PLIOCENE STRAIN TRANSFER IN THE RIO GRANDE RIFT OF NORTH-CENTRAL NEW MEXICO

The Espanola and Santo Domingo basins lie in a right step between larger Albuquerque and San Luis basins within the Cenozoic Rio Grande rift. The transfer of extension among these basins is poorly documented. The >700 sq km Cerros del Rio volcanic field (CdRVF) erupted mostly between 3 Ma and 1.1 Ma at the southwest margin of the Espanola basin where its lava flows have regional dip of <1° above more steeply dipping Miocene sediment. Paleomagnetic data from the CdRVF (grand mean of D = 352.8°, I = 49.7°, k= 14, a95 = 3.9, n = 97 sites) indicate that the field has been rotated counterclockwise 7.2° +/- 5.2° about a vertical axis relative to an expected dipole field direction. This result corroborates modest CCW rotations (8°-13°) detected by other paleomagnetic studies of late Oligocene igneous rocks and Miocene sediments within the Espanola basin, and indicates young vertical-axis rotation relative to the late Oligocene inception of the basin. Regional studies of fault kinematics in the Espanola, Santo Domingo and Albuquerque basins show that north-striking normal faults were predominant during rift evolution, but a subset of post-3-Ma faults cutting the CdRVF have large components of dextral and sinistral strike slip. Thus, both paleomagnetic and kinematic data indicate a Pliocene increase in lateral strain rate that we attribute to late-stage transfer of extension among rift basins. That strain transfer accelerated after 3 Ma is curious given rift formation in the late Oligocene. Possible causes are (1) linkage of the northern Espanola basin to the San Luis basin via the Embudo fault after 4 Ma, (2) inward migration of active rift faulting in the Pliocene that facilitated stress rotation and strain transfer among major faults of different basins, and/or (3) a change of crustal rheology related to voluminous Pliocene-Quaternary magmatism in the Jemez Mountains of the western Espanola basin.