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Abstract

Eocene right-lateral displacements occurred along several major fault zones in western and central Washington, including the Straight Creek fault, the Entiat-Leavenworth fault system, and probably the Puget fault, a covered north-trending structure in the Puget Lowland. Within this strike-slip framework, nonmarine sediments accumulated in the Chuckanut, Puget-Naches, Chiwaukum graben, and Swauk Basins to form some of the thickest (more than 6,000 m) alluvial sequences in North America. To varying degrees, the basins are characterized by (1) high sediment-accumulation rates, implying rapid subsidence; (2) abrupt local stratigraphic thickening and thinning; (3) intrabasinal and basin-margin unconformities; (4) abrupt facies changes; (5) fault-induced drainage re-organization; (6) intermittent internal drainage; and (7) interbedded and intrusive relationships with extension-related(?) volcanic rocks. Similar sandstone petrography between basins suggests a common sediment source with possible local or temporary connection between basins. Sedimentation and deformation throughout the province were diachronous, and the basins experienced rapid alternating subsidence and uplift. Orientations of folds and faults are consistent with regional right-lateral shear.

The 90-km-wide composite Chuckanut-Puget-Naches Basin probably formed as a pull-apart basin between the Straight Creek and Puget faults. The 50-km-wide Swauk Basin may have formed as a fault-wedge basin between the Straight Creek and Entiat-Leavenworth faults. These basins are large when compared to most modern and ancient basins controlled by strike-slip faults, suggesting that processes of strike-slip basin formation operate at a variety of scales. The 20-km-wide Chiwaukum graben probably formed as a pull-apart basin between the Entiat and Leavenworth faults.

Eocene strike-slip faulting was probably driven by oblique convergence of the Kula plate below North America. Although strike-slip basins in Washington occupied a forearc or possibly an intra-arc tectonic setting in this ancient continental margin, they differ significantly from typical forearc or intra-arc basins.

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