New Mexico Geological Society Annual Spring Meeting — Abstracts

We investigate the tectonic history of faults near Clara Peak and a west-down fault system on Lobato Mesa, located 16-22 km WNW of Española in the north-central Rio Grande rift. La Cañada del Almagre fault (CdAF) lies NE of Clara Peak, a late Miocene basaltic eruptive center in the Jemez Mountains, and appears to intersect the Santa Clara fault to the south. Previous studies have determined ~520 m of right-lateral offset of a 9.74±0.21 Ma basalt dike along this fault and greater than 350 m of east-down stratigraphic separation. We compare the relative stratigraphic heights of two contacts on either side of the fault to refine when this fault experienced throw. One contact separates the Ojo Caliente Sandstone and Chama-El Rito Members of the Tesuque Fm. This 13.3-13.5 Ma contact has experienced ~430-480 m of eastdown stratigraphic separation. The second contact, corresponding to the base of 9.7-11.0 Ma basalts of the Lobato Fm, has undergone ~60-70 m of east-down stratigraphic separation. Consequently, 370-410 m of the total 430-480 m stratigraphic separation occurred between 13.5 and ~10 Ma, a time of significant tectonic activity along this fault. Oblique-slip continued after ~10 Ma, but the throw component was at a comparably lower rate. Furthermore, the aforementioned basalts fill a 30 m-deep paleovalley on both the footwall and hanging wall of the CdAF. This deep incision implies uplift along the Santa Clara fault on the southern flanks of Clara Peak prior to 10 Ma.

The Cerritos fault dips to the east and trends NNW-SSE along the eastern margin of the Los Cerritos dacite center, located ~2.5 km west of Clara Peak. This fault appears to have been active since 9.65 Ma, which is the age of a distinctive Los Cerritos dacite lava flow vertically offset by >50 m across the fault. Older basalts, partly mantled by thick colluvium, appear to be of comparable thickness on either side of the Cerritos fault, suggesting no or very low rates of throw prior to 9.65 Ma.

A 2.5 km-wide system of west-down, normal faults offset the ~10 Ma basalts that cap Lobato Mesa by as much as 180 m. However, these do not offset 3-7 Ma Tchicoma dacite flows to the south. We do not know if this fault system was active before 10 Ma, but it is evident that these faults produced significant throw between 10 and ~5 Ma and have not been noticeably active in the Plio-Pleistocene.