EBTAG Annual Workshop and Field Trip
May 16-17, 2012

¹New Mexico Highlands University, Box 9000, Las Vegas, NM, 87701, USA, rhondatrujillo@hotmail.com

The western margin of the Rio Grande rift at the latitude of Espanola, NM, is characterized by a zone >17 km wide of oblique-slip faults. We investigated the possibility that this area experienced some degree of vertical axis rotation associated with rifting. We examined a suite of Miocene mafic dikes in the southern Española Basin using paleomagnetic, rock magnetic, field, and thin section data. We hypothesized that the mafic dikes experienced some degree of vertical axis rotation associated with mid-Miocene to recent rifting.

Paleomagnetic data provided constraints on potential components of vertical-axis rotation across structural blocks, between separate dikes, and along strike. Rock magnetic data provided constraints on the magnetic mineralogy responsible for carrying the remanence directions. Low-field susceptibility versus temperature experiments yielded a spectrum of results reflecting a thermomagnetic behavior typical of intermediate composition titanomagnetite as well as the presence of a Fe-sulfide phase. The Lowrie-Fuller test and acquisition of isothermal remanent magnetization (IRM) and backfield IRM experiments provided information to verify the magnetic mineralogy, domain state, and the coercivity of the remanence. These experiments, as well as other data, indicated that the remanence is carried by single domain to pseudo single domain magnetite and is likely a primary thermoremanent magnetization acquired during cooling and is thus geologically stable.

The dikes are composed of major Ca-plagiclase, augite, olivine, and magnetite. Swallowtail morphologies to the plagioclase laths as well as dendritic habit of the Fe-Ti oxide phases indicate undercooling of the host magmas consistent with shallow emplacement and rapid cooling.

The in situ paleomagnetic results for 20 paleomagnetic sites provide a group mean ofD=345.3°, I=43.1°, α95= 7.1°, k= 46.2, that is statistically distinct from the mid-Miocene expected field direction (D=358, I=58, A95 = 6.0) with an inferred rotation and flattening of -12.7°, and 14.9°, respectively. We interpret this discordant direction to indicate that some degree of counter- clockwise vertical axis rotation was associated with 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 vertical axis rotation between structural blocks.