Unusual volcanic instability and sector collapse configuration at Chimpa volcano, central Andes

Upper Miocene Chimpa volcano is one of the largest stratovolcanoes of the back-arc region of the central Andes. The gentle-sloped volcano underwent previously unrecognized volcanic instability, consisting of ? 2 km mass-wasting processes and catastrophic sector collapse, whose characteristics have been identified by means of stratigraphy, geological mapping and structural analysis. The origin of instability at Chimpa can be attributed to tectonic faulting, hydrothermal alteration and overloading. These common promoting and triggering factors have produced an unusual configuration of the volcano gravitational instability, characterized by parallel landslide scars delimiting unstable sectors on eastern and western volcano flanks, with large toreva blocks sliding in opposite directions, perpendicular to the flow direction of a subsequent long-run out debris avalanche. Even if the style of such a complex volcanic instability has never been described before, its identification may be also useful to study volcano sector failures and mass-wasting deposits in other volcanoes worldwide.