Collaborative Research: Collapse of the Valles Caldera: An Integrated Lithic and Pumice Compositional Study of the Bandelier Tuff

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Wolff9909700There is a lack of field data bearing on the details of how caldera collapse relates to eruption progress during large ignimbrite eruptions. We shall study the variation of both pumice and accidental rock fragment composition with stratigraphic height and geographic position in the chemically zoned Otowi Member of the Bandelier Tuff, erupted at 1.61 Ma during formation of the Valles caldera, New Mexico. Pumice compositional variations supply information about which part(s) of the magma body were sampled by an ignimbrite emplacement lobe. Rock fragment lithologies will be tied to pre-caldera geology to identify vent locations, and to study how vent geometries and positions change with time. We shall use these data to constrain the progress of caldera collapse as the magma body vented during the eruption. In the process, we shall test our new model of caldera formation through lateral block sliding following unbuttressing by establishment of dikes feeding an initial vent inside the developing caldera. New vents then open around the caldera margin as unbuttressed blocks move to fill the central void, allowing rapid eruption from deeper levels in the system. This model potentially has general applicability to all large continental calderas and their eruptive products.