ORIGIN AND EVOLUTION OF A LARGE SILICIC CALDERA COMPLEX: VALLES CALDERA AND THE ASSOCIATED UPPER BANDELIER TUFF
Brunstad, Keith Andrew
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The Jemez Mountains Volcanic Field, New Mexico, has been active for more than 20 Ma and it is a very complex system that has attracted much scientific attention for many decades, but some areas are remain poorly understood. This study focuses on 1) the Tshirege Member of the Bandelier Tuff by correlating the stratigraphy and chemistry from the well-known Pajarito Plateau to the less understood Jemez Plateau to reconstruct the latest caldera forming eruption (Section I.); 2) understanding the longevity of the Jemez Mountains Volcanic Field's magmatic system that led to cataclysmic eruptions producing the Bandelier Tuff and Valles caldera (Section II.); and 3) investigating the stratigraphy of the East Fork Member, the latest eruption from the Valles caldera (Section III.).An eruption history can be reconstructed for the Tshirege Member of the Bandelier Tuff across the entire Jemez Mountains, using stratigraphy and geochemistry. The Tshirege eruption sequence started with a Plinian phase that developed into an ignimbrite depositional phase during the collapse of Valles caldera. Early eruptions were focused to the northwest and east followed midway by deposition around the entire caldera, and ending with a depositional dispersal axes trending from the northeast to south-southeast and southwest. Integrated petrological, geochemical, geophysical, and tectonic information provided a base for developing a model that shows two predictable magma evolution environments a) the production of mafic to intermediate magmas from primitive mantle derived liquids, and b) the creation of silicic magmas from further magma-crust interaction at shallow levels. The established tectonic patterns are influencing the magma pathways throughout the history of the volcanic field, and as a feedback mechanism the presence of magma bodies may influence the fault patterns. This study was able to provide a more complete event stratigraphy for the East Fork Member of the Valles Rhyolite. The results show that during the earliest Plinian eruption pumice was dispersed over Valles caldera, southern Jemez Mountains, and the Rio Grande rift. Simultaneous pyroclastic fall and density current activity characterized the 55 ka event. The eruption progressed through two distinct stages separated by a short lull.