Stratigraphic and structural relationships of multiply deformed Precambrian metamorphic rocks in the Rio Mora area, New Mexico

Abstract

The contact between a Proterozoic greenstone belt and a Proterozoic quartzite terrane is exposed in the Rio Mora area, New Mexico. Metavolcanic rocks of the Vadito Group, forming the northern edge of the Pecos greenstone belt, crop out along the southern margin of the area. Minor meta-sedimentary rocks are interbedded. A thick section of quartzite, pelitic schist, and graphitic phyllite lies north of the Vadito rocks. This metasedimentary section can be correlated, bed for bed, with Ortega Group rocks of the nearby Picuris Range. The Vadito-Ortega contact appears to be conformable. All rocks are isoclinally folded about south-dipping axial planes, and so Ortega rocks are structurally below Vadito rocks. Most previous workers have accepted this structural superposition as evidence that the Vadito Group is younger. However, abundant cross-beds within Ortega Quartzite, which rests directly against the Vadito, and within quartzites of the overlying Rinconada Formation, show clearly that Ortega Group rocks are younger than Vadito Group rocks. The Vadito-Ortega contact occupies the southern limb of an overturned syncline. Metamorphic rocks have been deformed in two generations of tight to isoclinal folds, with subparallel axes plunging to the southwest. A single F1 syncline is exposed. Its axial plane is folded by an F2 antiform whose axial plane strikes N70°E and dips steeply to the south. Because the two folding events are nearly coaxial, the resulting map pattern appears similar to that formed from a single period of isoclinal folding. Only careful analysis of structural fabrics and structural facing relationships has revealed the multiply folded nature of the rocks.