New Mexico Geological Society Annual Spring Meeting — Abstracts

East of Socorro, NM, several east-dipping, normal-separation faults root into a subregionally developed low-angle fault system in evaporites of the Permian Yeso Formation (Cather, 2010, NMGS Guidebook). In nearly all known outcrops, the faults of this system cut down-section eastward and excise stratigraphy. This fault system was postulated to be either Laramide or mid-Tertiary but timing constraints are sparse.

Detailed and reconnaissance mapping along the western side of the Sierra Larga shows that the Neogene Bustos fault can be traced north to deeper structural levels where it soles into the Yeso decollement zone. The Bustos fault is an east-dipping, listric normal fault with 1–1.5 km of stratigraphic separation that cuts strata as young as upper Eocene-Oligocene, proving its Neogene age. In at least one place, a fault within the decollement zone cuts an andesitic(?) dike, also suggesting Neogene slip. Several levels of Quaternary terraces overlap the fault so it is probably not Quaternary in age.

Thus, at least the Blackington Hills–Sierra Larga portion of the Yeso decollement system has had significant slip in post-Laramide time, and our current preferred hypothesis is that the Yeso decollement here is mainly a rift-age structure. It is possible that this and other parts of the decollement were active earlier, however, and that the system in the Sierra Larga was reactivated by “capturing” the Bustos fault. It remains unknown whether the Yeso decollement is (1) a rooted fault system that accommodated regional crustal extension east of the Sierra Larga or (2) a superficial, gravity-driven feature that dies out eastward at shallow crustal levels in folds within the Yeso section.