Large-scale silicic volcanism - The result of thermal maturation of the crust

Large Silicic Volcanic Fields (LSVF) are common features of the circum-Pacific active continental margins in space and time. They are considered to be the surface manifestations of batholith formation at depth and are commonly associated with "ignimbrite flare-ups." The Late Miocene to Recent Altiplano-Puna Volcanic Complex (APVC) of the Central Andes is one of the largest and best preserved LSVF in the world. Here, available age and volume data on major ignimbrite eruptions in the APVC show that ignimbrite volcanism in the region initiated at ~10Ma with several large but regionally restricted units such as the Artola and Sifon ignimbrites and ignimbrites of the Vilama-Corutu center. Activity continued for 10Ma to the recent but appears to have "pulsed" with major episodes of activity at ~8, 6, and 4Ma. Activity since 4Ma has been minor with the largest eruptions being those of the Purico and Laguna Colorado centers at ~1Ma. Three characteristics of the available age and volume data are: (1) Pulsing of the ignimbrite eruptions with an approximate two million year period. (2) Trend to larger volume eruptions climaxing at about 4Ma. (3) Markedly diminished activity since 4Ma. Interestingly the pattern of sudden onset of spatially diffuse, volumetrically minor eruptions leading to a focused catastrophic episode that is followed by quiescence seems to be a feature of other large silicic volcanic fields. This suggests a consistency of process during ignimbrite flare-ups in space and time. We present a model of these large silcic volcanic fields as the result of progressive thermal (and mechanical) maturation of the crustal column due to advection of heat by magmatism and its effects on lithosphere strength. Elevation of the brittle-ductile transition to within a few kilometers of the surface leads to eventual catastrophic failure of the crust and explosive eruptions of thousands of cubic kilometers of magma as regionally extensive ignimbrites.