Deformation of the continental flood-basalt in the westernmost portion of the Columbia Plateau has resulted in regularly spaced anticlinal ridges. The periodic nature of the anticlines is characterized by dividing the Yakima fold belt into three domains on the basis of spacings and orientations: (1) the northern domain, made up of the eastern segments of Umtanum Ridge, the Saddle Mountains, and the Frenchman Hills; (2) the central domain, made up of segments of Rattlesnake Ridge, the eastern segments of Horse Heaven Hills, Yakima Ridge, the western segments of Umtanum Ridge, Cleman Mountain, Bethel Ridge, and Manastash Ridge; and (3) the southern domain, made up of Gordon Ridge, the Columbia Hills, the western segment of Horse Heaven Hills, Toppenish Ridge, and Ahtanum Ridge. The northern, central, and southern domains have mean spacings of 19.6, 11.6, and 27.6 km, respectively, with a total range of 4 to 36 km and a mean of 20.4 km ( n = 203). The basalts are modeled as a multilayer of thin linear elastic plates with frictionless contacts, resting on a mechanically weak elastic substrate of finite thickness, that has buckled at a critical wavelength of folding. Free slip between layers is assumed, based on the presence of thin sedimentary interbeds in the Grande Ronde Basalt separating groups of flows with an average thickness of roughly 280 m. Many of the observed spacings can be explained by this model, given that: (1) the ratio in Young’s modulus between the basalt and underlying sediments E/E o ⩾ 1,000, (2) the thickness of the Grande Ronde Basalt was between 1,200 and 2,300 m when the present wavelengths were established, and (3) the average thickness of a layer in the multilayer is between 200 and 400 m. The lack of well-developed anticline-syncline pairs in the shape of a sinusoid may be the result of plastic yielding in the cores of the anticlines after initial deformation of the basalts into low amplitude folds. Elastic buckling coupled with plastic yielding confined to the hinge area could account for the asymmetric fold geometry of many of the anticlines.

Abstract Eocene sedimentary and volcanic rocks on the eastern flank of the Cascade Range consist of five regional, unconformity-bounded formations of the Challis synthem. These formations define a series of northwesterly striking folds. Five anticlines are 9 to 28 km apart, have pre-Tertiary crystalline rocks in their cores, high-angle reverse faults on their steeper northeastern limbs, and pass down-plunge into more gentle folds in the Neogene Columbia River Basalt Group (CRBG). Such northwesterly trending folds extend from east of the Columbia River across the Cascade Range to the Puget Lowland. The Chiwaukum graben and Swauk basin, which heretofore were thought to be local, extensional, depositional basins, are, instead, the major northwesterly trending synclines in this series of folds. The Eocene formations were preserved, not deposited, in these synclines. Dextral, N-S faults cut the reverse faults and the pre-CRBG portion of some of the folds. The post-CRBG folds control the regional distribution of the Eocene formations. The Cascade Range is a southerly plunging, post-CRBG anticline. Clasts in the Thorp Gravel indicate that this anticline began to rise ca. 4 Ma. The anticline has an amplitude of ∼3.5 km, and it causes the plunges of the northwesterly striking post-CRBG folds. The northerly and northwesterly post-CRBG folds form a regional interference pattern, or “egg-crate,” that dominates the present topography of Washington State.

Abstract The Tertiary sedimentary and volcanic rocks of the Cascade Range unconformably overlie a crystalline basement of previously accreted terranes. The Tertiary strata are parts of four synthems, or interregional unconformity-bounded sequences of tectonic origin. Thus, the formations in these synthems were not deposited in local basins. The 55-38 Ma Challis synthem has five regional unconformity-bounded formations; the names with precedence are (from the base up) Swauk, Taneum, Teanaway, Roslyn, and Naches. Near Blewett Pass (nee Swauk Pass), the Challis fluvial and arkosic Swauk Formation is ~5 km thick and has several members in a generally upward-fining succession. The members of the Swauk do not interfinger, and some are separated by unconformities. The Oligocene to mid-Miocene andesitic and rhyolitic Kittitas synthem is almost absent in the area. The most voluminous lithostratigraphic unit in the mid-Miocene to Pliocene Walpapi synthem is the Columbia River Basalt Group. Clasts of Columbia River Basalt Group and older rocks in the ca. 4 Ma Thorp Formation of the High Cascade synthem record initial uplift of the Cascade Range to the west. North of Blewett Pass, the northwesterly segment of the Leavenworth fault is the Camas Creek reverse fault that places Swauk and Teanaway in the Blushastin anticline over a syncline in the Roslyn Formation. Northerly striking faults in the Leaven-worth fault zone are parts of a younger system that cuts the Camas Creek thrust and northwest-striking folds in Challis rocks. In style, scale, and age, the Camas Creek fault resembles the Easton Ridge thrust south of Cle Elum, the Eagle Creek fault in the Chiwaukum graben, and the Seattle fault in the Puget Lowland. These faults are on the steeper northeastern limbs of major anticlines in Challis rocks. Down plunge, these folds are more gentle in Walpapi rocks. These folds and faults are part of the regional Seattle-Wentachee-Kittitas fold-and-thrust belt. The Straight Creek fault is a major, north-south, dextral fault in the northern Cascade Range. The fault offsets all five of the Challis unconformity-bounded formations. The southeasterly curving discontinuity along which it was mapped east of Easton is due to unconformities at the base of the Taneum and Teanaway, not a fault. The Straight Creek fault is 2.7 km west of Easton and passes southward beneath Kittitas rocks. Although the fault dextrally displaces pre-Tertiary units >90 km, Tertiary displacement is &55 km. This may indicate two (or more) periods of displacement. Perhaps the displaced portion of the fault underlies Puget Sound. Two sets of post-Walpapi folds deform the Tertiary synthems. The Seattle-Wentachee-Kittitas fold-and-thrust belt is part of a set of northwest-striking folds. One of several north-trending regional anticlines causes the Cascade Range. The Cascade Range anticline, with an amplitude of ~3.5 km, has risen in approximately the past 3.5 m.y. This anticline causes the plunges of the Seattle-Wentachee-Kittitas fold-and-thrust belt folds. The two sets of folds cause a regional interference, or “egg-crate,” pattern that dominates the present topography of the Pacific Northwest.

Abstract A prevailing hypothesis for the central Cascade Range of Washington State is that it underwent regional extension or transtension during the Eocene. This hypothesis is based on the idea that kilometers-thick, clastic, Eocene formations were deposited syntectonically in local basins. Our mapping and structural analysis indicate that these formations are preserved in fault-bounded, regional synclines, not in separate depositional basins. Thus, the type area for the hypothesis, the so-called Chiwaukum graben, is here renamed the Chiwaukum Structural Low. The Eocene arkosic Chum-stick Formation, which was thought to have been syntectonically deposited in the graben, is the proximal equivalent of the Roslyn Formation 25 km southwest of the graben. Because the name “Roslyn Formation” has precedence, the name “Chumstick Formation” should be abandoned. Additionally, several areas previously mapped as Chumstick Formation in the Chiwaukum Structural Low probably are parts of the older Swauk Formation and younger Wenatchee Formation. The southwestern boundary of the Chiwaukum Structural Low includes the Leav-enworth fault zone, which consists of postdepositional, northwest-striking reverse faults with adjacent northwest-striking folds. The reverse faults place the regionally extensive early-Eocene, arkosic Swauk Formation over the mid-Eocene, arkosic Chumstick Formation. A diamictite, which previously was placed in the Chumstick Formation and inferred to have been syntectonically derived from the Leavenworth fault zone, is part of the older Swauk Formation. We mapped a 0.6–1-km-thick conglomerate-bearing sandstone as a robust marker unit in the Chumstick Formation; instead of being spatially related to the bounding faults, this unit has a >30 km strike length around the limbs of folds in the structural low. The northwest-striking reverse faults and fold hinges of the structural low are cut by north-striking strike-slip faults, which likely are late Eocene to Oligocene; these north-south faults partially bound the structural low. The Eocene folds and faults were reactivated by deformation of the Miocene Columbia River Basalt Group; this younger folding largely defines the regional map pattern, including the structural low. A model to account for the above characteristics is that all of the Eocene formations, not just the Roslyn Formation, are kilometers thick and are remnants of regional unconformity-bounded sequences that were deposited on the Eocene margin of this part of North America. Their present distribution is governed by younger faults, folds, and erosion. Thus, the Eocene to Recent history of the central Cascade region is characterized not by crustal extension, but by episodes of folding (with related reverse faults) and strike-slip faulting.