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Subduction zones represent steady-state convergence. Because subduction involves movement of material downward, it is difficult to observe the products of subduction in the present-day world. During arc-continent and continent-continent collision, units deformed in subduction zones may be uplifted and exposed. Significant effort has gone into identifying subduction zones in ancient orogens, because they have the potential for telling us about pre-collision geometries. We examine modern subduction zones first and then look at their possible ancient counterparts.
Notice that some arc regions are extending overall. However, the deformation in the accretionary prism always seems to involve at least local shortening. This will be our main focus.
The deep structure of subduction zones comes from theory and from earthquake studies.
The thermal structure of a subduction zone can be modelled based on what we know about the thermal conductivity of rocks and the temperature of the asthensosphere.
The accretionary wedge or accretionary prism is essentially a subaqueous zone of thrusting. However many other processes are involved in its structure.
Materials entering the accretionary wedge include:
Accretionary wedges have been analysed dynamically using the same critical wedge methods as are used in thrust belts.
Our best knowledge of modern accretionary wedges is from trenches where the fill is thick turbidite sediments.
In these trenches we see predominatly thrust faults that are listric and flatten into a decollement horizon that is often (not always) close to the top of the pelagic sediments.
At greater depth, material may be added at the base of the accretionary prism too, a process called tectonic underplating.
Example: Southern uplands of Scotland is located in the Appalachian-Caledonide orogen
Some accretionary wedges show few reflectors on seismic profiles and may contain chaotically deformed sediments known as broken formation or mélange similar to those known from ancient orogens
The term mélange was coined in 1919 to describe a unit of rocks in Wales, with large blocks (up to tens of metres diameter) supported in a matrix of slate: the Gwna mélange.
Mélange is defined on the basis of the following criteria
Some units described as mélange lack truly "exotic" lithologies. Some structural geologists use the term "broken formation" for such units. Another term is bimrock: short for "block-in-matrix" rock
In the mid-20th century, it was noted in the Alps and other orogens that the internal zones of orogens contained:
This association became known as the Steinmann trinity, after the geologist who pointed this out.
In California a unit called the Franciscan Group or Complex was also identified as a mélange. It occupies a large area of California east of the San Andreas fault. Blocks include blueschists. The mélange dates from a time when plate reconstructions now show there was subduction of the Farallon Plate along the continental margin.
Hence mélanges became associated with deformation in trenches and subduction zones.
Many mélanges have been surrounded by controversy. Essentially there are three main structural mechanisms that have been proposed for the fragmentation that characterizes mélanges.
This was the original model proposed by Greenly (1919) in Wales. It was suggested that mélange forms as a sort of giant fault-breccia in a thrust zone or subduction zone.
Some features that support this type of origin:
Mélanges interpreted to have formed by tectonic processes are sometimes called tectonosomes.
An olistostrome is an ancient sedimentary deposit consisting of diverse blocks immersed in a fine-grained matrix. The term comes from Greek words meaning 'slide-layer'. An olistolith is a block that slid. Most ancient olistostromes were probably debris flows (a type of sediment gravity flow).
Slope processes do not usually produce a strong tectonic fabric. However, in a tectonically active area, a debris flow or olistostrome may easily be overprinted with a cleavage or other tectonic fabric
Features favouring a sedimentary origin:
Materials in accretionary terrains subject to diapirism include
Examples of shale and serpentinite diapirism, and mud volcanoes, are known in modern arc environments. Features favouring a diapiric origin for a mélange include:
In practice, many of these features are difficult to establish unless there is exceptional exposure.
Nonetheless, mélanges are often found in convergent tectonic environment, in association with blueschists. Where blueschists are present, either as blocks or as a metamorphic overprint, a subduction environment is highly likely