Regional tectonic inferences for the 1.4 Ga-Holocene lateral slip history of the Picuris-Pecos and related faults, northern New Mexico

A BSTRACT . —Dextral discontinuities evident from analysis of aeromagnetic maps for northern New Mexico, corresponding primarily to the north-striking Picuris–Pecos, the Tusas–Picuris, and the Nacimiento fault systems, together yield at least ~55 km and perhaps as much as ~90 km of net dextral separation. A strike-slip origin for these separations, however, has been clearly demonstrated by geologic mapping only for the Picuris–Pecos fault and, less concisely, for the Tusas–Picuris fault. The age of the Picuris–Pecos fault is younger than ~1.45 Ga because it cuts dated foliations of that age in the Picuris Mountains. The lack of mylonites along the Picuris–Pecos fault also indicates it is not older than ~1.2–0.8 Ga, the age when the basement rocks of the Sangre de Cristo Mountains last cooled through temperatures characteristic of the brittle-ductile transition (300–200°C). To help unravel the reactivation history of the Picuris–Pecos and related faults, the directions of horizontal shortening and/or extension were analyzed for the eight major deformations that have affected the region from 1.4 Ga to the Holocene. For each of these eight tectonic episodes, the resolved lateral shear sense (dextral or sinistral) for north-striking faults in northern New Mexico was inferred. Although there is no direct evidence for Proterozoic slip on the Picuris–Pecos or related faults, such slip seems probable given that Phanerozoic movements often reactivated older structures. Mesoproterozoic slip (~1.4 Ga, ~1.1 Ga) was potentially sinistral based on regional deformation patterns and the postulated long-lived contractional plate margin along southern Laurentia. Neoproterozoic slip (~0.8 Ga) would have been extensional, with a possible sinistral component. Possible Cambrian slip accompanying the opening of the southern Oklahoma aulacogen would have been dextral but of small magnitude (a few km). The earliest documented slip on any of the faults (Picuris–Pecos, Nacimiento) is late Paleozoic. Lateral slip on north-striking faults in northern New Mexico during the late Mississippian–early Permian Ancestral Rocky Mountain orogeny was probably dextral and possibly of large magnitude. Laramide fault slip was also dextral and probably large (tens of km). The lateral slip component during the main phase of Rio Grande rifting (Miocene) was sinistral but of small magnitude. Lateral components during late rifting (latest Miocene–Holocene) are indeterminate, but small. The 55–90 km net dextral separation on north-striking faults in northern New Mexico is probably the cumulative result of numerous tectonic events, not all of them dextral. If our analysis is correct, then the Ancestral Rocky Mountain and Laramide events are most likely responsible for the majority of the dextral separations seen today. The relative importance of dextral contributions by these two orogenies, however, has not yet been determined.