A Shallow Magma Reservoir at Laguna del Maule Volcanic Field, and its Influence on Regional Fault Structures: Insights from Static and Time-Varying Gravity Models.

Abstract:

Static and time-varying gravity measurements from Laguna del Maule since 2013, reveal the internal architecture of the volcanic field, and time varying processes occurring within it. Static Bouguer gravity measurements show a 19 mGal gravity low coincident with the center of inflation (~20 cm/year). We designed a novel 3D inversion, inspired by conceptual models of silicic magma systems, where regions of low density contrast surround a higher density contrast body representing melt within a cumulate mush. The inversion model presents a 30 km³ body of density 1800-1900 kg/m³, adjacent to the regional scale Troncoso normal fault. Using thermodynamic models we interpret this anomaly as a high melt proportion (50-80%) magma reservoir, with 2-4% free vapour phase, spanning a region 2 to 5 km below the lake surface. This melt and volatile rich body sits within a larger, 115 km³body of low melt proportion (20%), interpreted as crystal mush. The modelled magma reservoir lies above the source of inflation, previously modelled as an inflating mafic sill at 5 km depth.

Time-varying gravity measurements show positive yearly increases (60 to 124 uGal), close to the center of inflation, indicating significant ongoing mass injection (> 1.5x10¹¹ kg). Source models of the gravity change are focussed along the Troncoso Fault at a depth of 1.5 to 2 km, and are best represented by a vertical rectangular prism. Mass injection into the inflating sill does not replicate the gravity anomalies, instead the shallow depth extent and small density change (156 to 307 kg/m³) suggest a mechanism of hydrothermal fluid intrusion into existing voids, or voids created by the substantial uplift. We explore a coupled magma-tectonic interaction mechanism that allows for shallow mass addition, facilitated by the deeper inflating sill. Computing the strain, and mean, normal, and Coulomb stress changes on the Troncoso fault, caused by the opening of a sill at 5 km depth, shows an increase in strain and mean and normal stresses along the fault, coincident with the areas of mass addition. Seismic swarms in mid-2012 may be responsible for dynamically increasing permeability on the Troncoso fault, promoting influx of hydrothermal fluids, which in turn causes larger gravity changes in the 2013 to 2014 interval, compared to subsequent intervals.