Abstract This field guide describes a three-day trip from Vancouver, British Columbia, to the Wells Gray–Clearwater volcanic field (WGCVF) in east-central British Columbia. The WGCVF is the site of transitional to alkali olivine basaltic volcanism erupted over the last three million years. The small volume magmas (<1 km 3 ) erupted along preexisting normal faults related to the late stages of Cordilleran terrane amalgamation, along the boundary between the miogeoclinal and pericratonic rocks of the Kootenay terrane and the allochthonous Slide Mountain and Quesnellia terranes west of ancestral North America. The magmas are highly enriched in incompatible elements, especially large-ion lithophile elements, and are interpreted as the result of low degrees of partial melting of a heterogeneous, metasomatized mantle. Upon ascent through the crust, they carried up both crustal and mantle xenoliths. During the eruptive period of the WGCVF, at least four glacial periods have occurred. The interplay between volcanism and glaciation is captured in the wide range of volcanic features found in the region. Field trip participants will view numerous diverse volcanic landforms and deposits: from tuyas to ice-marginal valley-edge deposits, volcanoclastic-lacustrine deposits, and associated pillow lavas and hyaloclastites.

Abstract Much of today will be spent in the plutons that are the principal host rocks of the western margin of the Tuolumne Intrusive Complex ( Fig. 3-1 ). These include the Sentinel and Yosemite Creek granodiorites (Kistler, 1973; Bateman, 1992) and the Yosemite Valley Intrusive Suite ( Bateman, 1992 ). The Sentinel and Yosemite Creek Granodiorites were tentatively included as part of the Tuolumne by Kistler and Fleck (1994) and Bateman (1992), but assignment of these plutons to the Tuolumne has been hampered by lack of detailed study. We and our students and colleagues have been mapping and conducting focused studies on the petrology, geochemistry, and structure of both plutons recently, but the results are presently only in theses and abstracts (Petsche, 2008; Fulmer and Kruijer, 2009; Bliekendaal, 2012; van der Linde, 2012; Johnson, 2013). Reasonably accessible exposures of these units can be seen along Highway 120 within Yosemite National Park

The Cordillera of western North America occupies the central 5000 km of the circum-Pacific orogenic belt, which extends for 25,000 km along a great-circle path from Taiwan to the Antarctic Peninsula. The North American Cordillera is anomalous because dextral transform faults along its western flank have supplanted subduction zones, the hallmark of circum-Pacific tectonism, along much of the Cordilleran continental margin since mid-Cenozoic time. The linear continuity of the Cordilleran orogen terminates on the north in the Arctic region and on the south in the Mesoamerican region at sinistral transform faults of Mesozoic and Cenozoic age, respectively. The Cordilleran margin of Laurentia was formed initially by rift breakup of the supercontinent Rodinia followed by development of the Neoproterozoic to early Paleozoic Cordilleran miogeocline along a passive continental margin, but it was modified in California and Mexico by Permian to Triassic transform truncation of Paleozoic tectonic trends. Late Paleozoic and Mesozoic accretion of oceanic island arcs and subduction complexes expanded the width of the Cordilleran orogen both before and after Triassic initiation of ancestral circum-Pacific subduction beneath the Cordilleran margin. Mesozoic to Cenozoic extensions and counterparts of Cordilleran accreted terranes extend southward into the Caribbean Antilles and northern South America. The development of successive forearc and retroforeland basins accompanied the progress of Cordilleran orogenesis over time, and coeval Mesozoic to Cenozoic batholith belts reflect continuing plate consumption at subduction zones along the continental margin. The assembly of subduction complexes along the Cordilleran continental margin continued into Cenozoic time, but dextral strike slip along the Pacific flank of the Cordilleran orogen displaced elongate coastal segments of the orogen northward during Cenozoic time. In the United States and Mexico, Laramide breakup of the Cordilleran foreland during shallow slab subduction and crustal extension within the Basin and Range taphrogen also expanded the width of the Cordilleran orogen during Cenozoic time.

The crystalline core of the North Cascades arc records the Cretaceous to Paleogene history of magmatism, deformation, and crustal growth along a segment of the North American Cordillera. The Nd isotopic compositions of granitoid plutons that intrude the Cascades core are a product of their source regions, and they provide probes of the crustal architecture. We present Sm-Nd isotopic data from 96 Ma to 45 Ma plutons and meta-igneous and metasedimentary terranes across the Cascades core. Sm-Nd data from all metamorphic terranes, excluding the much younger ca. 73 Ma Swakane terrane, yield mid-Cretaceous ε Nd values that range from +8.5 to −1.9 and indicate minor involvement of an enriched crustal component. Amphibolites from the Napeequa complex and Chiwaukum Schist yield near-depleted-mantle ε Nd values in the mid-Cretaceous, and ε Nd values from meta-clastic rocks from these terranes (+3.4 to −1.9) have an isotopic character that is intermediate between arc-derived and continental-shelf (miogeocline) sediments, reflecting a mixture of these two sources. Initial ε Nd values of the Swakane Gneiss range from +0.6 to −5.4 and reflect a significant input from the miogeocline. The initial ε Nd values of the Late Cretaceous to Paleogene plutons studied range from +1.5 to +6.3, consistent with geochemical studies that indicate the plutons were generated by mixing of mantle-derived melt and melt derived by anatexis of the underlying terranes. Initial ε Nd values of plutons from the NE part of the Cascades core generally decrease over time, suggesting a greater contribution of melt from evolved crustal sources, which may reflect a change in the physical parameters of melting. The metamorphic terranes of the North Cascades show a close affinity to the Late Triassic to Early Cretaceous arc terranes of the southern Coast Belt. The similarity in isotopic character supports the assumption that the North Cascades terranes formed in a position outboard of the North American craton but in close enough proximity to derive sediments from the miogeocline. Variations in Nd signature are also observed between the northern and southern Coast plutonic complex, and they indicate changes in the sources of crustal melting along the length of the Cretaceous arc.

The Mojave-Sonora megashear model, which implies left-lateral strike-slip motion of northern México in Jurassic time, remains one of the most influential ideas concerning the geology of México. A comprehensive review of the literature related to this topic does not yet allow resolution of the controversy over the validity of this hypothesis. A clear conclusion is that the original hypothesis was based on a relatively simplistic model of the geology of Sonora, as the basement of the Caborca terrane is not simply a fragment of the Mojave Precambrian basement province of eastern California. Attempts to use quantitative techniques in testing the model have yielded results contrary to the hypothesis, such as clockwise rotations indicated by paleomagnetic data, and the diversity and complexity of the basement of Caborca indicated by geochemical and geochronological data. Other quantitative methods such as zircon provenance studies in quartzites of the sedimentary cover yield inconclusive results. The main conclusion of the studies of detrital zircons is that Grenvillean zircons are relatively abundant, but that their presence cannot be attributed solely to sources in the Grenville province in a fixist model. Stratigraphic correlations of upper Paleozoic and Mesozoic rocks in Caborca with similar sequences in California and Nevada do not provide convincing arguments of large displacement, but should be evaluated in more detail. Elements that have the potential to test the hypothesis with greater certainty include detailed studies of basement rocks, a refined stratigraphy of the Jurassic volcanic and volcaniclastic arc rocks south of the inferred fault trace, and an increased understanding of depositional trends in the miogeoclinal sequence. Structural studies are sparse in this region. It is particularly important to gain a better understanding of the effects in time and space of Late Cretaceous–Tertiary contractional deformation. A tectonic evolution model that does not conflict with the existing data is the proposal that displacement of a para-autochthonous Caborca terrane may have occurred in the late Paleozoic. Nonetheless, available data and geologic relations in the Caborca region do not require Late Jurassic slip of several hundred kilometers. El modelo de la megacizalla Mojave-Sonora, el cual implica desplazamiento lateral izquierdo en el norte de México durante el Jurásico, permanece como una de las ideas más influyentes en la geología del país. Una revisión general de la literatura relacionada con el tema no permite aún resolver la controversia sobre la validez de la hipótesis, pero una conclusión clara es que la hipótesis original estaba basada en un modelo relativamente simplista de la geología de Sonora, ya que el basamento del terreno Caborca no es un simple fragmento de la corteza Mojave del este de California. Intentos de utilizar métodos cuantitativos han dado resultados contrarios a la hipótesis, como el de las rotaciones horarias indicadas por el paleomagnetismo y la diversidad de basamentos en Caborca que sugieren la geocronología y geoquímica; otros métodos producen resultados indeterminados, como la proveniencia de circones en las cuarcitas de la cobertura del terreno Caborca. La conclusión más relevante de esos estudios es la abundancia de circones de edad Grenvilleana, pero su presencia no puede simplemente atribuirse a fuentes en la Provincia Grenville en un modelo fijista. Las correlaciones estratigráficas entre secuencias Paleozoico tardío y Mesozoico en Caborca y secuencias similares en California y Nevada no producen argumentos convincentes a favor de grandes desplazamientos, pero deben considerarse con datos más detallados. Elementos que podrían evaluar la hipótesis con mayor contundencia son estudios más detallados del basamento, una estratigrafía fina del arco volcánico Jurásico y de las rocas volcanoclásticas al sur de la traza inferida de la falla y un mejor conocimiento de la secuencia miogeosinclinal. Son pocos los estudios estructurales en la región y en particular un problema importante es resolver en tiempo y espacio los efectos de la deformación compresional Cretácico-Terciario. Un modelo que no entra en conflicto con la evidencia existente es la propuesta de que el desplazamiento del terreno parautóctono Caborca haya ocurrido en el Paleozoico tardío. Sin embargo, los datos existentes y las relaciones geológicas en la región de Caborca, no requieren de un desplazamiento de cientos de kilómetros en el Jurásico Tardío.