The Embudo fault between Pilar and Arroyo Hondo, New Mexico--An active intracontinental transform fault
— Willilam R. Muehlberger

The N60E-trending Embudo fault zone is both a hinge zone and a transform fault that transfers the major displacement along this segment of the Rio Grande depression from the Taos fault (Kelley, 1954) along the eastern margin of the San Luis Valley basin to the western margin of the Velarde graben, the Los Alamos fault in the Espanola basin. Reversal of downthrow is shown by Kelley (1978) to be just west of Black Mesa near the confluence of the Chama and Ojo Caliente rivers. The amount of strike slip is unknown because the un-known shapes at depth of the border faults control the amount of horizontal extension. The Jemez caldera lies astride the southwestward projection of the Embudo fault zone.

The Jemez lineament (Chapin and others, 1978; Clark, 1968; Cordell, 1978; Lambert, 1966) (Springerville-Raton line of Lipman, 1979), an alignment of Pliocene-Holocene volcanic centers, extends diagonally across Arizona and New Mexico from near Springerville, Arizona (some include the Pinacate volcanic field of Mexico near the head of the Gulf of California), northeastward from Mt. Taylor and the Jemez caldera to the Raton-Clayton-Capulin volcanic field of northeastern New Mexico. The Embudo fault zone lies astride this lineament where it traverses the Rio Grande depression.

The segment of the fault zone between Pilar and Ranchos de Taos is anomalous in that it has reverse fault displacements that are best displayed in the roadcuts near Arroyo Hondo, about 8 mi (13 km) southwest of Ranchos de Taos. These displacements demonstrate that the Rio Grande depression is not being extended normal to its length (i.e. not extending east-west). If the Embudo fault was a typical transform con-necting two rifts, it would be pulling apart and be a zone of active volcanism because of its N60E strike. Instead, it is a zone of active compression in this segment, as shown by the overriding of the Taos Plateau to the north by the Picuris Mountains and its flanking sedimentary blanket.

Kelley (1952, 1977) has argued that longitudinal left slip along the Rio Grande rift is present, that the Laramide ancestry of this zone is significant and that the relationships between the successive deformations should be analyzed carefully. The present motions of the Picuris over the Taos Plateau support a component of left slip along the rift as well as a dominant extension across the rift.

The intersection of northeast-trending faults, such as the Embudo and Tijeras (near Albuquerque, fig. 1), and north-trending faults, such as the Picuris-Pecos and Los Alamos-La Bajada, produce diamond-shaped patterns. The one outlined by the faults named above has opposite corners high (Picuris and Sandia mountains) and low (Velarde graben of Manley (1978) and the Santa Fe embayment (see Kelley, 1978, fig. 2 for additional northeast-elongate diamonds)). Each of these bounding faults reverses throw along its length; for example, the Rio Grande enters and leaves the Espanola basin in reversal zones.