Ophiolites are distinctive assemblages of mafic, ultramafic, and felsic igneous rocks that are commonly thought to represent oceanic crust and mantle that has been accreted to a continental margin. The accretion of ophiolite and island arc terranes has been the primary mechanism of continental growth since the Proterozoic. My work focuses on the Coast Range ophiolite (CRO) of California, one of the most extensive ophiolites in North America.
Petrology, Geochemistry, and Age of Accreted Oceanic Crust, California Coast Ranges
The western margin of North America is characterized by extensive tracts of ophiolitic basement with radiometric ages of 155 to 170 my. The Coast Range Ophiolite of California is the most extensive of these terranes, which also include the Fidalgo ophiolite complex, the Smartville ophiolite, the Josephine ophiolite, the Preston Peak ophiolite, and the Cedros ophiolite. The regional extent of these ophiolite belts, and the narrow range in their ages of formation, make their petrogenesis one of the more important tectonic problems in the Cordillera.
The Coast Range Ophiolite (CRO) was first recognized by Bailey, Blake, and Jones (1970) and later studied in detail by Hopson, Mattinson, and Pessagno (1981). Early studies of the CRO emphasized its similarity to oceanic crust formed at mid-ocean ridge spreading centers, and tectonic interpretations focused on the obduction of intact oceanic lithosphere (e.g., Bailey and others, 1970; Page, 1972; Hopson and Frano, 1977; Hopson and others, 1981). These early studies provided the fundamental petrologic and structural framework for later investigations, and established the overall "oceanic" nature of the ophiolite.
The Elder Creek ophioliteis
the northern-most exposure of Coast Range ophiolite (CRO) in California,
and also one of the best preserved. It comprises a nearly complete ophiolite
sequence, with cumulate mafic and ultramafic rocks, non-cumulate gabbro
and diorite, dike complex, and volcanic rocks. In contrast, the Stonyford
Volcanic complex, which just 35 miles south of Elder Creek, is
unique within the CRO. It consists almost entirely of volcanic rocks, with
minor intercalations of chert and limestone. The focus of this project
is to determine the origin and history of these two disparate remnants
of CRO, and what they tell us about the tectonic evolution of oceanic crust
and the western Cordillera.
Elder Creek: Field relations
and geochemistry of the Elder Creek ophiolite show that four magmatic episodes
are required for its formation. The first magma series is represented by
cumulate dunite, wehrlite, and gabbro, and isotropic gabbro, and dike complex.
The second magmatic episode consists of clinopyroxenite intrusions with
less common gabbro and gabbro pegmatoid. The third magmatic episode is
represented by isotropic gabbro, agmatite with xenoliths of cumulate or
foliated gabbro and dike complex in an isotropic gabbro-diorite matrix,
diorite and quartz diorite stocks and dikes which intrude all of the older
lithologies, and felsite dikes which are marginal to the quartz diorite
plutons. The fourth magma series is represented by basaltic dikes which
cross-cut rocks of the older episodes. Geochemical data are consistent
with formation of the first three series in a supra-subduction zone (arc)
environment; dikes of the final magma series are characterized by MORB-like
major and trace element compositions. U-Pb zircon ages of plagiogranites
from stage 1 and the stage 3 quartz diorites require a narrow formation
interval, from 172 Ma to 165 Ma.
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Volcanic rocks are most commonly preserved as clasts in the Crowfoot Point breccia, a coarse, unsorted fault-scarp talus breccia which varies from <10 m to over 1000 m in thickness. This unit was deposited on an eroded surface which cross-cuts all other units of the ophiolite (from cumulate ultramafics through dike complex). Additional volcanic rocks crop out in fault-bounded blocks and in the dike complex. With the exception of the late, MORB-like dikes, all volcanic and hypabyssal rocks associated with the Elder Creek ophiolite are island arc tholeiite or calc-alkaline series basalts, andesites, or dacites.
The ophiolite formed in a fore-arc setting, above the east-dipping proto-Franciscan subduction zone, marginal to the western Cordillera. The first three magmatic episodes described above correspond to progressive stages in ophiolite formation: (1) formation of incipient island arc crust above a nascent subduction zone, (2) rifting and deformation of this crust, which allows the intrusion of primitive, unfractionated tholeiitic ankaramite magma, and (3) transition to more normal calc-alkaline magmatism. The fourth magmatic episode corresponds to a ridge subduction event which we have documented in other CRO localities (Stonyford, Berkeley Hills).
Stonyford: The Stonyford volcanic complex (SFVC) consists largely of volcanic flows with subordinate diabase, hyaloclastite breccia, and sedimentary intercalations. The rocks are exceptionally fresh for the Coast Range ophiolite, as shown by the preservation of primary igneous plagioclase, clinopyroxene, and basaltic glass in most of the volcanic rocks. Volcanic rocks of the SFVC comprise three distinct petrologic groups: (1) Enriched ocean island basalts (tholeiites), (2) Transitional alkali basalts and glasses, and (3) High-alumina, low-Ti tholeiites. Pb isotopic data for the volcanic glasses are similar to Pacific oceanic basalts currently found in off-axis seamounts and associated with LILE-rich mantle plumes. The REE, trace element, and Pb data indicate that the group 1 (OIT) and group 2 (alkalic) lavas of the Stonyford complex were derived from a heterogeneous mantle source with at least two components: (1) depleted MORB-asthenosphere and (2) an enriched OIB-like component similar to the present day Easter Island source. The group 3 (high-Al, low-Ti) lavas resemble second stage melts of MORB asthenosphere which form by melting plagioclase lherzolite at low pressures. These lavas also resemble high-Al island arc basalts. The trace element and Pb systematics show a OIB influence, which overprints generally depleted trace element characteristics. Sr and Nd isotopic determinations are inprogress.
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View looking NW at the Stonyford Volcanic Complex -- a remnant of the Coast Range Ophiolite in northern California. Low hills in foreground are serpentinite melange, mountains in background are Franciscan assemblage. |
The SFVC crops out as four large tectonic blocks (up to 5 x 3 km in areal extent) within or overlying a sheared serpentinite-matrix melange. Structurally below the largest blocks of SFVC are dismembered remnants of CRO plutonic and volcanic rock, including dunite, wehrlite, clinopyroxenite, gabbro, diorite, quartz diorite, and keratophyre pillow lava. Quartz diorite also occurs as dikes within melange blocks of isotropic gabbro. At other locations in the melange, tectonic blocks include unmetamorphosed volcanogenic sandstones (correlative with the Crowfoot Point breccia near Elder Creek), foliated metasediments (possibly correlative with the Galice formation), and pale green metavolcanic rocks.
40Ar/39Ar plateau ages for basalt glasses
from four localities within the complex show that they were erupted over
a short period of time, ranging from 163.0 ±0.8 to 164.8 ±0.6
Ma. U/Pb zircon ages for CRO diorites in the underlying melange are 166
to 172 Ma. This coincidence in ages, coupled with the occurrence of arc-like
high-Al, low-Ti basalts and the structural position of the SFVC overlying
dismembered CRO plutonics in the serpentinite melange, imply that formation
of the complex may have occurred in the upper plate of the CRO "arc", probably
in response to collision of the subduction zone with a spreading center.
This event, which has been documented in the Elder Creek ophiolite (above)
and in the Leona Rhyolite (Berkeley Hills), may have ended ophiolite formation
during the consequent change in plate motions.
Cuesta Ridge: The Cuesta Ridge ophiolite in central California is separated from the main ophiolite belt along the western margin of the Great Valley by the Salinia block and two major fault systems (San Andreas, Sur-Nacimiento). Layered gabbro is rare, but isotropic gabbro and tonalite are common, and comprise almost all of the sheeted sill complex at the east end of Cuesta Ridge. Volcanic rocks include tholeiitic basalts, calc-alkaline dacites, and boninites. The entire section is overlain by a radiolarian chert horizon that has been tectonically thickened by thrust faults. The ophiolite and its cover of late Cretaceous Toro formation (Great Valley equivalent) and lower Tertiary sandstones lies in thrust contact on top of the Franciscan complex and its non-conformable cover of Miocene Monterey formation and Pliocene sandstone. The thrust contact is marked by a thick unit of breccia and megabreccia that includes clasts of Monterey formation in the dominant peridotite breccia. These relations require that thrusting was post-early Pliocene in age, coincident with uplift and deformation throughout the Coast Ranges.
Lithologies in the Cuesta Ridge ophiolite show that it is a fore-arc or "supra-subduction zone" assemblage that formed during middle Jurassic convergence along the western margin of North America -- although its original site of formation may have been much farther to the south or north.
Publications Resulting From This Project
Shervais, J.W., Zoglman-Schuman, M.M., and Hanan , B.B., 2005, The Stonyford Volcanic Complex: A Forearc Seamount in the Northern California Coast Ranges , Journal of Petrology, v. 46 (10), 2091-2128.
Shervais, J.W., Murchey, B., Kimbrough, D.L., Renne, P., and Hanan, B.B., 2005, Radioisotopic and Biostratigraphic Age Relations in the Coast Range Ophiolite, Northern California: Implications for the Tectonic Evolution of the Western Cordillera, Geological Society of America Bulletin, v. 117, no 5/6, p.633-653.
Shervais, J.W., Kolesar, P., and Andreasen, K., 2005, Field and Chemical Study of Serepentinization – Stonyford, California: Chemical Fluxes and Mass Balance, International Geology Review, v 47, 1-26.
Shervais, J.W., Kimbrough, D.L., Renne, P. Murchey, B., and Hanan, B.B., 2004, Multi-stage Origin of the Coast Range Ophiolite, California and Oregon: Implications for the Life Cycle of Supra-subduction Zone Ophiolites. International Geology Review, v. 46, 289-315.
Shervais, J.W., 2001, Birth, Death, and Resurrection: The Life Cycle of Suprasubduction Zone Ophiolites, Geochemistry, Geophysics, Geosystems, vol. 2, (Paper number 2000GC000080), 20,925 words, 8 figures, 3 tables.
Shervais, J.W., 1990, Island arc and ocean crust ophiolites: contrasts in the petrology, geochemistry, and tectonic style of ophiolite assemblages in the California coast ranges. Ophiolites: Oceanic Crustal Analogues, Malpas, Moores, Panayiotou, and Xenophontos (eds.), The Geological Survey Department, Nicosia, Cyprus, 507-520.
Shervais, J.W. and Hanan, B.B., 1989, Jurassic volcanic glass from the Stonyford volcanic complex, Franciscan assemblage, northern California coast ranges, Geology, 17, 510-514.
