MIND THE MATRIX: FIELD RELATIONSHIPS ARE CRUCIAL FOR INTERPRETING MÉLANGES

Many interpret the mode of creation and mixing of blocks into the matrix of mélanges on the basis of studies of blocks. Data from such studies do not distinguish between the two primary mechanisms of block creation and mixing which are (1) the creation and mixing of blocks by faulting and deformation (tectonic mélange) or (2) the creation of blocks by exhumation and surface erosion followed by mixing into matrix by submarine landsliding (sedimentary mélange) prior to subsequent deformation. For example, some interpret a wide range of peak metamorphic conditions of blocks-in-mélange to indicate tectonic mixing, whereas others view the same data as evidence of sedimentary mixing. The matrix of mélanges receives far less attention, partly because exposures of it are comparatively rare, but such exposures may provide more definitive information on mélange-forming mechanisms than blocks, because the contacts between blocks and matrix can be examined across a range of scales in the field and petrographically. Key field relationships include preservation of clastic sedimentary block-matrix textures and sedimentary basal contacts of mélanges, variably overprinted with later deformation, contrasted to relationships showing creation and mixing of blocks by progressive deformation of formerly intact/coherent (not block-in-matrix) rock assemblages. Specific relationships include relative time indicators showing the timing of deformation relative to block formation and mixing, graded bedding, the presence or absence of matrix deformation within blocks, geometry of matrix fabric compared to block borders, and the geometry of truncation of internal block fabrics along block boundaries with matrix. Some relationships visible in outcrops are more clearly viewed in matrix at petrographic scales, but collection of samples and making of thin sections of matrix is commonly difficult.