TECTONIC FRAGMENTATION OF LANDSCAPE DRIVES SPECIES ENDEMISM IN DEATH VALLEY SPRINGS

Wetlands in the Death Valley System (including Amargosa, Owens, and Death Valley) support many endemic species (species that occur nowhere else) of fish and aquatic gastropods. Genetic studies show that their ancestral forms are in western Arizona and northeastern Mexico, and that they arrived in the Death Valley area during the Pliocene (~4-5 Ma). Endemism is driven by many biological and environmental factors, and we show here that tectonic fragmentation of once inter-connected hydrological systems exerted a first order control on the divergent evolution of local pupfish and spring snails.

Specific geological relationships support Pliocene disconnection of Amargosa Valley and opening of modern Death Valley (see also Knott et al., 2019 this session). These include: 1) minor extensional faulting in post-Miocene sediments of southern Amargosa Valley, 2) localization of the Black Mountains fault zone and rapid cooling of Badwater Turtleback at ca. 3.5-5 Ma, 3) a 3.1 Ma switch to locally sourced alluvial fan deposition in the Furnace Creek Formation, 4) new unconformities in Death Valley-bounding ranges after ca. 3.5 Ma, and 5) relatively undeformed Quaternary (ca. 2 Ma) lacustrine beds in Tecopa.

Using these geologic constraints and published extension rates as a guide, we have developed landscape evolution models (see Berry et al., 2019 this session) to simulate drainage disruption and hydrologic isolation in the Death Valley area. The models replicate the age, size, and shape of Late Miocene-Pliocene basins. Models organically generate a hydrological disconnection between eastern Amargosa Valley and Death Valley at ca. 3.5 Ma.

In arid climates, landscape evolution and hydrologic connectivity are strongly controlled by faulting because the escarpments are less smoothed by erosion and local isostatic imbalances are more pronounced. Aquatic ecosystems are often limited to isolated springs in endorheic basins, so localized topography due to feedbacks between tectonics and climate controls the isolation of populations. For spring species, this creates genetic divergence and endemism. Thus, the interrelationships between lithospheric geodynamics and island evolution can be expressed at a number of scales, from continental (e.g. Wegener, 1912 continental drift theory) to regional and local (this study).