Widespread Gravity Changes and CO2 Degassing at Laguna Del Maule, Chile, Accompanying Rapid Uplift

Abstract:

Laguna Del Maule (LdM), located on the Andes range crest in central Chile, is one of the most active rhyolite volcanic fields on Earth with 36 post glacial rhyolitic eruptions. Since 2007, LdM has accumulated over 1.8 m of uplift at rates of up to 300 mm per year. We hypothesize that this rapid uplift results from the injection of basaltic magma into the base of a rhyolite chamber. To test this hypothesis we undertook a dynamic gravity study, complimented with CO₂ soil gas measurements. We established a 35 station dynamic gravity and differential GPS network around the lake in April 2013 and undertook initial CO₂ measurements. We resurveyed the network in January 2014 and expanded the soil gas coverage. From these surveys we calculated 0.134 ± 0.030 mGal residual gravity change (Δg) accompanied by 281 ± 13 mm of uplift over the 10 month period. Statistical tests show that the results of the 2013 and 2014 surveys are different at p < 0.01. The Δg anomaly occupies an area of 5 km x 10 km, oriented E/W, and centred in the south eastern part of the lake, and is coincident with the area of maximum uplift. Gaussian integration of Δg yields an excess mass of ~1.2 x 10¹¹ kg. Assuming a density of 2700 kg/m³ this results in a volume of around 0.044 km³. In the 10 month time interval between surveys the calculated volume change rate was 41 ± 1 m³/s. We examine gravity / height change (Δg/Δh) relationships to determine if changes observed relate soley to increased mass or if density changes are involved.

In addition to the Δg and Δh measurements, CO₂ soil concentrations of up to 7 % are recorded around the entire lake basin. We will discuss modelling of the Δh and Δg data to explore the geometry and physical parameters of the mass and pressure source and discuss the relationship of CO₂ anomalies to these models.