GEOMETRY AND TIMING OF MOVEMENTS IN THE PROTEROZOIC MANZANO THRUST BELT, CENTRAL NEW MEXICO

Mapping and monazite geochronology of rocks of the Manzano Mountains provide new data for understanding Proterozoic stratigraphy, large-scale structure, and timing of movements in the Manzano thrust belt, a major NE-striking Proterozoic thrust belt. Three generations of structures have been identified from overprinting relationships. F1 folds are west-verging isoclinal nappe-style folds, preserved mainly in banded siliceous layers in the Blue Springs schist. F2 folds are upright NE-striking structures, with variable plunge. The S2 axial plane foliation is the dominant fabric in the range. D3 deformation tightened and refolded the earlier folds and fabrics, reactivated older foliations, and created kinks and NW- verging shear bands. All three deformational events postdate the 1656 Ma Monte Largo pluton. The Manzano thrust belt was active ca. 165O Ma based on the synkinematic nature of the Manzanita pluton and growth of 1.65 Ga syntectonic monazite in the aureole of the 1.66 Ga Ojito pluton. D1 and D2 predate the 1427 Ma Priest pluton based on truncation of S1 foliation and a macroscopic F2 syncline at the north end of the pluton. The large scale geometry of D2 folds and the Monte Largo thrust in the Manzano mountains is consistent with a ramp-flat geometry related to the NW-directed thrust belt. Renewed movements on the Monte Largo thrust, tightening of D2 folds and the D3 event are interpreted to have taken place during and after emplacement of the Priest pluton at ca. 1427 Ma. Monazite within andalusite in quartzite of the aureole of the Priest pluton is ca. 1420 Ma. Matrix monazite are younger, about 1380 Ma, suggesting that thrust movement, and monazite growth continued significantly after pluton emplacement. These dates are compatible with ~1380 Ma Ar ages on muscovite. We remain uncertain whether the microstructural evidence for multiple generations of staurolite and garnet is best explained by separate 1.65 and 1.4 Ga metamorphisms, or the protracted ca. 1.4 Ga event. The Manzano Mountains are an excellent example of a long-lived orogeny and reactivation of structures in the middle crust.