STRUCTURAL CONTEXT AND AGE CONSTRAINTS FOR THE SYNOROGENIC MARQUENAS FM. METACONGLOMERATE, PICURIS MTNS, NEW MEXICO

The Marquenas Formation in the southwestern United States represents a synorogenic conglomerate associated with the ca. 1.45–1.38 Ga Picuris Orogeny. We determined the U-Pb zircon age of metarhyolite clasts, U/Pb detrital zircon ages from quartzite clasts, and the 3D finite strain of clasts to better characterize the depositional age and nature of deformation across the Marquenas Formation. Detailed field observations, mapping, and structural measurements suggest that the base of the Mesoproterozoic Marquenas Formation is separated from the ca. 1.7 Ga Vadito Group by a 250 Ma angular unconformity. The Marquenas Formation and uppermost Vadito Group are overturned, lie in the hanging wall of the Plomo fault, and are juxtaposed against the Mesoproterozoic Piedra Lumbre Formation in the footwall. U-Pb ages from five metarhyolite clasts yield ages from ca. 1450 Ma to ca. 1715 Ma, and constrain the depositional age to be younger than about 1450 Ma ± 7 Ma. Quartzite clasts yield multimodal detrital zircon age populations with a minimum peak age of ca. 1650 Ma and multiple age peaks between 1700 and 1780 Ma, significantly different from the narrow unimodal detrital zircon age populations of quartzite from the underlying Vadito Group.

We examined the 3D finite strain based on deformed clasts in the upper and lower metaconglomerate members. Multiple flat planar surfaces were photographed at each study outcrop. The perimeters of 100-200 pebbles/cobbles were traced on each image and the shape matrix eigenvector method was used to determine 2D strain. 3D strain ellipsoids were subsequently calculated for multiple outcrops at varying distances from the Plomo Fault. Our results indicate the clasts are oblate, flattened in the plane of the S₂ foliation. The greatest principal strain axes are subhorizontal and close to strike parallel near the unconformable basal contact of the Marquenas. They rotate to more closely down the dip of the regional S₂ foliation at outcrops closest to the Plomo Fault. The finite strain increases and the ellipsoid is more elongate in the X/Y plane (roughly the plane of the S₂ foliation) with proximity to the Plomo Fault, consistent with dip slip movement between about 1450 and 1380 Ma.