A back-arc basin is an elongated, generally submarine depression located directly behind a volcanic arc in a subduction zone setting. This geological feature is characterized by crustal extension and spreading, which occurs despite being situated within an overall convergent plate boundary. Back-arc basins are zones where the crust of the overriding plate is stretched and thinned, sometimes leading to the formation of new seafloor. They are typically long and narrow, often stretching thousands of kilometers in length while remaining only a few hundred kilometers wide.
The Tectonic Environment
Back-arc basins form at convergent plate boundaries where oceanic crust descends beneath either continental or other oceanic crust (subduction). The point where the oceanic plate begins its descent is marked by a deep oceanic trench, which parallels the volcanic arc.
The subducting plate, or slab, sinks into the Earth’s mantle below the overriding plate. This movement triggers melting in the mantle wedge above the slab, generating magma that rises to form the volcanic arc. This arc separates the main convergent zone from the back-arc region.
The back-arc region is the area of the overriding plate situated landward of the volcanic arc. This arrangement of trench, slab, and arc is necessary for extensional forces to take effect. Not all subduction zones develop these basins; their presence relies on conditions like the age and density of the subducting oceanic plate.
Primary Mechanisms Driving Extension
The primary process responsible for stretching and rifting in the back-arc region is slab rollback, driven by the negative buoyancy of the subducting slab. As the cold, dense oceanic plate sinks into the warmer mantle, the weight of the slab pulls the entire subduction hinge vertically and oceanward. This movement causes the trench to retreat faster than the overriding plate can move forward.
This backward retreat creates an area of reduced pressure in the mantle wedge behind the volcanic arc. The overriding plate is stretched to fill this newly created space. This stretching thins the crust and initiates the rifting process that defines the back-arc basin. Older subducting oceanic crust (typically 55 million years or older) is denser, which often leads to a steeper descent angle and more pronounced rollback.
A second factor contributing to the extension involves the complex circulation of the mantle material, often referred to as the mantle wedge flow. As the slab sinks, it drags the surrounding mantle, inducing a circulation pattern in the mantle wedge situated between the arc and the slab. This flow exerts a shear force, sometimes called “trench suction,” on the base of the overriding plate.
This dynamic mantle flow pulls the overriding plate toward the retreating trench, enhancing the extensional stress caused by slab rollback. While the slab’s negative buoyancy is the main engine, this induced mantle circulation helps to sustain the rifting. The interplay between the slab’s gravitational pull and the mantle’s viscous drag ensures continued stretching.
Resulting Geological Characteristics
The sustained extension in the back-arc region results in a series of distinct geological features that characterize the basin. The most immediate effect is extreme crustal thinning and rifting of the overriding plate. As the crust is pulled apart, it fractures along normal faults, and the central portion drops down, forming the basin structure.
Seafloor Spreading
If extensional forces persist, the crust can thin completely, leading to the formation of new oceanic crust through seafloor spreading. Spreading rates vary significantly; for example, the Mariana Trough spreads slowly (a few centimeters per year), while the Lau Basin spreads much faster (up to 15 centimeters per year). The new crust formed is often basaltic, similar to mid-ocean ridge basalts, but is enriched in magmatic water due to its proximity to the subduction zone’s fluid-rich environment.
High Heat Flow and Volcanism
The thinning crust allows for significantly high heat flow and volcanism within the basin. Magma from the mantle rises closer to the surface through the rifts, creating volcanic ridges and intense hydrothermal activity. This magmatism is responsible for the unique composition of back-arc basin basalts and supports chemosynthetic ecosystems around deep-sea vents.
Sedimentation
The newly formed depression acts as a collection area for sedimentation. Material eroded from the nearby volcanic arc and adjacent continental landmasses is transported and deposited into the basin. These sediments accumulate over time, recording the history of the volcanic arc’s activity and the tectonic evolution of the basin.