Abstract Title: RayleighÐTaylor-type Instability during Continental Breakup: An 
Example from the Eastern California Shear Zone

Abstract Author(s): Zandt, George (U. of Arizona)- Jones, Craig (U. of 
Colorado)- Gilbert, Hersh (U. of Arizona)- Owens, Tom (U. of South Carolina)  

Abstract: RayleighÐTaylor-type instability is often described in the context of 
how mantle is removed beneath compressional or transpressional orogens (e.g., 
Tibet, New Zealand). Yet, one of the best examples of this phenomenon is 
provided by an ongoing removal of the dense batholithic root beneath the 
southern Sierra Nevada, located near the transtensional boundary between the 
Sierra Nevada-Great Valley (SNGV) block and the western edge of the Basin and 
Range. Geologic and geophysical data constrain the initiation of the removal 
between 10 and 3 Ma with a RayleighÐTaylor-type instability, but with a 
pronounced asymmetric flow into a mantle downwelling (drip) beneath the 
adjacent Great Valley. Seismic receiver function imaging reveals a nearly 
horizontal bottom-to-the-SW shear zone that accommodated the detachment of the 
dense ultramafic root from its granitoid batholith. With continuing flow into 
the mantle drip, viscous drag at the base of the remaining ~35-km-thick crust 
is driving present-day surface subsidence in the Great Valley, and peripheral 
uplift in the High Sierra. Counterflow upwelling of asthenosphere beneath the 
adjacent western edge of the Great Basin is associated with active volcanism 
(Big Pine and Long Valley) and zones of extreme extension (Death Valley). On a 
regional scale, the eastern California shear zone is accommodating 
transtensional moton of the SNGV block away from the Great Basin and may 
represent the early stages of the formation of a new plate boundary. 
RayleighÐTaylor-type instability may be a more general mechanism for removing 
lithospheric material and promoting the fast breakup of continental lithosphere.