Abstract

The geology of a strip of 15′ quadrangles across the Franciscan Complex in the Coast Ranges of northern California is the basis for interpretation of the significance of potassium-argon and fossil ages of glaucophane schists, ophiolitic materials, and associated sedimentary rocks. Whole-rock and mineral separates of Franciscan rocks were analyzed, and 69 potassium-argon ages were obtained. White micas and actinolites from blueschist blocks in mélange give apparent ages clustering about 142 and 153 m.y., but they probably represent a continuum. Blue amphibole ages from blocks range from 98 to 151 m.y.; the younger ages are from the eastern portion of the central mélange belt. Published U-Pb ages from the ophiolite belt give crystallization ages from 153 to 165 m.y.; K-Ar ages extend from 143 to 166 m.y. Whole-rock ages from the South Fork Mountain Schist range from 113 to 158 m.y. but cluster around 124 m.y. The association of older K-Ar ages with more coarsely crystalline schist suggests that metamorphism occurred prior to 124 m.y. ago.

Fossil evidence indicates that the basal sediments of the Great Valley Sequence were deposited upon the disrupted Coast Range Ophiolite within 5 to 10 m.y. of the oldest crystallization ages for ophiolite. Fossils within mélange units, broken formations, and unsubducted trench-slope deposits provide evidence for mélange development by the Early Cretaceous, with youngest mélange units generally lying to the west.

Available chronologic evidence is compatible with a tectonic model involving mélange formation in a restricted flow channel beneath the accretionary wedge of the subduction complex. Upflow of mélange returned blocks of coarsely crystalline metamorphic rocks that were accreted during the initial stages of subduction when the hanging wall was still hot. The viability of this and other proposed tectonic models can be assessed by continued careful field documentation of the nature of contacts between key lithotectonic units and by detailed application of temperature-sensitive geochronologic tools.

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