largest.

Using the distributions of edifice volumes and spatial vent densities, we then study spatially focused versus distributed volcanism throughout the arc. Although volcanic centers laterally extending ~100 km are persistent throughout the Quaternary, we find varying degrees and changes of volcanism style through time at the latitudes of Mt. Shasta / Medicine Lake and Three Sisters / Newberry. Furthermore, we observe a general shift to more focused volcanism in the northern part of the arc over the past 2 My Combined with the correlation analysis, this shift in volcanism style suggests a time-evolving spatial focusing of magma ascent, interacting with along-arc gradients in background tectonic stress state, throughout the arc.