THE KINEMATIC PARADOX OF THE SAN-ANDREAS FAULT

The Pacific-North America plate boundary along the San Andreas fault s ystem is notoriously a right-lateral transpressive margin where both a lmost pure thrust and strike-slip tectonics take place. The Pacific pl ate travels WNW, forming an angle of about 25 degrees with the boundar y. Since the Pacific is moving WNW faster than North America, right la teral transtension should result along the San Andreas system. North A merica, in turn, travels westward obliquely to the boundary and a left -lateral transpressive component would be expected along the same marg in. Therefore, the right-lateral transpression of the San Andreas syst em can be partitioned into (i) a sinistral transpression along the sou thwestern margin of the North America plate obliquely overriding (ii) a faster right lateral transtension occurring along the transfer margi n of the Pacific plate between the East Pacific rise in the California Gulf and the Gorda ridge to the north-west. This is due to the obliqu e trend of the Pacific and North America plate margins with respect to their motion in a absolute reference frame. The geodynamics of Califo rnia is marked by a unique setting in which there is a special subduct ion where, in contrast with classic subduction zones, the footwall of the subduction plane is obliquely diverging from the hanging wall in a n E-W section, while it is converging at slower rates in a NE-SW direc tion. The extensional E-W component is absorbed into the Basin and Ran ge rifting, whereas the compressive NE-SW component is mainly expresse d in the Coast Ranges and California offshore. The compression perpend icular to the San Andreas is then not intrinsic in the strike-slip mov ement, but it is rather an independent tectonic factor. Therefore, the San Andreas system cannot be considered as an archetype of a pure str ike slip fault.