Abstract Title: Constraints on Basin and Range lithospheric dynamics from the 
Global Positioning System

Abstract Author(s): Hammond, William (U.S. Geological Survey)  

Abstract: The spatial variation of strain rates measured with the Global 
Positioning System (GPS) places strong constraints on the horizontal variation 
of the Basin and Range lithospheric rheology. Previous results have shown that 
near 39? N latitude there are ~10 mm/yr of deformation concentrated in the 
westernmost 150 km of the Basin and Range. However, north of 41? N latitude, 
while the kinematic requirement to absorb ~10 mm/yr of relative motion between 
the eastern Basin and Range and the Sierra Nevada/Great Valley block is the 
same, the zone of deformation observed by GPS is much wider. I present a 
synthesis of newly obtained and previously published GPS observations obtained 
in California, Nevada, Oregon, and Utah by the USGS. I use these in combination 
with results from regional continuously recording networks to derive a regional 
strain rate field with a finite element approach. At 39? N latitude, 
comparisons between the observed and predicted deformation based on a 
two-dimensional thin viscous sheet under gravitational load suggest that the 
vertically averaged viscosity of the lithosphere is lowest in the westernmost 
150 km. Using new measurements from the northwest Basin and Range, I will test 
whether this rheologically weaker zone extends north of 41? N latitude by 
constraining the location of elevated shear and dilatational deformation. 
Compared to 39? N latitude, the broader and less intense deformation field at 
41? N latitude is consistent with greater lithospheric viscosity, or with a 
more gradual west to east viscosity increase.