Theory 02/Geology & Volcanology /Tectonics/3 min
Arcs Form Where Slabs Tear
Arc volcanoes are supposed to sit a fixed distance above the slab. The exceptions line up with something else.
Volcanic arcs are located by tears in the subducting slab rather than by the depth of the slab beneath: magma rises through the tear, so arc gaps and anomalous volcanism mark slab segmentation rather than changes in the mantle wedge.
01The evidence
- Slab segmentation has been linked to anomalous arc volcanism and to the giant porphyry copper deposits of Indonesia, which places the tectonic discontinuity and the magmatic anomaly in the same place.Earth and Planetary Science Letters, 2023
- The abrupt end of the Trans-Mexican Volcanic Belt and the interruption of arc volcanism near the transition from flat to normal subduction show that arc continuity is controlled by slab geometry rather than by wedge fertility alone.JGR, Mexican arc
02The tension
The standard model places the arc at a fixed depth above the slab, typically around a hundred kilometres, where dehydration triggers melting. That predicts continuous arcs following the slab contour. Real arcs have gaps, offsets and segments that do not follow the contour, and the gaps are usually attributed to local variations in the wedge.
03The theory
A subducting slab is not a continuous sheet. It segments along transform structures inherited from the seafloor, and where two segments meet, the slab can tear. A tear is a vertical pathway through the slab that connects the deep mantle to the shallow wedge.
If magma preferentially rises through tears, then the arc is a projection of the tear pattern onto the surface, not of the slab depth contour. That explains gaps where the slab is continuous, offsets where a tear is oblique, and the association of anomalous volcanism with slab segmentation.
The prediction is that arc segmentation should match slab segmentation, observed seismically, and that the largest porphyry deposits should sit at tear intersections where the pathway has been long-lived. That is a mapping prediction rather than a geochemical one.
It also predicts that arc migration, when it happens, should be stepwise and located at segment boundaries, because the tear is fixed while the slab geometry changes around it.
04What this is not
The depth model predicts arcs at a constant distance above the slab. This predicts arcs at tears, which is why arcs are segmented and why the segmentation lines up with seafloor structures rather than with slab contours.
05Predictions
- P1Arc gaps should coincide with seismically imaged slab tears more often than with wedge anomalies.
- P2The largest porphyry deposits should cluster at tear intersections.
- P3Arc migration should occur in steps at segment boundaries rather than continuously.
06What would refute it
Arcs that follow slab contours continuously, with gaps uncorrelated with slab structure, would restore the depth model.
07What it would change
It would make mineral exploration target slab structure, and reframe arc gaps as tectonic rather than magmatic features.
08Evidence base
- Slab segmentation, anomalous arc volcanism and giant porphyry copper deposits in Indonesia — EPSLhttps://www.sciencedirect.com/science/article/pii/S0012821X23005447
- The abrupt end of the Mexican volcanic belt and the flat-slab transition — JGRhttp://authors.caltech.edu/47476/1/jgrb50646.pdf