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KELSON, K.I., W. Lettis & Assoc., 1777 Botelho,Walnut Creek, CA 94596

JOHNSON, P., NM Bureau of Mines, Socorro, NM 87801

Three major fault systems intersect near Taos, NM: 1) the repeatedly reactivated, N-striking, 5-km-wide Picuris-Pecos fault (PPF); 2) the Holocene, eastern rift-bounding, Sangre de Cristo fault (SdCF); and 3) the Embudo fault (EF), the transfer zone between the San Luis and Española rift basins. The S end of the SdCF (Cañon & Hondo sections) is a 20-km-long, arc-shaped zone that defines the Taos embayment. The SdCF continues N (Questa section) along the linear range front. The transition between the NE-striking, left-oblique normal EF and the N-striking, normal Cañon section is a smooth curve that cuts the PPF in a structurally complex zone near Talpa. S of the EF/SdCF, the volcaniclastic Picuris Fm (34-18 Ma) of the Miranda graben is cut by strike- and oblique-slip faults of the PPF. The PPF projects N 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 S San Luis basin. The E edge of the graben (Town Yard fault) lacks Quaternary expression, but is in line with the PPF and the Questa section. Conceptual geologic model of the Taos valley: The PPF and SdCF are reactivated pre-Laramide faults. The Miranda and Taos grabens were originally parts of an oblique-slip Oligocene-to-Miocene basin. The PPF and Questa section are the exposed eastern edge of the graben; Town Yard fault is the buried intermediate section. Sometime after 18 Ma, rift kinematics changed, and the EMZ-Cañon/Hondo section severed the PPF, 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 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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