Structure and tectonic evolution of the Sierra Blanca Basin
— Daniel J. Koning, Geoffrey C. Rawling, Shari Kelley, Fraser Goff, William McIntosh, and Lisa Peters

The present structural geometry of the Sierra Blanca Basin is an inward plunging, asymmetric synclinorium, where the eastern limb generally dips more steeply than the western limb. Basin subsidence has preserved 900–1300 m of Cretaceous strata unconformably overlain by 140–800 m of lower to middle Eocene fluvial strata, which in turn are overlain by >1 km of lava flows and volcaniclastic sedimentary rocks related to the Sierra Blanca volcanic field. Northwest- to northtrending anticlines and synclines provide evidence for Laramide contraction on the western margin of the basin. Northeasttrending anticlines and synclines are also found along the eastern side of the basin, generally parallel to the transpressional Pecos buckles. The synclinal structure of the Sierra Blanca Basin has been modified by extensional faulting, characterized by five fault sets: (1) north- to northeast-striking, extensional faults along the eastern margin of the basin and in the interior of the Sierra Blanca massif, many coinciding with and paralleling a major dike swarm between Ruidoso and Capitan; (2) relatively long normal faults, such as the Alamogordo and Godfrey Peak faults, that strike northwest to northeast in the central and southwestern parts of the basin; (3) large-displacement, northwest-striking, oblique-normal faults north of the Three Rivers drainage; (4) low-displacement, west-striking faults south of Oscura that parallel dikes radiating from the center of the Sierra Blanca volcanic field; and (5) northeast-trending faults at the southern margin of the basin.

We interpret a more complex structural history for the basin than past workers, who have restricted basin formation to the Laramide, based largely on past interpretations that Sierra Blanca volcanic strata are not deformed and unconformably overlie lower-middle Eocene strata. We show that lower volcanic strata are indeed deformed and argue that the unconformity at the base of volcanic strata is likely restricted to the southern margin of the basin, where northward tilting created paleotopography. Northwest-oriented extension between 38 and 34 Ma resulted in normal throw for the northeast-striking faults of set 1, downdropping the eastern part of the basin; related fracturing and jointing facilitated ascension of voluminous magmas associated with Sierra Blanca volcanism. The common observation that dikes locally fill northeast-striking normal faults is consistent with dilation preceding and accompanying igneous activity. Subsidence rates may possibly have waned between 34 and 30 Ma, coincident with a change to more alkalic volcanic composition, based on slower stratal accumulation rates and the presence of unconformities. The north-striking Godfrey Peak fault was active after 28.2 Ma. The northwest-striking Jackass Mountain fault was active around 28.5 Ma, based on a dike of that age that intrudes the fault zone and is brecciated by movement along that fault. By Quaternary time only the Alamogordo fault remained active.