Diffuse plate boundary and microplate motion: is the Sierra Nevada an independent block?

The North America/ Pacific Plate boundary is an example of diffuse plate boundary where the motion is accommodated along localized shear zone dividing more rigid blocks. The Sierra Nevada Great Valley microplate (SNGV) is one of such blocks. While the western and south border of the microplate are clearly marked by the San Andreas Fault and its restraining bend, the eastern and northern boundary are more difficult to identify. The southern part of the eastern boundary is clearly marked by the presence of a fairly localized shear zone accommodating ~20% of the North America Pacific plate relative motion, the Eastern California Shear Zone. Moving northward the Walker Lane accommodates the relative displacement on a more diffuse zone and the northern extent of the microplate is not unequivocally identified. Using a compilation of geodetic measurement, we study the motion of the SNGV microplate and the strain accumulated at its borders to quantify to which extent we can really consider the SNGV an independent rigid block with respect to the North American Plate. Preliminary maximum shear strain rate computed using the method of Hackl et al 2009 and available GPS velocity field for the Western United States. Crosses indicate the direction of maximum shear strain rate.