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

Abstract

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.