An accretionary wedge is a massive, wedge-shaped accumulation of deformed sediment and rock that forms where tectonic plates converge. This geological feature results from a continuous process where material is scraped from a descending oceanic plate and plastered onto the edge of the overlying plate. The wedge represents a significant buildup of oceanic debris and is a major mechanism for adding material to the Earth’s crust.
The Tectonic Environment
Accretionary wedges form exclusively at convergent plate boundaries where oceanic lithosphere sinks beneath another plate, a process known as subduction. This setting provides the necessary compressional forces and the steady supply of sediment required to build the massive structure. The wedge is situated on the overriding plate, positioned between the deep-sea trench and the volcanic arc that typically forms further inland.
The oceanic trench marks the surface expression of the plate boundary and acts as the initial collection point for the sediment that will ultimately form the wedge. This trench is where the down-going oceanic plate begins its descent, dragging with it a layer of marine sediments. The existence of a substantial blanket of incoming sediment on the subducting plate is a prerequisite for a large, well-developed accretionary wedge.
The thickness and type of sediment on the subducting plate directly influence the resulting accretionary wedge’s characteristics. For instance, a plate carrying a thick layer of soft, fine-grained sediment often results in a broad, shallowly sloped wedge. Conversely, a thinner sediment layer or one composed of coarser, more cohesive material can lead to a narrower wedge with a steeper surface angle.
The Mechanics of Formation
The creation of an accretionary wedge is a mechanical process driven by the unrelenting convergence of the two tectonic plates. As the oceanic plate descends, the overlying plate acts like a bulldozer, scraping off the upper layers of sediment and rock. This scraped material is then compressed, folded, and faulted into the growing wedge structure.
The entire wedge mass slides along a major structural boundary called the décollement, which is the main detachment fault separating the deformed wedge material from the undeformed oceanic crust beneath it. Above this décollement, the sediment is systematically stacked through imbricate thrust faulting. These faults push older, more inland sections of the wedge upward and over younger material closer to the trench, creating the characteristic triangular cross-section.
The physical shape of the wedge is governed by the critical taper concept. This taper represents an equilibrium angle where the internal strength of the wedge, the angle of the basal décollement, and the external tectonic forces are balanced. The wedge constantly strives to maintain this stable geometry, deforming internally to adjust its slope if it becomes too shallow or too steep. Fluid pressure within the sediments is a major factor influencing this taper, as high pore fluid pressure weakens the material and results in a shallower wedge angle.
Internal Structure and Composition
The fully formed accretionary wedge exhibits a distinct internal architecture that reflects its compressional history. The outermost edge, facing the trench, is known as the wedge “toe” and consists of the youngest, most recently scraped-off material. From the toe, the wedge progressively thickens and rises toward the overriding plate, reaching its maximum elevation at a topographic high known as the forearc ridge.
Landward of this high, a depression often forms, called the forearc basin. This basin is a zone of relatively lower deformation that traps sediments eroded from the volcanic arc and the inner parts of the wedge.
The material making up the wedge is a chaotic mix of rock types. It primarily consists of marine sediments like turbidites and pelagic muds, along with fragments of oceanic crust, such as basalts from seamounts.
The intense compression and burial within the wedge cause a significant process of fluid expulsion, or dewatering. As the sediments are squeezed, water is forced out, often migrating upward along the numerous thrust faults. This fluid movement influences the mechanical strength of the wedge, and the presence of high-pressure, low-temperature minerals in some ancient wedges indicates the severe conditions of deformation at depth.