New Mexico Geological Society Annual Spring Meeting — Abstracts

The objective of this research is to address the inconsistencies between geologic and isotope-based evidence of the timing and elevation of the southern Sierra Nevada Mountains (Sierra) during the Miocene. Geologic evidence suggests that the southern Sierra uplifted ~2000 m in the last 20 Ma, whereas, isotope-based paleoaltimetry studies suggest little to no elevation change in the last 16 Ma. From incision rates in the Sierra, as well as sedimentation rates in the Great Valley, it is hypothesized that there have been two periods of uplift in the last 20 Ma. The first uplift event ca. 20 Ma is thought to be driven by the opening of a slab window. The second occurs post-3.5 Ma and is thought to be the result of mantle delamination beneath the southern Sierra. Isotope-based paleoaltimetry studies rely on the linear relationship between Δδ¹⁸O and net elevation. Records of paleometeoric waters from the leeward side of the Sierra show relatively little change in the δ¹⁸O values during the last 16 Ma which are interpreted to suggest that there has been relatively little change in elevation of the southern Sierra during the Miocene. There are several complications with interpreting isotope records for paleoaltimetry, most notably is that modern air trajectories for the Sierra are inconsistent with the underlying 2-D assumptions used in the isotope-based approach. Due to complexities introduced by 3-D flow patterns we suggest that the isotope-based approach may not be able to constrain the uplift history of the southern Sierra during the last 16 Ma.Atmospheric flow over topography can be understood, to a first order, in terms of the nondimensional flow parameter Nh/U; where N is the buoyancy frequency (s^-1), h is the mountain height (m), and U is the horizontal wind speed (m/s). Idealized models of flow around a topographic barrier demonstrate that when Nh/U << 1, flow is more likely to go over the topographic barrier but when Nh/U >> 1, flow is deflected around a topographic barrier. Using General Circulation Models (GCMs) for both Miocene (20 Ma to 14 Ma) and modern climate we characterize the annual average climate climates in terms of Nh/U. For the incoming offshore conditions for the Sierra we find that N_modern < N_Miocene , meaning that in flow is more deflected for Miocene climates. We also find that the annual average U wind profiles are very similar. Trajectory analyses from simulations of flow around an idealized Sierra, assuming that the Miocene Sierra were high, demonstrate that for annual average simulations for the Miocene and modern climate (N and U) that most of the incoming trajectories are deflected around the highest terrain. This suggests that during the Miocene the annual average climate state may not have been different enough from modern to support the underlying assumption of 2-D flow used in isotope-based paleoaltimetry studies.