New insights into La Pacana caldera inner structure based on a gravimetric study (central Andes, Chile)

作者：Delgado Francisco^*; Pavez Andres

来源：Andean Geology, 2015, 42(3): 313-328.

DOI：10.5027/andgeoV42n3-a02

• La Pacana caldera
• Volcanology
• Gravimetry
• Caldera collapse

La Pacana (central Andes, Northern Chile) is one of the largest resurgent calderas in the world, formed 4 Ma ago during an eruption with a VEI of 8.7. We undertake a gravimetric study to contribute new insights into the inner structure and evolution of this caldera. The La Pacana Bouguer residual anomaly is asymmetric and has an average amplitude of -12 to -14 mGal, which we interpret as being produced by the low-density intracaldera ignimbrite infill. A reinterpretation of the caldera stratigraphy plus the available geochronology suggests that the current shape of La Pacana was produced by the collapse of two nested calderas, roughly limited by the axis where the resurgent dome changes its orientation, with the oldest eruption in the southern part of La Pacana. The gravity data suggests that these southern and northern nested structures would have collapsed with downsag and trap-door geometries respectively, evidence of asymmetric subsidence. Intra caldera ignimbrite thicknesses were calculated with 2.5D forward models and show that the ignimbrite infill in its southern and northern parts reach similar to 0.6-1.1 km and similar to 2.5-3 km respectively. Using Gauss's theorem, we calculate the volume of the intra caldera ignimbrite infill is similar to 3,400-3,500 km(3), in agreement with previous estimates and with models that show that the larger the caldera diameter, the larger the erupted volume.

• 出版日期2015-9