How long do structures take to form in transpression zones? A cautionary tale from California

It is generally assumed that individual sets of coplanar and colinear defor mation structures form together during events that are of relatively short duration (1-5 m.y,). The record of deformation in a sequence of Late Cretac eous to Holocene sedimentary rocks from the northern Salinian block of Cali fornia spans at least 30 m.y. and illustrates that this assumption is somet imes incorrect. At different localities, geometrically and kinematically id entical contractional structures either predate or postdate local unconform ities of varying age within the succession, so that it is possible to defin e at least four chronologically distinct, but otherwise indistinguishable, deformation episodes. In the absence of the unconformities, the punctuated nature of the deformation would not be apparent, therefore suggesting that subparallel structures may form during successive, distinct deformations sp read out over long time periods. In the northern Salinian block, the inferr ed contractional strain held is approximately normal to the adjacent San An dreas fault and appears to have been consistently oriented in this directio n during deformation events recorded over the past 30-45 m.y. The strain pa ttern is most easily explained by efficient partitioning of transpressional strains into fault-normal shortening and right-lateral faulting during epi sodic regional deformations. We propose that reactivation of preexisting st ructural anisotropies controls the observed partitioning of deformation in many transpression zones.