GRADATION BETWEEN MéLANGES AND COHERENT UNITS: INSIGHT INTO CONVERGENT PLATE MARGIN TECTONICS

Block-in-matrix rock units known as mélanges form a fundamental component of the rock record of paleo subduction zones. Traditionally geologists have separated block-in-matrix mélange units from non block-in-matrix “coherent” units, but recent field observations raise serious questions about the significance of this separation. The position of some mélanges that separate coherent (non block-in-matrix) nappes within subduction complexes suggests that they collectively accommodate hundreds to thousands of km of subduction slip. Field and petrographic relationships in the Franciscan Complex of California, however, show a complete gradation between undeformed sedimentary breccia and conglomerate and strongly foliated sandstone, shale, and serpentinite matrix mélanges, with the same block types as present as clasts in the breccia and conglomerates. This indicates a sedimentary origin for the block-in-matrix fabric and introduction of exotic blocks in the mélanges. These and other field relationships preclude large displacement within these mélanges and indicate localization of megaslip in narrow (20 m thick or less) fault zones on the upper contacts of the mélanges where fault rocks are found. Examination of clast/block and matrix relationships shows that many blocks underwent subduction to blueschist facies or greater depths twice, followed by exhumation to the surface, and some blocks may record three such cycles. Sedimentary serpentinite occurs as multi-km scale mélange units, as well as blocks and clasts with Franciscan sandstones. Franciscan sandstones contain more recycled Franciscan material than previously assumed. The recycled nature of mélanges and clastic material is consistent with recent detrital zircon ages showing reworking of many Franciscan megafossil localities. Age relationships in the Franciscan mélanges, combined with the field and petrographic a long period of non-accretion following subduction initiation, and rare preservation of structurally thin, coherent high-grade nappes suggests minimal net tectonic erosion associated with this period.