SEDIMENTOLOGIC AND ISOTOPIC CONSTRAINTS on THE CENOZOIC TOPOGRAPHIC EVOLUTION OF THE WESTERN US CORDILLERA AT ~36°N

Widespread Cretaceous marine deposits require ~ 2 km of topographic uplift of the Colorado Plateau during the Cenozoic, however, the exact timing of this uplift remains poorly constrained, leaving open questions about the geodynamic driving forces of this uplift, and their relationship to the topographic and paleogeographic evolution of the neighboring central Basin and Range. Previous work has suggested the early Cenozoic northwestern margin of the Colorado Plateau was flanked by a high elevation (> 3 km) ‘Nevadaplano’ prior to mid-Cenozoic Basin and Range extension. Evidence for such a high elevation continental plateau at the latitudes of the central and southern Plateau (35-37°N), however, is lacking. We use a combined detrital zircon and stable isotopic (δ¹⁸O) approach to constrain the sedimentary provenance and paleoelevation of early Cenozoic basins from the western Colorado Plateau margin to the paleo-Pacific coast, ~250 km to the west in tectonically reconstructed coordinates. Detrital zircon provenance of near sea-level Paleocene-Eocene sedimentary rocks (now exposed at ~1200 m) within and adjacent to the southern Sierra Nevada indicate that coastal drainages were isolated from the continental interior, while δ¹⁸O values from Paleocene limestones on the leeward side of the Sierra Nevada suggest these basins were subject to rainout over an orographic barrier of modest elevations (1-2 km). Relatively constant δ¹⁸O values of Paleocene to mid-Miocene central Basin and Range meteoric water proxies suggest only minor changes (≤ + 1 km) in regional elevations through the mid-Cenozoic. This result implies that the southward continuation of a high-standing ‘Nevadaplano’ to the latitude of the central Basin and Range is unlikely, and that the paleoelevation history of the central Basin and Range contrasts with recent studies suggesting rapid, and significant, early Cenozoic uplift of the Colorado Plateau. The observed > 5‰ increase in central Basin and Range δ¹⁸O records from mid-Miocene to present, then, likely reflects a shift from Pacific dominated moisture sources to more southerly sources introduced as a result of crustal extension in coastal California and late Miocene opening of the Gulf of California, rather than significant regional topographic changes.