The role of the Troncoso fault on active faulting and magmatism at the Laguna del Maule volcanic field

Abstract:

Volcanic unrest at the tectonically active Laguna del Maule (LdM) volcanic field, located in the Southern Volcanic Zone of the Andes (36 °S), has caused uplift at rates >20 cm/yr since 2007. Geophysical surveys (GPS, Gravity, Seismic Tomography, Magnetotellurics, InSAR) have potentially located the magmatic reservoir between 2-7 km depth below the west-central side of LdM. The regional-scale, NE-striking Troncoso fault – recording a combination of dextral and normal offset - extends into the center of the field area and appears to end under the lake. Southeast of the fault, structural domes are observed, whereas northwest of the fault, and above the reservoir, abundant fault arrays cut young lake sediments. The fault arrays have small offsets (<10 cm) and cut dominantly subhorizontal clay, sand, gravel, ash-fall and lapilli deposits. These arrays of small faults rarely preserve kinematic indicators, which makes it difficult to constrain true displacements. The vast majority of small faults show apparent normal separation, strike ~NE, and dip 60° – 80° to the NW. Faults with apparent reverse offset are either parallel or perpendicular to the dominant normal fault arrays. We suggest that normal faults record NW elongation of the basin, whereas reverse faults indicate NE shortening. Faults with apparent strike-slip offsets strike either EW or NNW, with dips > 70°. The exact sense of motion of strike-slip faults in the basin is unknown, however their orientation is favorable for accommodation of NE shortening and NW elongation via sinistral motions on the EW set and conjugate dextral motions on the NNW set. These elongation and shortening directions are consistent with present elongation and shortening directions recorded by structures in the NS-oriented, dextral Antiñir-Copahue fault system (37° S), immediately south of the volcanic field. We attribute deformation recorded by faults in lake sediments northwest of the Troncoso fault to regional NS dextral shear. These observations suggest that the Troncoso fault 1) likely controls the localization of the magma reservoir, as interpreted from several geophysical methods; 2) it separates tectonically faulted (NW) and unfaulted (SE) domains, and 3) separates magmatically domed (SE) from un-domed (NW) areas. It is still unclear, however, how these three roles are related.