Origin of Central Andean collapse calderas

Abstract

Regional strains in tectonically active volcanic provinces may have a profound influence on the mode of collapse caldera formation. Conversely, the deformation pattern, more specifically, the symmetry of plan-view strain fields imparted to caldera floors may assist in elucidating the regional deformation active during caldera formation. The symmetry of plan-view strain fields is chiefly controlled by the mode of floor subsidence, particularly whether subsidence is uniform, symmetric or asymmetric, portraying collapse mechanisms known respectively as plate, downsag and trapdoor. Plate and downsag subsidence generates centro-symmetric strain fields characterized by radial and concentric discontinuities and subvolcanic dikes. Such strain fields appear to develop preferably where magma pressure controls collapse. By contrast, rectilinear horizontal strain fields form under unidirectional stretching and generate normal faults and subvolcanic dikes transverse to the stretching direction. Rectilinear strain fields are typical for trapdoor subsidence but also for straight orogenic belts and suggests that the formation of both may be related. This was tested for six central Andean collapse calderas that formed between 10.5 and 2Ma and are located on prominent NW–SE striking fault zones. A combined geochronological and structural analysis of the Miocene Negra Muerta Caldera in particular was designed to better understand caldera formation associated with the prominent Olacapato – El Toro Fault Zone...