The Crustal Magmatic Structure Beneath the Denali Volcanic Gap Imaged by a Dense Linear Seismic Array

Abstract

The crustal structure in south-central Alaska has been influenced by terrane accretion, flat slab subduction, and a modern strike-slip fault system. Within the active subduction system, the presence of the Denali Volcanic Gap (DVG), a ∼400 km region separating the active volcanism of the Aleutian Arc to the west and the Wrangell volcanoes to the east, remains enigmatic. To better understand the regional tectonics and the nature of the volcanic gap, we deployed a month-long north-south linear geophone array of 306 stations with an interstation distance of 1 km across the Alaska Range. By calculating multi-component noise cross-correlation and jointly inverting Rayleigh wave phase velocity and ellipticity across the array, we construct a 2-D shear wave velocity model along the transect down to ∼16 km depth. In the shallow crust, we observe low-velocity structures associated with sedimentary basins and image the Denali fault as a narrow localized low-velocity anomaly extending to at least 12 km depth. About 12 km, below the fold and thrust fault system in the northern flank of the Alaska Range, we observe a prominent low-velocity zone with more than 15% velocity reduction. Our velocity model is consistent with known geological features and reveals a previously unknown low-velocity zone that we interpret as a magmatic feature. Based on this feature's spatial relationship to the Buzzard Creek and Jumbo Dome volcanoes and the location above the subducting Pacific Plate, we interpret the low-velocity zone as a previously unknown subduction-related crustal magma reservoir located beneath the DVG.