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 Spatial distributions of widespread igneous arc rocks and high-pressure–low-temperature (HP/LT) metamafic rocks, combined with U-Pb maximum ages of deposition from detrital zircon and petrofacies of Jurassic–Miocene clastic sedimentary rocks, constrain the geologic development of the northern and central Californian accretionary margin: (1) Before ca. 175 Ma, transpressive plate subduction initiated construction of a magmatic arc astride the Klamath-Sierran crustal margin. (2) Paleo-Pacific oceanic-plate rocks were recrystallized under HP/LT conditions in an east-dipping subduction zone beneath the arc at ca. 170–155 Ma. Stored at depth, these HP/LT metamafic blocks returned surfaceward mainly during mid- and Late Cretaceous time as olistoliths and tectonic fragments entrained in circulating, buoyant Franciscan mud-matrix mélange. (3) By ca. 165 Ma and continuing to at least ca. 150 Ma, erosion of the volcanic arc supplied upper-crustal debris to the Mariposa-Galice and Myrtle arc-margin strata. (4) By ca. 140 Ma, the Klamath salient had moved ~80–100 km westward relative to the Sierran arc, initiating a new, outboard convergent plate junction, and trapping old oceanic crust on the south as the Great Valley Ophiolite. (5) Following end-of-Jurassic development of a new Farallon–North American east-dipping plate junction, terrigenous debris began to accumulate as the seaward Franciscan trench complex and landward Great Valley Group plus Hornbrook forearc clastic rocks. (6) Voluminous deposition and accretion of Franciscan Eastern and Central belt and Great Valley Group detritus occurred during vigorous Sierran igneous activity attending rapid, nearly orthogonal plate subduction starting at ca. 125 Ma. (7) Although minor traces of Grenville-age detrital zircon occur in other sandstones studied in this report, they are absent from post–120 Ma Franciscan strata. (8) Sierra Nevada magmatism ceased by ca. 85 Ma, signaling transition to subhorizontal eastward underflow attending Laramide orogeny farther inland. (9) Exposed Paleogene Franciscan Coastal belt sandstone accreted in a tectonic realm unaffected by HP/LT recrystallization. (10) Judging by petrofacies and zircon U-Pb ages, Franciscan Eastern belt rocks contain clasts derived chiefly from the Sierran and Klamath ranges. Detritus from the Sierra Nevada ± Idaho batholiths is present in some Central belt strata, whereas clasts from the Idaho batholith, Challis volcanics, and Cascade igneous arc appear in progressively younger Paleogene Coastal belt sandstone.

Based on relationships among volcanic-plutonic arc rocks, high-pressure–low-temperature (HP-LT) metamafic rocks, westward relative migration of the Klamath Mountains salient, and locations of the Mariposa-Galice, Great Valley Group, and Franciscan depositional basins, the following geologic evolution is inferred for the northern California continental edge: (1) By ca. 175 Ma, onset of transpressive plate underflow generated an Andean-type Klamath-Sierran arc along the margin. At ca. 165 Ma and continuing to ca. 150–140 Ma, erosion supplied volcanogenic debris to proximal Mariposa-Galice ± Myrtle overlap strata. (2) Oceanic crustal rocks were metamorphosed under HP-LT conditions in an inboard, east-inclined subduction zone from ca. 165 to 150 Ma. Most such mafic rocks remained stored at depth, and HP-LT tectonic blocks only returned surfaceward during the Late Cretaceous, chiefly entrained in circulating, buoyant Franciscan mud-matrix mélange. (3) At end-of-Jurassic time, before onset of paired Franciscan and Great Valley Group + Hornbrook deposition, the Klamath salient was deformed and displaced ∼100–200 km westward relative to the Sierran arc. (4) After this ca. 140 Ma seaward step-out of the Farallon–North American convergent plate junction—stranding preexisting oceanic crust on the south as the Coast Range ophiolite—terrigenous debris began to arrive at the Franciscan trench and intervening Great Valley forearc. Voluminous sedimentation and accretion of Franciscan Eastern + Central belt and Great Valley Group coeval detritus took place during paroxysmal igneous activity and rapid, nearly orthogonal plate convergence at ca. 125–80 Ma. (5) Sierran arc volcanism-plutonism ceased by ca. 80 Ma in northern California, signaling a transition to shallow, nearly subhorizontal eastward plate underflow attending Laramide orogeny far to the east. (6) Paleogene–Lower Miocene Franciscan Coastal belt sedimentary strata were deposited in a tectonic realm nearly unaffected by HP-LT subduction. (7) Grenville-age detrital zircons apparently are absent from the post–120 Ma Franciscan section. Detritus from the Pacific Northwest is not present in the Central belt sandstones, whereas zircons from the Idaho Batholith, the Challis volcanics, and the Cascade Range appear in progressively younger Paleogene–Lower Miocene Coastal belt sediments. This trend suggests the possible gradual NW dextral offset of Franciscan trench deposits of up to ∼1600 km relative to the autochthonous Great Valley Group forearc and basement terranes of the American Southwest.

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.

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 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.

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 growing body of evidence indicates that Middle Jurassic to Early Cretaceous plutons recorded changing sources during tectonic evolution of the Klamath Mountain province. The data set now includes U-Pb zircon ages and zircon trace element compositions determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Thirteen rock samples were dated, and these data refine thermal ionization mass spectrometry (TIMS) data where inheritance was problematic, or provide new U-Pb ages. Individual plutonic suites, previously defined on the basis of crystallization age, isotope and elemental compositions, and petrogenetic style, show characteristic inherited zircon age ranges and zircon trace element patterns. Moreover, ages of inherited zircons in these suites are distinct and, in at least three suites, indicate the presence of cryptic (unexposed) source rocks. The zircon data complement oxygen, Nd, and Sr isotope whole-rock data that, when taken together, suggest a number of major changes in the crustal column with time. Middle Jurassic magmatism began with the oceanic(?) arc-related western Hayfork terrane comprising volcanic, volcaniclastic, and plutonic components. After regional thrusting on the ca. 170-Ma Wilson Point thrust, the Ironside Mountain batholith and Wooley Creek suite of plutons were emplaced. The former shows little evidence of interaction with the crust, but the latter contains Middle Jurassic inheritance and Sr, Nd, and oxygen isotope signatures suggestive of interaction with metasedimentary crustal rocks. Following Nevadan thrusting (ca. 153–150 Ma), emplacement of western Klamath suite plutons in the western Klamath Mountains province involved significant assimilation of Galice Formation metasedimentary rocks. This activity was followed by emplacement of tonalite-trondhjemite-granodiorite (ttg) plutons in the eastern Klamath Mountains province, which were derived by partial melting of metabasic rocks. Their zircon trace element signatures indicate diverse magma histories and, at least locally, multiple magma sources. Inherited zircons in ttg plutons suggest late Middle Jurassic to Late Jurassic sources, younger than the Josephine ophiolite. The youngest magmatism in the Klamath Mountains province consists of broadly granodioritic plutons, which, on the basis of limited data, show variable petrogenesis and zircon inheritance. At least one of these plutons (136-Ma Yellow Butte pluton) contains a ca. 150-Ma inheritance that indicates the presence of Late Jurassic crustal rocks beneath the eastern Klamath terrane.

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.

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.

Marginal basin flysch deposits of the western Jurassic belt of the Klamath Mountains were thrust eastward beneath the western Paleozoic and Triassic belt during the Late Jurassic Nevadan orogeny. Nevadan underthrusting created two generations of nearly coaxial north-trending folds within the western Jurassic belt rocks. These structures formed at chlorite-grade, greenschist-facies conditions and have accompanying pressure solution and mineral recrystallization. The geometry of the Nevadan structures suggests that the thrusting direction was roughly west–east in present coordinates. This direction is perpendicular to the regional strike of the bounding thrust faults and is consistent across 35 km of dip exposure. Post-Nevadan structures include locally developed strike-slip faults and related third- and fourth-generation folds. These features have associated quartz and calcite veins but lack the metamorphic mineral growth associated with the Nevadan structures. Relatively young, high-angle normal faults are very common and appear to be contemporaneous with Neogene uplift of the entire range. Nevadan-age structures within the western Sierra Nevada Foothills terrane also formed in response to west–east thrusting. Global plate-circuit models suggest that the Nevadan Farallon–Pacific relative motion may have been orthogonal to the continental margin at the latitude of the Klamath Mountains. This convergence direction and the kinematic analyses suggest that the Klamath Mountains and Sierra Nevada Foothills were in their same relative orientation during the Nevadan orogeny.

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 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.