Tectonic and geologic evolution of the Espanola Basin, Rio Grande Rift: Structure, rate of extension, and relation to the state of stress in the western United States

Abstract

The Española basin of the Rio Grande rift began as a broad crustal downwarp in latest Oligocene time. Most of the basin is 2–3 km deep, but localized faulting allowed accumulation of up to 5 km of sedimentary fill in a central sub-basin. The localized early faulting ended before filling of the central Española basin was completed about 10 m.y. ago. Movement on faults that define the present western margin of the Española basin began ~ 10 m.y. ago. Jemez Mountain volcanism, in the western Española basin, also began at about this same time. West tilting of up to 30° occurred due to movement along pervasive N-trending intrabasin faults about 7.5 m.y. ago in conjunction with continued movement along the western border faults. Volcanism continued after this tilting, forming many of the large volcanic constructs of the Jemez Mountains. Regional uplift of the entire northern Rio Grande rift began ~ 7 m.y. ago. Movement on the Pajarito fault zone began about 5 m.y. ago and continues to the present. This fault zone defines the western margin of the velarde graben, a narrow central sub-basin where recent movement has been concentrated. Some volcanism also has occurred within the southern Velarde graben. Total extension across the Española basin since ~ 26 m.y. ago is estimated to have been ~ 5.5 km (roughly 10%) or between 3.5 and 8 km assuming high-angle planar faulting. The ~ 0.2 mm/yr averaged long term rate of extension has been separated into three periods of activity: 1. (1) ~ 0.14 mm/yr from 26 to 10 m.y. ago. 2. (2) ~ 0.5 mm/yr from 10 to 5 m.y. ago. 3. (3) ~ 0.14 mm/yr from 5 m.y. ago to present. A change in least principal stress direction from WSW-ENE to WNW-ESE that occurred throughout the western United States about 10 m.y. ago coincides with a roughly 3.5 times increase in the rate of extension, preferential development and movement of N- to NE-trending normal faults, and a few degrees of clockwise rotation of rocks in the western Española basin. Similar to the Española basin, initial basins of the southern Rio Grande rift were broad downwarps and rifting was greatly accelerated after ~ 10 m.y. ago. Accelerated uplift of the northern Rio Grande rift also occurred at about this time indicating that activity in the entire Rio Grande rift was modulated by this change in extension direction ~ 10 m.y. ago that appears related to Pacific-North American plate interactions. This modulation coupled with major faulting (~ 10 m.y. ago) preceding uplift (~ 7 m.y. ago) in the Española basin suggest a passive rifting process for the Rio Grande rift whereby stresses due to plate interactions elsewhere cause faulting in the lithosphere which leads to the development of a “passive” asthenospheric uplift. Furthermore, the roughly 20 m.y. pre-uplift history of sediment accumulation in basins of the central and southern rift, and the inherited character, trend, and geometry of the Rio Grande rift as a whole are also more consistent with a passive rifting process.