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.
[VDPM 17.4 - arc without back-arc basin]
[VDPM 17.16b - arc with back-arc basin and trench rollback]
Notice that some arc regions are extending overall. However, the deformation in the accretionary prism always seems to involve local shortening. This will be our main focus.
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
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.
[VDPM 17.12b] Interpreted seismic profile across a subduction zone from ocean drilling program
At greater depth, material may be added at the base of the accretionary prism too, a process called tectonic underplating.
[VDPM 17.12a] Diagram of accretionary prism with normal faults, underplating, and slumping.
Other seismic profiles have shown up normal faults, suggesting that wedges go through cycles of shortening and extension.
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 (see below).
Chapple, and subsequently Dahlen and co-workers, analysed the deformation of a deforming wedge of brittle material. They looked at the continuous growth of a wedge as material was added at a convergent plate boundary. The analysis is equally applicable to a pile of sand in front of a bulldozer or a pile of snow in front of a snowplow.
They also allowed the base of the wedge to be weak, because of fluid pressure. Internally, they assumed that the material of the wedge followed the Mohr-Coulomb failure criterion, typically illustrated by a Mohr circle construction with a failure envelope. This construction shows that rocks effectively increase in strength as lithostatic pressure rises; hence the rocks at the back end of the wedge are effectively stronger than those at the front.
For a given set of boundary conditions - slope of basal décollement, strength of basal layer, strength of wedge - there is critical angle between the top of the wedge and its base, known as the critical taper. If material is added to the wedge so as to increase the taper above this angle, gravity spreading will reduce it. Conversely, if the wedge extends so that the taper is reduced below the critical value, stress applied to the back edge will shorten it until the critical taper is achieved.
The equation relating all the various quantities is quite complex
surface slope is α
and décollement slope is β
The internal strength (yield stress) of wedge is k
Internal fluid pressure ratio is λi
Décollement fluid pressure ratio λb
Décollement friction μb
Traced to depth, most of what we know about subduction zones comes from theory and from earthquake studies.
It's possible to model the thermal structure of a subduction zone, based on what we know about the thermal conductivity of rocks and the temperature of the asthensosphere.
[VDPM 17.7] subduction zone thermal structure
Deeper in the subduction zone, hydrous minerals become dehydrated.
Ultimately the rocks of the slab and any sediments still attached will enter eclogite facies (characterized by the combination of garnet plus sodic pyroxene in mafic rocks). The expelled water contributes to partial melting in the mantle above the slab, producing the bulk of subduction zone magma.
Deep structure of slab shows varying extension and shortening regimes.
This variability is a result of the varying forces on the plates - in particular
In the mid-20th century, it was noted in the Alps and other orogens that the internal zones of orogens contained strange and unusual combinations of rocks. These included:
This association became known as the Steinmann trinity, after the geologist who pointed this out.
With the advent of plate tectonics, it was realized that this association marks ancient subduction zones.
We will focus on mélange - examples from the Appalachian-Caledonide orogen
The term mélange was coined in 1919 to describe a unit of rocks in Wales, with large blocks (up to 10s of metres diameter) supported in a matrix of slate: the Gwna mélange.
Subsequently identified in the Alpine Mediterranean system, in close association with ultramafic rocks, and with blueschists.
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 very rapidly with deformation in trenches and subduction zones.
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.
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.
It is clear that mélanges form under conditions of low temperature, because the metamorphic grade is low and because some mélanges are dominated by brittle fracturing - both extensional and shear fractures are present.
Brittle fracturing is favoured by high fluid pressures
The effective normal stress on all planes is reduced by an amount equal to the fluid pressure, thereby allowing shear fracture to occur at lower shear stress.
[Slides of Humber Arm Allochthon in W. Newfoundland - an example of a tectonic mélange]
Landslides and debris flows are known to produced fragmentation on land and under water. Numerous terms have been used to describe such deposits, and in some cases different terminology has been used in modern and ancient sediments
A debris flow is a deposit that contains a wide range of grain-sizes, in which the larger clasts are typically not in contact (matrix support) because when the flow was moving, the matrix behaved as a plastic material, with a finite yield-strength, that was able to support blocks.
[Photo of a debris flow]
An olistostrome is a sedimentary deposit consisting of blocks of diverse origin that are immersed in a fine-grained matrix (typically mudstone or serpentinite). The term comes from greek words meaning 'slide-layer'. A related term olistolith means a block that slid. Most ancient olistostromes were probably debris flows.
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:
[Slides of Carmanville mélange, E. Newfoundland; a mélange with a strong sedimentary mixing component]
Materials in accretionary terrains subject to diapirism include
These may form mélanges if strata are disrupted by diapirism. Less work has been done on this type of process than the others, though examples of both shale and serpentinite diapirism 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 trench environment is highly likely
Example: Southern uplands of Scotland is located in the Appalachian-Caledonide orogen
Consists of thick belts of immature sandstone (wacke) and rare conglomerate separated by narrow belts of black shale with planktonic fossils (graptolites), all of Ordovician to Silurian age.
[Cross section - comparison with modern accretionary wedge]
These characteristics show that the belt was deformed progressively, and that parts were deforming while others were still being deposited; these are features expected in an accretionary wedge.