During a post-Turonian (Late Cretaceous)/pre-Pliocene deformation, the pre-Cretaceous basement rocks near Ono, California, yielded along several northerly trending, right-lateral slip faults; but apparently these faults died out in the thick, marine mudstones and sandstones of the overlying Budden Canyon Formation (Cretaceous). Instead, a swarm of approximately 150 sandstone dikes, individually up to 8 feet wide and several miles long, cuts Budden Canyon strata. The dikes are vertical or nearly vertical and most strike 45°–65°E. of the fault trends. Apexes of the acute angles between the two sets of structures point toward the direction of fault slip. This structural arrangement closely resembles experimentally-produced deformation patterns and suggests that the dikes represent fillings of tension gashes (feather joints) developed in the incompetent marine unit in response to strike-slip faulting in the underlying competent basement complex.