At a convergent plate boundary, an accretionary wedge can form above the subduction zone. Fig. 5.3.1 illustrates this using the example of the convergent plate margin off the island of Java in Indonesia. Here, sediment material previously deposited on the subducting plate during plate drift is scraped off and transferred to the overriding upper plate. It is therefore not subducted, but rather incorporated into the accretionary wedge of the overriding plate. The term “wedge” derives from the wedge-shaped structure that this formation exhibits in a cross-section perpendicular to the subduction zone.

There are two different types of accretion: sedimentary and magmatic accretion.

Sedimentary accretion takes place in the uppermost part of the subduction zone, directly at the plate boundary and on the overriding plate. Here, sedimentary material deposited on the subducting plate is scraped off tectonically and transferred to the overriding plate. This forms the wedge-shaped structure which is called an accretionary wedge.

Figure 5.3.3 illustrates how the sediment deposited on the incoming plate is scraped off and transferred to the overriding plate. The thrust surface where the sediment layers are sheared off is called the décollement (from the French décoller = to detach). Here, the sediment is detached from the subducting plate.

The scraping of sediment at the plate boundary is often compared to a carpenter´s plane. In this analogy, the shavings represent the stacked sediment layers. The accretionary wedge is built up from the bottom, it grows from below.

The sediments deposited on the oceanic crust collide with the overriding plate as they begin to subduct into the subduction zone. This collision results in compression, which eventually leads to the formation of a shear plane where the sediment layers are thrust over one another (Fig. 5.3.4). The shear plane is the décollement shown in Fig. 5.3.3. This shear plane also forms the plate boundary between the subducting and overriding plates, because everything scraped off here is transferred to the overriding plate and thus to the other side of the plate boundary.

The process repeats itself several times, and the initially formed tectonic slices, also known as duplexes, migrate further and further back into the accretionary wedge, with the inactive shear plane gradually becoming steeper. The uppermost and furthest back duplexes, which can also be seen as tectonic nappes, are the first to be scraped away and therefore the oldest structures.

Similar structures to those found in the frontal thrust can also develop in deeper regions of the accretionary wedge. There, too, sediment packages are scraped from the subducting plate and transferred to the overriding plate. This process is called underplating.

A very similar pattern emerges (Fig. 5.3.5). The tectonic structures formed during underplating can also be duplexes. These structures vary greatly in size and characteristics, ranging from a few tens of meters to kilometers in diameter. However, the fundamental principle of material being added from below remains the same in all dimensions.

Example of duplex structures on the meter scale: On the Miura Peninsula on the southern coast of Japan, older tectonic slices of an accretionary wedge have been uplifted and are now exposed by coastal erosion. Here, the internal structures within an accretionary wedge can be studied. The structures shown in Fig. 5.4.6 exhibit duplexes arranged one behind the other like roof tiles. This is referred to as the imbrication of the tectonic slices.

Figure 3.5.7 shows a 3D block diagram of the Nankai accretionary wedge. Moore et al., (2007) interpreted the structures in the seismic profiles. The blue lines are the thrust faults that define the boundaries of the individual duplexes. Here, too, one can see the roof tile-like imbrication of the tectonic slices, only on a different scale as in this case, on the order of kilometers.

Figure 3.5.8 shows a seismic profile from the area of ​​the frontal thrust front in the Nankai accretionary wedge. The imbrication of the individual tectonic slices is particularly evident here. It was at this location that drilling through the décollement was first achieved in 1995. The borehole was drilled down to the oceanic crust. It was carried out with the research drilling vessel JOIDES Resolution as part of the then international research project Ocean Drilling Program (ODP). Later, the ODP became the IODP, initially called the Integrated Ocean Drilling Program, and subsequently renamed the still-active International Ocean Discovery Program.

Figure 3.5.9 shows a balanced cross section performed using a seismic profile in the Nankai accretionary wedge. This analysis pulls apart the overthrust tectonic duplexes, as if straightening a crumpled tablecloth. In this way, it is possible to calculate the degree to which the transported sediment layers were compressed, or shortened, at the subduction zone. For the Nankai accretionary wedge, this is approximately 30% in the frontal 20 km at the plate boundary alone. This shortening is achieved through imbrication, the tectonic stacking process.

Figure 5.3.10 illustrates the development of the Nankai accretionary wedge. It shows how the imbrication builds forward, enlarging the wedge. The most recently sheared slices are the outermost ones, located directly at the plate boundary. Here, the décollement is still quite shallow. Further back, the shear planes become inactive and steeper due to the increasing accumulation of material from below. In the rearward region, older tectonic slices can also be uplifted, as is the case, for example, on the Miura Peninsula in Japan (see Fig. 5.3.6). Additionally, so-called out-of-sequence thrusts can develop in the older parts of the accretionary wedge. This occurs when the pressure becomes too large and the frontal thrust can no longer accommodate all the load.

Furthermore, the shear planes become inactive and become steeper due to the increasing uplift. Beneath the décollement lies sedimentary material that is pulled down to greater depths in the subduction channel within the subduction zone. This demonstrates that not all of the incoming sedimentary material is transferred into the accretionary wedge; a more or less significant proportion always remains, being pulled down into the subduction zone.