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mélange Mélanges are complex, typically chaotic tectonic mixtures of sedimentary, volcanic, and other types of rocks in a highly sheared sedimentary or serpentinitic matrix. Mélanges show inclusions of material of widely diverse origins at many different scales, showing that mélanges are fractal systems, with the same patterns appearing at multiple scales of observation. Some mélanges may be sedimentary in origin, formed by the slumping of sedimentary sequences down marine escarpments. These mélanges are more aptly termed olistostromes. Tectonic mélanges are formed by structural mixing between widely different units, typically in subduction zone settings.
Tectonic mélanges are one of the hallmarks of convergent margins, yet understanding their genesis and relationships of specific structures to plate kinematic parameters has proven elusive because of the complex and seemingly chaotic nature of these units. Many field workers regard mélanges as too deformed to yield useful information, and simply map the distribution of mélange-type rocks without further investigation. Other workers map clasts and matrix types, search for fossils or metamorphic index minerals in the mélange, and assess the origin and original nature of the highly disturbed rocks. Recent studies have made progress in being able to relate some of the structural features in mélanges to the kinematics of the shearing and plate motion directions responsible for the deformation at plate boundaries.
one of the most persistent questions raised in mélange studies relates to the relative roles of soft-sediment versus tectonic processes of disruption and mixing. Many mélanges have been interpreted as deformed olistostromes or giant submarine landslide deposits, whereas other models attribute disruption entirely to tectonic or diapiric processes. Detailed structural studies have the potential to differentiate between these three end-member models, in that soft-
Mélange formed during Taconian Orogeny in the Cambrian Period at Lobster Cove Head, Gros Morne National Park, Newfoundland, Canada (François Gohier/Photo Researchers, Inc.)
sedimentary and some diapiric processes will produce clasts, which may then be subjected to later strains, whereas purely tectonic disruption will have a strain history beginning with continuous or semicontinuous layers that become extended parallel to initial layering. Detailed field, kinematic, and metamorphic studies may help further differentiate between mélanges of accretionary tectonic versus diapiric origin. Structural observations aimed at these questions should be completed at regional, outcrop, and hand-sample scales.
Analysis of deformational fabrics in tectonic mélange may also yield information about the kinematics of past plate interactions. Asymmetric fabrics generated during early stages of the mélange-forming process may relate to plate kinematic parameters such as the slip vector directions within an accre-tionary wedge setting. This information is useful for reconstructing the kinematic history of plate interactions along ancient plate boundaries, or how convergence was partitioned into belts of head-on and margin-parallel slip during oblique subduction.
See also accretionary wedge; convergent plate margin processes; structural geology.
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