SEDIMENTARY COMPARED TO TECTONIC SERPENTINITE MÉLANGES AND BLOCKS OF ONE TYPE OF MÉLANGE IN ANOTHER: COASTAL CALIFORNIA EXAMPLES

Sedimentary serpentinite mélanges of the Franciscan Complex of California contain blocks spanning a wide range of lithologies and metamorphic grades in a variably deformed matrix of serpentinite conglomerate, sandstone, pebbly sandstone, and sedimentary breccia. Internal fabrics within clasts and blocks are abruptly truncated by clast boundaries without drag features. In the more deformed mélanges, areas with least strain and best preservation of sedimentary textures are in the strain shadows adjacent to blocks (or clasts/grains at smaller scales). Serpentinite matrix grades into and is interbedded with siliciclastic matrix or turbidites. These clastic rocks comprise the trench-fill component of ocean plate stratigraphy that is imbricated and deformed across a range of scales. Tectonic serpentinite mélanges have blocks and matrix of the same metamorphic grade. Internal fabrics within clasts or blocks are commonly continuous with the matrix foliation or such fabrics show drag features in relationship to the matrix foliation. Such mélanges grade into and are part of ocean floor imbricates and duplexes composed of metamafic, metachert, and metaultramafic rocks. Within the Franciscan Complex metaultramafic-mafic-metachert imbricates with local block-in-matrix relationships are most commonly preserved within high-grade (amphibolite and eclogite) blocks in serpentinite and siliciclastic sedimentary mélanges. The metaultramafic components of such blocks have been described as blackwall “rinds” assuming they completely encircle the blocks, but such “rinds” make up only part of the block perimeter in addition to forming zones within the blocks. Some blocks have no “rind” but have internal metaultramafic zones. Such imbricate features locally show block-in-matrix (tectonic serpentinite mélange) geometry. Although such high-grade rocks are most common as blocks-in-mélange in the Franciscan, they also occur as rare intact sheets, imbricated at 100s of m scale in the manner observed in the blocks. Serpentinite-rich zones within such sheets probably have features similar to those preserved within the blocks-in-mélange. This type of HP ultramafic-mafic association and associated tectonic serpentinite matrix mélanges resemble those found in localities such as the Western Alps.