Published erosion rates suggest that acceleration of
river incision beginning some time before 3 Ma initiated
formation of the deep river canyons in the southern
Sierra Nevada. Such acceleration signals a change
in erosional efficacy but its initial timing is poorly constrained.
Increased erosional efficacy caused by elevation
gain is predicted by scenarios such as block faulting,
mantle lithosphere removal, and passage of a slab
window. The timing and magnitude of elevation gain
may be used to distinguish between competing mechanisms.
As in many landscapes, the small magnitude ( <
1.5 km) and antiquity of river incision in the Sierra
Nevada make the timing of landscape evolution and
its relation to tectonic scenarios inaccessible by most
methods. Until recently, we have lacked the potential
to 'see' erosional events that exhume leas than several
kilometers and that occur over several to several tens
of millions of years
