The eight field trips in this volume, associated with GSA Connects 2021 held in Portland, Oregon, USA, reflect the rich and varied geological legacy of the Pacific Northwest. The western margin of North America has had a complex subduction and transform history throughout the Phanerozoic, building a collage of terranes. The terrain has been modified by Cenozoic sedimentation, magmatism, and faulting related to Cascadia subduction, passage of the Yellowstone hot spot, and north and westward propagation of the Basin and Range province. The youngest flood basalt province on Earth also inundated the landscape, while the mighty Columbia watershed kept pace with arc construction and funneled epic ice-age floods from the craton to the coast. Additional erosive processes such as landslides continue to shape this dynamic geological wonderland.

The Mojave-Sonora megashear, which bounded the Jurassic southwestern margin of the North America plate from 170 to 148 Ma, may be linked northward to Alaska via the previously recognized discontinuity between the Insular and Intermontane terranes and co-genetic regional elements such as transtensional basins, transpressional uplifts, and overlapping correlative magmatic belts. The longer, continental-scale fault thus defined, which is called the Mexico-Alaska megashear, separated the North America plate from a proto-Pacific plate (the Klamath plate) and linked the axis of ocean-floor spreading within the developing Gulf of Mexico with a restraining bend above which mafic rocks were obducted eastward onto Alaskan sialic crust that converged against the Siberian platform. The fault, about 8000 km long, lies among more than a dozen large basins (and numerous smaller ones) many of which formed abruptly at ca. 169 Ma. The basins, commonly containing Middle and Late Jurassic and Cretaceous clastic and volcanic units, distinguish a locally broad belt along the western and southwestern margin of the North America plate. The basin margins commonly coincide with easterly striking normal and northwesterly striking sinistral faults although most have been reactivated during multiple episodes of movement. The pattern of intersecting faults and the rarely preserved record of displacements along them suggest that the basins are structural pull-aparts formed at releasing steps of a sinistral continental margin transform and are therefore transtensional. The width of the zone delineated by the basins is a few hundred km and extends west-northwesterly from the Gulf of Mexico across northern Mexico to southern California where it curves northward probably coincident with the San Andreas fault. Principal basins included within the southern part of the transtensional belt are recorded by strata of the Chihuahua trough, Valle San Marcos and La Mula uplift (Coahuila, Mexico), Batamote and San Antonio basins (Sonora, Mexico), Little Hatchet and East Potrillo Mountains and Chiricahua Mountains basins (New Mexico), Baboquivari Mountains Topawa Group (Arizona), regional Bisbee basin (Arizona, New Mexico, and Sonora, Mexico), Bedford Canyon, McCoy Mountains, Inyo Mountains volcanic complex and Mount Tallac basin (California). The latter probably extend into Nevada as part of the Pine Nut assemblage. At the southern margin of the Sierra Nevada of California, the inferred fault steps west then north, roughly along the Coast Range thrust and into the Klamath Mountains. The Great Valley (California) and Josephine ophiolites (Oregon) record these two major, releasing steps along the Mexico-Alaska megashear. From the northwestern Klamath Mountains, the Mexico-Alaska megashear turns east where Jurassic contractional structures exposed in the Blue Mountains indicate a restraining bend along which transpression is manifest as the Elko orogeny. Near the border with Idaho the fault returns to a northwest strike and crosses Washington, British Columbia, and southern Alaska. Along this segment the fault mainly coincides with the eastern limit of the Alexander-Wrangellia composite terrane. West of the fault trace in Washington, the Ingalls and Fidalgo ophiolites record separate or dismembered, co-genetic, oceanic basins. Correlative sedimentary units include Nooksack, Constitution, and Lummi Formations and the Newby Group, within the Methow basin. In British Columbia, the Relay Mountain Group of the Tyaughton basin, and Cayoosh, Brew, Nechako, Eskay, and Hotnarko strata record accumulation from Bajocian through Oxfordian within a northwestward-trending zone. From southern Alaska and northwestward correlative extension is recorded in basins by sections at Gravina, Dezadeash-Nutzotin, Wrangell Mountains, Matanuska Valley (southern Talkeetna Mountains), Tuxedni (Cook Inlet), and the southern Kahiltna domain. The pull-apart basins began to form abruptly after the Siskiyou orogeny that interrupted late Early to Middle Jurassic subduction-related magmatism. Convergence had begun at least by the Toarcian as an oceanic proto-Pacific plate subducted eastward beneath the margin of western North America. As subduction waned following collision, sinistral faulting was initiated abruptly and almost synchronously within the former magmatic belt as well as in adjacent oceanic and continental crust to the west and east, respectively. Where transtension resulted in deep rifts, oceanic crust formed and/or volcanic eruptions took place. Sediment was accumulating in the larger basins, in places above newly formed crust, as early as Callovian (ca. 165 Ma). The belt of pull-apart basins roughly parallels the somewhat older magmatic mid-Jurassic belt. However, in places the principal lateral faults obliquely transect the belt of arc rocks resulting in overlap (southern British Columbia; northwestern Mexico) or offset (northern Mexico) of the arc rocks of at least several hundreds of kilometers. The trace of the principal fault corresponds with fault segments, most of which have been extensively reactivated, including the following: Mojave-Sonora megashear, Melones-Bear Mountain, Wolf Creek, Bear Wallows–South Fork, Siskiyou and Soap Creek Ridge faults, Ross Lake fault zone, as well as Harrison Lake, Bridge River suture, Lillooet Lake, and Owl Creek faults. Northward within the Coast Range shear zone, pendants of continental margin assemblages are interpreted to mark the southwest wall of the inferred fault. Where the inferred trace approaches the coast, it corresponds with the megalineament along the southwest edge of the Coast Range batholithic complex. The Kitkatla and Sumdum thrust faults, which lie within the zone between the Wrangellia-Alexander-Peninsular Ranges composite terrane and Stikinia, probably formed initially as Late Jurassic strike-slip faults. The Denali fault and more northerly extensions including Talkeetna, and Chilchitna faults, which bound the northeastern margin of Wrangellia, coincide with the inferred trace of the older left-lateral fault that regionally separates the Intermontane terrane from the Wrangellia-Alexander-Peninsular Ranges composite terrane. During the Nevadan orogeny (ca. 153 ± 2 Ma), strong contraction, independent of the sinistral fault movement, overprinted the Mexico-Alaska megashear fault zone and induced subduction leading to a pulse of magmatism.

The Klamath Mountains province of northwestern California and southwestern Oregon is a classic example of a mountain belt that developed by the tectonic accretion of rock assemblages of oceanic affinity during progressive crustal growth along an active continental margin. Consequently, the Klamath Mountains province has served as an important model for the definition and application of the terrane concept as applied to the evolution of Phanerozoic orogenic belts. Early regional studies divided the Klamath Mountains province into four arcuate lithic belts of contrasting age (from east to west): the eastern Klamath, central metamorphic, western Paleozoic and Triassic, and western Jurassic belts. The lithic belts are bounded by regional thrust faults that commonly include ophiolitic assemblages in the hanging-wall block. The age of thrusting is a complex problem because of structural overprinting, but generally the age of regional thrust faulting is older in eastern parts of the province and younger to the west. The lithic belts were subsequently subdivided into many tectono-stratigraphic terranes, and these lithotectonic units are always fault-bounded. Few of the regional faults are fossil subduction zones, but multiple episodes of high pressure–low temperature (blueschist-facies) metamorphism are recognized in the Klamath Mountains province. The tectonostratigraphic terranes of the Klamath Mountains province are intruded by many composite, mafic to felsic, arc-related plutons, some of which reach batholithic dimensions. Many of these plutonic bodies were emplaced during the Jurassic; however, radiometric dates ranging from Neoproterozoic through Early Cretaceous have been determined from (meta)plutonic rocks of the Klamath Mountains province. The orogenic evolution of the province apparently involved the alternation of contraction and extension, as exemplified by the Jurassic history of the province. Widespread Middle Jurassic plutonism and metamorphism is associated with a poorly understood contractional history followed by the development of the Preston Peak–Josephine ophiolite and Upper Jurassic Galice Formation in a probable transtensional inter-arc basin. During the Late Jurassic Nevadan orogeny, this basin collapsed, and rocks of the Galice Formation were thrust beneath the Rattlesnake Creek terrane along the Orleans fault. During this regional deformation, the Galice Formation experienced polyphase deformation and was metamorphosed under lower greenschist-facies conditions. Immediately following thrusting, the hanging-wall and footwall blocks of the Orleans fault were intruded by a suite of composite, mafic to felsic plutons (i.e., western Klamath plutonic suite) that have oceanic-arc geochemical and isotopic characteristics, indicating a subduction-zone petrogenesis for the magmas. The western boundary of the Klamath Mountains province is a regional thrust fault that emplaced the rocks of the province above Early Cretaceous blueschist-facies rocks (South Fork Mountain Schist) of the Franciscan Complex. Neogene structural doming is manifested in the north-central Klamath Mountains by the Condrey Mountain window, which exposes the high pressure–low temperature Condrey Mountain Schist framed by chiefly amphibolite-facies metamorphic rocks of the Rattlesnake Creek terrane.

Irwin's (1972) terrane concept fathered tectonostratigraphy and has done much to decipher complex geology throughout the Circum-Pacific region, Caribbean, and elsewhere in the world. The present chapter analyzes the tectonostratigraphy of seemingly unrelated Jurassic terranes utilizing an accurate chronostratigraphic framework. Faunal paleolatitudinal data, paleomagnetic data (where available), and microfacies analysis have been utilized in the tectonostratigraphic interpretations. Four terranes/subterranes are analyzed herein: (1) the Izee terrane (Blue Mountains, northeastern Oregon), (2) the Rogue Valley subterrane (southwestern Oregon), (3) the Smith River subterrane (northwestern California), and (4) the San Pedro del Gallo terrane (east-central and central Mexico, western Cuba). Paleolatitudinal data indicate that all of these terranes/subterranes have undergone tectonic transport. The Izee terrane originated at low paleolatitudes (Central Tethyan province) during the Late Triassic and migrated to higher paleolatitudes (Southern Boreal province: 30° N) by the late Bathonian (Middle Jurassic). Northward movement is postulated to have occurred along a megashear analogous to the present-day San Andreas fault system. The Rogue Valley and Smith River subterranes both originated at low paleolatitudes (Central Tethyan province) during the Callovian and were transported to higher paleolatitudes (Southern Boreal province) by the the middle Oxfordian (early Late Jurassic). The Huayacocotla remnant of the San Pedro del Gallo terrane originated at high Southern Boreal paleolatitudes (30–40° N) during the late Bathonian or early Callovian and, subsequently was transported from northwest to southeast along the west side of the Walper megashear. By the latest Tithonian (Late Jurassic), the Huayacocotla remnant had been transported to low paleolatitudes (Central Tethyan province). Unconformities in the Bathonian to Callovian interval in the Izee terrane(?) and in the Huayacocotla remnant of the San Pedro del Gallo terrane are believed to reflect the breakup of Pangea. As demonstrated by the author in previous reports, each remnant of the San Pedro del Gallo terrane shows the same paleobathymetric fingerprint: (1) marine deposition at inner neritic depths during the Callovian to early Oxfordian (Middle to early Late Jurassic), (2) marine deposition at outer neritic depths during the late Oxfordian (Late Jurassic), and (3) sudden deepening to upper abyssal depths from the early Kimmeridgian (Late Jurassic) until the end of the Cretaceous. The sudden deepening event (outer neritic to upper abyssal) during the early Kimmeridgian is associated with a disconformity and hiatus in all San Pedro del Gallo remnants (early Kimmeridgian strata overlie middle Oxfordian strata). This event reflects the opening of the Gulf of Mexico. Combined faunal and floral data indicate that the San Pedro del Gallo terrane was in a back-arc position at approximately the same latitude as the Foothills terrane of the Sierra Nevada during the middle Oxfordian. Given the faunal data as well as some paleomagnetic data, it is probable that these San Pedro del Gallo remnants might represent some of the missing Upper Jurassic back-arc deposits from farther north (i.e., western Nevada?). In the Smith River subterrane, the disconformable contact between the volcanopelagic facies and Galice Formation sensu lato reflects a sudden influx of siliciclastic turbidite from a Jurassic volcanoplutonic arc source area and from older rocks of the accreted continental margin that lay inboard of the Nevadan island-arc complex. The same event is represented by the deposition of the siliciclastic turbidite of the Monte del Oro and Mariposa Formations (Foothills terrane, western Sierra Nevada) and by that of the Galice Formation sensu stricto (Rogue Valley subterrane).

The western Jurassic belt of the Klamath Mountains represents one of the Earth's best-preserved exposures of ancient marginal ocean basin lithosphere and chiefly consists of the coeval Rogue–Chetco volcanic-plutonic oceanic arc and Josephine ophiolite. This Late Jurassic ocean basin is hypothesized to have formed in response to rifting that initiated at ca. 165 Ma along the western margin of North America, disrupting a Middle Jurassic arc that had been constructed on older Klamath terranes and forming a marginal ocean basin with an active arc, inter-arc basin, and remnant arc. Previous workers characterized a “rift-edge” facies in the remnant-arc region. This chapter describes field, age, and geochemical data that suggest that a similar rift-edge facies exists in the vicinity of the active arc, on the opposite side of the marginal basin. The rift-edge facies in the active arc setting consists of two main lithotectonic units, herein named informally as the Onion Camp complex and Fiddler Mountain olistostrome. The Onion Camp complex is partly composed of a characteristic metabasalt and red chert association. Red chert yielded scarce radiolarians of Triassic(?) and Early Jurassic age. A distinct chert-pebble conglomerate occurs at scarce localities within metasedimentary rocks. Concordant, composite bodies of amphibolite and serpentinized peridotite represent another distinctive feature of the Onion Camp complex. The metamorphic and lithologic features of the Onion Camp complex are similar to the lower mélange unit of the Rattlesnake Creek terrane, and the units are interpreted to be correlative. The Fiddler Mountain olistostrome is composed of Late Jurassic (Kimmeridgian?) pelagic and hemipelagic rocks interlayered with ophiolite-clast breccia and megabreccia, similar in character to olistostromal deposits associated with the rift-edge facies of the remnant arc. The occurrence of the Rattlesnake Creek terrane and an associated olistostromal deposit within the western Jurassic belt of southwestern Oregon may therefore represent the rift-edge facies in the active arc setting, at the transition between the Rogue–Chetco arc and Josephine ophiolite, further corroborating previous models for the Late Jurassic tectonic evolution of the Klamath Mountains.