SUBSURFACE VIEWS OF A RIFT TRANSFER ZONE: INTEGRATION OF GEOPHYSICAL, GEOLOGICAL, AND BOREHOLE INFORMATION AT THE NORTHERN END OF THE EMBUDO FAULT ZONE NEAR TAOS, NEW MEXICO

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. 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 northern end of the Embudo fault zone extends ~20 km NE to ENE from Pilar, eventually curving northward to join the N-striking Sangre de Cristo fault zone. Slip is mainly left-oblique on near-vertical fault strands, with dominantly lateral slip giving way to dominantly NW-down normal slip from SW to NE. The major, pre-rift, N-S Picuris-Pecos fault system is truncated at its northern end near where the Embudo and Sangre de Cristo faults join. From geologic mapping, the Embudo fault zone reaches several kilometers in width with evidence for episodic reactivation, but displacements can only be documented as far back as 3 Ma.

Interpretations of aeromagnetic and gravity data in combination with geologic cross-sections and lithologic logs from water wells provide subsurface views of a much wider Embudo fault zone than evident at the surface. Faulting appears to extend an additional 4-7 km basinward of the mapped fault zone, bounded on the northwest by the fairly linear, SW-flowing Rio Pueblo, a tributary to the Rio Grande. The faults step the basin down ~1.3 km across this zone. Prominent NE linear aeromagnetic features arising from 3-5 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 along the Rio Pueblo. From gravity modeling, the basin deepens north of Rio Pueblo with a NE to E tilt, reaching the deepest part (~2 km) northwest of the Town of Taos, ~4.5 km west of the Sangre de Cristo range front. The basin floor is modified by a gentle ridge that emanates from the Picuris-Pecos fault zone and heads NNW. West of this ridge, W-down, N-S Quaternary faults cross the basin and the Embudo fault zone. East of the ridge, similar patterns of mostly concealed, N-S faults instead drop basalt layers down to the east. Subsurface views of the Embudo fault zone attest to a prolonged and complex history in the development of this rift transfer zone.