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1 New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA
2 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
3 New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA
Aeromagnetic maps from north-central New Mexico show east-northeast–trending linear features that are offset dextrally across the north-striking Picuris-Pecos, Tusas-Picuris, and (possibly) Nacimiento fault systems. Geologic structures that correspond to these regional aeromagnetic lineaments, where exposed in Phanerozoic uplifts, are major ductile shear zones of Proterozoic age that juxtapose folded metasedimentary rocks associated with areas of low aeromagnetic value with metavolcanic and metaplutonic rocks associated with areas of high aeromagnetic value. Regional aeromagnetic lineaments serve as piercing lines that suggest at least 
55 km and perhaps as much as 90 km of net dextral separation since 1.4 Ga. The restored aeromagnetic lineaments are interpreted to represent a series of east-northeast–striking, dominantly north-vergent, thrust-sense shear zones that formed initially during the ca. 1.65 Ga Mazatzal orogeny and were variably reactivated during the ca. 1.4 Ga magmatic and deformational event.
There is no direct evidence for Proterozoic strike or dip slip on the Picuris-Pecos fault, although such slip is possible. The lack of mylonites and other ductile deformation along the Picuris-Pecos fault indicates that it is not older than ca. 1.2–0.9 Ga, the age when the basement rocks of the Sangre de Cristo Mountains last cooled through temperatures characteristic of the lower limits of ductile deformation (300–250 °C).
The 55–90 km net dextral separation on major north-striking faults in northern New Mexico is the cumulative result of numerous tectonic events, not all of them dextral. The directions of horizontal shortening and/or extension were analyzed for the six major deformations that have affected the region since peak metamorphism at ca. 1.4 Ga. For each of these tectonic episodes, the resolved lateral shear sense on north-striking faults in northern New Mexico was inferred from regional deformation patterns. Grenville (ca. 1.1 Ga) and Neoproterozoic (ca. 0.7 Ga) slip potentially had sinistral components. Cambrian slip accompanying the opening of the southern Oklahoma aulacogen potentially had a small dextral component (a few kilometers). Lateral slip during the late Mississippian–Early Permian Ancestral Rocky Mountain orogeny was probably dextral and possibly of large magnitude. Laramide fault slip was dextral and probably of large magnitude (tens of kilometers). The lateral slip component during the main phase of Rio Grande rifting (Miocene) was sinistral, but of small magnitude. The Ancestral Rocky Mountain and Laramide events thus appear to have been largely responsible for the dextral separations seen today. The relative importance of dextral contributions by these two orogenies, however, has not yet been determined.
Keywords: aeromagnetic • Laramide • Grenville • Ancestral Rocky Mountains • strike slip
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