Back-arc basin
Submarine basins formed by extension behind volcanic arcs.
A back-arc basin is a geological basin that occurs at certain convergent plate boundaries. Today, every known example lies underwater, linked to island arcs and subduction zones, and many are located in the western Pacific Ocean. These basins mostly arise from pulling forces, driven by a process called oceanic trench rollback, in which a subduction zone shifts toward the plate being subducted. When first discovered, back-arc basins surprised geologists because convergent boundaries were thought to always be places of compression. In 1970, Dan Karig proposed a model that explained back-arc basins within the framework of plate tectonics.
These basins are usually very long and narrow—often thousands of kilometers in length but only a few hundred kilometers wide at most. A subduction zone must exist for back-arc extension to happen, though not every subduction zone produces such a basin. Back-arc basins form where the subducting oceanic crust is very old. Their limited width comes from the fact that magmatic activity depends on water and induced mantle convection, which restricts formation to areas along subduction zones. Spreading rates range from just a few centimeters per year (as in the Mariana Trough) up to 15 centimeters per year in the Lau Basin. The basalts erupted at spreading ridges inside these basins resemble those from mid-ocean ridges, but back-arc basin basalts are often much richer in magmatic water (typically 1–1.5 weight percent H₂O), whereas mid-ocean ridge basalt magmas are very dry (usually less than 0.3 weight percent H₂O). This high water content comes from water carried down the subduction zone and released into the overlying mantle wedge. Additional water may come from the eclogitization of amphiboles and micas in the subducting slab. Like mid-ocean ridges, back-arc basins host hydrothermal vents and the chemosynthetic communities that rely on them.
Evidence of seafloor spreading has been found in cores taken from the basin floor. Sediment thickness decreases toward the basin’s center, indicating a younger surface there. Harry Hess first proposed that the thickness and age of seafloor sediment relate to the age of the oceanic crust. Magnetic anomalies in crust formed in back-arc basins differ from those at mid-ocean ridges. In many places, the anomalies are not parallel, and their profiles lack symmetry or a central anomaly, pointing t
- type
- Geologic basin
- location
- Western Pacific Ocean (primarily)
- key_process
- Oceanic trench rollback
- first_model
- Dan Karig, 1970
- typical_length
- Thousands of kilometers
- typical_width
- Few hundred kilometers at most
- spreading_rate_range
- Few cm/year to 15 cm/year
Lore & Background
Back-arc basins are typically very long and relatively narrow, often thousands of kilometers long while only being a few hundred kilometers wide at most. For back-arc extension to form, a subduction zone is required, but not all subduction zones have a back-arc extension feature. Back-arc basins are found in areas where the subducting plate of oceanic crust is very old. The restricted width of back-arc basins is due to magmatic activity being reliant on water and induced mantle convection, limiting their formation to along subduction zones. Spreading rates vary from only a few centimeters per year (as in the Mariana Trough), to 15 cm/year in the Lau Basin. Spreading ridges within the basins erupt basalts that are similar to those erupted from the mid-ocean ridges; the main difference being back-arc basin basalts are often very rich in magmatic water (typically 1–1.5 weight % H2O), whereas mid-ocean ridge basalt magmas are very dry (typically <0.3 weight % H2O). The high water contents of back-arc basin basalt magmas is derived from water carried down the subduction zone and released into the overlying mantle wedge. Additional sources of water could be the eclogitization of amphiboles and micas in the subducting slab. Similar to mid-ocean ridges, back-arc basins have hydrothermal vents and associated chemosynthetic communities.
Reader's Guide
Back-arc basins are significant because they represent a key exception to the expectation that convergent plate boundaries are exclusively zones of compression. Their discovery and modeling by Dan Karig in 1970 helped refine plate tectonic theory, showing that extension can occur behind volcanic arcs due to trench rollback. The basins are characterized by asymmetric seafloor spreading, as evidenced by magnetic anomalies that lack symmetry and central anomalies typical of mid-ocean ridges. This asymmetry is variable even within single basins, such as the Mariana Trough and Lau Basin, and its causes remain poorly understood, involving factors like arc melt generation processes, heat flow, hydration gradients, and mantle wedge effects. The age of the subducting crust is critical: back-arc spreading requires subducting crust 55 million years old or older, explaining their concentration in the western Pacific. Sedimentation in these basins is strongly asymmetric, with most sediment supplied from the active volcanic arc. The basins also host hydrothermal vents and chemosynthetic communities, similar to mid-ocean ridges. Overall, back-arc basins provide insight into the complex dynamics of subduction zones and the processes that shape Earth's crust.
Did You Know?
- Back-arc basins were initially unexpected in plate tectonics because convergent boundaries were thought to be zones of compression.
- Spreading rates in back-arc basins range from a few centimeters per year (Mariana Trough) to 15 cm/year (Lau Basin).
- Back-arc basin basalts are rich in magmatic water (1–1.5 weight % H2O), unlike dry mid-ocean ridge basalts (<0.3 weight % H2O).
- The subducting crust must be at least 55 million years old for back-arc spreading to occur.
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