Abstract A ruling hypothesis for the central Cascade Range in Washington is that the Eocene arkosic formations, which are kilometers thick, were deposited in local grabens, such as the Chumstick Formation in the putative Chiwaukum graben. However, the formations are regional in extent and are preserved in less extensive northwesttrending synclines. The Chumstick Formation in the Peshastin syncline is a more proximal equivalent of the Roslyn Formation, which is preserved in the Kittitas Valley syncline 25 km to the southwest. The Chiwaukum structural low is partially bounded on the southwest by the Leavenworth fault zone, which consists of northwesterly striking, northeasterly verging reverse faults (with associated northwest-striking folds). The reverse faults and the hinges of the folds are cut by N-S, dextral strike-slip faults, which also partially bound the Chiwaukum structural low. Conglomeratic units in the Chumstick Formation are not proximal to either set of bounding faults. The Leavenworth fault occurs on the steeper northeastern limb of a northwesterly trending, basement-cored anticline. The Eagle Creek and Ainsley Canyon anticlines also have reverse faults on their steeper northeastern limbs. In the Puget Lowland, the Seattle reverse fault is in a similar anticline. The regional distribution of the Eocene formations and uplift of the Cascade Range are caused by folding of the Miocene Columbia River Basalt Group since 4 Ma. The remnant of a 4 Ma andesite on Natapoc Mountain shows that the present low topography of the Chiwaukum structural low is erosional and young.

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.

The Kettle River Range in Ferry County, Washington, is underlain by sillimanite-grade rocks of the Tenas Mary Creek sequence. Two >800-m-thick sheets of augen gneiss occur above and below feldspathic quartzite, biotitic gneiss, and minor marble. Polyphase deformation (including mylonites and east-trending lineations) and slightly uraniferous aplitic to pegmatitic bodies are common. Cataclasis is common, and rocks of the Tenas Mary Creek sequence appear to be in tectonic contact with overlying upper Paleozoic phyllitic rocks. Fine-grained biotitic metasedimentary rocks occurring locally between the phyllitic rocks and rocks of the Tenas Mary Creek probably are older than the late Paleozoic phyllitic rocks. Foliation and contacts in the Tenas Mary Creek sequence rarely dip >25° and define the flat-topped Kettle dome (which is >65 km long north-south, 27 km wide, and has about 3 km of structural relief). The Okanogan dome west of the Kettle dome consists primarily of orthogneisses and granitic plutons of Mesozoic(?) age. Rocks in the flat-topped Spokane dome along the Washington-Idaho border are lithologically similar to those of the Tenas Mary Creek and may be pre-Beltian. The Sanpoil syncline between the Kettle and Okanogan domes and a syncline on the northeastern margin of the Kettle dome contain Eocene sedimentary and volcanic rocks. Because the axes and structural reliefs of the Okanogan dome, the Sanpoil syncline, and the Kettle dome are similar, the present structural relief (as opposed to the internal structure and high-grade metamorphism) of the Kettle dome probably is due to post-Eocene folding. The gently synformal Tertiary Newport fault straddling the Washington-Idaho border may be a related structural feature. Four other low-angle faults, three of which cut Tertiary rocks, occur between the domes. The low-angle faults commonly are marked by cataclastic zones more than 100 m thick. Cataclasis occurred as the basement of batholiths and pre-Beltian(?) metamorphic rocks became decoupled from overlying Precambrian to Tertiary layered rocks. Whether this decoupling represents one or more zones of Tertiary decollement of regional extent is not yet known. Owing to post-Eocene (possibly late Miocene or younger) folding, the cataclastic zones crop out on the margins of the domes.