The kinematic paradox of the San Andreas Fault

ABSTRACT The Pacific‐North America plate boundary along the San Andreas fault system is notoriously a right‐lateral transpressive margin where both almost pure thrust and strike‐slip tectonics take place. The Pacific plate travels WNW, forming an angle of about 25° with the boundary. Since the Pacific is moving WNW faster than North America, right lateral transtension should result along the San Andreas system. North America, in turn, travels westward obliquely to the boundary and a left‐lateral transpressive component would be expected along the same margin. Therefore, the right‐lateral transpression of the San Andreas system can be partitioned into (i) a sinistral transpression along the southwestern margin of the North America plate obliquely overriding (ii) a faster right lateral transtension occurring along the transfer margin 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 oblique trend of the Pacific and North America plate margins with respect to their motion in a absolute reference frame. The geodynamics of California is marked by a unique setting in which there is a special subduction where, in contrast with classic subduction zones, the footwall of the subduction plane is obliquely diverging from the hanging wall in an E‐W section, while it is converging at slower rates in a NE‐SW direction. The extensional E‐W component is absorbed into the Basin and Range rifting, whereas the compressive NE‐SW component is mainly expressed in the Coast Ranges and California offshore. The compression perpendicular to the San Andreas is then not intrinsic in the strike‐slip movement, but it is rather an independent tectonic factor. Therefore, the San Andreas system cannot be considered as an archetype of a pure strike slip fault.