DISTINGUISHING THE MAZATZAL AND PICURIS OROGENIES IN POLYDEFORMED METASEDIMENTS AND A REGIONAL COMPILATION OF THE 1.4 GA PICURIS OROGENY

The southwestern U.S. Proterozoic exposures provide a record of orogenesis and terrane accretion along a long-lived southern convergent plate margin of Laurentia. The timing of orogenic events and processes of continental growth remain subjects of continued debate. Recent published work has redated the Yankee Jo Formation of AZ and the Trampas Group of NM to be ~1.5-1.46 Ga instead of as Paleoproterozoic metasedimentary rocks associated with the Mazatzal orogeny (1.65-1.6 Ga). These units were deposited after the 1.6-1.5 Ga magmatic gap and just before Picuris orogeny magmatism, deformation, and metamorphism from 1.46-1.35 Ga across a wide region of southern Laurentia. Although Mesoproterozoic tectonism was recognized previously, this has led to naming and new focus on the ~1.4 Ga Picuris orogeny. Here we present new compiled results for Picuris tectonism in the region and more detailed structural analysis of the basal contact of the Trampas Group in New Mexico. Compiled results emphasize the need to unravel a > 150 Ma interval of intracratonic tectonism inboard of the “Nd line” that extends through west Texas.

Structural analyses in polydeformed rocks in the southern Picuris has identified an early isoclinal fold generation in what we interpret to be the uppermost Paleoproterozoic rocks, the Rinconada Formation, that is not present in the lowermost Mesoproterozoic rocks, the Pilar Formation. This suggests a previously unrecognized low-angle angular unconformity below the Trampas Group that represents ~200 Ma hiatus. Compilation of Picuris data shows sedimentation as late as 1.46 Ga, peak triple point metamorphism ~ 1.45 Ga, intrusion of A-type granite ~ 1.45 Ga, and cooling through ~ 400 C by ~1.4 Ga. This implies thrust burial of sediments to ~ 12-15 km between 1.46 and 1.45 Ga followed by exhumation to < 10 km by 1.4 Ga. A similar post- Mazatzal angular unconformity has not been identified in AZ but is hinted at by data from Four Peaks where 1.68 Ga penetratively deformed rocks are overlain by the Yankee Jo Formation. Future work will focus on characterizing the structure and geochronology of field areas with exposed Mesoproterozoic rock that are not currently covered by this compiled dataset.