5.4 Back-Arc Basins
Extension Behind the Arc
Back-arc basins are extensional features that develop in the overriding plate behind volcanic arcs. Their existence posed a fundamental paradox when first discovered: how can extension occur in a convergent tectonic setting? The resolution lies in the dynamics of slab rollback and mantle wedge flow, which can produce extensional stresses in the overriding plate even as two plates converge.
Dan Karig (1971) was the first to propose that back-arc basins form by seafloor spreading behind island arcs, based on his studies of the Mariana Trough and the Philippine Sea. His recognition that these basins contain young oceanic crust with magnetic anomalies, thin sediment cover, and high heat flow — features identical to mid-ocean ridges — was a landmark in understanding convergent margin dynamics.
Driving Mechanisms
Two primary mechanisms have been proposed for back-arc extension, and both likely operate simultaneously in most settings:
Slab Rollback (Trench Retreat)
As discussed in Section 5.1, the slab's negative buoyancy causes it to sink and the trench to migrate toward the subducting plate. If the overriding plate cannot advance fast enough to keep pace with trench retreat, the overriding plate is stretched, creating back-arc extension. This is the dominant mechanism for most western Pacific back-arc basins. The rate of trench retreat directly controls the rate of back-arc opening.
Mantle Wedge Flow & Dynamic Suction
The corner flow induced by slab descent creates a dynamic pressure low in the mantle wedge. This "suction" effect can pull the overriding plate toward the trench, thinning the back-arc region. Additionally, the upwelling limb of the mantle wedge flow brings hot, buoyant material beneath the back-arc, thermally weakening the lithosphere and promoting extension.
Trench retreat velocity and back-arc opening rate:
\[ v_{\text{back-arc}} \approx v_{\text{rollback}} - v_{\text{overriding}} \]
where $v_{\text{rollback}}$ is the trench retreat rate (in a fixed mantle frame),$v_{\text{overriding}}$ is the velocity of the overriding plate toward the trench, and$v_{\text{back-arc}}$ is the resulting extension rate. When$v_{\text{rollback}} > v_{\text{overriding}}$, the back-arc extends; when$v_{\text{rollback}} < v_{\text{overriding}}$, the back-arc compresses.
Laboratory analog models (Funiciello et al., 2003; Schellart et al., 2007) demonstrate that back-arc extension depends strongly on slab width: narrow slabs allow toroidal mantle flow around the slab edges, facilitating rollback and back-arc opening, while very wide slabs impede rollback and tend to produce compression in the overriding plate.
Back-Arc Basin Basalts (BABB)
The basalts erupted at back-arc spreading centers have a distinctive geochemistry that is intermediate between mid-ocean ridge basalts (MORB) and arc basalts. These back-arc basin basalts (BABB) carry a characteristic "subduction component" that reflects contamination of the back-arc mantle source by fluids and/or melts from the subducting slab.
MORB
Low LILE, low water content (~0.1–0.3 wt% H2O), flat to depleted REE patterns. Generated by decompression melting of ambient depleted mantle at mid-ocean ridges.
BABB
Intermediate LILE enrichment, moderate water content (~0.5–1.5 wt% H2O), variable HFSE depletion. Generated by decompression melting of mantle that has been variably enriched by slab-derived fluids.
Arc Basalt
High LILE, high water content (~2–6 wt% H2O), strong HFSE depletions (Nb-Ta anomaly). Generated by flux melting of the mantle wedge directly above the dehydrating slab.
The subduction component in BABB can be quantified using trace element ratios:
\[ \text{Ba/Nb} \gg 1 \quad \text{(BABB, arc)} \quad \text{vs.} \quad \text{Ba/Nb} \approx 4\text{--}6 \quad \text{(MORB)} \]
Ba is fluid-mobile and enriched by slab fluids; Nb is fluid-immobile and retained in the slab. Elevated Ba/Nb ratios are a geochemical fingerprint of a slab-influenced mantle source.
Major Back-Arc Basins
Mariana Trough
An actively spreading back-arc basin behind the Mariana arc in the western Pacific. Spreading rate is ~3–5 cm/yr (full rate), asymmetric, and fastest in the south. The Mariana Trough is a type example of rollback-driven extension: the Pacific plate subducts steeply (~60–70°) and the trench retreats eastward, stretching the Philippine Sea Plate and generating new oceanic crust in the trough.
Lau Basin
Located behind the Tonga-Kermadec arc, the Lau Basin is one of the fastest-opening back-arc basins, with spreading rates up to ~9 cm/yr (full rate) in the northern part near the Tonga trench, where rollback is fastest. The Eastern Lau Spreading Center (ELSC) transitions from a MORB-like ridge in the south to a strongly arc-influenced ridge in the north as it approaches the volcanic front, providing a natural laboratory for studying the spectrum from MORB to BABB geochemistry.
Sea of Japan (Japan Sea)
Opened during the Miocene (~25–15 Ma) by back-arc spreading behind the Japanese island arc. The basin contains oceanic crust in its central part (Japan Basin) and thinned continental crust on its margins. The opening was accompanied by ~40–60° clockwise rotation of southwest Japan, documented by paleomagnetic studies (Otofuji et al., 1985). Back-arc spreading has since ceased, and the basin is now in a state of incipient compression.
Philippine Sea
The Philippine Sea Plate is largely composed of a mosaic of back-arc basins of different ages: the West Philippine Basin (~55–35 Ma), the Shikoku and Parece Vela Basins (~30–15 Ma), and the Mariana Trough (~6 Ma–present). This plate records the successive episodes of back-arc opening associated with the evolution of the Izu-Bonin-Mariana arc system.
Remnant Arcs & Arc Splitting
When back-arc extension splits a volcanic arc, the inactive portion that is stranded on the far side of the new basin is called a remnant arc (or abandoned arc). Remnant arcs preserve the geological and geochemical record of earlier arc magmatism and serve as markers for reconstructing the tectonic history of complex back-arc systems.
Lau Ridge
Remnant arc of the Tonga-Kermadec system. The Lau Basin opened between the active Tonga Ridge and the now-inactive Lau Ridge beginning ~6 Ma ago. The Lau Ridge preserves Miocene arc volcanic rocks that predate the onset of back-arc spreading.
West Mariana Ridge
Remnant arc separated from the active Mariana arc by the Mariana Trough. Before the Trough opened (~6 Ma), the West Mariana Ridge and the modern Mariana arc were a single continuous volcanic chain. The Parece Vela Basin, an older back-arc basin (~30–15 Ma), lies west of the West Mariana Ridge.
The process of arc splitting typically begins with rifting within or immediately behind the volcanic front, where the lithosphere is thinnest and weakest due to magmatic heating. Once rifting nucleates, the spreading center propagates along-strike, progressively separating the active and remnant arc segments. The new spreading center inherits a mantle source contaminated by slab fluids, which is why nascent back-arc spreading centers produce BABB rather than pure MORB.
Back-Arc Spreading vs. Mid-Ocean Ridge Spreading
Although back-arc spreading centers produce oceanic crust by the same fundamental process as mid-ocean ridges (decompression melting of upwelling mantle), there are important systematic differences:
| Property | Mid-Ocean Ridge | Back-Arc Spreading Center |
|---|---|---|
| Driving force | Ridge push + slab pull | Slab rollback + mantle wedge flow |
| Mantle source | Depleted MORB mantle (DMM) | DMM + slab fluid component |
| Water content | ~0.1–0.3 wt% | ~0.5–1.5 wt% |
| Ridge morphology | Axial valley (slow) or dome (fast) | Variable; often inflated due to higher melt supply |
| Crustal thickness | ~6–7 km | ~5–8 km (variable) |
| Longevity | ~200 Myr (Wilson cycle) | ~10–30 Myr (episodic) |
| Symmetry | Approximately symmetric | Often asymmetric (faster on arc side) |
Enhanced melt production in the back-arc due to water — parameterized productivity increase:
\[ \frac{\partial F}{\partial P}\bigg|_{\text{wet}} \approx \frac{\partial F}{\partial P}\bigg|_{\text{dry}} + \frac{X_{\text{H}_2\text{O}}}{D_{\text{H}_2\text{O}}} \cdot \frac{1}{\Delta T_{\text{solidus}}} \]
where $F$ is melt fraction, $P$ is pressure,$X_{\text{H}_2\text{O}}$ is water concentration in the source,$D_{\text{H}_2\text{O}}$ is the bulk partition coefficient for water, and$\Delta T_{\text{solidus}}$ is the solidus depression per unit water content.
Closure of Back-Arc Basins
Back-arc basins are transient features. They open when slab rollback is active and close when rollback ceases or reverses, or when a change in plate kinematics imposes compression across the basin. Closure can occur by:
Subduction Polarity Reversal
A new subduction zone initiates within the back-arc basin, consuming the basin floor. This has been proposed for the Solomon Sea and parts of the Philippine Sea. Polarity reversal can result from collision of the arc with a buoyant feature (continent, plateau) that jams the original subduction zone.
Rollback Cessation
When the driving force for rollback diminishes (e.g., the slab breaks off, the subducting plate becomes younger and more buoyant, or the trench reaches a continental margin), extension ceases and the basin cools. The Sea of Japan is an example: back-arc spreading stopped ~15 Ma and the basin is now being weakly compressed.
Obduction & Ophiolite Emplacement
In some cases, back-arc basin oceanic crust is thrust onto continental margins during collision events, forming ophiolite complexes. Many ophiolites show BABB geochemical signatures (e.g., the Troodos ophiolite of Cyprus), suggesting they originated in back-arc rather than mid-ocean ridge settings.
Key Numbers
~9 cm/yr
Max Lau Basin Spreading
~10–30 Myr
Typical Basin Lifespan
~25–15 Ma
Sea of Japan Opening
1971
Karig's Discovery