Rowland W. Tabor

Ages and stratigraphy of lower and middle Tertiary sedimentary and volcanic rocks of the central Cascades, Washington: Application to the tectonic history of the Straight Creek fault

In the central Cascade Range of Washington, three structural blocks of early Tertiary sedimentary and volcanic rocks help to define the position and history of the southern segment of the Straight Creek fault. East of the fault, in the Teanaway River block, the early Eocene fluviatile feld-spathic sandstone of the Swauk Formation is interbedded with largely dacitic volcanic rocks of the Silver Pass Volcanic Member. Zircon fission-track ages on the Silver Pass are about 52 m.y. Overlying the tightly folded Swauk and Silver Pass is the Teanaway Formation, a gently dipping accumulation of basalt, andesite, rare dacite, and rhyolite that yields middle Eocene (about 47 m.y.) whole-rock K-Ar ages. Conformably overlying the Teanaway is the Roslyn Formation, a coal-bearing fluviatile feld-spathic sandstone of probable middle to late Eocene age. The late Eocene and Oligocene(?) Naches Formation, exposed west of the Straight Creek fault in the Cabin Creek block, is rich in fluviatile feldspathic sandstone and rhyolite flows and tuffs in its lower part but grades upward into a sequence dominated by basalt but with some andesite. The basal Guye Sedimentary Member of the Naches Formation underlies and is interbedded with the Mount Catherine Rhyolite Member in the Snoqualmie Pass area. On the basis of zircon fission-track ages on silicic tuffs and whole-rock K-Ar ages on basalt, the basal part of the Naches is about 40 to 44 m.y. old. The formation is tightly folded and complexly faulted along the Straight Creek fault. The Straight Creek fault intersects the Olympic-Wallowa lineament in the strongly deformed Manastash River block. In this block, fluviatile coal-bearing feldspathic sandstone of the Manastash Formation is overlain by principally dacitic volcanic rocks of the Taneum Formation. Overlying the Taneum is the basalt of Frost Mountain. We correlate this threefold sequence, feldspathic sandstone-dacite-basalt, with the sequence in the Teanaway River block. Small patches of rhyolitic ash-flow tuff, dated at 33 m.y. (late Oligocene) and interbedded feldspathic sandstone overlie the Roslyn and Teanaway Formations in the Teanaway River block and are probably correlative with the late Oligocene Wenatchee Formation exposed in the Chiwaukum graben east of the Teanaway River block as well as with the late Oligocene volcanic rocks (30 m.y.) along the Cascade crest to the southwest. These latter rocks are little deformed and unconformably overlie the Naches Formation. The ages and depositional record in the three blocks indicate a Tertiary history of dominantly vertical movement along the Straight Creek fault and its southeasterly splays that merge with the Olympic-Wallowa lineament. The horst-and-graben structure of the blocks, as well as enechelon fold axes in the Swauk Formation, suggests some post–early Eocene, right-lateral shear along the fault, although there is no direct evidence of lateral offset. Vertical movement, significant since early Eocene Swauk deposition, followed late Eocene Naches deposition but tapered off by late Oligocene time and ceased by Miocene time, when the fault was intruded by the 25-m.y.-old Snoqualmie batholith and other plutons. However, structures with Olympic-Wallowa lineament trends appear to have influenced folding of the Miocene Yakima Basalt Subgroup.

Origin of Ridge-Top Depressions by Large-Scale Creep in the Olympic Mountains, Washington

In the high mountain area of the Olympic Mountains, Washington, there are many troughlike depressions on and essentially parallel to ridge tops. The troughs are mostly developed on rocks with strong planar anisotropy: slate, sandstone, and phyllite. Similar features in Europe, Japan, and New Zealand have been variously ascribed to erosion, slow movement along deep-seated shear planes, creep, and tectonic movements. In the Olympics, many depressions parallel structure; one wall is steeply dipping rocks, the other shattered, gently dipping rocks. These depressions seem to be the gaps left between undisturbed steeply dipping rocks and beds or cleavage bent valley-ward by creep. A few troughs may be the result of slow down-slope movement along deep-seated shear planes; this is a favorite explanation of eastern European workers. The Olympic ridge-top depressions testify to the importance of gravity in the degradation of high mountains carved from weak rocks.

Late Mesozoic and possible early Tertiary accretion in western Washington State: The Helena-Haystack mélange and the Darrington-Devils Mountain fault zone

The Helena-Haystack melange (HH melange) and coincident Darrington-Devils Mountain fault zone (DDMFZ) in northwestern Washington separate two terranes, the Northwest Cascade System (NWCS) and the western and eastern melange belts (WEMB). The two terranes of Paleozoic and Mesozoic rocks superficially resemble each other but record considerable differences in structural and metamorphic history. The HH melange is a serpentinite-matrix melange containing blocks of adjacent terranes but also exotic blocks of schistose metavolcanic rocks and Jurassic tonalite and associated amphibolite. The HH melange must have formed between early Cretaceous and late middle Eocene time, because it contains tectonic clasts of early Cretaceous Shuksan Greenschist and is overlain by late middle Eocene sedimentary and volcanic rocks. Less certain constraints on its age are a tectonic clast of metarhyolite that yields 90 Ma metamorphic ages and the presumption that the melange was emplaced before the outboard Olympic terrane arrived at about 50 Ma. The apparent continuity of the HH melange and the Decatur terrane of the San Juan Islands suggests that the melange is the strongly tectonized equivalent of the Fidalgo ophiolite. The out-crop pattern suggests that the HH melange overlies rocks of the NWCS and it may have formed when the WEMB terranes were thrust over rocks of the NWCS. Much of the exposed belt of the HH melange is overlain by late middle Eocene feldspathic sandstone and volcanic rocks of the Barlow Pass Volcanics of Vance (1957a), which are cut by numerous faults of the DDMFZ paralleling the melange. The Barlow Pass Volcanics appear to overlie the Straight Creek fault without large offset, but a displaced exotic block of amphibolite with attached early or early middle Eocene(?) sandstone in the melange suggests that strike-slip movement along the DDMFZ was synchronous with movement on the Straight Creek fault, and stretched cobbles in the conglomerates of the Barlow Pass Volcanics suggest post-Straight Creek movement. The possible continuation of the DDMFZ to the northwest as the San Juan and the West Coast faults on Vancouver Island suggests That the structure has had a major role in the emplacement of all the westernmost terranes in the Pacific Northwest. This major suture is strongly bowed to the northeast opposite the great oroclinal bend of the Olympic terrane, suggesting that the emplacement of that terrane may have deformed a once straighter strike-slip zone.

Late Cretaceous and early Tertiary plutonism and deformation in the Skagit Gneiss Complex, North Cascade Range, Washington and British Columbia

The Skagit Gneiss Complex forms a more-or-less continuous terrane within the northern, more deeply eroded part of the North Cascade Range. The complex comprises abundant plutons intruded at mid-crustal depths into a variety of metamorphosed supracrustal rocks of both oceanic and volcanic-arc origin. A plethora of syntectonic pegmatite, small plutons, and granitic dikes gives the complex a migmatitic aspect. U-Pb zircon ages from gneissic plutons within and near the Skagit Gneiss Complex indicate magmatic crystallization between 75 and 60 Ma. Deformation, recrystallization, and migmatization in part postdate intrusion of the 75-60 Ma plutons. This latest Cretaceous and earliest Tertiary plutonism and migmatization may reflect thermal relaxation following early Late Cretaceous orogeny documented else-where in the North Cascades. The complex was ductilely extended northwest-southeast shortly after intrusion of granite dikes at ∼45 Ma, but before emplacement of the earliest (∼34 Ma) plutons of the Cascade arc. Outcrops of Late Cretaceous and earliest Tertiary plutons, migmatites of the Skagit Gneiss Complex, and rocks with young ductile deformation are roughly coextensive, all apparently marking a region of greater middle Eocene unroofing. Unroofing was apparently contemporaneous with east-west extension in the Okanogan region to the east and north-south and northwest-southeast strike-slip faulting within the North Cascades.

Age of the Olympic Metamorphism, Washington: K-Ar Dating of Low-Grade Metamorphic Rocks

The complexly deformed core rocks of the Olympic Mountains grade from mildly sheared and recrystallized graywacke and slate to semi-schist and mica phyllite. K-Ar ages of sand-sized and matrix-sized fractions from 14 samples (28 ages) of the graywacke-semischist sequence decrease with increasing rank of metamorphism. The ages plotted against the rank of metamorphism (as determined in thin section) define a curve which, in a rough way, flattens to an asymptote at approximately 29 m.y. The ages of 8 samples of the slate-phyllite sequence define the same asymptote, which is assumed to represent the age of regional metamorphism. Four samples from brecciated and quartz-veined mica phyllite indicate a local resetting of the clock by faulting and resulting recrystallization at about 17 m.y. Lines of equal age plotted from the average ages of samples from the graywacke-semischist sequence denote a crude metamorphic zonation which corresponds approximately to the observed metamorphic gradation. The ages of the metamorphic events are reasonable in light of known stratigraphic ages of the rocks and the regional tectonic setting, suggesting this method may be applied to other areas of low-grade rocks.

Isotopic provenance of Paleogene sandstones from the accretionary core of the Olympic Mountains, Washington

Conventional modal sandstone data from Paleogene units of the Pacific Northwest are not precise enough to pinpoint source areas and constrain displacement histories of accreted sedimentary terranes. Isotopic provenance study, used in conjunction with traditional basin-analysis techniques, provides a powerful means of identifying source areas. Analysis of Rb-Sr data in both wholerock and detrital white micas of Paleogene sandstones from allochthonous units in the eastern core of the Olympic Mountains and coeval autochthonous sandstones from coastal Pacific Northwest shows that the Needles-Gray Wolf and Grand Valley lithic assemblages of the core came from the same source as sandstones of the Chuckanut Formation and Puget Group in northern Washington. The source of these units is isotopically distinct from the source for units farther south, such as the Tyee Formation in Oregon. Chemical compositions, conventional K-Ar age determinations, and oxygen- and hydrogen-isotope compositions of white micas support this conclusion. Similar analyses of sandstones from the Western Olympic lithic assemblage and the Yakutat terrane of southeastern Alaska suggest that these two units have a similar source, but that they differ slightly from sandstones of the eastern Olympic core and autochthonous Washington units. The overall composition of sandstones (lithic arkosic), and the very high initial-Sr values (>0.710), moderately high δ 18 O values (∼+9) and Mesozoic age for detrital white mica of Olympic core rocks and sandstone of the Yakutat terrane suggest a source in the high-rank metamorphic and plutonic rocks from the eastern part of the Omineca Crystalline Belt in southeastern British Columbia. Furthermore, rapid uplift of this source area during Eocene time is consistent with the depositional age of the Olympic rocks. Sediment derived from this source area was transported westward by major river systems that crossed the low-lying North Cascade Range and supplied the deposits of the autochthonous units of the northern Washington coast and the offshore equivalents before the latter were accreted to form the Olympic core. Limited data from the Yakutat terrane suggest that it lay offshore of southern British Columbia sometime during middle Eocene to early Oligocene time, consistent with paleomagnetic and some paleontologic interpretations, and subsequently migrated northward by plate motions.

LARGE QUARTZ DIORITE DIKE AND ASSOCIATED EXPLOSION BRECCIA, NORTHERN CASCADE MOUNTAINS, WASHINGTON

The bladelike shape of the large quartz diorite dike at Cascade Pass, Washington, and its trend, which is normal to regional metamorphic structures and folds, suggest emplacement by forceful intrusion along an ac fracture. Lit-par-lit structures and contact breccias along the margins indicate some sloping. K feldspar of possible deuteric origin is present in the main phase of the dike but is rare or absent in the chilled margins, suggesting that chilling prevented its formation. However, K feldspar is concentrated marginally in the top of the dike adjacent to an explosion breccia. K metasomatism of the hornfelsed schist fragments of the breccia suggests that the breccia formed during the deuteric stage in the cooling of the pluton. Hornfels occurs at the contact of the pluton, but farther away only small glomeroblastic patches of actinolitic hornblende and/or biotite superimposed on a schistose fabric show the thermal metamorphism. This outer portion of the contact aureole widens considerably in a deep valley, suggesting that the dike widens with depth.