The San Juan Islands expose a thick and regionally extensive sequence of Late Cretaceous thrust faults and nappes, referred to as the San Juan thrust system. This thrust system, which straddles the southeastern edge of the Wrangellia terrane of Vancouver Island, contains important information on the accretionary history of Wrangellia and other, related, far-traveled terranes. Nappes of the thrust system contain a diverse group of rocks ranging from early Paleozoic to middle Cretaceous in age. Based on contrasts in stratigraphy, metamorphism, and geochemistry, we identify five terranes within, and peripheral to, the thrust system. These terranes were widely separated from each other, and also from Wrangellia, until the Late Jurassic: (1) the Haro terrane, an Upper Triassic arc-volcanic sequence; (2) the Turtleback terrane, a Paleozoic arc-plutonic and volcanic unit; (3) the Deadman Bay terrane, a Permian to Lower Jurassic oceanic-island sequence containing Tethyan-fusulinid limestones; (4) the Garrison terrane, a Permo-Triassic, high-pressure metamorphic unit; and (5) the Decatur terrane, a Middle to Upper Jurassic ophiolite and superimposed arc-volcanic sequence. Thick Jura-Cretaceous clastic units are linked to these older San Juan terranes and to Wrangellia, either as directly overlapping units or by the presence of clastic material derived from the terranes. The voluminous amount of clastic material in these Jura-Cretaceous units requires a large, subaerially exposed source region. We infer that this source region was a continent-like landmass, presumably part of continental America (North or Central?).
Late Cretaceous thrusting juxtaposed these older terranes and disrupted the Jura-Cretaceous clastic units. Very low-temperature high-pressure metamorphic assemblages, including lawsonite and aragonite, were developed during this event, and formed as a direct result of thrust-related burial to depths of about 20 km. Stratigraphic evidence indicates that structural burial, metamorphism, and subsequent uplift back to the surface all occurred during a very short time interval, between 100 and 84 Ma, with average vertical transport rates of about 2 km/m.y. The Upper Cretaceous Nanaimo Group, a syn-orogenic basin to the north of the San Juan system, contains cobbles of metamorphosed rocks from the San Juan nappes, and therefore records the erosional unroofing of the thrust system.
We envision the San Juan system to be a short-lived collision-like orogen, rather than a long-lived subduction complex. This conclusion is based primarily on the diversity of rock units involved and the punctuated nature of the deformation. What remains unclear is the cause of this orogenic deformation. The clastic-rich Jura-Cretaceous units imply that Wrangellia and the older San Juan terranes were adjacent to or part of the American continent by latest Jurassic time. Moreover, there is no evidence for the arrival and collision of an exotic terrane during the Cretaceous. As a result, the San Juan system is considered to be the product of distributed deformation within an active continental margin. Two tectonic settings are examined: (1) a convergent margin where orogenic deformation is driven by greater coupling across the subduction boundary resulting in extensive shortening within the overriding continental plate, and (2) an irregular transform margin where terranes and overlap sequences are transported northward within a system of coast-parallel faults. In the latter case, orogenic shortening occurs when these fault slices collide with a reentrant in the margin, perhaps like the modern collision of the Yakutat block in the Gulf of Alaska.
