MELANGES AND EXOTIC BLOCKS
In strongly deformed rocks, originally continuous stratigraphic units are usually found to be disrupted to a large extent by faulting, boudinage or transposition. In certain deformed rocks, however, the destruction of stratigraphic continuity is a very prominent and pervasive feature and blocks or fragments of more brittle rock types are found dispersed and mixed in a more ductile matrix. The mixing process is sufficiently large-scale so that exotic blocks (originating remote from their present lithic surroundings) are found juxtaposed with blocks and matrix of entirely different lithology or age. Such bodies, where they originate by deformation, are called tectonic melanges. The discontinuity is a very prominent and pervasive feature and blocks or fragments of more brittle rock types are found dispersed and mixed in a more ductile matrix. The mixing process is sufficiently large-scale so that exotic blocks (originating remote from their present lithic surroundings) are found juxtaposed with blocks and matrix of entirely different lithology or age. The term melange was used first by Greenly (1919) for rocks on Anglesey and has since been applied especially to very big areas characterized by this kind of structure.
Melange structure seems to be characteristic of low-grade eugeosynclinal terrains made up dominantly of greywacke and siltstone and containing members of the ophiolite suite (cherty sediments, pillow lavas, gabbros, and ultramafic rocks). Blocks of high-pressure metamorphic rocks may also be present. The matrix between blocks of all these lithologies is typically fine-grained pelitic rock or serpentine. The blocks themselves range in size from microscopic fragments to flat slabs several kilometers in their maximum dimensions. In the Franciscan formation, there is a weak regional structural grain parallel to the outcrop belt of Franciscan rocks, defined by preferred orientation of the long axes of the larger blocks and by preferred orientation of bedding and shear surfaces. The blocks may or may not be internally deformed. Where internal deformation is prominent, as for example in glaucophane schist blocks in the Franciscan formation, it is abruptly truncated by the faults separating the block from the surrounding matrix. The matrix itself is pervasively sheared and often intricately folded. The shears are commonly approximately parallel to bedding and are seen in
Melange structure was attributed by Greenly (1919) to thrust faulting and was seen to be associated locally with imbricate thrust faulting by Bailey and McCallien (1950). Hsu and Ohrbom (1969) compared melange development to very large-scale land-sliding, such as occurred at Brientz, Switzerland, where a section of rock about 200 meters thick by 1.2 km long slid down a 20' slope, with much internal shearing and deformation. Turbidites of relatively local origin with fragments of oceanic crust and mantle, the ophiolite suite, emplaced from the west by large-scale horizontal movement of the sea floor. If this interpretation applies to a given melange, then its deformational history is likely to be long and complex. The ophiolitic components may show structures connected with faulting or other deformation predating their emplacement in the trench. The trench deposits proper may show early structures due to relatively shallow slumping or tectonic deformation. Superposed on such earlier structures, both components of the melange will show whatever structures result from their interaction at the trench. origin. A body consisting mainly of such fragments would properly be classed as a very coarse sedimentary breccia, or as a sedimentary melange or olistostrome,if there is marked mixing of rock types and chaotic structure due to submarine slumping [see Elter and Trevisian (1973)]. Tectonically sheared olistostromes may closely resemble ordinary tectonic melanges
The study of melange structure is still in its infancy. The details of the mixing process remain very obscure, it is hard to distinguish sedimentary from tectonic features, and serious questions can be raised about the origin of the ophiolitic components.