New Mexico Geological Society Annual Spring Meeting — Abstracts

Three major fault systems intersect in the Taos area of northern NM: 1) the repeatedly reactivated, N-striking, 5-km-wide Picuris-Pecos fault system (PPF); 2) the Holocene, eastern rift-bounding, Sangre de Cristo fault zone (SdCF); and 3) the Embudo fault zone (EF) which is the transfer zone between the San Luis and Espanola rift basins. The mountain front zone between Pilar and Canon provides preeminent exposures of the termination of a major rift transfer fault. New, detailed (1:12,000 and 1:6,000 scale) mapping of bedrock, rift fill, surficial deposits, and the faults that cut them all, provides important controls on the geometry and kinematics of Laramide to Holocene tectonism.

The southern end of the SdCF (Canon & Hondo sections of the SdCF) is a 20-km-Iong, arcshaped fault zone that defines the Taos embayment. The SdCF continues north (Questa section) into Colorado along a relatively linear range front. The transition between the NE-striking, left-oblique normal EF and the N-striking, normal Canon section is a smooth curve that cuts the PPF in a structurally complex zone near Talpa. South 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 northward across the Taos valley to align with the Questa section. The Taos graben, identified primarily by geophysics and drillholes, is a buried, N-trending, 13-km-wide, 5000 m-deep graben. The Taos graben is the major rift feature in the southern San Luis basin. The eastern edge of the graben (Town Yard fault) lacks Quaternary expression, but is aligned with the PPF to the south and the Questa section of the SdCF to the north.

A preliminary conceptual geologic model of the Taos valley is as follows. 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 of the SdCF represent the exposed eastern edge of the graben. The Town Yard fault is the buried intermediate section of the graben. Sometime after 18 Ma, rift kinematics changed, and the EZ-Canon/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 a variety of geologic, hydrogeologic and physiographic features of the Taos plateau, including the Rio Grande gorge, intrabasinal faults such as Los Cordovas faults,
Pliocene basalts, broad basinal warps, hot springs, ground-water flow and asymmetric drainages.