NEOGENE DECOLLEMENT-STYLE FAULTING IN PERMIAN YESO FORMATION, SIERRA LARGA, SOCORRO COUNTY, NEW MEXICO

East of Socorro, NM, several mostly E-dipping, normal faults that cut Permian and, locally, mid-Tertiary strata sole into a widespread low-angle decollement in evaporitic Permian Yeso Formation (Cather, 2010 NM Geol. Soc. Guidebook). Faults of the evaporite decollement zone excise stratigraphy and cut common, tight, 10-100 m-wavelength folds. This fault system likely originated in Laramide time, was active locally in mid- to late Tertiary "early Rio Grande rift time", and is dismembered by late-rift faults.

The E-dipping, listric, Neogene Bustos normal fault cuts rocks as young as late Oligocene in its hanging wall. High-standing Pleistocene(?) terraces overlap the fault so it probably is pre-Quaternary. Traced north, the Bustos fault soles into the Yeso decollement in the western Sierra Larga. The decollement can be traced to the E side of that range, extending its eastern extent by ~10 km. In at least one place, a fault in the decollement zone cuts an andesitic dike (Oligocene? dating in progress), also suggesting Neogene slip. In turn, the dike cross-cuts folds in the decollment zone, suggesting that folding, and presumably some decollement slip, are of Laramide origin.

We hypothesize that the N part of the breakaway for Neogene normal slip on this fault system lies in the Joyita Hills, where a Permian to Cretaceous section lies in fault contact over older Permian strata. The footwall immediately W of there dips E and was probably tilted E during formation of the Laramide Sierra Uplift farther W. The inferred Laramide slip in the decollement zone likely occurred during formation of the uplift.

The eastward termination and basement significance of the decollement is uncertain. Tightly folded evaporates of the Yeso formation underlie only broadly warped younger Permian strata for tens of km E of the Sierra Larga, suggesting a blind Laramide thrust-tip geometry (with the thrust rooted W in the Sierra Uplift). The E extent of extensional faulting above the decollement is presently unknown, but recognition of Neogene slip in the Sierra Larga shows that early-rift extension occurred well east of the modern Rio Grande rift. It is not known if this rooted into basement. Alternatively, either or both episodes of slip may have been gravity driven and represent only shallow crustal strain.