LATE QUATERNARY DEFORMATION, SADDLE MOUNTAINS ANTICLINE, SOUTH-CENTRAL WASHINGTON

Grabens and beheaded streams above a subjacent thrust fault illuminate late Quaternary deformation on the north limb of the Saddle Mountains anticline, Yakima fold belt, Washington. Geologic mapping and trenche s excavated across a scarp on the north flank of a 13-km-long graben a nd a +/-5-m-high scarp north of the graben, confirm that deformation i s related to coseismic slip on the underlying Saddle Mountains fault. Graben development began ca. 100 ha and continued into the Holocene. A t least 6.5 m of vertical displacement in a ca. 20-40 ka paleosol yiel ds minimum normal fault slip rates of 0.16-0.33 mm/yr, Assuming that n ormal slip in the hanging wall above the daylighting fault tip is rela ted to primary reverse slip on the fault, resolution of throw on the 3 0 degrees-dipping Saddle Mountains fault yields minimum slip of 13 m a nd minimum slip rates of 0.33-0.65 mm/yr, 2.3-9.4 times greater than s lip rates used in a recent seismic hazard analysis for the Hanford Res ervation.