Abstract

The southern California borderland consists of north-northwest-trending ridges and basins with a superimposed west-northwest crosstrend. This topography is the result of folding of a late Miocene to an early Pliocene erosional surface during Pliocene time. There is no direct topographic expression of faults associated with the folding. The crosstrends indicate right-lateral strain. Claycake experiments with simulated convergent wrenching produced folds with little surface expression of the break at the base of the claycake, similar to the structures of the borderland. It is concluded that the upper crust rides on deep-seated mini-plates that are separated by converging right-lateral wrench faults. The upper crust adjusts to the converging wrenching by folding to form long anticlinal ridges and intervening synclinal basins. The vertical component of faulting is small compared to uplift due to folding. The topographic crosstrends are the result of folding caused by local resistance to lateral fault movement.