PROTEROZOIC TECTONIC EVOLUTION OF THE PICURIS MOUNTAINS, NORTHERN NEW-MEXICO

The gross structure of the Picuris Range, defined by the buckled and imbricated km-thick Ortega Formation quartzite, is that of a tight synclinorium bounded by limb-parallel ductile shear zones. Folds are moderately inclined, horizontal, and are locally refolded, cleavage-transected, and cut by ductile faults. Three boundaries separate lithostratigraphic successions: (1) the south-dipping Pilar shear zone, which separates the Glenwoody Formation from overlying Hondo Group, shows both reverse and normal motion; (2) the Plomo fault is a south-dipping, ductile reverse fault, which separates the Hondo Group from the structurally overlying, but older, Vadito Group; and (3) the north-striking Picuris-Pecos fault, which truncates both of the other boundaries, separates supracrustal rocks from an eastern plutonic terrane. Granitoids (1680-1450 Ma) intrude Vadito Group rocks only. The first two boundaries do not juxtapose different tectonometamorphic terranes, even though they are major shear zones. Although the Picuris Range has common characteristics with adjacent ranges, several features mark it as unusual (cleavage-transected folds, a problematic distribution of plutons, and a map-scale anticline). These features may be manifestations of the strain heterogeneity characteristic of the Proterozoic rocks of New Mexico, rather than evidence of diverse tectonic histories. Folding and top-to-the-north shearing occurred during a progressive shortening event, perhaps related to the ca. 1650 Ma Mazatzal orogeny of Arizona and central New Mexico.