KINK-LIKE DEFORMATION IN THE OREGON-IDAHO GRABEN, MALHEUR COUNTY, OREGON

The Oregon-Idaho graben is a north trending structural low in eastern Oregon and western Idaho.  Structures and the stratigraphy indicate three development stages.  The second stage resulted in north-striking intragraben fault zones.  These fault zones are localized along pre-existing kink-like folds.  On average, the intragraben fault zones have structural displacements greater than 1 km, but are composed of individual faults with displacements ranging from less than 1 m to 10 m.  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 in the Dry Creek Buttes intragraben fault zone, here informally called the trap-door kink, is located along the north shore of the Lake Owyhee Dry Creek arm.

1:3650 structure mapping and a series of down-plunge views revealed kink-like folds containing joints, deformation bands, and high-angle faults.  Topographic relief in the area also provided details about changes down-section.  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 geometry continues along strike to the north.  The Burnt Mountain strand, 3.5 km to the east, is a portion of the Dry Creek Buttes intragraben fault zone.  Observations of the Burnt Mountain strand, also suggest that a fault formed along one hinge of kink-like folds.

The kinked sedimentary section overlies a sequence of peperites, hydrovolcanic vents, and basaltic sills and dikes.  Although it is not known if these igneous units are folded, field relations and numerical modeling suggest that deformation in the sedimentary section is related to this intrusive igneous activity.  The geometric change from south to north may suggest the growth of adjacent laccolithic-like intrusions, 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.  The trap-door kink and Burnt Mountain strand suggest local shortening.  It appears that the intragraben fault zones are, perhaps, a result of shortening in an en echelon fault system, or above the edges of intrusions.