TRANSITION FROM ACTIVE BASIN AND RANGE EXTENSION TO CONTRACTION AND BLOCK ROTATION, BLUE MOUNTAINS, NORTHEAST OREGON, SOUTHEAST WASHINGTON, AND WESTERN IDAHO

The transition between transcurrent and transtensional deformation in the California borderland, Sierra Nevada, and western Great Basin and transpressional deformation in Cascadia and the Inland Northwest is preserved in a diffuse belt of structures stretching from the Cascade Range east into the Blue Mountains of northeastern Oregon, southeastern Washington, and western Idaho. A belt of extension localized along the western margin of the continental crust of North America stretches north from the western Great Basin, west of the Owyhee Plateau of southwester Idaho, and forms the boundary between the central Blue Mountains and the western Idaho batholith. In northeastern Oregon and western Idaho, the extensional belt bifurcates with a NS-trending eastern arm forming the western flank of the Idaho batholith and a western arm trending WNW separating the central and northeastern Blue Mountains. Although largely aseismic, the extensional belt is underlain by half-grabens with bounding faults that cut Quaternary alluvium. A composite micro-seismic focal mechanism together with fault slip inversion indicates southwest-directed extension during the Neogene. In contrast, the northern flank of the Blue Mountains records a pattern of mid-Miocene to recent regional uplift and short-wavelength structures formed during thick-skinned regional shortening. Focal mechanisms and fault slip inversion indicate NW-SE contraction. Shortening in the northern Blue Mountains was accommodated by a complex array of faults and folds, whose orientations were controlled by crustal anisotropy formed during the Mesozoic and early Tertiary history of terrane accretion and thick-skinned thrusting in the northern Rocky Mountains. Contractional strain in the northeastern Blue Mountains progressively decreases from west to east and is consistent with clockwise vertical axis rotation about a pole located near Orofino in central-northern Idaho. Clockwise rotation of the Blue Mountain block resulted in separation from the Idaho batholith and development of extensional structures in the east and was accommodated by north-south shortening and formation of structures of the Yakima fold-thrust belt in south central Washington and north-central Oregon.