Calderas of the Marysvale Volcanic Field, west central Utah

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Abstract
Five calderas (Three Creeks, Big John, Monroe Peak, Mount Belknap, and Red Hills) have been identified and a sixth (Lion Flat) tentatively identified in the middle Tertiary Marysvale volcanic field of western Utah. The calderas are manifestations of separate magma bodies that worked their way to shallow depths between 27 and 19 Ma. No coherent ash flow field exists in the Marysvale volcanic field; instead, the ash flow units are interleaved at different places in the general volcanic sequence. The magma chambers that vented ash flows and collapsed as calderas formed discrete high‐level plutons; they are probably underlain by a deep composite batholith consisting of many individual plutons from two distinct petrogenetic types, calc‐alkalic and bimodal (mafic‐silicic). The four largest calderas can be separated into two groups by contrasting structural behavior. The Three Creeks and Big John calderas are broad faulted downwarps that sagged into relatively deep magma chambers whose roofs were sufficiently strong to inhibit development of ring faults. In contrast, the Monroe Peak and Mount Belknap calderas are subcircular calderas that collapsed into shallow magma chambers with thin roofs, so easily breached that large volumes of lava were erupted within the calderas. Neither type shows strong postcollapse resurgence; roofs above the deeper magma chambers were too strong to be uplifted, and those above the shallow chambers were too weak to contain the magmatic pressures.