THE NEVADAPLANO: WHEN, WHY, AND HOW HIGH?

Despite agreement that an elevated plateau occupied the hinterland of the Sevier belt in the area of the present-day Great Basin during Cenozoic time, questions persist about its height, the thickness of the crust that supported it, and how and when it developed. Suggestions that crust in this area reached thicknesses of 60 km or more are challenging to explain given the present ~28–34 km thick crust and estimated ≤ 50% extension. Moreover, implications posed by widespread, south-migrating middle Cenozoic ignimbrite flareup volcanism, which followed Mesozoic crustal shortening and preceded Neogene Basin and Range extension, are rarely considered. Thick volcanic successions were erupted across the Great Basin, with potentially much more plutonic material added to the crust, and heat flow increased sharply as indicated by partial melting in metamorphic core complexes. Here we integrate records of faulting, sedimentation, erosion, and magmatism, focusing on the Elko area of northeast Nevada, where we employ new geochronology and geologic mapping to improve age constraints for the sedimentary rocks targeted for studies of regional paleoelevation. We also present a compilation showing that the age of fill in paleovalleys developed across the Great Basin decreases southward roughly synchronous with migrating volcanism ca. 45–20 Ma. These datasets indicate that uplift accompanied volcanism, disrupting drainage systems and progressively shifting the north–south continental divide east into central Nevada from its Late Cretaceous location along the Sierra Nevada arc. Despite local faulting, the Great Basin was an elevated volcanic tableland until dissection by Basin and Range faulting began ca. 17 Ma. We suggest that changing boundary conditions and effects of magmatism including thermal weakening, crustal thickening, and thermally supported elevation were all responsible for the onset of Neogene extension. Because these causal factors arose long after Mesozoic shortening, they do not require development of a strongly elevated Nevadaplano until the onset of magmatism. Given the results of prior clumped isotope studies and the location of the Paleocene shoreline within the Mojave region slightly to the south, it is unlikely that elevations in Nevada exceeded ~2.0–2.5 km before middle Cenozoic magmatism.