Evaluation of Magma Flow and Emplacement Mechanisms of the Mafic Dulce-Platoro Dike Swarm, NW New Mexico and SW Colorado Using Magnetic Susceptibility Fabrics

Magnetic fabric investigations of magmatic dikes have been frequently employed for over four decades to estimate magma flow directions to better understand volcanic and magmatic systems. Anisotropy of magnetic susceptibility fabric data were collected from 32 sites of the approximately 125km N-S, 25km E-W Platoro-Dulce dike swarm of late Oligocene age in northern central New Mexico and southern central Colorado, USA, to estimate magma flow directions. The Platoro Caldera in the Southern Rocky Mountain Volcanic Field at the northern end of the swarm, and the incipient Rio Grande rift to the east of the swarm, represent two potential sources for this mafic dike swarm. This study helps by considering two models of emplacement: one involving a magma flow regime that predicts generally steep magma ascent with no bias to horizontal component of flow or, alternatively, one that predicts generally shallower magma ascent with a south bias to the horizontal component of flow. Rock magnetic, paleomagnetic, and petrographic inspection of the dikes all show that the measured rock magnetic fabric is of primary origin at time of emplacement and thus that these dikes are permissible for estimating magma flow directions. A tentative conclusion based on inferred flow directions which demonstrate no bias for north or south horizontal flow directions but bias for steeply inclined vertical flow is that the Rio Grande rift likely acted the magma source and was responsible for the extension dominated tectonic stress field in the eastern San Juan Basin that controlled emplacement of the dikes, although the Platoro Caldera likely influenced local stress field conditions.