Interpretation of the Isabella High Wave-Speed Anomaly as the Partially Delaminated High-Density Root of the Southern Sierra Nevada Batholith, California

High resolution tomography of the Sierra Nevada Earthscope Project (Reeg, 2008 & Jones et al., 2012) shows that the core area of the Isabella anomaly (Vp+4-6%) resembles a prolate antiformal slab that plunges steeply SE into the upper mantle to ~200 km depth, extending down from a zone of lower crustal attachment that runs along the southwestern Sierra Nevada and adjacent eastern San Joaquin basin. Receiver function, refraction and tomography also show that areas to the east and south of lower crustal attachment consist of ascended asthenosphere lying directly beneath tectonized Moho. The lower-velocity envelope of the anomaly (Vp+1-4%) extends to 250-300 km depths and covers cross-sectional areas locally in excess of 2x of the higher Vp core. We have leveraged lithospheric structure and geologic history against thermal-mechanical modeling in pursuit of an integrated story for the physical and geologic processes that are governing the development of the anomaly. Initial structure is constrained by mantle xenoliths, differentially exhumed lower crustal exposures, and deep cores in the basin. The initial state further recognizes that: 1. the sub-Sierra Nevada batholith mantle lithosphere, including a substantial thickness (35-40 km) of eclogitic (arclogite) cumulates that were produced during high magma flux arc activity, was cooled to a conductive geotherm by flat slab subduction at the end of the Cretaceous; and 2. the gravitationally metastable mantle lithosphere was thermally mobilized from beneath in the Neogene by the opening of a slab window, which also imposed a state of modest regional extension. We have resolved a class of models that successfully predicts the structure of the anomaly, the timing and kinematics of related lithospheric separation and focused extensional tectonism, the timing and source characteristics of related volcanism, and the spatial/temporal patterns of observed subsidence and uplift transients. A general aspect of most of our model runs is a chain of events that initiates with the basal thermal perturbation and load of the arclogite root inducing Rayleigh-Taylor (RT) instability within the peridotitic lithosphere, as well as the development of a lower crustal channel along the eastern margin of root, which draws lower crust into the eastern Sierra region from the adjacent Basin and Range. These lead to a lithospheric break-off event that corresponds to the ca. 10 Ma inception of the Sierra Nevada microplate, and which further promotes the east to west delamination of the arclogite root. Initial topography is shown to influence the asymmetry of delamination. Much of our model experimentation consists of testing the influence of crustal rheology on model results. We find that a relatively weak crust for the entire microplate best reproduces rock uplift and tectonic subsidence observations, as well as the timing and source characteristics of observed volcanism. We apply the findings of our 2-D models to 3-D relationships across the southern Sierra region in order to elucidate the time transgressive patterns in uplift, subsidence, volcanism and shallow thermal anomalies in relation to the 3-D delamination of the root, and the production of the higher Vp core of the anomaly. These relations suggest a significant compositional component to the core area of the anomaly (deformed arclogite slab), while the peridotitic envelope produces a broad thermally-induced wave-speed anomaly.