NEW VIEWS OF THE MAZATZAL OROGENY—FIELD AND GEOCHRONOLOGIC DATA FROM THE MANZANO THRUST BELT IN CENTRAL NEW MEXICO

New mapping of the relationship between the Los Pinos pluton and the surrounding supracrustal rocks of the Manzano Group, along with new geochronologic data, provide insight into the timing and nature of the deformation related to the Mazatzal orogeny in the Becker Quadrangle, central New Mexico. Previous U-Pb ages in the region (Shastri, 1993) are 1662 ± Ma for the Sevilleta metarhyolite, oldest unit in the quadrangle, and 1655 ± Ma for the Los Pinos pluton which was originally thought to be the youngest Proterozoic unit. New U-Pb zircon ages from the uppermost rhyolite of the Manzano Group show a nearly concordant age of ~1600 Ma. Detrital zircons from the Estadio Canyon quartz arenite, several hundred meters lower in the section, are very well rounded and have near-concordant youngest ages of ~1620 Ma. This suggests there may be an unrecognized unconformity between the lower dominantly volcanic and upper dominantly siliciclastic sections of the Manzano Group.

Three deformation events are recognized in fabrics of the rocks in this region. The first event involved layer parallel shear and local isoclinal folding of units and is interpreted to be related to early thrusting in the Manzano thrust belt. S1 fabrics were subsequently folded into a northeast-trending regional syncline with steeply SE dipping axial plane foliation that is the dominant foliation in the range. Mapping shows that strong S2 fabric in the Sevilleta Metarhyolite is truncated at map scale by the (less-foliated) Los Pinos pluton, suggesting that D2 was synchronous with pluton emplacement. Upper units of the Manzano Group (ca. 1.6 Ga) also contain a strong S2 foliation suggesting a protracted shortening event lasting for more than 50 Ma, from 1.65 to after 1.60 Ga. The third major deformational event is concentrated around the 1.43 Ga Priest pluton to the north; this resulted in localized tightening and refolding of F2 folds and variable development of S3 crenulation cleavage. Further detailed mapping and structural studies are in progress to improve understanding of timing of deformation within the Manzano thrust belt, syntectonic deposition of the Manzano Group, and syntectonic plutonism of the 1.66 to 1.64 Ga plutons within a protracted (1.66 to 1.60 Ga) Mazatzal orogeny.