Transform-normal extension and asymmetric basins: an alternative to pull-apart models

Continental transforms are commonly associated with relatively small scale pull-apart basins. However, much larger scale basins, which are not consistent with the pull-apart model, commonly exist adjacent to transforms. Structures associated with several basins along the Dead Sea rift and El Pilar fault illustrate this. These basins are remarkably asymmetric, bounded by essentially linear transform segments on one side and subparallel normal faults on the other, suggesting simultaneous strike-slip motion and transform-normal extension. This observation, which is incompatible with classical faulting theory, can be explained along divergent plate boundaries if the transform fault is much weaker than the adjacent crust. Analysis of the orientation of the horizontal principal stress direction near a weak transform fault embedded in an otherwise strong crust shows that convergent and divergent plate motion results in a stress field characterized by nearly fault-normal compression and extension, respectively. Structural styles along the San Andreas, Dead Sea, and El Pilar transform systems show that predictable changes between these two states have occurred through time along strike of the transforms. -Authors