Making rift connections: Geophysical insights into the geometry and temporal evolution of the Embudo transfer zone and the basin-bounding Sangre de Cristo fault, northern Rio Grande rift, New Mexico

Abstract:

The 64-km-long, NE-striking Embudo fault is a classic rift transfer zone that accommodates strain between the Española and San Luis Basins in the northern Rio Grande Rift near Taos, New Mexico. This dominantly left-slip fault zone forms the link between the east-down western border of the Española basin (Pajarito fault) and the west-down eastern border of the southern San Luis Basin (Sangre de Cristo fault). The southwest to northeast transition along the NE-striking Embudo fault system to the N-striking Sangre de Cristo fault system appears gradual. Slip on the Embudo fault is mainly left-oblique on near-vertical fault strands, with dominantly lateral slip giving way to dominantly NW-down normal slip as it approaches the Sangre de Cristo fault. Detailed geologic mapping shows that the surface Embudo fault traces comprise a zone about 2 to 3 km wide that exhibits evidence for episodic reactivation since at least 3 Ma.

Interpretations of aeromagnetic and gravity data in combination with geologic cross-sections and lithologic logs from water wells provide evidence of a much wider subsurface fault system than mapped at the surface. Aeromagnetic data suggest the Embudo fault zone extends an additional 4 to 7 km basinward (northwest) of the mapped fault traces, as a series of parallel, oblique-slip faults stepping down into the basin. Prominent NE linear aeromagnetic features arising from 3 to 5 Ma basalt layers <200 m deep provide evidence for probable Pliocene faulting along two main strands: one 2-4 km basinward of the currently active fault zone and the other beneath a fairly linear, SW-flowing tributary to the Rio Grande. On the northern side of the tributary, the gravity modeling suggests the Taos basin deepens toward the NE and E (maximum ~ 2 km). The geophysical data together indicate that the greatest displacement along the N-striking boundary of the subbasin occurs about 4.5 km basinward (west) of the Sangre de Cristo fault trace. The interpreted geophysical model of subbasin geometry suggests that activity on both the Embudo transfer zone and the Sangre de Cristo fault have shifted southeastward and eastward, respectively, toward the present-day range fronts over time. This interpretation attests to a prolonged and complex history in the development of the northern part of the rift transfer zone.