Stable isotopic evidence for Neogene surface downdrop in the central Basin and Range Province

Authigenic calcite and smectite stable isotopic records were determined for five Neogene terrestrial stratigraphic sections from the central Basin and Range Province and surrounding regions. We recognized a general pattern of increasing authigenic mineral delta O-18 values with decreasing age since the middle Miocene throughout the region. We suggest this pattern results from regional elevation decrease and associated changes in atmospheric circulation patterns. An anomalous pattern of decreasing delta O-18 values with decreasing age is observed in the Miocene Horse Spring Formation exposed in the Lake Mead Basin, which is contemporaneous with local magmatic activity. We suggest this pattern results from surficial uplift of local highlands feeding water into file Horse Spring depositional system. The topographic histories suggested here provide insight into the response or the land surface to crustal processes. Neogene extensional deformation in the central Basin and Range Province resulted in a net regional elevation decrease.