Benita L. Murchey

Multi-Stage Origin of the Coast Range Ophiolite, California: Implications for the Life Cycle of Supra-Subduction Zone Ophiolites

The Coast Range ophiolite of California is one of the most extensive ophiolite terranes in North America, extending over 700 km from the northernmost Sacramento Valley to the southern Transverse Ranges in central California. This ophiolite, and other ophiolite remnants with similar mid-Jurassic ages, represent a major but short-lived episode of oceanic crust formation that affected much of western North America. The history of this ophiolite is important for models of the tectonic evolution of western North America during the Mesozoic, and a range of conflicting interpretations have arisen. Current petrologic, geochemical, stratigraphic, and radiometric age data all favor the interpretation that the Coast Range ophiolite formed to a large extent by rapid extension in the fore-arc region of a nascent subduction zone. Closer inspection of these data, however, along with detailed studies of field relationships at several locales, show that formation of the ophiolite was more complex, and requires several stages of formation. Our work shows that exposures of the Coast Range ophiolite preserve evidence for four stages of magmatic development. The first three stages represent formation of the ophiolite above a nascent subduction zone. Rocks associated with the first stage include ophiolite layered gabbros, a sheeted complex, and volcanic rocks with arc tholeiitic or (more rarely) low-K calc-alkaline affinities. The second stage is characterized by intrusive wehrlite-clinopyroxenite complexes, intrusive gabbros, Cr-rich diorites, and volcanic rocks with high-Ca boninitic or tholeiitic ankaramite affinities. The third stage includes diorite and quartz diorite plutons, felsic dike and sill complexes, and calc-alkaline volcanic rocks. The first three stages of ophiolite formation were terminated by the intrusion of mid-ocean ridge basalt dikes, and the eruption of mid-ocean ridge basalt or ocean-island basalt volcanic suites. We interpret this final magmatic event (MORB dikes) to represent the collision of an active spreading ridge. Subsequent reorganization of relative plate motions led to sinistral transpression, along with renewed subduction and accretion of the Franciscan Complex. The latter event resulted in uplift and exhumation of the ophiolite by the process of accretionary uplift.

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.

Deposition of Franciscan Complex cherts along the paleoequator and accretion to the American margin at tropical paleolatitudes

Red radiolarian cherts from three localities within the Franciscan subduction complex of northern California contain three components of remanent magnetization which are best isolated by progressive thermal demagnetization. The first component, usually removed by 300 °C, has an in situ direction similar to the present axial-dipole field and is probably a recently acquired thermoviscous overprint. A second component, generally removed between 300 and 630 °C, has constant (normal) polarity and direction within each section and is interpreted to have been acquired by low-temperature chemical alteration during subduction and accretion at the continental margin. The third component, isolated between ∼560 and 680 °C, has both normal and reversed polarities, passes a fold test, and is inferred to have been acquired during or soon after deposition. The available paleomagnetic, biostratigraphic, and geochemical data indicate deposition of these cherts along the paleoequator (0°-2°N or S paleolatitude) between Pliensbachian and Oxfordian time as the oceanic plate moved eastward, relative to North America, beneath the equatorial zone of high biologic productivity. Between Bathonian and Cenomanian time, the chert sequences apparently moved progressively away from the paleoequator (2°-15°N or S), and were soon after accreted to the American continental margin. Plate reconstruction models for the Farallon plate corroborate low-paleolatitude trajectories from ridge crest to subduction zone (for example, from 3°S to 11°N), and they imply subsequent northward translation of the Franciscan Complex (> 4,000 km) by strike-slip faulting related to relative motions between the Farallon, Kula, Pacific, and North American plates.

A mid-Permian chert event: widespread deposition of biogenic siliceous sediments in coastal, island arc and oceanic basins

Abstract Radiolarian and conodont of Permian siliceous rocks from twenty-three areas in teh the circum-Pacific and Mediterranean regions reveal a widespread Permian Chert Event during the middle Leonardian to Wordian. Radiolarian- and (or) sponge spicule-rich siliceous sediments accumulated beneath high productivity zones in coastal, island arc and oceanic basins. Most of these deposits now crop out in fault-bounded accreted terranes. Biogenic siliceous sediments did not accumulate in terranes lying beneath infertile waters including the marine sequences in terranes of northern and central Alaska. The Permian Chert Event is coeval with major phosphorite deposition along the western margin of Pangea (Phosphoria Formation and related deposits). A well-known analogue for this event is middle Miocene deposition of biogenic siliceous sediments beneath high productivity zones in many parts of the Pacific and concurrent deposition of phosphatic as well as siliceous sediments in basins along the coast of California. Interrelated factors associated with both the Miocene and Permian depositional events include plate reorientations, small sea-level rises and cool polar waters.

Paleomagnetism of Jurassic radiolarian chert above the Coast Range ophiolite at Stanley Mountain, California, and implications for its paleogeographic origins

Upper Jurassic red tuffaceous chert above the Coast Range ophiolite at Stanley Mountain, California (lat 35°N, long 240°E), contains three components of remanent magnetization. The first component (A; removed by ≈100–≈200°C) has a direction near the present-day field for southern California and is probably a recently acquired thermoviscous magnetization. A second component (B; removed between ≈100 and ≈600°C) is identical to that observed by previous workers in samples of underlying pillow basalt and overlying terrigenous sedimentary rocks. This component has constant normal polarity and direction throughout the entire section, although these rocks were deposited during a mixed polarity interval of the geomagnetic field. The B magnetization, therefore, is inferred to be a secondary magnetization acquired during accretion, uplift, or Miocene volcanism prior to regional clockwise rotation. The highest temperature component (C; removed between ≈480 and 680°C) is of dual polarity and is tentatively interpreted as a primary magnetization, although it fails a reversal test possibly due to contamination by B. Separation of the B and C components is best shown by samples with negative-inclination C directions, and a corrected mean direction using only these samples indicates an initial paleolatitude of 32°N ± 8°. Paleobiogeographic models relating radiolarian faunal distribution patterns to paleolatitude have apparently been incorrectly calibrated using the overprint B component. Few other paleomagnetic data have been incorporated in these models, and faunal distribution patterns are poorly known and mostly unquantified. The available data, therefore, do not support formation of the Coast Range ophiolite at Stanley Mountain near the paleoequator or accretion at ≈10°N paleolatitude, as has been previously suggested based on paleomagnetic data, but indicate deposition near expected paleolatitudes for North America (35°N ± 4°) during Late Jurassic time.

Paleontologic evidence for complex tectonic interlayering of Mississippian to Permian deep-water rocks of the Golconda allochthon in Tobin Range, north-central Nevada

A detailed paleontologic study in the Hoffman Canyon area was undertaken to determine the degree of tectonic interlayering in a thick succession of largely deep-water chert, argillite, and silty limestone composing the Havallah sequence of the Mesozoic Golconda allochthon of western and north-central Nevada. The study focused on the type area of the Havallah Formation which, as originally defined, was considered to be largely a conformable sequence of Permian(?) rocks. The results indicate that the Havallah sequence in the Hoffman Canyon area consists of a complex tectonic accumulation characterized by repetition of Mississippian, Pennsylvanian, and Permian rocks in at least 12 superposed thrust plates. Such a style of deformation may be characteristic of much of the Golconda allochthon, although paleontologic evidence of this style is clear in only a few areas and is probably best defined in the Hoffman Canyon area. Detailed stratigraphic and paleontologic studies of the Havallah sequence at a few other localities in Nevada indicate considerable lithologic variability of comparable-age rocks in the sequence from area to area, indicating a complex depositional setting that at present is only partly understood.

Equatorial origin for Lower Jurassic radiolarian chert in the Franciscan Complex, San Rafael Mountains, southern California

Lower Jurassic radiolarian chert sampled at two localities in the San Rafael Mountains of southern California (∼20 km north of Santa Barbara) contains four components of remanent magnetization. Components A, B′, and B are inferred to represent uplift, Miocene volcanism, and subduction/accretion overprint magnetizations, respectively. The fourth component (C), isolated between 580° and 680°C, shows a magnetic polarity stratigraphy and is interpreted as a primary magnetization acquired by the chert during, or soon after, deposition. Both sequences are late Pliensbachian to middle Toarcian in age, and an average paleolatitude calculated from all tilt-corrected C components is 1° ± 3° north or south. This result is consistent with deposition of the cherts beneath the equatorial zone of high biologic productivity and is similar to initial paleolatitudes determined for chert blocks in northern California and Mexico. This result supports our model, in which deep-water Franciscan-type cherts were deposited on the Farallon plate as it moved eastward beneath the equatorial productivity high, were accreted to the continental margin at low paleolatitudes, and were subsequently distributed northward by strike-slip faulting associated with movements of the Kula, Farallon, and Pacific plates. Upper Cretaceous turbidites of the Cachuma Formation were sampled at Agua Caliente Canyon to determine a constraining paleolatitude for accretion of the Jurassic chert sequences. These apparently unaltered rocks, however, were found to be completely overprinted by the A component of magnetization. Similar in situ directions and demagnetization behaviors observed in samples of other Upper Cretaceous turbidite sequences in southern and Baja California imply that these rocks might also give unreliable results.

Setting and occurrence of Late Paleozoic radiolarians in the Sylvester allochthon, part of a proto-Pacific ocean floor terrane in the Canadian Cordillera

Abstract Late Paleozoic radiolarians have been used to establish th allochthon of the Slide Mountain terrane in British Columbia, and have thereby greatly clarified the geology and tectonic history of the terrane. As the Sylvester radiolarian fauna is limited, age assignments were based on a few distinctive and diagnostic robust forms. Radiolarians occur in cherts from a wide variety of different oceanic sequences that are structurally juxtaposed within the Sylvester allochthon. Like others in a suite of correlative terranes that lie along the length of the Cordillera, the Sylvester allochthon and the radiolarian bearing cherts in it derive from the telescoping together of slices from what was, in the late Paleozoic, a large area of the proto-Pacific ocean.

Chapter 8 Distribution, Age, and Depositional Environments of Radiolarian Chert in Western North America

Abstract Radiolarian chert is widespread in allochthonous accreted terranes throughout the Cordillera of western North America, where dated deposits range in age from Ordovician to middle Cretaceous. Six principal lithologic associations are described: (1) ophiolitic chert association; (2) alternating pillow-basalt/ chert association; (3) silicic volcanic/chert association; (4) subsidence association; (5) clastic/chert association; and (6) melange chert association. Each association may represent a distinct environment. the ages and lithologic associations of radiolarian chert deposits in 23 allochthonous Cordilleran terranes are summarized.