Geophysical Investigations Into the Structure, Activity and Hydrology of the Valles Caldera, New Mexico

The Valles Caldera lies within the Jemez Mountains along the western edge of the Rio Grande Rift, New Mexico, USA. Activity since the last caldera-forming eruption at 1.2 Ma includes a resurgent dome, a ring of time-progressive rhyolite domes, eruptive activity at 70 ky in the south caldera, and abundant hydrothermal activity. At least three Pleistocene lakes formed within intra-caldera valleys through damming of drainages by volcanic flows.

Since 2017, geophysical field investigations as part of the Summer of Applied Geophysical Experience program have been carried out to investigate the structure, activity and hydrology of the Valles Caldera. Magnetotelluric measurements within the caldera suggest up to 1 km of debris flow deposits atop Bandelier Tuff, rift-fill and pre-rift sedimentary units. Precambrian basement is estimated at 4 km depth, while a middle-crustal conductor at 15-20 km depth, ubiquitous beneath the Rio Grande Rift, does not appear to shallow beneath the caldera.

A nodal seismic experiment within Valle Grande, the southernmost intra-caldera valley, was carried out to map basin structure as well as look for seismicity within the notably aseismic caldera. Horizontal-to-vertical spectral ratios indicate 400-800 m of unconsolidated sedimentary fill, while a supervised machine-learning algorithm has revealed local earthquakes. A broadband seismic array has been recently deployed to search for additional seismicity.

Transient electromagnetic (TEM) soundings coincident with the nodal-seismic array image a conductive (clay) horizon at 2560-2580 m elevation that is remarkably uniform throughout Valle Grande and nowhere exceeds 100 m thickness. An underlying resistive unit thickens to 500+ m in the basin center. These models are consistent with drill holes in the valley that intercept 80-90 m of lacustrine mud and silt atop 250+ m of pumiceous sand and gravel. Along the northern edge of the valley, resistive rhyolite flows from a nearby rhyolite vent are imaged extending beneath the basin sediments. TEM soundings in valleys of the northern caldera reveal conductive clay horizons at lower elevation, suggestive of an older lake bed. Considerable variability in the resistivity models within the northern valleys refutes models suggesting a common Pleistocene history.