A group of plutons were emplaced in the western Klamath Mountains province during the waning stages of the Late Jurassic Nevadan orogeny. Published U-Pb (zircon) ages indicate that the “western Klamath plutonic suite” was emplaced in the age range of 151–144 Ma. Crosscutting relationships, development of contact metamorphic aureoles, and the presence of distinctive inherited zircon populations indicate that the magmas intruded the footwall and hanging-wall rocks of the principal Nevadan thrust fault. The plutons are chiefly gabbroic to dioritic in composition, but commonly include ultramafic rocks and contain smaller volumes of tonalite and granodiorite. Hornblende is the most common mafic phase, except for some ultramafic rocks in which clinopyroxene ± olivine are locally distinctive, the two-pyroxene dioritic to monzodioritic rocks of the Buck Lake unit of the Bear Mountain pluton, and the most felsic rocks in which biotite is the most abundant mafic phase. Compositions of fine-grained mafic dikes suggest the presence of two principal parental, H 2 O-rich magmas: primitive basalt and evolved basalt/basaltic andesite. The former was parental to the ultramafic rocks of this suite. It was also parental to the basalt/basaltic andesite magmas by deep-seated fractional crystallization processes. The latter magmas were parental to the gabbroic and dioritic units. Many of the felsic rocks show evidence of origins by partial melting of metabasaltic crustal rocks, particularly their low heavy rare-earth element concentrations and high Sr/Y ratios. Mixing of crustal melts with primitive basaltic magmas was locally important (e.g., Pony Peak pluton). The mafic parental magmas show trace element features typical of an origin by partial melting of a subduction-modified mantle wedge. It is unclear whether subduction was coeval with western Klamath magmatism or whether the subduction signature developed as the result of Middle Jurassic subduction.

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.

ABSTRACT The Klamath Mountains province and adjacent Franciscan subduction complex (northern California–southern Oregon) together contain a world-class archive of subduction-related growth and stabilization of continental lithosphere. These key elements of the North American Cordillera expanded significantly from Middle Jurassic to Early Cretaceous time, apparently by a combination of tectonic accretion and continental arc– plus rift-related magmatic additions. The purpose of this field trip is twofold: to showcase the rock record of continental growth in this region and to discuss unresolved regional geologic problems. The latter include: (1) the extent to which Mesozoic orogenesis (e.g., Siskiyou and Nevadan events plus the onset of Franciscan accretion) was driven by collision of continental or oceanic fragments versus changes in plate motion, (2) whether growth involved “accordion tectonics” whereby marginal basins (and associated fringing arcs) repeatedly opened and closed or was driven by the accretion of significant volumes of material exotic to North America, and (3) the origin of the Condrey Mountain schist, a composite low-grade unit occupying an enigmatic structural window in the central Klamaths—at odds with the east-dipping thrust sheet regional structural “rule.” Respectively, we assert that (1) if collision drove orogenesis, the requisite exotic materials are missing (we cannot rule out the possibility that such materials were removed via subduction and/or strike slip faulting); (2) opening and closure of the Josephine ophiolite-floored and Galice Formation–filled basin demonstrably occurred adjacent to North America; and (3) the inner Condrey Mountain schist domain is equivalent to the oldest clastic Franciscan subunit (the South Fork Mountain schist) and therefore represents trench assemblages underplated >100 km inboard of the subduction margin, presumably during a previously unrecognized phase of shallow-angle subduction. In aggregate, these relations suggest that the Klamath Mountains and adjacent Franciscan complex represent telescoped arc and forearc upper plate domains of a dynamic Mesozoic subduction zone, wherein the downgoing oceanic plate took a variety of trajectories into the mantle. We speculate that the downgoing plate contained alternating tracts of smooth and dense versus rough and buoyant lithosphere—the former gliding into the mantle (facilitating slab rollback and upper plate extension) and the latter enhancing basal traction (driving upper plate compression and slab-shallowing). Modern snapshots of similarly complex convergent settings are abundant in the western Pacific Ocean, with subduction of the Australian plate beneath New Guinea and adjacent island groups providing perhaps the best analog.

New, high-precision U-Pb titanite (sphene) and zircon dates from five samples of the Bear Mountain intrusive complex establish the timing and duration of magmatism. The oldest, magmatic date (150.5 ± 0.6 Ma) comes from dark-colored titanite from a biotite-hornblende tonalite that is part of a composite pluton that intrudes the Blue Ridge ultramafic-gabbroic intrusion. Pale titanite and zircon from this sample yielded a distinctly younger date of 149.3 ± 0.3 Ma. A similar pattern of mineral dates is also apparent in two samples of the areally extensive Punchbowl unit of the Bear Mountain pluton. Dark-colored titanite in one of these samples yielded a date of 149.5 ± 0.6 Ma, whereas the dates of pale titanite and zircon are 147.4 ± 0.3 Ma. The second sample of the Punchbowl unit only contained a single morphology of pale titanite, which yielded the same date as zircon (148.2 ± 0.3 Ma). The U-Pb zircon date of the Buck Lake unit of the Bear Mountain pluton, 148.2 ± 0.2 Ma, supports field evidence that the Buck Lake unit was emplaced synchronously with the Punchbowl unit. A lower age limit on magmatism in the Bear Mountain intrusive complex comes from a 145.4 ± 0.4-Ma zircon date from a late crosscutting mafic dike. All samples exhibit slight inheritance in the zircon data, with 152- to 150-Ma minimum ages. The mafic dike contains inherited components that are at least 264 Ma and possibly Paleoproterozoic in age. The new dates constrain magmatism in the Bear Mountain intrusive complex to the period from 151 to 147 Ma, with a minimum duration of 1.5 m.y. and a maximum of 6 m.y. The dates establish that the emplacement and crystallization of the Bear Mountain intrusive complex post-dated regional thrust faulting (Orleans fault) associated with the Nevadan orogeny, including the South Siskiyou Fork fault, which is interpreted as an oblique-slip tear fault associated with the Orleans (thrust) fault system. The pattern of mineral dates from the composite pluton intruded into the Blue Ridge intrusion as well as the areally more extensive Punchbowl unit indicate that crystallization of these bodies occurred over 1.5–2 m.y., due to either insulating effects of the intrusive complex and/or magma recharge.