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B. Murchey

Publications and source records attributed to B. Murchey.

3 recordsLinked to original sources
USGS2010

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.

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USGS2010

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 forearc 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 vith arc tholeiitic or (roore 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.

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