Understanding the tectono-magmatic system of the Transitional Southern Volcanic Zone (TSVZ), Chilean Andes, from seismicity analysis

Abstract

Historically the Argentinian side of the Andes has been deeply explored for its petroleum potential and favoured by the fact that theoutcropping geology is dominated by sedimentary units, facilitating the structural reconstruction of geological events (Farias etal., 2010). However, this is not true for the Chilean Andes. This lack of geophysical and core data means only a very vague pictureof the 3D geological structure in the Chilean Andes exists compared to the eastern side of the mountain chain. As Chile is one ofthe most active countries in terms of volcanism and seismicity, there is an urgent need to characterise the relationship betweenactive magmatic systems, the distribution of seismicity and the current interaction between tectonics and fluid transport, so thataccurate geological hazard maps can be designed and appropriate emergency strategies can be implemented. This also hasimportant implications for the generation and location of ore bodies, as well as for the geothermal exploration in the Andes. Themain uncertainties concerning active geological processes lie in the regional to local scale. Detailed geological maps are limitedand very often when compared they are incongruous between one and other. Regarding seismicity, there is a sparse permanentnetwork of monitoring seismometers all along the country and many of the seismology studies have focussed on the slab seismicity,where the largest earthquakes occur. Very few studies have looked or are looking at the crustal seismicity of the Main Cordillera,where the vast majority of the volcanoes are located and where significant seismicity can still occur. This project aims to shedsome light on the crustal tectono-magmatic configuration of the volcanic arc at the Central Andes by means of a high-resolutionseismicity study. The seismicity analysis will be focussed on the Tinguiririca-Teno rivers area, where there are evidences of activevolcanism, seismicity, porphyry emplacement and hydrothermal fluids release. In March 2017 twelve seismometers, belongingto GEF (UK NERC funded Geophysical Equipment Facility) pool managed by SEIS-UK at Leicester University, were deployed inthe area at a mean spacing of ~15km. The instruments will record until November 2018, providing us with more than 1.5 yearsof continuous seismic data. It is well-known that hypocentres location contributes to the identification of active faults geometry(Du et al., 2004 and Schaff et al., 2002) and focal mechanisms can be calculated from well constrained earthquakes, offeringvery valuable information on the kinematics of active structures. For these reasons, the resulting data represent a key input,particularly when combined with information on the past crustal processes (i.e. surface geology) to help us understand presentdaytectonic processes, their relationship with observable magmatic and hydrothermal features and help reconstruct the tectonicand structural evolution of the area. The study will be developed in three principal stages: 1) Accurately locate earthquakesthrough traditional and relocation techniques (e.g., HYPOINVERSE-2000, HYPODD) to identify seismogenic zones, their depths,shapes and distribution. 2) Calculate the focal mechanisms of well-constraint earthquakes to determine the kinematics of activefaults and contextualize according to the local and regional structural setting. 3) Compare and combine seismicity results withother geophysical and geological data. This project is associated to a broader Chile-UK collaborative research scheme, for whichthere are collaborators focussed on gravity and magnetotelluric methods, structural geologists improving the geological mapsand geochronologists studying the age of outcropping rocks and/or geological events (e.g., faulting). Combining the gravity,magnetotelluric and seismicity data will enable us to build an accurate 3D underground model of the crustal structure in thearea. Also, in order to get a complete and representative model, the surface geology will be used to constrain the geophysical interpretations