A Late-Pleistocene confined volcanic debris avalanche promoted by hydrothermal alteration at the Tinguiririca volcano (Andes of Central Chile)

Distinguishing volcanic debris avalanche deposits from other epiclastic breccia could be complex. For more than 60 years, the Tinguiririca deposit (sourced from the homonymous volcano) in the Andes of Central Chile has been described by different authors as glacial moraines, a lahar, a volcanic debris avalanche, and even a debris flow deposit. To decipher its obscured origin and emplacement dynamics, we have carried out a detailed investigation of its distribution, contact relationships, sedimentology, and facies. Our findings unravel that the 57 km-long deposit is 5 to 300 m thick, totalling a reconstructed volume of 3.64 ± 0.05 km³. It is composed of unsorted heterometric breccias formed by clasts and blocks arranged in mixed and matrix facies characterised by distinctive lithological domains. In general, three clasts lithologies are dominant, consisting of black and grey andesites and hydrothermally altered clasts with jigsaw cracks and fractures. The deposit overlies terraced colluvium along the valleys and forms hummocks and ridges. Emplacement velocities estimates range from 39.6 m/s to 108.4 m/s. Therefore, the Tinguiririca deposit should represent a massive volcanic debris avalanche that formed after a lateral collapse that affected the ancient Tinguiririca Volcanic Complex, during the Late Pleistocene (between 45 ± 18 and c. 19.2 ± 1.2 ka). The abundance of hydrothermal minerals within the deposit's matrix and clasts (i.e., illite, phengite, epidote, tridymite, chlorite, hematite, jarosite, and alunite) all represent the volcano's hydrothermal system that likely favoured rock weakness and edifice collapse. Finally, the new interpretation is valuable for evaluating volcanic hazards and requires further mapping and research efforts.