The mid-Tertiary ignimbrite flare-up and associated andesitic lava flows must have resulted from a complex interaction between magmatism and tectonics. Focusing on the San Juan volcanic field that was active in the Oligocene to Miocene, important questions (some still unresolved) include identifying the parent magma, determining important magmatic processes that altered the parent magma, and creating a tectonic framework to allowed this voluminous magmatism.
The andesites and ash flow tuffs of the San Juan volcanic field originated from partial melting of mantle-derived basalts (Riciputi et al., 1995; Askren et al., 1997). These magmas were then carried into lower and upper crustal magma chambers where crustal contamination and magma mixing occurred. Riciputi et al. (1995) required two caldera stages prior to eruption to explain the isotopic signatures of the volcanic rocks in the San Juan volcanic field.
Several processes or events could have permitted or retarded magmatic activity in the Cordilleran, including changes in plate velocities, plate boundary interactions, crustal density, pre-existing weaknesses (including thermal and mechanical), gravitational potential energy, and the thermal structure of the crust. Humphreys (1995) developed a model of the buckling of the subducted Farallon slab to explain the migration of two magmatic belts towards southern Nevada in the Cenozoic. As to the question whether gravitational collapse of the Cordilleran orogen was responsible for the flare-up, Liu (2001) determined that the amount of asthenospheric upwelling produced in numerical models of gravitational collapse was not enough to generate partial melting. However, thermal weakening and hardening of the crust might have been processes that assisted the flare-up.
Future efforts should be focused towards developing an improved tectonic model that not only permits the flare-up event, but also incorporates geologic events prior to and after the flare-up event.
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