Abstract

In northern California, the central belt of the Franciscan subduction complex of late Mesozoic to early Tertiary age is largely a zone of chaotically mixed pelitic-matrix melange. The melange contains a diverse assemblage of rocks of various sizes and degrees; of metamorphism. The most abundant rock types are graywacke and greenstone, but the exotics such as blueschist and eclogite are the most distinctive rocks in the belt. The melange belt is traceable for hundreds of kilometres along strike and is up to tens of kilometres wide. Elongate fragments of most rock types show pinch and swell structure that grades into boudinage. This suggests they behaved as relatively rigid bodies in a ductilely deforming matrix. The general planar preferred orientation of elongate blocks, development of foliation in the matrix, wide separation of blocks, and chaotic mixing indicate that large amounts of flow occurred during melange formation.

The major mineralogy of the matrix is quartz + albite + chlorite + white mica and rarely kaolinite. Minor amounts of pumpellyite and lawsonite are present. Comparison with burial metamorphic sequences and other blueschist terranes indicates that most of the blocks and matrix are compatible with conditions of metamorphism in the range of 100 < T < 250 °C and 2 < P < 8 kb.

A model is proposed in which the pelitic matrix melanges of the Franciscan represent zones where flow, driven by the movement of the descending plate, occurred in the accreted sediment pile. When flow occurs in a low-angle corner, a forced convection drives material back to the surface and transports blocks of blueschist and eclogite upward. A varied assemblage develops because fragments of various lithologies and grades of metamorphism are plucked from many points along the walls of the melange wedge and added to subducted material. Laminar flow causes particles to be dispersed. Most large clasts are broken into smaller, rounded fragments by boudinaging as the melange turns the corner to flow back towards the surface. Mixing is enhanced because differences in block size and density cause differential settling of large blocks within the nonuniform velocity field. Oblique convergence cause;s blocks to take helical paths and may explain the dispersal of blocks such as eclogite along the length of the Franciscan from a few localized sources. The thermal regime in a mature flow melange is such that the downward and upward P-T paths of contained material almost coincide. This accounts for the retrograde blueschist facies metamorphism of the high-grade blocks such as eclogite.

The Franciscan melange belt was probably initially produced by the subduction and deformation of a thick continental margin sequence with a high shale/sandstone ratio (the “Knoxville”). Similar chaotic zones that do not contain exotic blocks in other subduction complexes, may have originated as small flow melanges that developed in wide-angle wedges that did not go to great depth.

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