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.

ABSTRACT A growing body of evidence suggests that continental arc lower crust and underlying mantle wedge assemblages native to the Mojave Desert (i.e., the southern California batholith) were displaced eastward during Laramide shallow-angle subduction, and reattached to the base of the Colorado Plateau Transition Zone (central Arizona) and farther inboard. On this field trip, we highlight two xenolith localities from the Transition Zone (Camp Creek and Chino Valley) that likely contain remnants of the missing Mojave lithosphere. At these localities, nodules of garnet clinopyroxenite, the dominant xenolith type at both studied localities, yield low jadeite components in clinopyroxene, chemically homogeneous “type-B” garnet, and peak conditions of equilibration at 600–900 °C and 9–28 kbar. These relations strongly suggest a continental arc residue (“arclogite”), rather than a lower-plate subduction (“eclogite”), origin. Zircon grains extracted from these nodules yield a bimodal age distribution with peaks at ca. 75 and 150 Ma, overlapping southern California batholith pluton ages, and suggesting a consanguineous relationship. In contrast, Mesozoic and early Cenozoic igneous rocks native to SW Arizona, with age peaks at ca. 60 and 170 Ma, do not provide as close a match. In light of these results, we suggest that Transition Zone xenoliths: (1) began forming in Late Jurassic time as a mafic keel to continental arc magmas emplaced into the Mojave Desert and associated with eastward subduction of the Farallon plate; (2) experienced a second ca. 80–70 Ma pulse of growth associated with increased magmatism in the southern California batholith; (3) were transported ~500 km eastward along the leading edge of the shallowly subducting Farallon plate; and (4) were reaffixed to the base of the crust at the new location, in central Arizona. Cenozoic zircon U-Pb, garnet-whole rock Sm-Nd, and titanite U-Pb ages suggest that displaced arclogite remained at elevated temperature (>700 °C) for 10s of m.y., following its dispersal, and until late Oligocene entrainment in host latite. The lack of arclogite and abundance of spinel peridotite xenoliths in Miocene and younger mafic volcanic host rocks (such as those at the San Carlos xenolith locality), and the presence of seismically fast and vertically dipping features beneath the western Colorado Plateau, suggest that arclogite has been foundering into the mantle and being replaced by upwelling asthenosphere since Miocene time.

ABSTRACT The Sierra Nevada batholith of California represents the intrusive footprint of composite Mesozoic Cordilleran arcs built through pre-Mesozoic strata exposed in isolated pendants. Neoproterozoic to Permian strata, which formed the prebatholithic framework of the Sierran arc, were emplaced against the tectonically reorganized SW Laurentian continental margin in the late Paleozoic, culminating with final collapse of the fringing McCloud arc against SW Laurentia. Synthesis of 22 new and 135 existing detrital zircon U/Pb geochronology sample analyses clarifies the provenance, affinity, and history of Sierra Nevada framework rocks. Framework strata comprise terranes with distinct postdepositional histories and detrital zircon provenance that form three broad groups: allochthonous Neoproterozoic to lower Paleozoic strata with interpreted sediment sources from Idaho to northern British Columbia; Neoproterozoic to Permian strata parautochthonous to SW Laurentia; and middle to upper Paleozoic deposits related to the fringing McCloud arc. Only three sedimentary packages potentially contain detritus from rocks exotic to western Laurentia: the Sierra City mélange, chert-argillite unit, and Twin Lakes assemblage. We reject previous correlations of eastern Sierra Nevada strata with the Roberts Mountains and Golconda allochthons and find no evidence that these allochthons ever extended westward across Owens Valley. Snow Lake terrane detrital ages are consistent with interpreted provenance over a wide range from the Mojave Desert to central Idaho. The composite detrital zircon population of all analyses from pre-Mesozoic Sierran framework rocks is indistinguishable from that of the Neoproterozoic to Permian SW Laurentian margin, providing a strong link, in aggregate, between these strata and western Laurentia. These findings support interpretations that the Sierran arc was built into thick sediments underpinned by transitional to continental western Laurentian lithosphere. Thus, the Mesozoic Sierra Nevada arc is native to the SW Cordilleran margin, with the Sierran framework emplaced along SW Laurentia prior to Permian–Triassic initiation of Cordilleran arc activity.

ABSTRACT Forearc basins are first-order products of convergent-margin tectonics, and their sedimentary deposits offer unique perspectives on coeval evolution of adjacent arcs and subduction complexes. New detrital zircon U-Pb geochronologic data from 23 sandstones and 11 individual conglomerate clasts sampled from forearc basin strata of the Nacimiento block, an enigmatic stretch of the Cordilleran forearc exposed along the central California coast, place constraints on models for forearc deformation during evolution of the archetypical Cordilleran Mesozoic margin. Deposition and provenance of the Nacimiento forearc developed in three stages: (1) Late Jurassic–Valanginian deposition of lower Nacimiento forearc strata with zircon derived from the Jurassic–Early Cretaceous arc mixed with zircon recycled from Neoproterozoic–Paleozoic and Mesozoic sedimentary sources typical of the continental interior; (2) erosion or depositional hiatus from ca. 135 to 110 Ma; and (3) Albian–Santonian deposition of upper Nacimiento forearc strata with zircon derived primarily from the Late Cretaceous arc, accompanied by Middle Jurassic zircon during the late Albian–Cenomanian. These data are most consistent with sedimentary source terranes and a paleogeographic origin for the Nacimiento block south of the southern San Joaquin Basin in southern California or northernmost Mexico. This interpreted paleogeographic and depositional history of the Nacimiento block has several implications for the tectonic evolution of the southern California Mesozoic margin. First, the Nacimiento forearc depositional history places new timing constraints on the Early Cretaceous unconformity found in forearc basin strata from the San Joaquin Valley to Baja California. This timing constraint suggests a model in which forearc basin accommodation space was controlled by accretionary growth of the adjacent subduction complex, and where tectonic events in the forearc and the arc were linked through sediment supply rather than through orogenic-scale wedge dynamics. Second, a paleogeographic origin for the Nacimiento forearc south of the southern San Joaquin Valley places new constraints on end-member models for the kinematic evolution of the Sur-Nacimiento fault. Although this new paleogeographic reconstruction cannot distinguish between sinistral strike-slip and thrust models, it requires revision of existing sinistral-slip models for the Sur-Nacimiento fault, and it highlights unresolved problems with the thrust model.

ABSTRACT The Upper Cretaceous Las Tablas unit of the Franciscan Complex, a conglomerate-breccia containing a diverse array of clasts, is located in the central California Coast Ranges. The Las Tablas unit was originally deposited in southern California, where significant amounts of the western half of the Sierra Nevada batholith and coeval Great Valley forearc basin and basement are missing. The most likely explanation for this absence is that forearc and western arc assemblages were removed through a combination of surface and tectonic erosion that accompanied Laramide shallow subduction. Petrographic analysis of rounded to subrounded gabbro, quartz diorite, tonalite, granodiorite, and andesite clasts from the Las Tablas unit reveals a prehnite-pumpellyite–grade overprint of primary igneous textures. Furthermore, zircon grains derived from these clasts yield generally Late Jurassic to Early Cretaceous U-Pb ages and positive Hf isotopic values, with one sample yielding a Late Cretaceous age and a negative Hf value. These relations strongly suggest that the analyzed clasts experienced subduction zone metamorphism and were derived principally from the western and axial Sierra Nevada batholith, with possible additional input from forearc basement (the Coast Range ophiolite). The presence of western arc–derived detritus in the Las Tablas unit suggests that surface plus tectonic erosion removed a significant amount of these units and incorporated them into the subduction complex. Granitic clasts of the Las Tablas unit were likely introduced into previously subducted and exhumed Franciscan materials by sedimentary rather than tectonic processes.

ABSTRACT The Sierra Nevada batholith is an ~600-km-long, NNW-trending continental arc generally exposed from epizonal to mesozonal levels and showing a distinct strike-perpendicular zonation in structural, lithologic, petrologic, geochronologic, and isotopic patterns. South of 35.5° N, in the southern Sierra Nevada–northern Mojave Desert region, the depth of exposure increases markedly and a tectonostratigraphy consisting of three distinct, fault-bounded assemblages is observed. From high to low structural levels, these units are (1) fragments of shallow-level eastern Sierra Nevada batholith affinity rocks, (2) deeper-level western to axial zone rocks, and (3) subduction accretion assemblages (e.g., the Rand schist). This multi-tiered core complex is the product of shallow subduction that occurred over ~500 km of the plate margin in Late Cretaceous time. Slab shallowing was accompanied by intense contractile deformation within the crust and along the subduction megathrust; crustal thickening, uplift, and denudation of the residual arc to midcrustal levels; removal of the forearc and frontal arc by subduction erosion; and replacement of sub-batholithic mantle with underplated subduction assemblages. As the slab reverted to a “normal” trajectory, previously thickened crust no longer compensated at depth by a shallowly dipping slab became gravitationally unstable and underwent a profound phase of extensional collapse. Two subparallel shear zones, one separating assemblages 1 and 2 (the southern Sierra detachment) and the other juxtaposing units 2 and 3 (the Rand fault), comprise an integrated Late Cretaceous detachment system that accommodated extensional collapse. These Late Cretaceous events preconditioned the southern California crust for imprints of subsequent tectonic regimes. For example, subduction of the Pacific-Farallon slab window in early Neogene time created an extensional stress regime in the overriding plate, facilitating high-angle normal faulting across the previously extended region and volcanism associated with upwelling astheno-spheric material. The invasion of hot and buoyant asthenosphere destabilized dense sub-batholithic root material still affixed beneath the central Sierra Nevada batholith, leading to Pliocene–Quaternary delamination of the high-density rocks. Replacement of dense sub-batholithic root materials with asthenosphere has led to ~1 km of uplift across the southern Sierra Nevada and into the eastern San Joaquin Basin. The purpose of this trip is to highlight structural and petrologic records of multiple phases of tectonism in the southern Sierra Nevada–Mojave Desert region, illustrating the profound and lasting effect that shallow subduction may have on a continental margin.