Stanley Mountain Research Articles

Paleomagnetism, paleogeographic origins, and uplift history of the Coast Range ophiolite at Mount Diablo, California

Divergent paleogeographic origins have been proposed for the Coast Range ophiolite of western California which are testable using paleomagnetic methods. Paleomagnetic data for Middle Jurassic pillow lavas and diabase sills of the Coast Range ophiolite at Mount Diablo, northern California, indicate that they contain two components of remanent magnetization. The characteristic directions have normal and reversed polarities and apparently are carried by Ti‐poor magnetite. This magnetization is inferred to have been acquired during emplacement and seafloor alteration at an ancient spreading ridge. The paleolatitude calculated from its structurally corrected mean direction is 20°N ± 9° and agrees with the expected direction for stable North America; this result is also consistent with the concordant paleolatitude (32°N ± 8°) recently determined for Upper Jurassic Coast Range ophiolite at Stanley Mountain in southern California. In addition, clockwise vertical axis rotation of Mount Diablo (143° ± 11°) is indicated by the characteristic magnetization direction. An overprint component is inferred to have been acquired during uplift of Mount Diablo since the Miocene.

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

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

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.

Upper Jurassic Radiolarian biostratigraphy of Stanley Mountain, Southern California Coast Ranges

Over 400 species of middle Oxfordian to uppermost Tithonian Radiolaria have been documented and illustrated from volcanopelagic strata overlying the Coast Range Ophiolite at Stanley Mountain in the Coast Ranges of southern California. The volcanopelagic succession at this locality consists of approximately 130 m of chert and siliceous mudstones with interbeds and lenses of manganiferous limestones. This succession is transitional with and overlain by, strata of the Great Valley Supergroup. Utilizing the radiolarian biostratigraphic zonation of Pessagno et al. for North America, these assemblages are assigned to Zone 2, Subzone 2 gamma (Middle Oxfordian) to Zone 4, Subzone 4 alpha (uppermost Tithonian). Faunas near the base of the section are Tethyan in character, but become Boreal higher in the succession. An exception to this pattern occurs in the uppermost part of the succession, suggesting a strong paleoceanographic or tectonic influence in the development of these assemblages.

Remagnetization of the Coast Range ophiolite at Stanley Mountain, California, during accretion near 10°N paleolatitude

Paleomagnetic data are presented for a 50-m-thick sequence of Oxfordian to Tithonian sedimentary rocks conformably overlying Upper Jurassic pillow basalt within the Coast Range ophiolite at Stanley Mountain, California. These new data are similar in direction and polarity to previously published paleomagnetic data for the pillow basalt. The Jurassic sedimentary rocks were deposited during a mixed-polarity interval of the geomagnetic field, and uniformity of the remanent magnetization within the entire section of pillow basalt and sedimentary rocks indicates later remagnetization. Remagnetization of the Coast Range ophiolite is interpreted to have occurred during accretion to the continental margin, possibly by burial and low-temperature alteration related to this event. Similar paleolatitudes calculated for the ophiolite (11° ±3°) and for mid-Cretaceous sedimentary rocks of the Stanley Mountain terrane at Figueroa Mountain (6° ±5°) are consistent with remagnetization of the ophiolite at low paleo-latitudes. Uniform-polarity directions for other remnants of ophiolite in southern California and elsewhere along the Pacific coast imply that these rocks were also overprinted, and their magnetic inclinations suggest remagnetization at low paleolatitudes as well. The Coast Range ophiolite at Stanley Mountain is thus inferred to have been remagnetized along the North American margin near 10°N paleolatitude between earliest and mid-Cretaceous time and subsequently transported northward by strike-slip faulting related to relative motions between the Farallon, Kula, Pacific, and North American plates.

Chronostratigraphic Assignment of Volcanopelagic Strata Above the Coast Range Ophiolite

The chronostratigraphic assignments of radiolarian cherts, tuffaceous mudstones and volcaniclastic strata (referred to herein as volcanopelagic succession)' overlying the Coast Range ophiolite in the Coast Ranges of California have been refined on the basis of additional field data and recent revision of the Pessagno and others radiolarian biostratigraphic zonation for the Middle and Upper Jurassic of North America. Revision of this zonation scheme, accomplished through the integration of radiolarian data with ammonite biostratigraphy and geochronometry, has enabled close correlation of radiolarian assemblages in North America, Europe, and Japan. However, major differences still exist in the stage assignments of these assemblages among different zonations. The Pessagno and others scheme is utilized herein to discuss the current chronostratigraphic assignments at five localities in the Coast Ranges: Point Sal, Stanley Mountain, Cuesta Ridge, and Llanada in the southern Coast Ranges, and Black Mountain (Geyser Peak area) in the northern Coast Ranges of California. Stage assignments for volcanopelagic successions in these areas range from uppermost Callovian/lower Oxfordian to upper Tithonian.

Tectonics of formation, translation, and dispersal of the Coast Range Ophiolite of California

Data from the Coast Range ophiolite and its tectonic outliers in the northern California Coast Ranges suggest that the lower part of the ophiolite formed 169 to 163 Ma in a forearc or back arc setting at equatorial latitudes. Beginning about 156 Ma and continuing until 145 Ma, arc magmatism was superimposed on the ophiolite, and concurrently, a transform developed along the arc axis or in the back arc area. Rapid northward translation of this rifted active magmatic arc to middle latitudes culminated in its accretion to the California margin of North America at about 145 Ma. This Late Jurassic episode of translation, arc magmatism, and accretion coincided with the Nevadan orogeny and a proposed major plate reorganization in the eastern Pacific basin. The high rate of poleward motion necessary to translate the Coast Range ophiolite to middle latitudes during this time implies that the ophiolite traveled north on a fast‐moving plate of the eastern Pacific basin, here termed plate X. Plate X probably was driven by a cryptic ridge east‐northeast of the Pacific‐Farallon‐Izanagi ridge triple junction. Structural relations indicate that following Late Jurassic time, parts of the Coast Range ophiolite were displaced from the west side of the Great Valley province and incorporated into the Central belt of the Franciscan Complex along steep‐dipping to low‐angle reverse faults having dominant components of dextral shear. A northwest trending eastern zone of these right‐laterally displaced outliers shows strong affinities to the main Coast Range ophiolite of the northwestern Sacramento Valley (the Elder Creek terrane), in that the outliers include ophiolitic breccias of Oxfordian to Kimmeridgian age. A southwestern zone of outliers lacks ophiolitic breccia and instead includes latest Oxfordian or Kimmeridgian to Tithonian, arc‐derived volcanic rocks like those found in the Del Puerto and Stanley Mountain terranes of the main ophiolite. Whereas outliers of the northeastern outlier zone are right‐laterally displaced no more than 260 km from the western side of the Sacramento Valley, outliers of the southwestern zone are displaced a minimum of 169 to 249 km. This displacement occurred between about 60 and 52 Ma. Ophiolitic rocks in the Decatur terrane of western Washington that have recently been correlated with the Coast Range ophiolite and the Great Valley sequence of California were apparently displaced at least 950 to 1200 km from the west side of the Great Valley between early Tertiary and Early Cretaceous time. Derived rates of northward translation for the ophiolite outliers in California are in the range of 1 to 4 cm/yr. Rates for the Decatur terrane are in the range of 2.5 to 4 cm/yr if translation was initiated 90 Ma, but as much as 11.9 to 15 cm/yr if it was not initiated until 60 Ma. The lower rates for the Decatur terrane are consistent with the rates derived for the California outliers and with the northward component of relative motion between the Farallon and North American plates from 90 to 50 Ma. The higher rates require northward transport on the Kula plate or on a fast‐moving microplate. The higher translation rates derived for the Decatur terrane are also consistent with paleomagnetically determined rates for some limestones of the Central belt. This may indicate that outliers of the Coast Range ophiolite dispersed in the Central belt of the Franciscan Complex record only part of the total displacement which occurred along the Late Cretaceous to early Tertiary western margin of North America.

Tectonic and Paleogeographic Setting of a New Latest Cretaceous Floristic Province in North America

The latest Cretaceous (Santonian to Maastrichtian) Normapolles and Aquilapollenites floristic provinces are well known; they occupied the southeastern part and most of the northern and western parts, respectively, of North America. The continental margin floristic province of this paper differs from the other two provinces because of its largely endemic pollen species, its abundance of Proteacidites group pollen, and its generally low abundance or lack of pollen of the Aquilapollenites, Callistopollenites, Normapolles, wind-pollinated? triporate, and angiosperm monosulcate groups. Characteristic features and geographic distribution of the continental margin province are based on previously described assemblages from British Columbia and newly examined assemblages from California. Latest Cretaceous sediments of this province rest on rocks of the Wrangellia terrane and the Coast Plutonic Complex in southwestern British Columbia, on the Salinian terrane of the La Panza Range, and on the Stanley Mountain terrane of the southeastern San Rafael Mountains, all of which have been translated northward at least 2000 km since the Campanian according to paleomagnetic data. Thus, latest Cretaceous floras of the continental margin province were isolated from floras to the east partly because of oceanic and mountain barriers but probably also because they formed in more tropical climates than most floras of the other two provinces.

Geochemical evidence for the tectonic setting of the Coast Range ophiolite: A composite island arc–oceanic crust terrane in western California

The Middle to Late Jurassic age Coast Range ophiolite (CRO) of California contains two geochemically distinct volcanic rock associations that formed in different tectonic settings. Volcanic rocks from the southern CRO (Point Sal, Cuesta Ridge, Stanley Mountain, Llanada, Quinto Creek, and Del Puerto) and parts of the northern CRO (Healdsburg, Elder Creek) are similar to low-K tholeiites and calc-alkaline rocks of the island-arc suite. The thin volcanic sections of these ophiolite remnants suggest formation by intra-arc rifting. In contrast, volcanic rocks from Stonyford seamount and Paskenta in the northern CRO are transitional subalkaline metabasalts with geochemical characteristics similar to enriched mid-ocean ridge basalts or ocean-island tholeiites. These rocks are associated with Tithonian radiolarian cherts and may be part of the Franciscan Complex. Alternatively, they may represent a change in tectonic setting within the CRO during the Late Jurassic. Regardless, the CRO as currently conceived cannot be considered a single terrane with one mode of origin.

Structure of Franciscan Complex in the Stanley Mountain window, southern Coast Ranges, California

Three sets of deformational events are recognized in the Franciscan Complex of the Stanley Mt. area, S. Coast ranges, California. First, in pre-melange time, shortening of the relatively cohesive sequence of interbedded graywacke and mudstone formed isoclinal folds and an axial-plane slaty cleavage. Second, fragmentation of the once cohesive sequence, probably over a considerable period of time, produced the configuration now considered a melange. Third, after the melange developed, the Franciscan Complex was deformed along with the surrounding upper Mesozoic Great Valley sequence into the Stanley Mt. antiform. In the cohesive Upper Cretaceous Carrie Creek Formation, macroscopic and mesoscopic folds have 2 predominant orientations. The less cohesive Franciscan Complex attempted to fold, as shown by the distribution of shear foliations on stereographic projections, but lack of lithologic continuity and slip along previously formed shear fractures prevents the recognition of macroscopic folds. Hence, in the Franciscan Complex of the Stanley Mt. window, several lines of evidence show that the melange structure is tectonic in origin, not just a tectonic imprint superimposed upon already chaotic rocks of sedimentary origin (olistostromes). 43 references.

Exotic terranes of western California

Numerous distinct geological terranes compose the North American Cordillera1; there may be as many as 50 terranes in California alone2. Critical to deciphering the history of Cordilleran tectonic assembly is an understanding of the displacement history of individual terranes. It is therefore important to know: (1) whether a terrane has undergone significant motion with respect to the stable craton (that is, whether it is allochthonous or exotic); (2) if so, when relative motion started and stopped; (3) from where an individual terrane originated; and (4) the nature of interterrane movements. We consider here the problem of determining whether the now-juxtaposed Salinian and Stanley Mountain terranes of California became amalgamated at or near their present position with respect to cratonic North America, or if they collided at a considerable distance from their present positions and were later accreted to North America as a composite package. The palaeomagnetic data that we present indicate that the latter was the case.

Franciscan-Knoxville Problem

Franciscan-Knoxville Problem