Structural geology of the Tierra Amarilla Anticline, New Mexico
— John M. Bailey, Karl E. Karlstrom, Cameron C. Reed, and Matthew T. Heizler, [eds.]

Abstract—The Tierra Amarilla anticline of northwestern New Mexico is a well-exposed anticline in Mesozoic strata that lies near the boundary of the Colorado Plateau to the west and the Rio Grande rift to the east. Its axial trace appears to align with the southern extension of the Nacimiento fault zone, and its gentle south plunge mimics the south plunge of the Laramide Nacimiento uplift. This paper examines the relationship of the anticline to the apparent southern termination of the Nacimiento fault. New 1:6,000 mapping by the University of New Mexico Department of Earth and Planetary Sciences 2023 field camp suggests several new map interpretations that recognize three fault segments as a continuation of the Nacimiento fault system. The ~10-m-thick travertine carapace and aligned artesian carbonic springs in the western core of the fold are best explained by a concealed fault that we call the Tierra Amarilla segment. A Gypsum Hill segment is an east-side-up reverse fault in the center of the fold that repeats the Todilto Formation and has been partly inverted by east-side-down normal displacement during rifting. The East Limb segment is also an east-side-up reverse fault that repeats the Todilto and Summerville Formations. Both exposed fault zones are ~10 m wide and involve a dextral strike-slip displacement component. Recognizing that the strike-slip component precludes rigorous restoration of cross sections, we restore the dip-slip component and suggest that the Tierra Amarilla anticline is an inverted (collapsed) segment of the nacimiento uplift. A second new interpretation is constraining the age of the angular unconformity that bevels the folded strata and fault zones. We call this erosional surface the San Ysidro surface, and we dated the Cenozoic sediments that overlie it. The basal travertine carapace that was deposited on this surface yields U-series ages as old as 269±3 ka, which we interpret to be a minimum age for the erosional beveling. A detrital sanidine age (n = 1) from the Cenozoic sediment above the unconformity yields a youngest 40Ar/39Ar age of 261±34 ka, which is a maximum depositional age for the sediment and a similar minimum age for the erosion. The San Ysidro surface slopes northward toward the Rio Salado and is approximately graded to a travertine-cemented river strath terrace
that is 44 m above river level and gives a U-series age of 250±4 ka. The combined constraints suggest that the deposition on top of the San Ysidro surface took place 270–250 ka. The inception, duration, and rate of erosional beveling that created the angular unconformity are not constrained by our data but the formation of the San Ysidro surface may be related to rapid incision along the adjacent Rio Salado and Quaternary neotectonic uplift of the southern Nacimiento Mountains.