MAGMATO-TECTONIC LINKS: IGNIMBRITE CALDERAS, REGIONAL DIKE SWARMS, AND THE TRANSITION FROM ARC TO RIFT IN THE SOUTHERN ROCKY MOUNTAINS (Invited Presentation)

Compositionally diverse dikes and other intrusions, which radiate westward from the polycyclic Platoro caldera (source of five large ignimbrite sheets: 30.0-28.6 Ma) in the Southern Rocky Mountain volcanic field, merge in structural trend, composition, and age with the spectacular but little-studied Dulce dike swarm of trachybasalt that trends SW and S for ~125 km along the NE margin of the Colorado Plateau (San Juan Basin) from southern CO into northern NM. More than 100 dikes of the Platoro-Dulce swarm are depicted on regional maps. Some dikes, though only 1-2 m thick, are traceable for 20 km or more; recent fieldwork identified numerous smaller unmapped dikes. Subhorizontal grooves on walls of some Dulce dikes suggest rapid lateral injection for distances of >10 km, perhaps farther, even originating from the Platoro locus. Two ⁴⁰Ar/³⁹Ar biotite ages (24.94, 25.64 Ma) and published K-Ar dates (23-27 Ma) for Dulce dikes are within the range of five new sanidine ⁴⁰Ar/³⁹Ar ages from dacite dikes (20.31-26.53 Ma) radial to Platoro.

We suggest that the radial Platoro intrusions and Dulce dikes record the interrelated effects of uplift associated with prolonged solidification of an arc-related granitoid batholith beneath Platoro caldera concurrently with the transition to weak extension along the NE margin of the Colorado Plateau, satellitic to initiation of the Rio Grande rift and associated basaltic volcanism farther east. The potential genetic link between the Dulce swarm and the Platoro radial dikes provides opportunities to examine the role of transitional tectonics (convergent arc to continental rift) on mechanisms of magma emplacement and strain partitioning during waning of a large caldera complex. The implied link between tectonic and magmatic regimes, and associated impacts of abrupt shifts in either over geologically short times, has implications for continental structural evolution and geologic hazards. Rapid emplacement of such an enormous dike swarm, in conjunction with late evolution of a caldera-related batholith, could have generated extension-related earthquakes and triggered dispersed volcanism. Documentation of such events would define a previously unrecognized magmato-tectonic hazard that could occur near active calderas in the western USA such as Valles, Long Valley, or Yellowstone.