Syntectonic phreatic eruptions in the Tocomar geothermal field, Salta

Abstract

Pleistocene volcaniclastic breccias are described closely associated to the Tocomar geothermal field (24°10'S - 66°34'O). The absence of juvenile fragments (pumice and accessory lithics) attests an origin from an eruption developed by 1) an hydrothermal system violently depressurised or 2) by the interaction of hydrothermal fluids with overheated host rock (phreatic to hydro-geothermal explosion). Fragments produced by these phreatic explosions were transported by base surges. Planar bed structures, sand-wave structures and bomb sags are the most common depositional structures. Some pits perhaps related with this eruption were recognized, roughly prolate in shape 50 to 100 m in diameter. The phreatic eruptions were triggered by horizontal dilation associated to a tectonic transference between main Ntrending thrusts and the Calama-Olacapato-El Toro fault zone. The volcaniclastic deposits shows syn-depositional faulting. This interpretation is an additional evidence of the tectonic activity of the Calama-Olacapato-El Toro fault in the Pleistocene, reinforcing the geothermal potential of the Tocomar area.