THE LARAMIDE OROGENY IN SW NEW MEXICO: EVIDENCE FOR A BROAD OROGENIC PLATEAU

Geodynamic models seeking to explain the tectonics and crustal structure of Laramide orogenesis in western North America require geologic input in the form of temporal patterns of basin and uplift development within the orogen, duration of attendant crustal deformation, and paleoelevation reconstructions across the orogen as determined from multiple proxies archived in synorogenic strata. Spatial and temporal distribution of deformation, its structural style, and more recently, its paleoaltimetry are reasonably well understood in the classic Laramide provinces of the Colorado Plateau and Rocky Mtns, where iconic uplift-basin pairs have been studied for decades. In contrast, a large expanse of the Laramide orogen lies south of the Colorado Plateau in SE AZ and SW NM. In MT, WY, CO, UT, and northern NM we note a correspondence between the end of Laramide-style shortening, the timing of attainment of a high-standing plateau (the “Nevadaplano”) and the timing of late- and post-Laramide magmatism. This pattern continues in AZ and NM but without any information about paleoelevations. Our preliminary analysis suggests a much higher-standing land surface during Laramide deformation than the present in southern NM and our recent work suggests Laramide deformation continued to ~30 Ma. Paleosol carbonate from the fluvial Baca Formation (late? Eocene) in the Sawtooth Mtns of western NM yields δ¹⁸O values between -10 and -11‰. Early cements in sandstone from the same section returned much lower values, approximately -16‰. The paleosol results suggest paleoelevations of the late-Laramide Baca Basin perhaps 500 meters higher than current elevations of around 2400 masl. Lacustrine carbonates from the Oligocene Mineta Fm from the San Pedro valley in SE AZ returned δ¹⁸O values as low as -14‰, suggesting Oligocene elevations of ~2 km, about 1 km higher than now. Volcanic glasses have been obtained the Oligocene Lemitar tuff near Socorro, NM. These results returned reconstructed δD values of around -60‰ suggesting elevations similar to the current elevation of 1.5 km. We therefore hypothesize that the Nevadaplano of the northern Rockies, where the correspondence between deformation and plateau development is reasonably well established, extended into the southern Rockies of AZ, NM and northern Mexico but not as far east as Socorro.