Late Jurassic Nevadan Orogeny Research Articles

Middle Jurassic to Early Cretaceous orogenesis in the Klamath Mountains Province (Northern California–southern Oregon, USA) occurred by tectonic switching: Insights from detrital zircon U-Pb geochronology of the Condrey Mountain schist

The Klamath Mountains Province of Northern California and southern Oregon, USA, consists of generally east-dipping terranes assembled via Paleozoic to Mesozoic subduction along the western margin of North America. The Klamath Mountains Province more than doubled in mass from Middle Jurassic to Early Cretaceous time, due to alternating episodes of extension (e.g., rifting and formation of the Josephine ophiolite) and shortening (e.g., Siskiyou and Nevadan events). However, the tectonic mechanisms driving this profound Mesozoic growth of the Klamath Mountains Province are poorly understood. In this paper, we show that formation of the Condrey Mountain schist (CMS) of the central Klamath Mountains Province spanned this critical time period and use the archive contained within the CMS as a key to deciphering the Mesozoic tectonics of the Klamath Mountains Province. Igneous samples from the outer CMS subunit yield U-Pb zircon ages of ca. 175–170 Ma, which reflect volcanic protolith eruptive timing. One detrital sample from the same subunit contains abundant (~54% of zircon grains analyzed) Middle Jurassic ages with Paleozoic and Proterozoic grains comprising the remainder and yields a maximum depositional age (MDA) of ca. 170 Ma. These ages, in the context of lithologic and thermochronologic relations, suggest that outer CMS protoliths accumulated in an outboard rift basin and subsequently underthrust the Klamath Mountains Province during the Late Jurassic Nevadan orogeny. Five samples of the chiefly metasedimentary inner CMS yield MDAs ranging from 160 Ma to 130 Ma, with younger ages corresponding to deeper structural levels. Such inverted age zonation is common in subduction complexes and, considering existing K-Ar ages, suggests that the inner CMS was assembled by progressive underplating over a >10 m.y. timespan. Despite this age zonation, age spectra derived from structurally shallow and deep portions of the inner CMS closely overlap those derived from the oldest section of the Franciscan subduction complex (South Fork Mountain schist). These relations suggest that the inner CMS is a composite of South Fork Mountain schist slices that were sequentially underplated beneath the Klamath Mountains Province. The age, inboard position, and structural position (i.e., the CMS resides directly beneath Jurassic arc assemblages with no intervening mantle) of the CMS suggest that these rocks were emplaced during one or more previously unrecognized episodes of shallow-angle subduction restricted to the Klamath Mountains Province. Furthermore, emplacement of the deepest portions of the CMS corresponds with the ca. 136 Ma termination of magmatism in the Klamath Mountains Province, which we relate to the disruption of asthenospheric flow during slab shallowing. The timing of shallow-angle subduction shortly precedes that of the westward translation of the Klamath Mountains Province relative to correlative rocks in the northern Sierra Nevada Range, which suggests that subduction dynamics were responsible for relocating the Klamath Mountains Province from the arc to the forearc. In aggregate, the above relations require at least three distinct phases of extension and/or rifting, each followed by an episode of shallow-angle underthrusting. The dynamic upper-plate deformation envisioned here is best interpreted in the context of tectonic switching, whereby slab steepening and trench retreat alternate with slab shallowing due to recurrent subduction of buoyant oceanic features.

Radioisotopic and biostratigraphic age relations in the Coast Range Ophiolite, northern California: Implications for the tectonic evolution of the Western Cordillera

The Coast Range ophiolite (CRO) in northern California includes two distinct remnants. The Elder Creek ophiolite is a classic suprasubduction zone ophiolite with three sequential plutonic suites (layered gabbro, wehrlite-pyroxenite, quartz diorite), a mafi c to felsic dike complex, and mafi c-felsic volcanic rocks; the entire suite is cut by late mid-oceanic-ridge basalt (MORB) dikes and overlain by ophiolitic breccia. The Stonyford volcanic complex (SFVC) comprises three volcanic series with intercalated chert horizons that form a submarine volcano enclosed in sheared serpentinite. Structurally below this seamount are melange blocks of CRO similar to Elder Creek. U/Pb zircon ages from plagiogranite and quartz diorites at Elder Creek range in age from 165 Ma to 172 Ma. U/Pb zircon ages obtained from CRO melange blocks below the SFVC are similar (166‐172 Ma). 40 Ar- 39 Ar ages of alkali basalt glass in the upper SFVC are all younger at ≈164 Ma. Radiolarians extracted from chert lenses intercalated with basalt in the SFVC indicate that the sedimentary strata range in age from Bathonian (Unitary Association Zone 6‐6 of Baumgartner et al., 1995a) near the base of the complex to late Callovian to early Kimmeridgian (Unitary Association Zones 8‐10) in the upper part. The SFVC sedimentary record preserves evidence of a major faunal change wherein relatively small-sized, polytaxic radiolarian faunas were replaced by very robust, oligotaxic, nassellarian-dominated faunas that included Praeparvicingula spp. We suggest that CRO formation began after the early Middle Jurassic (172‐180 Ma) collision of an exotic or fringing arc with North America and initiation of a new or reconfi gured east-dipping subduction zone. The data show that the CRO formed prior to the Late Jurassic Nevadan orogeny, probably by rapid forearc extension above a nascent subduction zone. We infer that CRO spreading ended with the collision of an oceanic spreading center ca. 164 Ma, coincident with the oldest high-grade blocks in the structurally underlying Franciscan assemblage. We further suggest that the “classic” Nevadan orogeny represents a response to spreading center collision, with shallow subduction of young lithosphere causing the initial compressional deformation and with a subsequent change in North American plate motion to rapid northward drift (J2 cusp) causing sinistral transpression and transtension in the Sierra foothills. These data are not consistent with models for Late Jurassic arc collision in the Sierra foothills or a back-arc origin for the CRO.

Ophiolitic basement to a forearc basin and implications for continental growth: The Coast Range/Great Valley ophiolite, California

We present a compilation of 18 published models from the length of the Great Valley forearc basin, California, based on seismic reflection, borehole, seismic refraction, gravity, and aeromagnetic data to address long‐standing questions about the nature of the basement to the Great Valley, its origin, its tectonic history, and it's mechanism of incorporation into the North American continental margin. The geophysical models permit a 700‐km‐long, 70‐km‐wide, complete ophiolite sequence beneath the entire Great Valley. In the northern Great Valley, the ophiolite is overlain by ophiolitic breccia, the ophiolite crust is 7–8 km thick, and the mantle section is mostly unserpentinized. Beneath the southern Great Valley, there is no ophiolitic breccia, the crust may be up to 10 – 12 km thick, and the mantle section, if present at all, is serpentinized to such a degree that it cannot be distinguished from Sierran basement or mafic ophiolite members on the basis of velocity or density data. Geochemical, petrological, and paleomagnetic data support suprasubduction zone ophiolite formation at North American paleolatitudes, and geological data and geophysical models are consistent with ophiolite formation by back arc spreading behind an east facing arc. In the north, this was apparently followed by obduction of back arc crust onto older continental basement during the Late Jurassic Nevadan orogeny. In the south, the newly formed intraoceanic arc and back arc apparently collided with the continental margin during the Nevadan orogeny but were not obducted onto it. Instead, the arc and back arc “docked” with the continental margin leaving the arc itself to become the basement to the Great Valley basin. Cretaceous Sierran magmatism then intruded plutons beneath the docked ophiolite and mafic arc. Irrespective of the detailed accretionary history, our cross sections show a rapid pulse of continental growth by ophiolite accretion of more than 500 km³ km−1 in less than 10 Myr.

Phanerozoic Tectonic Evolution of Central California and Environs

The use of actualistic analog models for paleotectonic reconstruction produces significant advances in our understanding of evolutionary continental tectonics. The sequential application of such models is possible in a cross section from the central California coast to Utah. This transect represents one of the best-understood Phanerozoic continental margins on Earth. Excellent exposure, detailed local studies, and regional syntheses all contribute to the choice of appropriate plate-tectonic models for successive intervals from the latest Neoproterozoic to the Quaternary. These models include craton, terrestrial rift, nascent ocean, intraplate continental margin, intraoceanic magmatic arc intraplate margin, Devonian-Mississippian Antler orogeny (arc-continent suturing), Mississippian-Pennsylvanian intraplate margin, Pennsylvanian-Permian Ancestral Rockies orogeny (Ouachita-Marathon continental suturing), Permian-Triassic Sonoma orogeny (arc-continent suturing), Triassic-Jurassic continental-margin magmatic arc, Late Jurassic Nevadan orogeny (arc-arc suturing), latest Jurassic-Late Cretaceous continental-margin magmatic arc, latest Cretaceous-Eocene Laramide orogeny, Oligocene ignimbrite flare-up, and Miocene-Holocene triple-junction migration, transform boundary, and regional extension of the Great Basin. The integrated result of sequential superposition of the actualistic models produces a reasonable representation of the complexity of the study area. The discipline of applying actualistic models to the evolution of this continental margin provides new insights and forces one to consider new implications of the models. The complexity of the tectonic history of this continental margin argues against simplistic general models for the growth of continental crust.

U‐Pb geochronology of detrital zircon from Upper Jurassic synorogenic turbidites, Galice Formation, and related rocks, western Klamath Mountains: Correlation and Klamath Mountains provenance

Synorogenic turbidites of the Upper Jurassic Galice Formation overlie a variety of basement terranes within the western Klamath Mountains along the Oregon‐California border, including the early Late Jurassic ophiolite assemblages, ensimatic arc deposits, and sedimentary terranes. U‐Pb analyses of 68 multiple grain fractions from 11 samples of detrital zircon support the correlation of Galice Formation on these various basement terranes, although some new complexities in provenance are revealed. With one exception, upper intercept ages range from 1509−3+3 to 1675−8+8 Ma. Least squares regression of all fractions yields an upper intercept age of 1583±1 Ma, indicating the importance of an ultimately continental, recycled, and generally well‐mixed sedimentary source. Early Mesozoic lower intercept ages range between 183−2+2 and 263−3+4 and average 215±1 Ma. Results from Galice cover on sedimentary basement show significantly older 2.1 Ga Precambrian component, however, that may be locally derived from pre‐Late Jurassic basement rocks that are rich in recycled sedimentary debris. Existing isotopic data from older, zircon‐bearing Klamath units further indicate that Galice detritus was derived from immediate source terranes within the Klamath Mountains. Reworking of fragile limestone clasts from the biogeographically distinctive eastern Klamath terrane (McCloud Limestone) into Galice Formation substrate also supports early paleogeographic ties between terranes. Thus the tectonic setting of the Late Jurassic Nevadan orogeny in the Klamath Mountains is tightly constrained by original paleogeographic ties between subterranes of the western belt and by provenance ties to terranes to the east. Ultimately continent‐derived clastic debris and other distinctive tracers were recycled within this long‐lived ensimatic convergent margin system.

Crustal structure and composition of the southern Foothills Metamorphic Belt, Sierra Nevada, California, from seismic data

The Foothills Metamorphic Belt is an accreted terrane consisting of Paleozoic and Mesozoic metamorphic rocks that separates the Great Valley from the Sierra Nevada batholith in northern and central California. Until recently, the only available geophysical data for this area were reconnaissance refraction surveys, and gravity and magnetic data. New insights into the structure of the deep crust are provided by the interpretation of a seismic reflection profile (CC‐2), acquired in 1984 by the U.S. Geological Survey at the southern end of the Foothills Metamorphic Belt. Our interpretation is constrained by a new seismic velocity model derived from coincident microearthquake data. Earthquake hypocenters that occur at unusually great depths of 12 to 30 km make the data set particularly useful for obtaining deep crustal velocity information. The velocity model shows velocities of 5.2 to 6.3 km s−1 for the upper 12 km of the crust, and 6.7 to 6.8 km s−1 from 12 km to an estimated Moho at 32 km. The upper crustal velocities correspond to metamorphic rocks and serpentinites of the Foothills Metamorphic Belt as well as to diorites and granodiorites of the Sierra Nevada batholith, while the lower crustal velocities are interpreted to represent intermediate to mafic granulites. The majority of the earthquake hypocenters as well as a 6.7 km s−1 layer in the velocity model corresponds in depth to thick zones of west dipping midcrustal reflections that may represent major shear zones formed during the late Jurassic Nevadan orogeny or synbatholithic ductile shear zones that accommodated crustal extension associated with batholith intrusion. These reflections are truncated updip by an inferred subvertical contact that coincides with the western edge of the Sierra Nevada batholith and the southward trace of the Bear Mountains fault zone. The updip truncation of midcrustal shear zones and high lower crustal velocities indicate that strike‐slip faulting and magmatic underplating can be important processes during the docking and welding of an accreted terrane.

Jurassic Cordilleran dike swarm-shear zones: Implications for the Nevadan orogeny and North American plate motion

A cogenetic and coeval tonalitic and mafic dike swarm has been identified within a southern fragment (the Owens Mountain area) of the western Foothills terrane (California). The dikes were mylonitized and transposed (rotated into subparallel orientation) during emplacement, from 155 to 148 Ma (U-Pb zircon data), which coincides in time with the Nevadan orogeny. Steeply southeast-plunging fold axes and S-fold geometries indicate a sinistral-sense of shear, possibly with some dip-slip motion as well. This shear zone may be the southern and possibly deeper extension of the Bear Mountains fault zone. This and other Late Jurassic Cordilleran dike swarms record a complex pattern of sinistral-sense transtension-transpression that developed at the apparent-polar-wander J2 cusp (∼150 Ma) and during subsequent, rapid, northwestward acceleration of North America. The Late Jurassic Nevadan orogeny is a manifestation of these dramatic changes in magnitude and direction of North American motion.

Isotopic systematics of Pb/U (zircon) and 40Ar/39Ar (biotite-hornblende) from rocks of the central Foothills terrane, Sierra Nevada, California

Pb/U zircon and ^(40)Ar/^(39)Ar biotite-horn-blende studies in conjunction with detailed structural and textural studies clarify the age and structural relations of a unique cluster of plutons in the central Sierra Nevada western metamorphic belt. The plutons range from gabbro to tonalite/trondhjemite and include the Guadalupe Igneous Complex. Magmatic emplacement ages for the plutons are for the most part between ∼150 and ∼145 m.y., as recorded in both the zircon and hornblende isotopic systems. Additional ages of 138 and 123 m.y. were determined on small plutons having key structural settings. In general, the Pb/U and ^(40)Ar/^(39)Ar isotopic systems are well behaved. Second-order complications are locally present in the Pb/U systems, owing to the entrainment of older zircon from wall rocks, and in ^(40)Ar/^(39)Ar systems, owing to minor compositional differences in mica and amphibole phases. The age data and structural relations of the plutons in conjunction with ^(40)Ar/^(39)Ar metamorphic wall-rock biotite and hornblende age data constrain regional ductile deformation in the study area to have occurred between ∼150 and ∼125 Ma. Such deformation was accompanied by upper greenschist- to lower amphibolite-facies metamorphism and is at least in part closely related to pluton emplacement. Such deformation and metamorphism has in the past been attributed to the Late Jurassic Nevadan orogeny. This study shows clearly, however, that ductile deformation and metamorphism in the study area are post-Nevadan if modern concepts of the Nevadan orogeny are strictly adopted.

Palaeoceanography and tectonic setting of the Jurassic Coast Range ophiolite, central California: evidence from the extrusive rocks and the volcaniclastic sediment cover

The primary thesis of this paper is that the character of sedimentary rocks within or directly on top of an ophiolite complex can be used to constrain tectonic models of oceanic crustal genesis. Late Jurassic ( c. 169-161 Ma) Coast Range ophiolite remnants are exposed E and W of the San Andreas Fault in central California. Ophiolite volcanic rocks are mainly sub-alkaline tholeiitic basalts, but include subordinate intermediate and silicic flows and breccias. Available ‘immobile’ major and trace element data indicate that the Coast Range ophiolite formed above a subduction zone, possibly in a fore-arc setting, as suggested by the presence of high magnesian andesites and boninites. The ophiolite extrusives mainly comprise pillow lava and lava breccia formed on a rugged fault-controlled ocean floor. Interlava pelagic sediment is restricted to very small amounts of interstitial red chert and nannoplankton limestone. Widespread occurrence of vesicular lavas probably reflects high primary volatile contents rather than eruption in shallow water. Local interbeds of arc-derived silicic tephra in the uppermost extrusives suggest that the ophiolite and cover sediments formed together in a single tectonic setting. Depositionally overlying deep-water successions up to 600 m thick comprise radiolarian-rich tuffaceous and volcaniclastic sediments of ?Callovian-mid-Tithonian age (165-148 Ma). Depositional processes included airfall, turbidity currents, mass flow and current reworking. Small SiFeMn lenses along the lava-sediment contact formed by hydrothermal replacement of basal siliceous tuffaceous sediments or by direct chemical precipitation. At one locality (Llanada), the ophiolite is overlain by up to 1.8 km of younger (?Tithonian) basic-intermed, and acid lava flows, submarine lahars and mainly silicic pyroclastic sediments, of inferred volcanic arc origin. The volcaniclastic cover passes conformably upwards into late Tithonian and younger terrigenous deep-water clastic sediments of the Great Valley Group, deposited in a fore-arc setting. The Coast Range ophiolite and its cover are interpreted as an incipient intraoceanic volcanic arc, possibly developed above an eastward (continent-ward)-dipping subduction zone within the equatorial Pacific Ocean. In this scenario, short-lived (5–8 Ma) supra-subduction zone spreading to form the ophiolite was followed by ?Callovian-Tithonian arc volcanism, shedding tuffaceous debris onto the adjacent fore-arc. Subduction was active separately along the western American continental margin with the result that the Coast Range arc-ophiolite complex eventually collided with North America, triggering the Late Jurassic Nevadan orogeny and initiating Franciscan subduction-accretion. The Coast Range ophiolite remnants, at least those west of the San Andreas Fault, were apparently translated northwards to their present positions after Late Cretaceous.

An expanded view of Jurassic orogenesis in the western United States Cordillera: Middle Jurassic (pre-Nevadan) regional metamorphism and thrust faulting within an active arc environment, Klamath Mountains, California

Basaltic to basaltic andesitic volcaniclastic rocks and their contemporaneous mafic-ultramafic intrusive complexes delineate a Middle Jurassic arc terrane within the Klamath Mountain province of northern California. Exposures of the supracrustal volcaniclastic rocks are restricted to a single fault-bounded terrane, but the deeper level intrusive complexes were emplaced into most, if not all, the pre-Late Jurassic terranes of the Klamath Mountain region. The pre-Late Jurassic terranes thus constitute the basement of the Middle Jurassic arc. U-Pb zircon analyses of 39 zircon fractions from 12 intrusive complexes plus K-Ar dating of the volcaniclastic strata demonstrate magmatic activity over the interval of ∼177-159 Ma. The active arc and its basement were imbricated by a compressive deformational event, the signature of which included thrust faulting, isoclinal folding, and regional metamorphism. Several diverse lines of evidence, including K-Ar dating of metamorphic rocks, crosscutting relations of dated intrusive complexes to thrust faults, and U-Pb dating of synmetamorphic intrusive complexes, establish a distinctly pre-Nevadan Middle Jurassic age (ongoing at ∼169 Ma and over by at least 161 Ma) for this compressive deformational episode. Outboard and structurally beneath the Middle Jurassic arc and its basement are several terranes that collectively comprise the western Jurassic belt. These terranes were deformed and regionally metamorphosed during the Late Jurassic Nevadan orogeny, which occurred within the time interval of ∼157-150 Ma, as Upper Jurassic plutons with 150- to 142-m.y.-old zircon ages have contact aureoles that overprint the Nevadan fabric, and the ∼157-m.y.-old Rogue Formation was deformed in the Nevadan event. The Middle and Late Jurassic compressive deformational events were thus distinct and separated by as much as 15-20 m.y. The relation between Middle and Late Jurassic magmatism and deformation suggests that the Klamath Mountain province records the evolution of a considerably long-lived arc system that evolved above an east-dipping subduction zone. In addition, we suggest that this are system may represent an oceanic continuation of the long-recognized early Mesozoic arc terrane of the western U.S. Cordillera.

The Devils Elbow ophiolite remnant and overlying Galice Formation: New constraints on the Middle to Late Jurassic evolution of the Klamath Mountains, California

The Josephine ophiolite and its sedimentary cover (Galice Formation) of the western Jurassic belt, Klamath Mountains, northern California, are the expression of a Late Jurassic rift basin which developed between an active Late Jurassic arc to the west and a remnant Middle Jurassic are to the east. The Devils Elbow ophiolite remnant (DEO) and its sedimentary and volcanic cover represent the southern continuation of the Josephine ophiolite and overlying Galice Formation. The DEO consists of a dike complex, volcanic sequence, and ophiolitic breccia. Overlying strata consist of terrigenously derived turbidites and andesitic to dacitic volcaniclastic rocks. The following relations suggest formation of the DEO in a marginal basin setting, in accordance with the setting inferred for the Josephine ophiolite basin: (1) dikes and lavas are characterized by an abundance of clinopyroxene phenocrysts, by crystallization of clinopyroxene before plagioclase, and by trace-element geochemistry which is transitional between IAT and MORB; (2) overlying sedimentary rocks appear to have been derived from erosion of older Klamath Mountain terranes and were deposited into the DEO basin shortly after and/or during its formation; (3) the texture and composition of the volcanic deposits interlayered with sedimentary strata indicate the presence of a nearby active volcanic arc source. The period of rifting which culminated in the formation of the Josephine ophiolite/DEO basin began by at least 164 Ma, the age of the DEO, but is unlikely to have begun much earlier, as the region was experiencing a major period of compression just prior to this time. Rifting began at least 5 m.y. prior to cessation of magmatism in the rifted Middle Jurassic arc. The overlap in rifting and arc magmatism can be explained if spreading in the Josephine ophiolite/DEO basin was asymmetric relative to the axis of Middle Jurassic are magmatism and/or if transform motion approximately parallel to the Middle Jurassic are was important in the spreading geometry of the basin. The western boundary fault of the western Jurassic belt, and Klamath Mountain province, is the South Fork fault (previously referred to as part of the Coast Range thrust). In the southern western Jurassic belt, a serpentinite matrix meelange (SMM) occurs along this fault. Blocks in the SMM consist largely of greenstones with the trace-element geochemistry of MORB. Other block types include gabbroic to dioritic intrusive rocks, diabase, harzburgite, amphibolite, chert, sandstone, and shale. Generally similar serpentinite matrix melanges have been described from other locations along the South Fork fault. This spatial association suggests that the South Fork fault provided a pathway for the bouyant rise of serpentinite or serpentinite matrix melange from a source at depth. Obvious sources for the SMM include parts of the Franciscan complex, the Josephine ophiolite/DEO, or possibly the Coast Range ophiolite. The geochemistry and relative abundances of different lithologies in the SMM, however, indicate that none of these could represent the source. In contrast, the SMM is nearly identical in block content and block geochemistry to serpentinite matrix melange of the Rattlesnake Creek terrane which structurally overlies the western Jurassic belt to the east. This suggests that a previously unrecognized part of the Rattlesnake Creek terrane may underlie the Josephine ophiolite/DEO at depth. As the Rattlesnake Creek terrane formed the western part of the basement for the rifted Middle Jurassic arc, the part of the Rattlesnake Creek terrane which underlies the Josephine ophiolite/DEO may represent a fragment of crust which was rifted oceanward with the opening of the Josephine ophiolite/DEO basin and then subsequently thrust beneath the basin during the Late Jurassic Nevadan Orogeny.

Extent of the Nevadan orogeny, central Sierra Nevada, California

The Late Jurassic Nevadan orogeny has long been considered responsible for much of the deformation that occurred in the central Sierra Nevada. On the basis of our recent work and that of other workers, we suggest that the intensity of Nevadan deformation in the central Sierra Nevada decreases eastward within the foothills belt, is restricted to domainal deformation in rocks between the Melones fault zone and the batholith, and is virtually absent in the continental volcanic arc along the east side of the range. These data and interpretations suggest that in the central Sierra Nevada, Nevadan deformation does not represent as intense and widespread an orogeny as originally supposed.

Paleomagnetism of the Upper Jurassic Galice Formation, southwestern Oregon: Evidence for differential rotation of the eastern and western Klamath Mountains

Paleomagnetic studies of low-grade metatuff within the Upper Jurassic Galice Formation at the type section along the Rogue River, Oregon, indicate stable magnetizations at 13 sites with a mean orientation of D = 245.6°, I = 55.9°, and α 95 of 5.2°. Site mean directions are well grouped in situ despite the variable orientation of both bedding and cleavage. The magnetizations are therefore secondary and postdate Late Jurassic folding. The most likely times for remagnetization are during low-grade metamorphism of the Late Jurassic Nevadan orogeny (150 Ma) or during intrusion of the nearby Grants Pass pluton (139 Ma). The paleomagnetic pole position calculated from the formation mean direction lies at 40°N, 31°E, far from North American cratonic pole positions aged 150 Ma and younger. Clockwise rotation of the study area by 99°±10° and tilting or poleward displacement of 0°±7° are indicated by comparison with a reference pole aged 150 Ma. However, the amount of rotation is not significantly different for other reference poles aged 140 to 80 Ma. Thus, the data indicate that the Galice stratotype and vicinity have rotated approximately 100° since Late Jurassic time, 20° to 40° more than the eastern Klamath Mountains. An unrecognized fault may be required to accommodate the difference in rotation. In addition, restoration of the rotation places Nevadan structural trends in the western Klamaths at a high angle to Nevadan trends in the nearby Sierra Nevada and Blue Mountains of Oregon.

Mesoscopic structural fabric of early Paleozoic rocks in the northern Sierra Nevada, California and implications for Late Jurassic plate kinematics

Analysis of the mesoscopic structure of the early Paleozoic Shoo Fly complex, northern Sierra Nevada, California, reveals three phases of deformation and folding. The first phase of folding is pre-Late Devonian and the second two are constrained by regional relations as due to the Late Jurassic Nevadan orogeny. Main phase Nevadan deformation produced penetrative slaty cleavage which is steep, NNW-trending and parallel to tectonostratigraphic terranes of the region. Cleavage is axial-planar to ubiquitous isoclinal similar folds. Fold axes define a NNW-trending girdle with a distinct, near-vertical maximum. Main phase Nevadan folds have nearly ideal class 2 orthogonal thickness geometry although some class 1C forms exist in more competent units. The overall geometry of main phase folds suggests formation by progressive deformation in a flattening regime with cleavage as the flattening plane and a steep extension axis defined by the fold axis maximum. A steep extension axis direction for main phase Nevadan deformation is supported by analysis of interference relations where folds of this generation deform pre-Late Devonian folds. Late Nevadan folds range from kink flexures to ideal class 2 similar folds with incipient axial-planar cleavage. The kinematic significance of late Nevadan folds cannot be evaluated because of their varying style and orientation throughout the northern Sierra Nevada. Penetrative ductile deformation and near-vertical extension during the Nevadan orogeny was synchronous with accretion of oceanic and/or island arc rocks against the western margin of the northern Sierra Nevada. The kinematic framework of deformation defined for Nevadan deformation is consistent with essentially orthogonal convergence of these exotic terranes with the Sierran margin and argues against a transform/transpressive regime.

Middle to Late Jurassic tectonic evolution of the Klamath Mountains, California‐Oregon

The geochronology, stratigraphy, and spatial relationships of Middle and Late Jurassic terranes of the Klamath Mountains strongly suggest that they were formed in a single west‐facing magmatic arc built upon older accreted terranes. A Middle Jurassic arc complex is represented by the volcanic rocks of the western Hayfork terrane and consanguineous dioritic to peridotitic plutons. New U/Pb zircon dates indicate that the Middle Jurassic plutonic belt was active from 159 to 174 Ma and is much more extensive than previously thought. This plutonic belt became inactive just as the 157 Ma Josephine ophiolite, which lies west and structurally below the Middle Jurassic arc, was generated. Late Jurassic volcanic and plutonic arc rocks (Rogue Formation and Chetco intrusive complex) lie outboard and structurally beneath the Josephine ophiolite; U/Pb and K/Ar age data indicate that this arc complex is coeval with the Josephine ophiolite. Both the Late Jurassic arc complex and the Josephine ophiolite are overlain by the “Galice Formation,” a Late Jurassic flysch sequence, and are intruded by 150 Ma dikes and sills. The following tectonic model is presented that accounts for the age and distribution of these terranes: a Middle Jurassic arc built on older accreted terranes undergoes rifting at 160 Ma, resulting in formation of a remnant arc/back‐arc basin/island arc triad. This system collapsed during the Late Jurassic Nevadan Orogeny (150 Ma) and was strongly deformed and stacked into a series of east‐dipping thrust sheets. Arc magmatism was active both before and after the Nevadan Orogeny, but virtually ceased at 140 Ma.

Origin and metamorphism of peperite and associated rocks in the Devonian Elwell Formation, northern Sierra Nevada, California

The Late Devonian Elwell Formation is part of a late Paleozoic island-arc assemblage exposed in the northern Sierra Nevada, California. In the 10-km interval from Mount Elwell (Plumas County) to Packer Lake (Sierra County), it consists largely of numerous andesitic sills interleaved with phosphatic radiolarian chert and turbidity-current and debris-flow deposits. Peperite, formed by interaction of intrusive andesitic magma and still-moist, incompletely lithified “chert” not far beneath the sea floor, occurs sparingly. The whole assemblage was thoroughly recrystallized (greenschist-facies regional metamorphism) during the Late Jurassic Nevadan orogeny. The phosphatic chert is composed of white, fluorapatite-rich lenses and nodules enclosed in a black matrix consisting of microgranular quartz and carbonaceous material (probably graphite). Radiolaria (family Entactiniidae) are so abundant that the chert is thought to have originated as radiolarian ooze. The phosphatic lenses and nodules probably formed by direct inorganic precipitation of fluorapatite from pore water within decaying radiolarians at the sediment-water interface. The andesitic sills are generally aphyric and aphanitic; original textures varied from hyalopilitic in sill interiors to pilotaxitic at sill margins. The andesite was charged with plagioclase (now albite) microlites, and quench morphologies occur where sills intruded the wettest “chert.” Relatively thick sills commonly display columnar structure, whereas a few originally more viscous sills are flow laminated. The top of a sill emplaced essentially at the sediment-water interface is pillowed. Many sills bear xenoliths of phosphatic chert, and scarce quartz megacrysts (evidently derived from the underlying, dacitic Sierra Buttes Formation) appear nearly everywhere. Concentrations of major-element oxides and immobile trace elements indicate that the andesite originated as part of the tholeiitic rock association of island arcs; rare-earth–abundance patterns are particularly diagnostic of this paleotectonic setting. Most peperite has poorly sorted (coarse ash to coarse block) andesitic fragments uniformly distributed through a voluminous black chert matrix. Where invaded “chert” retained the most pore water, quenched andesitic magma formed irregularly shaped lapilli and small pillows that initially exhibited ductile behavior. Rapid cooling below the softening point of glass resulted in brittle fracture of the lapilli during dispersal through the “chert” by steam explosions. Steam explosions also incorporated into the peperite rigid clasts of locally dewatered chert, and shattered already-formed phosphatic nodules. Where the invaded “chert” was better lithified, freezing sill margins simply granulated quietly in situ by thermal-contraction cracking of microlitic glass; fluid “chert” injected widely spaced contraction joints deep within the sills. In all cases, there was sufficient heated connate water to cause palagonitization and chloritization of andesitic fragments. Greenschist-facies metamorphism produced two mineral assemblages in the interiors of andesitic sills, the second perhaps reflecting locally higher Pco 2 /Ph 2 o during metamorphism: (1) albite + quartz + epidote + actinolite + chlorite + Ti-rich mineral ± calcite ± stilpnomelane ± white mica, and (2) albite + quartz + chlorite + Ti-rich mineral + calcite ± stilpnomelane ± white mica. Epidote-rich specks occurring sporadically in the sills represent metasomatism on a scale of millimetres to centimetres; such specks are mineralogically zoned in andesitic fragments in intensely recrystallized peperite. Additional examples of local meta-somatic activity include syntectonic crystallization of calcite rhombohedra across original vesicle walls in a sill. Cleavage developed locally during regional metamorphism is parallel to axial planes of closely appressed folds that deformed andesitic fragments in peperite and flow laminae in sills.

Bear Mountain Igneous Complex, Klamath Mountains, California: an Ultrabasic to Silicic Cale-Alkaline Suite

The Bear Mountain igneous complex, Klamath Mountains, California, can be divided into distinct lithologic suites (order according to apparent relative age): (1) satellitic masses of clinopyroxene-rich ultramafic and gabbroic rocks with subordinate dunite and hornblende-plagioclase pegmatoid; (2) two-pyroxene-biotite diorite and monzodiorite; (3) heterogeneous hornblende-rich rocks varying from gabbro to diorite; (4) leucocratic rocks, chiefly consisting of biotite tonalite and granodiorite; and (5) late dikes (mafic to felsic). Elongate masses of unit (1) flank a composite pluton consisting of units (2–4), while the late dikes (unit 5) intrude the adjacent country rocks. The rocks of the complex invaded an ophiolite allochthon during the Late Jurassic Nevadan orogeny, and well-defined contact aureoles surround the complex. Lower greenschist facies rocks, chiefly metabasalt, impure siliceous metasedimentary rocks, and serpentinized peridotite, have been dynamothermally metamorphosed to mineral assemblages indicative of hornblende-hornfels facies and locally pyroxene-hornfels facies. The emplacement of the igneous complex was chiefly by forcible shouldering aside, although local tectonic features such as faults in the ophiolite allochthon were instrumental in the emplacement history. The ultramafic and gabbroic rocks are interpreted as crystal cumulates of a fractionated basaltic magma. Mineral compositions and whole-rock chemical characteristics of the proposed cumulates suggest that the Mg/Fe ratio of the parental basaltic liquid was high. The activity of silica was low, while water vapor pressure apparently increased through time until it was moderately high during the late magmatic stage. These cumulates were subsequently remobilized during lateral tectonic compression and emplaced higher in the crust as hot, semisolid aggregates. A diverse array of data, including pyroxene compositions, major-, minor-, and rare-earth-element abundances and field relations, suggest that the two-pyroxene-biotite diorite/monzodiorite unit was consanguineous with the clinopyroxene-rich ultramafic and gabbroic rocks. The diorite/monzodiorite unit, therefore, is an intermediate differentiate of an early primitive basalt. Furthermore, major-, trace, and rare-earth-element data characteristic of the diorite/monzodionte unit indicate strong similarities to low-Si andesite and clearly suggest a calc-alkaline affinity. Age relations indicate that the hornblende-rich and leucocratic units are younger and represent the intrusion of other magmas into the same igneous locus. Petrographic and geochemical data from the hornblende-rich unit suggest recrystallization from hydrous magmas similar in composition to high-Al basalt and basaltic andesite. The leucocratic suite, consisting chiefly of calc-alkaline tonalitic rocks, is similar to other quartz-rich felsic rocks widespread throughout the Klamath Mountains-western Sierra Nevada. The available petrographic and geochemical data are consistent with formation of these rocks by either fractional crystallization of a wet basaltic magma or partial melting of amphibolite or eclogite. The Bear Mountain igneous complex is an example of a diverse but distinctive association of ultrabasic to silicic rocks which characterize numerous plutonic complexes in the Klamath Mountains-western Sierra Nevada. These intrusive complexes invade older ensimatic rocks and appear to define the roots of a complex, Middle to Late Jurassic calc-alkaline magmatic arc. The ultramafic and gabbroic rocks characteristic of this plutonic association are similar to Alaskan-type complexes but differ in detail. More significantly, these rocks are important clues to the composition of early magmas as well as the complex processes operative in reservoirs that form the core of calc-alkaline magmatic centers.

Lawsonite blueschist in the northern Sierra Nevada, California

Lawsonite-bearing blueschist metamorphic rocks along the North Fork Yuba River in the northern Sierra Nevada consist of three lithologic types. Most abundant is quartz-muscovite phyllite that possibly originated as part of a chert and shale sequence. These rocks rarely contain blueschist minerals. Metavolcanic rocks including massive lava and volcanic breccia are second in abundance and are typified by fracture-filling growths of crossite. Rare metaclastic rocks contain abundant lawsonite and quartz and minor glaucophane. The rocks were deformed inhomogeneously in three principal phases. Blueschist metamorphic recrystallization accompanied the earliest deformation. The later structural events, possibly related to the Late Jurassic Nevadan orogeny, deformed the blueschist minerals. Despite uncertainty regarding time(s) of Ar loss, K-Ar dates indicate that the high-pressure metamorphism occurred before 174 m.y. B.P. and possibly before 190 m.y. B.P. (Bajocian time or earlier), during Andean-type magmatic arc activity in eastern California and western Nevada. The blueschist rocks provide the first direct evidence for pre-Franciscan subduction in the Sierra Nevada region.

Early Mesozoic tectonic evolution of the western Sierra Nevada, California

Prebatholithic rocks of Mesozoic age in the Sierra Nevada can be interpreted as remnants of ancient volcanic arcs, subduction complexes, and sequences of oceanic lithosphere. Two partly coeval subparallel volcanic arcs, one in the western foothills and the other in the northern and eastern Sierra Nevada, are juxtaposed. The western arc was an east-facing island-arc complex that evolved through a series of steps including formation of a remnant arc and interarc basin. The eastern arc was a west-facing marginal arc that was constructed on the edge of North America. Both arc-subduction complexes consumed intervening oceanic lithosphere and collided during the Late Jurassic Nevadan orogeny. Generation of magmas in both arcs apparently ceased at about this time, and renewed subduction was initiated west of the island arc in latest Jurassic time, giving birth to the Franciscan-Sierran arc-trench complex. Fault zones and melanges in the western Sierra Nevada reflect the complex suturing at the collision boundary. Pre-Tithonian ophiolite at the base of the Great Valley sequence in the Coast Ranges originated in a back-arc or marginal basin setting with respect to the coeval Sierran foothills arc.

Zones of Alpine Tectonism and Metamorphism Klamath Mountains, Southwestern Oregon

Mesozoic, eugeosynclinal, metasedimentary, and metavolcanic strata are intruded by elongate and arcuate northeast-trending alpine ultramafic belts in the Klamath Mountains of southwestern Oregon. The ultramafic belts conform to the regional patterns of alternating antiforms, synforms, and conformable high-angle faults. Throughout the region, the belts of alpine ultramafic rocks occupy axes of maximum meta-morphic intensity. The highly deformed, higher-grade metavolcanic-metasedimentary strata and centralized ultramafic-mafic intrusions form distinctive associations which define zones of tectonism. In several zones (herein termed "zones of alpine tectonism") where the intensity of tectonism varies from brittle to plastic, the grade of metamorphism varies from chlorite zone to staurolite zone. Patterns of mesoscopic structures in all zones of alpine tectonism are consistent with deformation during Late Jurassic Nevadan orogeny and suggest a chronological sequence. Isoclinal folding of ultramafic-mafic sheets and intense plastic deformation of adjacent eugeosynclinal strata are recognized in the cores of certain tectonic zones and are considered representative of the first phase of Nevadan orogeny. The ultramafic-mafic sheets were folded by diapiric intrusion and served as loci for continued movement during a later stage of Nevadan orogeny. Later movement was responsible for the occurrence of the more widespread, cataclasized chlorite-zone assemblages which occur within and surrounding the zones of tectonism.