Three-Dimensional Fissure Eruption Geometries in the San Rafael (Utah) Subvolcanic Field Revealed by Nonlinear Inversion of Magnetic Anomaly Data

Abstract:

Distributed volcanic fields are common on Earth and nearby planetary bodies. Unlike their central-vent counterparts, these volcanic centers are comprised of many individual basaltic magmatic dikes, which are often only expressed at the surface in the form of vents, domes, and lava flows. In situ imaging of the shallow (<1 km) subsurface can reveal important details about the 3D geometry of fissure systems that feed distributed eruptive centers, with implications for the nature of these eruptions: their mass flow rates, explosivity, durations, and volcanotectonic interaction. Luckily, dikes, sills, conduits and related near-surface structures tend to carry high remanent magnetizations, creating magnetic anomalies at the surface where sufficient magnetic contrast exists with the host rocks they intrude. In the San Rafael Sub-volcanic field (SRVF), basaltic dikes intrude fractured and horizontally bedded Jurassic sandstones, now eroded to a depth of about 800 m beneath the paleo-surface. Detailed mapping and profiles with a cs-vapor magnetometer reveal far more complex anomalies than can be attributed to simple planar dikes, including: wing-like sills, buds, and domes. We image these geometries using MagCube-parallel, an open-source nonlinear inversion code we developed that models complex geometry with multiple (e.g., 1,000) vertical-sided prisms. We show one normally polarized fissure system to include along strike: a 10 m thick, 50 m wide laccolith at ~7-14 m deep, a roughly vertical conduit ~100 m wide near the center of the mapped fissure system, and a 1-5 m. thick sill at ~3-13 m depth and thinning to the north. While model depth and thickness vary with magnetization contrast, the main geometric relationships do not. Magnetic mapping of a nearby fissure reveals the same types of structures. The implication of these structures is that the small-volume fissure eruptions were likely pulsatory, with episodes of horizontal intrusion of sills, and sufficient time to develop gravitational instabilities. We suggest much can be learned about fissure eruptions on other planetary bodies with high-resolution magnetic anomaly mapping.