Paleomagnetic Data Bearing on Vertical Axis Rotation of the Rio del Oso dike swarm, Western Espa�ola Basin, New Mexico

The Española Basin is one of a series of interconnected, asymmetrical basins in the Rio Grande rift that includes a number of northand northeast-striking faults that accommodated block tilting and basin subsidence. The western margin of the Española Basin, in particular, is characterized by a greater than 17-km wide zone of normal and oblique-slip faults. To clarify the involvement of block rotation in the tectonic evolution of the Española Basin, we carried out a paleomagnetic study of mafic intrusions (Rio del Oso dike swarm) that are genetically related to regionally extensive basalt flows of the mid-Miocene Lobato Formation. The primary hypothesis tested was that these intrusions experienced some degree of vertical axis rotation associated with mid-Miocene to recent continental rifting. In situ paleomagnetic results from forty-two sites yield a group mean declination (D) of 344.0°, an inclination (I) of 41.1°, α95 of 6.1°, and k of 14.1. The group mean result is discordant to the <10 Ma pole of D=356.0°, I=54.4°, α95 = 3.3° with a statistically significant inferred rotation (R) of -12.0°± 7.2° and flattening of +13.3° ± 5.5° relative to the <10 Ma pole field direction. These discordant results indicate that a modest degree of counter-clockwise vertical axis rotation occurred in this region, which is likely associated with Rio Grande rifting north of the Jemez Mountains. It is possible that oblique motion along the Santa Clara fault and/ or the Cañada del Almagre fault facilitated the vertical axis rotation. The results from this study imply that vertical axis rotation is common to extensional rift systems and should be considered when modeling continental extension.