A SERIAL CROSS-SECTION ANALYSIS OF THE LEWISTON STRUCTURE, CLARKSTON, WA

The Lewiston Structure is located in southeastern Washington / west-central Idaho and is a generally E-W trending asymmetric, non-cylindrical anticline in the Columbia River Basalt Group (CRBG) that transfers displacement into the Limekiln fault system to the southeast. A serial cross-section analysis and 3-D construction of this structure shows how the fold varies along its trend and sheds light on the deformational history of the Lewiston Basin. Construction of the fold’s 3-D form shows that the fold’s wavelength increases and amplitude decreases eastward along its trend. Balanced cross-sections show approximately 5% shortening across the structure which is consistent with the Yakima Fold Belt (YFB). Although the structure is similar to the YFB, it does not form part of a belt and its local nature has been suggested to mark the cratonic boundary of the Cretaceous.

Discovery of an angular unconformity in the Grande Ronde Basalt – reverse polarity unit 1 (GRB-R1) proves that the NE trending section of the fold was deforming during emplacement of GRB-R1 and allows separation of the fold in to two structural domains. Analysis of the two domains using fault-slip data and the Gauss method for heterogeneous fault-slip data indicate NW-SE shortening during GRB-R1 time and N-S shortening for post CRBG emplacement. Furthermore, slip data for strain-inversion and specification of spatial-distribution patterns help identify the existence of a transpressional tectonic environment.

The nature of faulting associated with the Lewiston Structure is a topic of some debate, namely the presence of a high-angle reverse fault on the southern limb of the fold conspicuously hidden by the Snake River. The reverse fault under debate outcrops to the East of the field area and is GRB-R2 (reverse polarity unit 2) thrust over Pliocene gravels. Better control on unit thicknesses and map contacts rule out the possibility of a reverse fault on the southern limb of the fold in the field area. Thus, this major fault dies out before reaching the ID-WA border and the conspicuous change in attitude from the north side of the river to the south is accommodated by an abrupt fold hinge beneath the river.