Syn-eruptive laccolith growth at Cordón Caulle, Chile

Abstract:

The 4 June 2011 eruption of Cordón Caulle, Chile was the largest event recorded that year, and marked just the second time a rhyolite eruption has been scientifically monitored. Ground-based and remote sensing observations (Terra SAR-X, Landsat 7) of this event indicate that a shallow magmatic intrusion occurred beneath the active vent in the second week of the eruption (~18-24 June) and lead to significant, localised uplift (10 to >200 m) and complex surface deformation across an area of about 2 km². Shallow intrusion coincided with the first days of lava effusion, with effusion rates estimated at 20-80 m³s-1 and simultaneous pyroclastic venting producing 5-6 km-high ash plumes. Despite this contemporaneous, high magmatic flux out of the vent, the intrusion continued to grow, implying greater rates of magma input than eruption. High-resolution digital elevation data extracted from Pléiades satellite imagery in March 2014 along with detailed surface mapping of ground deformation patterns help determine the depth, form, and mechanism of intrusion. These data collectively indicate a laccolith-shaped body emplaced at a minimum depth of 300 m and with an asymmetrical form controlled by broad monoclines on the southern end and thrust and high-angle growth faults to the northern side. Structures such as these have long been recognized in dissected laccoliths in the southwestern U.S. and are interpreted here to indicate shallow magma emplacement involving: 1) firstly, the formation of a sill-shaped body, 2) lateral spreading of the sill to a critical width, and 3) upward bending of overburden and eventual failure along growth faults at the margins of the laccolith. A 2D mechanical model will be presented that determines the stress required to grow the laccolith, which in turn will provide an estimate of the magma overpressure in the shallow subsurface.