Gregory D. Harper

Formation and emplacement of the Josephine ophiolite and the Nevadan orogeny in the Klamath Mountains, California-Oregon : U/Pb zircon and 40Ar/39Ar geochronology

Cordilleran ophiolites typically occur as basement for accreted terranes. In the Klamath Mountains, ophiolitic terranes were progressively accreted by underthrusting beneath North America. The Josephine ophiolite is the youngest of the Klamath ophiolites and forms the basement for a thick Late Jurassic flysch sequence (Galice Formation). This ophiolite-flysch terrane forms an east dipping thrust sheet sandwiched between older rocks of the Klamath Mountains above and a coeval plutonic-volcanic arc complex below. The outcrop pattern of the roof (Orleans) thrust indicates a minimum displacement of 40 km, and geophysical studies suggest >110 km of displacement. The basal (Madstone Cabin) thrust is associated with an amphibolitic sole and has a minimum displacement of 12 km. A rapid sequence of events, from ophiolite generation to thrust emplacement, has been determined using ^(40)Ar/^(39)Ar and Pb/U geochronology. Ophiolite generation occurred at 162–164 Ma, a thin hemipelagic sequence was deposited from 162 to 157 Ma, and flysch deposition took place between 157 and 150 Ma. Tight age constraints on thrusting and low-grade metamorphism associated with ophiolite emplacement (Nevadan orogeny) are provided by abundant calc-alkaline dikes and plutons ranging in age from 151 to 139 Ma. Deformation and metamorphism related to the Nevadan orogeny appears to have extended from ∼155 to 135 Ma. Most of the crustal shortening took place by thrusting, constrained to have occurred from ∼155 to 150 Ma on both the roof and basal thrusts. Minimum rates of displacement are 2.4 and 3.6 mm/year for the basal and roof thrusts, respectively, but correlations with coeval thrusts yield rates of 8.4 and 22 mm/year (within the range of plate velocities). The high displacement rates and synchronous movement along the basal and roof thrusts suggest that the ophiolite may have behaved as a microplate situated between western North America and an active arc from ∼155 to 150 Ma. A steep thermal gradient was present in the Josephine-Galice thrust sheet from ∼155 to 150 Ma, with amphibolite facies conditions developed along the basal thrust. After accretion of the ophiolite by underthrusting, the ophiolite and overlying flysch underwent low-grade dynamothermal regional metamorphism from 150 to 135 Ma. The upper age limit is tightly constrained by a 135 Ma K-feldspar cooling age, syntectonic plutons as young as 139 Ma, and a Lower Cretaceous angular unconformity. Very rapid exhumation is indicated by the late Valanginian to Hauterivian age (∼130 Ma) of the unconformably overlying strata, suggesting unroofing by extensional tectonics.

Fe-Ti basalts and propagating-rift tectonics in the Josephine Ophiolite

The Jurassic Josephine Ophiolite of northwest California and southwest Oregon is one of the largest and most complete ophiolites in North America and clearly formed in a suprasubduction-zone setting. The geochemistry of lavas and dikes is highly variable because of a range in magma types and different degrees of fractionation. Basalts rich in Fe and Ti occur in the uppermost part of the extrusive sequence and as late dikes cutting oceanic serpentinite. They have MORB (mid-oceanic-ridge basalt) affinities and appear to be unrelated to the rest of the ophiolite that has affinities dominantly transitional between IAT and MORB (IAT = island-arc-tholeiite). This late Fe-Ti-MORB suite was erupted while the ophiolite was undergoing the last half of ∼50° of tilting of the entire crustal sequence and after widespread serpentinization of ultramafic cumulates and uppermantle peridotite related to freezing of the axial magma chamber. Fe-Ti basalts are characteristic of propagating spreading centers on mid-ocean ridges and in at least one backarc basin. Geochemical, structural, stratigraphic, and regional geologic constraints suggest formation of the Josephine Ophiolite by propagation of a spreading center into rifted island-arc lithosphere that is preserved along the margin of the ophiolite. Another possible indication of propagating-rift tectonics is the presence of off-axis metalliferous sedimentary rocks that may have been deposited as hydrothermal plume fall-out from a second propagating spreading center. The Lau Basin provides modern analogues, including spreading centers propagating into rifted arc lithosphere, into older backarc ocean crust, and across the arc into the forearc.

Oceanic versus emplacement age serpentinization in the Josephine ophiolite: Implications for the nature of the Moho at intermediate and slow spreading ridges

We present field, petrographie, and geochemical evidence for oceanic serpentinization in the 162 Ma Josephine ophiolite of NW California and SW Oregon. Undeformed and unrodingitized dikes that intruded into serpentinized shear zones provide time markers for serpentinization and deformation. The dikes intruding serpentinites are of two types: (1) Fe-Ti enriched dikes with normal mid-ocean ridge basalt (NMORB) magmatic affinity which are geochemically linked to the uppermost lavas and a late Fe-Ti dike within the crustal sequence of the ophiolite; and (2) hornblende-bearing, calc-alkaline dikes intruded during ophiolite emplacement between 150 and 146 Ma. Based on crosscutting relationships between dikes and serpentinites, serpentinization of upper mantle peridotites took place at or near the ridge axis, during periods of amagmatic structural extension in the absence of a magma chamber. Lizardite-bearing seipentinites in the northern peridotite are constrained to be oceanic and indicate temperatures of <350°C within the oceanic upper mantle. Oceanic, lizardite-bearing, serpentinized shear zones, from the ultramafic cumulate section to the basal sole, indicate that the entire Josephine peridotite may have been transected by serpentinized shear zones prior to ophiolite emplacement The ultramafic cumulate sequence was completely serpentinized prior to ophiolite emplacement, and we suggest that the paleo-Moho in the Josephine ophiolite is a serpentinization boundary. The basal sole is interpreted to be a reactivated oceanic fault along which antigorite mylonites formed from a preexisting serpentinite during ophiolite emplacement. Oceanic serpentinization in the Josephine ophiolite took place beneath a 2–3 km crustal sequence and suggests that serpentinization may be an important feature at intermediate spreading rate mid-ocean ridges, as well as at slower spreading ridges.

Geochemistry of Upper Proterozoic rift-related volcanics, northern Utah and southeastern Idaho

Mafic volcanic rocks associated with Upper Proterozoic diamictites are widespread in western North America, and a continental rift setting has been proposed for their origin. We report trace-element and rare-earth-element geochemistry for these volcanics from northern Utah and southeastern Idaho. The analyzed samples are high-Ti within-plate basalts, and they vary from transitional tholelitic-alkaline in the south to alkaline in the north. The geochemistry, together with the geologic setting of the volcanics, indicates a rift setting, but the timing of this rifting and its relationship to final continental separation are still controversial.

Detachment faulting and amagmatic extension at mid-ocean ridges: The Josephine ophiolite as an example

Lithospheric extension at the Josephine paleo-spreading center occurred by a combination of magmatic and amagmatic processes. The amount of extension by amagmatic processes appears at least as large as that from magmatic processes. The structural processes responsible for amagmatic extension in the absence of a magma chamber appear to be normal faulting and block rotation in the brittle upper lithosphere and ductile flow in the lower lithosphere. An extensive detachment shear zone occurs beneath the fault blocks, approximately 1 km below the base of the crustal sequence. The amount of amagmatic extension can be approximated from a simple geometric model relating extension to rotations of sheeted dikes. Similarly, attenuated crustal thicknesses can be related to fault-block rotations. The results suggest that thin crustal sequences commonly observed in ophiolites and near fracture zones may result from attenuation during amagmatic extension. Furthermore, detachment faulting and block rotation may be characteristic of spreading centers where the magma budget is low.

Tectonic implications of boninite, arc tholeiite, and MORB magma types in the Josephine Ophiolite, California-Oregon

Abstract The Josephine Ophiolite is a large complete ophiolite flanked by arc complexes, including rifted arc facies, and overlain by volcanopelagic and volcaniclastic sedimentary rocks. The extrusive sequence and sheeted dyke complex record a wide range in magma types and degree of fractionation. The upper part of the extrusive sequence, as well as late dykes in the ophiolite, have mid-ocean ridge basalt (MORB) affinities and include unusual highly fractionated Fe-Ti basalts. The sheeted dyke complex and lower pillow lavas are dominated by transitional island-arc tholeiite (IAT) to MORB, but about 10% consist of low-Ti, high-Mg basalts and andesites. Whole-rock chemistry and Cr-spinel compositions indicate that the low-Ti rocks range from boninite (BON) to primitive arc basalt. The low-Ti samples have trace element characteristics indicating a greater subduction component than the IAT-MORB or MORB samples, as well as derivation from a wide range of sources ranging from depleted to enriched relative to an average N-MORB mantle source. Mixing of low-Ti and MORB magmas may have produced the IAT-MORB magma type that is most characteristic of the ophiolite. Podiform chromites and late magmatic features in the mantle peridotite, described by previous workers, appear to have been formed from the low-Ti magmas. Regional geological relationships and the presence of boninitic magmas suggest that arc rifting and initial sea-floor spreading to form the Josephine Ophiolite occurred in the forearc of a west-facing arc built on edge of the North American plate. Arc magmatism appears to have jumped westward, at which time the Josephine basin became situated in a back-arc setting, analogous to the inferred evolution of the modern Lau back-arc basin. Alternatively, the Josephine Ophiolite may have formed in a setting analogous to the north end of the Tonga Trench or the south end of the North Fiji basin, both sites of modern boninites, where a back-arc spreading centre has propagated across an arc into the forearc. Rift propagation during formation of the Josephine Ophiolite is consistent with the presence of highly fractionated Fe-Ti basalts.

Geochemistry and Tectonic Setting of the Ophiolitic Ingalls Complex, North Cascades, Washington: Implications for Correlations of Jurassic Cordilleran Ophiolites

This study synthesizes new and existing chemical data to interpret the tectonic setting of the Late Jurassic Ingalls Complex of the Northwest Cascades, Washington, and to compare it with other Cordilleran ophiolites of similar age. Mafic rocks of the Ingalls Complex represent a spectrum of magma types including normal‐type and enriched mid‐ocean ridge basalt, within‐plate basalt, and island arc basalt. Based on mafic magma chemistry, compositions of Cr‐spinels in associated peridotite, and field relations, the Ingalls Complex is a suprasubduction zone ophiolite formed in a back‐arc basin cut by an oceanic fracture zone. Regional tectonic relations also support this hypothesis. Our synthesis of available chronologic, geochemical, and stratigraphic data for all Jurassic Cordilleran ophiolites shows that the Ingalls Complex has similarities and differences with each. The most compelling likeness is between the Ingalls Complex and Josephine ophiolite, both of which exhibit a wide range of magmatic affinities, Cr‐spinel compositions that are consistent with a suprasubduction zone origin in a back‐arc basin setting, and positions inboard of a coeval Jurassic arc.

Oceanic faulting and fault‐controlled subseafloor hydrothermal alteration in the sheeted dike complex of the Josephine Ophiolite

Based upon detailed mapping (1:10 and 1:100) of a large water-polished outcrop of the sheeted dike/gabbro transition zone in the Josephine ophiolite of NW California and SW Oregon, the following history of alternating episodes of magmatic, structural, and hydrothermal events has been documented using crosscutting relationships, petrography, geochemistry, and strontium and oxygen isotopic data: (1) crystallization of gabbro and later subvertical mafic dike injection, (2) amphibolite facies metamorphism, (3) extensional faulting and tilting of dikes, (4) continued faulting, tilting, and dike injection associated with retrograde metamorphism at greenschist facies conditions, (5) continued extensional faulting and tilting synchronous with the development of a variety of hydrothermal veins at decreasing temperature and increasing fluid/rock ratios, and (6) subvertical injection of a highly fractionated dike which truncates all previous features (features 1–5). Trace element geochemistry indicates the highly fractionated dike (feature 6) is genetically related to the other dikes and thus was intruded at or near the paleospreading axis. Hence, all previous events (features 1–5) can be constrained to have occurred at the rift axis, including large-scale tilting (∼ 50°) of the sheeted dikes and extensional faulting. The [87Sr/86Sr]initial ratios from recrystallized whole rocks and from hornblende, epidote, and prehnite separates from veins display a systematic increase with relative age, from 0.7033 for altered gabbro screens and mafic dikes to 0.7049 for prehnite in the youngest oceanic fault rock. Calculated oxygen isotope fluid compositions for the same suite of samples range from δ18Ofluid = +5 to −1 with time, indicating a change to a seawater-dominated hydrothermal system with time, consistent with observed increased permeability due to seafloor extensional faulting. The crosscutting relationships, alteration mineral assemblages, and isotopic data suggest (1) an early stage of high-temperature (≥450°C) hydrothermal alteration with low permeability (i.e., grain-scale flow), followed by (2) a decrease in temperature (∼350to ≤200°C), and an increase in permeability due to faulting and accompanied by tectonic tilting at the rift axis. The consistency of these crosscutting relationships at similar pseudo-stratigraphic levels at different localities in the Josephine ophiolite suggests that alternating magmatic and structural extension with synchronous retrograde alteration is common in crust formed at similar rates of spreading (slow- to intermediate-spreading centers), such as the modern Mid-Atlantic Ridge.

Episodic magma chambers and amagmatic extension in the Josephine ophiolite

Geochemical and structural aspects of the Josephine ophiolite indicate that it formed by alternating cycles of magmatic and structural (amagmatic) extension. Episodic magma chambers are indicated by the presence of very primitive lavas in the lower pillow lavas which would have mixed into a magma chamber. Fe-Ti basalts that occur in the uppermost extrusive rocks and plagiogranites that occur along the sheeted-dike/gabbro contact represent highly fractionated magmas produced during the last stages of crystallization of a magma chamber. During periods of amagmatic extension, breccias accumulated and the entire crustal sequence was tilted >50°, probably by faulting above an upper-mantle detachment. More than 1000 m of uplift and down-dropping between magmatic cycles is suggested by the presence of out-of-place screens in the sheeted-dike complex.

The Josephine ophiolite: an ancient analogue for slow- to intermediate-spreading oceanic ridges

Abstract The 162 Ma Josephine ophiolite, NW California and SW Oregon, consists of harzburgite tectonite (>800 km2), cumulates, high-level gabbro, a sheeted dyke complex having a consistent dyke orientation over hundreds of square kilometres, and pillow lavas. The ophiolite is conformably overlain by hemipelagics which grade upward into synorogenic turbidites. Open folding of the ophiolite and sediments provides ideal conditions to reconstruct the structural geometry of oceanic features in the ophiolite. Episodic axial magma chambers, structural extension, and episodic hydrothermal circulation is indicated by: (1) the local presence of both highly fractionated and very primitive lavas and dykes, (2) thick talus(?) breccias, and (3) multiple stacked massive sulphide deposits, each overlain by up to 5 m of mudstone within the pillow lavas. The effects of structural extension are evident from: (1) a 50° tilting of the entire crustal sequence except for the uppermost lava flows, (2) oceanic normal and transfer faults, and (3) shear zones within the harzburgite consisting of peridotite (± talc) mylonite and high-temperature serpentine mylonite. A regionally extensive, subhorizontal serpentine mylonite zone within the upper c. 1 km of the harzburgite formed at c. 500°C and may represent an oceanic detachment fault. The 50° tilting of the entire crustal sequence occurred prior to eruption of the uppermost lava flows and implies extreme attenuation (c. 100%) at the spreading ridge. This extension would account for the present thin crustal sequence of the Josephine ophiolite (c. 3 km thick). It is important to note that where deep faulting occurs the actual oceanic crustal thickness, as defined by seismic velocities, is likely to coincide with the lower limit of hydrothermal serpentine alteration in the harzburgite tectonite. Subseafloor hydrothermal alteration resulting from flow of discharging fluids is especially localized along brittle oceanic faults in the Josephine ophiolite. Seafloor massive sulphide deposits in the pillow lavas and mineralized stockworks in the upper dyke complex apparently formed by discharging black smoker fluids localized along oceanic fault zones. Relatively late oceanic faults have been observed in the sheeted dyke complex and are extensively altered to quartz + sulphides + sericite. They are tentatively interpreted as off-axis faults and may represent pathways for lower temperature fluids which vented to the seafloor and formed metalliferous sediments 8–23 m above the uppermost pillow basalts of the ophiolite. Although the regional setting and geochemistry of the Josephine ophiolite indicate that it formed in a suprasubduction zone setting, the following features suggest that it may provide a useful structural analogue for slow-to intermediate-spreading mid-ocean ridges: (1) episodic magmatism, (2) extensive faulting and talus deposits, (3) normal faulting extending into the upper mantle, (4) fault-controlled hydrothermal discharge, and (5) large-scale tilting of the entire crustal sequence. The high degree of structural extension distinguishes the Josephine ophiolite from the Oman and Bay of Islands ophiolites, which probably formed at fast-spreading centres.