Allochthonous Jurassic ophiolite in northwest Washington

摘要: Fragments of Jurassic ophiolite having U-Pb zircon ages narrowly grouped at 160 to 170 m.y. are widespread over parts northwest Washington. The Haystack thrust fault is inferred mark the base in San Juan Islands and adjacent Cascade foothills; other bodies mafic ultramafic rock western Cascades may be klippen plate. probably structurally highest possibly most extensive yet recognized within a family Late Cretaceous faults its time emplacement (bracketed between about 100 88 m.y.) suggest similarity with Coast Range California which Upper Great Valley sedimentary sequence Franciscan assemblage. However, relations complicated by extraordinarily diverse character lower plate rocks, very few resemble Franciscan. We conclude that an original subduction system was modified later tectonic activity so variety terranes juxtaposed as rootless thrusts, forming, least some areas, uppermost structural unit. Perhaps Wrangellia, allochthonous microcontinent west Islands, caused thrusting.