If the Western US is kept fixed and the global circuit reconstruction method is used to determine the position of the East Pacific Rise it can be seen that there is a significant overlap of the two ( fig 1).

If we accept that there cannot be a spreading center beneath a continent then it has to be deduced that the Western US has experienced a large amount of deformation in response to the plate interactions at its boundary.

There is indeed a plethora of literature that is concerned with deformation especially concentrating on the Basin and Range, coastal and offshore California and the San Andreas itself. These mainly deal with specific areas to unravel the geological tale for the past 30 Ma. The trick is to combine all the available data to reconcile the plate boundaries leaving no overlap and honor the majority of data found from smaller studies. This has been attempted by Atwater and Stock (1998) and the rest of this page will present their assumptions for Western US deformation which in themselves have been synthesized from many workers.

There are some large scale assumptions that can be dealt with first regarding the maximum and minimum deformation allowable. Looking at coast perpendicular ( assumed to be N60E ) deformation within the North American plate, it can be seen that this must equal or exceed the coast perpendicular motion calculated in the plate circuit solution. If it can be assumed that shear is caused only by the plate interactions and if an unknown quantity of that shear is accommodated offshore then the total deformation parallel to the to the coast must be less than that calculated for the Pacific — North American motion. When the two become fully coupled ( ~18Ma ) it should be expected that the coast parallel components should be equal. With these restrictions in place the following local assumptions have been made.

1. The Colorado plateau is assumed to be rigid block with little internal deformation.
2. This is also held true for the Sierra Nevada-Great Valley block although to accommodate variation in Basin and Range extensional it is assumed that some deformation in the Southern Sierras did occur.
3. Basin and Range extensional values can be seen to vary both through time and spatially with a range between 150 — 235 km in a coast perpendicular direction since 24 Ma.
4. Extension within the Rio Grande rift can be seen to be small relative to the other deformations. A value of 15km of extension is found in a coast perpendicular direction for the past 24 Ma.
5. The San Andreas fault has seen 315 ± 10 km of strike slip displacement within the last 23 ma based on reconnecting piercing points ( the Neenach volcanics ). The majority of this slip budget occurred later than 8 Ma ( ~200km )
6. Independent estimates of Californian deformation can be found using the rotation of the Transverse Ranges ( 110 degrees clockwise — Dickinson, 1996 ) and the partly associated extension ( ~100km ) in the Inner Californian borderland ( Crouch and Suppe, 1993 )
7. There are other assumptions concerning the rotations of the Tehachapi mountains and the Oregon Coast ranges along with the translation of Baja California but the majority have been covered. All the above assumptions are included in figure 2, which compares the coast parallel and perpendicular motions for both the global circuit reconstructions and the Western US deformation.