Abstract During the period lasting from about 150 to 80 Ma, two contemporaneous but geographically distinct tectonic regimes dominated the Cordillera (Fig. 1). In the eastern part, the largely thin-skinned Cordilleran fold and thrust belt and a coeval foreland basin developed as Proterozoic, Paleozoic, and lower Mesozoic strata were contracted and emplaced eastward onto the platformal cover of the North American craton. In the western Cordillera, subduction of oceanic crust along the continental margin gave rise to the magmatic arc, forearc-basin deposits, and subduction complex that are preserved respectively in the Sierran batholithic belt, Great Valley Group, and Franciscan Complex. Upper Jurassic, Lower Cretaceous, and lower Upper Cretaceous rocks elsewhere in the western Cordillera, including Washington and southwestern California, reside in large composite terranes that may have been displaced northward with respect to continental North America on the order of 1, 000 km or more since about 80 Ma. This hypothesis, supported chiefly by paleomagnetic data, is definitely controversial.

Abstract The Cordilleran orogen, with an evolution that has spanned the entire Phanerozoic, is one of the longest lived orogenic belts on the planet. A reason for its long history is that since at least Cambrian time the Re has been a large region underlain by oceanic lithosphere to the west of continental North America, and such continent-ocean lithospheric boundaries are tectonically unstable. Interaction between these two types of lithosphere resulted in intermittent tectonic activity during Paleozoic time, and nearly continuous tectonic activity during the Mesozoic and Cenozoic. Because Pacific oceanic plates are still present west of North America, the Cordilleran orogen continues to evolve. Plateboundary interactions along the western boundary of North America have included diverse types of convergent, divergent, and transform activity as well as combinations of these plateboundary interactions. Fortunately, the preservation of a large tract of Mesozoic and Cenozoic oceanic lithosphere west of North America enables the evolution of Pacific Basin plates to be reconstructed for Cenozoic time and, with less certainty, back to middle Mesozoic time (Engebretson and othe Rs, 1985; Stock and Molnar, 1988). Such reconstructions permit correlation between continental geology and lithospheric plate interactions to a degree uncommon in most post-Paleozoic orogens, and provide a testing ground for relating oceanic plate tectonics to continental deformation and evolution.

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.

Abstract The San Juan Islands are located at the north end of Puget Sound in northwestern Washington State. This field guide in-eludes four localities: one on Lopez Island and three on San Juan Island. To reach the islands (Fig. 1 1), take I–5 to Exit 230, thendrive west on Washington 20 to the Anacortes ferry terminal.Washington State ferries provide frequent daily service for carsand passengers to Lopez and San Juan Islands. Long waits shouldbe expected on summer weekends.

Abstract The San Juan Islands are located at the north end of Puget Sound in northwestern Washington State. This field guide in-eludes four localities: one on Lopez Island and three on San Juan Island. To reach the islands (Fig. 1 1), take I–5 to Exit 230, thendrive west on Washington 20 to the Anacortes ferry terminal.Washington State ferries provide frequent daily service for carsand passengers to Lopez and San Juan Islands. Long waits shouldbe expected on summer weekends.

Scaly fabrics are present in DSDP cores from the Barbados, southern Mexico, Guatemala, and Mariana forearcs. Where independently documented, the scaly fabrics occur adjacent to faults. Scaly foliation surfaces in mudstone are planes of slip comparable to surfaces experimentally produced in shear boxes at low confining pressures. Microscopic and scanning electron microscope (SEM) images of scaly folia in sediments indicate strong preferred orientation of phyllosilicates both parallel to and at low angles to the slip surfaces. The clays within the slip surfaces are not new mineral phases but are produced by the reorientation of existing minerals and perhaps the disruption of clay aggregates. SEM studies and physical property data indicate loss of porosity during the development of scaly fabrics in sediments. At DSDP sites, scaly mudstone formed at temperatures of less than 25 °C, pressures less than 4 MPa, and strain rates of about 10 −13 ; scaly fabrics develop typically in underconsolidated sediment. Scaly fabrics preferentially occur in weak smectitic mudstones as opposed to stronger calcareous or silty mudstones. In sediments, scaly fabrics apparently develop by the lateral propagation of faults in which individual slip surfaces are formed and abandoned after a limited amount of displacement. Slip may cease on scaly folia because of increasing coefficient of friction, because of decrease in pore pressure, or because of reorientation of the slip surface relative to the stress field. The propagation of scaly fabric occurs because the surrounding undeformed sediment matrix is weaker and/or has more favorably oriented potential failure surfaces. Conversely, in hard rocks fault zones would be less likely to propagate laterally and develop broad scaly fabric zones because they would not exceed the strength of the country rock.

Abstract The theme of these explanatory notes is the westward growth of the North American continent in the region transected by the strip maps and cross-sections on the accompanying displays. The focus is on continent-ocean interactions and on the transition from the Precambrian sialic basement of North America to accreted terranes during Mesozoic and Cenozoic time. The first section describes the active plate boundaries along which the Juan de Fuca plate is being created and consumed. Next is a brief review of major tectonic events during which diverse terranes were accreted to the margin of North America.