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

The gross structure of the Picuris Range, defined by the buckled and i mbricated km-thick Ortega Formation quartzite, is that of a tight sync linorium bounded by limb-parallel ductile shear zones. Folds are moder ately inclined, horizontal, and are locally refolded, cleavage-transec ted, and cut by ductile faults. Three boundaries separate lithostratig raphic successions: (1) the south-dipping Pilar shear zone, which sepa rates the Glenwoody Formation from overlying Hondo Group, shows both r everse and normal motion; (2) the Plomo fault is a south-dipping, duct ile reverse fault, which separates the Hondo Group from the structural ly overlying, but older, Vadito Group; and (3) the north-striking Picu ris-Pecos fault, which truncates both of the other boundaries, separat es supracrustal rocks from an eastern plutonic terrane. Granitoids (16 80-1450 Ma) intrude Vadito Group rocks only. The first two boundaries do not juxtapose different tectonometamorphic terranes, even though th ey are major shear zones. Although the Picuris Range has common charac teristics with adjacent ranges, several features mark it as unusual (c leavage-transected folds, a problematic distribution of plutons, and a map-scale anticline). These features may be manifestations of the str ain heterogeneity characteristic of the Proterozoic rocks of New Mexic o, 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 ce ntral New Mexico.