A serial cross-section analysis of the Lewiston structure, Clarkston, Washington

Michael Robert Alloway

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 and the Silcott fault system to the southwest. 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 near its eastern and western boundaries. 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 North American continental margin of the Cretaceous Discovery of an angular unconformity below the Grande Ronde Basalt - normal polarity unit 1 (GRB-N1) in addition to a variation of N1 unit thickness across the structure suggests that the fold was deforming before emplacement of N1. Analysis of structural data using the Gauss method for heterogeneous fault-slip data indicate N-S shortening prior to and after N1 emplacement. Analysis of 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 reverse fault on the southern limb of the fold. The reverse fault under debate outcrops to the east of the field area and is GRB-R2 (reverse polarity unit 2) juxtapose with Pliocene (?) gravels. Better control on unit thicknesses and map contacts rule out the surface exposure of a reverse fault on the southern limb of the fold in the field area. This major fault dies out or becomes blind before reaching the ID-WA border and the change in flow attitude from the north side of the river to the south is interpreted to be accommodated by an abrupt fold hinge beneath the Snake River.

A serial cross-section analysis of the Lewiston structure, Clarkston, Washington

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