Published:July 01, 2008
The composite California Coast Range ophiolite consists of remnants of Middle Jurassic oceanic lithosphere, a Late Jurassic deep-sea volcanopelagic sediment cover, and Late Jurassic intrusive sheets that invade the ophiolite and volcano-pelagic succession. The dismembered Middle Jurassic Coast Range ophiolite remnants (161–168 Ma) were parts of the axial sequence of an oceanic spreading center that consisted of basaltic submarine lava, subvolcanic intrusive sheets, and gabbro, and coeval but off-axis upper lava, dunite-wehrlite mantle transition zone, peridotite restite, and dikes rooted in the mantle transition zone that fed the upper lava. Hydrothermal metamorphism overprints the lavas, subvolcanic sheets, and part of the gabbro. The nearly complete magmatic pseudostratigraphy with minimal syngenetic internal deformation accords with a “hot” thermal structure and robust magma budget, indicative of fast spreading.
Upper Jurassic volcanopelagic strata composed of tuffaceous radiolarian mud-stone and chert (volcanopelagic distal facies) overlie the ophiolite lava disconformably and grade up locally into arc-derived deep-marine volcaniclastics (volcanopelagic proximal facies). An ophiolitic breccia unit at northern Coast Range ophiolite localities caps shallow to deep levels of fault-disrupted Middle Jurassic oceanic crust. The Late Jurassic igneous rocks (ca. 152–144 Ma) are mafic to felsic subvolcanic intrusive sheets that invade the Middle Jurassic ophiolite, its Late Jurassic volcanopelagic cover, and locally the ophiolitic breccia unit. Hydrothermal metamorphism of volcanopelagic beds and underlying ophiolite meta-igneous rocks accompanied the Late Jurassic deep-sea magmatic events.
The Middle Jurassic ophiolite formed at a spreading ocean ridge (inferred from its Jurassic plate stratigraphy). Intralava sediment and thin volcanopelagic strata atop the Coast Range ophiolite lava record an 11–16 m.y. progression from an open-ocean setting to the distant submarine apron of an active volcanic arc, i.e., the sediments accumulated upon oceanic lithosphere being drawn progressively closer to a subduction zone in front of an ocean-facing arc. Trace-element signatures of Coast Range ophiolite lavas that purportedly link ocean-crust formation to a suprasubduction-zone setting were influenced also by processes controlled by upper-mantle dynamics, especially the mode and depth of melt extraction. The polygenetic geochemical evidence does not decisively determine tectonic setting.
Paleomagnetic and biostratigraphic evidence constrains the paleolatitudes of Coast Range ophiolite magmatism and volcanopelagic sedimentation. Primary remanent magnetism in ophiolite lavas at Point Sal and Llanada Coast Range ophiolite remnants records eruption within a few degrees of the Middle Jurassic paleoequator. The volcanopelagic succession at Coast Range ophiolite remnants consistently shows upward progression from Central Tethyan to Southern Boreal radiolarian assemblages, recording Late Jurassic northward plate motion from the warm-water paleo-equatorial realm. Northward seafloor spreading was interrupted by local Late Jurassic rift propagation through the Middle Jurassic oceanic lithosphere. Coast Range ophiolite crust with volcanopelagic soft-sediment cover that lay in the path of propagating rifts hosted rifting-related magmatic intrusions and hydrothermal metamorphism. The advancing broad deformation zone between propagating and failing rifts left paths of pervasive crustal deformation marked now by fault-disrupted ophiolite covered by depression-filling ophiolitic breccias, found at northern Coast Range ophiolite remnants. Coast Range ophiolite lithosphere that lay outside the propagating and failed rift zones lacks those features. The rift-related magmatism and crustal deformation took place at ephemeral spreading-center offsets along a transform fault.
Late Jurassic seafloor spreading carried Middle Jurassic oceanic lithosphere northeastward toward a subduction zone in front of the Middle to Late Jurassic arc that fringed southwestern North America. Termination of oblique subduction during the late Kimmeridgian, replaced by dextral transform faulting, left a Coast Range ophiolite plate segment stranded in front (west) of the trench. The trench was then filled and locally bridged by the arc’s submarine sediment apron by the latest Jurassic, allowing coarse volcaniclastic (proximal volcanopelagic) deposits to lap onto earlier, plate-transported tuffaceous radiolarian chert (distal volcanopelagic) deposits. Deep-marine terrigenous muds and sands from southwestern Cordilleran sources then buried the stranded Coast Range ophiolite–volcanopelagic–ophiolitic breccia unit oceanic crust during latest Jurassic northward dextral displacement, which proceeded offshore. Those basal Great Valley Group strata record lower continental-slope and basin-plain marine sedimentation on Jurassic oceanic basement, i.e., the Coast Range ophiolite and adjacent Franciscan oceanic lithosphere (Coast Range serpentinite belt). Forearc basin deposition did not begin until the mid–Early Cretaceous, when the inception of outboard Franciscan subduction lifted and tilted the Coast Range ophiolite–volcanopelagic–ophiolitic breccia unit–basal Great Valley Group succession and Coast Range serpentinite belt to form a basin-bounding forearc ridge. Thereafter, Cretaceous Franciscan subduction and accretionary wedge growth operated in front (west) of the submerged ridge, and Great Valley Group forearc basin terrigenous sediments accumulated behind it.