NEW CONSTRAINTS ON THE MIDDLE–LATE EOCENE (~46–36 MA) TECTONIC TRANSITION FROM LATE-STAGE LARAMIDE DEFORMATION TO BIMODAL RIFT VOLCANISM (RIO GRANDE RIFT) IN SOUTHERN NEW MEXICO

The final Paleocene–middle Eocene deformational stage of the Laramide orogeny in southernmost New Mexico has been well documented and is recorded by nonmarine clastic strata that are preserved in a series of short-wavelength, wedge-top basins (e.g., Potrillo, and Love Ranch basins). However, while there is a near-continuous stratigraphic record of Paleocene–middle Eocene synorogenic sedimentation throughout south-central New Mexico, very little is known about the age and duration of middle–late Eocene magmatism, volcanism, and volcaniclastic sedimentation that occurred just after Laramide deformation and prior to the onset of latest Eocene–Oligocene bimodal volcanism and initiation of the Rio Grande rift in this region at ~36 Ma. Presented here are new U-Pb ages from volcanogenic and volcaniclastic strata of the middle–late Eocene Palm Park Formation (and equivalent units). All ages are presented in the context of newly measured stratigraphic sections.

Nine new U-Pb igneous zircon ages (n=247 analyses) from intermediate volcanic rocks of the Palm Park Formation fall with a range of 45.0±0.7 to 39.6±0.5 Ma. These new ages together with existing biostratigraphic constraint bracket this phase of post-Laramide/pre-rift volcanism and sedimentation to a ~10 m.y. period between ~46–36 Ma. Four new U-Pb detrital zircon ages (n=403 analyses) from volcaniclastic strata of the Palm Park Formation all exhibit Eocene primary peak ages that fall between 45–41 Ma with rare, isolated occurrences of Jurassic–Cretaceous, Mesoproterozoic, and Paleoproterozoic ages. The abundance of Eocene age detrital zircons compared with the paucity of pre-Eocene age grains is interpreted to reflect little to no detrital contributions from Laramide basement-block uplifts by middle Eocene time. Data presented here support a model where much of the middle–late Eocene landscape in southern New Mexico was dominated by active stratovolcanoes with little to no evidence for deformation associated with Laramide structures. Although the record of Late Cretaceous–Paleocene Laramide magmatism in southern New Mexico is somewhat sparse, it cannot be ruled out that widespread occurrences of middle–late Eocene intermediate rocks in this region record a brief re-introduction of the mantle wedge just prior to the onset of the Rio Grande rift.