INSIGHT INTO FRANCISCAN MéLANGE DEVELOPMENT FROM SEDIMENTARY BRECCIAS, FIELD RELATIONS, AND BLOCK TYPES

Franciscan Complex mélanges have a wide range of structural associations, matrix, and block population characteristics. All have a sheared matrix, most commonly shale, but serpentinite matrix occurs, as does mixed serpentinite-shale matrix. Thicknesses vary from meters to kilometers, and the structural settings include (1) boundaries between coherent units, (2) between imbricate sheets within coherent units, and (3) possibly cutting the structural grain of coherent units. Many mélange blocks appear similar to coherent rocks bounding the mélange, but exotic blocks are common. Blocks are commonly isofacial with the bounding coherent rocks and the matrix, but blocks of higher metamorphic grade than either occur. In category 1 above, the youngest elements present in any given mélange appear to young structurally downward with structural level within the Franciscan. Category 1 mélanges occupy shear zones that have accommodated many km of movement based on the lack of correlation between bounding coherent units and, in some cases, multi-km differences in burial depth between these units. Given that even category 2 mélanges are along strike with thrust imbrications in coherent units, it is little wonder that some have advocated an entirely tectonic origin for mélanges, whereas others have argued for earlier olistostromal deposition, followed by tectonic strain. In exposures in the eastern San Francisco Bay area, blueschist facies sedimentary breccias include those with shale matrix (most common) with subordinate sandstone matrix and mafic breccias. These breccias range from slightly sheared to strongly deformed ones wherein the shale matrix types are similar in appearance to typical shale matrix mélange. Although most of the clasts in the breccias appear of similar metamorphic grade, a few higher-grade clasts are found. At a locality in Sonoma County, a breccia is found on one side of a blueschist block in serpentinite-matrix mélange. Growth of some blueschist facies minerals postdates breccia development, and the clasts are composed of mainly blueschist, similar to the attached block, and some serpentinite. This breccia exhibits little shearing. The occurrence of these breccias suggest that at least some Franciscan mélanges had an early olistostromal mixing stage, followed by tectonism.