Taking the conclusions made from observations the blurred picture begins to clear.  It is clear that the present topography was created in the last 10 to 20 Ma.  This coincides well with initiation of Basin and Range extension as well as the creation of the "slab window" beneath the Sierra with the end of Subduction.  This window could allow upwelling of hot lower mantle material previously shielded by the slab.  In addition, the extension occurring to the east created a mass deficit that needed to be filled.  A logical choice would seem the newly active lower mantle.  This upwelling could increase extension rates forcing overall deformation to be accomodated in the lithosphere as well as the crust.  This thinning of the mantle lithosphere could have taken place underneath the Sierra allowing Bouyant,hot material to take the place of the removed upper mantle.  In addition this upward convection could have forced the displaced lithosphere down possibly explaining the Isabella anomaly.  This convection process can be seen in a numerical model done by Bertram Schott.  This is a very nice fit to the earlier conclusions.  The gravity observations predicted a recent upper mantle feature providing support for the Sierra.  Magnetotelluric data was best fit with a partially molten body in the upper mantle which could be the result of decompression melting.  Pn velocities are indicative of a thermal anomaly in the upper mantle.  After considering these as well as many other observations, [Jones et al.,1998] proposed the following model based on previous work of Brian Wernicke.

 
 
 

 
 
 
 
 
 

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