Abstract

In the lower Kings River area, rocks older than the Sierra Nevada batholith include a disrupted and metamorphosed ophiolite. The Kings River ophiolite consists of tectonically emplaced slabs as much as 20 km long that are separated by serpentinite-matrix melange zones and by crosscutting plutons of the batholith. Within the slabs, various segments of the original ophiolite section are preserved. From the base upward, the reconstructed section consists of (1) a harzburgite zone (more than 4 km thick), (2) a transition zone between ultramafic and mafic tectonites and cumulates (2.5 km thick), (3) a gabbro zone with cumulates (2 km thick), (4) a mafic-dike zone (0.7 km thick), and (5) a pillow-basalt zone (1.8 km thick). The pillow basalt is overlain by at least 20 m of metalliferous radiolarian chert. After tectonic mixing and emplacement into the Sierran terrane, the ophiolite was metamorphosed to the hornblende-hornfels facies by the batholith.

The Kings River ophiolite is interpreted as a disrupted fragment of oceanic crust and upper mantle. Isotopic ages along with structural and petrographic data indicate that the igneous part of the section originated in latest Paleozoic or possibly earliest Mesozoic time. Intense deformation of the ophiolite began at its point of origin. Deep levels of the ophiolite were penetratively mylonitized, intermediate levels were deformed by ductile faulting, and upper levels were deformed by brittle shear. As deformation and disruption progressed, serpentinization of the ophiolite's lower levels also progressed. Serpentinization and differential tectonic movements were concentrated along zones that became serpentinite-matrix melange. The inclusion of only ophiolite-assemblage rocks in the melange zones indicates that the melange mixing was oceanic.

The ophiolite originated and began its deformational history at a mid-ocean spreading center where that center was cut by a transverse fracture zone. The progression from brittle to ductile behavior with stratigraphic depth during initial deformation is attributed to a steep thermal gradient, typical of an ocean ridge. Progressive deformation and disruption and, ultimately, ophiolite emplacement occurred along a wrench zone that cut obliquely into western North America and truncated earlier-formed tectonic elements. The wrench zone is believed to have been an extension of the mid-ocean fracture zone that widened and became more complex with time. During the later stages of wrench movement, a component of eastward underthrusting commenced. Disrupted ocean floor of the wrench zone was left as an accretionary hanging wall of a newly formed subduction zone. A Jurassic volcanic arc was built across the already weakened oceanic basement as it underwent transverse shortening and continued wrench movements in response to oblique subduction. Final truncation of North American tectonic elements and emplacement of the ophiolite probably overlapped in time with arc activity. Similar deformation and truncation zones are a common feature in modern subduction-arc complexes of the circum-Pacific.

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