Hydrothermal and magmatic reservoirs at Lazufre volcanic area, revealed by a high-resolution seismic noise tomography

- First high-resolution Vs model of the Lazufre volcanic zone for the first 7 km.
- Imaging of two reservoirs (hydrothermal and magmatic) in the upper crust.
- New insights for the origin of gases at Lastarria volcano.

We determine here for the first time the geometry and location of the hydrothermal and magmatic reservoirs in the Lazufre volcanic area. This furthers the understanding of the origin of one of the largest worldwide volcanic uplift regions, both in space and amplitude. The exact locations and shapes of the sources generating a double-wide uplift region in the Lazufre found by past deformation data (InSAR and GPS) and generating hydrothermal and magmatic fluids found by geochemical gas analysis have not been well-delimited. In this study, we use seismological data to perform a 3-D high-resolution S-wave velocity model, which allows defining better the locations and shapes of the sources of the deformations and the hydrothermal and magmatic reservoirs. We find three anomalies. Two of them (with S-wave velocity of about 1.2-1.8 km/s) are located below the Lastarria volcano. The shallow one (<1 km below the volcano base) has a funnel-like shape. The deeper one is located between a depth of 3 and 6 km below the volcano base. Both are strongly elliptical in an EW direction and separated by a 2-3 km thick zone with Vs of ∼1.5-2 km/s. As far as these anomalies are located under the hydrothermal activity of Lastarria volcano, they are interpreted as a double hydrothermal (the shallow part) and magmatic source (the deeper part). The latter can feed the former. This double hydrothermal and magmatic source is in agreement with previous geochemical, deformation (GPS and InSAR) and magneto-telluric studies. In particular, it explains the double origin of the gases (hydrothermal and magmatic). The third low-velocity zone (with S-wave velocity of about 2.3 km/s) located at 5 km depth and deeper is centered beneath an area of surface uplift as determined by InSAR data. We compare the seismic tomography and InSAR results to propose that this low-velocity zone is at the top of a large reservoir, hosting hydrothermal fluids and possibly also magma.