Some Thoughts on Structural Evolution of Eastern Boundary of Columbia Basin

Richard L. Thiessen, David R. Gaylord, Gregory B. Mohl

The geologic evolution of the western North American cratonic boundary has had an important influence on the character of the sedimentary basins that developed near the eastern margin of the Columbia basin. Latest Precambrian rifting, followed by deposition of a miogeoclinal wedge, formed the cratonic margin. Subsequently, during the mid to late Mesozoic, allochthonous terranes were accreted onto the craton causing extensive deformation, intrusion, and metamorphism. Directly to the east of the Columbia basin there is a gap in accreted terranes, and the miogeoclinal wedge of the cratonic margin appears not to have been subjected to these orogenic effects. However, the miogeoclinal wedge may have undergone transform motions at various times in its history. The cratonic marg n in British Columbia and northeast Washington is characterized by a line of gravity maximum and a parallel gradient eastward toward the continent. A similar signature is seen for the north-south portion of the continental margin in west-central Idaho, and we have extended this gravity signature westward in southeastern Washington under the basalt, to where it intersects the Klamath-Blue Mountain lineament. The gravity signature then turns to the north or northeast forming a wedge-shaped portion of craton that extends into Washington. Near Spokane, Washington, this signature again turns to the west, and intersects with the north-northeast-oriented craton/Quesnellia Terrane suture. The irregular shape of the cratonic margin may reflect initial Precambrian rifting, impact of accreted terra es during the Mesozoic, Eocene shearing, extension and rotation, and later rotations due to shearing of the Cordillera and opening of the Basin and Range.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.