Early Mesozoic arc magmatism of the southern Sierra Nevada region records the onset of plate convergence–driven magmatism resulting from subduction initiation near the end of Permian time along a prior transform margin. We provisionally adopt the term California-Coahuila transform for this complex boundary transform system, which bounded the southwest margin of the Cordilleran passive margin, its offshore marginal basin, and fringing island arc. In Pennsylvanian–Early Permian time, this transform cut into the arc-marginal basin and adjacent shelf system, calved off a series of strike-slip ribbons, and transported them differentially southward through ∼500–1000-km-scale sinistral displacements. These strike-slip ribbons constitute the principal Neoproterozoic–Paleozoic metamorphic framework terranes for the superposed Mesozoic batholithic belt in the Sierra Nevada and Mojave plateau regions. The southern Sierra Nevada batholith intruded along the transform truncation zone where marginal basin ribbons were juxtaposed against the truncated shelf. Strike-slip ribbons, or blocks, liberated from the truncated shelf occur today as the Caborca block in northwest Mexico, and possibly parts of the Chortis block, farther south. The oldest arc plutons in the Sierra region were emplaced between 256 and 248 Ma, which matches well with ca. 255 Ma high-pressure metamorphism recorded in the western Sierra Foothills ophiolite belt, interpreted to approximate the time of subduction initiation. The initial phases of arc plutonism were accompanied by regional transpressive fold-and-thrust deformation, kinematically marking the transition from transform to oblique convergent plate motion. Early arc volcanism is sparsely recorded owing to fold-and-thrust–driven exhumation having accompanied the early phases of arc activity. By Late Triassic time, the volcanic record became quite prolific, owing to regional subsidence of the arc into marine conditions, and the ponding of volcanics in a regional arc graben system. The arc graben system is but one mark of regional suprasubduction-zone extension that affected the early SW Cordilleran convergent margin from Late Triassic to early Middle Jurassic time. We interpret this extension to have been a dynamic consequence of the subduction of exceptionally aged Panthalassa abyssal lithosphere, which is well represented in the Foothills ophiolite belt and other ophiolitic remnants of the SW Cordillera. Middle and Late Jurassic time was characterized by important tangential displacements along the SW Cordil-leran convergent margin. In Middle Jurassic time, dextral impingement of the Insular superterrane intra-oceanic arc drove a migrating welt of transpressional deformation through the SW Cordillera while the superterrane was en route to its Pacific Northwest accretionary site. Dextral transtensional spreading in the wake of the obliquely colliding and translating arc opened the Coast Range and Josephine ophiolite basins. In Late Jurassic time, a northwestward acceleration in the absolute motion of the North American plate resulted in an ∼15 m.y. period of profound sinistral shear along the Cordilleran convergent margin. This shear is recorded in the southern Cordillera by the Mojave-Sonora megashear system. Late Jurassic intrusive units of the southern Sierra region record sinistral shear during their magmatic emplacement, but we have not observed evidence for major Late Jurassic sinistral displacements having run through the Sierran framework. Possible displacements related to the megashear in the California to Washington regions are likely to have: (1) followed preexisting transforms in the Coast Range ophiolite basin and (2) been accommodated by oblique closure of the Josephine ophiolite basin, and the northern reaches of the Coast Range ophiolite basin, proximal to the southern Insular superterrane, which in Late Jurassic–earliest Cretaceous time was obliquely accreting to the inner Cordillera terranes of the Pacific Northwest.

Sedimentary rocks occurred throughout much of the Late Jurassic Cordilleran margin of Laurasia. Their tectonic setting and provenance are critical to understanding the evolution of the Cordilleran margin during this time. We review published detrital zircon ages and new and published whole-rock geochemistry of the Peshastin Formation and Darrington Phyllite, Cascade Mountains, Washington State, with the goal of better understanding the tectonic development of the Cordillera and strengthening regional correlations of these sedimentary units. The Peshastin Formation conformably overlies the ca. 161 Ma Ingalls ophiolite complex. Published dating of detrital zircons from a Peshastin Formation sandstone provided a youngest U-Pb age distribution of ca. 152 Ma and a significant U-Pb age distribution of ca. 232 Ma. The Darrington Phyllite is structurally above the Shuksan Greenschist; however, this unit also occurs interbedded with the Shuksan Greenschist. The Darrington Phyllite and Shuksan Greenschist have been grouped into the Easton Metamorphic Suite. Published detrital zircons from a Darrington Phyllite metasandstone have a youngest U-Pb age distribution of ca. 155 Ma and a significant U-Pb age distribution of ca. 238 Ma. New major- and trace-element geochemistry and previously published sandstone petrography suggest that these units were derived from Late Jurassic volcanic arc sources that were predominantly transitional between mafic and intermediate compositions. Middle to Late Triassic detrital zircon ages and detrital modes suggest that some recycling of older accreted arc terranes also contributed to these sediments; however, this Middle to Late Triassic component could also be first cycle. These units consistently plot on geochemical diagrams in fields defined by modern back-arc basin turbidites. The youngest detrital zircon age distributions, detrital sandstone petrography, and geochemistry of these units suggest they formed in Late Jurassic arc-fed basins. We suggest that the Peshastin Formation and Darrington Phyllite are age correlative and formed in an arc-proximal back-arc basin that could have initiated by forearc rifting. Postulated restoration of latest Cretaceous to Cenozoic faulting places these Late Jurassic basins near the Galice Formation and underlying Josephine ophi-olite, Klamath Mountains, Oregon-California. The Galice Formation and underlying Josephine ophiolite have been correlated with the Peshastin Formation and Ingalls ophiolite complex. After postulated Late Jurassic accretion to the North American margin, the Peshastin Formation and Darrington Phyllite were dextrally displaced to the north before they were emplaced in their current position by thrust faulting during the Late Cretaceous.

ABSTRACT This field guide describes stops in the Oregon Klamath Mountains that visit near-complete ophiolite sections, pre- and post-accretion arc plutons, greenschist- to amphibolite-grade metamorphosed wallrocks, arc volcanic rocks, and interbedded chert, argillite, and olistostromal deposits. Structural features at these stops include local- and regional-scale folds and faults, as well as penetrative metamorphic fabrics such as slaty cleavage, gneissic layering, and mineral lineations. The geologic history here reveals a period of Late Triassic and Jurassic ophiolite and oceanic-arc formation followed by Middle Jurassic terrane accretion, tectonic mélange formation, and continued oceanic arc magmatism. Rifting from ca. 165 to 160 Ma produced the Rogue-Chetco arc, Josephine ophiolite, and remnant arc comprised of older Klamath Mountains terranes. Deformation and magmatism during the Late Jurassic Neva-dan orogeny accreted this active arc–inter-arc basin–remnant arc triad to western North America, producing the lithotectonic belts observed today. The Oregon Klam-ath Mountains therefore provide an exceptional opportunity to examine the deep to shallow levels of multi-phase oceanic lithosphere and deformational features related to the accretion of these terranes to the continental margin.

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.

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.

The Upper Jurassic Galice Formation of the Klamath Mountains, Oregon-California, overlies the ca. 162-Ma Josephine ophiolite and the slightly younger Rogue–Chetco volcano-plutonic arc complex. The Galice Formation that overlies the Josephine ophiolite consists of a siliceous hemipelagic sequence, which grades upward into a thick turbidite sequence. Bedded hemipelagic rocks and scarce sandstone, however, also occur at several localities within the Josephine ophiolite pillow basalts. Corrected paleoflow current data suggest that the Galice Formation was derived predominantly from the east and north. Detrital modes of sandstones from the Galice Formation indicate an arc source as well as a predominantly chert-argillite source with minor metamorphic rocks. A sandstone located ∼20 m below the top of the Josephine ophiolite has detrital modes and heavy mineral suites similar to the turbidite sandstones. Detrital Cr-spinel compositions from the turbidite and intra-pillow lava sandstones are also similar, indicating supra-subduction zone mantle peridotite and volcanic sources. Published detrital zircon data from a turbidite sandstone chiefly give a bimodal age distribution of 153 Ma and ca. 227 Ma but with a minor Proterozoic component. Whole-rock geochemistry from intra-pillow lava sedimentary rocks, the hemipelagic sequence, and the turbidites suggest a mixture between mafic and cratonic sources. It is suggested that the source area for the intra-pillow lava sedimentary rocks, hemipelagic sequence, and turbidites resulted from the mixing of arc and accreted terranes. These data indicate that the source areas for the Galice Formation were already established by ca. 162 Ma, probably during a Middle Jurassic orogeny that predated formation of the Josephine basin.

The Galice Formation is characterized by slaty cleavage, overturned tight-to-isoclinal folds having variable hingeline orientations, and a south–southeast-trending stretching lineation formed during the Nevadan orogeny. Calc-alkaline dikes and sills (151–146 Ma) that intruded the Galice Formation and its basement (Josephine ophiolite) are regionally metamorphosed, and some are deformed; however, some plutons of this age also overprint slaty cleavage, suggesting syntectonic intrusion. Amoeboid margins on some sills suggest intrusion began prior to lithification of the Galice Formation. Some dikes are intruded into pre-existing small thrust faults that predate the slaty cleavage. Dikes show a wide range of orientations, and poles to dikes are consistently oriented at a high angle to poles to extension veins and to the stretching lineation in the Galice Formation. Poles to dikes define two quadrants on an equal-area, lower-hemisphere projection separated by planes oriented at right angles. These planes are analogous to nodal planes of a fault-plane solution, and thus allow determination of P- and T- axes. Restoration of structures to their original (Nevadan) orientation results in the P - and T -axes, stretching lineations, poles to extension veins, poles to small syn-cleavage faults, and poles to cleavage all being essentially coplanar with the “movement plane” that strikes to the northwest and dips steeply. The “fault-plane solution” derived from dike orientations indicates northwest-southeast contraction, consistent with slip directions for most small faults having slickenfibers. A wide range of fold hingeline orientations and slip directions on small pre-cleavage faults, however, may record early west-directed shortening.

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.

Field mapping and structural analysis indicate that the Snowcamp remnant of the Coast Range ophiolite, in the vicinity of Game Lake, southwest Oregon, was thrust to the northeast over mafic phyllonite and amphibolite correlated with the Chetco complex. The contact between the upper ophiolitic and lower mafic sections is marked by a ∼5- to 10-m-thick serpentinite unit and paralleled by mylonitic foliations in the lower mafic section. Syndeformational tonalites intrude the lower plate. The fabric-forming metamorphic mineral assemblages (amphibolite facies) in the mafic section suggest a temperature range of ∼550–600+ °C and medium pressure, whereas quantitative thermobarometry indicates 615–720 °C and 3.5–6 kb (11.5–20 km depth). A mylonitic greenschist-facies mineral assemblage in phyllonites below the thrust overprints the amphibolite-facies fabric, indicating that thrusting continued during cooling. The 40 Ar/ 39 Ar analyses indicate that thrusting and syntectonic intrusion occurred by at least 154.2 ± 2.0 and 149.1 ± 0.4 Ma, respectively. The character of this thrust strongly resembles that of the Madstone Cabin thrust that emplaced the Josephine ophiolite over the Rogue–Chetco arc complex. The proposed correlation between the two faults implies that either the Snowcamp remnant of the Coast Range ophiolite has undergone a similar emplacement history as the Josephine ophiolite or that the ophiolitic rocks near Game Lake may be an outlier of the Josephine ophiolite juxtaposed by postemplacement faulting against a Coast Range remnant around Snowcamp Mountain to the south.

Pumpellyite and prehnite are associated closely with epidosite in two well-exposed sections of the Josephine ophiolite and are interpreted to have formed during hydrothermal metamorphism beneath a spreading axis. In the upper 75 m of the extrusive sequence, epidosite grades upward into “pumpellyosite” (granoblastic pumpellyite + quartz + chlorite ± epidote rock) and, in interpillow hyaloclastite, into “prehnitite” (granoblastic prehnite + quartz + epidote ± chlorite rock). Probable hydrothermal pumpellyite also occurs in the lower hematitic pillow lavas as amygdules that contain pumpellyite + chlorite ± epidote ± chalcopyrite. The second occurrence of pumpellyosite and prehnitite is in the basal sheeted dike complex, where these minerals formed during the late stages of retrograde hydrothermal metamorphism. The bulk-rock composition of pumpellyosite and prehnitite shows extensive Ca metasomatism very similar to that of epidosite. Like epidosites, these rocks are inferred to have formed by interaction with large volumes of upwelling, highly reacted hydrothermal fluids, similar to those at modern high-temperature hot springs on mid-ocean ridges. The change in the upper 75 m of the pillow lavas from epidosite to pumpellyosite and prehnitite may reflect cooling of upwelling fluids to less than ~315 °C. The presence of interpillow prehnitite immediately below sediments overlying the ophiolite implies that these fluids leaked directly onto the sea floor. The Josephine pumpellyosites and prehnitites formed at temperatures between 200 and 315 °C, within the overlapping stability fields of epidote, prehnite, and pumpellyite. The influence of fluid composition on mineral equilibria is evaluated at 250 °C and 500 bar using an a Ca 2 + / a H + 2 versus a Fe 3 + / a H + 3 diagram modified from Rose and Bird (1987). The topology of the pumpellyite-epidote and pumpellyite-prehnite phase boundaries was derived using compositions of coexisting Ca-Al silicates in the Josephine samples. The pumpellyite and prehnite stability fields are generally at lower a Fe 3 + / a H + 3 than epidote, whereas the pumpellyite stability field is generally at lower a Ca 2 + / a H + 3 than prehnite.

The 162-Ma Josephine ophiolite was emplaced over an active mafic batholith (Chetco River complex) along the Madstone thrust in southwestern Oregon during the Nevadan orogeny, beginning at ∼155 Ma. Strongly deformed amphibolite and minor quartzite occur between the ophiolite and the batholith and are interpreted to make up a metamorphic sole formed during thrusting. Retrograde metamorphism is ubiquitous, and amphibolite has been locally converted to greenschist-facies mafic phyllonite adjacent to the Madstone thrust. Pegmatite dikes locally cut the amphibolite but are also penetratively deformed, indicating syntectonic intrusion. A geochronologic study (Harper and others, 1989) indicates cooling from ∼450°C at 153 Ma, intrusion of the pegmatite at 150 Ma, and cooling to ∼350°C at 146 Ma. Geobarometry, using amphibole composition and phengite content of muscovite, indicates relatively low P/T metamorphism. The lower contact of the amphibolite sole with the Chetco River complex, as described by previous workers, is intrusive and syntectonic with deformation of the amphibolite sole. In the hanging wall of the Madstone thrust, 20 to 40 m of high-T serpentinite mylonite occurs along the base of the Josephine Peridotite. The serpentinite apparently formed during ophiolite emplacement because it is structurally concordant with the underlying amphibolite and phyllonite. In addition, the serpentinite locally shows metasomatism, which probably resulted from interaction with fluids derived from the amphibolite sole. The amphibolite shows two generations of folds having fold hinges parallel to a NNE-stretching lineation. These structures, along with grain-size reduction and asymmetric fabrics, indicate that the amphibolites are mylonites formed by progressive simple shear. The lineations and sense-of-shear criteria for the amphibolite and serpentinite mylonite indicate thrusting of the Josephine ophiolite toward the north-northeast, over the Chetco River complex. Continued north-northeast thrusting during greenschist retrograde metamorphism is indicated by lineations and microstructures in phyllonites and a pegmatite dike. A minimum displacement of 12 km is inferred from the outcrop pattern of the Madstone thrust. The metamorphic sole and regional geologic setting of the Josephine ophiolite are distinct from other ophiolites. There is no inverted gradient, maximum temperatures were lower, syntectonic magmas were intruded into both the metamorphic sole and the ophiolite, and the ophiolite was thrust over an active magmatic arc rather than a continental margin. In addition, the ophiolite and overlying Galice Formation were thrust beneath the North American continent by >40 km along the roof thrust (Orleans fault) and regionally metamorphosed to low grade. Geochronologic and structural studies indicate that the basal Madstone thrust and the roof thrust were both active at 150 ± 1 Ma, but the thrusting direction along the roof thrust appears to have been west or northwest. The cause and tectonic significance of nearly orthogonal thrusting directions between the basal and roof thrusts of the ophiolite is enigmatic. One possibility is that thrusting occurred during sinistral oblique subduction, and the Josephine thrust sheet was effectively decoupled along the roof thrust due to high pore-fluid pressures in the Galice Formation.