WHAT IS AN EXOTIC BLOCK? IMPORTANCE IN EVALUATING ORIGINS OF MÉLANGE

Distinguishing a block-in-mélange as "exotic" or "foreign" to the matrix versus "cogenetic" (part of broken formation) aids in evaluating mélange origins and convergent plate margin processes. However, this distinction is not easily made. For example, chert and/or basalt blocks may share a common ocean plate stratigraphy with their shale and/or sandstone matrix ("cogenetic") or not ("exotic"). Serpentinite blocks in a sandstone and/or shale matrix are probably exotic, but the serpentinite may also be cogenetic, derived from ocean plate stratigraphy that included serpentinite (such as oceanic crust formed at oceanic core complexes or low magma supply ridge segments). Blocks of the same metamorphic grade as the matrix ("isofacial") may be cogenetic or exotic, but a block of higher metamorphic grade than matrix ("high-grade") is unambiguously exotic. Introduction of higher-grade metamorphic blocks into lower grade matrix cannot be easily explained by large-scale deformation or displacement because of the absence of high-grade equivalents of the matrix lithology; amphibolite and eclogite blocks in shale and sandstone matrix mélanges of the Franciscan Complex are good examples. Many exposures of mélange with such blocks in the Franciscan Complex show evidence of sedimentary origins with a gradation between undeformed sedimentary breccia and conglomerate with high-grade metamorphic clasts/blocks, and deformed mélange with the same block populations. Accordingly, introduction of high-grade blocks into matrix appears to have resulted from submarine sliding. An exotic origin can be recognized for many isofacial blocks if field relationships analogous to those seen for high-grade blocks are observed. Progressive deformation of parent strata results in formation of cogenetic isofacial blocks, such as sandstone blocks in shale matrix formed by deformation of interbedded sandstone and shale, or chert and basalt blocks in sandstone or shale matrix, or chert blocks in basalt matrix, formed by progressive deformation of ocean plate stratigraphy. Thus, "tectonic mélanges" have cogenetic blocks and are the equivalent of "broken formation" whereas exotic blocks suggest a sedimentary origin for the mélange.