New Mexico Geological Society Annual Spring Meeting — Abstracts

We use borehole data, Bouguer gravity data, and previous geologic mapping of surrounding mountains to preliminarily characterize the geologic structure underneath the eastern San Agustin Plains. Home to the Very Large Array, the eastern San Agustin Plains occupies the 1400 km² basin east of Datil. It is surrounded by the Datil Mountains and Gallinas Mountains to the northwest and northeast, respectively, Tres Montosas to the east, and the Mount Withington area of the northern San Mateo Mountains to the southeast. The 14 km-wide C-N embayment on the southwest side of the basin extends 16 km between the Luera and northern San Mateo Mountains. Sandy basin-fill correlative to the Santa Fe Group underlies most of the eastern San Agustin Plains, whereas the surrounding mountains are composed of ignimbrites, volcaniclastic sediment, and lava flows of the Mogollon-Datil volcanic field.

We interpret at least two, possibly three, structural grabens underneath the eastern San Agustin Plains. Some of the faults bounding these grabens have experienced Quaternary activity. A subsurface bedrock high trends northeast across the center of the eastern San Agustin Plains, separating the North graben to the north from the C-N graben to the south. This bedrock high is manifested in the gravity data and collaborated by borehole lithologic data near the VLA headquarters. Southeast of the eastern extension of this bedrock high, relatively low gravity values suggest a third possible fault-bounded graben, which we call the White Lake graben, between Tres Montosas and Mount Withington.

The North graben is an east-tilted half-graben, tilted towards a north-northeast striking, 3 km-wide fault located 8-9 km east of Datil. A deep exploratory borehole indicates at least 3500 ft of basin-fill immediately east of this master fault zone. Although west-down, northwest-trending faults are present on the east side of North graben, their respective throws are relatively minor and do not produce notable gravity gradients. On the north end of North graben, the North Lake playa lies in a minor graben between two of these eastern faults.

The C-N graben underlies the northern C-N embayment. Rather than a simple half-graben, we interpret a northeast-elongated, fully-fault bounded graben. Its northern bounding fault corresponds to a previously inferred, northeast-striking, normal or normal-oblique fault that bounds the southern side of the aforementioned subsurface bedrock high. Its eastern bounding fault corresponds to the VLA fault, which has formed 40 m-tall scarps in middle Pleistocene alluvium. The gravity anomally associated with the C-N graben is similar to that of the western North graben, so we infer similar basin-fill thicknesses of 3500-4000 ft.

These grabens formed over the past 30 Ma during west-east extension associated with Rio Grande rifting. The rectangular shape of the C-N graben suggests a pull-apart structure created by oblique slip along its northeast-striking, north bounding fault. This fault was likely a reactivation of earlier structures associated with the northeast-trending San Agustine lineament of Chapin (1971), which approximates the northwestern boundary of the Laramide-age Morenci uplift of Cather and Johnson (1984).