Primary and ‘forced folds’ of the Columbia River basalt province, eastern Washington, USA

Abstract

The Yakima fold belt is a series of asymmetric, east-west anticlines separated by much broader synclines in the thick and otherwise horizontal sequence of Columbia River basalt flows. The folds have grown progressively over the last 17 Ma in response to regional north-south compression and east-west extension. This stress and strain pattern is reflected on a regional scale by the highly oriented north-northwest south-southeast basalt feeder dykes, by the WNW-(right-lateral) and NNE-(left-lateral) trending conjugate fault system, and by the presence of a similar strain field measurable in the basalt flows today.Peripheral to the Yakima fold belt, pre-basalt structures of different orientations project under the basalt pile. Some of these structures have continued to develop during and after basalt extrusion and have influenced structures in the basalt flows which are, therefore, 'forced'.

Large‐scale splay faults on a strike‐slip fault system: The Yakima Folds, Washington State

“…The early growth of the YF coincided with early to mid Miocene CRBG extrusion, Basin and Range extension, and westward migration of the Oregon Coast Range [ Reidel , 1984; Reidel et al , 1989, 1994; Hooper and Conrey , 1989; Wells and Heller , 1988]. Early Miocene extension in the Basin and Range south of the OWL should have caused right‐lateral slip on the OWL and parallel structures [Lawrence, 1976; Hooper and Conrey, 1989; Watkinson and Hooper, 2000]. The ~N10°W orientation of the early to mid Miocene CRBG feeder dikes (Figure 2a) [ Camp , 1995; Petcovic et al , 2001] indicates that the principal compressive stress was oriented NNW during the early and mid Miocene [ Hooper and Camp , 1981; Barrash et al , 1983; Hooper and Conrey , 1989].…”