Tectonic Evolution of the western Margin of the Espaņola Basin, Rio Grande Rift, NM: Rock Magnetic, Paleomagnetic, and Petrologic Data

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 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.