KINK-LIKE DEFORMATION ABOVE A BLIND FRACTURE, MALHEUR COUNTY, OREGON

North trending intragraben fault zones within the Oregon-Idaho graben are localized along pre-existing kink-like folds. Many of the fault zones are vertical; therefore, a displacement on these faults produces little shortening or extension normal to their strike. A faulted-kink, here informally called the trap-door kink, is located along the north shore of the Lake Owyhee Dry Creek arm. Structurally, the trap-door kink is in the Dry Creek Buttes intragraben fault zone.

1:3650 structure mapping revealed kink-like folds containing joints, deformation bands, and high-angle faults. The folding is above peperites in basaltic tephra deposits, and tuffaceous and arkosic fluvial layers. At its southern margin, the trap-door kink has dip changes greater than 35°; intermediate to mafic sills and dikes on the western limb; sharply kinked layers in the middle; and relatively constant dip on the eastern limb. This bedding geometry continues along strike to the north.

Most NNE and NW joints are at greater than 75° to bedding; a few are at about 55° to bedding. Joints are spaced about 0.08 m near faults and kink hinges, and 0.5 m where the beds are not tilted. Deformation bands are only found in the coarse-grained (greater than 4F) layers. They increase in thickness approaching faults from approximately 1 cm to 3 cm. Based on field mapping and cross-sections, faults appear to have developed after a kink-like structure had developed.

The trap-door kink suggests shortening. This is consistent with observations of the Burnt Mountain strand, 3.5 km to the east. The Oregon-Idaho graben kink-like folds are, perhaps, a result of local shortening in an en echelon fault system, or above the edges of intrusions. Peperites below the section, hydrovolcanic vents, and basaltic sills and dikes all suggest deformation related to intrusive igneous activity. The geometric change from south to north may suggest the growth of adjacent structures (laccoliths), where the trap-door kink is a structural response between two evolving laccoliths. The geometry of the trap-door kink is consistent with the geometry expected for deformation above a blind fracture.