Gravity Studies of Two Silicic Volcanic Complexes

Gravity data from two silicic volcanic complexes, the Valles Caldera, New Mexico and the Breiddalur central volcano, Iceland, have been modelled and interpreted. The Rubio Volcano (4 Ma to 1.5 Ma) in the area of the Toledo Embayment in the Jemez Mountains formed the centre of a depression which grew southwestwards and then collapsed to form the Toledo Caldera (1.45 Ma) and then the Valles Caldera (1.12 Ma). Published gravity data covering the Valles Caldera are used to generate three profiles across the caldera. Models in 2½D show a combination of chaotic collapse and a trapdoor caldera up to 3,600 m deep, hinged to the west with some faulted basement blocks. The maximum depth of tuffs filling the caldera are up to 1,000 m less than in previously published models, although in other places the tuffs are found to be over 1,000 m thicker. Gravity data from Breiddalur in southeastern Iceland indicate that the partly silicic, andesitic volcano was about 2,000 m thick, with a 500 m thick silicic core and that at least two other buried silicic centres may have developed during the volcano's history. The generation and storage of silicic magmas in these contrasting volcanic provinces is considered. Much more basic material is required at Valles than at Breiddalur to produce the inferred volumes of silicic material. The consequences of this difference are discussed.