THE MARQUENAS FORMATION: A CA. 1.4 GA SYN-OROGENIC CONGLOMERATE IN THE PICURIS MOUNTAINS OF NORTHERN NEW MEXICO

The Marquenas Formation, exposed in the Picuris Mountains of northern New Mexico, consists of 500 meters of polymictic boulder conglomerate, micaceous quartzite, and pebble-to-cobble conglomerate. To the south, the Marquenas Formation is in contact with metasedimentary and metavolcanic rocks of the Vadito Group. To the north, it is in contact with Ortega Formation quartzite across the Plomo fault. Quartzite clasts in the Marquenas Formation are strongly flattened in a mylonitic foliation that parallels the fault contact, and elongated clasts define a prominent south-plunging stretching lineation with reverse-sense (top-up-to-the-north) kinematics. The unit also contains a penetrative tectonic foliation that is similar in orientation to S2 foliation in rocks to the north and south. New detrital zircon data indicates that the Marquenas Formation is not correlated with the Vadito Group as previously proposed. Hondo and Vadito Group samples were derived primarily from >1700 Ma sources, but abundant <1700 Ma detrital zircon in the Marquenas Formation suggest that it was derived from a mixture of local Yavapai-aged sources and younger Mazatzal-aged sources to the south. Furthermore, a 1459 Ma age peak (n=10) in the upper Marquenas Formation requires a Mesoproterozoic depositional age. Thus, the southern contact with the Vadito Group must be either an unconformity or shear zone. We interpret the Marquenas Formation to represent a syn-orogenic succession that was deposited during regional deformation, metamorphism, and magmatism in the southern Rocky Mountains ca. 1.4 Ga. We interpret the boulder conglomerate and associated quartzite to represent an alluvial fan deposit that formed in response to a growing orogenic highland to the south. These highlands may be associated with ca. 1.4 Ga movement along the Plomo fault locally or the Manzano thrust belt to the south. Deformation and metamorphism of the Marquenas Formation involved juxtaposition against poly-deformed, amphibolite-facies rocks of the Hondo Group along the ductile Plomo fault and must have occurred shortly after deposition. This finding represents a new line of evidence in support of regional arguments for orogenesis at ca. 1.4 Ga in a convergent or transpressional tectonic setting.