TECTONIC EVOLUTION OF THE WESTERN MARGIN OF THE ESPANOLA 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 of oblique-slip faults spanning a width of ~ 17 km. We hypothesize that this area experienced some degree of vertical axis rotation associated with rifting. Data from a suite of Miocene mafic dikes in the southern Española Basin include paleomagnetic, rock magnetic, field, and thin section results. Paleomagnetic data provided constraints on potential components of vertical-axis rotation across structural blocks. Rock magnetic data aid with identifying the magnetic mineralogy responsible for carrying the remanence directions. Low-field susceptibility versus temperature experiments yield a spectrum of results that reflect 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. Paleomagnetic results from twenty sites provide an in situ group mean of D=345.3°, I=43.1°, α₉₅= 7.1°, k= 46.2, that is statistically distinct, at the 95% confidence level, from the mid-Miocene expected field direction (D=358°, I=58°, A₉₅ = 6.0°) with an inferred rotation (R) and flattening (F) of R = -12.7° and F = 14.9°, respectively. We interpret the discordant paleomagnetic data to indicate that counter-clockwise vertical axis rotation was associated with rifting north of the Jemez Mountains. It is possible that oblique-slip left lateral motion along the Santa Clara fault and/or right lateral slip along the Cañada del Almagre fault facilitated vertical axis rotation between structural blocks. We argue that extensional strain was accommodated via rotation in this area. Additional paleomagnetic studies should help constrain the regional extent of the inferred vertical axis rotation.