Complex, typically chaotic tectonic mixtures of
sedimentary, volcanic, and other types of rocks, typically in a
highly sheared sedimentary or serpentinitic matrix, are
known as mélanges. Mélanges must be mappable units, and
most show inclusions of material of widely diverse origins at
many different scales, suggesting that mélanges are fractal
systems. 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 the 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
proved 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, 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-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 semi-continuous layers which
become extended parallel to initial layering. Detailed field,
kinematic, and metamorphic studies may be able to further
differentiate between mélanges of accretionary tectonic versus
diapiric origin. Structural observations aimed at these questions
should be completed at regional, outcrop, and handsample
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 accretionary 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 CONVERGENT PLATE MARGIN PROCESSES.
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