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Paul W. Bauer, New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro NM 87801, bauer@nmt.edu

Keith I. Kelson, William Lettis & Associates, Inc., 1777 Botelho, Walnut Creek CA 94596, kelson@lettis.com

Peggy S. Johnson, New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro NM 87801, peggy@gis.nmt.edu

Three major fault systems intersect near Taos, New Mexico:

1. The repeatedly reactivated, N-striking, 5-km-wide Picuris-Pecos fault
2. The Holocene, eastern rift-bounding, Sangre de Cristo fault
3. The Embudo fault, the transfer zone between the San Luis and Española rift basins.

The southern end of the Sangre de Cristo fault (Cañon & Hondo sections) is a 20-km-long, arc-shaped zone that defines the Taos embayment. The Sangre de Cristo fault continues north (Questa section) along the linear range front. The transition between the NE-striking, left-oblique normal Embudo fault and the N-striking, normal Cañon section is a smooth curve that cuts the Picuris-Pecos fault in a structurally complex zone near Talpa. South of the Embudo fault/Sangre de Cristo fault, the volcaniclastic Picuris Fm (34-18 Ma) of the Miranda graben is cut by strike- and oblique-slip faults of the Picuris-Pecos fault. The Picuris/Pecos fault projects northward across the Taos valley to the Questa section. The Taos graben, identified by geophysics and drillholes, is a buried, N-trending, 13-km-wide, 5000 m-deep graben, and the major rift feature in the southern San Luis basin. The eastern edge of the graben (Town Yard fault) lacks Quaternary expression, but is in line with the Picuris-Pecos fault and the Questa section.

A conceptual geologic model of the Taos valley is as follows. The Picuris-Pecos fault and Sangre de Cristo fault are reactivated pre-Laramide faults. The Miranda and Taos grabens were originally parts of an oblique-slip Oligocene-to-Miocene basin. The Picuris-Pecos fault and the Questa section represent the exposed eastern edge of the graben; the Town Yard fault is the buried intermediate section. Sometime after 18 Ma, rift kinematics changed, and the Embudo fault-Cañon/Hondo section severed the Picuris-Pecos fault, leading to its extinction. As the rift widened and extension slowed, the Taos graben was abandoned and faulting migrated eastward to form the Taos embayment.

This model explains many of the geologic, hydrogeologic and physiographic features of the Taos plateau, including the Rio Grande gorge, intrabasinal faults, Pliocene basalts, broad basinal warps, hot springs, groundwater flow and asymmetric drainages.

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Revised: 12 May, 2008

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Copyright © 2007 - 2008 New Mexico Bureau of Geology & Mineral Resources unless otherwise specified.