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.

Neogene doming in the north-central Klamath Mountains, California, tilted the Rattlesnake Creek terrane, chiefly an ophiolitic mélange, exposing an oblique cross section through disrupted and metamorphosed oceanic crust and mantle. The deepest section of the tilted terrane, in the Kangaroo Mountain area near Seiad Valley, contains tectonic slices of ultramafic, mafic, and sedimentary rocks that were penetratively deformed and metamorphosed under upper-amphibolite- to granulite-facies conditions. This section, called the Seiad complex, is the ophiolitic basement of an accreted Mesozoic island arc, and its polygenetic history reflects the magmatic and tectonic processes that occur during island-arc construction and evolution. The presence of metarodingite and metaserpentinite, and the concordance of structural elements and metamorphic grade among all units of the Seiad complex, indicate that initial tectonic disruption of the ophiolitic suite occurred in the upper crust and subsequent penetrative deformation and metamorphism occurred under high-temperature conditions in the deep crust. Crustal granulite-facies metamorphism is indicated by two-pyroxene metagabbroic bodies and two-pyroxene metasedimentary paragneiss. Geothermobarometric data from garnet amphibolite and granulite-facies metagabbro within the ophiolitic suite yielded pressure and temperature conditions of ~5–7 kb and ~650–750 °C. Geochemical data from samples of granulite, amphibolite, and leucotrondhjemite suggest a supra-subduction origin, although there is significant variation among the amphibolite samples, indicating multiple magma types. Crosscutting, radiometrically dated plutons and the regional geologic context suggest that high-grade metamorphism and deformation of these disrupted ophiolitic rocks occurred in the Middle Jurassic (ca. 172–167 Ma). This time interval broadly corresponds with contraction along several regional thrust faults in the Klamath Mountains province and juxtaposition of the Rattlesnake Creek terrane with terranes to the east. A U-Pb zircon age of 152.7 ± 1.8 Ma on a sample of a crosscutting leucotrondhjemitic dike swarm and published 40 Ar/ 39 Ar hornblende age spectra of ca. 150 ± 2 Ma from amphibolite indicate that magmatism and an accompanying thermal flux continued to affect this region into the Late Jurassic. Compared with the deep-crustal sections of the well-studied Kohistan and Tal-keetna arc complexes, the widespread mélange character of the Rattlesnake Creek terrane (including the Seiad complex) is unique. However, ophiolitic rocks, including mantle ultramafic rocks, are common components in the basal parts of these classic arc crustal sections. Hornblende gabbro/diorite and clinopyroxenite in the Seiad complex may be small-scale melt conduits that fed middle- and upper-crustal components of the arc, analogous to the relationship seen in Kohistan between deep-crustal ultramafic-mafic bodies and mid-crustal magma chambers.

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.

Nd and Sr isotopic data are presented for argillaceous rocks from two terranes in the Klamath Mountains: the western Klamath terrane and Rattlesnake Creek terrane. In the collage of terranes exposed in the Klamath Mountains, these terranes are located farthest outboard from the North American craton and include (meta)igneous rocks of clear oceanic affinity. Nevertheless, the argillaceous rocks from these terranes preserve a strong isotopic signal of terrigenous sedimentary input. The lowermost portion of the Rattlesnake Creek terrane, the serpentinite-matrix mélange, is interpreted to have formed within an oceanic fracture zone. The argillaceous sediment incorporated into this tectonic assemblage was probably derived partly from eolian dust eroded from the continents and partly from local juvenile detritus shed from topographic highs along the fracture zone. The Upper Triassic–Lower Jurassic “cover sequence” of the Rattlesnake Creek terrane has been interpreted as an oceanic-arc assemblage, but the argillaceous rocks of the cover sequence have the most negative ϵ Nd (−8.3) and radiogenic 87 Sr/ 86 Sr (0.7114) of any samples analyzed in this study. We infer that cratonic sediment was delivered to the depocenter of the Rattlesnake Creek terrane arc, probably transported by river systems. This situation suggests proximity to a continental landmass during arc magmatism. The Galice Formation, a thick turbiditic sequence above the Late Jurassic Josephine ophiolite, appears to be composed of detritus shed from both the Rogue–Chetco oceanic arc on the west (in present geographic coordinates) and previously accreted Klamath Mountains terranes and/or North American craton to the east. The continental isotopic signal is stronger in the argillaceous rocks than in the (meta)graywackes, suggesting that the finer-grained rocks contain a greater proportion of cratonic debris, material that may have been reduced to mud-sized particles during sediment recycling. The presence of continental-derived sediment in these otherwise ensimatic terranes indicates that although continental crustal growth by accretion of oceanic terranes is an important process, such accreted terranes commonly are not composed entirely of juvenile crust.

A newly discovered dike complex and serpentinite-matrix mélange may represent basement for the Upper Jurassic Galice Formation in the Elk outlier of the western Klamath terrane, which lies far to the west of the Klamath Mountains province. The fault-bounded dike complex consists of virtually 100% parallel dikes with scarce diorite screens and amphibole-rich hydrothermal metamorphic assemblages, all consistent with it being a sheeted dike complex of an ophiolite. Magmatic affinities of the dikes are transitional island-arc tholeiite (IAT) to mid-ocean ridge basalt (MORB) and boninitic, similar to the Josephine ophiolite that underlies the Galice Formation elsewhere. The mélange includes blocks of mafic volcanic (many pillowed) and hypabyssal rocks, igneous breccia, amphibolite, pyroxenite, peridotite, and metasandstone in a sheared serpentinite matrix. Volcanic rocks and amphibolites have MORB magmatic affinities, and some of the pillow lavas are unusual highly fractionated Fe-Ti basalts. Hypabyssal blocks have MORB, within-plate, and calc-alkaline affinities. The magmatic affinities of the mélange blocks are distinct from the dike complex, and thus the mélange and dike complex are unrelated petrogenetically. The mélange may belong to the underlying Pickett Peak terrane of the Franciscan Complex, or it may be part of the western Klamath terrane and correlative to Lower Mesozoic ophiolitic rocks found elsewhere in “rift-edge facies” of the Josephine ophiolite. The Elk outlier is far to the north of the part of the western Klamath terrane that contains the Josephine ophiolite. Thus, the presence of a remnant of the Josephine ophiolite in the Elk outlier would be consistent with Cretaceous dextral strike-slip displacement proposed by previous workers or with the Josephine extending farther north than previously recognized.

The Bear Peak intrusive complex is a Late Jurassic (ca. 144 Ma) composite plutonic suite that ranges in composition from ultramafic to silicic. Clinopyroxene- and hornblende-rich ultramafic cumulate rocks form an intrusion breccia that is complexly intruded by multiple generations of crosscutting gabbroic to dioritic dikes. The bulk of the intrusive complex consists of mappable gabbroic to quartz dioritic to tonalitic/granodioritic units. The Bear Peak intrusive complex was emplaced into rocks of the Rattlesnake Creek terrane, producing a dynamothermal contact aureole. Contact metamorphism was chiefly at hornblende-hornfels-facies conditions and grades into regional greenschist-facies metamorphism. Andalusite, cordierite, and chloritoid form small porphyroblasts in some of the more aluminous metasedimentary rocks, indicating low-pressure contact metamorphism (<4 kb). Al-in-hornblende geobarometry in quartz dioritic to tonalitic rocks also suggests pressure conditions of ∼4 kb. Pseudomorphs of original chiastolite porphyroblasts developed during contact metamorphism of pelitic horizons in the Upper Jurassic Galice Formation, which lies in the footwall of the regional Orleans thrust fault, indicate that the Bear Peak intrusive complex was emplaced after regional contraction related to the Nevadan orogeny. The Bear Peak intrusive complex is an example of the extended compositional range characteristic of some oceanic-arc plutonic suites and demonstrates how multiple, chiefly magmatic processes, can yield a broad range of rock compositions within a single intrusive complex. Mafic magmatic enclaves are common in most of the plutonic units of the Bear Peak intrusive complex, and distinctive migmatitic amphibolite enclaves indicate that magma temperatures were sufficient to facilitate dehydration-melting of metabasic rocks. The distribution of host-rock enclaves and screens suggest that much of the gabbroic to quartz dioritic parts of the Bear Peak intrusive complex were emplaced as magmatic sheets that coalesced into mappable, relatively homogeneous units that grew by piecemeal intrusion. Ultramafic-mafic cumulates and hornblende gabbro crystallized from a high-Mg, low-Al basaltic parent, whereas high-Al, low-Mg contents in quartz dioritic rocks suggest an evolved basaltic or basaltic andesite parent. The biotite tonalite/granodiorite rocks have high Sr values (>700 ppm), large Sr/Y and Ba/Y ratios, and reverse J-shaped rare-earth-element (REE) patterns. These features are characteristic of partial melting of metabasic rocks in which amphibole ± garnet are residual phases. Thus, major, trace, and REE compositions indicate at least two batches of magma were involved in the petrogenesis of the Bear Peak intrusive complex. Complex field relationships and geochemical data suggest that multiple magmas passed through the cumulates and presumably fed structurally higher mafic units in the complex.

New field mapping and bulk-rock geochemical investigations in the southern Klamath Mountains and central Sierran Foothills combined with previous structural, petrochemical, and geochronologic studies allow the distinction between three Triassic–Jurassic basaltic arcs built along the continental edge versus two roughly coeval basaltic complexes that formed farther off the Californian margin. The three Klamath Mountains arcs are: (1) The Hayfork Summit–Salmon River segment of the southern North Fork terrane, formed offshore as a sequence of interlayered chert, volcaniclastic strata and shale, and ocean island basalt (OIB), deposited on a mélanged and serpentinized basement containing blocks of 310- to 265-Ma mid-ocean ridge basalt (MORB). (2) Northward, the Sawyers Bar sector of the central North Fork terrane formed closer to the continental margin; this mafic arc originated at ca. 200–170 Ma as a stack of interdigitated island-arc tholeiites (IAT) and minor OIBs interstratified with, and largely overlying, distal turbidites derived from eastern Klamath terranes. (3) The currently farther outboard Rattlesnake Creek terrane consists of continent-sourced, Lower Jurassic metasedimentary quartzose strata interbedded with island-arc volcanic rocks; this near-shore section was laid down on older ophiolitic basement consisting of tectonized serpentinite and MORB blocks. The remaining two arcs are in the Sierran Foothills: (4) The offshore Peñon Blanco arc consists of cherty and volcaniclastic sedimentary strata interlayered with 200-Ma mafic volcanic-plutonic arc rocks, all resting on a 300-Ma ophiolitic basement; suturing against the structurally higher Mariposa Formation took place after deformation of the latter at ca. 150 Ma. (5) The Slate Creek complex, and possibly the Lake Combie, Owl Gulch, and Sullivan Creek entities, formed along the margin of North America; superjacent units consist chiefly of 207- to 170-Ma volcaniclastic, sedimentary, and arc volcanic rocks deposited on an ophiolitic mélange basement. Metamorphic belts of the central Klamath Mountains and Sierran Foothills evidently contain both near-shore and offshore oceanic arcs.

The Bear Mountain intrusive complex, Klamath Mountains, California, is a multiphase, ultramafic to silicic plutonic suite emplaced into the Rattlesnake Creek terrane during the Late Jurassic (ca. 151–147 Ma). The intrusive complex includes five plutonic units: (1) elongated, flanking bodies of ultramafic to gabbroic rocks (Blue Ridge, Clear Creek, and Cedar Creek intrusions); (2) biotite + two-pyroxene diorite/monzodiorite of the Buck Lake plutonic unit; (3) biotite-bearing hornblende ± pyroxene gabbro/diorite of the Punchbowl plutonic unit; (4) biotite + hornblende ± pyroxene (± quartz) diorite of the Doe Flat plutonic unit; and (5) minor biotite ± hornblende quartz diorite to tonalite/granodiorite of the Wilderness Falls plutonic unit. Units 2, 3, and 4 constitute the Bear Mountain pluton. The petrogenesis of these units involved emplacement of distinct batches of magma into the same magmatic center during a complex, multistage intrusion history. Inclusions of older units are found within younger units, and dikes related to younger units intrude older units. Mineral assemblages and Al-in-hornblende barometry indicate early crystallization of pyroxene at >1100 °C and late crystallization of hornblende and biotite at >700 °C between 3 and 5 kb. A dynamothermal aureole extends for ∼600–1000 m from the external intrusive contact of the Bear Mountain pluton. Metabasic rocks within this dynamo-thermal aureole have been strongly deformed and recrystallized to well-foliated and lineated hornblende schist or fine-grained amphibolite and exhibit a “pluton-down” sense of shear (i.e., the Bear Mountain pluton). Adjacent to the intrusive complex, contact-metamorphic conditions were hornblende-hornfels facies and locally reached pyroxene-hornfels facies. At some localities in the inner aureole, anatexis occurred, forming migmatitic amphibolite gneiss. The effects of thermal metamorphism apparently extend >2 km from the intrusive contact and are evidenced by various mineral assemblages in metaserpentinite. A concentric, margin-parallel magmatic foliation is found throughout the main body of the Bear Mountain pluton. Map-scale evidence suggests emplacement of the Bear Mountain pluton as a series of sheetlike intrusive bodies that subsequently sagged into an oval-shaped pluton due to negative buoyancy and overall isostatic downward displacement. The three-dimensional shape of the complex is poorly known, but it may be funnel-shaped with a dense root of ultramafic rocks. The ultramafic rocks of the Bear Mountain intrusive complex are primarily cumulates from H 2 O- and MgO-rich basaltic parental magmas. Differentiation of these magmas gave rise to H 2 O- and Al 2 O 3 -rich basalt/basaltic andesite from which rocks of the Punchbowl unit accumulated. Further differentiation of this magma type resulted in the development of the parental magma for the Doe Flat unit. Magmas parental to the Buck Lake unit were similar to the Punchbowl parent, except that Buck Lake magmas were H 2 O poor. All of these magmas have trace-element, Sr, Nd, and oxygen isotope values characteristic of a supra-subduction zone tectonic setting. The late-stage tonalite/granodiorite magmas have geochemical signatures consistent with an origin by partial melting of metabasic crustal rocks with residual garnet; therefore, they cannot be explained by local, in situ anatexis of host rock metabasites.